452 IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 9, NO. 4, APRIL 1997 Wavelength Conversion up to 18 nm at 10 Gb/s by Four-Wave Mixing in a Semiconductor Optical Ampliﬁer David F. Geraghty, Robert B. Lee, Kerry J. Vahala, Member, IEEE, Marc Verdiell, Member, IEEE, Mehrdad Ziari, Member, IEEE, and Atul Mathur, Member, IEEE Abstract—We characterize the conversion bandwidth of a four- wave mixing semiconductor optical ampliﬁer wavelength con- verter. Conversion of 10-Gb/s signals with bit-error-rate (BER) performance of 10 is demonstrated for wavelength down- shifts of up to 18 nm and upshifts of up to 10 nm. Index Terms—Communication systems, frequency conversion, optical mixing, semiconductor optical ampliﬁer. A WORLDWIDE consensus is emerging on the use of wavelength-division multiplexing (WDM) technologies for more effective utilization of bandwidth in existing and future ﬁber-optic telecommunications infrastructure. A key element in the implementation of all-optical WDM systems is a wavelength converter [1]. Wavelength converters have been demonstrated utilizing a wide variety of mechanisms [2]. Optoelectronic, cross-gain saturation, and cross-phase saturation wavelength converters are candidate technologies that have demonstrated excellent performance. However, all of these converters suffer from the limitation that they are not transparent to either bit-rate or modulation format. Complete transparency is offered only by ultra-fast wave mixing tech- niques. Wavelength conversion by four-wave mixing (FWM) has been demonstrated in single-mode ﬁber [3], semiconductor lasers [4], and semiconductor optical ampliﬁers (SOA’s) [5], [6]. Of these FWM converters, only the SOA allows a tunable pump source. For a wavelength converter to allow completely ﬂexible switching between channels in a WDM system, a tunable pump is required in order to provide access to a continuous range of converted signal wavelengths. A central issue to the implementation of FWM SOA wave- length converters is conversion efﬁciency. With third order susceptibilities ﬁve to seven orders of magnitude larger than that of silica ﬁber, it is possible to obtain useful conversion efﬁciencies in SOA’s that are less than 1 mm in length [7]. The efﬁciency decreases with increased signal-pump wave detun- ing (more rapidly for wavelength upshifts than for downshifts due to destructive phase interference between the contributions Manuscript received October 7, 1996; revised December 11, 1996. This work was supported by DARPA under Contract DAAL 01-94-K-03 430 and the National Science Foundation under Grant ECS-9 412 862. D. F. Geraghty, R. B. Lee, and K. J. Vahala are with the Department of Applied Physics, California Institute of Technology, Pasadena, CA 91125 USA. M. Verdiell, M. Ziari, and A. Mathur are with SDL, San Jose, CA 95134 USA. Publisher Item Identiﬁer S 1041-1135(97)02448-8. from the multiple mechanisms participating in the FWM process [8]). This, in turn, can prevent detection of the converted signal with a low bit-error rate (BER). Previous work has demonstrated conversion of 622-Mb/s signals over 20 nm [5], and 10-Gb/s signals over 4 nm [6]. Here, we demonstrate and characterize wavelength conversion of 10- Gb/s signals over a record 18-nm downshift and a 10-nm upshift. To our knowledge, this is the ﬁrst reported system demonstration with a wavelength upshift by SOA FWM. In addtion, we characterize the converter’s power margin, or the difference between the in-ﬁber converted signal power at the SOA output and the power required at that wavelength shift for 10 BER performance. The wavelength converter is shown in Fig. 1. The pump source is a tunable, external-cavity semiconductor laser with about 3 dBm in-ﬁber power. This pump source and the input signal are coupled together in a bidirectional coupler (BDC) after each individually goes through a mechanical polarization controller. The combined signals are then ampliﬁed in a high- power erbium-doped ﬁber ampliﬁer (EDFA). The ampliﬁed signals are coupled through a 10-nm-wide bandpass ﬁlter (BPF). This is done to suppress the ASE from the EDFA in the spectral region of the converted signal. The ASE preﬁltering, ﬁrst described and demonstrated by Zhou et al [9], provides an increase in the optical SNR of the converted signal of over 5 dB. After ASE preﬁltering, the pump and input signal are coupled into the SOA with a combined power of approximately 13 dBm. The SOA is a ﬁber pigtailed unit from SDL based on a multiquantum-well compressively strained gain medium with 25-dB ﬁber-to-ﬁber gain. The wavelength conversion efﬁciency is polarization dependent, and both the pump and probe polarizations must be aligned to the TE axis of the SOA. Following the SOA, these signals are coupled through a 1-nm-wide BPF to suppress the pump and input signal at the wavelength converter output. To characterize the performance of this converter, it is introduced into the simple optical link shown in Fig. 1. Signal generation, error detection, and eye analysis are done with a 10-Gb/s bit-error-rate tester (BERT) and a microwave tran- sition analyzer. The BERT generates a 10 Gb/s NRZ PRBS which is ampliﬁed and used to drive a LiNbO Mach–Zehnder external modulator. This modulates the optical signal source, which in this case is a distributed feedback laser (DFB). The ﬁxed wavelength of the DFB prevents us from being able to compare the converted signal performance to a baseline, 1041–1135/97$10.00  1997 IEEE