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Polarisation Characteristics and Feedback Dynamics of

VCSELs Student: Mark Logan

Supervisor: Thorsten Ackemann Date: 14/01/2014

Student Number: 200914452

An investigation into the effects of optical feedback on a vertical cavity surface emitting laser with emphasis on polarisation characteristics and the possibility that the laser displays bistable emission. Optical feedback can be used to reduce the threshold current of a laser and allow it to be in a state where the laser can both be on and off acting as an optical flip-flop. The external cavity consists of a diffraction grating which allows the first order mode of the laser to be reflected back into the laser cavity which results in less loss in the gain medium. This reduction of losses allows the threshold current to become smaller as is demonstrated in this report. A VCSEL operates intrinsically in a single mode and is more inclined to show bistability than a conventional edge emitting laser which has multiple modes.

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Contents Table of Figures ...................................................................................................................................... 4

Table of Equations .................................................................................................................................. 5

Introduction ............................................................................................................................................ 6

History .................................................................................................................................................... 6

Applications ............................................................................................................................................ 7

Theory ..................................................................................................................................................... 7

Evolution of Edge Emitting Laser to VCSEL ......................................................................................... 7

VCSEL Manufacture ............................................................................................................................ 9

Active Region .................................................................................................................................... 11

Long Wavelength VCSELs .................................................................................................................. 14

Buried Tunnel Junction ..................................................................................................................... 15

Polarisation ....................................................................................................................................... 16

Frequency Selective Feedback.......................................................................................................... 18

Bistability .......................................................................................................................................... 19

Laser Properties Changed by Feedback ............................................................................................ 20

Grating Position ................................................................................................................................ 20

Last Years Results ................................................................................................................................ 21

Experiment ........................................................................................................................................... 22

Thresholdless Laser .......................................................................................................................... 26

Fundamental Laser ........................................................................................................................... 27

Addition of Mirror in Feedback Arm ................................................................................................. 28

Feedback with Mirror ....................................................................................................................... 29

Addition of Spectrum Analyser ......................................................................................................... 30

Fundamental Laser Change .............................................................................................................. 31

Addition of Grating in Littrow Configuration in Feedback Arm ........................................................ 32

Feedback with Grating in Littrow Configuration .............................................................................. 33

Thermal Effects ................................................................................................................................. 34

Feedback with Grating in Littrow Configuration and Thermal Effects Adjusted .............................. 36

Addition of Grating in Littmann Configuration in Feedback Arm ..................................................... 38

Feedback with Grating in Littmann Configuration ........................................................................... 39

Polarisation ....................................................................................................................................... 40

Discussion ............................................................................................................................................. 41

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Conclusions ........................................................................................................................................... 43

Acknowledgments ................................................................................................................................ 43

Works Cited .......................................................................................................................................... 44

Appendix .............................................................................................................................................. 47

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Table of Figures

Figure 1: The diagram shows output modes of both a Fabry Perot laser on the left and a VCSEL on

the right. The red line shows multimodes in the gain bandwidth for the Fabry Perot laser and only

one for the VCSEL. ................................................................................................................................... 9

Figure 2: A simple diagram showing the cross section of an early VCSEL with each layer labelled on

the diagram and explained below. ......................................................................................................... 9

Figure 3: Threshold current density optimisation for the active layer thickness of DFB laser ............. 13

Figure 4: Band gap energy of materials plotted as a function of lattice match to GaAs and InP

substrates. ............................................................................................................................................. 15

Figure 5 : An example of a VCSEL with a buried tunnel junction on a gold heatsink as is used in this

report. ................................................................................................................................................... 17

Figure 6: The figure shows output power for increasing injection current with an abrupt turn on

apparent at around 6.3mA. .................................................................................................................. 19

Figure 7: Different configurations to induce frequency selective feedback, on the left the Littrow and

on the right the Littman. ....................................................................................................................... 20

Figure 8: The graph on the right is a plot of feedback strength against threshold current, it clearly

shows as feedback strength increases the threshold current decreases. The plot on the right shows

feedback strength against wavelength and it shows as feedback strength increases the wavelength

also increases. ....................................................................................................................................... 21

Figure 9: The left plot shows the threshold current decreasing from the blue line to the red line as

the grating produces feedback. On the right the plot shows the single longitudinal mode being tuned

to different wavelengths within a range of around 8nm...................................................................... 22

Figure 10: A close-up through the eye piece of a microscope the contact pads of a semiconductor

material showing the method in which the probes were connected................................................... 23

Figure 11: Current versus Output showing a linear relationship for the semiconductor practice wafer.

.............................................................................................................................................................. 23

Figure 12: The left side of the figure shows the wafer of interest in this report and on the right side

shows the layout of different aperture sizes on the wafer .................................................................. 24

Figure 13: A close up of an individual VCSEL showing the layout of contact pads, the separation

between adjacent VCSELs and also the dicing pattern. ........................................................................ 25

Figure 14: The first set up shown schematically. The VCSEL was simply focussed onto a photo

detector to monitor output power. ...................................................................................................... 25

Figure 15: Plot of threshold current against output power showing very low power and little

indication of a defined threshold where stimulated emission takes over from spontaneous emission

.............................................................................................................................................................. 26

Figure 16: LI curve of the first laser to emit light with substantial output power, allowing the laser to

be visible with use of an infrared viewer .............................................................................................. 27

Figure 17: Addition of a 50/50 beam splitter to introduce feedback using a flat mirror ..................... 28

Figure 18: LI curve showing mirror induced feedback reduces the threshold current of the laser, the

red line being most feedback, black line some feedback and blue line no feedback. ......................... 29

Figure 19: Adding a spectrum analyser to obtain spectra introduced many elements as shown in the

above schematic diagram and are fully explained in the text .............................................................. 30

Figure 20: LI curve of the second laser to emit substantial output power after the first laser broke. 31

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Figure 21: The flat mirror in the previous set up is replaced by a 1350 lines per millimetre grating

which is set up in the Littrow configuration, the arrow corresponds to the rest of the set up not

shown on the diagram .......................................................................................................................... 32

Figure 22: similar to the previous figure the LI curves shows a reduction in threshold current as the

amount of feedback increases. The increase in this figure however is larger than before. ................. 33

Figure 23: Plot shows the thermal effects, as the current increases the thermal effects mean that the

wavelength increases. ........................................................................................................................... 34

Figure 24: plot showing that increasing the frequency modulation speed give a single mode which is

the average of the thermal effects induced by increasing the current. ............................................... 35

Figure 25: Threshold current versus output power, similar to figure 15 but this time the graph was

obtained from an oscilloscope .............................................................................................................. 36

Figure 26: Different spectra obtained from rotating the grating, the tuning shows no coherence to

the angle of the grating......................................................................................................................... 37

Figure 27: The grating from the previous section is now set up in the Littman configuration, the

arrow corresponds to the rest of the set up not shown on the diagram. ............................................ 38

Figure 28: Current plotted as a function of Output Power showing the grating taking affect on the

threshold current .................................................................................................................................. 39

Figure 29: The red line shows the case for the favoured polarisation being allowed through the wave

plate and the blue line shows the polarisation when the wave plate was rotated to let the

orthogonal polarisation through. ......................................................................................................... 40

Figure 30: Pictures of the full set up and also a close up of the wafer with the probes attached ....... 47

Figure 31: Screenshot of the software used, Labview and Originpro................................................... 48

Table of Equations

l = (1) ....................................................................................................................................... 8

s+m (2) ...................................................................................................................................... 8

m= (3) .................................................................................................................. 8

