Eye Openings in WDM Transmission Systems with Optical SSB Modulation Using a Hilbert Transformer and with Electrical Low-Frequency Band Equalization Kikuji Tanaka, 1 Katsumi Takano, 2 Kazuhiro Kondo, 2 and Kiyoshi Nakagawa 2 1 Photonics Research & Development Group, Tokyo Communication Equipment Mfg. Co., Ltd., Yonezawa, 992-0003 Japan 2 Faculty of Engineering, Yamagata University, Yonezawa, 992-8510 Japan SUMMARY A narrowband optical modulation model was devised based on an optical single sideband (SSB) modulator using a finite-order Hilbert transformer. The model’s wavelength multiplexing transmission characteristics were analyzed by evaluating the eye openings obtained by computer simula- tion. First, we evaluated a transmission model with no adjacent channels and found that the transmission eye open- ings after demodulation were degraded by the charac- teristics of the optical SSB modulation and the frequency response of a channel separation filter. The degradation of the eye openings could clearly be corrected by applying electrical attenuation equalization in the low-frequency band after baseband demodulation. Next, we evaluated the eye openings obtained by the optimum channel separation filter determined as a function of the order of the Hilbert transformer by evaluating the transmission model extended to three-channel multiplexing that includes adjacent chan- nels. It was found that applying electrical attenuation equalization improved the received signal characteristics as the order of the Hilbert transformer increased. © 2003 Wiley Periodicals, Inc. Electron Comm Jpn Pt 1, 86(5): 1–11, 2003; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/ecja.10058 Key words: optical SSB modulation; narrowband optical modulation/demodulation; Hilbert transformer; channel separation filter; electrical attenuation equaliza- tion. 1. Introduction Single sideband (SSB) modulation is a communica- tion scheme featuring a narrower transmitted signal band- width and lower transmitted power, and has been a research topic particularly in the wireless communication field for a long time. Over the past few years, these features have been effective even in the optical communication field that has focused on high-density wavelength multiplexed transmis- sion. In particular, superior effects such as more wavelength channels in wavelength division multiplexing (WDM) and a longer fiber propagation distance due to improved robust- ness to chromatic dispersion can be expected because the spectral efficiency can be increased. A recent paper discussed the construction of an inte- grated optical circuit prototype and the evaluation of an optical SSB modulator [1]. Other papers have reported that signal degradation caused by chromatic dispersion could be expected due to the electrical dispersion compensator after detection in single channel transmission using optical SSB modulation [2, 3]. The SSB signal generation method called phase shifting [4] that is used in this optical SSB modula- tion requires a 90° phase shifter called a Hilbert transformer for shifting with respect to the baseband signal. On the other hand, since the frequency response of the Hilbert trans- © 2003 Wiley Periodicals, Inc. Electronics and Communications in Japan, Part 1, Vol. 86, No. 5, 2003 Translated from Denshi Joho Tsushin Gakkai Ronbunshi, Vol. J85-B, No. 2, February 2002, pp. 159–168 1