M.Sc Thesis


M.Sc StudentSydnev Artiom
SubjectUltra-Coherent Semiconductor Laser Locked to a Fiber
Mach-Zehnder Interferometer with Active Fiber
Noise Cancellation
DepartmentDepartment of Electrical and Computer Engineering
Supervisor PROFESSOR EMERITUS Gad Eisenstein


Abstract

The linewidth of semiconductor lasers, as the main measure of their coherence level, has been a subject of extensive research for several decades due to a vast number of applications for narrow linewidth lasers. This progress made highly coherent lasers, with linewidth on the order of single kHz, readily available off the shelf. However, to achieve higher level of stability these lasers must by coupled to an external cavity in order to diminish frequency noise fluctuations and reduce the linewidth even further. While many different approaches for laser stabilization have been demonstrated, narrowing the linewidth to single-Hz values always require external cavities with exceptional level of stability and can be performed only in highly controlled laboratory environment. In addition, the measurement of ultra-narrow linewidths becomes dubious, as classical self-heterodyne method fails to estimate the linewidth correctly, and reference sources with linewidth of this magnitude are rarely available.

In this master thesis we propose and demonstrate a technique to reduce the linewidth of a commercially available semiconductor laser operating at 1550 nm from 20 kHz to the regime of sub to single-Hz values by locking it to an all-fiber Mach-Zehnder interferometer with a short path length imbalance of 5 meters. Such locking stabilizes the inherent frequency noise of the laser but induces fiber noise in its place. In order to measure the noise resulting from the fiber fluctuations, a second laser is locked to a different fringe of the interferometer. In this configuration the beat-note signal between the two lasers includes contributions of the phase noise, as well as noise resulting from mechanical and thermal fiber fluctuations. The beat signal is then down converted to the DC region and serves as the error signal of a phase locked loop, that controls a piezo electric fiber stretcher stabilizing the MZI. The width of the beat note under locked conditions is in the sub to single Hz range, which means that the relative linewidth of each laser is of that order.