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Polarization-insensitive 160-Gb/s wavelength converter with all-optical repolarizing function using circular-birefringence highly nonlinear fiber Takuo Tanemura, Ju Han Lee, Dexiang Wang, Kazuhiro Katoh, and Kazuro Kikuchi Research Center for Advanced Science and Technology, University of Tokyo, 4-6-1 Komaba, Meguro-Ku, Tokyo 1538904, Japan [email protected]

Abstract: A circular-birefringence highly nonlinear fiber (CB-HNLF) with the nonlinear coefficient of 12 /W/km is fabricated successfully by twisting a commercial silica-based highly nonlinear fiber. Using the cross-phase modulation in a 100-m-long CB-HNLF and subsequent optical filtering, we realize error-free pulsewidth-maintaining wavelength conversion of 160Gb/s signal with only 0.7-dB polarization sensitivity. In addition to the simplicity and stability, the demonstrated scheme features an ultrafast repolarizing functionality, where the degree-of-polarization of the input signal is restored in an all-optical manner. These advantages make the scheme highly attractive to be employed in the future photonic networks. ©2006 Optical Society of America OCIS codes: (060.4370) Nonlinear optics, fibers; (060.0060) Fiber optics and optical communications; (230.1150) All-optical devices; (060.2420) Fibers, polarization-maintaining.

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C. Schubert, R. Ludwig, S. Watanabe, F. Futami, C. Schmidt, J. Berger, C. Boerner, S. Ferber and H.G. Weber, “160 Gbit/s wavelength converter with 3R-regenerating capability,” Electron. Lett. 38, 903-904 (2002). H. Sotobayashi, W. Chujo, and T. Ozeki, “Inter-wavelength-band conversion and demultiplexings of 640 Gbit/s OTDM signals,” in Proc. Optical Fiber Communications Conf. (OFC) 2002, Paper WM2 (2002). L. Rau, W. Wang, S. Camatel, H. Poulsen, and D. J. Blumenthal, “All-optical 160-Gb/s phase reconstructing wavelength conversion using cross-phase modulation (XPM) in dispersion-shifted fiber,” IEEE Photonics Technol. Lett. 16, 2520-2522 (2004). F. Futami, R. Okabe, Y. Takita, and S. Watanabe, “Transparent wavelength conversion at up to 160 Gb/s by using supercontinuum generation in a nonlinear fiber,” in Proc. Optical Amplifier and their Applications (OAA) 2003, Paper MD07 (2003). S. Nakamura, Y. Ueno, and K. Tajima, “168-Gb/s all-optical wavelength conversion with a symmetricMach-Zehnder-type switch,” IEEE Photonics Technol. Lett. 13, 1091-1093 (2001). J. Leuthold, L. Möller, J. Jaques, S. Cabot, L. Zhang, P. Bernasconi, M. Cappuzzo, L. Gomez, E. Laskowski, E. Chen, A. Wong-Foy, and A. Griffin, “160 Gbit/s SOA all-optical wavelength converter and assessment of its regenerative properties,” Electron. Lett. 40, 554-555 (2004). I. Brener, B. Mikkelsen, G. Raybon, R. Harel, K. Parameswaran, J.R. Kurz, and M.M. Fejer, “160 Gbit/s wavelength shifting and phase conjugation using periodically poled LiNbO3 waveguide parametric converter,” Electron. Lett. 36, 1788-1790 (2000). T. Tanemura, J. Suzuki, K. Katoh, and K. Kikuchi, “Polarization-insensitive all-optical wavelength conversion using cross-phase modulation in twisted fiber and optical filtering,” IEEE Photonics Technol. Lett. 17, 1052-1054 (2005). C. S. Goh, S. Y. Set, and K. Kikuchi, “Widely tunable optical filters based on fiber Bragg gratings,” IEEE Photonics Technol. Lett. 14, 1306-1308 (2002). T. Tanemura, K. Katoh, and K. Kikuchi, “Polarization-insensitive asymmetric four-wave mixing using circularly polarized pumps in a twisted fiber,” Opt. Express 13, 7497-7505 (2005), http://www.opticsexpress.org/abstract.cfm?URI=OPEX-13-19-7497. B. E. Olsson and D. J. Blumenthal, “All-optical demultiplexing using fiber cross-phase modulation (XPM) and optical filtering,” IEEE Photonics Techonol. Lett. 13, 875-877 (2001).

