Wideband silicon photonic polarization beamsplitter ... - OSA Publishing

Wideband silicon photonic polarization beamsplitter based on point-symmetric cascaded broadband couplers Zeqin Lu,∗ Yun Wang, Fan Zhang, Nicolas A. F. Jaeger, and Lukas Chrostowski Department of Electrical and Computer Engineering, University of British Columbia, Vancouver BC, V6T1Z4, Canada ∗ [email protected]

Abstract: We design and demonstrate a wideband silicon photonic polarization beamsplitter on a silicon-on-insulator platform. The device consists of two 3 dB broadband couplers cascaded in a point-symmetric network. The transverse electric (TE) and transverse magnetic (TM) modes are coupled to different output ports due to a large difference between their coupling strengths. The device exhibits large isolation at both the two output ports, of more than 20 dB over a large bandwidth of 125 nm, and a small excess loss, of less than 0.5 dB for the entire C-band. © 2015 Optical Society of America OCIS codes: (130.3120) Integrated optics devices; (230.5440) Polarization-selective devices.

References and links 1. J. Wang, S. He, and D. Dai, “On-chip silicon 8-channel hybrid (de)multiplexer enabling simultaneous mode- and polarization-division-multiplexing,” Laser Photonics Rev. 8, L18–L22 (2014). 2. T. Barwicz, M. R. Watts, P. 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). 3. A. Hosseini, S. Rahimi, X. Xu, D. Kwong, J. Covey, and R. T. Chen, “Ultracompact and fabrication-tolerant integrated polarization splitter,” Opt. Lett. 36, 4047–4049 (2011). 4. Y. Huang, Z. Tu, H. Yi, Y. Li, X. Wang, and W. Hu, “High extinction ratio polarization beam splitter with multimode interference coupler on SOI,” Opt. Commun. 307, 46–49 (2013). 5. M. Yin, W. Yang, Y. Li, X. Wang, and H. Li, “CMOS-compatible and fabrication-tolerant MMI-based polarization beam splitter,” Opt. Commun. 335, 48–52 (2015). 6. H. Fukuda, K. Yamada, T. Tsuchizawa, T. Watanabe, H. Shinojima, and S. Itabashi, “Ultrasmall polarization splitter based on silicon wire waveguides,” Opt. Express 14, 12401–12408 (2006). 7. J. Wang and D. Dai, “Ultra-small silicon polarization beam splitter based on cascaded asymmetry directional couplers,” in Optical Fiber Communication Conference 2013 (Optical Society of America, 2013), paper OTh4I.1. 8. D. Dai, “Silicon polarization beam splitter based on an asymmetrical evanescent coupling system with three optical waveguides,” J. Lightwave Technol. 30, 3281–3287 (2012). 9. T. Liang and H. Tsang, “Integrated polarization beam splitter in high index contrast silicon-on-insulator waveguides,” IEEE Photonics Technol. Lett. 17, 393–395 (2005). 10. D. Dai, Z. Wang, J. Peters, and J. Bowers, “Compact polarization beam splitter using an asymmetrical machzehnder interferometer based on silicon-on-insulator waveguides,” IEEE Photonics Technol. Lett. 24, 673–675 (2012). 11. D. Dai, Z. Wang, and J. Bowers, “Considerations for the design of asymmetrical mach–zehnder interferometers used as polarization beam splitters on a submicrometer silicon-on-insulator platform,” J. Lightwave Technol. 29, 1808–1817 (2011). 12. B. Shen, P. Wang, R. Polson, and R. Menon, “An integrated-nanophotonics polarization beamsplitter with 2.4×2.4 µm2 footprint,” Nat. Photonics 9, 378–382 (2015). 13. J. Wang, D. Liang, Y. Tang, D. Dai, and J. E. Bowers, “Realization of an ultra-short silicon polarization beam splitter with an asymmetrical bent directional coupler,” Opt. Lett. 38, 4–6 (2013).

