This paper evaluates several signal processing techniques for optical dual-polarization quadrature phase-shift keying (DP-QPSK) coherent receivers, using offline post-processing of experimental data. In addition to electronically separating the two polarization multiplexed signals, the investigated signal processing algorithms also compensate for chromatic dispersion (CD) and polarization mode dispersion (PMD). In particular, we investigate two distinct architectures: 1) Constant modulus algorithm (CMA) equalization followed by feedforward phase recovery. 2) Decision-directed least mean square (DD-LMS) equalization combined with decision-directed phase recovery. The interplay of both architectures with bulk dispersion compensators and carrier recovery algorithms is also analyzed.

Nonlinear phase noise can severely degrade the performance of multicarrier optical signals by introducing both inband and out-of-band distortion. In this paper we study analytically the impact of nonlinear phase distortion on CO-OFDM (Coherent Optical Orthogonal Frequency Division Multiplexing) schemes with large number of subcarriers. By using the Bussgang theorem, we derive the power spectral densities of the useful and the distortion term of the nonlinearly distorted signal. The traditional approach in conventional CO-OFDM implementations is to treat nonlinear distortion as an undesirable noise term that leads to performance degradation. However, it can be shown that this distortion has information on the transmitted signals that can be used to improve the performance. Surprisingly, we show that the distortion inherent to the nonlinear phase noise can lead to performance improvements relatively to the ideal, linear CO-OFDM transmissions. We present a closed-form expression for the average asymptotic gain associated to the optimum detection of nonlinearly distorted CO-OFDM signals and we derive the approximated bit-error-rate using the distribution of the the asymptotic gain.

In this paper we report the use of a monolithic system that combines the demultiplexing operation with the simultaneous photodetection and self amplification of the signal. The device is a double pi’n/pin a-SiC:H heterostructure with optical gate connections for light triggering in different spectral regions. Results show that when a polychromatic combination of different pulsed channels impinges on the device the output signal has a strong nonlinear dependence on the light absorption profile, (wavelength, bit rate and intensity). This effect is due to the self biasing of the junctions under unbalanced light generation of carriers. Self optical bias amplification under uniform irradiation and transient conditions is achieved. An optoelectronic model based on four essential elements: a voltage supply, a monolithic double pin photodiode, optical connections for light triggering, and optical power sources for light bias explains the operation of the optical system.