(4) ................................................................................................................ 8

(5) ............................................................................................................................ 9

(6) .............................................................................................................. 10

(7) ......................................................................................... 10

(8) ...................................................................................................................... 11

(9) .............................................................................................................. 11

(10) ....................................................................................................... 11

(11) ........................................................................................... 12

(12) .................................................................................................................... 12

(13) ....................................................................... 13

(14) ............................................................................................................................ 14

(15) .................................................................. 16

(16) ......................................................................................................... 16

(17) ................................................................................................................. 20

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Introduction This report investigates polarisation characteristics and also feedback dynamics of a vertical

cavity surface emitting laser (VSCEL) at telecommunication wavelengths, mainly focussing

on the 1.3m wavelength. The investigation builds on last years project where the area of

focus was conventional edge emitting lasers. There will be clear similarities between the

properties shown in both types of laser, however the VCSEL may show signs of polarisation

switching as the polarisation could be weakly pinned to an axis and also the VCSEL may also

show signs of bistability where the edge emitting laser did not. The motivation for studying

telecommunication wavelength VCSELs is the fact they inherit all the same characteristics of

short wavelength VCSELs, however, with the longer wavelength the transverse and

polarisation become easier to control making them a low cost substitute for distributed

feedback lasers (DFBs) [1]. Short wavelength VCSELs are gallium arsenide based structures

for around 800nm wavelengths and Indium gallium arsenide for around 1200nm

wavelengths. For larger wavelengths it is necessary to use Indium phosphide. Using this

material however can be problematic as will be discussed later in this paper. These

problems can be addressed and a VCSEL suitable for a wide range of applications

manufactured.

History To fully understand the relevance and significance of studying the VCSEL it is important to

have an understanding of when the concept originated and how it has developed over the

last 40 years. The need to produce a laser with a wavelength that exceeded 1m was

established in 1976 when it was predicted the transmission loss of an optical fibre would be

low for long wavelengths [2]. The idea of a surface emitting laser came to light in 1977 and

was investigated immediately, however the laser conditions were crucial and there were

difficulties in gaining a low threshold, this led to it being deemed to have a 50% chance of

being possible and so serious research into the field was not immediately followed up. In

addition to this, the market place had no need for another laser as there were already a

large number of lasers looking for suitable applications. This led to difficulties in obtaining

finance and resources [3]. Research however continued and in 1986 a laser with a threshold

1/10th of that of a conventional semiconductor laser was achieved [2]. The height of the

VCSEL was only 7m with a diameter of 6m and this paved the way for lasers to be

fabricated in the order of only a few microns. Continuing research yielded greater, more

positive results and continuous operation was realised in 1988 [4]. From this point short

wavelength VCSELs based on GaAs were researched intensely and took priority over

conventional lasers in a number of applications. Long wave length lasers which is the topic

of this report is the newest area of research with the first VCSEL grown on InP fabricated in

1999 and with continuous wave operation being discovered the following year [5]. From the

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millennium the goal of research has been and remains to increase the Gigabits per second

as this is important for a vast number of applications. In the last ten years the Gigabits per

second has increased from 2.5Gbps to 25Gbps [5] with 56Gbps claimed to be achieved in

the near future.

Applications VCSELs are becoming more widely used than conventional edge emitting lasers due to a

number of reasons. One reason is that edge emitting lasers have an elliptical output beam

due to the narrow height of the cavity; allowing a small confinement area and also a wide

structure to gain a large output. On top of this the edge emitting laser has a beam that is

highly divergent. Whereas VCSELs, unlike edge emitting lasers, have a circular beam this

leads to lower divergence and since it is the same shape as the optical fiber, which is also

circular, it can be seamlessly used in applications. Looking at cost, the VCSEL is by far the

cheaper option as it can be tested on wafer and does not need to be cleaved and packaged

before testing like edge emitting lasers. VCSELs can therefore be created in two dimensional

arrays which make them easier and cheaper to mass produce, additionally in the 2

dimensional arrays there can be many elements, for example in paper [6] it demonstrates

19 elements with an active diameter of 50m producing powers of over 1W which can be

used to pump an external laser . Furthermore VCSELs have a short resonator so they can be

modulated into frequencies in the gigahertz range [7]. This is the main reason they are ideal

for optical fiber communications such as local area networks, optical links and mobile links.

Computer optics also utilise VCSELs in optical interconnects, high speed/parallel data

transfer and storage area networks. Other fields include, optical memory, optoelectronic

equipment, optical information processing, displays, illumination and also automotive

systems [2]. As listed, there are a vast number of fields where the VCSEL has become the

favoured option due to a large number of characteristics which make it more practical than

other conventional lasers. Most of the above mentioned applications are currently under

1m or medium wavelength. Expanding on the long wavelength VCSEL opens up

metropolitan area networks (MANs). The reason for this is the ease in which the long

wavelength lasers can be tuned over a wide range. Also the ability to be modulated at

25Gbts per second speeds make research into high density interconnects in computer chips

one of great interest [7].

Theory The following sections offer a more detailed explanation of how a VCSEL is fabricated and

operates plus the theory behind the characteristics and properties that feedback induces.

Evolution of Edge Emitting Laser to VCSEL

A vertical cavity surface emitting laser is a variation of a Fabry Perot laser. The differences

between both the Fabry Perot structure and VCSEL structure is the direction of laser

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emission and also the size and thickness of the gain region. The Fabry Perot cavity is

horizontal with frequency selective reflection layers with thickness=

at each end of the

cavity. These layers are Bragg stacks and act as mirrors reflecting the light back and forth in

the cavity. In a Fabry Perot laser the mirror reflectivity is around 32%, this means that

around 68% of the light is lost. To compensate for this loss the cavity length is very long

relative to the VCSEL cavity length. Normal lengths of Fabry Perot cavity are in the region of

300-500 microns; this length is to make up for the losses in the resonator. If we rotate the

Fabry Perot cavity by 90 degrees the layers being deposited by epitaxial growth cannot grow

layers thicker than 10 microns, most often the thickest being around 5 microns [8]. This

means that growing a Fabry Perot gain cavity around 300 microns onto a substrate is

impossible and so the gain medium must be reduced.

In all lasers the condition for laser oscillation to occur states that the laser resonator loss

must equal the resonator gain,

l = (1)

Where resonator gain is given by and resonator loss is given by l which can be broke

down into two components.

s+m (2)

With s denoting the losses due to scattering and diffraction losses while m represents the

loss due to the mirrors calculated by:

m=

(3)

Where l refers to the length of the cavity, and R1 and R2 refer to the mirror reflectivity [9].

If for example we take the gain to be 50 cm-1 and let s equal 10cm-1 and m equal 40cm

-1

while assuming that both R1 and R2 are equal we can calculate the length of the cavity by

rearranging the above equation to equal length:

(4)

When R is equal to 0.32 then the length is 285m. If the reflectivitys of the mirrors increase

then the length of the cavity will decrease. For VCSELs the reflectivity of the mirrors can be

as high as 90% allowing the cavity to be reduced to 26m and in more recent VCSELs the

reflectivity has been 99% reducing the cavity length to just 2.5m and even further with

reflectivitys as high as 0.996 leaving cavities at 1 m [2].

As the reflectivity affects the cavity length, the cavity length affects the free spectral range

of the laser. Free spectral range is the frequency spacing between resonator modes. The

cavity length is inversely proportional to the free spectral range [10] by the equation:

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(5)

Where f is the free spectral range, c is the speed of light, n is the refractive index and l is

the length

As the length of the cavity in a Fabry Perot laser is large, around 300 microns the free

spectral range is small. This leads to multiple longitudinal modes being present in the gain

curve.

