Date of Award


Document Type

Doctoral Thesis

Degree Name

Doctor of Philosophy


Physical Sciences

First Advisor

Dr. Ian O'Driscoll

Second Advisor

Dr. Guillaume Huyet


In this thesis, the effect of operating two-section InAs quantum dot passively mode- locked semiconductor lasers in the random population regime is examined in detail in an attempt to achieve wide optical bandwidths and ultrashort optical pulses. It is demonstrated in this work that large increases in optical bandwidths can be observed in the random population regime. Random population was achieved with an undoped device by lowering the operating temperature to 20 K. An emission bandwidth of 27 nm was measured using an undoped device at 20 K, with corresponding pulses of 290 fs, shorter than any previously reported with a standard two-section passively mode-locked quantum dot device.

In order to unlock the full potential of the random population regime and attempt to achieve the widest bandwidths and shortest pulses possible, without changes to the growth structure of the devices, a detailed study was performed on the relationship between the measured optical gain spectra, mode-locked optical emission spectra, and cavity loss as a function of temperature. By operating lasers with the same growth structure in the random population regime, it is shown that optimising the parameters intrinsic in the make-up of the laser can have a dramatic effect on the optical bandwidth and on the width of the optical pulses measured. At 80 K, for two devices with gain mediums of different lengths, a doubling of the optical bandwidth and a 30-40 % decrease in pulsewidths were observed in the longer device, compared to the shorter device.

Random population in an undoped device was confirmed at 20 K, but in order to demonstrate random population at elevated temperatures, mode-locked lasers that had been doped with p and n dopants were used. It is shown that random population was achieved at 100 K with these doped devices, with optical bandwidths of 53 nm noted at this temperature. Short pulses were successfully detected with p-doped devices at elevated temperatures compared to the undoped devices. Additionally, optical bandwidths of 38 nm were observed at 300 K using n-doped quantum dot devices, more than four times the bandwidth found using undoped devices at the same temperature.

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