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Optics Express

  • Editor: Andrew M. Weiner
  • Vol. 22, Iss. 11 — Jun. 2, 2014
  • pp: 12799–12807
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Improved 8-channel silicon mode demultiplexer with grating polarizers

Jian Wang, Pengxin Chen, Sitao Chen, Yaocheng Shi, and Daoxin Dai  »View Author Affiliations


Optics Express, Vol. 22, Issue 11, pp. 12799-12807 (2014)
http://dx.doi.org/10.1364/OE.22.012799


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Abstract

An improved 8-channel silicon mode demultiplexer is realized with TE-type and TM-type grating polarizers at the output ends, and these gratings serve as fiber-chip couplers simultaneously. The present 8-channel silicon mode demultiplexer includes a three-waveguide PBS (for separating the TE0 and TM0 modes) and six cascaded ADCs (for demultiplexing the high-order modes of both polarizations). The grating polarizers with high extinction ratios are used to filter out the polarization crosstalk in the 8-channel hybrid multiplexer efficiently and the measured crosstalk for all the mode-channels of the improved 8-channel mode multiplexer is reduced greatly to ~−20dB in a ~100nm bandwidth.

© 2014 Optical Society of America

1. Introduction

2. Structure and analysis

Figure 1 shows the 8-channel silicon hybrid demultiplexer including a three-waveguide PBS and six cascaded ADCs [15

15. J. Wang, S. He, and D. Dai, “On-chip silicon 8-channel hybrid (de)multiplexer enabling simultaneous mode- and polarization-division-multiplexing,” Laser Photon. Rev. 8(2), L18–L22 (2014). [CrossRef]

].
Fig. 1 Schematic configuration of an 8-channel hybrid demultiplexer [15].
The PBS is used to separate the fundamental modes for TE and TM polarizations (TE0 and TM0) while the six ADCs are designed to demultiplex the high-order modes of both polarizations (i.e., the TE1, TE2, TE3, TM1, TM2, and TM3 modes). According to the phase matching condition, the parameters for the PBS and the ADCs are chosen optimally, as shown in Ref. [15

15. J. Wang, S. He, and D. Dai, “On-chip silicon 8-channel hybrid (de)multiplexer enabling simultaneous mode- and polarization-division-multiplexing,” Laser Photon. Rev. 8(2), L18–L22 (2014). [CrossRef]

]. The TE3, TE2, TE1, and TE0 modes are dominantly output from ports O1~O4 respectively while the TM0, TM1, TM2, and TM3 modes are dominantly output from ports O5~O8, respectively.

Figures 2(a)2(h) show the calculated transmission responses at the eight output ports (O1~O8) of the designed hybrid demultiplexer, respectively when all of the modes (TE3, TE2, TE1, TE0, TM0, TM1, TM2, and TM3 modes) are launched from the input port of the bus waveguide at the left, respectively.
Fig. 2 The transmission responses at the eight output ports for the 8-channel hybrid demultiplexer shown in Fig. 1 (without any polarizer at the output ports) (a) O1 (TE3 mode channel), (b) O2 (TE2 mode channel), (c) O3 (TE1 mode channel), (d) O4 (TE0 mode channel), (e) O5 (TM0 mode channel), (f) O6 (TM1 mode channel), (g) O7 (TM2 mode channel), and (h) O8 (TM3 mode channel) when all of the used modes are launched at the left of the bus waveguide, respectively.
Here a three-dimensional finite-difference time-domain (3D-FDTD) method is used for the calculation. One should note that some higher-order modes might be cut-off as the bus waveguide is tapered down. Therefore, the crosstalk from the cut-off higher-order modes to the following output ports will be negligible. For example, when the TM3 mode is launched from the input port of the bus waveguide, it will be dominantly dropped by the first ADC (which is designed for the TM3 mode) and received by the O8 port (as shown in Fig. 2(h)) while there is a small part of the TM3 mode power resident in the bus waveguide (due to the fabrication deviations). This resident power is still carried by the TM3 mode and propagates forward. When it goes through the following adiabatic taper, the bus waveguide will become too narrow to support the TM3 mode and the resident power carried by the TM3 mode becomes radiated. In this case, little crosstalk will be introduced to the following ports. Therefore, here the transmissions to some output ports are too low to be shown in Figs. 2(a)2(h).

