Lehigh UniversityLehigh Preserve

Theses and Dissertations

1993

Coupling light emitting diodes to multimodeoptical fibersStephen J. WetzelLehigh University

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Recommended CitationWetzel, Stephen J., "Coupling light emitting diodes to multimode optical fibers" (1993). Theses and Dissertations. Paper 165.

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C upling ight Emittin

ids to Multim de

ptical Fibers

TE: .May 30,1993

Coupling Light Emitting Diodes to

Multimode Optical Fibers

by

Stephen J. Wetzel

A Thesis

Presented to the Graduate and Research Committee

of Lehigh University

in Candidacy for the Degree of

Master of Science

in

Electrical Engineering

Lehigh University

May 14,1993

ACKNOWLEDGEMENTS

Numerous interactions contributed significantly to this work. During the

period when I was developing experimental methods, C.F. Flores provided

insight and creative suggestions. He recommended optical coupling

geometries for analysis. At the same time, D.A. Snyder provided valuable

knowledge and recommendations for implementation. The simulation

program, which I modified and applied in this study, was conceived and

originally written by E. Rice. The simulation program was rewritten and

generalized by E.E. Bergmann. I acknowledge the significant efforts of

these two men. Useful discussions with D.R. Decker led to deeper

understanding of the applicability of the simulation program and aided the

comparison of simulation results with experimental results. also

acknowledge the support and encouragement of R.H. Knerr and D.J.

Wasser.

- 111 -

CONTENTS

Title Page iApproval Page iiAcknowledgements iiiTable of Contents ivList of Tables vList of Figures viAbstract 1

I. Overview of LED-to-Multimode Optical Fiber Coupling 3

1. Light Emitting Diodes as Optical Sources 4

2. Optical Fiber Characteristics 9

3. LED-to-Optical Fiber Coupling 11

II. Coupling Model 13

1. Simulation Program Description 14

2. Application of Simulation to LED-to-Fiber Coupling 17

3. LED Emission Profile Results 19

4. Lensed LED Coupling Results 20

III. Experimental Measurements of LED-to-Fiber Coupling 24

1. Light Emitting Diodes Studied 24

2. Measurements of Optical Coupling 24

3. LED Butt-Coupled to Multimode Fiber Results 27

4. Microspherically Lensed LED Coupling Results 28

5. LED Lensed with an Imaging Sphere Results 32

6. Two-Lens LED Coupling Results 35

IV. Discussion 38

1. Comments on the LED Model 38

2. Accuracy of Lensed Coupling Simulations 39

3. Recommendations for Future Work 44

References 76Author Biography 79

- IV -

Table 1:

Table 2:

Table 3:

Table 4:

Table 5:

Table 6:

Table 7:

Table 8:

Table 9:

LIST OF TABLES

Ray-Tracing Integration Parameters 15

Microspherical Lenses Examined 29

Lateral Fiber Tolerance Comparison for Imaging Geometry 34

Coupling Results for Two-Lens Approach 36

Coupling in a Two-Lens Arrangement Using a BK-7 1mmDiameter Macrolens 37

Comparison of Results for Axial Tolerance of MicrosphericalLens Geometry 40

Comparison of Lateral Tolerance Results for MicrosphericalLensing Geometry 41

Lateral Fiber Tolerance for Imaging Geometry 42

Lateral Tolerances of Two-Lens Geometry 43

- v -

53

Figure 1:

Figure 2:

Figure 3:

Figure 4:

Figure 5:

Figure 6:

Figure 7:

Figure 8:

Figure 9:

Figure 10:

Figure 11:

LIST OF FIGURES

Light Emitting Diode as an Optical Source 45

Optical Fiber Coupling Characteristics 46

LED-to-Fiber Lensing Geometries 47

Ray Parameter Notation for Simulation Program 48

Step Index Fiber Model of a Light Emitting Diode 49

LED-to-Fiber Coupling Test Set 50

Simulation Results: Normalized LED Internal and ExternalRadiance 51

Lateral Fiber Tolerance of Butt-Coupled Geometry 52

Experimental Lateral Tolerance of Butt-Coupled Geometry

Axial Tolerance for Microsphere Lens Materials 54

Axial Tolerance using Microsphere Lenses of VariousDiameters 55

Figure 12: Simulation Results: Lateral Fiber Tolerance using MicrosphereLenses of Various Materials 56

Figure 13: Experimental Results: Lateral Fiber Tolerance usingMicrosphere Lenses of Various Materials 57

Figure 14: Simulation Results: Lateral Fiber Tolerance for MicrosphereLenses of Various Diameters 58

Figure 15: Experimental Results: Lateral Fiber Tolerance for MicrosphereLenses of Various Diameters 59

Figure 16: Experimental Launched Power and Lateral Fiber Tolerance forMicrospherically-Lensed Diodes 60

Figure 17: Experimental Results: Effect of pin Contact Misalignment onFiber Coupling of Microspherically-Lensed LEOs 61

Figure 18: Fiber Lateral Tolerance for LEOs Microspherically-Lensed using300 ~m diameter Sapphire Lenses 62

Figure 19: Fiber Lateral Tolerance for LEOs Microspherically-Lensed using300 ~m diameter Zirconia Lenses 63

- VI -

Figure 20: Fiber Lateral Tolerance for LEOs Microspherically-Lensed usingGK-19 250 J.l.m diameter Lens 64

Figure 21: Fiber Lateral Tolerance for LEOs Microspherically-Lensed usingGK-19 100 J.l.m diameter Lens 65

Figure 22: Fiber Lateral Tolerance for LEOs Microspherically-Lensed usingGK-19 60 J.l.m diameter Lens 66

Figure 23: Experimental Results; LEO-to-Lens Axial Tolerance of ImagingGeometry 67

Figure 24: Axial Fiber Tolerance of Sapphire Imaging Geometry 68

Figure 25: Lateral Fiber Tolerance for Imaging Geometry using 1mmSapphire Lenses 69

Figure 26: Lateral Fiber Tolerance for Imaging Geometry using 1mmLaSF-18 Glass Lenses 70

Figure 27: Lateral Tolerances of Two-Lens GK-19/BK-7 250/250Geometry 71

Figure 28: Lateral Tolerances of Two-Lens GK-19/BK-7 508/356Geometry 72

Figure 29: Lateral Tolerances of Two-Lens GK-19/BK-7 762/485Geometry 73

Figure 30: Lateral Tolerances of Two-Lens GK-19/BK-7 965/658Geometry 74

Figure 31: Lateral Tolerance of Two-Lens GK-19/Sapphire 250/90Geometry 75

- Vll -

ABSTRACT

Experimental measurements of optical power launched into the core of a

multimode optical fiber from a light emitting diode are compared to power

values determined by a ray-tracing computer program. The simulation

models the LED as a step-index optical fiber having a "core" emitting area

slightly larger than the size of the current-defining contact. The optical

power emitted from the source is assumed to be a function of the current

density in the active layer of the diode. The current density, in turn, is

assumed to have a constant value over the central 75% of the source area

and to be a rapidly decreasing function of radius for the remaining source

area. Power coupling is examined for three LED-to-fiber lensing geometries:

a microspherically-Iensed diode; a diode coupled through a larger, isolated,

imaging lens; and a combined microsphere/imaging two-lens geometry. The

coupling capability of the various lensing geometries is'judged on their ability

to couple power and on their sensitivity to alignment.

Modeled diode radiance (power per steradian per unit source area)

decreases as cos 3 (8) rather than the first order cosine dependence of a

Lambertian emitter. The simulated butt-coupled launched power differs from

experiment by 0.5 dB when the fiber is laterally aligned with the diode.

fredicted launched powers for any of the three lensing geometries generally

- 1 -

differ from experiment by 0.5 - 3.0 dB. The powers predicted during lateral

tolerance analysis for the microsphere coupling geometry deviate from

.experiment by no more than 4.4 dl? Axial tolerances are predicted to be

values within a factor of three from the experimental tolerances. Again, the

worst performance is for microspherical lensing. Lateral tolerances are the

most accurately predicted parameter for LED-to-fiber coupling. For all

lensing geometries, lateral tolerances are within ± 50% of the experimental

tolerance.

The magnitude of the simulation's predictions differs from experiment

resulting from the simulation's tight angular distribution of power. Increasing

the power radiated at larger angles from the surface normal, making the

diode appear more Lambertian, should improve the model's ability to predict

LED-to-fiber coupling. An additional improvement would be to increase the

radiant flux density (power per unit source area) of the source.

- 2 -

1. Overview of LED-to Multlmode Optical Fiber Coupling

Light Emitting Diodes (LEOs) are effective sources for fiber optic

communication links of moderate bit rates (less than 200 Mb/s) and

moderate lengths (less than 2 km). Design of communication links using

LEOs must consider the coupling of the diodes into the optical fiber. This

design issue results from the differing spatial characteristics of the LED

emission and the fiber acceptance cone. A light emitting diode has a very

divergent light emission. Its radiance (power per unit solid angle per unit

source area) degrades to one-half its maximum value by moving from normal

incidence to an observation angle of 60 degrees. In contrast, optical fiber

exhibits a rather narrow angle of acceptance (12-20 degrees is typical). The

optical fiber transmits only those rays incident on its core area with

propagation angles less than the fiber's limiting acceptance angle.

This work examines LEO-to-multimode optical fiber coupling using both

experimentation and simulation. Coupling will be predicted using a ray­

tracing simulation for several LEO-to-fiber lensing geometries. The

simulation results will be compared to experimental measurements of LEO­

to-fiber coupling in similarly-lensed configurations. The results of these

comparisons will be used to evaluate the assumptions of the simulation

model and recommend improvements to the simulation model.

- 3 -

1.1 Light Emitting Diodes as Optical Sources

Surface-emitting light emitting diodes used as optical fiber communication

sources have evolved from homojunction broad-area devices with etched

wells (Burrus and Dawson 1970) to double-heterojunction planar devices

with current confinement (Keramidas, Berkstresser and Zipfel 1980; Escher

et al. 1982). The devices examined in this study are double-heterostructure

devices in the 0.87 Ilm wavelength Ga1_xAlxAslGaAs material system,

although similar concepts and results apply to InGa xAs 1_ xPllnP devices.

The devices are grown by Liquid Phase Epitaxy on n-type GaAs substrates,

and they consist of three epitaxial layers (Figure 1). Working downward

from the substrate (upon which the window layer is grown prior to its

removal), the first layer is a thick n-Ga1-xAlxAs window layer. The next

layer grown is the p-type GaAs active layer. The third layer is a p­

Ga1_xAlxAs confining layer. The window and confining layers form

heterojunctions with the active layer. Thus, their larger bandgaps contain

carriers within the active layer. In addition, the window layer provides

support to the structure while maintaining transparency to the emitted

radiation.

After epitaxial growth, the substrate is chemically etched; and contacts

are provided on the top and bottom of the diode. The n-contact metallization

- 4 -

is applied directly to the n-type window layer. This contact extends radially-

outward from a circular opening for the emitted light to escape. The bottom-

side p-contact is a circular dot of photolithographically-controlled size. The

size of the p-contact determines, to a large extent, the emitting region of the

active layer, and isolation is placed around the p-contact to prevent

unwanted currents. The currents can be assumed constant in the region of

the active layer directly above the p-contact.

