polarization rotator-splitters in standard active …...polarization rotator-splitters in standard...

Polarization rotator-splitters in standardactive silicon photonics platforms

Wesley D. Sacher,1,∗ Tymon Barwicz,2 Benjamin J. F. Taylor,1 andJoyce K. S. Poon1

1Department of Electrical and Computer Engineering, University of Toronto,10 King’s College Road, Toronto, Ontario, M5S 3G4, Canada

2IBM Thomas J. Watson Research Center, 1101 Kitchawan Road,Yorktown Heights, New York 10598, USA

∗[email protected]

Abstract: We demonstrate various silicon-on-insulator polarizationmanagement structures based on a polarization rotator-splitter that uses abi-level taper TM0-TE1 mode converter. The designs are fully compatiblewith standard active silicon photonics platforms with no new levels re-quired and were implemented in the IME baseline and IME-OpSIS siliconphotonics processes. We demonstrate a polarization rotator-splitter withpolarization crosstalk < −13 dB over a bandwidth of 50 nm. Then, weimprove the crosstalk to < −22 dB over a bandwidth of 80 nm by inte-grating the polarization rotator-splitter with directional coupler polarizationfilters. Finally, we demonstrate a polarization controller by integrating thepolarization rotator-splitters with directional couplers, thermal tuners, andPIN diode phase shifters.

© 2014 Optical Society of America

OCIS codes: (130.3120) Integrated optics devices; (230.5440) Polarization-selective devices.

References and links1. Editorial, “Simply silicon,” Nat. Photonics 4, 491 (2010).2. T. Baehr-Jones, L. Pinguet, T., G.-Q., S. Danziger, D. Prather, and M. Hochberg, “Myths and rumours of silicon

photonics,” Nat. Photonics 6, 206–208 (2012).3. T. Barwicz, M. R. Watts, M. A. Popovic, P. T. Rakich, L. Socci, F. X. Kartner, E. P. Ippen, and H. I. Smith,

“Polarization-transparent microphotonic devices in the strong confinement limit,” Nat. Photonics 1, 57–60(2007).

4. M. R. Watts and H. A. Haus, “Integrated mode-evolution-based polarization rotators,” Opt. Lett. 30, 139–140(2005).

5. M. R. Watts, H. A. Haus, and E. P. Ippen, “Integrated mode-evolution-based polarization splitter,” Opt. Lett. 30,967–969 (2005).

6. L. Chen, C. R. Doerr, and Y.-K. Chen, “Compact polarization rotator on silicon for polarization-diversified cir-cuits,” Opt. Lett. 36, 469–471 (2011).

7. H. Fukuda, K. Yamada, T. Tsuchizawa, T. Watanabe, H. Shinojima, and S. Itabashi, “Silicon photonic circuitwith polarization diversity,” Opt. Express 16, 4872–4880 (2008).

8. D. Vermeulen, S. Selvaraja, P. Verheyen, P. Absil, W. Bogaerts, D. Van Thourhout, and G. Roelkens, “Silicon-on-insulator polarization rotator based on a symmetry breaking silicon overlay,” IEEE Photon. Technol. Lett. 24,482–484 (2012).

9. L. Liu, Y. Ding, K. Yvind, and J. M. Hvam, “Efficient and compact TE-TM polarization converter built onsilicon-on-insulator platform with a simple fabrication process,” Opt. Lett. 36, 1059–1061 (2011).

10. D. Dai and J. E. Bowers, “Novel concept for ultracompact polarization splitter-rotator based on siliconnanowires,” Opt. Express 19, 10940– 10949 (2011).

11. Y. Ding, H. Ou, and C. Peucheret, “Wideband polarization splitter and rotator with large fabrication toleranceand simple fabrication process,” Opt. Lett. 38, 1227–1229 (2013).

#202433 - $15.00 USD Received 3 Dec 2013; revised 19 Jan 2014; accepted 20 Jan 2014; published 10 Feb 2014(C) 2014 OSA 24 February 2014 | Vol. 22, No. 4 | DOI:10.1364/OE.22.003777 | OPTICS EXPRESS 3777

12. T.-Y. Liow, K.-W. Ang, Q. Fang, J.-F. Song, Y.-Z. Xiong, M.-B. Yu, G.-Q. Lo, and D.-L. Kwong, “Siliconmodulators and germanium photodetectors on soi: monolithic integration, compatibility, and performance opti-mization,” IEEE J. Sel. Top. Quantum Electron. 16, 307–315 (2010).

