High-Speed Optical Sampling Techniques Enabled By Ultrafast Semiconductor Lasers

Abstract

In this thesis, novel optical sampling techniques are theorised and demonstrated using quantum dot two-section passively mode-locked laser diodes (MLLDs) as an inexpensive and compact source of ultrashort light pulses on the order of a few picoseconds duration.

The first technique, OSBERT (Optical Sampling By Electronic Repetition-Rate Tuning) is a single-laser optical sampling technique which was demonstrated using a MLLD whose repetition rate may be modulated electronically by varying the reverse bias applied to the saturable absorber section, whilst a constant forward bias is applied to the gain section. This is compared to standard repetition rate tuning which is typically performed by varying an internal or external laser cavity length, often using a motorised mirror. Splitting the pulse train into two copies and directing them through a highly imbalanced Michelson interferometer then gives rise to a temporal scan range which is proportional to the repetition rate modulation, scaled by the length of the passive delay line (the longer of the two interferometer arms). The technique was proven as an entirely non-mechanical alternative to traditional motorised translation stage methods of optical sampling as both a single and a two-photon absorption cross-correlator. Whilst scan ranges of up to 200ps were realised, the OSBERT method also successfully demonstrated distance measurements at scan rates of 10kHz, where target displacements between 100μm and 1.0mm were detected at effective free-space distances of over 3m and 36m respectively, exhibiting accuracies of 10μm and 100μm respectively. Finally, the ability to detect cross-correlations at up to 1MHz scan rates was shown, suggesting great potential for MHz level scan rates under future development.

The second technique, SLECOPS (Single Laser Electronically Controlled OPtical Sampling) was developed from a theoretical basis. Whilst OSBERT is a technique specifically invented for the use of MLLDs, SLECOPS is potentially suited to any type of laser whose repetition rate may be varied via square wave modulation, and thus could be adopted by a vast variety of laser systems and their respective optical sampling applications. The theory outlines the potential to contend with the OSBERT technique in scan rate and in minimising characteristic laser output differences between pulse pairs used during each cross-correlation event.

SLECOPS is particularly interesting in that the technique involves asynchronous-type scanning (such as that used in two-laser sampling systems such as Asynchronous Optical Sampling and Electronically Controlled Optical Sampling) but uses only one laser. This led to the conceptualisation of the ‘virtual laser’, where just one laser can be said to function as two within the context of optical sampling.

The selectable scan range and competitive scan rates of both OSBERT and SLECOPS as single laser optical sampling techniques thus pave the way for two section passively MLLDs to be deployed in fast, low-cost and compact optical sampling systems.

Date of Award	2016
Original language	English
Sponsors	Philip Leverhulme Prize
Supervisor	Maria Cataluna (Supervisor)