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Received 11 Aug 2015; revised 14 Oct 2015; accepted 15 Oct 2015; published 2 Nov 2015 16 November 2015 | Vol. 23, No. 23 | DOI:10.1364/OE.22.029413 | OPTICS EXPRESS 29413

14. T. Uematsu, T. Kitayama, Y. Ishizaka, and K. Saitoh, “Ultra-broadband silicon-wire polarization beam combiner/splitter based on a wavelength insensitive coupler with a point-symmetrical configuration,” IEEE Photonics J. 6, 1–8 (2014). 15. Z. Su, E. Timurdogan, E. S. Hosseini, J. Sun, G. Leake, D. D. Coolbaugh, and M. R. Watts, “Four-port integrated polarizing beam splitter,” Opt. Lett. 39, 965–968 (2014). 16. M. R. Watts, H. A. Haus, and E. P. Ippen, “Integrated mode-evolution-based polarization splitter,” Opt. Lett. 30, 967–969 (2005). 17. K. Jinguji, N. Takato, Y. Hida, T. Kitoh, and M. Kawachi, “Two-port optical wavelength circuits composed of cascaded Mach-Zehnder interferometers with point-symmetrical configurations,” J. Lightwave Technol. 14, 2301–2310 (1996). 18. Z. Lu, H. Yun, Y. Wang, Z. Chen, F. Zhang, N. A. F. Jaeger, and L. Chrostowski, “Broadband silicon photonic directional coupler using asymmetric-waveguide based phase control,” Opt. Express 23, 3795–3808 (2015). 19. K. Jinguji and M. Kawachi, “Synthesis of coherent two-port lattice-form optical delay-line circuit,” J. Lightwave Technol. 13, 73–82 (1995). 20. https://www.lumerical.com/tcad-products/fdtd/. 21. R. J. Bojko, J. Li, L. He, T. Baehr-Jones, M. Hochberg, and Y. Aida, “Electron beam lithography writing strategies for low loss, high confinement silicon optical waveguides,” J. Vac. Sci. Technol. B 29, 06F309 (2011). 22. Y. Wang, H. Yun, Z. Lu, R. Bojko, W. Shi, X. Wang, J. Flueckiger, F. Zhang, M. Caverley, N. A. F. Jaeger, and L. Chrostowski, “Apodized focusing fully etched subwavelength grating couplers,” IEEE Photonics J. 7, 1–10 (2015). 23. B. E. Little and T. Murphy, “Design rules for maximally flat wavelength-insensitive optical power dividers using Mach-Zehnder structures,” IEEE Photonics Technol. Lett. 9,1607–1609 (1997). 24. M. Tormen and M. Cherchi, “Wavelength-flattened directional couplers for mirror-symmetric interferometers,” J. Lightwave Technol. 23, 4387–4392 (2005).

1.

Introduction

Silicon-on-insulator (SOI) holds significant promise for the development of dense integration of photonic components. The high refractive index contrast of SOI permits sharp waveguide bends and ultrasmall device sizes and, thus, allows for such high density integration. However, the high refractive index contrast results in strong birefringence, leading to polarization-sensitive performance, which has both advantages and disadvantages for photonic integrated circuits (PICs). On the one hand, the polarization sensitivity can be a useful tool to increase spectral efficiency of PICs, for example, by multiplexing [1] two orthogonal modes, such as fundamental transverse electric (TE) and fundamental transverse magnetic (TM) modes, the on-chip spectral efficiency can potentially be doubled. On the other hand, the polarization sensitivity is a problem for compatibility between on-chip PICs and optical fibres [2], because polarization states can change randomly in optical fibres. For both of the above cases, polarization control is essential. In PICs, the polarization beamsplitter (PBS), which splits or combines the orthogonal TE and TM modes, is a fundamental component for polarization control. Ideally, a PBS should be broadband (i.e., operate over a wide wavelength range), low loss, compact in size, easy to fabricate, and have high isolation (i.e., efficiently separate the two mode types). In recent years, various types of PBS [3–16] have been reported on SOI platforms. Among these works, multimode interferometer (MMI) based PBSs [3–5] are simple in structure. But most demonstrated devices have large excess losses, more than 1 dB, and narrow operating bandwidth, about 50 nm. These narrow bandwidths are due to dispersion. Symmetric [6] and asymmetric [7, 8] directional coupler based PBSs can efficiently separate the two mode types and, thus, be compact in size. However, the coupling coefficients are wavelength dependent, which results to them having narrow bandwidths. Mach-Zehnder interferometers (MZIs) [9–11] were used for polarization beamsplitting, but both their bandwidths and isolations need to be further improved. Other demonstrated devices, such as a reverse-design splitter [12], a bent coupler [13], and an adiabatic splitter [15], have issues such as large footprint, narrow bandwidth and/or low isolation. Also, while a simulated device reported in [16] has a larger footprint than the one that we are reporting on, it is possible that one with a similar footprint to ours could be modeled. In this work, we demonstrate a PBS that is broadband, low loss, easy to fabricate, and has #247577 (C) 2015 OSA


large isolation. The device has a point-symmetric configuration [17], and is based on SOI strip waveguides fabricated using electron beam lithography. This paper is organized as follows. The principles on which our PBS is based on will be discussed in section 2. This is followed by the device design and simulation results which are presented in section 3. In section 4, our measurement results will be presented and discussed. Finally, our conclusions will be presented in section 5. 2.

