Narrow Filtered DPSK Implements Order-1 CAPS Optical Line Coding

Abstract

A novel family of optical line codes has been presented elsewhere, here referred to as combined amplitude-phase shift (CAPS) codes. We show here that narrow filtering of a differential phase shift keying signal with bandwidth equal to about 2/3 of the bit rate turns out to closely implement the order-1 CAPS line coding. Performance of the two systems is compared for various types of optical filters.

Simultaneous Demodulation and Clock-Recovery of 40-Gb/s NRZ-DPSK Signals Using a Multiwavelength Gaussian Filter

“…The BER performance for this modulation format was then almost constant up to 150-ps/nm total dispersion. We see from the insets the evolution of the eye diagrams showing eye opening in propagation as expected from [4]. Penalty starts to increase at 200 ps/nm.…”

“…1(a)] [3]. It works selecting the part of the signal spectrum corresponding to phase persistences while rejecting the phase transitions, which leads to phase-to-intensity conversion [2]- [4]. The OSNR sensitivity at bit-error rate (BER) for a number of sample wavelengths in the -band with a PRBS sequence is reported in Fig.…”

“…In respect to this value, we measured an OSNR penalty ranging from 1.5 to 3.7 dB. Part of this penalty ( 1.5 dB) is known to come from the narrow filtering technique [4]. All the measurements at this point are made using a common electrical CR circuit.…”

“…The additional penalty comes from slight filter bandwidth and shape inhomogeneities (e.g., filter bandwidth slightly increase at longer wavelengths from 24 to 28 GHz). However, despite this back-to-back OSNR penalty, the signals show a significantly larger resilience to chromatic dispersion compared to both conventional OOK and usual DPSK demodulated by the conventional one-bit delay interferometer [2]- [4]. Using different spools of fiber, we analyzed the resilience to chromatic dispersion of the narrow-filtered DPSK compared to the usual NRZ-OOK.…”

“…We recently reported that 10-Gb/s nonreturn-to-zero (NRZ)-DPSK signals can also be demodulated using a Gaussian-shaped optical filter having bandwidth of around 60% of the bit rate [3]. When using this demodulation scheme, we actually obtain an optical duobinary-like or CAPS code modulation format, largely increasing the signal tolerance to chromatic dispersion [2]- [4]. To accomplish filtering demodulation, we used a fiber Bragg grating (FBG) filter in reflection centered at the carrier wavelength.…”