From Figs. 2(a)2(h), it can be seen that the crosstalk from orthogonal polarization modes is dominant for some output ports. For example, the dominant crosstalk for the TM2 mode-channel (port O7) comes from the TE3 mode-channel and the polarization crosstalk is ~−11dB @1550nm, as shown in Fig. 2(g). The reason is that the second-stage ADC designed for the TM2 mode does not have a significant phase-mismatch between the TE3 mode in the bus waveguide and the TE0 mode of the access waveguide. Consequently, some part of power carried by the TE3 mode is dropped to port O7 by the second-stage ADC designed for the TM2 mode. From Fig. 2(a) we also note that the crosstalk from the launched TM2 mode in the bus waveguide to port O1 (which is for the TE3 mode channel) is much lower (<−20dB @1550nm) because most power of the TM2 mode has been dropped by the second-stage ADC before it arrives at the third ADC working for the TE3 mode-channel. Consequently very little power carried by the TM2 mode-channel arrives at the third ADC and thus little crosstalk is introduced to the TE3-mode channel.

Since the dominant crosstalk is from the orthogonal polarization mode, we realize that such polarization crosstalk can be filtered out by introducing polarization-selective devices (such as polarizers and PBSs) at the end of the output ports of the mode (de)multiplexer. As it well known, various polarizers [16

16. K. Rollke and W. Sohler, “Metal-clad waveguide as cutoff polarizer for integrated optics,” IEEE J. Quantum Electron. 13(4), 141–145 (1977). [CrossRef]

20

20. C. H. Chen, L. Pang, C. H. Tsai, U. Levy, and Y. Fainman, “Compact and integrated TM-pass waveguide polarizer,” Opt. Express 13(14), 5347–5352 (2005). [CrossRef] [PubMed]

] and PBSs [21

21. S. Lin, J. Hu, and K. B. Crozier, “Ultracompact, broadband slot waveguide polarization splitter,” Appl. Phys. Lett. 98(15), 151101 (2011). [CrossRef]

24

24. X. Guan, H. Wu, Y. Shi, L. Wosinski, and D. Dai, “Ultracompact and broadband polarization beam splitter utilizing the evanescent coupling between a hybrid plasmonic waveguide and a silicon nanowire,” Opt. Lett. 38(16), 3005–3008 (2013). [CrossRef] [PubMed]

] have been realized on SOI platform. In this paper, we use high extinction-ratio TE-type and TM-type polarizers based on grating, which also serve as fiber-chip couplers simultaneously. According to the design given in Ref. [25

25. D. Taillaert, F. Van Laere, M. Ayre, W. Bogaerts, D. Van Thourhout, P. Bienstman, and R. Baets, “Grating couplers for coupling between optical fibers and nanophotonic waveguides,” Jpn. J. Appl. Phys. 45(8A), 6071–6077 (2006). [CrossRef]

], the gratings have a duty cycle of 0.5 and an etching depth of 70nm. And the optimized grating periods are 0.63μm and 1.0μm for the TE- and TM-type polarizers, respectively. Figures 3(a) and 3(b) show the calculated transmissions of the designed TE-type and TM-type grating polarizers when the TE0 or TM0 mode is launched with a 10° incident angle, respectively.
Fig. 3 The calculated transmission of the designed grating polarizer when the TE0 or TM0 mode is launched, respectively. (a) TE-type, (b) TM-type.
For this calculation, a 3D-FDTD simulation is used. From Figs. 3(a) and 3(b), it can be seen that the designed TE-type and TM-type grating polarizers have high extinction ratios over a broad band. The extinction ratio at the central wavelength (~1550nm) is higher than 25dB and 30dB for the TE-type and TM-type polarizers respectively. This is favorable to filter out the polarization crosstalk and significantly improve the performance of the 8-channel hybrid demultiplexer. The performance of the grating polarizers can be even improved further by some specific designs as demonstrated in Refs. [26

26. G. Roelkens, D. Van Thourhout, and R. Baets, “High efficiency silicon-on-insulator grating coupler based on a poly-Silicon overlay,” Opt. Express 14(24), 11622–11630 (2006). [CrossRef] [PubMed]

, 27

27. Y. Tang, Z. Wang, L. Wosinski, U. Westergren, and S. He, “Highly efficient nonuniform grating coupler for silicon-on-insulator nanophotonic circuits,” Opt. Lett. 35(8), 1290–1292 (2010). [CrossRef] [PubMed]

].