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Received 19 December 2005; accepted 28 January 2006

20 February 2006 / Vol. 14, No. 4 / OPTICS EXPRESS 1408

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J. Suzuki, T. Tanemura, K. Taira, Y. Ozeki, and K. Kikuchi, “All-optical regenerator using wavelength shift induced by cross-phase modulation in highly nonlinear dispersion-shifted fiber,” IEEE Photonics Technol. Lett. 17, 423-425 (2005).

1. Introduction For the future high-bit-rate photonic networks, there is a strong demand for simple and costeffective all-optical wavelength converters operating at data rates beyond 160 Gb/s, where the optical-to-electrical-to-optical (OEO) conversion is difficult. Such ultrafast wavelength conversion (WC) has been demonstrated successfully by using an optical fiber [1-4], semiconductor optical amplifier (SOA) [5,6], and periodically poled LiNbO3 (PPLN) waveguide [7]. Among these schemes, however, the nonlinear polarization rotation [1]; fourwave mixing [2]; and cross-phase modulation (XPM) in a standard fiber [3], and the cascaded χ(2) process in a PPLN waveguide [7] are intrinsically sensitive to the input signal polarization, and thus require either an automatic polarization stabilizer or the polarizationdiversity architecture. While the SOA-based WC can, in principle, be made polarizationinsensitive by proper engineering of SOA fabrication, it seems quite challenging in practice to achieve polarization-insensitive operation at 160 Gb/s [5,6]. The one based on self-phase modulation (SPM) in fiber exhibits inherent polarization insensitivity [4]; however, it has a drawback of comparatively large signal power requirement to achieve sufficient WC efficiency and broad bandwidth. On the other hand, we have recently demonstrated a relatively simple and efficient scheme of polarization-insensitive WC at 40 Gb/s, based on XPM in a circular-birefringence fiber (CBF) [8]. It utilizes the unique feature of CBF, that the XPM effect between two circularly polarized waves becomes nearly independent of relative states-of-polarization (SOPs). By aligning the CW probe light to a circular SOP, polarization-insensitive WC is achieved in a simple straight-line configuration. In this first experiment, however, CBF was fabricated by twisting a 1-km-long conventional dispersion-shifted fiber (DSF) with a small nonlinear coefficient (γ = 3.0 /W/km). As a result, the polarization-mode dispersion (PMD) and the walk-off effect in the long CBF, made it difficult to achieve higher-bit-rate WC. In this paper, we demonstrate 160-Gb/s WC with 0.7-dB polarization sensitivity by using a 100-m length of a newly developed circular-birefringence highly nonlinear fiber (CBHNLF). The CB-HNLF is fabricated successfully by twisting a commercial silica-based HNLF. Owing to the reduced PMD and walk-off effect in the short CB-HNLF, we achieve, for the first time to our knowledge, error-free pulsewidth-maintaining WC of a polarizationscrambled 160-Gb/s signal without employing an automatic signal-polarization stabilizer. The scheme also offers an ultrafast repolarizing function, where an input signal with low degree-of-polarization (DOP) is converted to a high-DOP signal in an all-optical manner. 2. Experimental setup The experimental setup is shown in Fig. 1. A pulse train (1545 nm) generated from a 10-GHz regeneratively mode-locked erbium-doped fiber ring laser (EFRL) was intensity-modulated with a 10-Gb/s 231-1 pseudorandom bit sequence (PRBS) and multiplexed to a singlepolarization 160-Gb/s signal by using a passive fiber interleaver (MUX). They were then transmitted through a polarization scrambler (Fiberpro PS-155-A-B), followed by a polarization controller (PC1). In the wavelength converter, the signal was amplified with an erbium-doped fiber amplifier (EDFA), combined with the CW light (1556 nm), and launched on a 100-m CB-HNLF. Average input powers of the signal and CW light were 24.7 dBm and 12.3 dBm, respectively. At the CB-HNLF output, the longer-wavelength sideband of the CW light was filtered out by using two thin-film-based bandpass filters (BPFs) and a tunable fiber Bragg grating (TFBG) with a sharp cutoff characteristic [9]. These filters resulted in the total 3-dB transmission bandwidth of 2.5 nm and the center wavelength at 1557.5 nm.