Photons are emitted within the active region of a light emitting diode

when radiative recombination occurs. For high quantum efficiency and high

power output, the types of recombination processes must be controlled.· For

thin active layer diodes, non-radiative surface recombination at the

heterointerfaces reduces the quantum efficiency. For highly-doped active

layers, non-radiative Auger recombination processes may also be

introduced. The ratio of radiative to non-radiative recombination events is

maximized. These recombination events are spontaneous, and the emitted

photons have arbitrary orientation. However, the active region of a

semiconductor diode has a high diameter-to-width aspect ratio. For this

reason, the diode may be viewed as a surface of point sources. The LED

active layer's radiance (power per unit area per unit steradian) within the

source material decreases as a cosine function of the viewing angle. These

emitter characteristics are termed Lambertian. An approximate model of a

- 5 -

..t•

light emitting diode represents the radiance emitted from the active region asj-,.

the following decreasing function of viewing angle:

8(8) = 8(r) * cos(8)

where B(r) is the radiance of the diode junction at a given radius, r, from the

center of the emitting area (Miller and Kaminow 1988). The work of Lee and

Dentai suggests that the light intensity profile, B(r), within the semiconductor

material, above the active region, is proportional to the current density (Lee

and Dentai 1978).

Thus, knowledge of the current distribution allows prediction of the light

(optical power) distribution, and many competing factors internal to the diode

influence the current distribution. These factors include, but are not limited

to: doping concentrations, current densities, current confinement, layer

thicknesses, differential bandgaps, diffusion lengths, and recombination

processes. The active layer doping concentration influences the

speed/power tradeoff. The high-level of active layer doping necessary for

high-speed operation introduces non-radiative recombination processes,

which reduce the power of the device. Alternatively, the low-doped active

layer necessary for high-power operation reduces the speed of the device.

Since current density is directly related to the output power, as stated

above, increased power is achieved by current confinement both parallel and

- 6 -

perpendicular to the junction plane. Parallel to the junction plane, current is

confined by controlling the size of the p-contact by using dielectric isolation

or Schottky barriers. The p-dot size can be made too small for a given

current, causing the diode series resistance to increase. This decreases the

output power by increasing the energy bandgap by internal heating.

Perpendicular to the junction plane, current is confined by double

heterojunctions, where the larger bandgap of the materials outside the active

layer provides a potential barrier maintaining the carriers within the active

layer. The current distribution in the plane of the junction will be modeled

using the results of Joyce and Wemple. As more thoroughly discussed in

section 2.2, this model allows for current to flow through the active layer in a

region larger than the size of the p-contact. Current density in the active

layer region directly above the p-contact is assumed constant; while, regions

of the active layer radially outside of the p-contact have rapidly decreasing

current density (Figure 5).

The active layer thickness influences output power through two

competing factors. Increased active layer thickness provides increased area

for recombination to occur as carriers diffuse through the junction. (This

effect is minimized since the minority carrier density decreases with distance

from the junction.) Also, the active region absorbs its emitted radiation.

Thus, increased active layer thickness may reduce output power. The ratio

- 7 -

of active layer width to diffusion length influences the interfacial surface

non-radiative recombination (Tsang 1985).

In addition to these concerns influencing the current density and optical

power within the semiconductor material, the semiconductor / air material

interface affects the power distribution external to the diode material.

Semiconductor materials have high refractive indices relative to air. (GaAs

has a refractive index of 3.3 -3.65 depending on doping.) Light traversing a

high to low refractive index boundary will be refracted toward higher angles

from the interface normal. At incidence angles above the critical angle, light

rays will be totally-internally reflected within the semiconductor material.

Due to the index of refraction difference at the GaAs/Air interface,

radiance of the diode external to the GaAs is expected to degrade with a

cosine dependence on viewing angle (Barnoski 1976). Thus, for GaAs, only

rays having divergence angles less than or equal to 16 degrees will be

emitted into the air above the semiconductor, while rays of higher divergence

angle are totally-internally reflected. Further, the energy propagating at

"angles" less than the 16 degree critical angle will be refracted throughout

the full hemisphere (1t radians) above the diode surface.

- 8 -

1.2 Optical Fiber Characteristics

An optical fiber is a dielectric waveguide of cylindrical cross-section

consisting of an inner core region of higher refractive index than the

surrounding cladding region (Figure 2). Electromagnetic radiation in the

optical frequency range is contained within the fiber waveguide by total

internal reflection within the core or at the core/cladding interface. Detailed

presentations of the characteristics and operation of optical fiber waveguides

are readily available in the literature (see for example, Miller and Kaminow

1988). Since this simulation of optical fiber light transmission is based on

ray-tracing techniques, only the refractive index properties of optical fibers

will be considered.

The refractive index profile of the core of an optical fiber typically has one

of two characteristic patterns: step-function or graded-function of radius

(Figure 2). In both cases, the central fiber core is of higher refractive index

than the surrounding cladding material. In the conceptually simplier step­

index fiber, the refractive index has a constant value in the core region and

abruptly changes to a lower value at the core/cladding interface. The

refractive index profile of this type of fiber has a step-function appearance.

Optical energy propagates at various angles through a step-index fiber core,

undergoing refraction at the core/cladding interface. A graded-index fiber,

- 9 -

on the other hand, has an index of refraction which is highest in value at the

center of the core and diminishes toward the perimeter of the core. The

variation of the core refractive index with radius may take ali several forms,

the most common being. a parabolic profile. The core refractive index. is

always greater than the cladding refractive index. Optical energy propagates(

through this type of fiber by continuous refraction as the index continuously

changes until total internal reflection occurs.

The conditions for total internal reflection indicate the requirements on

rays within the fiber core to continue their propagation within the fiber.

These internal conditions, in turn, dictate the requirements on light entering

the fiber such that the light will propagate in the waveguide.

The angle a ray makes with the fiber axis when entering the fiber can be

related to the ray's critical angle for total internal reflection. The angle of

such an extreme ray is the half-angle of a cone of acceptable rays entering a

fiber waveguide. The cone of acceptable rays describes the "numerical

aperture" of the fiber which is defined to be NA = n sin(8 c ) for a step index

fiber (where n is the index of the medium into which emission occurs) and

NA = n1 (2~)112'\h -(r/a)2 for a parabolic graded-index fiber. For the

graded index fiber, n1 is the refractive index along the fiber axis, ~ is a

parameter determining the scale of the profile change, r is a given radial

- 10 -

position, and a is the radius of the fiber core (Palais 1988).

Rays which enter the fiber at angles greater than that required for total

internal reflection will not be retained within the fiber core. These rays may,

however, be totally internally reflected at the c1adding-to-buffer material

interface. Such rays propagate a short distance through the fiber, but they

soon diminish in power due to the high attenuation they encounter in the

cladding material. In the experimental measurements for this study, long

lengths of fiber are used to attenuate these cladding modes.

1.3 LED-to-Optlcal Fiber Coupling

A surface emitting LED is a highly divergent source as discussed in

section 1.1 above. For fiber optic communication, the LED is coupled to a

fiber waveguide that has a numerical aperture limiting the angular

divergence of propagated rays. Therefore, significant optical power is lost

upon LEO-to-fiber coupling (coupling efficiencies are typically 5-10%).

However, properly designed LEOs in combination with appropriate LED-to­

fiber lensing provide sufficient power coupling for LEOs to serve as cost­

effective light sources for moderate bit rate, moderate distance fiber optic

communication.

Optical power is "Butt-coupled" into a fiber when the fiber is nearly

"butted" against the LED surface in an attempt to capture emitted light before

- 11 -

it diverges outside the fiber core radius. This simple coupling technique

provides near optimal coupling when the source radius is greater in size than

the fiber core, since lenses cannot increase the radiance above that at the

LED surface. In situations when the diode emitting area is smaller than the

fiber core, lensing can be used to increase the power coupled. A source

radius / fiber core radius ratio less than one is favorable, since equivalent

currents channeled through smaller active regions increase the radiance of

the diode. Lensing can be used to provide increased coupling into the fiber

due to the increased radiance of the source (Tsang 1985).

The divergence of optical power emitted by an LED can be reduced (that

is, the beam can be made more convergent), although the radiance (power

per steradian per unit area) of the LED just above its active layer provides a

limit to the radiance obtainable. Many lensing schemes have been used to

increase coupled power between small area emitters and optical fibers.

Lenses can be placed on the emitter surface, the fiber surface, or within the

region between the source and fiber.

This study will examine three lensed coupling arrangements: a

microspherically-Iensed LED, an imaging geometry, and a two-lens geometry

(Figure 3). A microspherically-Iensed LED has a small (100-500 M-m

diameter) high refractive index lens mounted directly on the LED surface. A

- 12 -

convergent beam is created from the highly divergent, Lambertian LED

emission, so that there is a very high coupling point axially removed from the

LED surface. At positions near this point, launched power can be traded for

lateral tolerance. An imaging geometry places a larger (approximately 1 mm

diameter) high refractive index lens a distance just greater than the lens

focal length from the LED surface. A real image of the LED is projected

aXially toward the fiber. Highest coupling occurs at the image point, and

coupling can be traded for lateral tolerance with axial fiber movement. A

two-lens geometry uses both a microspherical lens located on the LED and a

larger imaging lens to provide a large lateral tolerance between th~ lenses.

The results of computer simulation of coupling in these three lens

configurations will be compared to experimental measurements. The lensing

geometries will be evaluated for power coupled into multimode fiber and for

sensitivity to misalignment.

2. Coupling Model

---.A ray-tracing computer program is modified to predict power coupling

from a light-emitting diode into a multimode optical fiber. Light rays originate

from point sources on the LED surface within an area defined by the diode's

current distribution. Light rays emerge from all point sources in all angular

directions from normal emission down to a limiting angle determined by the

- 13 -

program input. The location and propagation direction of the rays are traced

through an optical system between the LED and fiber. The location and

angle of propagation of all rays is compared to the dimensions and

acceptance angle of the receiving fiber in the plane of the receiving fiber to

determine which rays are successfully coupled into the fiber. The intensities

of successfully coupled rays are summed and compared to the total power

emitted by the diode into the hemisphere above its surface. The power

coupled in all simulations for this study is then compared to the power butt-

coupled into the fiber core.

2.1 Simulation Program Description

A ray tracing program designed to evaluate coupling loss between two

optical fibers is used to predict LED-to-multimode optical fiber coupling. 1

The program identifies rays by source location and propagation direction.

Source location is specified by the two polar coordinates, rand <1> l' in the

source plane. A given ray's propagation direction is specified by the two

angles, e and <1> 2' as shown in Figure 4. The program performs a four­

dimensional, recursive integration over the parameters r, <1> l' e, and <1> 2'

1. This AT&T Program is written in the C language and runs under MS-DOS. A recentimprovement to the user interface expands the program's applicability beyond fiber-to-fibercoupling to include source-to-fiber and fiber-to-detector analyses. This thesis work providesinitial results applying the program to LED-to-multimode fiber coupling.

- 14 -

Limits for these parameters are dictated by the coupling geometry as

tabulated below.