Principles of operation

Our approach toward realizing polarization beamsplitting is to use the large difference between the coupling strengths achievable for the fundamental TE and TM modes in properly designed SOI directional couplers. Our couplers are designed so that the crossover length for the fundamental TM mode is much shorter than that for the fundamental TE mode. As a result, these two modes can be separated by a coupler with a crossover length designed for the fundamental TM mode. In order for our PBS to have a broadband response, broadband crossover-coupling is needed for the fundamental TM mode. Broadband couplers for the fundamental TM mode have been demonstrated [18], however, their broadband performance can only be achieved for low cross-coupling ratios, e.g., less than or equal to 50%. Therefore, when cascading two broadband couplers [18] in a mirror-symmetric way [17], the input light will be cross-coupled in the first coupler and then will be coupled back in the second coupler, due to reciprocity. To realize 100% broadband cross-coupling, we cascade two easy to achieve, broadband, 3 dB, TM mode couplers [18] in a point-symmetric way [17]. In the point-symmetric network, the light is crosscoupled by 50% in the first 3 dB coupler and the cross-coupling continues in the second 3 dB coupler, and, as a result, the light is 100% cross-coupled via the point-symmetric network. The point-symmetric network also has a quadratic behavior to the unbalance of the 3 dB couplers, which leads to a broader bandwidth, as discussed below. 1

2x2 coupler

E1

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Fig. 1. (a) Schematic of a point-symmetric network; (b) responses of a 3 dB 2×2 coupler and its pointsymmetric network. The shadow regions mark out the variations of their respective cross-coupling powers.

Figure 1(a) shows a schematic of a point-symmetric network consisting of two components. The component on the left is an arbitrary 2 × 2 coupler and the component on the right is the point-symmetry-transformed version of the coupler on the left. The unitary transfer matrix for the arbitrary 2 × 2 coupler is given by [17, 19]: t(λ ) −κ ∗ (λ ) T= (1) κ(λ ) t ∗ (λ ) where t(λ ) and κ(λ ) are the complex straight-through coupling coefficient and the crosscoupling coefficient, respectively, and t ∗ (λ ) and κ ∗ (λ ) are their complex conjugates. λ is the wavelength. The transfer matrix for the point-symmetry-transformed coupler is given by [17]: ∗ t (λ ) −κ ∗ (λ ) Tpoint−symmetric = (2) κ(λ ) t(λ ) #247577 (C) 2015 OSA


The transfer matrix for the point-symmetric network, as shown in Fig. 1(a), is expressed as: t(λ )t ∗ (λ ) − κ(λ )κ ∗ (λ ) −2 t(λ )κ ∗ (λ ) T · Tpoint−symmetric = (3) 2 t ∗ (λ )κ(λ ) t(λ )t ∗ (λ ) − κ(λ )κ ∗ (λ ) Given a normalized input electric field at one of the inputs, i.e., E1 = 1 and E2 = 0, as shown in Fig. 1(a), we have: E3 1 E5 1 =T· , = T · Tpoint−symmetric · (4) E4 0 E6 0 E3 (λ ) = t(λ ), E4 (λ ) = κ(λ ), E5 (λ ) = t(λ )t ∗ (λ ) − κ(λ )κ ∗ (λ ), E6 (λ ) = 2 t ∗ (λ )κ(λ ) (5) where E3 and E4 are the electric fields at the through port and cross port of the 2×2 coupler, respectively. E5 and E6 are the electric fields at the through port and cross port of the pointsymmetric network, respectively. P3 (λ ) and P4 (λ ) are the straight-through and cross-coupling power of the 2 × 2 coupler, respectively, and for convenience they are taken to be: P3 (λ ) = |E3 (λ )|2 = |t(λ )|2 , P4 (λ ) = |E4 (λ )|2 = |κ(λ )|2

(6)

Assuming that there is no coupling loss, i.e., |t(λ )|2 +|κ(λ )|2 =1, from Eqs. (5) and (6) we obtain the straight-through power, P5 (λ ), and the cross-coupling power, P6 (λ ), of the pointsymmetric network: P5 (λ ) = |E5 (λ )|2 = (P3 (λ ) − P4 (λ ))2 = (∆P(λ ))2