13. T. Baehr-Jones, R. Ding, Y. Liu, A. Ayazi, T. Pinguet, N. C. Harris, M. Streshinsky, P. Lee, Y. Zhang, A. E. Lim,T. Y. Liow, S. H. Teo, G. Q. Lo, and M. Hochberg, “Ultralow drive voltage silicon traveling-wave modulator,”Opt. Express 20, 12014–12020 (2012).

14. www.opsisfoundry.org.15. W. Yuan, K. Kojima, B. Wang, T. Koike-Akino, K. Parsons, S. Nishikawa, and E. Yagyu, “Mode-evolution-based

polarization rotator-splitter design via simple fabrication process,” Opt. Express 20, 10163–10169 (2012).16. D. Dai, Y. Yang, and J. E. Bowers, “Mode conversion in tapered submicron silicon ridge optical waveguides,”

Opt. Express 20, 13425–13439 (2012).17. W. Sacher, T. Barwicz, and J. K. Poon, “Silicon-on-insulator polarization splitter-rotator based on TM0-TE1

mode conversion in a bi-level taper,” in “Conference on Lasers and Electro-Optics, OSA Technical Digest,”(2013), p. CTu3F.3.

18. N. Walker and G. Walker, “Polarization control for coherent communications,” J. Lightwave Technol. 8, 438–458(1990).

19. T. Saida, K. Takiguchi, S. Kuwahara, Y. Kisaka, Y. Miyamoto, Y. Hashizume, T. Shibata, and K. Okamoto,“Planar lightwave circuit polarization-mode dispersion compensator,” IEEE Photon. Technol. Lett. 14, 507–509(2002).

20. C. Doerr and L. Chen, “Monolithic PDM-DQPSK receiver in silicon,” in “Optical Communication (ECOC),2010 36th European Conference and Exhibition on,” (2010), pp. 1–3.

21. C. Doerr, N. Fontaine, and L. Buhl, “PDM-DQPSK silicon receiver with integrated monitor and minimum num-ber of controls,” IEEE Photon. Technol. Lett. 24, 697–699 (2012).

1. Introduction

Silicon-on-insulator (SOI) is becoming a common platform for integrated photonic circuits be-cause the high refractive index contrast enables compact devices and the growing availabilityof foundry services allows complex silicon photonic chips to be fabricated at low costs [1, 2].However, high index contrast waveguides also possesses a large birefringence. Because thepolarization of the input light to a chip from an optical fiber is not usually fixed, polariza-tion transparent photonic devices and circuits are needed at the receiver and along an opticalcommunication link. To this end, polarization diversity can be implemented [3–5]. Essentialelements in polarization diverse photonic circuits are polarization splitters and polarization ro-tators. A challenge for polarization splitters and rotators in SOI is that they often require highaspect ratio features, extra layers, or an air cladding [3–11], which are not compatible withcommon foundry processes.

In this work, we combine the splitter and rotator functionalities into a polarization rotator-splitter (PRS) that is fully compatible with standard foundry processes. The PRS requires onlya single silicon (Si) material layer with top and bottom SiO2 cladding. This Si layer must bepatterned with both a full and partially-etched level; no high aspect ratio features are required.The compatibility with standard foundry processes enables us to demonstrate the PRS and anactive polarization controller using the IME baseline and IME-OpSIS silicon photonics pro-cesses [12–14]. The PRS uses a bi-level taper that converts a fundamental TM mode (TM0)input into a first-order TE mode (TE1) output, as proposed in [15, 16]. Our work is the firstdemonstration of PRSs using this mode conversion in bi-level tapers, and we presented a pre-liminary report in [17]. Although TM0-TE1 mode conversion in bi-level tapers was demon-strated in [16], a full PRS was not demonstrated. Overall, our work paves the way for polariza-tion diversity, polarization controllers, and polarization-multiplexed transmitters and receiversin standard active SOI photonic platforms.

The paper is organized as follows: we describe our adiabatic bi-level taper PRS design anddemonstration in Section 2; then, we show the PRS integrated with directional coupler polar-ization filters for improved polarization crosstalk in Section 3; finally, we apply the PRS to anactive polarization controller in Section 4.


