Optical differential phase-shift keying (DPSK) is a promising modulation format that offers high receiver sensitivity, high tolerance to major nonlinear effects in high-speed transmissions, and high tolerance to coherent crosstalk. Optical DPSK modulation includes differential binary phase shift keying (DBPSK), differential quadrature phase shift keying (DQPSK), and other related format variants.
In optical DPSK transmission, data information is carried by the optical phase difference between adjacent bits. For direct detection of an optical DPSK signal by conventional intensity detectors a demodulator for converting the phase-coded signal into an intensity-coded signal is needed. Such a demodulator is typically a delay interferometer. The free spectral range (FSR) of the interferometer is the inverse of the delay. In optical 1-bit delay interferometers (O-1bit-DI), the FSR is equal to the SR, where SR is the symbol rate of the DPSK signal to be demodulated.
The SR of an OC-768 DBPSK signal is usually 40 GHz (or 42.7 GHz when a 7% overhead is used to for forward-error correction), and the SR of an OC-768 DQPSK signal is usually 20 GHz (or 21.3 GHz). In wavelength-division multiplexed (WDM) systems which adhere to ITU-T Recommendation G.692, the minimum frequency channel spacing between two WDM channels on the ITU grid is 50 GHz, which is not equal to the FSR of a O-1bit-DI commonly used for decoding OC-768 DBPSK signals as discussed above. Thus, an O-1bit-DI cannot be used for demodulating any one of the channels on the ITU grid without readjusting (i.e. tuning) the passband center frequency of the O-1bit-DI. The center frequency readjustment requires sophisticated monitoring and feedback control, which increases the complexity and cost for the DPSK demodulation.
Conventional O-1bit-DIs are typically based on an all-fiber designs or planar lightwave circuit (PLC) designs. These designs are intrinsically temperature sensitive since the index of refraction of the material used to construct these interferometers (i.e. the optical paths) is temperature dependent. Thus, the temperature-induced optical phase changes of signals propagated in the optical paths of the ODI, which are different in length in order to obtain the 1-bit delay, are different. Consequently, precise control of the phase difference between the two optical paths of the ODI is required. To precisely control the phase difference between the optical paths, accurate temperature control and stabilization of the ODI are required, which significantly adds to the cost and complexity of the ODI.