Hybrid Integrated Photonic Components Based on a Polymer Platform

8/2/2019 Hybrid Integrated Photonic Components Based on a Polymer Platform

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Hybrid integrated photonic components based on a polymer platform

Louay Eldada*

DuPont Photonics Technologies, 100 Fordham Road, Wilmington, MA 01887, USA

ABSTRACT

We report on a polymer-on-silicon optical bench platform that enables the hybrid integration of elemental passive and

active optical functions. Planar polymer circuits are produced photolithographically, and slots are formed in them for

the insertion of chips and films of a variety of materials. The polymer circuits provide interconnects, static routing

elements such as couplers, taps, and multi/demultiplexers, as well as thermo-optically dynamic elements such as

switches, variable optical attenuators, and tunable notch filters. Crystal-ion-sliced thin films of lithium niobate are

inserted in the polymer circuit for polarization control or for electro-optic modulation. Films of yttrium iron garnet and

neodymium iron boron magnets are inserted in order to magneto-optically achieve non-reciprocal operation for

isolation and circulation. Indium phosphide and gallium arsenide chips are inserted for light generation, amplification,

and detection, as well as wavelength conversion. The functions enabled by this multi-material platform span the range

of the building blocks needed in optical circuits, while using the highest-performance material system for each

function. We demonstrated complex-functionality photonic components based on this technology, including a metroring node module and a tunable optical transmitter. The metro ring node chip includes switches, variable optical

attenuators, taps, and detectors; it enables optical add/drop multiplexing, power monitoring, and automatic load

balancing, and it supports shared and dedicated protection protocols in two-fiber metro ring optical networks. The

tunable optical transmitter chip includes a tunable external cavity laser, an isolator, and a high-speed modulator.

Keywords: polymer, integrated optics, hybrid integration, integration platform, subsystem on a chip, thermo-optics,

electro-optics, magneto-optics, photodiodes

1. INTRODUCTION

The great potential of optical integration has been recognized for decades, with the most notable advantages being size

reduction, reduction in cost, performance improvement (lower loss due to fewer interfaces, reduction in transient errors

when all interconnects are printed, etc.), increase in yields (due to the increase in uniformity and repeatability),

increase in production volumes, reduction in need for manual labor, and faster time to market. Turning this promiseinto a commercial reality relies on having a versatile integration platform where any optical element can be integrated

without sacrificing any performance characteristic relative to discrete components. Polymer-based planar lightwave

circuits (PLCs) have advantages over their silica-based counterparts in terms of cost, tunability, and polarization

dependence. Hybrid integrated devices based on a polymer waveguide platform can have a significant impact on the

future development of multifunctional photonics ICs. The polymer optical bench we have developed allows the

integration of films of any material, permitting the integration of all optical building blocks in the best-performing

material for each function, resulting in a hybrid subsystem on a chip with uncompromised performance and all the

advantages of integration. We describe in this manuscript advances in optical polymers and polymer devices, as well

as our polymer hybrid integration platform and its use to form integrated isolators, modulators, tunable lasers,

photodiodes, and subsystems on chip including a metro ring node module and a tunable optical transmitter.

2. POLYMERIC MATERIALS AND WAVEGUIDESGlass optical fibers are routinely used today for high-speed data transfer. Although these fibers provide a convenient

means for carrying optical information over long distances, they are inconvenient for complex high-density circuitry.

In addition to being fragile, glass fiber devices are difficult to fabricate, especially when they have a high port count,

and in most cases require active fabrication, resulting in high cost. Integrated optics is becoming the choice approach

for advanced photonic components. Several material systems are being pursued as integrated optic platforms, and their

properties are summarized in Table 1.1

The most widely used integrated optic platform is silica on silicon. Whereas

* [email protected]; phone 978.203.1300; fax 978.988.1040; www.photonics.dupont.com

Invited Paper

Photonics Packaging and Integration III, Randy A. Heyler, David J. Robbins, Ghassan E. Jabbour,Editors, Proceedings of SPIE Vol. 4997 (2003) 2003 SPIE 0277-786X/03/$15.00

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the choice of this platform in the early days of integrated optics was quite natural and justifiable, because of its kinship

to silica fiber, the cons of this technology outweigh the pros, even after three decades of extensive global research.

Silica fiber has a propagation loss of 0.15 dB/km at 1550 nm wavelength, a remarkably low number that is achieved

due to the fact that the fiber has no stress and no roughness. When silica is grown on asilicon wafer, typically at high

temperature by chemical vapor deposition (CVD), a high level of stress is obtained in the wafer when it cools down to

room temperature due to the coefficient of thermal expansion (CTE) mismatch between silica and silicon, which stressresults in polarization-dependent behavior, stress-induced scattering, and high coupling loss when pigtailing to fiber

blocks due to warpage in the substrate. The reactive ion etching (RIE) patterning of waveguides in silica creates rough

edge walls that create scattering loss and further polarization dependence. Another limitation is that the highest

contrast achieved to date in this technology is only 1.5%.2

And yields in this technology have historically been low,

especially in large interferometric devices such as arrayed waveguide gratings (AWGs), where numbers below 10%

are the norm. Other platforms for integrated optics are being pursued, including silicon on insulator, silicon

oxynitride, lithium niobate, indium phosphide, gallium arsenide, sol-gels, and polymers. A comprehensive analysis of

these platform can be found elsewhere.3

Polymer is the material of choice for integrated optics because, when

synthesized and processed properly, it offers high performance (the loss in state-of-the-art polymers is on par with the

state-of-the-art loss achieved in silica, the birefringence is smaller than that of silica by two orders of magnitude), tunability

(thermo-optic coefficient 10-40 times larger than in silica), environmental stability (see next section), high yields, and low cost.

