Integrated tunable optical add/drop filter for ... · 5. N. A. Riza and S. F. Yuan, “Reconfigurable wavelength add-drop filtering based on a Banyan network topology ... #257890

Integrated tunable optical add/drop filter for polarization and wavelength multiplexed signals

Yaguang Qin, Yu Yu,* Wenhao Wu, and Xinliang Zhang Wuhan National Laboratory for Optoelectronics and School of Optical and Electrical Information, Huazhong

University of Science and Technology, Wuhan, 430074, China *[email protected]

Abstract: We propose and demonstrate an integrated tunable optical filter which is promising for reconfigurable optical add/drop multiplexer (ROADM) targeting polarization and wavelength multiplexed signals. The proposed filter is comprised of a polarization diversity scheme and two tunable microring resonators (MRRs). The polarization scheme is implemented by two dimensional (2D) grating couplers which are functioning as signals import, output and add/drop ports, while the MRRs are signals processing units. The add/drop function can be applied on either polarization tribute or any wavelength by controlling the resonate wavelengths of ring resonators. For demonstration, dual polarizations and four-wavelength signals are experimentally added and dropped with good performance and reasonable power penalties.

©2016 Optical Society of America

OCIS codes: (130.3120) Integrated optics devices; (130.5440) Polarization-selective devices; (130.7408) Wavelength filtering devices.

References and links 1. P. N. Ji, Y. Aono, and T. Wang, “Reconfigurable Optical Add/Drop Multiplexer Based on Bidirectional

Wavelength Selective Switches,” in Integrated Photonics Research, Silicon and Nanophotonics and Photonics in Switching, OSA Technical Digest (CD) (Optical Society of America, 2010), PWB1.

2. T. Goh, T. Kitoh, M. Kohtoku, M. Ishii, T. Mizuno, and A. Kaneko, “Port Scalable PLC-Based Wavelength Selective Switch with Low Extension Loss for Multi-Degree ROADM/WXC,” in Optical Fiber Communication Conference/National Fiber Optic Engineers Conference, OSA Technical Digest (CD) (Optical Society of America, 2008), OWC6.

3. F. Xiao, B. Juswardy, K. Alameh, and Y. T. Lee, “Novel broadband reconfigurable optical add-drop multiplexer employing custom fiber arrays and Opto-VLSI processors,” Opt. Express 16(16), 11703–11708 (2008).

4. T. An Vu, Z. Wen De, R. Tucker, and S. Kai, “Reconfigurable multichannel optical add-drop multiplexers incorporating eight-port optical circulators and fiber Bragg gratings,” IEEE Photonics Technol. Lett. 13(10), 1100–1102 (2001).

5. N. A. Riza and S. F. Yuan, “Reconfigurable wavelength add-drop filtering based on a Banyan network topology and ferroelectric liquid crystal fiber-optic switches,” J. Lightwave Technol. 17(9), 1575–1584 (1999).

6. M. Muha, B. Chiang, and R. Schleicher, “MEMS Based Channelized ROADM Platform,” in Optical Fiber Communication Conference/National Fiber Optic Engineers Conference, OSA Technical Digest (CD) (Optical Society of America, 2008), JThA24.

7. H. Qiu, G. Jiang, T. Hu, H. Shao, P. Yu, J. Yang, and X. Jiang, “FSR-free add-drop filter based on silicon grating-assisted contradirectional couplers,” Opt. Lett. 38(1), 1–3 (2013).

8. H. Yamada, T. Chu, S. Ishida, and Y. Arakawa, “Optical add-drop multiplexers based on Si-wire waveguides,” Appl. Phys. Lett. 86(19), 191107 (2005).

9. E. J. Klein, D. H. Geuzebroek, H. Kelderman, G. Sengo, N. Baker, and A. Driessen, “Reconfigurable optical add-drop multiplexer using microring resonators,” IEEE Photonics Technol. Lett. 17(11), 2358–2360 (2005).

10. T. Okoshi, S. Ryu, and K. Kikuchi, “Polarization-diversity receiver for heterodyne/coherent optical fiber communications,” Paper 30C3–2, IOOC 83, 386–387 (1983).