3. Fabrication and characterization

In order to characterize the 8-channel hybrid multiplexer with grating polarizers, we designed and fabricated a PIC including the 8-channel hybrid multiplexer, a 100μm-long multimode waveguide and a hybrid demultiplexer on a silicon-on-insulator wafer which has a 220nm-thick top-silicon upon a 2μm-thick buried oxide layer. The fabrication processes include: (1) An E-beam lithography patterning for the waveguides; (2) An ICP etching process to etch the top silicon layer down to buried oxide layer; (3) A second E-beam lithography patterning for the grating polarizers (couplers); (4) A shallow-etching process with 70nm etching-depth for the grating patterns; (5) PECVD deposition for the 2.3μm-thick SiO2 upper-cladding. Figure 4 shows the microscope image of the fabricated hybrid (de)multiplexer with grating polarizers.
Fig. 4 Microscope image of the fabricated PIC consisting of a mode multiplexer and a mode demultiplexer with grating polarizers.
Each single-mode access waveguide has a TE or TM grating polarizer at the end, and a 300μm-long adiabatic taper is used to connect the 10μm-wide grating and the 500nm-wide single-mode access waveguide.

Generally speaking, the lithography process is one of the most important steps for the fabrication of any photonic integrated device to control the waveguide width within the fabrication tolerance to achieve good performances as designed. For the 8-channel hybrid multiplexer, the simulation results given in Ref. [28

28. D. Dai, J. Wang, and S. He, “Silicon multimode photonic integrated devices for on-chip mode-division- multiplexed optical interconnects,” Prog. Electromagn. Res. 143, 773–819 (2013). [CrossRef]

] show that the 8-channel hybrid multiplexer without polarizers has a fabrication tolerance of ± 5~10nm for the waveguide width when the crosstalk is required to be lower than −10dB. In our experiment, the deviation of the patterning linewidth is less than ± 5nm by utilizing the E-beam lithography patterning process carefully. Furthermore, the improved 8-channel mode demultiplexer demonstrated here has a larger fabrication tolerance because of the help from the grating polarizers. For the grating polarizer, the second shallow-etching process to achieve a 70nm etching-depth is a key step. Fortunately, the extinction ratio of the grating polarizer is not very sensitive to the etching depth. For example, the grating polarizer has an extinction ratio of >20dB when the etching depth varies from 60nm to 80nm. In experiment, the etching depth can be controlled accurately by slowing down the etching rate. In our fabrication process, the etching rate is slowed down to ~2nm/s and the deviation of the etching depth is less than ± 6nm in our lab.

For the measurement, a tunable laser (Agilent 81940A) is used as the light source and a powermeter (Agilent 8163A) is used at the terminal to monitor the output. Single-mode fibers are aligned with a 10° incident angle to couple light to/from the chip. The polarization state of input light is adjusted by a polarization controller. When measuring the 8-channel hybrid multiplexer, the light is launched from an input port Ii (i = 1,…, 8) and the transmission responses at the output ports (O1~O8) are measured one by one. Figures 5(a)5(h) show the measured transmission responsesat a fixed output port Oi (i = 1,…, 8), respectively, when light is launched from any of the input ports (I1~I8).
Fig. 5 The normalized transmission responses with respect to the straight waveguide on the same chip at the eight output ports (a) O1; (b) O2; (c) O3; (d) O4; (e) O5; (f) O6; (g) O7; (h) O8 when all of the input ports from I1 to I8 are launched. Here these transmission responses are normalized by the transmission of a straight bus waveguide (w = 2.363μm) with TE-type or TM-type grating polarizers at both ends on the same chip.
Here these transmission responses are normalized by the transmission of a straight bus waveguide (w = 2.363μm) with TE-type or TM-type grating polarizers at both ends on the same chip. Since the output powers at some non-major output ports are beyond the power range of our powermeter, the corresponding transmission responses are not shown in Figs. 5(a)5(h). As expected, Figs. 5(a)5(d) show that the TE(4-i) mode in the bus waveguide is excited dominantly and dropped to output port Oi when TE-polarized light is launched from input port Ii (i = 1, 2, 3, 4). Similarly, from Figs. 5(e)5(h), it can be seen that the TM(i−5) mode in the bus waveguide will be excited dominantly and dropped to output port Oi when TM-polarized light is launched from input port Ii (i = 5, 6, 7, 8). The measured excess losses around 1560nm are about 3.1dB, 2.2dB, 3.5dB, 0.2dB, 0.7dB, 2.1dB, 1.5dB, and 1.4dB for the TE3, TE2, TE1, TE0, TM0, TM1, TM2, and TM3 mode channels, respectively. The loss is mainly from the insufficient cross-coupling due to the fabrication deviation.

