Optoelectronic receiver in short reach optical fiber communications

Communication systems with the information encoded in the amplitude of the carrier and a receiver based on direct detection are called intensitymodulated and direct detected (IM-DD) systems. For IM-DD systems, the chromatic dispersion (CD) is one of the main impairments operating with single-mode fiber at the frequency region of 1.55 µm. The reason is that the interaction of intensity-modulated signals and the square-law detection of the photodetectors (PD) create nulls in the received signal’s spectrum — severely impacting the system performance. These nulls are known as the power fading effect. This thesis contributes to the state-of-the-art equalization techniques for IM-DD systems with digital-only, optical-only, and optoelectronic equalizers. For digital-only equalizers, we show original contributions by proposing and studying reservoir computing for IM-DD systems. We point out the advantage in the training process of the algorithm, which can be beneficial for dynamic systems, such as optical communications. Numerical analyses of the reservoir’s memory capacity and equalization performance are given and compared to time-delay neural networks. For optical-only equalization, we show the use of multiple Mach–Zehnder delay interferometer (MZDI) for mitigating the CD in the optical domain, which avoids the power fading effect. The original contribution is training the time delay and the phase shift components that mitigates the CD through an adaptive algorithm that tries to reduce the loss function of the system. For the optoelectronic system, we have proposed two directions. The first is using the same structure used for optical-only equalization for CD mitigation, together with nonlinear equalizers — the original contribution is in the joint training of time delay and phase shift of the optical structure and the nonlinear equalizer. The other direction is also an original contribution where we propose that the optical signal is divided into smaller subbands (through optical filters), and an individual PD detects each. A nonlinear equalizer is then used to reconstruct the signal and recover the information.