PropagationLoss

(dB/cm)

PigtailLoss

(dB/chip)

RefractiveIndex

(n)

Index ContrastRange in Waveguide

(n)

Birefringence(nTE - nTM)

T/O Coef.dn/dT(K

-1)

MaximumModulationFrequency

Passive/ Active

Silica[SiO2]

0.1 0.5 1.50-1.5%

(Channel)10

-4-10

-210

-5 1 kHz(T/O)

Y/Y

Silicon[Si]

0.1 1.0 3.570% [range = 0]

(Silicon on Insulator Rib)10

-4-10

-2 1.810

-4

1 kHz(T/O)

Y/N

Silicon Oxynitride[SiOxNy]

0.1 1.0SiO2:1.5Si3N4:2.0

0-30% [30%: Si3N4 core](Channel, SiO2 clad)

10-3

-510-6

10-5 1 kHz

(T/O)Y/N

Sol-Gels 0.1 0.5 1.2-1.50-1.5%

(Channel)10

-4-10

-210

-5

1 kHz(T/O)

Y/Y

Polymers 0.1 0.5 1.3-1.70-35%

(Channel)10

-6-10

-2-1-410

-4

>100 GHz (E/O)1 kHz (T/O)

Y/Y

Lithium Niobate[LiNbO3]

0.5 2.0 2.20-0.5%

(Channel)10

-2-10

-110

-5

40 GHz(E/O)

Y/Y

Indium Phosphide[InP]

3 10 3.10-3%

(Channel)10

-3 0.810

-4

40 GHz(E/O)

Y/Y

Gallium Arsenide[GaAs]

0.5 2.0 3.40-14% [14%: AlAs clad]

(Rib)10

-3 2.510

-4

20 GHz(E/O)

Y/Y

Table 1. Properties at 1550 nm wavelength of key integrated opticmaterials (T/O = thermo-optic, E/O = electro-optic).

Optical polymers were engineered in many laboratories worldwide (Table 2) and some are available commercially.4

Classes of polymers used in integrated optics include acrylates, polyimides, polycarbonates, and olefins (e.g.,

cyclobutene). Some polymers, such as most polyimides and polycarbonates, are not photosensitive, and are typically

processed using photoresist patterning and RIE etching. These polymers have most of the problems of the silica on

silicon technology in terms of roughness-induced and stress-induced scattering loss and polarization dependence.

Other polymers are photosensitive and as such are directly photopatternable, resulting in a full cycle time of about

30 minutes per multi-layer optical circuit on a wafer. These materials have an obvious advantage in throughput,

producing wafers between 10 and 1000 times faster than other planar technologies. Furthermore, this technology uses

low-cost materials and low-cost processing equipment (e.g., spin-coater and UV lamp vs. CVD growth system).

Optical polymers can be highly transparent, with absorption loss values around or below 0.1 dB/cm at all the key

communication wavelengths (840, 1310, and 1550 nm).5

The scattering loss can be minimized in polymer waveguides

by using direct photopatterning as opposed to surface-roughness-inducing RIE etching. The effect of the little

roughness that is obtained can be further minimized by the use of a graded index, a natural process in direct polymer

lithography where inter-layer diffusion is easily achieved. The graded index profile results in weak confinement of the

Proc. of SPIE Vol. 4997 89


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ManufacturerPolymer Type[Trade Name]

PatterningTechniques

Propagation Loss, Single-Mode Waveguide, dB/cm

[wavelength, nm]

Other Properties[wavelength, nm]

AmocoFluorinated polyimide

[UltradelTM

]Photoexposure /

wet etch0.4 [1300]1.0 [1550]

Birefringence: 0.025Crosslinked, Thermally stable

AcrylatePhotoexposure /wet etch, RIE,

laser ablation

0.02 [840]0.3 [1300]

0.8 [1550]

Birefringence: 0.0002 [1550]Crosslinked, Tg: 25C

Environmentally stableCorning(formerly AlliedSignal)

Halogenated acrylatePhotoexposure /wet etch, RIE,laser ablation


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in planar polymer components can closely approach the value of the absorption loss in the waveguides when fabrication

techniques are optimized. Fig. 1a shows the insertion loss at 1550 nm of a straight waveguide cut back to different

lengths, exhibiting 0.11 dB/cm propagation loss (slope) and 0.29 dB fiber coupling loss for both sides of the chip (y

intercept). Fig. 1b depicts the insertion loss of a 3-cm-long straight waveguide tested between 1500 and 1570 nm

wavelength, exhibiting flatness across this wavelength range.