Table 1

Ray-Tracing Integration Parameters

Variable Symbol Min MaxSource Radius r a User-Supplied Radius

Source Polar Angle <1>1 a 271:

Ray Axial Angle e a User-Supplied "NA" of Source

Ray Polar Angle <1>2 a 271:

The rays traced through the system are chosen by the program to

comprehensively cover variable values within the limits on the four

integration variables previously described. Traces are done on rays in the

following sequence:

rmax 21t 8 max

J J Jo 0 0

21t

J (Ray intensity & Acceptance) d<l> 2 de r d<j> 1 dro

As each ray is traced through the optical system, its position, direction,

and intensity change due to the shapes and characteristics of material

interfaces. Direction and intensity change due to refraction and reflection at

optical interfaces, respectively. A ray's position, tracked by (x,y) coordinates

- 15 -

about the optical (z) axis, changes as the ray propagates. Its angles of

propagation, e and <1>2' change due to refraction at material interfaces; and

the ray's intensity diminishes at each interface due to reflection loss. At the

receiving element, each ray's position and propagation angles are compared

to the receiving element's size and acceptance angle to determine if the ray

is successfully accepted. The total intensity of accepted rays is calculated

as a percentage of the intensity of all rays leaving the source's surface.

The program reports the coupling loss (dB) for each coupling geometry

and alignment situation described in the input file. The reference power

used for reporting the power ratio is the sum of the number of all rays leaving

the source under the specified conditions of source size and numerical

aperture. (All source rays have initial intensity one.)

Weighting factors may be used to modify the magnitude (power) of

accepted rays as functions of axial angle of propagation, e, or source radius

position, r. They are exponential factors which are multiplied by the intensity

immediately after integration of a given axial propagation angle or source

radius value. The multiplicative weighting factors decrease from a value of

one at integration variable values of zero to a value of exp [- c x], wherex limit

X'imit is the maximum value of the integration variable and c is a constant

specified by the program user. The constant is chosen to improve the

- 16 -

simulation's agreement with experiment.

The user provides input to the program through a structured ASCII file

describing the materials and geometry of surfaces along the optical path.

For analysis of coupling between two optical fibers, information on the input

and output fiber refractive indices, refractive index profiles, numerical

apertures, and core sizes are given. Sections of the input file describe air

gaps, spherical surfaces, apertures, and coatings encountered between the

input and output optical fibers.

2.2 Application of Simulation to LED-to-Flber Coupling

In order to apply this ray-tracing program to the LED-to-Multimode fiber

coupling problem, I made changes to allow large values of input "fiber"

refractive index and to include a weighting factor more accurately describing

LED emitted power. In addition, I changed the output to report linear power

values, relative power (dB), or linear power per steradian of solid angle

subtended by the receiving element.

The LED can thus be described similarly to a step index optical fiber, with

refractive index of 3.3 for GaAs. The emitting surface of the diode is not

AR-coated. The angle of LED emission is limited to the critical angle for total

internal reflection at the GaAs-to-air interface. This maximum angle of

emission from the LED is then converted to an input "fiber" numerical

- 17 -

aperture using: na = n sin(Sc)' The "fiber core" radius specifies the region

of the active layer which is a source of emitted radiation. As a first

approximation, this "core" radius was taken to be the radius of the p-contact.

However, I obtained better agreement with experiment by using an active

region larger than the size of the p-contact. This increased active area is

due to current spreading. Since the circular n-contact metallization has a

larger radius than the circular p-contact dot, I allowed the radius of the active

layer to extend beyond that of the p-contact. I assumed that the fields

created by the potential between the contacts will spread carriers outward up

to a radius limit defined by a straight line drawn between the n- and p-

contact metallizations. Using r1 as the radius of the p-contact metallization, I

defined '2 to be the radius of the emitting portion of the active layer, as

described above. Then, using the results of Joyce and Wemple (1970),

current density is assumed constant in the active region which lies directly

above the p-contact; but current density in the active layer radially beyond

the p-contact drops off at radial positions r outside the p-contact radius

according to the equation:

sin 2 [tan- 1 (k) + k In~]/, = [r;r '_1_

sin2 [tan- 1 (k) + k In r: ]

- 18 -

The factor k is as described by Joyce and Wemple. The parameters for this

modeled LED device are shown schematically in Figure 5.

2.3 LED Emission Profile Results

Radiance (power per steradian per unit source area) emitted by the LED

active layer into the subsequent semiconductor material and into the air

above the diode was predicted by simulation. The results reflect the

decrease in radiance with viewing angle that is expected for a Lambertian

emitter (Figure 7). A Lambertian source emits isotropically within the source

medium. It has equal radiant intensity (power per unit area) across its

surface when viewed perpendicular to the source surface. The Lambertian

source will, however, show a decrease in radiance due to the decreasing

effective source area when viewed obliquely. The predicted diode radiance

decreases with viewing angle at a greater rate than the Lambertian cosine

dependence (Figure 7).

The diode internal radiance distribution is refracted at the diode/air

interface. As a result, power at divergence angles greater than the critical

angle is internally reflected, and power at divergence angles less than the

critical angle is refracted an amount determined by Snell's Law. For the

relatively small angles involved, the resultant propagation angles in air are all

equally increased by the GaAs/air refractive index ratio. As a result, the

- 19 -

radiance external to the diode will be less in magnitude, but it will show the

same functional dependence as the radiance internal to the diode. The

simulation program correctly predicts the external diode radiance relative to

the internal radiance (Figure 7).

The simulation program predicts that a properly aligned butt-coupled fiber

located 0.5 Ilm above the diode surface loses -13.3 dB of power during

coupling. The simulation's reference power is the power emitted by the LED

into the entire hemisphere above its surface. A 13.3 dB loss indicates that

4.7% coupling occurs. The model predicts that the butt-coupled geometry

has a 3 dB lateral tolerance of ± 30 Ilm at an axial location of 254 Ilm (Figure

8).

2.4 Lensed LED Coupling Results

Power coupled, axial tolerance and lateral fiber tolerance were predicted

for microsphere lensing using lenses ranging in diameter from 60-300 Ilm

and in index from 1.7-1.9. The axial fiber tolerance results for various lens

materials indicate potential coupling improvement of 3 dB over butt coupled

for lens materials with refractive index of 1.9 or larger (Figure 10). Lower

index materials reach a peak coupling equal to butt-coupling. For all lens

materials, peak microspherically lensed coupling is axially removed from the

lens surface, and coupling degrades sharply beyond the optimal axial

- 20 -

coupling location. Optimal power coupling occurs at an axial separation of

700 Ilm for a sapphire (n=1.7) 300 Ilm diameter lens and at axial separations

of approximately 300 Ilm for GK-19 (n=1.9) or Zirconia (n=2.1) lenses of

similar diameters.

Axial tolerance of microspherically lensed coupling is predicted by the

model to have higher peak coupled powers as lens diameter is increased.

The peak axial coupling location moves farther from the diode surface as

lens diameter increases (Figure 11). Peak power coupled ranges from -1.0

dB for a 60 Ilm diameter GK-19 lens to +3.0 dB for a 250 11m diameter GK­

19 lens. The peak axial coupling point is 70llm for the 60 11m diameter lens

and 300 Ilm for the 250 11m diameter lens.

Lateral tolerance results from simulated microspherical lensing indicate

the increased power coupling and decreased lateral tolerance of higher

index lens materials. Sapphire lenses of 300 /lm diameter couple -1.0 dB

relative power when aligned and have a ±80 /lm 3 dB lateral tolerance.

Higher index GK-19 and Zirconia lenses of similar diameter have +1.0 dB

relative power coupling capability, but their 3 dB lateral tolerance is reduced

to approximately ±45 Ilm (Figure 12). Simulation results as a function of lens

diameter indicate that power coupled and 3 dB lateral fiber tolerance both

decrease with decreasing lens diameter (Figure 14).

- 21 -

Power coupled, axial fiber tolerance, and lateral fiber tolerance were

determined using the simulation model for Imaging Geometries using 1mm

Sapphire (n=1.7) and 1mm GK-19 (n=1.9) lenses. Axial fiber tolerance

results for Sapphire indicate maximum coupling of -1.0 dB occurs at a lens­

fiber separation of 1550 11m (Figure 24). Lateral fiber tolerance results for

the sapphire lens indicate peak power coupling of -6.7 and -2.0 dB for LED­

lens/lens-fiber spacings of 254/1000 11m and 406/749 11m, respectively. The

3 dB lateral fiber tolerance of ±80 11m for the 254/1000 11m configuration

exceeds the ±33 11m lateral tolerance of the 406/749 11m configuration

(Figure 25). These findings indicate the power versus tolerance tradeoff.

LaSF-18 lenses (n=1.9) couple more power and have reduced lateral

tolerances in comparison to the sapphire lens. The LaSF-18 2541lm LED­

lens / 851 Ilm lens-fiber configuration has +0.6 dB power coupling capability

with respect to butt coupling and ±28 11m lateral tolerance. The LaSF-18

50811m LED-lens / 445 11m lens-fiber configuration has a -1.5 dB power

coupling capability and a ±20 11m lateral tolerance (Figure 26).

Predictions of power coupled and lateral tolerances for the two-lens

geometry were made for GK-19 (n=1.9) 250llm diameter microlenses

combined with either 1mm BK-7 (n=1.5) or 1mm Sapphire (n=1.7)

macrolenses. The lateral tolerances between the microlens and macrolens

- 22 -

as well as between the macrolens and fiber were determined. In all two-lens

material and spacing geometries, the interlens lateral tolerance initially

exceeds the macrolens-to-fiber lateral tolerance. For GK-19/BK-7 two-lens

geometries with interlens spacings less than 800 ~m, coupled powers of -4.0

dB with respect to butt-coupled values are attained when in alignment. The

lateral tolerance between the lenses increases from approximately ±120 ~m

for the 762 ~m lens-lens axial spacing / 485 ~m macrolens-fiber spacing

geometry (Figure 29) to approximately ±150 ~m for the 250 ~m / 250 ~m

geometry using the same lenses (Figure 27). The macrolens-to-fiber lateral

tolerance simultaneously decreases from approximately ±80~m for the 762

Jlm /485 Jlm geometry (Figure 29) to approximately ±75 ~m for the 250 ~m /

250 Jlm geometry (Figure 27).

Simulation results for two-lens coupling with different macrolens materials

indicates that a larger refractive index macrolens (Sapphire vs. BK-7)

decreases the lateral interlens tolerance by approximately 30 ~m, and

increases the optimal power coupled by approximately 1.0 dB (Figures 27,

31 ).

- 23 -

3. Experimental Measurements of LED-to-Fiber Coupling

3.1 Light Emitting Diodes Studied

Small-area Double-Heterostructure GaAIAs Light Emitting Diodes have

been extensively reviewed in the literature for use as sources for optical fiber

communication (Burrus and Miller 1971; Tsang 1985; Miller and Kaminow

1988). The diodes used in this work are similar to the Burrus diode" except

that the highly absorbing n-type GaAs substrate layer is replaced by a

transmissive n-doped GaAIAs "window" layer. Thus, etching is not required;

and a planar device results (Keramidas, Berkstresser and Zipfel 1980).

These devices incorporate current confinement for increased current density

by dielectric contact isolation which improves the power coupled into small

core, limited NA optical fibers. At 60 mA d.c. typical operating current, 100

IlW of optical power is normally coupled from the diodes used in this study

into a 62.5 Ilm glass optical fiber having a numerical aperture of 0.29. The

optical power emitted into the hemisphere above the diode was measured

using an integrating sphere to be approximately 3 mW.