50μm 50μm 300μm

Opticalinput Optical

outputsTE0,TM0

TE0 TE0

TM0 TE1 TE0

450nm 550nm

850nm 650nm 500nm

500nm200nm1.55μm

90nm Partiallyetched slab

220nm Si

200nmTE0,TE1

Bi level taper Adiabatic coupler

(a)

Mode 1TE0

Mode 1TE0

Mode 1TE0

Mode 1TE0

Mode 2TM0

Mode 2TE1

Mode 2TE1

Mode 2TE1

(b)

0(450) 125(475) 250(500) 375(525) 500(550)1.4

1.8

2.2

2.6

Partially−etched Si fin width (Si rib width) (nm)n ef

f

TE0Mode 2Mode 3

TM0

TE1

TE1

TM0Hybridized

Hybridized

(c)

Fig. 1. (a) Schematic of the polarization rotator-splitter (PRS). Widths are labeled in red andpurple; lengths use green labels. (b) Schematic showing the profiles of the modes with thefirst and second highest effective indices (i.e., “mode 1” and “mode 2”) at different pointsalong the PRS. In the adiabatic coupler, “mode 1” and “mode 2” refer to supermodes of thecomposite waveguide. (c) Effective indices (ne f f ) along the first half of the bi-level taperfor modes 1 to 3 at a wavelength of 1550 nm.

2. Adiabatic bi-level taper polarization rotator-splitter

Our PRS design is shown in Fig. 1(a), where the red regions represent the full height of the Siand the purple regions represent the partially-etched level of Si. We designed the PRS for theIME baseline and OpSIS silicon photonics processes, which have a top silicon thickness of 220nm and a partially-etched thickness of 90 nm. Our PRS uses a bi-level taper for TM0-TE1 modeconversion and symmetric SiO2 cladding. After the bi-level taper, the TE0 and TE1 modes areseparated into two waveguides using an adiabatic coupler instead of the directional coupler pro-posed in [10], the Y-branch proposed in [15], or the Y-branch and multi-mode interferometerin [11]. Overall, our PRS design is entirely adiabatic. This is a key distinction from the earlierPRS designs in [10, 11, 15], which have non-adiabatic elements that limit the bandwidth, in-crease the senstivity to variations in waveguide dimensions, and increase the insertion loss. Thetotal length of the PRS design is about 475 μm.

The remainder of this section is organized as follows: in Section 2.1, we provide a detaileddescription of the PRS design and operating principles, and in Section 2.2, we describe ourexperimental demonstration of a bi-level taper PRS.

2.1. Detailed polarization rotator-splitter design and operation

The PRS operation relies on the principle of mode evolution [4,5]. The evolution of the modeswith the first and second highest effective indices (i.e., “mode 1” and “mode 2”) in the PRS is


illustrated in Fig. 1(b). In the first half of the bi-level taper, the Si ridge and partially-etched slabcontinuously widen, and this is where the TM0-TE1 mode conversion occurs. The partially-etched slab breaks the vertical symmetry of the waveguide [10, 16], which produces a largedifference in the effective indices of modes 2 and 3 throughout the structure, as shown in Fig.1(c). This allows a TM0 input to remain in mode 2 all along the bi-level taper and evolve into,first, a “hybridized” mode with TM0 and TE1 features, and finally, the TE1 mode. A TE0 inputsimply remains in mode 1 and exits the bi-level taper in the TE0 mode. The second half of thebi-level taper, where the Si ridge continues to widen and the partially-etched slab narrows, isused to provide a fully-etched, wide waveguide as the input to the adiabatic coupler.

The adiabatic coupler follows the bi-level taper and consists entirely of fully-etched Siwaveguides with symmetric SiO2 cladding, which prevents crosstalk between the TE1 andTM0 modes. The mode evolution in the adiabatic coupler can be understood from the modeprofiles in Fig. 1(b). Here, TE0 and TE1 refer to supermodes of the composite two-waveguidestructure. At the start of the coupler, a “narrow” 200 nm wide waveguide begins with a blunttip next to a “broad” 850 nm wide waveguide; the gap between the waveguides is 200 nm. TheTE0 and TE1 modes are well confined in the broad waveguide and have little overlap with thenarrow waveguide. Then, the broad waveguide is narrowed to a 650 nm width and the narrowwaveguide is widened to a 500 nm width; the gap is held constant at 200 nm. At this point, theTE0 mode is well confined in the broad waveguide while the TE1 mode is well confined in thenarrow waveguide. Finally, the narrow waveguide is bent away from the broad waveguide usingan arc with a radius of 450 μm. As the waveguides separate, the TE0 and TE1 supermodes ofthe adiabatic coupler evolve into the TE0 modes of the isolated top and bottom waveguides,respectively.

