OSA/OFC/NFOEC 2011
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Compact PLC-based Transponder Aggregator for Colorless and Directionless ROADM Toshio Watanabe, Kenya Suzuki, Takashi Goh, Kuninori Hattori, Atsushi Mori, Tetsuo Takahashi, Tadashi Sakamoto NTT Photonics Laboratories, NTT Corporation, 3-1 Morinosato-Wakamiya, Atsugi, Kanagawa, 243-0198 Japan e-mail:
[email protected]
Keiichi Morita, Shunichi Sohma, and Shin Kamei NTT Electronics Corporation, 6700-2, To, Naka, Ibaraki, 311-0122 Japan
Abstract: We describe a compact transponder aggregator comprising a splitter-switch and a tunable filter array based on PLC technology. We realize a colorless, directionless and contentionless multi-degree ROADM node cost-effectively with a small footprint. OCIS codes: (060.1810) Buffers, couplers, routers, switches, and multiplexers; (130.4815) Optical switching devices; (230.7390) Waveguides, planar
1. Introduction The increasing demand for optical networks with large capacity and high flexibility has led to the evolution of the reconfigurable optical add/drop multiplexer (ROADM) node. Multi-degree ROADM nodes have been deployed in long haul and metro networks. These nodes have a wavelength cross-connect (WXC) core between multiple input and output fibers, which enables any WDM signal from any direction to be routed in any direction. However, the transponders connected to add/drop ports are still colored and directed: their wavelengths and incoming/outgoing fibers are assigned in terms of the port to which they are connected. To realize a colorless and directionless ROADM node, an optical aggregation switch (hereafter, called it a transponder aggregator, TPA) should be installed between the WXC core and the add/drop transponders, as shown in Fig. 1. The TPA can assign any wavelength and any input/output fiber without signal contention. Previously reported TPAs use a wavelength selective switch (WSS) [1] or a large matrix switch [2]. The former requires a large number of WSSs, and the latter requires a matrix switch with a high port count. These switches are based on free space optics, which generally require a large footprint. They are obstacles to the compact and costeffective deployment of colorless and directionless ROADM node. In this paper, we present a TPA based on a novel, compact splitter-switch architecture. It is composed of arrays of optical splitters, switches, and tunable filters. All these components are fabricated using planar lightwave circuit (PLC) technology, which offers high-density integration and good reliability. This integrated TPA enables us to construct a colorless, directionless and contentionless ROADM node cost-effectively with a small footprint. WXC
Input fibers
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Rx Fig. 1. Colorless and directionless ROADM with TPA.
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Fig. 2. TPA based on splitter-switch architecture.
2. TPA Architecture Figure 2 shows a TPA based on the splitter-switch architecture. On the drop side of the TPA, incoming WDM signals from the WXC core are broadcasted by a 1xM splitter array, and selected by an Nx1 optical switch. Then the
OSA/OFC/NFOEC 2011
OTuD3.pdf OTuD3.pdf
signal is fed into the transponder after passing through a tunable filter. On the add side, the output signals from the transponders are routed by a 1xN switch array, combined with different wavelength by an Mx1 coupler array, and then launched into the WXC core. As shown in Fig. 2, the splitter-switch architecture includes a shuffle interconnection between the splitter and switch arrays. This makes them difficult to integrate on a single waveguide chip, because it requires many waveguide crossings. A conventional splitter-switch [3-4] is fabricated using separate switch and splitter chips connected together with a fiber circuit sheet. Here we propose a new circuit configuration that integrates the splitter and switch as shown in Fig. 3. We divided a 1x8 splitter into 1x2 splitting elements, and placed them between the 9 stages of the switch elements. This new circuit configuration allows the single chip integration of the splitter and switch arrays, which eliminates the fiber circuit sheet and fiber splice. The TPA also has the tunable filter arrays on the drop side, as shown in Fig. 2. Realizing a compact tunable filter array with high wavelength channel isolation has been a challenge, however, the recent development of a coherent detection technique has greatly relaxed the crosstalk requirements for the optical filter in front of the optical receiver [5]. In this scheme, the optical signal is selected in the electrical domain, and so an optical filter with a wide passband is allowed. Here we used a tunable filter consisting of a 3-stage Mach-Zehnder interferometer (MZI). The free spectral ranges (FSR) of each stage were 1600, 3200, and 6400 GHz, respectively. The center wavelength of each MZI was tuned by activating the thermo-optic phase shifter. After passing through the 3-stage MZI, the calculated 3 dB passband was 700 GHz. Although there were sidelobes whose crosstalk was -12 dB, the total optical output power within the 6400-GHz FSR was suppressed to 1/8 while the center wavelength power remained same. This should improve the dynamic range limitation and common-mode noise endurance of the coherent receiver.