3.2 Measurements of Optical Coupling

The optical power coupled from a surface-emitting GaAIAs Double

Heterostructure Light Emitting Diode into the core of a 62.5 Ilm glass optical

fiber was measured using the test set schematically shown in Figure 6. For

- 24 -

these measurements, the emitting diodes were rigidly clamped, and they

were d.c. biased using an HP6141 C Current Source. The receiving optical

fiber was mounted on a three-axis micrometer stage manufactured by

Newport Research Corporation. The optical power, coupled through a one

kilometer length of fiber to attenuate cladding mode power, was measured

using an Anritsu Model ML93A Optical Power Meter and sensor Model

MA95A. All work was performed on an air-suspension optical table.

Coupled power measurements were made in all three optical lensing

configurations described in section 1.3. This required precise location and

movement of a single lens as well as simultaneous use of two lenses. Two

methods were used for locating lenses. For the first method, I epoxied

lenses onto ultra-fine capillary tubes. These tube/lens assemblies were

attached to a Line Tool Co. three-axis micrometer stage for precise

movement. For the second technique, I epoxied small lenses directly onto

the surface of the emitting diode. This second technique was initially used to

measure coupling capability of a microspherically lensed LED and was

subsequently used during measurement of coupling in a two-lens geometry.

Measurements made in the imaging and two-lens geometries required

that both the fiber and the large lens be aligned in the plane of the diode

junction to establish the optical axis. The initial alignment was established in

- 25 -

the following way:

1. The fiber was initially aligned with the LED to achieve an optimal Butt­

coupled power. The power, fiber x-location, and fiber y-Iocation were

noted.

2. The fiber was then moved the minimum axial distance from the LED to

allow the lens to be inserted. The lens was inserted in this c1ose­

coupled position, and the lens' lateral position was optimized. The

lens' x-location and y-Iocation were noted.

3. Working under a microscope, the lens was moved axially toward the

LED to a near contact position, and the fiber was moved axially until it

nearly contacted the lens. These lens and fiber axial (z) locations were

used to establish proper LED-to-Iens and lens-to-fiber axial spacings.

4. The fiber was backed out axially to establish the desired coupling

distance, and the lens was backed out from the LED to establish the

desired LED-to-Iens and lens-to-fiber spacings.

5. Having established the desired axial positions of the lens and fiber, the

optimal fiber lateral position was verified.

The lens designs, described in section 1.3, were compared for their

power-coupling capability at various LED-to-Iens, interlens, or lens-to-fiber

- 26 -

spacings. Power measurements made with various amounts of lateral (in

the plane of the LED) or axial (along the optical axis) misalignment of one of

the components enabled me to evaluate each lens configuration's sensitivity

to alignment.

3.3 LED Butt-Coupled Into Multimode Fiber Results

Butt-coupling power from an LED into an optical fiber provides a

repeatable reference power for comparison of coupling geometries.

measured optical power launched with the fiber laterally aligned and

approximately 0.5 Ilm axially removed from the diode as part of the

calibration steps preceding any lens-coupled measurements. The diodes

used for this study typically had butt-coupled launched powers of -10.0 dBm.

I measured the sensitivity of butt-coupling to lateral fiber misalignment.

Lateral scans of power coupled into the 62.5 Ilm core diameter fiber were

completed on three diodes. These measurements were made at an axial

distance of 254 Ilm so that the fiber would not hit the top-side wire bond of

the diode. A typical single-axis lateral scan result for the butt-coupled

configuration indicates that the 3 dB lateral fiber tolerance for butt-coupling is

±33 Ilm (Figure 8). Measurements of power coupled as a function of both x­

and y-offset were taken at fiber displacement increments of 0.5 mils. A

perspective plot of the power coupled as a function of simultaneous x- and y-

- 27 -

misalignment (Figure 9) shows the symmetry and high sensitivity of the butt­

coupled arrangement.

3.4 Mlcrospherlcally Lensed LED Coupling Results

Initial tests were performed by gluing microspherical lenses to the surface

of the emitting diodes using optically transmissive epoxy having an index of

refraction, n=1.55. Five types of lenses were examined (Table 2). They

had diameters from 60 ~m to 300 ~m and ranged in refractive index from 1.7

to 2.1. The lens material designations BK-7, LaSF-18, and GK-19 refer to

optical materials defined by Schott Optical Glass, Inc. The lens' surface

finish was Grade 10 or Grade 25 as rated by the Anti-Friction Bearing

Manufacturer's Association. (A.F.B.M.A.) An average of three lenses of

each type were studied. Measurements were made to determine coupling

capability and sensitivity to lateral and axial misalignment.

Axial tolerance data (Figures 10, 11) indicates that a refractive index of at

least 1.9 is necessary for a lens of 250-300 ~m diameter to cause

convergence of the LED beam. These data show that sapphire, with a

refractive index of 1.7, has insufficient refractive power to narrow the beam

and cause an axial peak in power launched. These data also indicate that

GK-19 glass (n=1.9) and zirconia (n=2.1) have increasing ability to converge

the beam. It is important to restate at this point that the radiance of the

- 28 -

Table 2

Microspherical Lenses Examined

Material Refractive Diameter GradeIndex ()J.m)

Sapphire 1.7 300 25GK19 1.9 60 10

100 10250 10

Zirconia 2.1 300 25

emitted light can not be increased. Rather, the LED output is a rapidly

diverging beam; the lenses only affect the beam divergence. Radiance at

the peak power point cannot exceed the radiance emitted by the LED (Tsang

1985).

For materials of sufficiently high refractive index to refract the LED

emission, smaller diameter lenses cause greater convergence. The glass

material, GK-19, with a refractive index of 1.9, can cause an axial peak in

power for the LED in this study when the GK-19 lens diameter is at least 100

)J.m. The 60 )J.m diameter lens curve has no axial power peak, the 100 /lm

diameter lens curve peaks at z=40 /lm, and the 250 /lm diameter lens curve

peaks at z=1 OO)J.m (Figure 11).

The trends of increased launched power and decreased alignment

- 29 -

tolerance as functions of increased lens refractive index are noted in lateral

tolerance results (Figures 13, 15). Data are presented for various Sapphire,

GK-19, and Zirconia lens diameters. The increasing ability of higher index

". lenses to more effectively narrow the beam is evident by comparing lateral

tolerance data for nearly equivalent diameter lenses of sapphire, GK-19, and

zirconia. This is manifested by higher launched powers and smaller lateral

tolerances. Based on the GK-19 data, larger diameter lenses have greater

ability to collimate the LED output.

Experimental results of power coupled and lateral tolerance for diodes

lensed with microsphere lenses located above the diode surface on capillary

tubes are documented in Figure 16. Mounting the lenses on capillary tubes

allowed precise location of the lenses. Measurements using the lenses on

capillary tubes directly measured 3 dB lateral tolerance. Experimentally­

determined lateral tolerance results for diodes having microspherical lenses

epoxied on their surface are plotted in Figures 18-22. The tests providing

these data were organized to study the shape of the lateral tolerance curve

for each lens. Typically one diode, but in no cases more than two diodes,

were studied for lateral tolerance. For each diode studied, lateral tolerance

was measured in the ±x and ±y lateral directions. These four or eight lateral

tolerance measurements at each axial fiber location were averaged for these

plots. The error bars are drawn to ±1 standard deviation determined from

- 30 -

these measurements.

The LED and lens have been properly (laterally) aligned for all data

presented to this point. Further studies examine the effects of lateral

misalignments of the lens with respect to the active area of the LED. Such

results are useful for evaluating variations in coupling which could be

expected in manufacture of emitting diodes with attached spherical lenses

when the diode's top-surface lens-locating feature is misaligned from the

bottom-surface electrical contact. In Figure 17, I compare reduction in

coupled power and lateral tolerance which is caused by this diode contact­

to-lens misalignment. (Data for lenses that are properly centered are plotted

with small bullets. Data for LED active area-to-Iens lateral misalignment of

20 ~m are plotted with triangles, and data for diode-to-Iens misalignment of

40 ~m are plotted with boxes.)

In the left column of plots in this figure, reduction in coupled power is

plotted as a function of the axial separation between the lens and fiber. As

expected, the axial location for acceptable coupled power moves farther

from the LED as the refractive index of the lens decreases. These data

indicate the greater decrease in coupled power occurring for lower index

materials as a result of the lens-LED misalignment. This last result is due to

the lower ability of the lower index lenses to refract the angularly-diverging

- 31 -

light from a poorly aligned contact.

The right-hand column of plots shows the lateral fiber offset resulting in a

3 dB drop in coupled power as a function of axial lens-fiber separation. The

results indicate that greater lateral tolerance exists for lens materials of lower

index. Also, lateral fiber tolerance degrades more sharply for lower index

lenses as the lens and LED are laterally misaligned. The lateral tolerance is

greatly reduced for low index lenses when misalignment occurs due to the

lens material's inability to redirect angularly diverging light. The lateral

tolerance becomes asymmetric with the smaller value greatly suffering with

pin contact misalignment for lenses of low index.

3.5 LED Lensed with an Imaging Sphere Results

Spherical lenses with diameters on the order of 1mm have been used to

couple light from a GaAIAs LED into a 62.511m core diameter multimode

optical fiber in an "imaging" geometry. An imaging sphere is spatially

removed from both the source and fiber, and an image of the source is

created in the vicinity of the entrance plane of the fiber (positioning the

image precisely on the fiber plane decreases lateral tolerances). Lenses

with refractive indices of 1.5, 1.7, and 1.9 were considered, and none of the

lenses were anti-reflection (AR) coated. The lenses were evaluated for their

ability to couple optical power as well as provide both lateral and axial

- 32 -

tolerances.

Variables for the imaging geometry include: lens material, lens size,

LED-to-Iens spacing, lens-fiber spacing, LED lateral misalignment, lens

lateral misalignment, lens axial misalignment, fiber lateral misalignment, and

fiber axial misalignment. My measurements provide information on the

effects of LED-to-Iens spacing, lens-to-fiber spacing, and fiber lateral

misalignment on coupled power.

Results for LED-to-Iens separation (Figure 23) indicate that BK-7 glass

(n=1.5) has insufficient refractive power to bring significant LED radiation

within the fiber's NA. However, higher index sapphire (n=1.7) and LaSF-18

(n=1.9) provide acceptable powers at LED-to-Iens separations which are

practical for most packaging applications. For these measurements, lens­

fiber separation was optimized for each LED-to-Iens position measurement.

An additional power penalty, which must be considered in evaluating all

tolerances involved in packaging a 1 mm imaging lens design, is caused by

variation from the nominal axial lens-to-fiber ,separation. This loss can be

significant as demonstrated by the results for a 1 mm sapphire lens (Figure

24).