11. T. Barwicz, M. R. Watts, Popovi, M. A. Cacute, P. T. Rakich, L. Socci, F. X. Kärtner, E. P. Ippen, and H. I. Smith, “Polarization-transparent microphotonic devices in the strong confinement limit,” Nat. Photonics 1(1), 57–60 (2007).

12. Lumerical Solutions, Inc., http://www.lumerical.com/. 13. Y. Qin, Y. Yu, J. Zou, M. Ye, L. Xiang, and X. Zhang, “Silicon based polarization insensitive filter for WDM-

PDM signal processing,” Opt. Express 21(22), 25727–25733 (2013).

#257890 Received 21 Jan 2016; revised 13 Mar 2016; accepted 15 Mar 2016; published 24 Mar 2016 (C) 2016 OSA 4 Apr 2016 | Vol. 24, No. 7 | DOI:10.1364/OE.24.007069 | OPTICS EXPRESS 7069

14. D. Taillaert, W. Bogaerts, P. Bienstman, T. F. Krauss, P. V. Daele, I. Moerman, S. Verstuyft, K. D. Mesel, and R. Baets, “An out-of-plane grating coupler for efficient butt-coupling between compact planar waveguides and single-mode fibers,” IEEE J. Quantum Electron. 38(7), 949–955 (2002).

15. J. Zou, Y. Yu, M. Ye, L. Liu, S. Deng, and X. Zhang, “A Four-port Polarization Diversity Coupler for Vertical Fiber-Chip Coupling,” in Optical Fiber Communication Conference, OSA Technical Digest (online) (Optical Society of America, 2015), W2A.10.

1. Introduction

The optical add/drop multiplexer (OADM) is a key component that allows locally adding/dropping one or more channels to/from optical paths in a dense wavelength division multiplexing (DWDM) network [1]. Conventional OADM has fixed configuration thus requiring manual operation to reconfigure the add/drop channels. To achieve a more flexible and cost-efficient network, reconfigurable OADM (ROADM) is essentially desired. ROADMs have been widely investigated and many schemes implementing various technologies have been proposed, including planar lightwave circuit (PLC) [2], fiber arrays [3], fiber gratings [4], liquid crystal [5], microelectromechanical systems (MEMS) [6] etc. However, the ROADMs based on these schemes suffer from either large footprint size or alignment and packaging complexity. The prosperous development of silicon photonics seems to provide a feasible solution to dramatically reduce power consumption and device size. The silicon on insulator (SOI) has become a competitive candidate for all-optical signal processing and integration platform thanks to the complementary metal oxide semiconductor (CMOS) compatible fabrication technology, high yields and high refractive index contrast. The development of OADMs based on SOI platform can lead to further integration of WDM subsystems, and several researches have been reported, such as grating-assisted contradirectional couplers [7] and Bragg grating/MZI configurations [8]. However, a very limited transmission isolation of ~8 dB was achieved. Being the essential building block of a integrated ROADM, tunable add/drop filters based on integrated microring resonator (MRR) have superior performance compared to other candidates in terms of compactness and power consumption, and it is essentially suitable for WDM applications due to the intrinsically periodic characteristics [9].

On the other hand, current add/drop functions are mostly performed on wavelength domain, while multi-dimensional multiplexing is utilized nowadays to increase the transmission capacity, for instance the WDM combined with polarization division multiplexed (PDM) techniques. However, the polarization dependent issues for silicon nano-scale waveguide restricts the applicability of PDM signals. To tackle this problem, the polarization diversity scheme, where the orthogonal polarization states are firstly split and processed separately and then combined at output port, is introduced [10,11]. The two-dimensional grating coupler (2D GC), which couples the two orthogonal polarization modes from a single-mode fiber into the identical mode but two different paths of the waveguide, is an outstanding candidate for simultaneous coupling and polarization diversity. Arising from this, we propose a tunable add/drop filter for PDM and WDM signals, utilizing the polarization diversity configuration comprised of 2D GCs and MRRs. The proposed filter can be an important building block of ROADM circuit suitable for multi-dimensional system. As demonstration, 20 Gb/s On-Off Keying (OOK) dual polarizations and four-wavelength signals are experimentally processed. The adding and dropping functionalities can be successfully applied to different polarization and wavelength. The bit error rate (BER) measurements show an error free operation, indicating the good performance of the proposed filter.


