High power, high frequency mode-locked semiconductor lasers

Pusino, Vincenzo (2014) High power, high frequency mode-locked semiconductor lasers. PhD thesis, University of Glasgow.

Abstract

Integrated mode-locked laser diodes are effective sources of periodic sequences of optical pulses, which have always been of great interest for a range of spectroscopy, imaging and optical communications applications. However, some disadvantages prevent their widespread use, such as the restricted tuning of their repetition rate and their
output power levels never exceeding a few mW. This thesis reports on the work done to address those limitations. Two main findings are presented, the first being the generation of ultra-high repetition rate optical signals through external injection of two continuous wave signals. This mechanism is much simpler than other techniques previously
proposed to increase the repetition rate of monolithic modelocked laser, and has proved successful in generating optical signals up to quasi-THz. It is based on injection of two continuous wave signals whose spacing is an integer multiple of the pulsed cavity free spectral range and whose injection wavelengths coincide with two of the monolithic laser modes. This technique allows discrete tunability of the repetition rate with a step equal to the injected cavity free spectral range, and the injected laser has been shown to lock up to a repetition rate of 936 GHz, corresponding to 26 times that of the free-running semiconductor laser (36 GHz). The presented scheme is suitable for integration, opening the way for a successful on-chip generation of ultra-high repetition rate optical signals exploiting coupled cavity phenomena. The second main finding of this thesis regards the changes induced on the pulsed operation of monolithic passively mode-locked lasers by a blue bandgap detuning applied to their saturable absorber. The quantum well intermixing technique has been used for attaining an
area-selective bandgap shift on the fabricated chip, being fully postgrowth. The lasers with a detuned absorber were found to have an extended range of gain section currents and absorber voltages in which stable mode-locking operation took place. Furthermore, a comparison of mode-locked devices fabricated on the same chip, respectively with and without a bandgap detuned absorber, showed that the emitted pulses had greater peak power and were less affected by optical chirp
when the bandgap of the absorbing section was shifted. A new intermixing technique has also been developed as part of this work to address some inconsistencies of the pre-existing one; the newly introduced approach has been found to provide better spatial resolution and a more precise control of the attained bandgap shift.

• Item Type: Thesis (PhD)
• Qualification Level: Doctoral
• Keywords: Semiconductor Lasers, Mode-Locking
• Subjects: Q Science > QC Physics; T Technology > TK Electrical engineering. Electronics Nuclear engineering
• Colleges/Schools: College of Science and Engineering > School of Engineering > Electronics and Nanoscale Engineering
• Supervisor's Name: Sorel, Dr Marc
• Date of Award: 2014
• Depositing User: Mr Vincenzo Pusino
• Unique ID: glathesis:2014-5174
• Copyright: Copyright of this thesis is held by the author.
• Date Deposited: 22 May 2014 12:46
• Last Modified: 22 May 2014 12:47
• URI: https://theses.gla.ac.uk/id/eprint/5174