Monolithic Colliding Pulse Mode-Locking of AlGaAs/GaAs and InGaAs/InGaAsP Quantum Well Lasers

McDougall, Stewart Duncan (1997) Monolithic Colliding Pulse Mode-Locking of AlGaAs/GaAs and InGaAs/InGaAsP Quantum Well Lasers. PhD thesis, University of Glasgow.

Abstract

The fabrication of T-section colliding pulse mode-locked (T-CPM) GaAs/AlGaAs quantum well lasers is described. These devices have MOVPE grown ridge waveguide laser structures, with monolithically integrated side-injection waveguides, orthogonal to the laser waveguide, passing through the saturable absorber from the edge of the laser chip. This orthogonal waveguide is included to enable synchronisation applications which involve the injection of external optical pulse streams into mode-locked diode lasers. Intensity autocorrelations are performed on these devices, the first time for monolithic CPM lasers in this material system, showing a 400 mum long device mode-locked at a repetition rate of 175 GHz producing near transform-limited 1 ps pulses at wavelengths around 0.87 mum. A new technique for measuring the gain and absorption spectra in semiconductor material is presented, which utilises the multi-section laser fabrication technology developed for the CPM lasers. Using this new technique the TE and TM absorption spectra of the MOVPE grown T-CPM laser saturable absorber is measured in situ under the influence of increasing reverse bias, showing large exciton absorption features, which shift and broaden in accordance with the QCSE. With the measurement of the forward bias gain spectra, the sub-linear form of the gain - current density relationship was directly measured for the mode-locked laser wafer. The crucial role of quantum well doping in the achievement of high repetition rate monolithic mode-locking has been demonstrated by the realisation of short cavity CPM and multiple colliding pulse (MCPM) lasers in MBE grown GaAs/AlGaAs quantum well material only through background doping of the active region to simulate the effects of the residual carbon impurities incorporated in MOVPE grown AlGaAs. By inspection of the absorption spectra, it is shown that a distinct broadening of the exciton absorption peaks occur with the presence of background doping in MBE material. This indicates that the corresponding decease in the exciton lifetime has a beneficial effect on saturable absorber recovery, allowing high repetition mode-locking to take place. Low damage dry etching and wet etching techniques have been developed to enable the successful fabrication of ridge waveguide CPM lasers in InP/InGaAs/InGaAsP laser material. Spectral measurements show the mode-locked operation of CPM and T-CPM lasers in strained and lattice-matched material at wavelengths around 1.5 mum. Also, spectral results are presented showing harmonic CPM operation at repetition rates of 150 GHz and 300 GHz (two and four pulse CPM action), from a 600 mum MCPM laser (emitting at a wavelength of 1.53 mum). This is the first reported demonstration of an MCPM laser diode in the long wavelength region around 1.55 mum.

• Item Type: Thesis (PhD)
• Qualification Level: Doctoral
• Additional Information: Adviser: Charles Ironside
• Keywords: Electrical engineering
• Date of Award: 1997
• Depositing User: Enlighten Team
• Unique ID: glathesis:1997-75292
• Copyright: Copyright of this thesis is held by the author.
• Date Deposited: 19 Nov 2019 21:19
• Last Modified: 19 Nov 2019 21:19
• URI: https://theses.gla.ac.uk/id/eprint/75292