In addition to the power degradations caused by variation in LED-to-Iens

and lens-to-fiber axial spacings, the coupled power will drop when there is

- 33 -

/Table 3

Lateral Fiber Tolerance Comparisonfor Imaging Geometry

..LED-Lens Lens-Fiber Optimal Rei 3 dB Fiber

Lens Separation Separation Lens-Fiber Pwr Lat TolJlm (mils) 11m (mils) Separation dB ±Jlm

LaSF-18 254 (10) 749 (29.5) 1.3 38800 (31.5) * 1.1851 (33.5) 1.2 41

381 (15) 564 (22.2) 1.4 29612 (24.1) * 1.1665 (26.2) 1.4 29691 (27.2) 1.6 30

508 (20) 445 (17.5) 1.9 25495 (19.5) * 1.4546 (21.5) 2.0 25

635 (25) 330 (13) 3.6 25406 (16) * 2.2457 (18) 2.9 24

Sapphire 254 (10) 1016 (40) 1.5 491067 (42) * 1.2 46

356 (14) 818 (32.2) 1.4 41869 (34.2) * 1.2 41

406 (16) 749 (29.5) 1.3 36800 (31.5) * 1.1 33

508 (20) 597 (23.5) 1.4 25648 (25.5) * 1.0699 (27.5) 1.3 28

635 (25) 495 (19.5) 1.9 22549 (21.6) * 1.3 20599 (23.6) 22

- 34 -

lateral misalignment between the components (Table 3). That is, coupled

power drops if any of the elements (LED, lens, or fiber) is not centered along

the optical axis. If an LED is packaged in a connectorized package, LED-to­

lens lateral alignment will remain fixed during multiple insertions of a fiber. In

this implementation, the lens-to-fiber lateral alignment will vary. Lateral

tolerances have been measured at geometries where the fiber is axially

slightly removed from its optimal power point. This slight offset increases

lateral tolerance, though there is a corresponding power penalty. These

data show that for both lens types, lateral fiber tolerance decreases as the

lens is moved farther from the LED.

3.6 Two-Lens LED Coupling Results

A two-lens LED-to-fiber coupling design, as presented in section 1.3, has

four optical components: LED, microlens, macrolens, and fiber. A brief

study of the capabilities of a two-lens design was completed. Light-emitting

diodes with integrally-mounted 250 11m diameter GK-19 lenses were used for

these studies. The macrolens, mounted on a capillary tube mounted to a

three axis micrometer stage as discussed in section 3.2, allowed coupling

and lateral tolerances to be studied as functions of interlens and macrolens­

to-fiber spacings.

The results given in Table 4 show the tradeoff between coupled power

- 35 -

and lateral tolerance that exists for the two-lens design. Note that interlens

tolerance exceeds fiber lateral tolerance and that the two-lens lateral fiber

tolerance exceeds that of imaging or microspherical geometries. Further

investigation concentrated on use of the BK7 macrolens because of the

greater lateral tolerances it provides.

Table 4

Coupling Results for Two-Lens Approach

Integral Interlens Macrolens Macrolens Power Interlens FiberLens Spacing Description to Fiber Drop Lat. Tol. Lat. Tol.Type Ilm (mils) Type/DiaiGrade Ilm (mils) (dB) ±Ilm ±Ilm

GK19/250 250 (10) BK7/1 mrn/25 250 (10) 3.0 122 98GK19/250 250 (10) Sapphire/1 mrn/1 0 90 (3.5) 1.8 102 91

The two-lens design using a BK7 macrolens could provide coupled power

within 0.6 dB of the maximum power coupled using only a microlens. Table

5 lists data on the power coupled and lateral tolerances for the two-lens

design using a BK7 macrolens at a variety of spacings. Note that the 3 dB

lateral fiber tolerance remained nearly constant at ± 100 !lm, while the 3 dB

interlens lateral tolerance varied from ± 65 to ±130 !lm. The greater lateral

tolerance in a two-lens design exists between the two lenses, where the

power-to-tolerance tradeoff is least sensitive.

- 36 -

Table 5

Coupling in a Two-LensArrangement Using a BK-7 1mm Diameter Macrolens

Interlens Macrolens-to- Power 3 dB Lat. Tol. 3 dB FiberSeparation Fiber Separation Drop 1 Between Lenses Lat. Tol.J.lm (mils) J.lm (mils) (dB) ±J.lm ±J.lm

203 (8) 148 (5.8) 2.7 125 100254 (10) 148 (5.8) 2.8 120 102254 (10) 254 (10) 2.9 130 105254 (10) 305 (12) 3.0 130 na305 (12) 135 (5.3) 2.8 120 104356 (14) 226 (8.9) 2.9 120 100406 (16) 302 (11.9) 2.9 120 100457 (18) 345 (13.6) 2.8 115 100508 (20) 356 (14) 2.8 110 100559 (22) 399 (15.7) 2.7 90 100610(24) 439 (17.3) 2.6 105 na660 (26) 455 (17.9) 2.5 100 100711 (28) 495 (19.5) 2.4 95 100762 (30) 48b (19.1) ~ 2.4 90 104813 (32) 516 (20.3) 2.3 90 114864 (34) 737 (29) 2.2 70 112914 (36) 686 (27) 2.0 70 107965 (38) 658 (25.9) 1.9 65 100

1321 (52) 2 584 (23) 0.6 70 45

Note 1: Reduction in coupled power from the axial peak of the microlensed LED (-11.78 dBm).Note 2: Optimal geometry found by optimizing both the macrolens and fiber along three axes.

- 37 -

4. Discussion

4.1 Comments on the LED Model

Computer simulations indicate the proper equivalence between internal

and external diode radiance; however, both internal and external radiance

results deviate from the expected Lambertian profile by a factor cos2 (8)

(Figure 7). The greater angular degradation of the simulation indicates that

the step index fiber model for the diode does not "emit" sufficient photons at

large angles. This deficiency results from the different aspect ratios of a

fiber core and a diode active area. The fiber core is infinitely deeper than it

is wide, while the diode active layer is far thinner than it is broad (typically 1

Ilm thick and 50llm diameter wide). This thickness difference causes energy

emitted from a fiber core to be relatively more concentrated about the

surface normal due to wave propagation along the fiber axis. Emission from

a flat diode active layer, on the other hand, can be viewed as coming from a

surface of point sources. This energy profile pattern will be richer in wide

angle emission.

The model predicts a 13.3 dB optical power loss when butt-coupling the

diode into an optical fiber located 0.5 11m from the diode surface. This

means that 4.7% of the LED's total emitted power is captured by the 62.5

Ilm fiber core. Experimental measurements indicate that 2.3% of the diode's

- 38 -

total emitted power is butt-coupled into the fiber. The simulation's butt­

coupled power for a diode-to-fiber separation of 254 11m underestimates the

experimentally measured power by 0.5 dB, indicating that the simulation

predicts a coupled power that is 90% of the experimentally-determined

power (Figure 8). The combination of the simulation's greater percentage of

power coupled and lower absolute power coupled suggests that the LED

model is deficient both in large angle power emission and in power density.

The simulation's butt-coupled lateral fiber tolerance is 90% of the

experimentally determined value (Figure 8). An increased angular

distribution of emitted power would increase the simulation's predicted lateral

tolerance, making the results more closely resemble experimental results.

4.2 Accuracy of Lensed-Coupling Simulations

Simulation results of power coupled as a function of axial fiber position in

the microspherically-Iensed geometry have trends similar to experimental

results; however, simulation results consistently overestimate peak axial

power coupled by approximately 2.5 dB (Table 6). Overestimated peak

axial powers occur since the low angularly-divergent diode emission can be

made convergent. High power coupling occurs when the fiber is located

axially near the position of minimum beam width. The simulation's predicted

peak power axial positions are twice as large as experimentally determined

- 39 -

Table 6

Comparison of Results for Axial Toleranceof Microspherical Lens Geometry

Simulation Experiment ComparisonLens Lens Rei Peak 3 dB Rei Peak 3 dB Pwr Peak 3 dB

Material Size Pwr Axial Axial Pwr Axial Axial Rei Axial AxialJlrn dB 11m Jlrn dB Jlrn Jlrn dB ratio ratio

Sapphire 300 0.0 700 550 -2.2 400 1080 2.2 1.75 0.51

GK-19 250 3.0 300 275 0.3 100 550 2.7 3 0.5100

60

Zirconia 300 3.0 300 275 0.25 200 550 2.75 1.5 0.5

values (Table 6). The shape of the simulation's axial power coupling curves

is similar to experiment for the higher index GK-19 and zirconia lenses

(Figure 10). The predicted axial coupling curve for the lower index sapphire

lens has lower power values than experiment for axial separations less than

600 ~m (Figure 10). The model's axial coupling predictions for

microspherically-Iensed diodes could be improved if the diode were given a

greater angular distribution of power. Power emitted at higher angles from

normal emission would be hard for the low index sapphire lens to collimate,

and power of high emission angle would bring peak coupling locations closer

to the lens for higher index lens materials.

Predicted microspherically-Iensed power coupled at axial lens-fiber

- 40 -

separations of 500 Ilm overestimates experimental results slightly (0.4 dB)

for the sapphire lens and overestimates experimental results moderately

(3.0-4.0 dB) for GK-19 or zirconia lenses. The simulation program predicts

lateral fiber tolerances that are 5% too large for sapphire 300 Ilm diameter

lenses and 40% too small for GK-19 or zirconia lenses of similar diameter

(Table 7, Figure 16).

Table 7

Comparison of Lateral Tolerance Resultsfor Microspherical Lensing Geometry

Simulation Experimental ComparisonLens Lens Rei 3 dB Rei 3 dB Rei 3 dB

Material Size Pwr Tol Pwr Tol Pwr Tol

11m dB ±/lm dB ±/lm dB ratio

Sapphire 300 -0.8 80 -1.2 76 +0.4 1.05

GK-19 250 0.8 55 -2.2 86 +3.0 0.64100 -7.4 50 -9.4 82 +2.0 0.61

60 -12.7 20 -11.8 100 -0.9 0.20

Zirconia 300 1.3 40 -3.1 65 +4.4 0.62

The simulation results indicate increasing power coupling accuracy and

decreasing lateral tolerance accuracy for GK-19 lenses as the lens diameter

decreases (Table 7). Comparison of experimental results of

microspherically-Iensed lateral tolerance which include ± 1 standard

deviation error bars to simulation results of microspherically-Iensed lateral

- 41 -

tolerance indicates that the simulation results are not within the uncertainty

of the experimental measurements, with the exception of the results for the

GK-19 100 Jlm diameter lens (Figures 18-22).

Simulation of the axial tolerance of the imaging geometry overestimates

the peak power coupled by 0.2 dB, predicts an axial peak power coupling

location that is 45% farther from the diode than the experimental value, and

predicts the axial movement producing a 3 dB loss from the peak power

which is 5% in error (Figure 24). Predicted lateral fiber tolerances for the

imaging geometry are generally about 80% of the experimental value (Table

8).

Table 8

Lateral Fiber Tolerancefor Imaging Geometry

Simulation Experiment ComparisonLens LED to Lens to Rei 3 dB Rei 3 dB Rei 3 dB

Material Lens Fiber Pwr Tal Pwr Tol Pwr Tal

11m 11m dB ±Ilm dB ±Ilm dB ratio

Sapphire 254 1000 -5.5 85 -1.75 52 -3.75 1.63406 749 -2.0 30 -1.25 35 -0.75 0.86

LaSF-18 254 851 0.6 30 -1.2 41 1.8 0.73508 445 -1.5 20 -2.0 25 0.5 0.80

- 42 -

Modifying the LED model to have greater power at large divergence angles

would increase the lateral tolerance, increasing correspondence of

simulation results with experiment.