Diagram taken from Olympus micro website

Figure 1: The diagram shows output modes of both a Fabry Perot laser on the left and a VCSEL on the right. The red line shows multimodes in the gain bandwidth for the Fabry Perot laser and only one for the VCSEL.

The diagram demonstrates the case for a Fabry Perot laser on the left with multimode

output due to the free spectral range being short and the cavity being long. The right side of

the diagram shows a single longitudinal mode as the free spectral range is large and the

cavity short.

VCSEL manufacture

A VCSEL is basically a Fabry Perot structure rotated through 90 degrees as mentioned before

and as the diagram below illustrates.

Diagram taken from department of physics IIT Delhi

Figure 2: A simple diagram showing the cross section of an early VCSEL with each layer labelled on the diagram and explained below.

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The diagram above shows the simplest type of VCSEL which exhibits the important elements

of all VCSELs. The bottom region is the substrate this is the material in which the VCSEL is

grown by epitaxial growth, it usually has a thickness of 50 to 60 microns. The thin grey lines

are the bottom n contacts. The bottom Bragg stack consists of 25 alternating pairs of high

and low refractive index layers [11]. Commonly used is AlAs with a refractive index of 3.2

and AlGaAs with a higher refractive index of 3.5. The thickness of each of these layers is

calculated by:

(6)

Where is wavelength and n is the refractive index.

Taking an 800nm laser the thickness of each layer is approximately 625 angstrom and 560

angstrom respectively. Adding both these layers gives a period thickness of around 0.12m

and multiplying this by 25 gives a thickness for the Bragg stack of 3m. The reflectivity of the

bottom Bragg stack is made to be as near 100% reflectivity as possible [12] and can be

worked out using the equation:

(7)

Where N is the period, nh and nl are the refractive indexes both high and low respectively.

The active region is a double heterostructure of a low band gap material inserted between

high band gap materials typically GaAs and AlGaAs and has between 3 and 6 quantum wells

[11]. This section of the VCSEL is described in more detail in the following section.

Below the quantum well there is an oxide layer, often referred to as a blocking layer. This

layer is not included on the diagram above but is required to confine the carriers to the

central region of the device, leading to more concentrated emission from the aperture.

The upper Bragg stack has 15 periods so has a thickness of 1.8m for an 800nm laser [11].

The reflectivity is lower than that of the bottom Bragg stack, this is required so the

reflectivity is lower, allowing the laser to emit light from the aperture at the top of the

structure. The top contact is an annular electrode. Totalling the thickness of each individual

component gives an overall thickness of 6-7m. Comparing this thickness with that of the

previously calculated cavity length for a Fabry Perot it is apparent the difference in size, with

the VCSEL becoming nearly 50 times smaller than the Fabry Perot.

There are a couple of issues that develop when manufacturing VCSELs, one is growing many

lasers on a substrate on a 2D array, and the other is controlling the transverse mode. As the

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device is not a wave guided structure like a Fabry Perot the transverse mode is unknown

[13].

Active Region

The active region in a VCSEL is made up of a multiple quantum well structure, this allows it

to be defined as a quantum well laser which is a semiconductor double heterostructure

laser where the thickness of the active layer is less than or on the order of de Broglie

wavelength of electrons which restricts the motion of carriers in the direction normal to the

well [14].

The thickness of the quantum well defines the wavelength of the laser, the wavelength can

also be influenced by the composition of materials but it is easier to adjust the thickness of

the layers through epitaxial growth than to change the composition. For example a laser

made from InGaAs based on InP is lattice matched and has band gap energies of 0.74eV and

1.35eV respectively. Taking a bulk double heterostructure as reference with a thickness of

1000 angstroms then the band gap energy is 0.74eV and using the equation:

(8)

Where is the wavelength, Eg is the band gap energy difference and 1.24 a constant for

planks constant multiplied by the speed of light.

Wavelength can be calculated to be 1.67m. Reducing the thickness of the quantum well to

100 angstrom so it is then no longer a bulk structure but a quantum well structure with

discrete energy levels as shown by equations:

(9)

(10)

Where Mx represents Mc the conduction band and Mv represents the valence band. It is

important to note that the mass of holes is much higher than the mass of electrons, so

energy is lower in the valence band.

Assuming Ec is 50meV and Ev is 10meV then the band gap energy is 0.74+0.06 giving an

effective Eg of 0.8 therefore the corresponding wavelength by equation (8) is 1.55m.

Further reducing the thickness to 80 angstrom means the energy level Ec goes further up

and Ev will further decrease so Ec roughly equals 85meV and Ev equals 15meV giving a

difference of 100meV so 0.74+0.1 then the effective Eg is equal to 0.84 and the wavelength

is 1.46m. Through this example it is clear that the wavelength changes from 1.67m to

1.47m by changing the thickness from 1000 angstrom to 80 angstroms.

The use of quantum well lasers has two main advantages, one being low threshold currents

and the other they are less sensitive to temperature variation. For a GaAs Fabry Perot laser

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the characteristic temperature T0 which determines the quality degradation [15] is in the

order of 160 Kelvin and for quantum well lasers T0 is 385 Kelvin [16]. The relationship

between threshold current and temperature is:

(11)

Where represents the threshold current density at an unknown temperature and

represents the threshold current density at a known temperature. T is the new

temperature, TR the reference temperature and T0 is specific for quantum well lasers [17].

From the equation it is clear that if T0 is large then the threshold current at a given

temperature will be very close to the threshold current at the reference temperature. For

example if T0 was infinity then the threshold current would be the same regardless of

temperature, indicating the temperature would have no effect on the threshold current.

There are two main reasons for this.

One of the most important reasons is that because of the discrete energy levels the carrier

distribution inside the quantum well structure is not sensitive to temperature. If considering

a comparison between a bulk heterostructure to a quantum well structure, the bulk

heterostructure is sensitive to temperature. At any finite temperature, phonons will allow

electrons to go up and down states through emission and absorption continuously, meaning

a small change in temperature changes carrier distribution. In quantum well structures,

phonons have to make transitions between discrete levels therefore it is not continuous

making it much less sensitive to temperature changes unless phonons have the sufficient

energy to overcome the band gap. This means carrier distribution is less sensitive to

temperature and so threshold current is also much less sensitive to temperature.

The second reason is that in bulk heterostructures lasers the temperature change allows

non-radiative transitions to occur, in discrete quantum well structures there are no non-

radiative transitions as every transition must be an allowed one. The internal quantum

efficiency i is very large in quantum well structures and i is less sensitive to temperature

as the transitions are quantum mechanically allowed transitions so are all radative

transitions. Both characteristics mean T0 is larger for quantum well lasers than bulk double

heterostructure lasers.

The threshold current in VCSELs is lower due to the size of the cavity. As the equation below

shows, there is a linear relationship between threshold charge density and active region

thickness.

(12)

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Where JT is the threshold charge density, delta nT is carrier density e is the charge of an

electron and d is the thickness, tow is carrier lifetime near transparency [18].

For Fabry Perot lasers the thickness is large so the threshold current is large compared with

a VCSEL where Jt is small due to the thickness of the active region being small as it is a linear

relationship.

The confinement factor gamma varies inversely with threshold current. If gamma is small

the threshold current density will go up, this is true for double heterostructure.

(13)

Where Jt is threshold current density, J0 is current density, d is active region thickness, in is

internal quantum efficiency, g0 is the gain coefficient, is confinement factor, is loss

coefficient, l is length of cavity and R1 and R2 are mirror reflectivitys [19].

