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We propose a novel approach for implementing unipolar-encoding/bipolar-decoding for optical code division multiple access (CDMA). In the proposed system, an electrooptic phase modulator (EOPM) and two fiber Bragg grating (FBG) arrays are employed. At the transmitter, a low-bit-rate data sequence modulates the phase of optical carriers by the EOPM, and is then wavelength-mapped to a high-bit-rate optical phase sequence by the encoder FBG array in a unipolar way. At the receiver, the second FBG array acts as a series of frequency discriminators to convert the phase-modulated optical signals to intensity-modulated signals, as well as a matched filter to perform optical decoding. Bipolar decoding is achieved by locating the optical carriers at either the positive or the negative slopes of the reflection responses of the decoder. The proposed encoding/decoding scheme is equivalent to a sequence-inversion-keyed (SIK) CDMA, which has the potential to provide an improved performance compared with the conventional incoherent scheme using optical orthogonal codes. Both theoretical and experimental studies of the proposed SIK scheme are presented.