Ph.D Thesis


Ph.D StudentKodanev Anna
SubjectSelf-Coherent-Heterodyne System via Optical
Quadrature Front End for Frequency and
Intensity Noise Measurement
DepartmentDepartment of Electrical and Computer Engineering
Supervisors PROFESSOR EMERITUS Moshe Nazarathy
PROF. Meir Orenstein


Abstract

The frequency and the intensity noise of the laser sources are the limiting factor for satisfactory performance of many modern photonic applications. Coherent optical communication requires frequency-stable optical sources and precisely synchronized transmitter and receiver oscillators. Conventional methods of measuring laser frequency noise require a laser with similar or narrower linewidth or/and long delay-line incoherent interferometers, acousto-optic modulators and other complex and insufficiently inaccurate solutions. Some recent works in phase laser have shown promise in adopting delay-line interferometers terminated in coherent IQ-Hybrid (IQH), as an alternative to conventional passive optical combining.

We propose a novel self-coherent-heterodyne (SCOH-HET) laser measurement/stabilization system. The photonic front-end of our OFS consists of a Phase Modulator (PD) followed by coherent Delay-line Interferometer (DI), feeding the IQH input signal and reference to extract and digitize the I and Q quadrature components. The magnitude and instantaneous optical frequency variations are accurately sensed by applying   novel digital signal processing.

We develop comprehensive models and compare our scheme with two different state of the art self-coherent IQH-based schemes. The rigorous analysis of the impact of multiple types of impairments and their mitigation is a key topic of our work. We present an analysis of the photodetection shot-noise fundamental limits of the three systems and derive the degradation of the shot-noise limit in case receiver thermal noise is non-negligible. Moreover, we model and evaluate the impact of all dominant impairments affecting all three IQH-based systems, as well as proposing mitigation measures in the DSP and/or the optical front-end. In addition, for the first time we propose a novel method of measuring the differential group delay fluctuations, in order to generate the second-order statistics of these signals for the purpose of laser characterization.