Diagram taken from [20]

Figure 3: Threshold current density optimisation for the active layer thickness of DFB laser

Threshold current comes out to be a minimum at an active region thickness of 0.3m or

0.4m for a bulk double heterostructure as shown in the above diagram. For a quantum

well laser the minimum is less than that of a bulk double heterostructure so we would

expect the threshold to go up as the thickness decreases and therefore the factor of gamma

will also decrease. For a double heterostructure the optical mode fractional energy is 0.8 or

0.9. In quantum well structures the thickness decreases and the optical mode spreads more

evenly over the channel which means the gamma factor reduces to the order of 0.4 in

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quantum well structures. From this fact and the equation we would expect that the

confinement factor decreases the threshold current will go up, this is true for double

heterostructure lasers. For quantum well lasers the gain is large for low currents therefore

the gain can compensate for loss by the equation

(14)

Where a factor of gamma has been added to the previously shown equation in (1).

As mentioned, most lasers have multiple quantum wells, usually three to six. The quantum

wells are spaced by barriers and are all identical. The electric field of each quantum well is

isolated but the optical fields interact so strongly that the volume of quantum wells is

viewed as one large quantum well. The advantage of this system is that all properties of the

quantum wells are enhanced. More power can be obtained from the multiple quantum

wells and the gamma factor increases, if the gamma factor increases the threshold will

reduce further.

Long Wavelength VCSELs

The previously described manufacture of a VCSEL holds true for all structures. However,

when developing and manufacturing long wavelength VCSELs it is not possible to use the

same materials to construct the long wavelength lasers. For short wavelength lasers,

generally those under 1m, it is possible to base them on GaAs. The short wavelength GaAs

devices benefit from a large refractive index difference between GaAs and AlAs. This allows

the mirrors to become highly reflective with a small amount of layers, around 20-30 as

previously calculated in equation (7). The long wavelengths based on InP have an index

difference almost two times smaller than those of InGaAsP or InGaAlAs mirror layers.

Coupled with the fact the longer wavelength leads to thicker periods by equation (6) the

poor reflectivity means more periods are needed, up to 20-40 times the amount of periods

than that of the 800nm laser used as reference in the previous section. Consequently the

total size of the laser diode could increase considerably from 6-7m. The diagram below

shows the band gap energies plotted against lattice matching for common materials.

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Diagram taken from MIT open course ware

Figure 4: Band gap energy of materials plotted as a function of lattice match to GaAs and InP substrates.

In the diagram it can be observed that GaAs and AlAs are lattice matched and this is the

reason they are used in short wavelength devices. Long wavelength devices must use InP

which again from the diagram can be observed to not be lattice matched with GaAs or AlAs.

To overcome the problems induced by the poorly reflective mirrors created by poor lattice

matching of the semiconductor materials, a buried tunnel junction is added to the device

which solves this problem by reducing the resistance.

Buried Tunnel Junction

A buried tunnel junction comprises of one heavily doped p and one heavily doped n low

band gap InGa(Al)As layer . The junction is placed in the p side of the diode and the junction

has a smaller diameter than the diode diameter. The buried tunnel junction converts a large

part of the p side confinement layers from p to n, this is desirable because p conducting

semiconductor materials have high losses for long wavelengths. [12]. This is owed to the

fact free carrier absorption increases as wavelength increases and for p type material the

hole mobility is smaller.

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(15)

Where is the free carrier absorption, n is refractive index, me and mh are electron and

hold masses respectively as e and h are conductivity, with N and P referring to doping

concentrations with the known constants e, c and 0 [12].

The tunnel junction converts hole current into electron current so is the reason why p

material is replaced by n material. The junction is placed in the node of the optical wave and

in this way the losses are low even with high band gap materials with high optical losses [1].

With lower hole mobility and doping concentration the losses are smaller so the electrical

conductivity is lower, given by

(16)

Where all symbols are the same as above and E is the electric field while is electrical

conductivity [12].

With a low electrical conductivity it means that the ohmic heating is low making it ideal for

high speed modulation applications. The last benefit of a BTJ is that in InP based devices

forming apertures from oxidation is not feasible. Hence in the BTJ a highly doped layer is

etched away where current is not supposed to flow, this creates a p+np reverse bias diode

that does not allow current to flow in the area around the tunnel junction, effectively

creating an aperture in the same way the oxide layers did in the short wavelength VCSEL.

Current still however flows across the junction due to the tunnelling effect [12].

Polarisation

As discussed briefly before a main difference between conventional edge emitting lasers

and VCSELs is the polarisation characteristics of both. The edge emitting laser has a stable

polarisation while the VCSEL has an unstable polarisation due to the geometry. This

becomes a problem when applications dictate that the polarisation must be stable, such

applications as high bit data transmission, smart pixel based free space optical interconnects

and magneto optical disc computer memories [21]. There have been methods proposed to

control the polarisation of the VCSEL, these methods include; polarisation sensitive DBR

mirrors, geometrical or stress induced anisotropies and engineering the semiconductor

material to favour a polarisation direction [22]. A main motivation is to instead of suppress a

polarisation direction learn how to switch between them, this would open up a wider range

of applications.

There are a couple of main factors which determine the polarisation state of light; the first is

angular momentum of quantum states involved in the emission or absorption optical

transitions. The second factor is a combination of the anisotropies; these are geometry,

detuning and wave guiding effects of the cavity which lead to a preferred polarisation state.

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It has been shown that short wavelength VCSELs have a greater tendency to switch

polarisation than long wavelength VCSELs [1].

To control the polarisation in quantum well lasers it is essential to introduce gain

anisotropy. This can be introduced in two ways. Anisotropy stress in the active layer is

created by an elliptically etched hole in the substrate. The other method for achieving

anisotropic gain is using unconventional crystal orientations such as the non-(001) quantum

well layers which have an anisotropic gain property due to the asymmetric valence band. It

is possible to select transverse electric or transverse magnetic polarisation by applying

compression stress or tensile strain on the active layers, this in turn alters the levels of the

light holes and heavy holes in the valence band [3]. A further advantage of a strained active

layer is the radiative recombination from the non lasing transition is reduced because of its

energy separation from the valence Fermi energy as the discrete levels have higher energies

to overcome. Below is a diagram of a VCSEL with a buried tunnel junction included.

Diagram taken from [1]

Figure 5 : An example of a VCSEL with a buried tunnel junction on a gold heatsink as is used in this report.

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Frequency Selective Feedback

Frequency selective optical feedback was proposed for edge emitting lasers to stabilise

single longitudinal mode operation as edge emitting lasers are inherently multimode. This

was shown in last years investigation to be accurate as can be viewed later in this paper in

figure 9. The proposed methods to obtain frequency selective feedback as reported briefly

last year are a narrow band resonator, a grating, a fiber Bragg grating reflector or by locking

the frequency of the diode to transition lines of a medium placed within the external cavity

[23]. As a VCSEL already has a single longitudinal mode the need for frequency selective

optical feedback is not as vital as for the edge emitting laser, it does stabilise the single

mode but frequency selective optical feedback plays a more pivotal role in selecting the

polarisation. As described above the polarisation for VCSELs is weakly pinned to an axis due

to the circular geometry of the beam, this often leads to polarisation switching. Frequency

selective optical feedback can determine which polarisation is selected due to the frequency

splitting between the two sets of external cavity modes with orthogonal polarisation. The

feedback can be introduced in two ways, one method is monolithically integrating the

feedback grating and the laser as is shown in [24] the other is to insert the grating into a

delayed external cavity, the latter has been performed and is described in this report. The

main reason for doing it in this way is because it creates competition between external

cavity modes, this drastically increases laser performance and additional switching

properties arise. Furthermore, optical feedback has been shown to facilitate multi-stability

such as bistability which is covered in the following section.