From Figs. 5(a)5(h), it can also be seen that the present 8-channel mode demultiplexer with grating polarizers has low crosstalk. The crosstalk is defined as usual to be the difference between the powers at a fixed output port (Oi) when light is launched from the major input port (Ii) and another input port (Ij, ji). The dominant crosstalks (around 1560nm) for the TE3, TE2, TE1, TE0, TM0, TM1, TM2, and TM3 mode channels are about −20.8dB, −20.3dB, −18dB, −29.3dB, −36.5dB, −40.6dB, −17.7dB, and −20.9dB, respectively. The accumulated crosstalk from all the other non-major mode channels are −20.5dB, −20.2dB, −16.6dB, −29dB, −33.1dB, −38.3dB, −16.9dB, and −20.6dB for the TE3, TE2, TE1, TE0, TM0, TM1, TM2, and TM3 mode channels, respectively. And the crosstalk is insensitive to the wavelength over a broad band from 1520nm to 1620nm.

4. Conclusion

In summary, we have demonstrated an improved 8-channel hybrid demultiplexer with the assistance of TE-type and TM-type grating polarizers (which also serve as fiber-chip couplers). The experimental results have shown that the grating polarizers with high extinction ratios filter out the polarization crosstalk and the crosstalk of the 8-channel hybrid demultiplexer is reduced greatly. The dominant crosstalks (around 1560nm) from the TE3, TE2, TE1, TE0, TM0, TM1, TM2, and TM3 mode channels are about −20.8dB, −20.3dB, −18dB, −29.3dB, −36.5dB, −40.6dB, −17.7dB, and −20.9dB, respectively. And the crosstalk is insensitive to the wavelength from 1520nm to 1620nm. On-chip polarizers or PBSs can also be used to filter out the polarization crosstalk of the hybrid demultiplexer when needed.

Acknowledgments

This project was partially supported by a 863 project (No. 2011AA010301), the Nature Science Foundation of China (No. 11374263), Zhejiang provincial grant (Z201121938), the Doctoral Fund of Ministry of Education of China (No. 20120101110094).

References and links

1.

D. J. Richardson, J. M. Fini, and L. E. Nelson, “Space-division multiplexing in optical fibres,” Nat. Photon. 7(5), 354–362 (2013). [CrossRef]

2.

J. Sakaguchi, B. Puttnam, W. Klaus, Y. Awaji, N. Wada, A. Kanno, T. Kawanishi, K. Imamura, H. Inaba, K. Mukasa, R. Sugizaki, T. Kobayashi, and M. Watanabe, “305 Tb/s space division multiplexed transmission using homogeneous 19-core fiber,” J. Lightwave Technol. 31(4), 554–562 (2013). [CrossRef]

3.

K. S. Abedin, J. M. Fini, T. F. Thierry, B. Zhu, M. F. Yan, L. Bansal, F. V. Dimarcello, E. M. Monberg, and D. J. DiGiovanni, “Seven-core erbium-doped double-clad fiber amplifier pumped simultaneously by side-coupled multimode fiber,” Opt. Lett. 39(4), 993–996 (2014). [CrossRef] [PubMed]

4.

S. Randel, R. Ryf, A. Sierra, P. J. Winzer, A. H. Gnauck, C. A. Bolle, R. J. Essiambre, D. W. Peckham, A. McCurdy, and R. Lingle Jr., “6×56-Gb/s mode-division multiplexed transmission over 33-km few-mode fiber enabled by 6×6 MIMO equalization,” Opt. Express 19(17), 16697–16707 (2011). [CrossRef] [PubMed]

5.

