1 Metro-Access and Short Range Systems, Department of Photonics Engineering, Technical University of Denmark2 Department of Photonics Engineering, Technical University of Denmark3 Optical Transmission and Network Elements, Department of Photonics Engineering, Technical University of Denmark4 Nanophotonics Theory and Signal Processing, Department of Photonics Engineering, Technical University of Denmark5 High-Speed Optical Communication, Department of Photonics Engineering, Technical University of Denmark
We present a modelling technique and noise analysis of a clock recovery scheme based on an optoelectronic phase-locked loop. We treat the prob- lem using techniques from stochastic processes and stochastic differential equations. A set of stochastic differential (Langevin) equations describing the optoelectronic phase-locked loop are derived. By using small-signal analysis, the Langevin equations are linearized and the associated system of stochastic differential equations is solved using Fourier techniques. Nu- merical simulations are then used to investigate the performance of the optoelectronic phase-locked loop with noise at a bit-rate of 160 Gb/s. It has been shown that it is important to reduce the time delay in the loop since it results in the increased timing jitter of the recovered clock signal. We also investigate the requirement for the free-running timing jitter of the local electrical and optical oscillator. We show that it is possible to obtain recovered clock signal with less timing jitter then the input data signal as long as the jitter of the free-running electrical oscillator is less than the input data signal timing jitter. Using the guidelines form the nu- merical simulations, optoelectronic phase-locked loop based clock recovery operating at 320 Gb/s is demonstrated. Optical regenerator with clock recovery, based on an optoelectronic phase- locked loop, is also described using techniques from stochastic calculus. An analytical expression for the power spectral density of the retimed data sig- nal is derived. We use numerical simulation to investigate the performance of the optical regenerator operating at 40 Gb/s and 160 Gb/s. We have shown that for flat-top input data signal pulses and sufficiently narrow op- tical clock signal pulses, the timing jitter of the retimed data optical data signal can be significantly reduced compared to the jitter of the degraded input data signal. The optical clock signal pulse width needs to be rela- tively short compared to the optical data signal pulse width in order for the retimed data signal timing jitter to coincide with the recovered clocktiming jitter, i.e. 3.5 ps at 40 Gb/s and 0.5 ps at 160 Gb/s. In the last part of the thesis, a novel phase-locked coherent optical phase demodulator with feedback and sampling, to be used in phase-modulated radio-over-fibre optical links, is also presented, theoretically investigated and experimentally demonstrated. It is experimentally shown that the proposed approach results in 18 dB of spur-free-dynamic range improve- ment compared to a traditional demodulator without feedback. A new time-domain, large signal, numerical model of the phase locked coherent demodulator is developed and shown to be in excellent agreement with ex- perimental results. Numerical simulations are used to investigate how loop gain, LO phase-modulator non-linearities, amplitude modulation, ampli- tude and timing jitter influence the dynamical behavior of the demodulator in terms of the signal-to-intermodulation ratio and signal to-noise ratio of the demodulated signal. Furthermore, in order to alleviate non-linearities associated with the LO phase-modulator, we report on a novel technique for cancelation of the 3rd order intermodulation product of the demodulated signal. The proposed cancelation technique does not depend on input RF signal power and frequency.