19

Bistability

Bistability means simply something that can be resting in either of two states, for optical

bistability this relates to the fact a laser can both be non-lasing and lasing with the input

determining which state is adopted. Using frequency selective feedback it is possible to tune

the grating frequency close to the free running laser frequency. If the polarisation favoured

by the free running laser benefits from the feedback and the feedback efficiency is optimal

then an abrupt turn on of the laser can be seen.

Diagram taken from [23]

Figure 6: The figure shows output power for increasing injection current with an abrupt turn on apparent at around 6.3mA.

The figure from the paper [23] shows the abrupt turn on of the laser denoted by the black

line, the grey line denotes the free running laser. This abrupt turn on of the laser is a result

of the grating frequency being tuned close to the free running laser frequency resulting in

the cavity frequency hopping to the adjacent mode.

20

Laser properties changed by feedback Feedback changes many properties about a laser allowing it to be tuned over a range of

wavelengths. The external cavity reduces losses in the gain medium this leads to: the charge carrier

density decreasing; the refractive index increasing; the frequency decreasing; the wavelength

increasing and the threshold current to decrease. All of the factors listed before can be changed

when a grating introduced into the system is rotated and changes the frequency selective feedback.

Research has shown the ability to tune a laser between two adjacent modes by use of a diffraction

grating shown in [25].

Grating Position

When placing the grating into the experimental set up there are two configurations that can

be used, the Littrow configuration and also the Littman-Metcalf configuration. The diagram

below demonstrates both configurations.

Diagram taken from RP photonics

Figure 7: Different configurations to induce frequency selective feedback, on the left the Littrow and on the right the Littman.

The one which is simpler to configure is the Littrow, this is because it minimises the number

of optical elements required to construct. Here the grating is placed so that light is incident

on the grating and the first order mode is directed straight back into the laser output to

create feedback, the zero order then becomes the output. To calculate the angle at which

the grating needs to be relative to the incident beam for the first order beam to be reflected

into the cavity, the grating equation is used with m equal to 1:

(17)

Where g is the separation between the grooves, is the angle, m is the mode and is the

wavelength.

Once set up to provide maximum feedback the grating can be adjusted by rotating it

through a small angle, this will then tune the frequency and change the wavelength.

Advantages of the Littrow configuration is that it offers high efficiency, high power and a

better focus. However one disadvantage of this configuration is that the output changes

position as the diffraction grating is rotated, this means coupling the output to an

application is troublesome.

The second configuration is the Littman configuration. This is a more complicated setup with

the addition of a mirror. The grating is placed close to the grazing angle and is fixed in this

position. The laser output is incident on the grating giving both the zero order and first

21

order diffraction beams, the zero order is used as the new output and the first order in this

configuration is focussed onto a mirror and reflected back onto the grating which then

projects it back into the laser. Rotating the mirror in this configuration allows the laser to be

tuned. Unlike the Littrow configuration the output beam remains fixed in this set up so is

more common when coupling to applications and additionally the resolution is better as

more grooves are illuminated on the grating as consequence of the large angle. Drawbacks

of this set up however are a higher number of losses as the beam interacts with the grating

twice and also the angle in which the beam interacts with the grating. Both factors mean

that the laser has lower output power than the previously mentioned configuration [27].

Last Years Results Research from last year investigated the feedback dynamics and polarisation characteristics

of an edge emitting laser. As expected there were no polarisation induced effects as the

beam is elliptical the polarisation is pinned to an axis however the feedback did reduce the

threshold current and a single longitudinal mode was realised with the addition of a grating

to provide the optical feedback. Important results from last years research which have

similarities to this year are as follows:

Figure 8: The graph on the right is a plot of feedback strength against threshold current, it clearly shows as feedback strength increases the threshold current decreases. The plot on the right shows feedback strength against wavelength

and it shows as feedback strength increases the wavelength also increases.

The graph on the left clearly shows the relationship between feedback strength and

threshold current. As the feedback strength is increased the threshold current reduces

around 11% for 60% feedback. The graph on the right side shows the relationship between

the wavelength with feedback strength. Again it is clear that the feedback brings a change in

wavelength. These results were calculated using a simple mirror to create the feedback

external cavity and the wavelength change was 3nm for 60% feedback strength.

Changing the mirror for a diffraction grating to create the external cavity the laser became

single mode as the graph below illustrates.

22

Figure 9: The left plot shows the threshold current decreasing from the blue line to the red line as the grating produces feedback. On the right the plot shows the single longitudinal mode being tuned to different wavelengths within a range

of around 8nm

The graph on the left shows the feedback reduction with the grating providing no feedback

(blue line) and around 60% feedback (red line). The reduction from 5mA to 4.15mA shows a

17% reduction in threshold. The graph on the right demonstrates how the laser has a single

longitudinal mode and can be tuned between 1306nm to 1311nm after this range the

fundamental multimode lasing took over.

These results have significance to this years research as the feedback is expected to lower

the threshold of the VCSEL and bistability may be possible.

Experiment The experiment was carried out over a 13 week period with the large majority of results

being obtained in the last quarter of that time due to difficulties with the set up which will

be described in greater depth in the discussion appearing later in the report.

As in last years experiment the set up comprised predominantly of optical fibers, there was

little need to use lenses to collimate the beam and develop a complex arrangement of

mirrors to direct the beam from source to detection. Also, last year the laser was already

packaged unlike this years experiment where the laser was still on wafer and had to be

contacted using probes. The latter requirement made it important to get practise

positioning the probes onto the contact pads. To avoid destroying the VCSELs it was decided

to firstly practise on a semiconductor material. A microscope was used and both probes

were mounted on x,y,z mounts to give a high range of mobility. Below is the view obtained

through the microscope.

23

Figure 10: A close-up through the eye piece of a microscope the contact pads of a semiconductor material showing the method in which the probes were connected.

The view in the above diagram is taken through the lens of the microscope and it shows the

practice wafer with both probes contacting either side of the contacts. It is important to

mention that this wafer only had two contacts whereas the proper one below has three,

additionally the contact pads shown here are larger than those in the proper wafer and so

the probes shown here are also larger than those eventually chosen for experiments.

Results from the above experiment are shown below but it is important to note that their

value is limited and have no real bearing on the main outcome of this report.

Figure 11: Current versus Output showing a linear relationship for the semiconductor practice wafer.

24

The graph shows a linear relationship between output voltage and current, this was

expected as the semiconductor material was not a laser and had the same profile and an

LED.

Moving on to the experiment with the wafer comprising of VCSELs, it can be seen in the

diagram below how the wafer is secured to the metal mount with no external heat sink

included. In the diagram the wafer can be seen to have multiple lines running the length of

it, these are rows of contact pads. The diagram on the right is a map of the wafer. A marker

on the top right has been included as reference to make it possible to navigate the probes

to the correct aperture. The aperture size ranged from 2m to 120m. The wafer was

broken into a co-ordinate style system with each section having a layout similar to that on

the right diagram.

Figure 12: The left side of the figure shows the wafer of interest in this report and on the right side shows the layout of different aperture sizes on the wafer

The x co-ordinates were 7, 8 and 9 and the y co-ordinates were 2, 3 and 4. The directions

were X,Y,x,y For example, a laser with co-ordinates 2_7_4_6 would be in the upper left of

the wafer and be on the 4 column 6th row making it a 5.5m laser.