Simulation results for two-lens coupling estimate coupled power with a

systematic 2.0 dB error and estimate lateral tolerances within 50% of the

true values (Table 9).

Table 9

Lateral Tolerancesof Two-Lens Geometry

Simulation Experiment ComparisonMacro- Lens- Lens- Rei Lens Fiber Rei Lens Fiber Rei Lens FiberLens Lens Fiber Pwr Tol Tol Pwr Tol Tol Pwr Tol Tol(size) Ilm Ilm dB ±Ilm ±Ilm dB ±/lm ±/lm dB ±Ilm ±/lm

BK-7 250 250 -4.0 150 75 -1.7 130 105 -2.3 1.15 0.71(1mm) 508 356 -4.0 140 75 -1.5 110 100 -2.5 1.27 0.75

762 485 -3.6 120 80 -1.2 90 104 -2.4 1.15 0.77965 658 -2.1 95 85 -0.7 65 100 -1.4 1.46 0.85

Sapp 250 90 -3.0 120 65 -1.0 102 91 -2.0 1.18 0.71(1mm)

The lateral tolerance results are too large for the microlens-to-macrolens

tolerance, and they are too low to the macrolens-to-fiber tolerance. These

patterns result when coupling a restricted angular beam through a two-lens

system. The initial lens nearly collimates the beam pattern. A large second

lens can move large distances laterally and still capture the optical power.

- 43 -

The fiber, however, loses power coupling more quickly when moving laterally

in a more tightly-restricted angular beam.

4.3 Recommendations for Future Work

The results of this study simulating LED-to-fiber coupling using a step­

index fiber model for the emitting diode properly predict trends in coupling

parameters. Power coupling accuracy varies from 0.2 dB to 4.4 dB and

tolerance predictions range from ±5 % to 75% in accuracy. Interpretation of

the results suggests that the model should be revised to increase the surface

radiant flux density (power per unit area) of the diode and to increase the

angular divergence of emission. The increased surface radiant flux density

could be achieved by decreasing the radius of the active layer of the diode.

This decrease should occur only over the central portion of the active region

which has constant emitted ray density. Increased angular divergence could

be achieved using weighting factors in the beam direction angle integration

to increase the emitted power at large angles sufficiently to produce

Lambertian emission patterns.

- 44 -

e

I---L---------:.------¥-_l--l n-contact

p-activep-confining

isolation

heat sink

p-contact

'-

Figure 1: Light Emitting Diode asan Optical Source

- 45 -

The half-angle, 8, of the acceptance coneis defined such that the internal reflection

angle, <p, is the critical angle for Total Internal Reflection.

Ste Index Fiber

core

cladding

Index of Refraction Profiles for Step-Index andGraded-Index Fibers are of the form:

Step-Index

cladding

Graded-Index

-----.-~-- --------------------- -- -----

core

- - - - - -'-----,

Figure 2: Optical Fiber Coupling Characteristics

- 46 -

---&::~'. Imaging Geometry

----------~~~~1~---------------·.. ,

fCi'"---ICl-.... Two-Lens Geometry

~~f--------------------------

Figure 3: LE D-to-Fiber Lensing Geometries

- 47 -

Source Location Parameters: r, <1>1

Ray Direction Parameters: e, <1>2

A:. Y : / J+.2 .'if

: I '+' .... x

J e /\ ....../ .'

......... .f; ::.~:: ::>Z

Figure 4: Ray Parameter Notationfor Simulation Program

- 48-

Model Parameters:

n-contact

core radius = r2

na = n sin (8 sub c)

n=3.3

\\\

\\\\\\

n-window "layer \

\\

\\

\\

p-contact

: \

r1

8

r.

II

II

II

II

II

II

II

I

II

I

r2

Figure 5: Step Index Fiber Model ofa Light Emitting Diode

- 49 -

Current Source

IlrJJ~11 0

Figure 6: LED-to-Fiber Coupling Test Set

- 50 -

1.2

0.6

1.0

L

(/) 0.8'c-

Q)

~o

CL.

\JQ)N

oEo 0.4z

.. ·····:O·"..D.················ ..···············,·..· ·· ,................................... '-1

..~

0.2

1.00.80.60.40.20.0 [--I..---'-----'----'----'------L---'-------L.-.-L.-_

0.0

Angle of Obser. (rod)

Legend: o Junction Layer (Internal) Radianceo Radiance External to LED

cosi ne(angle)

Figure 7: Simulation Results; :!\ornlalized LEDInternal and External Radiance

- 51-

-6

-8

o-2 ~ , , , .; , , , .

- 4 ~ , , t!I'r''Y= , :~...,~ : : : .

:r:w -12~

o -140-

w -16>~ -18

~ -20

-22

-24

-26

-28'------'----'---'----"---'--.....L----'--.L..--I---'---'---'---'---'--....L-.L.-...I----'---'----'

-100-80 -60 -40 -20 0 20 40 60 80 100

...--..CD~ -1()

Fiber Offset (urn)

Legend:o Simulation o Experiment

Figure 8: Lateral Fiber Tolerance ofButt-Coupled Geometry

- 52-

Perspective Plot of Power Emitted from an LEDinto a 62.5 11m core Multimode Optical Fiber

(Total x- and y-movement is ±O.003 ";tic interval is 0.0005";Power scale is linear.)

Figure 9: Experimental Lateral Tolerance of Butt-

Coupled Geometry

- 53 -

1000 1500 2000 2500500o

4r-----------------------..32 )- ; ;J;

1o

-1,.-... -2CD\J -3~

I.. -4v~ -5o

0... -6

~ -7:;:;o - 8 1- ........................•.............................,.........................•...............'"'- '....""

v0::: - 9

-10-11-12 1- ; .

-13

-14- 15 l.-.----'--_-'--~_--'--------'-_--'----'_--'-_'----'-_'-......J

-500

Lens-Fiber Distance um

Legend:Sapphire 300 um diaGK19 250 um diaZirconia 300 um dia

a Simul"<\l Simulo Simul

G Experv Expero Exper

Figure 10: Axial Tolerance forMicrosphere Lens Materials

- 54-

4

2

0

-2

-4",-..... -6CD"D -8'-"~

-10v~0 -12CL

v -14>:.:; -160v -18a:::

-20

-22-24

-26

-28

0 0 0 0 0 0 0 0 0 0 0 0 00 0 0 0 0 0 0 0 0 0 0 0N N '<t (0 0Cl 0 N '<t (0 co 0 NI r- r- .- r- .- N N

Fiber Offset (um)

Legend:60 um die GK19100 um die GK 19250 urn die GK19

o Simul"V Simulo Simul

o Experv Exper

a Exper

Figure 11: Axial Tolerance usingMicrosphere Lenses ofVarious Diameters

- 55 -

25020015010050a

4

2

o- 2 ~ !... \:7:'it¥1 -!- ..•...•................... -!- , .

-4 ~ ·········.··········i·· .. ·· ..····················i·"-.:.:l

- 6 ~ : : \ t····· :r"l , , .

-8 ~ ! ! \ \ , ", , , .

~ -1 0 ~ : .IV~ -12o

Q.. - 14 ~... ·.. ······.. ·i, • ..• • · " ~

IV> - 16 1- < : \ H ·· ·· ·\·:· ·..· ·..· · ..·:· ·.. · ·..· ·

:;:;

~ -180:: - 20 ~ ..·· ..· · ·,·· ·..·..· ·..·: · ·..·..··..· ·,1

_ 22 ~ ; ! 'v

- 24 ~ ; ; , ; ; ! .

- 26 ~ ! ! ; : ~ ; .

-28 1- ,.

- 30 '-----'--_.L.----'-_.L.----'-_'-----'-_.L.----'-_.L.-----'----'

-50

Lat Fiber Offset (urn)

Axial Lens-Fiber Separation = 500 um

Legend:o Sopphire 300 urn dia"V GK-19 250 um diao Zirconia 300 um dia

Figure 12: Simulation Results; Lateral Fi berTolerance using Microsphere Lensesof Various Materials

- 56-

250200150100

. .···!·····························i············· oj-

50o

••• - .•.. j•••.• ,-,

4

2o (j} ! ! ! ..

-2-4-6 : roo .

-8~ -10v~ -12

Q.-14 , .

v>:.:; -16 .o~ - 18 j.. . j J.

-20············!·....

-22··· .. ·····: :

24 ; i- ·····················..··i·················..··········r ··········r················ .. - - .

-26- 28 I- :! ! ! - .

- 30 L.---L._-'----'-_--'-----'_-L-_.L---'--_.L---'--_....L..----'

-50

Lat Fiber Offset (um)

Axial Lens_Fiber Separation = 500 urn

Legend:• Sapphire 300 um dia ..., GK-19 250 urn dia.m Zirconia 300 urn dia

Figure 13: Experimental Resulls; Lateral FiberTolerance using Microsphere Lensesof Various Materials

- 57 -

5

.. . 1"" 2 _ ~ ..············_······t····················

. .

N--fl r -+--

I: :..................... -= ~........................ . .; ...•.•................ .:. ..•. _ 1. .....••..•.....••....•.

f '

io : . 1 • I I

----r- -1---r----r--rr----···················+··················r····· : y... 2.....•...............L...···············r··················

~ 1 ;

-20

-. -5CD"U.........I..V~0 -10a.v>~0v -150::

100 150 200 250 30050o- 25 ~-J..-----l---L_.L---'----'------.L_.L---L.--J---ll....-..L....--.L-----J

-50

Lateral Fbr Offset (um)

at 500 um Axial Lens-Fiber Separation

Legend: 6. 60 urn diameter GK19o 100 urn diameter GK19

o 250.um diameter GK19

Figure 14: Simulation Results; LateralFiber Tolerance for Micro­sphere Lenses of VariousDiameters

- 58 -

If) 0 If) 0 If) 0 If) 0 If) 0N N If) ~ 0 N If) ~ 0I ~ ~ ~ ~ N

. . .··· .. ·· ..··~··· ..·..··· ·l······.. . ~ , , ·· ..t· .. ···..· ; · ·····, ·..·· ·.. · · ·..........·:..........·1·..· ··..·..( --1· · ( ."'i I .

.............: 1" .

............: .

..................... .....L : : ' .

:;;::r 1-:;··..........·..··..r ·..·....:..·..·· ..·'I"·........ 'I' ..· ........·..,......

.: :.::..=.::.:.:: :::::::..,1":.: ::.::::::,,'::.. ".:~":::'","::.::.:.:: ..""!,"::':.:"::':" !."':":.:..:.:: , , ........_ ~ A: ·..· ··· ..·~ ~ iii ..............1' : .

.............................. ', -... . j' ~ ,"j'- ': .- ,-- .