Chip Length (mm)

0 10 20 30

InsertionLoss(dB)

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

Wavelength (nm)1490 1500 1510 1520 1530 1540 1550 1560 1570 1580

InsertionLoss(dB)

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

(a) (b)

Fig. 1. (a) Results of a cut-back experiment on a polymer optical waveguide, revealing 0.11 dB/cm propagation loss and 0.29 dB total

chip-to-fiber coupling loss. (b) Insertion loss of a polymer waveguide between 1500 and 1570 nm, demonstrating wavelength flatness.

The polarization dependent loss (PDL = lossTE lossTM) varies with processing conditions. The TE loss measured in

planar waveguides can be higher than the TM loss when the vertical walls of the core have a higher degree of

roughness than the horizontal boundaries, and it can be lower when the vertical evanescent tails overlap with an

absorptive substrate or superstrate. Waveguides that are well optimized by having minimal edge roughness and a well-

confining material stack can have PDL values that are immeasurably small. The birefringence (nTE - nTM) can be

extremely low (on the order of 10-6

) in polymers that undergo little molecular orientation during processing, as is

common in three-dimensionally cross-linked polymers.

The environmental stability of optical polymers (i.e., the stability of their optical and mechanical characteristics with

temperature and humidity) is an important issue because most polymers do not have properties that are appropriate for

operation in communication environments. A key characteristic for practical applications is the thermal stability of the

optical properties since organic materials may be subject to yellowing upon thermal aging due to oxidation. Thepresence of hydrogen in a polymer allows the formation of H-Halogen elimination products, which result in carbon

double bonds, which are subject to oxidation. Fortunately, the absorbing species from thermal decomposition are

centered near the blue region of the spectrum, and the thermal stability can be high at the datacom wavelength of

840 nm and even greater at the telecom wavelengths of 1300 and 1550 nm. On the humidity front, the resistance of

polymers to water incursion is critical since optical absorption results from the overtone bands of the OH-stretch of

water. However, polymers that stand up to 85C 85% RH (relative humidity) conditions have been demonstrated, and

some polymers passed the Bellcore 1209 and 1221 environmental tests.

0 500 1000 1500 2000 2500 3000 3500 4000 4500 5000

Time (hours)

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

Transmission(dB)

5000 Hours, 175C

Fig. 2. Transmission of four planar polymeric optical waveguides held at 175C for 5000 hours, revealing no degradation.



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Extensive materials research has yielded polymers that are highly transparent and reliable, to the extent that they are

no longer the limiting factor in components lifetime. Figure 2 shows the results of accelerated aging tests performed

on a DuPont optical polymer. This figure shows the optical transmission values relative to pre-bake values, as

measured in-situ on pigtailed 10-cm-long single-mode waveguides in an environmental chamber set at 175C. The

initial sharp increase in transmission is due to the relaxation of fabrication stresses. The step around 2000 hours is due

to stopping the experiment, then resuming after cleaning the connectors. As can be seen, no degradation in theinsertion loss of the components was observed after 5000 hours of testing.

Another important feature of polymers is the tunability of the refractive index contrast, which can have values up to

35%, enabling high-density compact waveguiding structures with small radii of curvature. Polymers also allow simple

high-speed fabrication of 3-D circuits with vertical couplers, which are needed with high-index-contrast waveguides

where 2-D circuits would require dimensional control, resolution, and aspect ratios that are beyond the levels

achievable with todays technologies. Furthermore, the unique mechanical properties of polymers allow them to be

processed by unconventional forming techniques such as molding, casting, stamping, and embossing, permitting rapid,

low-cost shaping for both waveguide formation and material removal for grafting of other materials such as thin-film

active layers, Faraday rotators, or half-wave plates.

The production of commercially viable polymeric optical components is a complex task because optical polymers need

to meet simultaneously many key properties. Table 3 summarizes the 25 main properties that a polymer needs to meetin order to be a viable thermo-optic polymer with potential for commercial deployment in communication applications.

1. Low absorption loss

2. Low polarization dependent loss (PDL)

3. Low chromatic dispersion

4. Low polarization mode dispersion (PMD)

5. Low birefringence

6. Low stress

7. Low pigtail loss

8. Variable refractive index difference (n)

9. Closeness of refractive index to silica fiber refractive index

10. Ability to vary refractive index profile

11. Low refractive index dispersion (dn/d)12. Thermal stability

13. Stability with humidity

14. Stability with optical power

15. Adhesion (to substrates, self, electrodes)

16. Full curability

17. Low shrinkage

18. Contrast in patterning

19. Process latitude in patterning

20. Patternability with low scattering loss

21. Machinability (cleaving, dicing, polishing)

22. Compatibility with electrode patterning

23. Large T/O coefficient (dn/dT)

24. Optimal thermal conductivity

25. Manufacturability with repeatable properties

Table 3. The top 25 list of properties of thermo-optic polymers.

3. POLYMERIC OPTICAL COMPONENTSOne of the earlier drivers for work with polymer waveguides was the concept of fiber-to-the-home, which raised the

possible future need for low cost passive optical components such as splitters. This perception of the need for low cost

devices has driven much of the early effort on the development of low cost replication technologies.6

When this

opportunity did not appear as soon as expected, other higher value applications requiring greater functionality were

pursued. The early commercialization of polymer waveguide technology came with the introduction of thermo-optic

switches.7

Integrated polymer components produced to date for the optical communication industry include couplers,

switches, attenuators, filters, polarization controllers, modulators, lasers, amplifiers, and detectors.

