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Tolerance to optical filtering of a DPSK system can be increased significantly by decreasing the differential delay of the traditional MZI-demodulator to less than one bit-period. It is shown that a 43 Gbit/s DPSK signal can support 50 GHz channel spacing if the delay is decreased to 65% of a bit-period.
Introduction: DWDM fibre transmission systems operating at a channel rate of 40 Gbit=s are receiving increasing attention because of the prospect of higher fibre capacity and the promise of lower cost per transmitted bit. To be compatible with existing 10 Gbit=s transport architectures, the chosen 40 Gbit=s modulation format must have an excellent OSNR sensitivity and be tolerant to optical filtering since existing systems might include optical multiplexers and demultiplexers for 50 GHz channels spacing as well as many cascaded optical add-drop multiplexers. The phase-shaped-binary-transmission (PSBT) format has been considered because of its narrow spectrum [1] whereas the differential-phase-shift-keying (DPSK) format has been widely considered because of its excellent OSNR sensitivity [2, 3]. However, PSBT has a relatively poor OSNR sensitivity that depends on the applied optical filtering [1] and DPSK lacks the filter tolerance [4]. In this Letter, we suggest a modified implementation of DPSK that provides excellent OSNR sensitivity and filter tolerance simultaneously.
Traditionally, a DPSK receiver is realised using an optical demodulator with a one-bit delay between the two arms of a Mach-Zehnder interferometer (MZI). Here we demonstrate that decreasing the differential delay to less than one bit-period significantly increases the tolerance to narrow optical filtering. Specifically, we show that decreasing the delay to 2=3 of a bit-period permits DPSK to be transmitted with 50 GHz channel spacing while achieving excellent sensitivity. This modification also enhances the tolerance to chromatic dispersion.
Experimental setup: In this Letter, we refer to classical DPSK (CDPSK) when the differential delay, t, of the MZI-based demodulator is equal to the bit-period and we refer to partial DPSK (PDPSK) when t is less than the bit-period. When the delay is less than the bitperiod, a single bit partly interferes with its neighbour bit and partly with itself at the demodulator outputs, hence the name 'Partial differential PSK'. Note that the free-spectral-range (FSR) of an MZI-based demodulator is equal to 1=t.
The experimental setup used...