4

2o

-2-4-6

CD -83 -10

I..

v -12~

o -140-v -16>~ -18otl -20a::

-22-24-26-28-30- 32 L-;L-...I---L---..1.~---L..----1----L--J----'--~..J.-..L..-..L-..L..-~L-...I--L---'

oIf)

I

Lot Fiber Offset (um)

Axial Lens-Fiber Separation = 500 um

Legend: v 60 um diameter GK 19II 100 um diameter GK 19o 250 um diameter GK 19

Figure 15: Experimental Results; LateralFiber Tolerance for MicrosphereLenses of Various Diameters

- 59 -

LateralTolerance

GK19.....,., ;

·.•••••.•••.•••.••·.:!!.:.•;:.•.::·.••..·.;.:J.1.·•.:.••.::.:.••.:O.•..·.: .••.•.••.::5.:•..•.••.:.d.••.••,..B..·:.·.••.••.;.•.;;[/ !!::~l;~=:b..i::: ::.:..:::1..·.:.-6.•::.:.::.:::•. ::.•.:.;::::O.dB.:.::::>::::> ~<'::};::::::::;.:-:::.::: .

.... ' :-:.;. ..:.::.:-: -:.:-:- :-:-:-:-:-:-:.:-: ;.:-:.:; "' -.-.,.:....::.:.:.:: (u··.ti·::····:i~t

I±120fUTl

-m~~~~if--"[+i~il-3~._'1~8I I ··········:r::::::··/:·

I I

200 400 600 800Axial Displacement (!J.rn)

Figure 16: Experimental Launched Power and LateralFiber Tolerance for Microspherically­Lensed Diodes

Sapphire 300 ~. Grade 25

-2Reduction

in -4 -----..._~

Power -6(dB) -8

Axial Lens-Fiber Separation UUn)

GK19 300 ~. Grade 10

-2Reduction

in -4----~Power -6

(dB) -8

Axial Lens-Fiber Separation UUn)

GK19 250 ~. Grade 10

ReductiOli2

in -4 __-_

Power -6(dB) -8

Axial Lens-Fiber Separation UUn)

Zirconia 300 ~. Grade 10

-2Reduction ~----

in -4Power -6(dB) -8

Sapphire 300 ~. Grade 25

120 .......--4

3 dB 100Lateral 80Fiber 60

Toleranc~O

20

Axial Lens-Fiber Separation (J..un)

GK19 300 Jll1l. Gratie 10

1203 dB 100

Lateral 80Fiber 60

ToleranceW20

Axial Lens-Fiber Separation (J..un)

GK19 250 Jll1l. Grade 10

1203 dB 100

Lateral 80Fiber 60Tol~O

20

Axial Lens-Fiber Separation (J..un)

Zirconia 300 Jll1l

1203 dB 100

Lateral 80Fiber 60

Toleranc~O

20

Axial Lens-Fiber Separation UUn) Axial Lens-Fiber Separation (J..un)

lens centered over LED contact[). 20 J.UIl pin contact offsetEll 40 J.UIl pin contact offset

Figure 17: Experimental Results; Effect of pin ContactMisalignment on Fiber Coupling ofMicrospherically-Lensed LEDs

- 61-

20015010050o

........... ..1. ; L i J .!...........1.._ .:.. i .

:i:~~·t··:·+~·:~-==j::tt·].-:·r··==1 1 j: ~ : . :

.............(" ).._ ~ ~....... ... ;.. ..._( _.. ··· ·_·_·T.. --·········;···_..·····.._-·············i·········· ···~.·············· .. i·····_· .. ·.· i .. _.... . f:.:: ':::.· ••••...•••••••.•..

~.: :.1. . .

: :••••• ••••••• j ! ••••.• ···········1' j j ..••.. .•..••..••••. ~.: •....••.. _._._.~: .

j ~ ~ I ! :.............; i · i. i i _ _ i.. . ~ ! .

I . 1 1 i ~i . : : :: :·············,···_············1·········_······,······_·······..I···_··········~I-········_···-l.. _... .... ··!·················r·············.•··

....... ·1·············,···········+·············/·······1···· j ····r ············1·················

.............................- -········1··_· ·······+_·_··········.··············· . - ..! j j i i j ~ ~

...............................;. ·········i············· ~·············· ..·r······· ~ 1 ··r················j·· .

··········..+···············1············..···:.. ··..····..·..·.:....···..····..·+········..··.. ·1··..·..·······1··· ··1···············"!-_··.. ·· ..····..; : : . : . . : j

.............! !..... ···· 1··· ..········ ~ ! _ j !..... . ! ~ _ .1 1

.............i i i _.. i i i ····.·i i ..j .i . ! ~ :. .= j .: ','~

......._ .; i _ .i l _t l _ J l._ _ l .~ ~ i . i . j ~ i

4

2

o-2-4-6-8

L. - 10v~ - 12o

Q. - 14v~ -16

~ -180: -20

-22-24

-26-28-30

Lat Fiber Offset (um)

Axial Lens-Fiber Separation = 500 um

Legend: o Simulation Resultse Experimental Results

Figure 18: Fiber Lateral Tolerancefor LEDs MicrosphericallyLensed using 300 umdiameter Sapphire Lenses

- 62 -

! : . j ; : • : !·······_··· ..·········...······r····..······-t····- ···_·.. i·················~·················l··········· ·····+·········_·····i·················~······

1 j 1!·..·_~ _..·l ..·..·..·..i..···_· ·"·l..· j · ·..·,· ·-r ·_·__·I _j _ ·..····..· r ·..r..· ·:..·· ··"l..· ·..· r· T T-..-- r ··..·T .. ·· .........._..,.................•.................,.. . " ·..·············.·················1················+·..·.·····_··1··········.······1·······_····.· .

._ -1.... 1 ..1 - .-' I .i. _ L _..L.._._ l -

..........-J ! J i _.~ -,- i -l .i ! .

1-)1 1 11- 1-11--

Ei=:_:]::::I::r::::::l:~:E:-J:. , i . 1 i ::•. , __ __•.......•........ ····1·.·_..·_· ,· .. ············ ,·R __ .' ·.1····••.. ·········,.·.•.. ·..·..•····.

.....H ; .. H H ..J. J ! H· T......... J... H ..

.............i i i i i i. L__ .L i .~ 1 ~ ~ . ; ~ i j

.............t ~ ~ 1 ···t········ j , ~- .....•••.•••.••~••••.••.••. -••... j .: . : : : : : ; .

.............i. i t .i i l _ .l._.._ .l i _ .

....·..·_·i-..·_ _·f..· _j..·..· ·r-· j· i · ~· · ~ l _..··..

4

2

0-2

-4-6

,-....CD -81)'-"~ -104.>~ -120

0.. -144.>> -16~0

-18~

cr:-20-22-24

-26-28-30

o 50 100 150 200

Lat Fiber Offset (um)

Axial Lens-Fiber Separation = 500 um

Legend: 0 Sim ulation Results111 Experimental Results

Figure 19 Fiber Lateral Tolerancefor LEDs MicrosphericallyLensed using 300 urndiameter Zirconia Lenses

- 63-

4

20

-2-4-6

",,-... -8CDiJ

-10'-'"

...v -12~0 -140...

v -16>...., -180I1J -20cr

-22-24-26-28-30-32

lJ") 0 lJ") 0 If) 0 If) 0 11) 0 tf)

N N lJ") r-- 0 N l1") r--. 0 NI ..- ..- ..- N N

Lateral Fiber Offset um

Axial Lens-Fiber Separation = 500 um

Legend: o Simulation ResultsG Experimental Results

Figure 20: Fiber Lateral Tolerance forLEDs Microspherically - Lensedusing GK--19 250 urn diameterLens

-64-

ooN

o l()If) ,.....

r-

If)N

r-

oo

o l() 0N l()

4 ..

2o

-2-4-6

CD -8~ -10L.

~ -12~

o -14Q.

~ -16>

:;:; -18o"i) -20a:::

-22-24

=~~EI]li[]]:J-:I:~r-:EFI]=I-[-i ; i : ~ : 1 : 1 : ~ : 1 : ; : ;

- 32 L.--.L..-.L..-.L..-.L---'--.L--..L--~~--'---L--'--'---l--L--L--L---I

If)NI

Axial Lens-Fiber Separation = 500 um

Legend: L::. Simulation ResultsA Experimental Results

Figure 21: Fiber Lateral Tolerancefor LEDs Micros2herically Lensedusing GK -19 100 urn diameter Lens

- 65 -

'i: I I I ! i i··_···_····~· ..···... ······l·················~·······_······_!·· ..·············i·_···· ..·······!······_···.. ···j·----·r·····_···_····i·············_·

IJ") 0 If) 0 If'> 0 If'> 0 If) 0N N If) f""- a N If) f""- aI ~ ..- ..- ..- N

Lateral Fiber Offset urn

Axial Lens-Fiber Separation = 500 urn

Legend: 0 Simulation ResultsII Experimental REsults

4

2o

-2-4-6

CD -8~ -10\-v -12~

o -14Q.

~ -16~ -18o"i) -200:::

-22-24-26-28-.30- 32 L.-I..-....-I'---'---I.-..L--'---L.--'---'---L-......L.---L--'--~..L--~'---'---I.---'

otoI

Figure 22: Fiber Lateral Tolerance forLEDs Microspherically- Lensedusing GK-19 60 urn diameterLens

- 66 -

Or-------------------.. .. ....................................... , ··1····· ...... : .

. . . .. .· ~ : :. . : ~ ~ ~ ~ ; : , ~ .

.. ..

:::::::::::~:::BK~7:'.~:::::;:::::;:::::~:::::::::::~::::::~::::: :::::; :::::-8 ~ : : : ~ ~ ~ ~ : : ; .

. . . . .

. ....• •••• , ••••• ~ •••••• : •••••• ~ ., ., ~ ••••• ~ ••••• ~ ••••••••••• 0' •••••• : ••••• , ••••• • ••••

. . .· , -., . . _..... . .-6 : : : ; ; ~ : : ~ .. ...... ' : -: " ; : : : :- -: ' .

. . .....• •••••••••• ~ ••••• '7" •••• '7' ••••••••••• 7••••• : ••••• : ••••• '":' •••• ':'" • • • •• ••••• • ••••

-4 .. :::::::::~:::-: :~::::: :_~:: .. ;:: :_::~..::.....-~.....:;/.....: .....~: ::~~~:c::: ::::: ::: ..PowerDegradation

fromButt

Coupled(dB)

. . . . . .. .·.... ~ ..... ~. .... . ..... ~ ..... ~ ..... : ..... : ..... :..... ~..... ':" .... , ..... : ...... . . . . . . .. .·...... .... . . ''- " '" ..... . .

-10 : ·····:······:······:······~···.·i·····~······:······:· : .. . . . . . . . ., .· ~ .- : ~ ~ -: ' , : .. . .. .

100 200 300 400 500 600 700 800 900 1000 1100 1200

LED-Lens Separation (um)

Figure 23: Experimental Results; LED-to-Lens Axial

Tolerance of Imaging Geometry

- 67 -

0.0

-0.5 r- ; ~ .; : : ~ + : ~ ~ .; : ~ ., .; .

- 1.0 I- ~ L .; : ~ ., , ..~

-1.5

-... -2.0CD-0........,L -2.5v~0 -3.0Q.

v-3.5>:.:;

0Q) -4.0(Y

-4.5

-5.0

-5.5

-6.00 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 00 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0co (j) 0 ,-- N I"") '<t If) lD r-- co (j) a ,-- N 1'0 ~

,-- ,-- ,-- ,-- ,-- r- .-- ,-- ,-- ,-- N N N N N

Lens-Fiber Distance (um)

Axial LED-to-Lens spacing = 250 urn

[ Legend:

Figure 24:

o Simulation Resultso Experimental Results

Axial Fiber Tolerance ofSapphire Imaging Geometry

- 68 -

14012010080604020

o-2

-4

-6 ..~ ..."