1x2 switch 8-arrayed 1x8 switch
64-fiber 8-arrayed circuit sheet 8x1 splittler
Gate switch
2x1 splitting element
Fig. 3. Conventional (left) and proposed (right) circuit configurations of 8x8 splitter-switch.
Fig. 4. Fabricated PLC chips of 8-arrayed tunable filter (top) and 8x8 splitter-switch (bottom).
3. Experiments We fabricated an 8x8 splitter-switch and an 8-arrayed tunable filter using 1.5 %-Δ silica-on-silicon waveguide. Figure 4 shows a photograph of the fabricated chips. The 8x8 splitter-switch and the 8-arrayed tunable filter were 110 mm x 15 mm and 55 mm x 20 mm, respectively. Figure 5(a) shows the measured performance of the splitter-switch and tunable optical filter. The average insertion loss of the splitter-switch was 11.7 dB including an intrinsic splitting loss of 9 dB. This is the same as the (a)
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Fig. 5. (a) Measured optical characteristics of 8x8 splitter-switch. (b) Measured transmission spectra of tunable filter.
OSA/OFC/NFOEC 2011
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previously reported value. The waveguide crossing induced loss was estimated to be 1 dB. The average and minimum off-state losses were 62.9 dB and 49.8 dB, respectively. The average electrical switching power was 0.2 W per path, and the total power was 1.6 W. The center wavelength of the filter can be tuned across the entire C-band, as shown in Fig. 5(b). The average insertion loss of the tunable filter array was only 1.4 dB. The 0.5 and 3 dB bandwidths were 300 GHz and 700 GHz, respectively. The sidelobe crosstalk was -12 dB. These values are almost the same for all 8 arrays, and identical to the calculated value as designed. The maximum electrical power for wavelength tuning was 0.3 W per array, and the total power was 2.4 W. We launched 100-GHz spacing, 40 ch WDM signals into the tunable filter. Figure 6(a) and (b) shows the measured optical spectra before and after the signals passed through the tunable filter. At the filter input, the total and individual channel powers of the signals were -3.8 dBm and -19.8 dBm, respectively. After passing through the filter, the total and center channel output powers were -12.2 dBm and -21.2 dBm, respectively. The signal to total power ratio was reduced from -16 dB (40 wavelengths) to -9 dB (which corresponds to 8 wavelengths), resulting in a 7 dB improvement. This result indicates that our tunable filter with a 3 dB passband of 700 GHz improved the dynamic range of the receiver by 7 dB. Optical Power (dBm)
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Fig. 6. Transmission spectra of 100 GHz 40 ch WDM signals (a) before and (b) after passing through tunable filter.
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Fig. 7. 112 Gb/s DP-QPSK signal transmission performance with and without TPA.
We also measured the optical signal transmission performance through the TPA. A single wavelength 112 Gb/s dual polarization (DP)-QPSK signal was received with and without TPA. Figure 7 shows the Q-factor as a function of the optical signal-to-noise-ratio (OSNR) when loaded with the ASE noise of an optical amplifier. The results show that there was no degradation in the signals after they passed through the TPA including the splitter-switch and tunable filter. 4. Conclusion We fabricated a TPA consisting of a compact 8x8 splitter-switch and an 8-arrayed tunable filter based on PLC technology. This integrated TPA enables us to realize a colorless, directionless and contentionless ROADM node cost-effectively with a small footprint. References [1] P. Roorda and B. Collings, “Evolution to colorless and directionless RODAM architectures,” Proc. NFC/NFOEC, Paper NWE2 (2008). [2] R. Jensen, A. Lord, and N. Parsons, “Colourless, directionless, contentionless ROADM architecture using low-loss optical matrix switches,” Proc. ECOC, Paper Mo.2.D.2 (2010). [3] T. Watanabe, “Silica-based PLC optical switches designed and fabricated for OADM and OXC,” Proc. OECC/COIN, paper 13F2-3 (2004). [4] S. Sohma, S. Mino, T. Watanabe, M. Ishii, T. Shibata, and H. Takahashi, “Solid-state optical switches using planar lightwave circuit and ICon-PLC technology,” Proc. SPIE, vol. 5625, pp. 767-775 (2004). [5] L. E. Nelson, S. L. Woodward, P. D. Magill, S. Foo, M. Moyer, and M. O'Sullivan, “Real-time detection of a 40 Gbps intradyne channel in the presence of multiple received WDM channels,” Proc. NFC/NFOEC, Paper OMJ1 (2010).