A. M. J. Koonen, H. Chen, H. P. A. van den Boom, and O. Raz, “Silicon photonic integrated mode multiplexer and demultiplexer,” IEEE Photon. Technol. Lett. 24(21), 1961–1964 (2012). [CrossRef]

6.

N. Hanzawa, K. Saitoh, T. Sakamoto, T. Matsui, K. Tsujikawa, M. Koshiba, and F. Yamamoto, “Two-mode PLC-based mode multi/demultiplexer for mode and wavelength division multiplexed transmission,” Opt. Express 21(22), 25752–25760 (2013). [CrossRef] [PubMed]

7.

A. Li, J. Ye, X. Chen, and W. Shieh, “Low-loss fused mode coupler for few-mode transmission,” in Optical Fiber Communication Conference/National Fiber Optic Engineers Conference 2013, OSA Technical Digest (online) (Optical Society of America, 2013), OTu3G.4. [CrossRef]

8.

D. Dai, “Silicon mode-(de)multiplexer for a hybrid multiplexing system to achieve ultrahigh capacity photonic networks-on-chip with a single-wavelength-carrier light,” in Asia Communications and Photonics Conference, OSA Technical Digest (online) (Optical Society of America, 2012), ATh3B.3. [CrossRef]

9.

T. Uematsu, Y. Ishizaka, Y. Kawaguchi, K. Saitoh, and M. Koshiba, “Design of a compact two-mode multi/demultiplexer consisting of multi-mode interference waveguides and a wavelength insensitive phase shifter for mode-division multiplexing transmission,” J. Lightwave Technol. 30(15), 2421–2426 (2012). [CrossRef]

10.

J. B. Driscoll, R. R. Grote, B. Souhan, J. I. Dadap, M. Lu, and R. M. Osgood, “Asymmetric Y junctions in silicon waveguides for on-chip mode-division multiplexing,” Opt. Lett. 38(11), 1854–1856 (2013). [CrossRef] [PubMed]

11.

J. Xing, Z. Li, X. Xiao, J. Yu, and Y. Yu, “Two-mode multiplexer and demultiplexer based on adiabatic couplers,” Opt. Lett. 38(17), 3468–3470 (2013). [CrossRef] [PubMed]

12.

D. Dai, J. Wang, and Y. Shi, “Silicon mode (de)multiplexer enabling high capacity photonic networks-on-chip with a single-wavelength-carrier light,” Opt. Lett. 38(9), 1422–1424 (2013). [CrossRef] [PubMed]

13.

H. Qiu, H. Yu, T. Hu, G. Jiang, H. Shao, P. Yu, J. Yang, and X. Jiang, “Silicon mode multi/demultiplexer based on multimode grating-assisted couplers,” Opt. Express 21(15), 17904–17911 (2013). [CrossRef] [PubMed]

14.

L.-W. Luo, N. Ophir, C. P. Chen, L. H. Gabrielli, C. B. Poitras, K. Bergmen, and M. Lipson, “WDM-compatible mode-division multiplexing on a silicon chip,” Nat. Commun. 5, 3069 (2014). [CrossRef] [PubMed]

15.

J. Wang, S. He, and D. Dai, “On-chip silicon 8-channel hybrid (de)multiplexer enabling simultaneous mode- and polarization-division-multiplexing,” Laser Photon. Rev. 8(2), L18–L22 (2014). [CrossRef]

16.

K. Rollke and W. Sohler, “Metal-clad waveguide as cutoff polarizer for integrated optics,” IEEE J. Quantum Electron. 13(4), 141–145 (1977). [CrossRef]

17.

D. Dai, Z. Wang, N. Julian, and J. E. Bowers, “Compact broadband polarizer based on shallowly-etched silicon-on-insulator ridge optical waveguides,” Opt. Express 18(26), 27404–27415 (2010). [CrossRef] [PubMed]

18.

M. Z. Alam, J. S. Aitchison, and M. Mojahedi, “Compact and silicon-on-insulator-compatible hybrid plasmonic TE-pass polarizer,” Opt. Lett. 37(1), 55–57 (2012). [CrossRef] [PubMed]

19.