(7)

P6 (λ ) = Pin − P5 (λ ) = 1 − (∆P(λ ))2

(8)

where Pin is the normalized input power. ∆P(λ ) is the unbalance of coupling for the 2×2 coupler. According to Eqs. (7) and (8), when ∆P(λ ) = 0, we have P5 (λ ) = 0 and P6 (λ ) = 1, and, therefore, crossover-coupling is achieved. When the deviation of ∆P(λ ) from 0 is small, then the deviation of P6 (λ ) from 1 is also small due to their quadratic relationship; in other words, the cross-coupling power P6 (λ ) is less sensitive to unbalanced coupling in the 3 dB 2×2 coupler. As an example, Fig, 1(b) shows the cross-coupling power of such a 3 dB 2 × 2 coupler and that of its point-symmetric network. Over a large wavelength span, the ∆P(λ ) varies by ±0.1, as shown by the red shadow region, while P6 (λ ) remains between 0.96 and 1, as shown by the blue shadow region. At this point, we have seen that broadband crossover-coupling can be obtained by cascading two 3 dB couplers in a point-symmetric network, as shown in Fig. 1. Combining the broadband crossover-coupling of the point-symmetric network and polarization beamsplitting using coupling, we can design a broadband PBS that crossover-couples the TM mode into the cross port over a broad bandwidth while leaving the TE mode to propagate to the through port without being significantly coupled. Such a design will be discussed in the following section. 3.

Design

Our PBS is based on 220-nm-high SOI strip waveguides. As shown in Fig. 2(a), it consists of two identical 3 dB broadband couplers [18] cascaded in a point-symmetric network as described in section 2. As shown in Fig. 2(b), the first 3 dB broadband coupler consists of two coupling sections and an asymmetric-waveguide based phase shifter in between them. Tapered waveguides are used to connect the two coupling sections and the phase shifter. It needs to be mentioned that, here, the tapered waveguide design for the transitions between the coupling sections and the phase shifter is different from that in our previous work [18]. In this work, both sides of the waveguides are tapered in order to reduce mode mismatch loss. An s-bend #247577 (C) 2015 OSA


bro t 3 dB

adba

n d co

upler nd co adba formed) o r b ns dB ic-tra nd 3 Seco t-symmetr (poin

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Fig. 2. (a) Schematic of our broadband PBS; (b) schematic of the first 3 dB broadband coupler [18]. TM mode 0

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(a) (b) Fig. 3. FDTD simulation results of the 3 dB broadband coupler for (a) the TE mode and (b) the TM mode.

waveguide is used at one of the inputs and a straight waveguide is used at the other input. As can be seen in Fig. 2(a), the second 3 dB broadband coupler is identical to the first coupler but is flipped around both axis, i.e., it is the point-symmetry-transformed version of the first coupler. We design our device in a 2 steps process. To begin with, we design the 3 dB broadband couplers using a commercially available three-dimensional finite-difference time-domain (3D FDTD) software package [20]. The methodology for designing such a broadband coupler was described in our previous work [18]. The geometric parameters of the 3 dB broadband couplers are detailed in Fig. 2(b). In the design we optimized the geometric parameters of the 3 dB broadband coupler to achieve 3 dB power coupling for the TM mode over a large wavelength span. A large waveguide spacing of 500 nm is used in the 3 dB broadband coupler design to achieve weak coupling for the TE mode, as a result, the TE mode propagates through the coupler without significant cross-coupling. Figures 3(a) and 3(b) show simulation results for the 3 dB broadband coupler for both the TE and TM modes, respectively. As shown in Fig. 3(a), the cross-coupling power for the TE mode is less than -22 dB across a wavelength range from 1480 nm to 1580 nm, whereas, 3 dB power coupling is achieved for the TM mode in the same wavelength range, as shown in Fig. 3(b). Then, we model our PBS by cascading two 3 dB broadband couplers with a point-symmetric configuration as shown in Fig. 2(a). We simulate the PBS using the FDTD software, and results

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TE mode

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Fig. 4. FDTD simulation results for the PBS. (a) Output spectra for the TE mode; (b) output spectra for the TM mode; (c) isolation spectrum at the through port; (d) isolation spectrum at the cross port.