2. Operation principle

23

1

Arrayed fibers

Thermal phase shifter

(a)

X polY pol

45

6

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Add

Drop

...x

yPath1

Y pol

Path2

Path1

Path2

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...

Fig. 1. (a) Schematic configuration of the proposed filter, (b) 3D FDTD simulated electric field distribution of coupled light with different input polarization via 2D GC, (c) schematic of operation principle on both polarization and wavelength domain.

The schematic configuration and the principle of the proposed scheme are illustrated in Fig. 1. The circuit is comprised of six 2D GCs (marked from No. 1 to No. 6 and these six corresponding ports are also marked from No.1 to No.6), two thermal tunable MRRs (marked as MRR-1 and MRR-2) and connecting access waveguides. No. 2 and 5 2D GCs are used for coupling main stream signals into and out of the chip. No. 1 and 4 2D GCs are functioning as drop and add port for MRR-1, while No. 3 and 6 are for MRR-2, respectively. To be noted, only ports 2 and 5 function as a polarizartion diversity ports, and thus 2D GCs are utilized. The remaining ports are normal coupling ports and conventional 1D GCs are competent. However, for a coincident performance, all the six ports are designed with 2D GCs. When injecting into the circuit via No. 2 2D GC, the signals would be coupled into two orthogonal access waveguide and propagate along two paths (Path 1 and Path 2), according to input state of the polarization (SOP). As depicted in Fig. 1(b), the coupling mechanism of a 2D GC is investigated and the simulation results are obtained utilizing 3D FDTD method [12]. When input light is X polarized, which is orthogonal to the direction of Path 1, it tends to be fully coupled into Path 2. If input light is Y polarized, the coupled light would propagate in Path 1. By aligning PDM tributaries to the polarization planes of 2D GCs, it is possible to couple the specific polarization tributary to the desired path [13]. The MRR-1 and MRR-2 are designed to have relatively large 3dB bandwidth and free spectrum range (FSR). Figure 1(c) shows the schematic of operation principle of proposed filter for wavelength and polarization. Since the two polarization tributaries of PDM signal would couple into different paths on the chip, it is possible to process them separately. By aligning the resonant wavelength of MRR-1 and MRR-2 to the desired wavelength, the signal of specific polarization and wavelength tributary can be added or dropped. Due to the presence of thermal phase shifter, the whole circuit is tunable within a large wavelength range.

3. Device fabrication and experimental results

The proposed filter is fabricated on SOI platform and a commercial SOI wafer with top silicon layer of 220 nm is used. The buried oxide layer is of 2µm thick, and the upper cladding material is silicon oxide as well. All the waveguides are slab ones with 90 nm thick slab to enhance the coupling strength of the MRRs. The optical micrographs and scanning electron micrographs (SEM) of the fabricated device are illustrated in Fig. 2. Figure 2(a) shows the top view, it is seen that all six 2D GCs are placed intentionally to match the fiber array so that add and drop functionalities can be performed simultaneously. The red dash boxes represent the reference MRRs with identical parameters as the ones in the proposed filter. Figures 2(b) and 2(d) provide the details of the tunable MRRs. The radii of MRR-1 and


MRR-2 are 20 µm and designed gap between the straight and bended waveguides for the coupling region is 200 nm. The calculated coupling strength is 0.246122. The designed 3dB bandwidth and FSR are 0.4 nm and 4.48 nm respectively. Figures 2(c) and 2(e) show the 2D GC with a square array of round holes and an etch depth of 90 nm. The designed diameter of the air holes are 340 nm and the lattice period is 840 nm. To improve coupling efficiency, Bragg grating structures (BGSs) are adopted [14].

The tunable spectral response of MRRs are firstly investigated. As depicted in Fig. 3(a), the optical spectrum at the through port of a reference MRR is measured. The FSR is measured to be 4.9 nm. When the bias voltage varies from 0 to 5V, the resonant wavelength shifts to longer wavelength nearly within a FSR, and the tuning efficiency is measured to be 60 mW/FSR. The extinction ratio is as large as 22 dB and the 3dB bandwidth is about 0.4 nm, indicating its capability of processing signals up to 40 Gb/s. As for the drop port, a visible tunable spectrum is also obtained, as shown in Fig. 3(b).