92 Proc. of SPIE Vol. 4997


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An important property of polymers is that they have a large negative thermo-optic coefficient (dn/dT = -1-410-4) thatis 10-40 times larger (in absolute value) than that of more conventional optical materials such as glass, resulting in

low-power-consumption thermally-actuated optical elements. In this section, we describe a variety of thermally

actuated polymeric components, including switches, variable optical attenuators (VOAs), and tunable filters.

3.1. SwitchesThermo-optic NN switches can be interferometric switches based on directional couplers or Mach-Zehnder interferometers(MZIs), or they can be digital optical switches (DOSs) based on X junctions or Y junctions.

8The most widely used

switch design is the Y-junction-based DOS (Y-DOS), because of its simplicity and its digital behavior.

The building-block element in a Y-DOS is a small-angle (typically about 0.1) 12 splitter with heaters on its arms.9A schematic diagram of such a device is shown in Fig. 3a. These splitters can be connected with bends and crossings

to form MN switching matrices. Each 12 splitter relies on adiabatic evolution of the mode profile in its twowaveguides into the mode of the ON guide (the guide with the higher effective refractive index) when the OFF guide is

heated to reduce its index, as shown in the computer simulation of Fig. 3b.

Heater

Electrode

(ON)

Heater

Electrode

(OFF)

Bonding

Pad

Channel

Waveguide

Optical

Signal

IN

Optical

Signal

OUT

~0.1

(a) (b)

Fig. 3. (a) Schematic diagram of a 12 Y-DOS and (b) computer simulation of such a device where the left arm is heated, allowing thelight to exit the right arm.

The device is considered to have switched once it reaches the desired isolation value, which occurs at some level of

electrical power dissipation in the electrodes, beyond which power level the device maintains the isolation, resulting in

its well-known digital behavior. DOSs in a 12N configuration can be fabricated with 2N-1 12s, and one electrode

at each stage needs to be heated to perform the switching. For instance a 14 switch can be built with three 12s, asshown in Fig. 4a, where the upper electrode in the first stage and the lower electrode in the second stage are powered to

switch the light from port 1 to port 3. As another example, a strictly non-blocking NN switch can be fabricated with

2N(N-1) 12s.10 Fig. 4b shows a 22 (or cross-bar) DOS built with four 12s. This switch is operated in the barstate by powering the four inner electrodes, while powering the four outer electrodes results in the cross state.

1'

2'

3'

4'

1 1'

2 2'

(a) (b)

Fig. 4. Schematic layouts of (a) a 14 DOS and (b) a 22 DOS. The dark electrodes are powered. The 12 is shown switching fromport 1 to port 3 and the 22 is in the bar state.



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Fig. 5 depicts the operational characteristics at 1550 nm of a 22 Y-DOS (shown schematically in Fig. 4b), exhibiting

high isolation of 57 dB at 66 mW heater electrical power consumption. When no electrical power is applied, the

insertion loss is 6 dB below the minimum insertion, as each of the two Y branches in a path behaves as a 3 dB splitter.

The isolation level is maintained at higher electrical power levels; this digital behavior makes the switch reliable and

insensitive to variations in temperature, electrical power, wavelength, and polarization. A typical maximum time for

restoration and restructuring of multiple circuit connections for SONET is 50 ms. The measured response time forthese thermo-optic switches is approximately 5 ms, a value that is adequate for system restoration.

Heater Power (mW)

0 10 20 30 40 50 60

OpticalOutput(dB)

60-

50-

40-

30-

20-

10-

0

On State

Off State

Fig. 5. Operational characteristics at 1550 nm of a 22 Y-branch-based DOS.

The small value of dn/dT in silica PLCs has kept 1616 switches from being implemented with DOS 12 switches.

Planar 1616 switches implemented to date have been based on MZIs.11 We describe here the implementation of the

first DOS-based 1616 switch.12 This switching matrix consists of 480 12 switches, interlinked with 704 S-bends that

intersect at 227 locations to provide a strictly non-blocking connectivity. Fig. 6 shows the 1616 switch bothschematically (a) and to scale (b), as well as a photograph of the heaters in a fabricated device (c).

S t a g e

1

2

3

4

5

6

7

8

(a) (b) (c)

Fig. 6. Design of the first 1616 DOS-based switch matrix. (a) Schematic diagram and (b) CAD layout of the recursive tree structure thatmakes up the waveguiding circuit, and (c) electrode layer of a fabricated device.

In the characterization of the switching matrix, the performance of the DOS and crossing building blocks were first

measured. In a fully functional 1616 switch, each path is defined by heating 8 heaters, thus 128 heaters are

continuously being used. Since the crosstalk is not limited by the switches, an extinction of 15 dB per 12 stagealready yields sufficient effective extinction due to the concatenation of the 8 switching/combining stages, and taking

into account the additional crosstalk due to the crossings. At a power dissipation of 50 mW per DOS, the insertion loss

is 6 dB and the extinction is 30 dB, mainly limited by the crosstalk due to crossings in the design. The total nominal

power consumption is 6.4 Watt.