for both the TE and TM modes at the two output ports are shown in Figs. 4(a) and 4(b), respectively. We use isolation and isolation bandwidth to evaluate the performance of our PBS. The isolation at the through port and cross port are defined as follows: (Isolation at the through port) = 10 log10 (Isolation at the cross port) = 10 log10

P T E,through P T M,through P T M,cross P T E,cross

(9) (10)

where P T E,through and P T E,cross are the output powers for the TE mode at the through port and cross port, respectively, as shown in Fig. 4(a). Similarly, P T M,through and P T M,cross are the output powers for the TM mode at the through port and cross port, respectively, as shown in Fig. 4(b). The isolation bandwidth is defined as the wavelength span over which the isolation is above a certain value, here we use 20 dB. Figures 4(c) and 4(d) show the isolation spectra at the through port and cross port, respectively. As shown in Fig. 4(c), the device has a 20 dB isolation bandwidth of 125 nm at the through port over the wavelength span 1465 nm to 1590 nm, as well as a maximum isolation of over 35 dB at 1495 nm. As shown in Fig. 4(d), the device has a 20 dB isolation bandwidth of 110 nm at the cross port in the wavelength span 1460 nm to 1570 nm. As a result, the 20 dB isolation bandwidth common to both the two output ports is 105 nm, ranging from 1465 nm to 1570 nm. As we can see, over a wide wavelength range most of the TE light propagates through the device and exits from the through port, while most of the TM light is coupled to the cross port. Hence, broadband polarization beamsplitting is possible. 4. 4.1.

Fabrication and measurement results Fabrication

Our PBSs were fabricated using electron-beam lithography [21] on an SOI wafer with 220 nm thick silicon on a 3 µm thick buried oxide layer. After etching, the chip had a 2 µm thick silicon

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500 nm 500 nm 500 nm

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Fig. 5. Scanning electron microscope (SEM) images for one of our fabricated test devices.

dioxide layer deposited on the waveguides using plasma-enhanced chemical vapor deposition. Figure 5 shows scanning electron microscope (SEM) images for one of our fabricated test devices. PM fiber (slow to slow)

Agilent 81600B

TE cross port

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20 µm spacing PM fiber (slow to fast)

TE through port TM cross port

TM mode Input Two identical PBSs under test

TM through port

Detector Agilent 81635A

Fig. 6. Sketch of measurement setup. The yellow and pink triangles are the on-chip grating couplers for the TE and TM modes, respectively.

Figure 6 shows a sketch of our measurement setup. Our device test set includes two identical PBSs separated by a small 20 µm spacing on the SOI wafer. For such a small spacing, the fabrication variation for the two identical PBSs is negligible. One of the PBSs is used for the characterization for the TE mode, while the other is used for the TM mode. To study the repeatability of the fabricated PBSs, we had three copies of the device test set fabricated on the same SOI wafer with a separation distance of 3 mm from each other. On-chip grating couplers [22], which also work as TE-pass or TM-pass polarizers due to their strong polarization dependence, were used to couple light into and out of our test devices. We also fabricated a pair of TE mode and a pair of TM mode grating couplers, connected by short waveguides for calibrating the insertion losses. To characterize our devices, we used an Agilent 81600B broadband laser as the input source and both channels of an Agilent 81635A optical power sensor as the output detectors. The laser output is TE polarized. The slow to slow polarization maintaining (PM) fiber keeps the polarization state of the light, and the slow to fast PM fiber rotates the polarization state of the light by 90 degrees at the outputs of our fiber that we used to inject light into the grating couplers. 4.2. Measurement Devices were measured over the wavelength range from 1460 nm to 1635 nm, with a measurement resolution of 10 pm and an input power at 0 dBm. Figures 7(a) and 7(b) present the measured output spectra of test set 1 for the TE and TM modes, respectively, in which the insertion losses introduced by the grating couplers have been calibrated out. It is found that the excess loss of our PBSs is less than 0.5 dB across the C-band for both the TE and TM modes. In Figs. 7(c) and 7(d) we plot and compare the measured isolation spectra with the simulated isolation spectra at the through port and cross port, respectively. The simulation results are shown by dash green lines while the measurement results are shown by solid green lines. Good agreement is seen between the simulated and the measured results. For the measured isolation spectrum at the through port, as shown in Fig. 7(c), our PBS has a 20 dB isolation bandwidth of 125 nm over the wavelength span 1477 nm to 1602 nm. For the measured isolation spectrum at the cross port, as shown in Fig. 7(d), our PBS has a 20 dB isolation bandwidth of 142 nm over