Fig. 2. Optical micrographs of (a) the layout of proposed filter, the red dash boxes are the reference MRRs, (b) tunable MRR, (c) 2D GC with BGSs, and the SEM images of (d) coupling region of reference MRR, (e) air holes of 2D GC.

(a) (b)

Fig. 3. Spectral response of reference MRR when bias voltage varies from 0V to 5V, (a) through port of MRR (b) drop port of MRR.


DATA1

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Isolator

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CSA

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λ1

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λ4 DATA2

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BPG2

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PBSPC4

DCF

Fig. 4. The experimental setup.

The experimental setup is shown in Fig. 4. Four CW lights at 1533.7, 1534.7, 1535.7 and 1536.7 nm are firstly combined by an arrayed waveguide grating (AWG) and are split into two Mach-Zehnder modulators (MZMs) driven by two independent bit pattern generators (BPGs) to obtain independent WDM OOK signals. The driven data are PRBS 231-1 at 20 Gbaud. A dispersion compensation fiber (DCF) with length of ~200 m and dispersion parameter of −80 ps/nm/km is utilized after each MZM. Although the induced delay cannot achieve a complete decorrelation, it is helpful to distinguish whether the proposed filter is effective to perform the drop operation with low crosstalk and large isolation from adjacent channels. The polarization states of the two signals are optimized by the polarization controllers (PC1 and PC2) and combined by a polarization beam combiner (PBC), forming the WDM-PDM signals. By vertically coupling via the 2D GC, the signals are coupled into the chip, assisting by another PC (PC3) to align the two orthogonal polarizations in the fiber to the polarization axes of the 2D GC in the silicon waveguide. If the circuit is not applied for the PDM signal processing, the PC3 is not necessary and the scheme performs as a polarization insensitive tunable filter. An optical isolator is used here to eliminate the reflection. At the output ports, the processed signals are coupled out by another set of 2D GCs, and the output power is optimized by the erbium-doped fiber amplifier (EDFA) and the attenuator (ATT). Assisting by the subsequent PC (PC4) and a polarization beam splitter (PBS), the output dual polarization signals can be detected. A band pass filter (BPF) is further used for the wavelength demultiplexing and an optical spectrum analyzer (OSA) and a communication signal analyzer (CSA) are also used for monitoring.

For signal demonstration, the X-polarized (as shown in Fig. 1(a)) tributary of PDM signals is firstly processed while the Y-polarized tributary remains unchanged. This can be done by thermally tunning the MRR-1and MRR-2, so that we can align only the resonate wavelength of MRR-1 with the signal wavelength. The dropped signal can be detected at No.1 2D GC while the added signal is added at No.4 2D GC.


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Fig. 5. The spectral response of dropped X-polarized signal at different sigal wavelengths: (a) wavelength 1, (b) wavelength 2, (c) wavelength 3, (d) wavelength 4; (e) the eye diagram of four signal wavelengths.

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Fig. 6. The spectral response of X-polarized signal after the filtering at different signal wavelengths: (a) wavelength 1, (b) wavelength 2, (c) wavelength 3, (d) wavelength 4.


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Fig. 7. The add functionality of X-polarizaed tributary at four signal wavelengths via bias voltage tuning, the spectral response of different signal wavelengths: (a) wavelength 1, (b) wavelength 2, (c) wavelength 3, (d) wavelength 4; (e) the eye diagram of four signal wavelengths.

The input PDM signals are fed into chip via No.2 2D GC and detected at No.1 2D GC. The drop functionality of the proposed filter for X-polarized signals is illustrated in Figs. 5(a) to 5(d). The axes of input signal and 2D GC are aligned by adjusting PC3. Consequently, X-polarized signal is guided into MRR-1 and Y-polarized signal is guided into MRR-2. The black curves are the spectra of input signals, while the red ones represent the dropped signals at four different wavelengths, respectively. The insertion loss at drop port, which is defined by the power dissipation between input signal and dropped signal at specific wavelength, is measured to be ~18 dB. It originates mainly from the coupling loss of two 2D GCs and might be improved by further optimizing the design of 2D GC [15]. The drop loss is 3.6 dB and the measured isolation from adjacent channel (considering 1 nm channel spacing) is 15 to 20 dB, for different channels. Figure 5(e) shows the eye diagrams of four dropped signals at different wavelengths (WL1 to WL4). To be noted, the inputs are PDM signals but the eye diagram is only single polarization, since the oscilloscope can only be triggered by either pattern generator. It can be seen that all the eye diagrams are clear and open, indicating the effective dropping functionality of signals. Then, the output is switched to No.5 port instead of No.1 to investigate the optical spectrum after filtering by MRR-1. Figures 6(a) to 6(d) show the spectral evolution of the signals processing. The black and red curves represent the input and output signals after dropping. The input signals can be fully downloaded one by one according to the results in Fig. 6. The measured crosstalk between dropped and undropped signal at same signal wavelength, which is defined as through crosstalk, is less than −14 dB. The insertion loss at through port (defined as the power attenuation between input and unfiltered signal at same wavelength) is measured to be ~15 dB.