The wafer of VCSELs was mounted onto a 15mm thick mount which was secured onto a

larger fixture which raised the wafer into a position whereby the tungsten probes were able

to make contact with the p and n contacts appearing on the surface of the wafer. Each

contact pad was only tens of microns thick and so the probes used were 7m in diameter to

ensure an appropriate contact. The diagram below shows a close up view of one individual

VCSEL on the wafer.

25

Figure 13: A close up of an individual VCSEL showing the layout of contact pads, the separation between adjacent VCSELs and also the dicing pattern.

The probes were joined to stages which allowed them to be moved in the x,y and z

direction, giving a large range of mobility. The contact pads on each individual VCSEL were

arrange p n p such that the positive probe was attached to the outside and the negative

probe to the inside. The probes were then connected to a laser diode controller made by

Thor labs, the VITC002. This probe arrangement remained constant throughout the

experiments and is included in the box entitled VCSEL. A CCD was first used as a camera to

position the probes with aid of an LCD screen and thereafter the probes were connected

and subsequently a photo detector was connected to an oscilloscope to monitor output

power.

Figure 14: The first set up shown schematically. The VCSEL was simply focussed onto a photo detector to monitor output power.

The VCSEL aperture was 5.5m in diameter and as the beam diverged rapidly a 35mm focal

length lens was placed to collimate the light. The beam was then reflected at a right angle

by a mirror and into a photo detector, the photo detector was connected to a computer

which used software called Labview to both control the input and record the output of the

laser.

26

Thresholdless Laser

When selecting a VCSEL from the wafer as described above, the sixth row of each section

had the 5.5m lasers. The first laser selected was next to the marker for easy reference,

however, this laser and a number of preceding lasers did not emit the laser power expected

as is shown in the graph below.

Figure 15: Plot of threshold current against output power showing very low power and little indication of a defined threshold where stimulated emission takes over from spontaneous emission

For driving currents of 15mA the power was under a third of a milliwatt when the

expectation was well over a milliwatt. Looking at the graph it is clear that there is no defined

threshold where the laser changes from spontaneous emission to stimulated emission and

appeared similar to results in the paper. The laser light was not visible on card or on the

infrared viewer. This result looks similar to a thresholdless laser which is when a photon

emitted by the gain medium is funnelled into the lasing mode [27] and when spontaneous

emission is made to take place mainly in the spatial mode which is defined by the laser

resonator [27]. Thresholdless lasers achieve this by having a microcavity around the gain

medium which modifies the mode structure of the gain medium; even with multiple modes

the spontaneous emission chooses the one with the highest Q factor. The idea of the

thresholdless laser is hypothetical and the lowest thresholds attained by VCSELs are 36uA

27

for a 2.5um device and 92uA for a 4um device [28]. Realistically the result shown above

demonstrates the same characteristics of an LED which also doesnt have a threshold. The

graph would be expected to be linear with a slight curving at the beginning; this could be a

combination of the signal to noise ratio being poor and also the power meter having an

offset.

Fundamental Laser

A laser was eventually found which emitted power of just under 1.4mW for driving currents

of 9mA. The graph below is a plot of current versus output power as the photo detector was

used to monitor power instead of a dedicated power meter the results in preceding graphs

will be recorded in volts, this made recording of results automated and removed the need

for manually recording data.

Figure 16: LI curve of the first laser to emit light with substantial output power, allowing the laser to be visible with use of an infrared viewer

The graph shows the LI curve of the laser as it scans up to 10mA and then back down to

0mA. It levels off between 9mA and 10mA because the laser was limited to 9.09mA by the

laser diode board yet the software programme ran data points to 10mA. The up scan is

shown on the right side of the curve and makes the threshold 5.57mA. The down scan is on

the left side. This is unusual as the down scan is usually at higher current due to thermal

heating. One reason for this could be the mode resting to the right of the gain curve and as

28

the device heats up the mode moves to the left at 0.1nmK-1 while the gain curve moves right

at 0.3nmK-1 allowing the mode to be more central in the gain curve allowing higher output

at lower current.

Addition of Mirror in Feedback arm

Once the VCSEL was emitting with sufficient power an external cavity was created to

provide feedback. In the first configuration a flat mirror was used to reflect the laser light

back towards the incident beam providing a source of optical feedback.

Figure 17: Addition of a 50/50 beam splitter to introduce feedback using a flat mirror

The diagram above shows the VCSEL emitting and being collimated as described previously.

The beam splitter is a 50/50 split meaning half the output goes to the feedback branch of

the experiment and the remaining half is collected by the photo detector for monitoring.

The distance at which the mirror was placed in relation to the lens was critical as for high

optical feedback the incident laser beam should be focussed onto the mirror. The reason for

this is that plane wave fronts must match before and after they interact with the mirror,

meaning they have the same size. The wave plate was added to increase the coupling

efficiency.

29

Feedback with Mirror

Placing a flat mirror into the feedback arm as is shown in figure 17 provides feedback which

lowers the threshold current of the laser and also increases the output power as is shown in

the below figure.

Figure 18: LI curve showing mirror induced feedback reduces the threshold current of the laser, the red line being most feedback, black line some feedback and blue line no feedback.

From the graph the blue line represents the fundamental laser, the one with no optical

feedback. This is done by covering the mirror so no feedback enters the laser similar to

figure 14 it has a threshold current of 5.48mA. Placing the mirror at a small angle to the

incident beam allows some feedback to enter the laser cavity; this reduced the threshold

current to 5.1mA and increased the output power from 1.6V to 1.8V. Placing the mirror

perpendicular to the incident beam allows the most feedback to go into the laser cavity, this

reduced the threshold current further to 4.67mA a total reduction of 15%. The output

power also increased to 1.9V and increase of 16%. The above threshold reduction agrees

with the properties mentioned earlier in this report on page 20 and is similar to the light

current characteristic results shown in paper [29].

30

Addition of Spectrum Analyser

At this stage efforts were made to monitor the spectra associated with results and so the set

up was altered so that part of the laser light was coupled to an optical fiber. The set up was

challenging as the first mirror was at a height of 30cm above the table and the beam had to

be directed downwards to a height of 10cm parallel to the table top. This was achieved as

shown below.

Figure 19: Adding a spectrum analyser to obtain spectra introduced many elements as shown in the above schematic diagram and are fully explained in the text

The second beam splitter was again a 50/50 splitter which provided 50% of the light to be

monitored by a photo detector and 50% of the light to be fiber coupled. Originally the light

was to be coupled into a monochoromator however the power was not sufficient to achieve

this so a spectrum analyser was used, denoted as SPA in the figure. The optical isolator

denoted P in the diagram was included to stop back reflections from the flat end of the

fiber.

31

Fundamental Laser Change

At this point in the experimental process the probes became detached from the contact

pads and as suggested by the manufacturer of the wafer a new VCSEL was used as the

previous one was most likely destroyed. The LI curve of the new laser is shown below.

Figure 20: LI curve of the second laser to emit substantial output power after the first laser broke.

The threshold is lower than the previous laser at 4.87mA and the computer interface was

adjusted to drive the laser to the point at which the laser diode controller limited the

current, removing the unnecessary levelling at the top of the previously shown fundamental

laser graph. The output here is also lower as the set up has changed and the laser light

interacts with another beam splitter before being monitored.