-8 ····•••••••·•·· •• 1····.·.······.··..··.···•. ·.··.···....·.. ;.···· ..···\..·.·.·· ..··.00··•••~ =;~jf-E..............T'··.·.·~.. ·:..::: :::::::.::.:::::::.:::::::~ - 14 ....·........+......···· ..)· .. ·....· f ·..~ .. ·····..······ ..·+·.. ··..·.. ·······..··j·· ·· .. ······· f··· ;

-16 ~.........................+ :~ ~ : " ..

:- - 18....+....... ..1 + :.......'1'.. , :.o ··········.. ·· .. ···f···· .. ···············:········ .. ······ , : \., : .~ - 20 ~ L ~ ..

0:: .. ..; :.... _..... ..-22 _ .

-24I- , : ..

-26 1- .

-28r ........ ·....+ ........·..·........ :· ..........·........ ~· ..·..· ........·..·:·

- 30 L.-..I....-...L----L.---!--~--L.--'--'---'_L..-::l...--...l.._...L----L.---!-----l

-20 0

Lot. Fiber Offset (um)

Legend for LED_lens / lens_fiber Spacings:254/1000 um 0 Simul (') Exper406/749 um \l Simul .., Exper

Figure 25: Lateral Fiber Tolerance forImaging Geometry usingImm Sapphire Lenses

- 69-

4 I­

2 -

-~ :.~: •............. -.

-4 ~T~~p ." :.: ..: ..- 6 -". '"\ \. "... ··i.. ·.. · ..· ..···,..

-8 . \ \\ r-

. .........

806020 40-20 0

V -10 \ "\~ \~ \ ; ~ ! ..

o -12 \ \Q.~ - 14 , ,\ , ,.(~) , ..

........ ,... .. , \ ,. ,"'(' , , ..:g - 16 - . . ~ i .;..\, \.\ .; ; ~ ..

lU .. ..., \.\ i \'..... , .

0::: -18 f- \ \........................................ \ ' \ , ; .;

- 20 1-.. A ,... ..\l: ~ .; ..

- 22 ";. ":.:.. :: .. ;:.. ::.. :\\\: .. :~ :: ..: : : .;".:: :.: :- 24 -..............' i···· ..~J).,i .. · .. ····· .. ·. i ~~ .

-26 _ ,....... :

- 28 1-. I ..······ 'j'" ... .. j .. "".j

100 120 140

Lot. Fiber Offset (um)

Legend for LED_lens / lens_fiber Spacings:254/851 um: 0 Simul 0 Exper508/445 um t:> Simul A Exper

Figure 26: Lateral Fiber Tolerance forImaging Geometry using1mm LaSF-18 Glass Lenses

- 70 -

0-2-4

-6

-8-10

,,-...,CD -12\J........,

-14~

v -16~0

-18Q.

v -20>~0 -22Q)

0::: -24-26-28-30-32-34

-50 a 50 100 150 200 250 300 350

Lateral Offset (um)

Conditions: GK -19 250 um MicrQlensBK-7 1 mm Mocrolens

250 um lens-lens spocing250 um macrolens-fiber spacing

Legend:o Simul. Lens-Lenso Simul. Lens- Fiber

o Exper. Lens-LensEI Exper. Lens-Fiber

Figure 27: Lateral Tolerances ofTwo--Lens GK-19 / BK-7250/250 Geometry

- 71 -

50 100 150 200 250 300 350

Lateral Offset (urn)

0-2-4-6-8

-10-12,......

CD -14-0"--" -16

L-

v -18~0 -20Q.

v -22> -24:.;:;0

-26Q.)

0::: -28-30-32-34-36-38-40

-50 0

Conditions: GK-19 250 um Microlens;BK7 1 mm Macrolens;

508 um Lens-Lens spacing'356 um macrolens-fiber spacing

Legend:o Simul. Lens-Lenso Simul. Lens - Fiber

o Exper. Lens-LensIII Exper. Lens- Fiber

Figure 28: Lateral Tolerances ofTwo-Lens GK-19 / BK7508/356 Geometry

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0-2-4-6-8

-10,,-.... -121Il -14"0......... -16~

rv -18~0 -200-

rv -22> -24....,0

-26rva::: -28

-30-32-34-36-38-40

-50 a 50 100 150 200 250 300 350

Lateral Offset (urn)

Conditions: GK-19 250 um Micro/ens;BK7 1 mm Macrolens;

762 um Lens- Lens spacing;485 um macrolens-fiber spacing

Legend:o Simul. Lens-Lenso Simul. Lens-Fiber

o Exper. Lens-Lens£I Exper. Lens-Fiber

Figure 29: Lateral Tolerances ofTwo-Lens GK-19 / BK7762/485 Geometry

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50 100 150 200 250 300 350

0-2-4-6-8

-10-12,-....

CD -14u'--" -16

L

IV -18~0 -200...

IV -22.~ -24oW

0v -26

a:: -28-30-32-34-36-38-40

-50 0

Lateral Offset (urn)

Conditions: GK-19 250 urn Microlens;BK7 1 rnm Mocrolens;

965 urn lens-lens spacing;658 urn macrolens-fiber spacing

Legend:o Simul. Lens- Lenso Simul. Lens-Fiber

G Exper. Lens-LensEI Exper. Lens-Fiber

Figure 30: Lateral Tolerances ofTwo-Lens GK-19/BK7965/658 Geometry- 74-

0-2-4-6-8

-10-12..-....

CD -14-0'--" -16

L11) -18~0 -20Q.

11) -22> -24+J

0

IV -26a:: -28

-.30-32-34-36-38-40

-50 0 50 100 150 200 250 300 350

Lateral Offset (um)

Conditions: GK -19 250 um Microlens;Sapphire 1mm Macrolens;250 um Lens-Lens spacing;90 um macrolens-fiber spacing

Legend:o Simul. Lens - Lenso Sirn'JI. Lens-Fiber

o Exper. Lens-LensEJ Exper. Lens-Fiber

Figure 31: Lateral Tolerance ofTwo-Lens GK--19 / Sapphire250 / 90 Geometry

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REFERENCES

Abe, M. et al. July 1977. High-efficiency long-lived GaAIAs LED's for fiber­optical communications, IEEE Transactions on Electron Devices ED-24(7): 990-994.

Abram, A.A., R.W. Allen, and A.C. Goodfellow. August 1975. The coupling oflight-emitting diodes to optical fibers using sphere lenses. Journal ofApplied Physics 46 (8): 3468-3474.

Ackenhusen, J.G. November 1979. Microlenses to improve LED-to-fiberoptical coupling and alignment tolerance. Applied Optics 18 (21): 3694­3699.

Barnoski, M.K., ed. 1976. Fundamentals of Optical Fiber Communications.New York, NY: Academic Press, Inc.

Berg, H.M., G.L. Lewis, and C.W. Mitchell. December 1981. A highperformance connectorized LED package for fiber optics. IEEETransactions on Components, Hybrids, and Manufacturing TechnologyCHMT-4 (4): 337-344.

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Borsuk, J.A. April 1983. Light intensity profiles of surface-emitting InGaAsPLED's: Impact on coupling to optical fibers. IEEE Transactions onElectron Devices ED-30 (4): 296-303.

Burrus, C.A and B.1. Miller. December 1971. Small area double­heterostructure aluminum-gallium arsenide electroluminescent diodesources for optical-fiber transmission lines. Optics Communications 4 (4):307-309.

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Escher, J.S., H.M. Berg, G.L. Lewis, C.D. Moyer, T.U. Robertson, and H.A.Wey. September 1982. Junction-current-confinement planar light­emitting diodes and optical coupling into large-core diameter fibers usinglenses. IEEE Transactions on Electron Devices ED-19 (9): 1463-1469.

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Horiuchi, S., K. Ikeda, T. Tanaka, and W. Susaki. July 1977. A new LEDstructure with a self-aligned sphere lens for efficient coupling to opticalfibers. IEEE Transactions on Electron Devices ED-24 (7): 986-990.

Joyce, W.B. May 1980. Current-crowded carrier confinement in double­heterostructure lasers. Journal of Applied Physics 51 (5): 2394-2401.

____, and S.H. Wemple. August 1970. Steady-state junction-currentdistributions in thin resistive films on semiconductor junctions. Journal ofApplied Physics 41 (9): 3818-3830.

Keramidas, V.G., G.W. Berkstresser, and C.L. Zipfel. November 1980. Planar,fast, reliable, single-hetrojunction light emitting diodes for optical links.Bell System Technical Journal 59 (9): 1549-1557.

King, W.C., B.H. Chin, I. Camlibel, and C.L. Zipfel. July 1985. High speed highpower 1.3 ~m InGaAsP/lnP surface emitting LED's for short-haul wide­bandwidth optical-fiber communications. IEEE Electron Device LettersEDL-6 (7): 335-337.

Kressel, H., ed. 1987. Semiconductor Devices for Optical Communication. Vol.39, Topics in Applied Physics, Second Edition. New York, NY: Springer­Verlag.

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Palais, J.C. 1988. Fiber Optic Communications, Second Edition. EnglewoodCliffs, NJ: Prentice Hall.

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Saul, R.H. April 1983. Recent advances in the performance and reliability ofInGaAsP LED's for lightwave communication systems. IEEETransactions on Electron Device ED-30 (4): 285-295.

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Wada, 0., H. Hamaguchi, Y. Nishitani, and 1. Sakurai. May 1982. High SpeedResponse InGaAsP/lnP DH LED's in the 111m Wavelength Region. IEEEElectron Device Letters EDL-3 (5): 129-131.

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AUTHOR BIOGRAPHY

Mr. Stephen J. Wetzel was born in Allentown, Pennsylvania in 1960. His

parents are the late Doris A. (Graver) Wetzel and the Reverend Willard W.

Wetzel, currently of Northampton, Pennsylvania. Mr. Wetzel received an A.B.

in Physics, Magna cum laude, from Franklin & Marshall College, Lancaster,

Pennsylvania, in 1983. He has been employed by AT&T Bell Laboratories

since 1984, and currently works at the AT&T location in Breinigsville,

Pennsylvania. Mr. Wetzel has been involved in the design, introduction to

manufacture, and qualification of optical multiplexers and optical data link

products. He has been responsible for optical subassembly design, and he has

specified a light-emitting diode for use as both the transmitting and receiving

device in a data link product. Mr. Wetzel has developed a procedure to qualify

data link transceivers for wave solder assembly. Mr. Wetzel has published the

following articles on optical networking and optical data link design: S.Y. Suh et

aI., Aug. 1987, Active star coupler based fiber-optic local area network, Journal

ofLightwave Technology, LT-5 (8): 1050-1061; and C.F. Flores, et aI., 1987,

ODL RS-232-2/02X: a single fiber RS-232 optical modem, in EFOC/LAN 87

Proceedings, 173-176. Mr. Wetzel was elected to Phi Beta Kappa in 1983, and

he is a member of the Sigma Pi Sigma National Physics Honor Society.

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