2.2. Polarization rotator-splitter measurements

PRSs were fabricated in the IME baseline process, and optical micrographs of the PRS areshown in Fig. 2. The PRS inputs and outputs lead to edge couplers with 220 nm wide squaretips. We measured the PRS by coupling light from a tunable laser from free-space using ob-jective lenses. Manually-adjustable, free-space, linear polarizers were placed at the input and

100 m

Bi level taper Adiabatic coupler

(a)

50 m

(b)

50 m

(c)

Fig. 2. (a) An optical micrograph of the polarization rotator-splitter fabricated in the IMEbaseline process. Magnified optical micrographs are shown for (b), the bi-level taper, and(c), the end of the adiabatic coupler.


1500 1520 1540 1560 1580−60

−40

−20

0

Wavelength (nm)

Tra

nsm

issi

on (

dB)

TE branch output

TE−>TETE−>TMTM−>TETM−>TM

(a)

1500 1520 1540 1560 1580−60

−40

−20

0

Wavelength (nm)

Tra

nsm

issi

on (

dB)

TM branch output


(b)

1530 1540 1550 1560 1570 1580−3

−2

−1

0

1

Wavelength (nm)

Tra

nsm

issi

on (

dB)

TE branch output (TE −> TE)

Raw dataFabry−Perot removed

(c)

1530 1540 1550 1560 1570 1580−3

−2

−1

0

1

Wavelength (nm)T

rans

mis

sion

(dB

)

TM branch output (TM −> TE)


(d)

Fig. 3. Measurement data for the PRS in Fig. 2. (a) Transmission spectra of the PRS TEbranch (top) output. (b) Transmission spectra of the PRS TM branch (bottom) output. (c)Magnified TE component of the TE branch transmission for a TE input. (d) Magnified TEcomponent of the TM branch transmission for a TM input. The legends in (a) and (b) indi-cate the settings of the input and output polarizers (i.e., TE→TM means we had a TE inputand measured the TM component of the output). (c) and (d) represent the PRS insertionloss, and the red curves have been post-processed to remove Fabry-Perot oscillations fromthe edge coupler facets and the measurement apparatus.

output of the chip to control the input polarization and analyze the output polarization.Figure 3 shows the measured transmission spectra of the two PRS outputs for TE and TM

inputs. The transmission spectra have been normalized to the transmission spectra of the edgecouplers to extract the spectral characteristics of the PRS only. From Figs. 3(a) and 3(b), the po-larization crosstalk at both output ports was less than -13 dB over a wavelength range between1530 nm and 1580 nm; the crosstalk increased to about -10 dB for wavelengths between 1500nm and 1530 nm. Due to inaccuracies in aligning the coupling lenses between measurements,the error in the transmission values was about ± 0.5 dB. Other than normalizing out the edgecoupler transmission, no post-processing was applied to the data in Figs. 3(a) and 3(b).

The extracted insertion loss of the PRS is shown in Figs. 3(c) and 3(d). The raw transmissionspectra in the black curves overestimate the PRS insertion loss since Fabry-Perot oscillationsfrom the chip facets and measurement apparatus were not fully removed by normalizing thedata to the transmission of the edge couplers. The edge coupler loss calibration structures andPRS had different Fabry-Perot oscillations, and the Fabry-Perot oscillations of the measurementsetup changed between measurements due to realignments. We post-processed the raw trans-mission spectra of the PRS and edge couplers to reduce the contribution of the Fabry-Perotoscillations and obtained the more accurate insertion loss data in the red curves. Chip facet


Fabry-Perot oscillations were easily identified from the waveguide lengths and group indices,and oscillations that differed little between devices and polarization settings were attributed tothe measurement apparatus. From the post-processed data, the insertion loss and polarization-dependent loss (PDL) were less than 1.5 dB and 1.6 dB, respectively, over a wavelength rangefrom 1530 nm to 1580 nm. Our ± 0.5 dB realignment error estimate is evident from the redcurves, which have some points with transmission > 0 dB. The large-period oscillations inFig. 3(d) may be Fabry-Perot oscillations from reflections at the chip facets and the waveguidediscontinuity at the beginning of the adiabatic coupler or within the bi-level taper.