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3.2. Variable Optical Attenuators

With the increasing complexity of WDM optical networks comes an increasing need for reliable, low cost VOAs that

adjust the power level of optical signals with high accuracy and repeatability. VOAs can be based on any switching

principle including interferometry, modal transition, or mode confinement. An interferometric approach involves

using an MZI where heat can be applied to at least one of the arms to induce a phase shift between the two arms before

they recombine, thereby controlling the level of optical power exiting the output guide. Fig. 7a shows a simulation ofthis device when power is applied to thermally induce a phase difference between the optical signals in the two arms,which recombine to form an asymmetric mode that radiates out into the cladding since it is not supported by the single-

mode output waveguide, resulting in full attenuation. Fig. 7b shows the operational characteristics of devices in an 8-

channel VOA array, revealing a very low power consumption of about 1.4 mW for 30 dB attenuation.13

One

performance specification that is typically difficult to achieve in VOAs is low PDL. The PDL achieved in polymeric VOAs

is under 0.2 dB across the entire attenuation range, a value that is lower than that achieved in any other material system.

Heater Power (mW)

0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4

OpticalOutput(dB)

30-

20-

10-

0

(a) (b)

Fig. 7. (a) Computer simulation of an MZI VOA where heat is used to induce a phase difference between the interferometer arms for fullattenuation, and (b) attenuation curve of such a VOA.

3.3. Tunable Filters

Tunable filters can be based on Bragg gratings, diffraction gratings, arrayed waveguide gratings (AWGs), microring

resonators, or photonic crystals. We describe below the characteristics of tunable Bragg gratings. By alternating in a

waveguide the refractive index periodically about an average effective refractive index of n, an in-line series of weakly

reflecting mirrors (a Bragg grating) of spacing is created. The cumulative effect of the mirrors is to maximally

reflect wavelengths , equal to 1/N multiples of 2n , where N1 is an integer indicating the order of the gratingperiod. Gratings in planar polymers can be produced by a variety of techniques such as casting, molding, embossing,

e-beam writing and photochemical processes.14-16

The first three techniques produce surface relief gratings while the

last two can produce either relief gratings or bulk index gratings across the waveguide core. Photochemical fabrication

processes for bulk index gratings utilize two-beam interference to induce an index modulation. This effect can be

achieved either through the use of a phase mask (where two beams corresponding to the +1st

and -1st

diffraction orders

are allowed to interfere) or through the use of direct interference of split laser beams. For a waveguide refractive index

of n = 1.5, the period of the grating corrugation (the Bragg period ) is approximately 0.52 m. The magnitude and

the filter width of the Bragg grating can be predicted using the following relations. Given a grating length of L = 10 mm, an

index modulation ofn = 110-4, and a center design wavelength of0 = 1.55 m, the grating has a coupling constant

= ( n)/(20) = 110-4

[m-1

]. At the Bragg wavelength, the power reflection coefficient is R = tanh2( L) = 0.58,

and the FWHM bandwidth of the filter can be approximated by FWHM = (0n)/(2n) = 50 pm. When tuning isdesired, a hybrid device of the type shown in Fig. 8a or an integrated device of the type shown in Fig. 8b can act as a

thermally tunable optical add/drop multiplexer (OADM). In the hybrid approach, the chip consists of a grating

produced in an integrated single-mode polymeric waveguide with a deposited thin film heater. This chip, combined

with two three-port optical circulators, forms a tunable OADM. In this design, the only critical requirement for the die

is to have a grating with the desired spectral response, and the circulators provide the add and drop functions in a

reliable fashion. In the integrated approach, a Mach-Zehnder interferometer (MZI) with a grating across its arms forms the

OADM. In this design, the couplers must be fabricated carefully for 3-dB operation in order to achieve good performance.



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Substrate Heater

Grating

Circulator Circulator

Input Pass

Drop Add

Waveguide

Input Add

Drop Pass

gratingheaterwaveguide

substrate (a) (b)

Fig. 8. OADM configurations: (a) hybrid design consisting of a chip (with a Bragg grating in a polymeric waveguide and a thin film heater)and two 3-port optical circulators, and (b) fully integrated design consisting of a grating in the arms of a polymeric MZI and thin film heaters.

Tuning of the grating is achieved by powering a heater that is fabricated in the proximity of the grating. Broad thermal

tuning of polymeric gratings is possible because of the large dn/dT. The value of dn/dT in the polymers used is

-3.110-4 /C (about 30 times larger than in glass), resulting in a channel tuning rate of -0.36 nm/C. This figuretranslates into tuning across the entire C band (1528-1565 nm) with a temperature range of about 100C. Fig. 9

depicts the transmission spectra obtained when tuning the filter between 20 and 120C, demonstrating that a

wavelength window of 37 nm can be covered with a single tunable filter.

Fig. 9. Transmission spectra for a tunable filter operated between 20 and 120C, covering the C band between 1528 and 1565 nm.