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The CB-HNLF was fabricated by twisting a silica-based commercial HNLF (Furukawa, nonlinear coefficient = 12 /W/km, zero-dispersion wavelength = 1558 nm, dispersion slope = 0.024 ps/nm2/km, propagation loss = 0.39 dB/km) at a rate of 15 turns/m. The twist introduces a torsional stress, which via the photoelastic effect, leads to high circular birefringence. To confirm the circular-polarization-maintaining property of CB-HNLF, we performed a polarimetric optical-time-domain reflectometry (P-OTDR) measurement [10]. The setup and measured traces are shown in Figs. 2(a) and 2(b), respectively. Because of the polarizer inserted at the OTDR output/input port, the received optical power becomes a function of the polarization ellipticity at the Rayleigh-scattered point of the fiber. Smooth POTDR traces without any ripple in the CB-HNLF section clearly indicates that the ellipticity of polarization is maintained excellently along CB-HNLF due to the sufficient circular birefringence. We found well-defined eigen-polarization modes by injecting a CW light into CB-HNLF and observing the wavelength dependence of the output SOP. The total amount of PMD was measured to be 0.18 ps (1.8 ps/km) by the fixed-analyzer method. In the WC experiment, input SOP of the CW light was adjusted to one of the eigen-modes by using PC2 and polarimeter, so that the CW light was circularly polarized inside CB-HNLF. 160-Gb/ s Polarization-insensi tive Wavelength Conversion

160-Gb/ s Signal Transmitter 9.953 Gbit/s PRBS 31

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Fig. 1. Experimental setup of the 160-Gb/s polarization-insensitive WC. MZ: Mach-Zehnder intensity modulator. ECL: external-cavity CW laser, VOA: variable optical attenuator.

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Distance [20 m/div]

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Fig. 2. Setup of the P-OTDR measurement (a) and the measured traces for two different adjustments of PC (b). Smooth traces without any ripple in the CB-HNLF section clearly indicates that the ellipticity of polarization is maintained excellently along CB-HNLF.

3. Results and discussion We first disable the polarization scrambler so that the input signal SOP is fixed. Figure 3(a) shows the optical spectra observed at the CB-HNLF input (broken), output (black and gray bold), and after TFBG (thin solid). Black and gray bold curves show, respectively, the two

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extreme cases, in which the spectral broadening of the CW light becomes the maximum and minimum as we vary the input signal SOP using PC1. When the CW light is aligned to a circular SOP, the spectral broadening of the CW light is almost independent of the signal SOP as shown in Fig. 3(a). Consequently, the polarization sensitivity of the wavelength-converted power after TFBG is measured to be as small as 0.7 dB. Figure 3(b) shows the eye-diagrams of the wavelength-converted signal with the polarization scrambler OFF (left) and ON (right), observed using an 80-GHz electric sampling oscilloscope. We see clear eye opening of the wavelength-converted signal even with the input SOP scrambled. Figure 4 shows the autocorrelation traces of the input and wavelength-converted signals. From the analytical curves (broken curves), which are calculated assuming Gaussian pulses, we estimate the full width at half the maximum (FWHM) of the input and wavelength-converted signal to be 2.6 ps and 2.8 ps, respectively, indicating that nearly pulsewidth-maintaining WC is achieved. 20