Through this first demonstration of a bi-level taper PRS, we can identify four simple designimprovements to reduce the crosstalk and increase the bandwidth. First, the blunt-tip at the startof the adiabatic coupler in Fig. 1(a) could be replaced by an arc with a large radius to eliminateany mode coupling caused by the waveguide discontinuity. Second, reducing the waveguidegap in the adiabatic coupler will reduce the crosstalk or the coupler length required to achievethe crosstalk we demonstrated; this is due to an increase in the effective index difference be-tween the TE0 and TE1 modes [4, 5]. Third, the widths and length of the bi-level taper can beoptimized; from Fig. 3(a), the incomplete TM0-TE1 mode conversion in the bi-level taper isa large component of the crosstalk. Finally, the PRS can be cascaded with additional PRSs orother types of polarization clean-up filters [6]. This latter approach is demonstrated in Section3 where the PRS is integrated with directional coupler clean-up filters for reduced crosstalk.

3. Polarization splitter-rotator with improved crosstalk

One approach to improving the performance of the PRS is to cascade it with polarization clean-up filters. This is demonstrated here using directional coupler clean-up filters [6, 7] placed infront of the PRS as shown in Fig. 4. In this configuration, it is more appropriate to refer tothis structure as a polarization splitter-rotator (PSR) than a polarization rotator-splitter (PRS).In contrast to the PRS by itself, which converts the TM polarization to the TE1 mode beforesplitting, here, the polarizations are first split with a directional coupler before TM is rotatedinto TE. This device was fabricated with the IME baseline process as well.

The detailed operation of the PSR can be understood from the annotated micrograph in Fig.4. The input is first separated into TE (top branch) and TM (bottom branch) polarizations usinga directional coupler. Directional coupler clean-up filters are integrated into both branches toreduce the polarization crosstalk. All directional couplers in the PSR are nominally identicaland use 440 nm wide strip waveguides, 10 μm long coupling regions, 400 nm wide couplinggaps, and 10 μm radius bends leading to and from the coupling region. The TE (top) branchuses four clean-up filters. The TM (bottom) branch uses two clean-up filters followed by a PRS

100 mDirectional coupler

TE pass filters

Directional couplerTM pass filters

Directional couplerTE pass filters

PRS used as a polarization rotator

Directional couplerpolarization splitter

TE0 +TM0 TE0

TM0 TM0 TE1 TE0

TE0

TE0

Fig. 4. Annotated optical micrograph of the polarization splitter-rotator (PSR) with im-proved crosstalk.


1500 1520 1540 1560 1580−80

−60

−40

−20

0

Wavelength (nm)

Tra

nsm

issi

on (

dB)

TE branch output


(a)

1500 1520 1540 1560 1580−80

−60

−40

−20

0

Wavelength (nm)

Tra

nsm

issi

on (

dB)

TM branch output


(b)

1530 1540 1550 1560 1570 1580−3

−2

−1

0

1

Wavelength (nm)

Tra

nsm

issi

on (

dB)

TE branch output (TE −> TE)


(c)

1530 1540 1550 1560 1570 1580−3

−2

−1

0

1

Wavelength (nm)T

rans

mis

sion

(dB

)

TM branch output (TM −> TE)


(d)

Fig. 5. Measurement data for the PSR in Fig. 4. (a) Transmission spectra of the PSR TEbranch (top) output. (b) Transmission spectra of the PSR TM branch (bottom) output. (c)Magnified TE component of the TE branch transmission for a TE input. (d) MagnifiedTE component of the TM branch transmission for a TM input. The legends in (a) and(b) indicate the settings of the input and output polarizers (i.e., TE→TM means we had aTE input and measured the TM component of the output). The red curves in (c) and (d)have been post-processed to remove Fabry-Perot oscillations from the chip facets and themeasurement setup.

for polarization rotation and then two additional clean-up filters. The unused ports of the PRSand directional couplers are terminated with waveguide tapers leading to 200 nm wide blunttips. The PRS is nominally identical to the PRS demonstrated in Section 2. The inputs andoutputs of the whole PSR lead to edge couplers with 220 nm wide tips.