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TE mode

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Fig. 7. Measurement results for the PBS test set 1. (a) Output spectra for the TE mode; (b) output spectra for the TM mode; (c) isolation spectra at the through port; (d) isolation spectra at the cross port. Through port

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Fig. 8. Measurement results for the three identical test sets. (a) Isolation spectra at the through ports; (b) isolation spectra at the cross ports.

the wavelength span 1460 nm to 1602 nm. As a result, the measured 20 dB isolation bandwidth common to both the two output ports is 125 nm, ranging from 1477 nm to 1602 nm. In Figs. 8(a) and 8(b) we compare the measured isolation spectra at the through ports and cross ports of the three test sets, respectively. The performance of the three test sets are found to be consistent, except for small variations on the 20 dB isolation bandwidth, which are likely due to the fabrication variation across the wafer. Please note that in this very first design, we did not intend to fabricate the test devices all over the wafer for a yield analysis. However, in our future work, a yield analysis would be interesting to investigate. In Table 1, we compare the performance of our PBS with those of other demonstrated PBSs. The measurement data of our test set 1 is used in the comparison. Here, we use extinction ratio (ER) bandwidth in the comparison, since it is given by all of the PBSs [3, 5, 6, 9, 10, 12, 13, 15]

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to which comparison being made. The ERs for the TE and TM modes are defined as follows: (ER f or the T E mode) = |10 log10

P T E,through | P T E,cross

(11)

(ER f or the T M mode) = |10 log10

P T M,through | P T M,cross

(12)

Accordingly, the measured ER spectra of the test set 1 for the TE and TM modes are shown in Figs. 9(a) and 9(b), respectively. The ER bandwidth is defined as the wavelength span over which the ER is above a certain value; here both 10 dB and 20 dB are used in the comparision. According to Table 1, our device shows high performance in both insertion loss and bandwidth. To the best of our knowledge, our PBS is the first device on an SOI platform that can provide 20 dB ER for both the TE and TM modes over a bandwidth of more than 120 nm. TM mode 35

30

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(a) (b) Fig. 9. ER spectra for the test set 1 for the (a) TE and (b) TM modes. Table 1. Comparison of demonstrated PBSs on SOI. Reference

Device Principles

Excess loss (dB)

20 dB ER bandwidth (nm)

10 dB ER bandwidth (nm)

Device Length (µm)

[3]

MMI

1.7

NA

50

0.94

[5]

MMI

2.2

NA

26

132.64

[9]

MZI

1.1

NA

45

> 6000

[10]

MZI

NA

NA

40

∼200

[15]

Mode evolution

3.4

NA

150

1400

[12]

Reverse-design

1.48

NA

32

2.4

[6]

Coupling

0.5

NA

100

32

[13]

Coupling

NA

NA

∼35

10.1

This work*

Coupling + pointsymmetric network

175

97.4

NA: data not available; 20 dB ER bandwidth: wavelength span over which the ER is above 20 dB; 10 dB ER bandwidth: wavelength span over which the ER is above 10 dB.

5. Conclusion We have demonstrated a high performance, broadband, silicon photonic PBS on a 220 nm SOI platform. The device shows a large bandwidth, of over 125 nm with an isolation of over 20 dB. The device also has a low insertion loss of less than 0.5 dB for the entire C-band. Our devices have simple structures, compact footprints, and are easy to fabricate. Broadband PBSs with other cladding materials and different Si wafer thicknesses can also be realized using our

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design approach, presented in sections 2 and 3. The bandwidth of the demonstrated PBS can be further increased by optimizing [23, 24] the bandwidths of the 3 dB broadband couplers. Additionally, the point-symmetric approach might be achieved using adiabatic 3 dB couplers that only operate for the TM polarization. As a fundamental component for polarization control, the demonstrated broadband PBS would likely find many applications in areas, such as mode division multiplexing [1] and polarization transparency [2]. Acknowledgments We acknowledge the financial support of the Natural Sciences and Engineering Research Council of Canada (NSERC), including the Silicon Electronic-Photonic Integrated Circuits (SiEPIC) Program. The devices were fabricated by Richard Bojko at the University of Washington WNF, part of the NSF NNIN. Zeqin Lu appreciates the funding support of the China Scholarship Council (CSC), and thanks Han Yun at the University of British Columbia for discussions. We acknowledge CMC Microsystems, Lumerical Solutions, Inc., and Mentor Graphics for the design software.

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