IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 17, NO. 8, AUGUST 2005 1683 Enhancing the Frequency Response of Cross-Polarization Wavelength Conversion Chia Chien Wei, Ming Fang Huang, and Jason (Jyehong) Chen Abstract—This work presents a novel wavelength conversion scheme, differential cross-polarization modulation (DXPoM), using an extra birefringence delay to enhance the performance of the conventional cross-polarization modulation. Simulation and experimental results confirm that the predicted performance is enhanced. Using the proposed scheme improves rise time by over 300%, reduces timing jitter by 50%, and increases extinction ratio by 9%. Index Terms—Birefringence, frequency conversion, semicon- ductor optical amplifiers (SOAs), wavelength-division multi- plexing (WDM). I. INTRODUCTION A LL-OPTICAL wavelength converters (AOWCs) are ex- pected to become key components in future wavelength- division-multiplexing (WDM) networks [1]. Wavelength con- verters will increase the flexibility and the capacity of WDM networks, and could be used in wavelength routers that manage wavelength paths through optical networks based on complex meshes, rather than point-to-point architectures. However, opto- electronic conversion methods, due to their bit-rate dependence, dramatically increase costs when the system is upgraded and the costs increase as the bit rate rises. Several AOWCs based on semiconductor optical amplifiers (SOAs) have been proposed, such as cross-gain modulation (XGM) [2], cross-phase mod- ulation (XPM) [2], four-wave mixing (FWM) [3], and cross- polarization modulation (XPoM) [4]. Each scheme has its own advantages and disadvantages. For example, FWM has low con- version efficiency, and the conversion speed of XGM, XPM, and XPoM are limited by the carrier’s recovery time. Among these parameters, conversion speed is considered to be the most im- portant factor; insufficient speed response causes larger timing jitter and, thus, limits cascadability [5]. This study presents a novel wavelength conversion scheme by adding an extra birefringence delay line in the standard XPoM method. While XPoM and XPM are both based on interferometric principle, time differential between two arms in XPM [6], [7] also works in XPoM. Therefore, with the extra delay line, the conversion speed of XPoM is enhanced signifi- cantly. Since the proposed approach adjusts the time differential of XPoM between transverse electronic (TE) and transverse Manuscript received October 12, 2004; revised Febuary 25, 2005. This work was supported by the National Science Council of the Republic of China, Taiwan, under Contract NSC 93-2215-E-009-027. The authors are with the Institute of Electro-Optical Engineering and Depart- ment of Photonics, National Chiao-Tung University, Hsin-Chu, Taiwan 300, R.O.C. (e-mail: mgyso.eo91g@nctu.edu.tw; mfhuang.eo90g@nctu.edu.tw; jchen@mail.nctu.edu.tw). Digital Object Identifier 10.1109/LPT.2005.851049 Fig. 1. Configuration of a DXPoM wavelength converter. magnetic (TM) modes, the new technique is referred to here as differential cross-polarization modulation (DXPoM). The new scheme functions as a 2R regenerator that provides both pulse reamplification and reshaping functions. Both simulation and experimental results corroborate that the predicted performance is enhanced. Compared to the conventional XPoM method, the experimental results for DXPoM improve rise time by over 300%, reduce timing jitter by 50%, and increase extinction ratio (ER) by 9%. II. OPERATING SCHEME AND SIMULATION Fig. 1 illustrates the configuration of DXPoM. As in a typ- ical XPoM, a continuous-wave (CW) probe laser beam at wave- length and a signal pump laser beam at wavelength were fed into an SOA. Proper control of the polarization states of and allowed the injected pump light to introduce additional birefringence in the SOA, and resulted in a differ- ential refractive index change between the TE and TM modes of the probe beam. At the polarizer, the two orthogonal modes are partially combined coherently. The proposed scheme and the conventional XPoM differ in that the proposed method adds an extra birefringence delay line in front of the polarizer. Based on the interferometric principle similar to the Mach–Zehnder inter- ferometer, the XPoM exploits the phase difference between the TE and TM modes when passed through an SOA. By adding an extra delay between TE and TM modes, the polarization state of the output CW beam after passing the delay line was rotated more rapidly with the variation of the signal power and, there- fore, overcome the speed limitation due to the carrier’s recovery time. Consequently, DXPoM was expected to have a better ER, a higher conversion speed, and a lower timing jitter. The large signal simulation is carried out to obtain details of DXPoM. In our simulation, the transfer matrix method is adopted and the basic rate equations are [8] (1) 1041-1135/$20.00 © 2005 IEEE