32

Addition of Grating in Littrow Configuration in Feedback arm

Inserting a grating with 1350 lines per mm in the Littrow configuration was then adopted

and is shown schematically in the diagram below. When selecting the grating there were

three choices: a 1200, 1350 or 1400 lines per millimetre grating. Last year when

experimenting with the edge emitting laser it was found that the 1200 grating provided a

reduction in threshold and the best results, however, repeating the experiment this year

with the VCSEL found that there was little difference in threshold reduction between the

three gratings with the 1350 grating giving a marginally smaller threshold reduction when

placed in the set up.

Figure 21: The flat mirror in the previous set up is replaced by a 1350 lines per millimetre grating which is set up in the Littrow configuration, the arrow corresponds to the rest of the set up not shown on the diagram

The addition of the grating was carried out as described in the section entitled grating

position. The first order mode was directed back into the laser cavity by setting up the

grating at an angle of 60 degrees in compliance with the grating equation.

33

Feedback with Grating in Littrow Configuration

After obtaining a threshold reduction with feedback induced by a mirror, the mirror was

removed and a grating placed where the mirror had been. As described above there are two

configurations for the grating to be placed, either in Littrow or Littman. Results below were

generated when the grating was placed in Littrow configuration.

Figure 22: similar to the previous figure the LI curves shows a reduction in threshold current as the amount of feedback increases. The increase in this figure however is larger than before.

The blue line again represents no feedback, when the grating was covered. This gave a

threshold current of 4.87mA. The reduction when the grating was rotated to give some

feedback was a lot larger than when the mirror in the previous set up was rotated to give

some feedback, this time the threshold current was 3.84mA, a reduction of 22%. Placing the

grating at the grating angle so that the first order was aligned with the incident laser light

gave a threshold reduction of 26% reducing the threshold current to 3.61mA. Results

recorded showed the output power was unaffected by the feedback in this configuration

and that total output power was roughly half of the fundamental output power as the

detector was placed after another beam splitter. Also the results shown above reflect the

properties previously discussed and again agree with the paper [30].

34

Thermal Effects

The set up and design of the wafer itself made controlling the temperature of the laser

difficult as there was no external heat sink, only the gold of the wafer to disperse heat. This

situation led to a relatively large change in wavelength as the driving current increased.

Figure 23: Plot shows the thermal effects, as the current increases the thermal effects mean that the wavelength increases.

The graph above shows that as current increased the wavelength also increased. This is

what we expect because as temperature increases the refractive index changes and as a

result the frequency decreases as shown in the theory section. The graph shows that for

both feedback and no feedback there were no change; the feedback had no additional

effects on the thermal shift of the wavelength. For a VCSEL the shift is usually 0.3nm/K so

for the above case the shift was around 3nm therefore a temperature increase of 10 Kelvin.

To try and limit the thermal effects induced by raising the driving current the computer

interface was no longer used to drive the current in a continuous wave but instead the laser

diode driver was configured to modulate the frequency rapidly, this meant that the

temperature rise of the laser was more stable thus giving more accurate results by shifting

the wavelength less. Another benefit of changing to this system is that it gave a real time

view of what was happening on the oscilloscope and changes to the grating were seen

35

instantly on the screen as opposed to waiting and analysing results on the software as was

necessary previously.

The graph below shows different plots as the modulation frequency was increased on the

laser diode driver.

Figure 24: plot showing that increasing the frequency modulation speed give a single mode which is the average of the thermal effects induced by increasing the current.

Four of the five plots shown above appear to have multiple modes but it is in fact the same

mode changing position as the frequency is modulated. It was expected that at lower

frequency modulation the plot would be shifted to higher wavelengths however this is not

reflected on the graph, possibly due to the spectrum analyser not measuring the peak at its

highest point. The red plot shows where the frequency modulation was at the highest value

possible and the one in which all following experiments were carried out. It shows only one

peak as a result of the frequency modulating too quickly for the spectrum analyser scans to

keep up and therefore averaging the range in which it modulated. Further research on the

thermal effects on VCSELs can be found in the paper [31].

36

Feedback with Grating in Littrow Configuration and Thermal Effects

Adjusted

The plot shown below is the same as figure 21 with the only difference being the way in

which the data was recorded. This time, as stated above, the results were taken from an

oscilloscope enabling changes in real time to be observed. This is advantageous as any

bistable emission would be immediately seen within a small range of the diffraction grating

being rotated.

Figure 25: Threshold current versus output power, similar to figure 15 but this time the graph was obtained from an oscilloscope

In the graph above there is still a difference in the up and down scan of the laser so the

thermal effects are not overcome, in fact, the gap between the up and down scan appears

to have become wider as a result of modulating the frequency at a higher rate and indicates

it is still not modulating faster than the heating cycles. The blue graph above depicts the

situation where the grating is covered and no feedback is present; the red line shows the

case where most feedback enters the laser cavity. In these results, as the output is

monitored on an oscilloscope, the up scan is shown on the left of each line and the down

scan on the right side. The case for no feedback gave a threshold current value of 5.04mA

which as before agrees with the fundamental laser. With the grating at the grating angle of

60 degrees allowing the first order mode to enter the laser, the threshold current dropped

37

to 3.61mA, a reduction of 29%. This reduction is greater than the previous set up possibly

due to the laser being set to modulate and thus the laser is less affected by thermal effects

as it operates at a lower average temperature.

The graph below shows the spectra associated with each rotation of the grating. In last

years experiment, it was observed that each turn of the grating resulted in the wavelength

being tuned either up or down depending on the direction of rotation.

Figure 26: Different spectra obtained from rotating the grating, the tuning shows no coherence to the angle of the

grating.

The results graphed above bear little resemblance to last years results when the grating

was rotated as the system is relatively insensitive to the grating angle. The red and blue plot

defines the range in which the grating was rotated and it is conclusive from the graph that

there is no coherence between the wavelength and the grating position. One reason why

the system is insensitive to the grating is because in the Littrow configuration the frequency

resolution is poor because there are fewer lines illuminated as a result of the spot being

focussed on the grating. The line width shown on the graph is over 1nm in size when it

should be 0.2nm for the best chance at bistable emission.

As the grating in the Littrow configuration did not offer any hints at bistable emission or

wavelength tuning it was decided to add an additional mirror and create the feedback cavity

with the grating in the Littmann setup as shown below.

38

Addition of Grating in Littmann Configuration in Feedback arm

The below diagram is very similar to the Littrow configuration with the only exception being

the addition of the mirror.

Figure 27: The grating from the previous section is now set up in the Littman configuration, the arrow corresponds to the rest of the set up not shown on the diagram.

The grating was placed at a grazing angle so a large number of the lines were illuminated;

this increased the resolution and gave a better chance of the wavelength being tuned. The

argument between feedback strength and frequency resolution depends on the separation

of the mirror and grating. For higher feedback strength it is favourable to have the beam

focussed on the grating as before in Littrow, however, for better frequency resolution more

of the lines have to be illuminated and the beam then focussed on the mirror. For the best

results in Littman both factors must be kept in mind and the separation should be a trade-

off between both properties. It is also apparent from the diagram that the second lens has

been removed so that the beam no longer had a focus so close to the grating.

39

Feedback with Grating in Littmann Configuration

The graph below clearly shows the grating beginning to influence the free running laser as is

clear below. Ideally, a free running laser plot would be added as a reference but was

unavaliable.

Figure 28: Current plotted as a function of Output Power showing the grating taking affect on the threshold current

The plot shows three different cases with the diffraction grating rotated around one full turn

between recording each set of results. It is clear that the grating is beginning to dictate the

threshold of the laser. The threshold current of the free running laser based on the linear

part of the curve is around 4.77mA. The threshold based on where stimulated emission

begins for the blue curve is 3.48mA, for the red it is 2.78mA and for the green it is 2.61mA.

