DOI: 10.25549/usctheses-c127-675092
title: Electronically assisted relative and absolute phase control of semiconductor lasers

description: Electronically assisted precise phase control of the output of semiconductor lasers (SCLs) is explored in this thesis. Coherent power combining of multiple semiconductor lasers, phase modulation or demodulation, and steerable laser arrays are the potential applications that require accurate frequency locking and relative phase adjustment of SCLs. Electro-Optical Phase Locked Loops (EOPLL), the equivalent of electrical Phase Locked Loops in the optical domain, can be used to frequency and phase lock an SCL to a reference laser. However, undesired effects such as optical propagation delay in the loop, laser phase noise, and the non-uniform FM response of SCLs, limit the EOPLL bandwidth and acquisition range and make the design of a robust EOPLL rather challenging. After studying these undesired effects, architectures are proposed that can electronically correct for the performance deterioration caused by these undesired optical effects. An integrated version of the electronic circuitry of an EOPLL is designed and implemented on a 130nm CMOS process, where the proposed frequency Aided Acquisition System improves the acquisition range of the EOPLL from 14MHz to 1GHz. The phase of a locked SCL in a heterodyne EOPLL can be adjusted electronically. Therefore, array of SCLs can be locked using heterodyne EOPLLs to a reference laser and coherent optical power combining and electronically controlled optical beam steering can be performed. Two architectures are proposed for coherent power combining of two SCLs and 94% combining efficiency is experimentally demonstrated. An SCL with low phase noise is highly desired in many applications such as coherent optical communication, interferometric sensing, LIDAR, and mm-wave signal generation. Since the invention of lasers, scientists and engineers have demonstrated different methods to reduce the linewidth of lasers. The most commonly used phase noise reduction schemes may be categorized into three classes; optical feedback, electrical feedback, and phase locking to a clean laser. The common issue among all mentioned phase noise reduction schemes is that first, they are light source dependent, that is, the laser is part of the phase noise reduction system and the design is done around the laser characteristics such as its FM response, and second, due to feedback action, large phase noise cancellation limits the phase noise cancellation bandwidth. The limited phase noise cancellation bandwidth could be an important concern in some applications such as coherent communication and low phase noise THz signal generation. New laser phase noise cancellation architectures based on electro-optical feed-forward technique are proposed. In the feed-forward technique, the laser phase noise is measured and subtracted from the phase of the laser in a feed-forward configuration. The experiments on a commercially available 1550nm Distributed feedback laser show linewidth reduction from 7.5MHz to 1.8KHz without using large optical cavity resonators. The feed-forward phase noise reduction scheme performs wideband phase noise cancellation independent of the light source and as such, it is compatible with the original laser source tunability without requiring tunable optical components. By placing the proposed phase noise reduction system after a commercial tunable laser, a tunable coherent light source with KHz linewidth over a tunning range of 1530nm-1570nm is demonstrated.