Differential phase shift keyed transmission (DPSK) is currently under serious consideration as a deployable datamodulation
format for high-capacity optical communication systems due mainly to its 3 dB OSNR advantage over
intensity modulation. However DPSK OSNR requirements are still 3 dB higher than its coherent counter part, PSK.
Some strategies have been proposed to reduce this penalty through multichip soft detection but the improvement is
limited to 0.3dB at BER 10-3. Better performance is expected from other soft-detection schemes using feedback control
but the implementation is not straight forward. We present here an optical multipath error correction technique for
differentially encoded modulation formats such as differential-phase-shift-keying (DPSK) and differential polarization
shift keying (DPolSK) for fiber-based and free-space communication. This multipath error correction method combines
optical and electronic logic gates. The scheme can easily be implemented using commercially available interferometers
and high speed logic gates and does not require any data overhead therefore does not affect the effective bandwidth of
the transmitted data. It is not merely compatible but also complementary to error correction codes commonly used in
optical transmission systems such as forward-error-correction (FEC). The technique consists of separating the
demodulation at the receiver in multiple paths. Each path consists of a Mach-Zehnder interferometer with an integer bit
delay and a different delay is used in each path. Some basic logical operations follow and the three paths are compared
using a simple majority vote algorithm. Receiver sensitivity is improved by 0.35 dB in simulations and 1.5 dB
experimentally at BER of 10-3.
Differential-phase-shift-keyed optical modulation (DPSK) has generated a lot of attention in fiber optic transmission over the past few years mainly because of its 3dB optical signal-to-noise ratio (OSNR) improvement over standard intensity modulated transmission  offering high receiver sensitivity, high tolerance to major nonlinear effects in high-speed transmissions , and high tolerance to coherent crosstalk .
To demodulate DPSK, a delay-line interferometer is usually employed to provide a one-bit delay such that a bit interferes on the following bit to provide constructive or destructive interference depending on the phase difference . However, with the use of logical pre-coding a multi-bit delay can be used instead of a single-bit delay. It was recently reported that a two- or four-bit delay might be advantageous in allowing polarization interleaving between bits to lessen the detrimental effects of fiber nonlinearities in fiber optic transmission [5-7]. Multi-bit delay was also proposed as a method to correct errors from amplified spontaneous emission (ASE) noise-limited transmission [8-9].
We present here simulation and experimental results on the penalty of using multi-bit delay demodulation for DPSK detection. We present Q-factor degradation as a function of delay and find that the Q penalty scales with 0.5 x delay for integer delays. We also present results of the detrimental effect of spectral filtering from the reduced free-spectral-range (FSR) in a multi-bit delay interferometer. We also find that it exhibits reduced dispersion tolerance. If not taken into account, these important limitations and more stringent tolerance on the frequency offset may reduce the effectiveness of multi-bit delay methods and prohibit practical implementation.