Y. Huang, S. Zhu, H. Zhang, T. Y. Liow, and G. Q. Lo, “CMOS compatible horizontal nanoplasmonic slot waveguides TE-pass polarizer on silicon-on-insulator platform,” Opt. Express 21(10), 12790–12796 (2013). [CrossRef] [PubMed]

20.

C. H. Chen, L. Pang, C. H. Tsai, U. Levy, and Y. Fainman, “Compact and integrated TM-pass waveguide polarizer,” Opt. Express 13(14), 5347–5352 (2005). [CrossRef] [PubMed]

21.

S. Lin, J. Hu, and K. B. Crozier, “Ultracompact, broadband slot waveguide polarization splitter,” Appl. Phys. Lett. 98(15), 151101 (2011). [CrossRef]

22.

D. Dai, Z. Wang, and J. E. Bowers, “Ultrashort broadband polarization beam splitter based on an asymmetrical directional coupler,” Opt. Lett. 36(13), 2590–2592 (2011). [CrossRef] [PubMed]

23.

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(1), 4–6 (2013). [CrossRef] [PubMed]

24.

X. Guan, H. Wu, Y. Shi, L. Wosinski, and D. Dai, “Ultracompact and broadband polarization beam splitter utilizing the evanescent coupling between a hybrid plasmonic waveguide and a silicon nanowire,” Opt. Lett. 38(16), 3005–3008 (2013). [CrossRef] [PubMed]

25.

D. Taillaert, F. Van Laere, M. Ayre, W. Bogaerts, D. Van Thourhout, P. Bienstman, and R. Baets, “Grating couplers for coupling between optical fibers and nanophotonic waveguides,” Jpn. J. Appl. Phys. 45(8A), 6071–6077 (2006). [CrossRef]

26.

G. Roelkens, D. Van Thourhout, and R. Baets, “High efficiency silicon-on-insulator grating coupler based on a poly-Silicon overlay,” Opt. Express 14(24), 11622–11630 (2006). [CrossRef] [PubMed]

27.

Y. Tang, Z. Wang, L. Wosinski, U. Westergren, and S. He, “Highly efficient nonuniform grating coupler for silicon-on-insulator nanophotonic circuits,” Opt. Lett. 35(8), 1290–1292 (2010). [CrossRef] [PubMed]

28.

D. Dai, J. Wang, and S. He, “Silicon multimode photonic integrated devices for on-chip mode-division- multiplexed optical interconnects,” Prog. Electromagn. Res. 143, 773–819 (2013). [CrossRef]

OCIS Codes
(030.4070) Coherence and statistical optics : Modes
(060.4230) Fiber optics and optical communications : Multiplexing
(130.3120) Integrated optics : Integrated optics devices
(230.5440) Optical devices : Polarization-selective devices

ToC Category:
Integrated Optics

History
Original Manuscript: March 18, 2014
Revised Manuscript: May 6, 2014
Manuscript Accepted: May 11, 2014
Published: May 19, 2014

Citation
Jian Wang, Pengxin Chen, Sitao Chen, Yaocheng Shi, and Daoxin Dai, "Improved 8-channel silicon mode demultiplexer with grating polarizers," Opt. Express 22, 12799-12807 (2014)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-22-11-12799