Finally, another input at different wavelength is fed into the chip via No.4 2D GC and detected at No.5 2D GC to test the add functionality, with results shown in Fig. 7. WL1 to WL4 represent the added signals at different wavelengths, recording from the output port 5 assisted by the polarization and wavelength demultiplexing. The measured optical spectrum and clear eye diagrams reveal the good adding performance.


We then foucus on the feasibility of Y-polarized tributrary of PDM signals processed by the MRR-2. Similar results on spectra and eye diagrams are obtained, as shown in Figs. 8, 9 and 10. The drop functionality of the proposed filter for Y-polarized signals is illustrated in Figs. 8(a) to 8(e), including the spectra and eye diagrams. Figures 9(a) to 9(d) show the spectral evolution of the signals processing. The black and red curves represent the input and output signals after dropping. The add functionality for Y-polarized signals is presented in Fig. 10, for different wavelength cases. The drop loss and isolation from adjacent channel is 4.2 dB and 13 to 18 dB for different channels. The through crosstalk is measured to be less than −12 dB. It is seen that results for Y-polarized signal is consistant with the ones for X-polarized, verifying the capability of handling dual polarization and multiple wavelengths. In order to quantitative investigate the performance of the proposed scheme, the BER measurements are perform for the adding and dropping functions, respectively.The measure results are depicted in Fig. 11. For simplification, only results at one wavelength (WL2) are presented, while the dispersion for different wavelengths are less than 0.7 dB. The power penlty is 1.2 and 1.5 dB for adding and dropping operation, respectively.

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Fig. 8. The spectral response of dropped Y-polarized signal at different sigal wavelengths: (a) wavelength 1, (b) wavelength 2, (c) wavelength 3, (d) wavelength 4; (e) the eye diagram of four signal wavelengths.


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Fig. 9. The spectral response of Y-polarized signal after the filtering at different signal wavelengths: (a) wavelength 1, (b) wavelength 2, (c) wavelength 3, (d) wavelength 4.

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Fig. 10. The add functionality of Y-polarizaed tributary at four signal wavelengths via bias voltage tuning, the spectral response of different signal wavelengths: (a) wavelength 1, (b) wavelength 2, (c) wavelength 3, (d) wavelength 4; (e) the eye diagram of four signal wavelengths.


-15 -14 -13 -12 -11 -10 -9 -8 -7 -6 -5

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X-pol drop Y-pol drop X-pol add Y-pol add BtB

Fig. 11. The measured power penalty.

4. Conclusion

In conclusion, we have propose and demonstrate a tunable add/drop filter that can be used for both wavelength and polarization multiplexed signals. 20 Gbit/s WDM-PDM signals can be successfully added and dropped with clear eyediagrams. Arising from this, many other polarization handling devices for PDM and WDM signal processing can be achieved by changing the configuration of this proposed scheme, and we believe that can be advantageous in monolithic integrated circuit for signal processing in the future optical transport networks utilizing multi-dimensional multiplexing.

Acknowledgments

This work was supported by the National Natural Science Foundation of China (NSFC) (Grant No. 61475050 and 61275072), the New Century Excellent Talent Project in Ministry of Education of China (NCET-13-0240), the Fundamental Research Funds for the Central Universities (HUST2015TS079), and Huawei Technologies Co. Ltd..


Documents

Integrated tunable optical add/drop filter for ... · 5. N. A. Riza and S. F. Yuan, “Reconfigurable wavelength add-drop filtering based on a Banyan network topology ... #257890