4. POLYMER OPTICAL BENCH FOR HYBRID INTEGRATION

The photolithographically defined planar polymer circuits can be machined to form slots in which chips and films of a

variety of materials can be inserted, thereby providing a hybrid integration platform. The polymer circuits provideinterconnects, static routing elements such as couplers, taps, and multi/demultiplexers, as well as thermo-optically

dynamic elements such as switches, VOAs, and tunable notch filters. Films of yttrium iron garnet (YIG) and

neodymium iron boron (NdFeB) magnets are inserted in order to magneto-optically achieve non-reciprocal operation

through polarization rotation (Faraday rotation), non-reciprocal phase shift, or guided-mode-to-radiation-mode

conversion, enabling isolation and circulation. CIS thin films of LiNbO3 are inserted in the polymer circuit to achieve

half-wave retardation for polarization-independent operation or polarization mode splitting, and they enable electro-

optic actuation for modulation. InP/InGaAsP laser chips or semiconductor optical amplifier (SOA) chips are inserted

for light generation or amplification, as well as wavelength conversion, and InP/InGaAs photodiode chips are inserted

for light detection. This multi-material platform (Fig. 10) permits the fabrication of any building block needed in

optical circuits, while using the highest-performance material system for each function.

4.1. Polymer Processing for Hybrid Integration

When optical chips or thin films are inserted into a polymer circuit with one of their long dimensions (> 1 mm) alongthe optical path, RIE processing is used to create a large opening for insertion. When however thin-film components

are inserted with their small dimension (10 m 1 mm) along the optical path, grooves with widths down to 10 m can be

fabricated with a standard dicing saw. The dicing process features excellent width control and yields remarkably smooth

sidewalls with very few defects. Typical results are shown in Fig. 11. After the insertion of a film in a groove, liquid

monomer of the type used in the waveguide core is poured into the groove and cured under UV light, thereby healing

the cut by providing perfect refractive index matching. Insertion loss measurements performed on such assemblies are

in good agreement with the calculated diffraction loss, indicating that the scattering loss is negligible (


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C i r c u l a t o r P o l a r i z a t i o n

C o n t r o l l e r

P h o t o d i o d e M o d u l a t o r

I s o l a t o r T u n a b l e

L a s e r

A m p l i f i e r

P o w e r

T a p

T u n a b l e

C o u p l e r

%

M u l t i /

D e m u l t i p l e x e r

T u n a b l e

F i l t e r

V a r i a b l e

A t t e n u a t o r

C r o s s b a r

S w i t c h

1 x N

S w i t c h

P o w e r

S p l i t t e r

W a v e l e n g t h

C o n v e r t e r

S t a t i c s & T h e r m o - O p t i c s

M a t e r i a l : p o l y m e r

M a g n e t o - O p t i c s

M a t e r i a l s : Y I G

( F a r a d a y r o t a t o r s ) ,

N d F e B o r S m C o

( m a g n e t s )

E l e c t r o - O p t i c s

M a t e r i a l : L i N b O

3

P o l a r i z a t i o n O p t i c s

M a t e r i a l s : A g / G l a s s

( p o l a r i z e r s ) , L i N b O

3

( h a l f - w a v e p l a t e s )

A c t i v e s

M a t e r i a l s : I n P , G a A s ,

r a r e - e a r t h - d o p e d p o l y m e r

Figure 10. Polymeric platform for hybrid integration of optical building block functions.

(a) (b)

Figure 11. (a) Scanning electron micrograph of grooves fabricated by dicing (groovedepth: 200m, width: 40m, tilt angle: 8), and (b) close up view of a diced facet.

4.2. Integrated Passive Optical Elements

Passive optical elements integrated in our polymeric platform include statics, thermo-optics, magneto-optics,

polarization optics, and electro-optics. The former two types of elements are formed in the polymer itself, not requiring

material hybridization. This section focuses on the latter three classes.

4.2.1. Hybrid Integration for Magneto-Optics

The hybrid integration of isolators into polymeric optical circuits is described here.17

All concepts for polarization

independent isolators and circulators that have been proposed so far utilize interferometers. In one proposed

implementation, such a polarization independent device consists of a Mach-Zehnder interferometer with a half-wave

retarder and a Faraday rotator in each arm, as shown in Fig. 12a.18

By having the two wave plates on opposite sides of

the Faraday rotator and at a 45 angle between their slow axes, constructive and destructive interference in forward andbackward propagation direction is achieved, respectively. Another design that resembles the functionality of the micro-

optical isolators and circulators is shown in Fig. 12b. It consists of two integrated polarization splitters, a 45 Faradayrotator and a half-wave retarder. The first polarization splitter splits the TE and TM components of the incoming light

into two separate arms. The light in each arm passes through the combination of Faraday rotator and half-wave plate

where, depending on the propagation direction, the polarization remains unchanged or is rotated by 90. The secondpolarization splitter then recombines the light from the two arms into one of the two output ports. The main advantage

of this design over the previous one is that it can achieve much higher isolation ratios.



11/15

Figure 12. Circulator designs: The top design (a) consists of Mach-Zehnder interferometer with half wave retarders and Faraday rotator.The bottom design (b) consists of two polarization splitters, a Faraday rotator, and a half wave retarder.

We demonstrated the hybrid integration of nonreciprocal components into polymer-based PLCs by realizing an isolator

within our polymer hybrid integration platform (Fig. 13). In this experiment, a commercial Bi:YIG crystal with a

thickness of 450 m was used as the 45 Faraday rotator. The field strength required to saturate the magnetization wasabout 500 Oe. In order to minimize the diffraction losses, waveguides with a mode diameter of 20 m were fabricated

on a silicon substrate and grooves to accommodate the thin-film elements were diced perpendicularly to the

waveguides. The groove for the Faraday rotator was 460 m wide, all other grooves were 40 m wide. All grooves

were separated from each other by 100 m. A thin LiNbO3 film was used as the half-wave plate.19

Its slow axis was

set at an angle of 22.5 relative to the Lamipol polarizers at the input and output of the device. Liquid monomer waspoured into the grooves and solidified by UV curing in order to fix the components and minimize the scattering losses.

