1832 IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 22, NO. 24, DECEMBER 15, 2010 Wavelength Converter for Polarization-Multiplexed 100-G Transmission With Multilevel Modulation Using a Bismuth Oxide-Based Nonlinear Fiber Ming-Fang Huang, Jianjun Yu, Yue-Kai Huang, Ezra Ip, and Gee-Kung Chang Abstract—We demonstrate a four-wave-mixing-based wave- length converter suitable for coherent optical transmission at 100 Gb s and beyond that uses phase-diversity and noncon- stant amplitude modulation formats. Our setup consists of 2 m of bismuth oxide-based nonlinear fiber and two copolarized external cavity pump lasers. We demonstrate wavelength conversion of 112-Gb/s polarization-multiplexed return-to-zero quadrature phase-shift keying and 144-Gb/s 16-ary quadrature-amplitude modulation signals. Less than 0.4-dB optical signal-to-noise ratio penalty at BER due to wavelength conversion was measured. Index Terms—Bismuth oxide-based nonlinear fiber (Bi-NLF), coherent detection, four-wave mixing (FWM), phase-shift keying, quadrature-amplitude modulation (QAM), wavelength conversion (WC). I. INTRODUCTION T HE popularity of high-definition video-on-demand and peer-to-peer multimedia file sharing has resulted in ever-increasing bandwidth demand on core and access networks. Future dense-wavelength-division multiplexed (DWDM) optical networks will transmit at data bit rates of 100 Gb s and beyond. To achieve high spectral efficiency, polarization-multiplexed (PolMux) transmission will be used as it enables the data rate per channel to be doubled with little increase in system complexity. To improve network efficiency, reconfigurability and system robustness, future networks will also employ multidegree, colorless, and directionless reconfigurable add/drop multiplexers (ROADMs). One of the enabling technologies that facilitate wavelength reuse and nonblocking ROADMs is all-optical wavelength conversion (AOWC) [1]–[6]. AOWC using self-phase modulation (SPM), cross-phase modulation (XPM), four-wave mixing (FWM), and cross-gain modulation (XGM) in optical fiber have been demonstrated [7]–[9]. The FWM method is one of the most promising as it is transparent to the signal’s baud rate and modulation format, and does not result in spectral inversion [10], [11]. Until recently, silica-based highly nonlinear fiber (HNLF) diameter was used as the nonlinear medium for the Manuscript received May 03, 2010; revised June 12, 2010; accepted October 11, 2010. Date of publication October 28, 2010; date of current version De- cember 02, 2010. M.-F. Huang, Y.-K. Huang, and E. Ip are with NEC Laboratories America, Inc., Princeton, NJ 08540 USA (e-mail: mhuang@nec-labs.com). J. Yu and G.-K. Chang with the School of Electrical and Computer Engi- neering, Georgia Institute of Technology, Atlanta, GA 30332 USA. Color versions of one or more of the figures in this letter are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/LPT.2010.2088384 Fig. 1. Four-level electrical signal generation. (Att.: attenuator; MZM: Mach–Zehnder modulator; OC: optical coupler; AWG: arrayed waveguide grating.) Inserted eye diagrams are (a) two-level and (b) four-level signals. WC [12]. However, due to the small nonlinear coefficient of HNLF, high power intensity is required to obtain good conver- sion efficiency. The usable pump power is ultimately limited by stimulated Brillouin scattering (SBS). Bismuth oxide-based nonlinear fiber (Bi-NLF) has increased nonlinearity and high SBS threshold, and has emerged an attractive medium for fiber-based nonlinear optical devices [11]. Devices fabricated using Bi-NLF can be more compact and power-efficient. Recently, WC of multigigabit/s signals using constant-ampli- tude modulation formats such as PolMux return-to-zero -ary differential phase-shift keying (PolMux-RZ- PSK) have been reported [4]–[6]. For nonconstant amplitude modulation formats such as quadrature-amplitude modulation (QAM), it is critical to have linear amplitude response to avoid distor- tion of the signal constellation. In this letter, we demonstrate a Bi-NLF-based WC using a copolarized pumping scheme that was developed in [1] and [2]. We present experimental results for PolMux-RZ-QPSK and PolMux-RZ-16-QAM using coherent detection. To our knowledge, this is the first time that WC has been demonstrated for nonconstant amplitude modulation formats at a data rate above 100 Gb s per . II. FOUR-LEVEL ELECTRICAL SIGNAL GENERATION Fig. 1 shows the setup used to generate a high-quality four- level electrical signal at 18 Gbaud. Two distributed-feedback (DFB) lasers at 1552.87 and 1553.68 nm were combined in an optical coupler (OC) and modulated with a Mach–Zehnder in- tensity modulator (MZM) driven by an 18-Gb/s binary electrical signal produced by multiplexing two pseudorandom bi- nary sequences (PRBS) with 5-bit delay for decorrelation. The 1041-1135/$26.00 © 2010 IEEE