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References

  1. D. J. Richardson, J. M. Fini, L. E. Nelson, “Space-division multiplexing in optical fibres,” Nat. Photon. 7(5), 354–362 (2013). [CrossRef]
  2. J. Sakaguchi, B. Puttnam, W. Klaus, Y. Awaji, N. Wada, A. Kanno, T. Kawanishi, K. Imamura, H. Inaba, K. Mukasa, R. Sugizaki, T. Kobayashi, M. Watanabe, “305 Tb/s space division multiplexed transmission using homogeneous 19-core fiber,” J. Lightwave Technol. 31(4), 554–562 (2013). [CrossRef]
  3. K. S. Abedin, J. M. Fini, T. F. Thierry, B. Zhu, M. F. Yan, L. Bansal, F. V. Dimarcello, E. M. Monberg, D. J. DiGiovanni, “Seven-core erbium-doped double-clad fiber amplifier pumped simultaneously by side-coupled multimode fiber,” Opt. Lett. 39(4), 993–996 (2014). [CrossRef] [PubMed]
  4. S. Randel, R. Ryf, A. Sierra, P. J. Winzer, A. H. Gnauck, C. A. Bolle, R. J. Essiambre, D. W. Peckham, A. McCurdy, R. Lingle., “6×56-Gb/s mode-division multiplexed transmission over 33-km few-mode fiber enabled by 6×6 MIMO equalization,” Opt. Express 19(17), 16697–16707 (2011). [CrossRef] [PubMed]
  5. A. M. J. Koonen, H. Chen, H. P. A. van den Boom, O. Raz, “Silicon photonic integrated mode multiplexer and demultiplexer,” IEEE Photon. Technol. Lett. 24(21), 1961–1964 (2012). [CrossRef]
  6. N. Hanzawa, K. Saitoh, T. Sakamoto, T. Matsui, K. Tsujikawa, M. Koshiba, F. Yamamoto, “Two-mode PLC-based mode multi/demultiplexer for mode and wavelength division multiplexed transmission,” Opt. Express 21(22), 25752–25760 (2013). [CrossRef] [PubMed]
  7. A. Li, J. Ye, X. Chen, and W. Shieh, “Low-loss fused mode coupler for few-mode transmission,” in Optical Fiber Communication Conference/National Fiber Optic Engineers Conference 2013, OSA Technical Digest (online) (Optical Society of America, 2013), OTu3G.4. [CrossRef]
  8. D. Dai, “Silicon mode-(de)multiplexer for a hybrid multiplexing system to achieve ultrahigh capacity photonic networks-on-chip with a single-wavelength-carrier light,” in Asia Communications and Photonics Conference, OSA Technical Digest (online) (Optical Society of America, 2012), ATh3B.3. [CrossRef]
  9. T. Uematsu, Y. Ishizaka, Y. Kawaguchi, K. Saitoh, M. Koshiba, “Design of a compact two-mode multi/demultiplexer consisting of multi-mode interference waveguides and a wavelength insensitive phase shifter for mode-division multiplexing transmission,” J. Lightwave Technol. 30(15), 2421–2426 (2012). [CrossRef]
  10. J. B. Driscoll, R. R. Grote, B. Souhan, J. I. Dadap, M. Lu, R. M. Osgood, “Asymmetric Y junctions in silicon waveguides for on-chip mode-division multiplexing,” Opt. Lett. 38(11), 1854–1856 (2013). [CrossRef] [PubMed]
  11. J. Xing, Z. Li, X. Xiao, J. Yu, Y. Yu, “Two-mode multiplexer and demultiplexer based on adiabatic couplers,” Opt. Lett. 38(17), 3468–3470 (2013). [CrossRef] [PubMed]
  12. D. Dai, J. Wang, Y. Shi, “Silicon mode (de)multiplexer enabling high capacity photonic networks-on-chip with a single-wavelength-carrier light,” Opt. Lett. 38(9), 1422–1424 (2013). [CrossRef] [PubMed]
  13. H. Qiu, H. Yu, T. Hu, G. Jiang, H. Shao, P. Yu, J. Yang, X. Jiang, “Silicon mode multi/demultiplexer based on multimode grating-assisted couplers,” Opt. Express 21(15), 17904–17911 (2013). [CrossRef] [PubMed]
  14. L.-W. Luo, N. Ophir, C. P. Chen, L. H. Gabrielli, C. B. Poitras, K. Bergmen, M. Lipson, “WDM-compatible mode-division multiplexing on a silicon chip,” Nat. Commun. 5, 3069 (2014). [CrossRef] [PubMed]
  15. J. Wang, S. He, D. Dai, “On-chip silicon 8-channel hybrid (de)multiplexer enabling simultaneous mode- and polarization-division-multiplexing,” Laser Photon. Rev. 8(2), L18–L22 (2014). [CrossRef]
  16. K. Rollke, W. Sohler, “Metal-clad waveguide as cutoff polarizer for integrated optics,” IEEE J. Quantum Electron. 13(4), 141–145 (1977). [CrossRef]
  17. D. Dai, Z. Wang, N. Julian, J. E. Bowers, “Compact broadband polarizer based on shallowly-etched silicon-on-insulator ridge optical waveguides,” Opt. Express 18(26), 27404–27415 (2010). [CrossRef] [PubMed]
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