Finally, two NdFeB magnets were glued to the surface with a glass plate between them as a spacer. The observed

magnetic field across the gap was 2 kOe, which was sufficient to saturate the Faraday rotator. This device was fully

packaged and optically measured at = 1.55 m. It exhibited an isolation of 15 dB and an excess loss of 2.2 dB. Theexcess loss is defined as the difference between the theoretically expected loss and the measured loss. It includes the

fiber-connector losses, imperfect polarization rotation at the half-wave retarder and angular alignment errors of

polarizing elements. The diffraction losses of 2 dB can be significantly reduced when the 450 m thick Faraday rotator

is replaced by multiple thin rotators. For instance, using three films of 150 m thickness reduces the diffraction loss by

1.2 dB. Calculations show that a Ce:YIG film Faraday rotator would reduce the overall insertion losses to less than 1 dB.

Figure 13. Fabricated hybrid integrated isolator based on DuPonts polymeric platform.

4.2.2. Hybrid Integration for Polarization Optics and Electro-Optics

The CIS process allows the production of thin crystal slices with optical properties identical to those of bulk crystals.Since the CIS process is an important tool in our hybrid integration technology, as it allows slicing practically any

crystal for insertion in our polymer circuits, it is worthwhile discussing this process in some detail. The CIS process

employs the formation and subsequent preferential etching of a buried damaged sacrificial layer obtained by implanting

highly energetic although light He+

particles into a single-crystal bulk material. As the ions penetrate into the target

they scatter and lose energy. At higher energies the scattering is primarily electronic with no lattice distortion, while at

lower energies, the dominant stopping mechanism is governed by Rutherford scattering. The host nuclei thus become

significantly dislodged towards the end of the ionic range, leading to the formation of a narrow damaged sacrificial

(a)

(b)



12/15

layer well beneath the surface, which exhibits preferential etching upon immersion into an etchant solution. The energy

of the implantation can be adjusted to select a desired film thickness, while the total dose determines the amount of

damage introduced in the sacrificial layer. After implantation, the sample can be wet-etched, and a deep undercut will

form in the sacrificial layer during the etch process and uniformly progress with time, resulting in a freestanding CIS

film.20,21

An alternate route to lift-off the thin CIS films uses thermal shock.22,23

This process results in a lateral

pressure release in the sacrificial (implanted) layer, and a subsequent separation of the CIS film from its parent wafer.The films obtained by both processes are of comparable quality. Figure 14a features a scanning electron micrograph of

a CIS film of LiNbO3 placed on a glass platform. As can be inferred from the figure, the CIS process does not degrade

the facet film quality (polished prior to liftoff). In the case of wet-etch CIS liftoff, the top film surface can generally be

affected by the etchant, but this effect can be readily suppressed by applying an appropriate protective layer. This is

particularly important in fabricating pre-patterned active optical circuits in thin CIS films.24

The underside film surface

corresponding to the sacrificial layer is somewhat rougher; however, it is still quite smooth relative to the wavelengths of

interest (~ 1550 nm), with a typical rms roughness value of


13/15

4.3.2. Photodiode Arrays

We have hybridly integrated photodiode arrays in our polymer platform for power monitoring, by flip-chip mounting

the array chips on top of out-of-plane mirrors at the ends of tap waveguides, as shown in Fig. 15. Various techniques

have been proposed for the fabrication of out-of-plane mirrors in PLCs.27

The mirror shown in Fig. 15a was fabricated

by Excimer laser ablation of the polymer. The ablation is followed by surface treatment for planarization, then by

metalization for high reflectivity.

(a) (b)

Fig. 15. Out-of-plane mirror fabricated by Excimer laser ablation (a) with flip-chip mounted photodiode arrays (b).

The mirror quality has been characterized by coupling light from a fiber into the input facet of a chip, and monitoring

the tap-transmitted power by detection with the photodiodes. The measured excess loss and PDL were 0.3 dB and 0.1

dB, respectively, a performance level that is adequate for our applications.

4.4. Applications

The proposed polymer-on-silicon platform for hybrid integration can be used to produce a variety of compact, high-

performance systems-on-a-chip. We describe in this section two such implementations, namely a metro ring node

module, and an integrated tunable optical transmitter.