The PSR was measured using the same method as Section 2.2 (i.e., free-space coupling withlinear polarizers at the input and output of the chip). Figures 5(a) and 5(b) show the measuredtransmission spectra of the two PSR outputs normalized to the transmission spectra of the edgecouplers for TE and TM inputs. The polarization crosstalk at both outputs was less than -22 dBover a wavelength range from 1500 nm to 1580 nm, which was an improvement of 9 dB overthe PRS in Section 2.2. In principle, the clean-up filters should have provided a significantlylower crosstalk, but the measurements may have been limited by the accuracy of the input andoutput polarizers.

The insertion loss of the PSR is shown in Figs. 5(c) and 5(d). The black curves are raw dataand the red curves have been post-processed to remove Fabry-Perot oscillations from the chipfacets and the measurement apparatus, as explained in Section 2.2. From the post-processeddata, the insertion loss and PDL of the PSR were less than 2.3 dB and 1.9 dB over a wavelengthrange from 1530 nm to 1580 nm; the error in the insertion loss was roughly ± 0.5 dB due torealignment error of the coupling lenses. Compared to the PRS in Section 2.2, the insertion


loss of the PSR in this section was larger and varied more with wavelength due to the loss andlimited bandwidth of the directional coupler polarization splitter and clean-up filters.

A more optimal design that has low polarization crosstalk, low insertion loss, and a broadbandwidth will likely involve optimizing our PRS design using the methods we described atthe end of Section 2.2 and then cascading the PRSs (i.e., the two outputs of a PRS are routedto additional PRSs, which act as polarization clean-up filters). This will result in an entirelyadiabatic design that is not subject to the bandwidth and insertion loss limitations of directionalcouplers.

4. Polarization controller

Finally, as an example of integration of the PRS with tuning and modulation elements, wedemonstrate the simple polarization controller shown in Fig. 6. The polarization controller con-sists of a PRS followed by a variable 2×2 Mach-Zehnder interferometer (MZI), phase-shifters,and a second PRS to combine the two branches. The MZI and phase-shifters modify the relativeamplitudes and phases of the output TE and TM-components to control the output polarization.Although this design uses both PIN diode and thermal phase-shifters to demonstrate the com-patibility of the PRS with a standard active Si photonic platform, in practice, only one type ofphase-shifter (thermal tuner or PIN diode) would be needed depending on the desired tuningspeed.

The polarization controllers were fabricated at IME using the OpSIS service [14]. The ther-mal tuners and PIN diodes are each 500 μm long and use the same etch depths as the PRS. The3-dB directional couplers use 500 nm wide fully-etched waveguides, a 13.5 μm long couplingregion with a 200 nm gap, and 20 μm radius S-bends leading to and from the coupling region.Tuning voltages were only applied to the top thermal tuners and PIN diodes in Fig. 6(a), whilethe bottom PIN diodes and thermal tuners balanced the loss in the two arms of the polarizationcontroller. Figure 7(a) shows the measured current-voltage characteristics of the top-left PINdiode and thermal tuner in the polarization controller.

We measured the polarization controllers using the method in Section 2.2 (i.e., free-spacecoupling with linear polarizers at the input and output of the chip); the input wavelength was

PRS3-dB

DC

PIN diode(500 m)

PRS Input Output

3-dB

DC

Thermal tuner(500 m)

PIN diode(500 m)


PIN diode(500 m)


PIN diode(500 m)


2 x 2 MZI controls amplitude ratio of TE and TM Controls phase between TE and TM

(a)

500 m

PRS PRS

PIN diode

3 dB DC

Thermaltuner PIN diode

Thermaltuner

3 dB DC

(b)

Fig. 6. (a) Schematic of the polarization controller. “3-dB DC” is a 3 dB directional coupler.(b) Optical micrograph of the polarizaton controller fabricated in the IME-OpSIS process.


fixed at 1570 nm and the input was chosen to be TE-polarized for simplicity. This simplemeasurement setup did not allow for the extraction of the phase between the output TE and TMpolarization components. The polarization controller insertion loss was < 2.5 dB. By drivingthe top-left and top-right thermal tuners, we generated TM-polarized, -45◦ linearly-polarized,and circularly-polarized outputs, which is evident from the output power as a function of theoutput polarizer angle in Fig. 7(b). 0◦ corresponds to a horizontal (TE) polarization axis; theuncertainty in the angle was about ±2◦. Crosstalk in the PRSs and non-ideal 3-dB directionalcouplers limited the extinction for the TM and -45◦ measurements. The red curve correspondsto a circularly-polarized output since the power only fluctuates by about 0.2 dB over all outputpolarizer angles. It was achieved with powers of 15 mW and 12 mW dissipated in the top-leftand top-right thermal tuners, respectively.