The green line looks similar to the plot shown before from the paper [23]. In this paper the

graph shows an abrupt on at the grating frequency then the power begins to fall as it is

tuned out of resonance. The same thing appears to happen in the green plot with the

heating moving the mode out of resonance before it joins the free running laser line well

above threshold and the grating no longer as any affect.

40

Polarisation

Unlike last year where the polarisation was strongly pinned to an axis due to the elliptical

nature of the edge emitting lasers output beam, a VCSEL has a weakly pinned polarisation

due to a circular output beam. Although the phenomenon is more prevalent in short

wavelength VCSELs it is still possible to make an appearance in long wavelength lasers. The

graph below shows the results of the polarisation.

Figure 29: The red line shows the case for the favoured polarisation being allowed through the wave plate and the blue line shows the polarisation when the wave plate was rotated to let the orthogonal polarisation through.

The above graph shows the output power of the laser as the current is increased, it is clear

that one polarisation is dominant and there is no indication of polarisation switching as

described in the theory section of this report. However the disfavoured polarisation is not

totally zero as there is a slight rise in output power, this is most likely the result of the wave

plate not being totally optimised as the mount it was placed in did not have high precision

thus the favoured polarisation was hard to suppress fully leading to a slight rise in output

power.

41

Discussion This section contains an overview of the results; an explanation as to why these results were

obtained; a description of the challenges encountered in the experiments generating the

results; recommendations on how to improve the method; and lastly, recommendations for

further research that would be beneficial to carry out.

The project offered up substantial challenges starting firstly with the set up. As the laser

beam was not packaged and still part of a wafer it was mounted flat to the table giving

vertical emission. It was hoped that a microscope could be used to place the probes while

the emission was directed parallel to the table using a mirror that let visible light pass and

infrared light reflected. This however was not possible and an alternative had to be found.

The alternative consisted of placing a lens to collimate the light as it diverged and use a CCD

camera and LCD screen to position the probes. However, this focal length was larger than

would have been ideal because space had to remain for the probes to contact the wafer and

this led to a small spot size. Secondly, the lasers that were first contacted had very low

output powers as is demonstrated in figure 14 so seeing the laser beam, aligning it with lens

and mirrors and thereafter detecting the output was problematic. Eventually a change in

laser diode driver solved this problem and a laser with sufficient power was found and used.

This laser had a threshold of 5.57mA and emitted just over 1mW in power. From this basic

set up there was then the addition of a beam splitter to direct some output for feedback

and some feedback for detection. This raised the height of the set up to around 30cm which

is three times the height of normal laser experiments. With the mirror bringing about

feedback the threshold current was successfully reduced to 4.67mA.

At this stage the probes became detached from the contact pads and a new laser had to be

selected requiring the replacing of the photodiode detector with the CCD which was a

process that took some time to realign. With the new laser emitting the set up was changed

to include a spectrum analyser. This was a time consuming and risky step because of the

time needed for alignment and because of the risk that the probes would detach again and

consequently the camera would have to be included and every alignment recalculated. The

last step in the new set up was coupling the light to an optical fiber so it could be used in a

monochoromator providing a spectrum associated with each result. This was also

problematic as the output power was too weak and could not be coupled to a

monochoromator so a spectrum analyser was utilised to overcome this problem.

Including a grating in the set up in Littrow configuration reduced the threshold more than

the mirror and achieved a threshold current of 3.61mA this being 5% more than the mirror

was able to achieve. The thermal effects were then studied with results showing that

feedback did not have any effect on the temperature change and that as driving current

increased from 0 to 9mA it had an associated temperature rise of 10K. This rise in

temperature is not optimal as this changes the number of charge carriers and affects the

wavelength and so the laser changed from continuous wave to modulated wave by the use

42

of the laser diode driver. It was hoped that this would average the temperature change and

result in less temperature sensitive results particularly as there was no external heat sink to

dissipate heat. With the data now in real time it was easier to view the changes the grating

made to the threshold current and the recorded threshold current then remained at 3.61mA

when recorded again. The spectra did not show any signs of tuning as the grating was

rotated, this was not surprising as the line width was large as a consequence of the small

beam size. Switching from the Littrow configuration to the Littman configuration offered

signs that the grating was controlling the laser frequency giving low threshold current values

of 2.78mA, however when optimising the external cavity length the probes unfortunately

became detached and with the time constraints on this experiment it was impossible to

realign everything and obtain spectra to accompany these results.

It is noticeable that in all LI curves there is a curved shape which is not expected of lasers. As

in last years results shown in figure 9 the stimulated emission section of the plot, above

threshold is linear. The reason this years graphs show a curved shape could be as a result of

thermal heating, meaning the mode moves out of resonance as the device heats up,

alternatively it could be as a result of electrons escaping from the quantum well as the

device increases in temperature.

To overcome the set up challenges and other problems mentioned above a number of

improvements are recommended.

Firstly, the setup height of the initial stage was too high, this led to unstable mounts holding

lenses and other components which were easily knocked out of alignment based on the high

height and long length of pole. The recommendation is to reduce the height of the cavity

allowing more secure mounts with shorter lengths thus reducing the freedom of movement.

Secondly, throughout the experiment period, construction work was taking place within the

building which led to large vibrations that the workbench could not filter out. This had a

pronounced effect on some graphs acquired as the probes vibrated and the resistance and

voltage changed to compensate. The research was impacted as some results had to be

recorded numerous times and the vibrations may also have played a role in the detachment

of the probes. It is ideal to carry out sensitive experiments at a time of no construction work

or other external influences which could have a pronounced effect on results obtained.

Additionally, a more robust method of securing the probes could be used or better still to

use a VCSEL that has been packaged.

Lastly, it was disappointing the experiment broke down when it did with insufficient time to

recover the set up. With more time the experiment would have been recovered and

bistability may have been achieved by increasing the length of the external cavity in Littman

configuration thus allowing the resolution to become clearer and reducing the line width.

The spectra analysis would also have been interesting to discuss had the experiment not

have broken down at such a critical stage. The recommendation here as above would be to

43

find a more secure method of attaching the probes or to use a VCSEL that has been

packaged.

Conclusions This report explains the evolution from the Fabry Perot laser to a VCSEL and the

developments in VCSEL from short wavelength to long wavelength. Detailing numerous

applications and the key properties associated with the.

Experiments conducted demonstrated that the threshold current of the laser can be

reduced by 20% with the use of a flat mirror and by 25% with the use of a 1350 lines per

millimetre diffraction grating set up in the Littrow configuration laser and up to 42%

threshold current reduction in Littmann configuration.

Thermal effects and polarisation effects have been presented which show no polarisation

switching and a large shift in wavelength as the driving current is increased, this is attributed

to the heating of the laser.

Finally, it was briefly shown that a diffraction grating in Littman configuration can influence

the frequency of the fundamental laser, consequently leading to a decrease in threshold

current and the ability to be tuned in an unknown range.

Acknowledgments

I would like to take this opportunity to thank the people who contributed to my work.

Firstly, I would like to thank my project supervisor, Thorsten Ackemann who assisted at various

stages of the set up and who guided me through each challenge and problem I encountered. In

addition to assisting to set up the practical experiment his knowledge of VCSELs and the

patience demonstrated when explaining topics were also vitally important.

Thanks must also go to Pedro Gomes who rescued me a few times when I was struggling

to work with the computer programs and also for his support and availability whenever I

had a question no matter how tr