IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 16, NO. 7, JULY 2004 1739 A Novel Wavelength Shift Keying Transmitter Based on Optical Phase Modulation Wai Hung, Student Member, IEEE, Ning Deng, Student Member, IEEE, Chun-Kit Chan, Member, IEEE, and Lian-Kuan Chen, Member, IEEE Abstract—We propose a novel transmitter structure to generate constant-intensity wavelength shift keying signal based on optical phase modulation and demonstrate its application in a wavelength- division-multiplexed passive optical network to facilitate upstream data remodulation. The feasibility and system performance are ex- perimentally investigated. Index Terms—Passive optical networks (PONs), wavelength-di- vision multiplexing (WDM), wavelength shift keying (WSK). I. INTRODUCTION B INARY wavelength shift keying (WSK) is a modulation format which employs two optical wavelengths: One carries a nonreturn-to-zero (NRZ) data stream while the other synchronously carries the same data stream but in its complementary form. Therefore, the resultant composite signal exhibits constant intensity in time domain. It is sometimes also regarded as optical frequency-shift keying. As this modulation format has a constant optical intensity, it has been utilized in combating pattern effects induced by semiconductor optical amplified transmission link [1] and data remodulation [2] in a wavelength-division-multiplexed passive optical network (WDM-PON) with centralized light sources (CLS) [3]. When comparing with another popular constant-intensity modulation format, optical differential phase-shift keying (DPSK), WSK results in much easier management in the optical network unit (ONU) as only an optical bandpass filter is needed for demod- ulation, whereas optical DPSK requires an actively-controlled delay interferometer (DI). Conventionally, a WSK transmitter was implemented by combining two complementarily modulated data patterns at different wavelengths which required two discrete intensity modulators and precise delay between two intensity modulated signals [2]. Another scheme [1] made use of a proprietary dual-input intensity modulator driven by complementary data signals. Both schemes require two sets of modulator drivers, making both of the schemes expensive. In this letter, we propose a novel WSK transmitter based on optical phase mod- ulation. The transmitter utilizes only a straight-line LiNbO optical phase modulator and one set of modulator driver. This transmitter architecture poses significant cost savings over the Manuscript received February 13, 2004; revised March 19, 2004. This work was supported in part by a Grant from the Research Grants Council of Hong Kong SAR, China (Project CUHK4191/01E). The authors are with the Department of Information Engineering, The Chinese University of Hong Kong, Shatin, N.T., Hong Kong (e-mail: ckchan@ie.cuhk.edu.hk). Digital Object Identifier 10.1109/LPT.2004.828499 Fig. 1. Proposed PM-WSK transmitter based on optical phase modulation. Inset shows the periodic frequency response of a typical DI and the arrangement of the two input wavelengths. : relative arm delay difference of the DI. previous implementations, appearing as an attractive scheme for practical applications. The feasibility and system perfor- mance of the proposed WSK transmitter were experimentally investigated at 10 Gb/s. In addition, we demonstrated its application in a CLS-PON as the downstream transmitter to facilitate upstream data remodulation. II. PROPOSED WSK TRANSMITTER AND THE REMODULATION SCHEME IN WDM-PON Fig. 1 illustrates the operation principle of the proposed WSK transmitter. Two continuous-wave (CW) light beams with narrow wavelength spacing are combined and fed into a LiNbO optical phase modulator. Through the modulator, both of the wavelengths are phase modulated with the differentially precoded data signal at b/s at a phase modulation depth of rad. The output then passes through a DI with a relative arm delay approximately corresponding to the data period. The wavelengths of the two optical carriers are chosen in such a way that one wavelength ( ) coincides with the minimum transmission point of the DI, whereas the other wavelength ( ) coincides with the maximum transmission point (as shown in Fig. 1). This choice of wavelength is equivalent to the condi- tions: and , where is the propagation distance of one data bit in the DI; and are the respective refractive indexes experienced by and in the DI; and and are positive integers. Therefore, the DI effectively converts the phase-modulated optical signals on and into intensity-modulated ones. In addition, the outputs produced by these two wavelengths are complementary to each other. As a result, the combined output exhibits constant optical intensity. Such WSK signals can be easily demodulated by means of simple optical filtering. In order to ensure optimal modulation, the DI has to be temperature-controlled to stabilize its frequency response. For the rest of this letter, we address the signal generated by this scheme as WSK based on optical phase modulation (PM-WSK). 1041-1135/04$20.00 © 2004 IEEE