(a) Power [dBm]

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Fig. 3. (a) Optical spectra observed at the CB-HNLF input (broken), output (bold black and gray), and after the BPF (thin) (resolution = 0.1 nm). The polarization scrambler is turned off. The black and gray bold curves show the two extreme cases where the XPM-induced spectral broadening of the CW light became the maximum and minimum as we vary the input signal SOP using PC1. (b) Eye diagrams of the 160-Gb/s wavelength-converted signal with the polarization scrambler OFF (left) and ON (right). 1

1

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Fig. 4. Autocorrelation traces of the input (left) and wavelength-converted (right) signals.

It should be noted that this WC scheme also offers the repolarizing function: The output signal has a nearly fixed SOP even when the input signal is depolarized. In fact, the input signal DOP is reduced to less than 5% when we enable the polarization scrambler, while that of the wavelength-converted signal is measured to be higher than 80 %. Although this value is slightly lower than that of the original CW light (possibly due to the nonlinear birefringence induced by the randomly polarized input signal), such repolarizing functionality is an

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attractive feature that cannot be obtained by the SPM-based WC scheme or those employing the polarization-diversity architecture. Finally, the wavelength-converted 160-Gb/s signal is demultiplexed to 10-Gb/s tributaries by using the XPM-induced spectral shift in a 150-m non-twisted HNLF [11]. Figure 5(a) shows the eye diagrams of a demultiplexed tributary with the input signal SOP fixed (left) and scrambled (right). Once again, we see clear eye opening of the demultiplexed tributary even for the scrambled case. The received power sensitivity at bit-error-rate (BER) of 10-9 is shown in Fig. 5(b) for all tributaries, while Fig. 5(c) shows the BER curves of the best (Ch 5) and worst (Ch 11) tributaries. In the back-to-back measurement, a 0.6-nm BPF is inserted at the 10-Gb/s transmitter, so that the FWHM of the received pulses is ~6 ps for all cases. We also confirm that the noise figure of the receiver preamplifier is constant at both 1545 nm and 1560 nm within ±0.2-dB measurement accuracy. From Figs. 5(b) and (c), error-free WC of the polarization-scrambled signal is achieved for all tributaries with 4.0- to 5.8-dB penalty. We assume that approximately 2 dB of the penalty comes from the MUX + DEMUX processes due to the relatively broad pulses employed in our experiment. The residual penalty in the WC process should be attributed to the non-optimized optical filters as well as insufficient input signal power to utilize the reshaping characteristic that this WC scheme potentially has [3]. We, thus, expect further reduction of the penalty by using a higher-power EDFA and/or proper optimization of the optical filters and CB-HNLF length.

10 Gb/s back-to-back (1545 nm)

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(a) Demultiplexed tributary with the input SOP fixed (left) and scrambled (right)

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(c) BER curves for the best (Ch5) and worst (Ch11) tributaries

Fig. 5. (a) Eye-diagrams of a demultiplexed tributary with the polarization scrambler OFF (left) and ON (right). (b) Receiver sensitivities for all tributaries with the polarization scrambler OFF (circles) and ON (dots). (c) BER curves for the best (Ch5, circles) and worst (Ch11, triangles) tributaries of the wave-length-converted signal with the polarization scrambler OFF (white) and ON (black).

4. Conclusions We have demonstrated 160-Gb/s pulsewidth-maintaining WC with 0.7-dB polarization sensitivity by using XPM-induced spectral broadening in a newly developed CB-HNLF. Error-free WC of a polarization-scrambled 160-Gb/s signal is realized, for the first time, without employing an automatic signal-polarization stabilizer. In addition to the simplicity and stability, the demonstrated scheme features the repolarizing functionality, where a depolarized signal is converted to a high-DOP light in an all-optical manner. Owing to the ultrafast response of the fiber XPM, the scheme should be scalable to higher bit-rate. Moreover, it can be readily upgraded to a polarization-insensitive 3R regenerator by replacing the CW light with a clock pulse train [12].

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