Next, as a simple demonstration of switching between TM and TE-polarized outputs, weinput TE light into the polarization controller and applied voltages only to the top-left thermaltuner [Fig. 7(c)] or the top-left PIN diode [Fig. 7(d)]; the other tuning elements were not driven.The applied voltage was swept with the output polarizer fixed to pass either TE or TM (marked

0 0.5 1 1.5 20

25

50

75

100

Voltage (V)

Cur

rent

(m

A)

Thermal tunerPIN diode

(a)

0 90 180 270 360−40

−30

−20

−10

0

Output polarizer angle (degrees)

Nor

mal

ized

ou

tput

pow

er (

dB)

TM−45 deg.Circular

(b)

0 20 40 60−20

−15

−10

−5

0

Thermal tuner power (mW)

Nor

mal

ized

ou

tput

pow

er (

dB)

Total outTE outTM out

TE −> TETE −> TM

(c)

0 10 20 30 40−20

−15

−10

−5

0

Diode current (mA)

Nor

mal

ized

ou

tput

pow

er (

dB)

Total outTE outTM out

TE −> TETE −> TM

(d)

Fig. 7. Polarization controller measurement data. (a) Current-voltage characteristics ofthe top-left thermal tuner and PIN diode. (b) Normalized output power as the output po-larizer was rotated. With a TE-polarized input, bias conditions were chosen to obtain aTM-polarized output (black curve), a -45◦ linearly-polarized output (blue curve), and acircularly-polarized output (red curve). (c) Normalized output power as the top-left ther-mal tuner power was swept. (d) Normalized output power as the top-left PIN diode currentwas swept. In (c) and (d), the output polarizer was set to pass either TE or TM or removedfrom the optical path (“Total out”). The optical output power curves were normalized to themaximum value in each plot. The magenta labels and dashed lines indicate points wherea TM or TE output was generated from the TE input (marked “TE→TM” and “TE→TE”,respectively).


“TE out” and “TM out” in the plots) or with the output polarizer removed from the optical path(marked “Total out” in the plots). Increasing the voltage on the thermal tuner or the PIN diodeshifted the output polarization between TM and TE. At PIN diode currents beyond 10 mA, theoptical loss increased substantially, which imbalanced the MZI and increased the total insertionloss of the polarization controller.

The polarization controller presented here is intended to show the full compatability of thePRS with a standard Si photonics platform. Its simple design limits its optical bandwidth andthe polarization states that it can create. A complete polarization controller can be achieved withtwo simple design modifications. First, the optical bandwidth can be extended by compensatingfor the group delay differences between the TE0 and TM0/TE1 modes in the PRS. This com-pensation should be applied to both PRSs and can be implemented as an extra length of straightwaveguide at one of the PRS outputs. Second, an additional set of phase-shifters should be in-cluded before the 2×2 MZI to create the full range of relative weights between the TE and TMcomponents required to convert any arbitrary input polarization to an arbitrary output polariza-tion. An endless polarization controller, which can be used in polarization-division multiplexedreceivers for polarization-tracking, would further require two extra sets of phase-shifters andtwo extra directional couplers [18–21].

5. Conclusion

In summary, we have demonstrated the first polarization rotator-splitter using a TM0-TE1mode converter based on an adiabatic bi-level taper and extended the concept to a polarizationcontroller and a polarization splitter-rotator with improved crosstalk using directional couplerclean-up filters. The main advantage of the designs in this work is that they are fully compat-ible with standard silicon photonic foundry processes and do not require specialty high aspectratio features, extra layers, or an air cladding. Although the adiabatic transitions make the po-larization rotator-splitter long, the design is inherently broadband and tolerant to dimensionalvariations.

Acknowledgments

The devices described in Sections 2 and 3 were fabricated using the IME baseline process withaccess sponsored by CMC Microsystems. The devices described in Section 4 were fabricatedusing the OpSIS service at IME A*STAR in Singapore. W.D.S., B.J.F.T, and J.K.S.P. are grate-ful for the financial support of CMC Microsystems and the Natural Sciences and EngineeringResearch Council of Canada. The assistance of Dan Deptuck of CMC Microsystems is grate-fully acknowledged.


polarization rotator-splitters in standard active …...polarization rotator-splitters in standard...

Documents