4.4.1. Metro Ring Node Module

The Metro Ring Node module contains an array of N optical circuits that handle in parallel N wavelength channels

passing through a network node. Each of the N circuits has three input ports and three output ports. Two of the

input/output ports are used to support bi-directional signal propagation within a ring network, while the remainingports on the input and the output are reserved for adding and dropping a signal, respectively. As shown in Fig. 16, the

device is realized with all inputs on one side of the chip and all outputs on the other side, with a middle input/output

port reserved for the add/drop function. Each of the N circuits consists of a variable 12 splitter, a 12 switch, two 22

switches, two VOAs, three optical power taps, and three integrated photodiodes. The allowed states for the variable

12 splitter include 50/50 splitting and complete switching into one of the two output ports. The device functions as an

OADM by allowing both inputs to remain in the ring network or by selecting one of the two inputs to exit at the drop

port while the signal from the add port exits the selected output port. It can also function as a dedicated and shared

protection switch by having the same functionality as the OADM, while being able to split the add signal. The VOAs,

taps and photodiodes allow for optical power monitoring, equalization of the power distribution across all the output

channels, and activation of the alarm function when the output signals are degraded. A detailed description of the

functionality can be found elsewhere.28

Four-channel TRN chips were fabricated in the polymer-on-silicon hybrid integration platform, and they werecharacterized.

29They measured 2.6 cm in length and 1.3 cm in width. The measured overall insertion loss values for

different paths in a TRN are shown in Fig. 17. The insertion loss consists of the propagation loss (polymer absorption

loss, waveguide scattering and radiation loss), fiber coupling loss, and excess loss at waveguide crossings, switches,

VOAs, and taps. Based on the straight waveguide measurements, the combined effect of the propagation and coupling

losses amounts to 0.55 dB. The waveguide crossing loss occurs at 22 switches, and it depends on the angle between

the intersecting guides as well as their geometry. At the present stage of development, our measurements indicate a

loss of 0.15 dB for this element. The maximum excess loss at switches and VOAs are measured to be 0.1 dB per



14/15

element. The excess loss at the 4% taps (not including the tapped optical power) is measured at 0.05 dB, and is found

wavelength insensitive across 1500-1570nm (< 0.05 dB). It should be noted that the measured insertion loss valuesreported in Fig. 17 are superior to those of typical loss specifications based on discrete fiber-optic elements (~1.75 dB).

12 Switch

22 Switch

Variable 12 Splitter

Variable Optical Attenuator

Optical Power Tap

Integrated Photodiode

In West

Add

In East

Out East

Drop

Out West

1

In West

Add

In East

Out East

Drop

Out West

N

.

.

.

.

.

.

Fig. 16. Layout of an N-channel TRN chip.

In WestOut East: 1.25 dB insertion loss

< -50 dB isolation

Drop: 1.25 dB insertion loss< -50 dB isolation

Out West: 1.10 dB insertion loss

< -50 dB isolationIn East

FromTo

In West

Out West

In East

Add

Out East Drop

1.10 dB

1.10 dB

1.25 dB

1.25 dB

1.25 dB

1.25 dB

Add

(a) (b)

Fig. 17. (a) Insertion loss values measured for different paths in a TRN. (b) Insertion loss and isolation values for one TRN state. Theattenuation levels of all the VOAs are set at 0 dB.

In order to determine the worst-case PDL, each element in the integrated TRN was individually characterized. Thepolarization dependent loss of a polymer waveguide itself is minimal (< 0.04 dB). The PDL of the VOA is measured to

be < 0.2 dB for attenuation levels up to 15 dB. The measured PDL of the 4% taps is also 0.2 dB. No significant

(< 0.05 dB) wavelength dependence of the PDL is observed for the wavelength range 1500-1570 nm.

4.4.2. Integrated Tunable Optical Transmitters

An all-inclusive tunable optical transmitter integrated on a single chip was proposed.26

It consists of an active element

(laser chip), thermo-optic elements (a phase controller and a tunable filter enabling tuning of the laser through a

tunable external cavity), a magneto-optic element (optical isolator), and an electro-optic element (modulator). The

entire circuit is built into our polymer platform. A block-diagram schematic of the integrated circuit is shown in Fig. 18.

P o l y m e r O p t i c a l B e n c h C h i p

LaserDiode

TunableFilter

Isolator

Modulator

M o d u l a t e d

S i g n a l

O u t

Tunable External Cavity Laser

Waveguide

Fig. 18. Block diagram of the hybrid integrated tunable optical transmitter subsystem on a chip.



15/15

The tunable optical transmitter subsystem on a chip is shown in Fig. 19. This is the first complete tunable transmitter

to be integrated in a single planar lightwave circuit.

+V

Polymer TunableBragg Grating

PolymerPhase Shifter

InP/InGaAsPMQW Chip

NdFeBMagnet

NdFeBMagnet

YIGFaraday Rotator

(45)

M M

LiNbO3Half-Wave Plate

(fast axis @22.5to TE)

Glass Plate

Ag-GlassPolarizer

(TE)

Ag-GlassPolarizer

(TE)

LiNbO3Modulator

Tunable External Cavity Laser

Isolator

PolymerWaveguide

SiliconSubstrate

Fig. 19. Tunable optical transmitter comprising a tunable laser, an isolator, and a modulator, integrated on a single silicon chip using apolymer optical bench platform.

5. CONCLUSION

We presented a polymer-on-silicon platform that enables the hybrid integration of elemental passive and active optical

functions. We discussed the properties of the polymers and their use to form interconnects, static routing functions,

and thermo-optically dynamic elements. We also discussed the hybrid integration of magneto-optics, polarization

optics, electro-optics, and actives. We demonstrated complex-functionality subsystems-on-a-chip based on this

technology, including a metro ring node module and a tunable optical transmitter.

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Hybrid Integrated Photonic Components Based on a Polymer Platform