Monolithic dual-wavelength InP/AlGaInP quantum dot lasers

This thesis describes the development of a monolithic dual-wavelength laser based on an InP/AlGaInP quantum dot (QD) laser structure. Each wavelength is sourced from the same active region and can be operated simultaneously or independently, with light being emitted from a common aperture. The inhomogeneity of the QD material provides a wide distribution of energies, resulting in a broad and relatively flat-topped gain spectrum, which is ideal for sourcing multiple wavelengths. Measurements of optical absorption, gain and laser threshold current densities were used to characterise the optical properties of InP/AlGaInP QDs and ascertain a suitable structure from which to fabricate the dual-wavelength source. A growth temperature of 710 °C resulted in the lowest threshold current densities, and the incorporation of tensile strain into the upper confining layers was found to reduce the temperature dependence. Optical gain measurements were used to assess how state-filling and temperature govern the gain-peak wavelength. For a fixed gain at low injection the wavelength dependence follows that of the band gap (≈ 0.17 nm/K), but at higher levels of injection it becomes relatively temperature-insensitive. A minima in wavelength sensitivity corresponded to a net gain of ≈ 28 cm-1. Edge-emitting lasers with a wavelength temperature dependence as low as 0.03 nm/K were demonstrated for temperatures up to 107 °C (380 K). An Ar-Cl2 based inductively-coupled plasma (ICP) etch process, suitable for fabricating sub-micron features, was developed to create the necessary device architecture. Using the effects of state-filling and spectrally preferential feedback, coupled-cavity ridgewaveguide lasers with unequal length sections were used to generate two wavelengths, with separations up to 61.5 ± 0.2 nm. Time resolved spectra were used to demonstrate dual-mode operation, where both wavelengths were observed to emit simultaneously. This is a promising result as it suggests that this device could potentially be used as a compact terahertz sourc

InP quantum dot mode-locked lasers and materials studies

InP/GaInP quantum dot laser structures exhibiting broad optical gain spectra suitable for mode-locking have been demonstrated. Two-section narrow ridge passive mode-locked lasers were fabricated from this material. Mode-locking conditions have been investigated for devices with different cavity lengths, with maximum frequency of 15.21 GHz

InP quantum dot lasers with temperature insensitive operating wavelength

We quantify the mechanisms that govern the lasing wavelength in edge-emitting InP/AlGaInP quantum dot (QD) lasers, by characterising the constituent factors controlling the temperature dependence of the gain peak wavelength. We show that a regime exists where the temperature coefficient of the bandgap can be compensated by the increasing wavelength-shift associated with state-filling in the QD ensemble, necessary to recover the gain peak magnitude. We demonstrate cleaved-facet edge-emitting lasers with a wavelength temperature dependence of 0.03 nm/K, similar to the temperature dependence of a Bragg stack fabricated in this material and approximately a sixth of the dependence of the bandgap

Degradation studies of InAs/GaAs QD lasers grown on Si

Lowering the threshold gain of InAs quantum dot lasers grown on Silicon, significantly extends device lifetime. Measurements on degraded devices show increased optical mode loss is responsible for degradation and a consequent shortening of lasing wavelength

Monolithic growth of InAs quantum dots lasers on (001) silicon emitting at 1.55 μm

Broad-area 1.55 μm InAs quantum dots (QDs) lasers were fabricated based on monolithic growth of InAs/InAlGaAs/InP active structures on nano-patterned (001) silicon substrates. Device optoelectronic properties and materials' optical gain and absorption features were studied to provide experimental support for further optimizations in laser design

Exploring the wavelength range of InP/AlGaInP QDs and application to dual-state lasing

We explore the accessible wavelength range offered by InP/AlGaInP quantum dots (QD)s grown by metal–organic vapour phase epitaxy and explain how changes in growth temperature and wafer design can be used to influence the transition energy of the dot states and improve the performance of edge-emitting lasers. The self assembly growth method of these structures creates a multi-modal distribution of inhomogeneously broadened dot sizes, and via the effects of state-filling, allows access to a large range of lasing wavelengths. By characterising the optical properties of these dots, we have designed and demonstrated dual-wavelength lasers which operate at various difference-wavelengths between 8 and 63 nm. We show that the nature of QDs allows the difference-wavelength to be tuned by altering the operating temperature at a rate of up to 0.12 nm K−1 and we investigate the factors affecting intensity stability of the competing modes

Degradation of III-V quantum dot lasers grown directly on silicon substrates

Initial age-related degradation mechanisms for InAs quantum dot lasers grown on silicon substrates emitting at 1.3-μm are investigated. The rate of degradation is observed to increase for devices operated at higher carrier densities and is therefore dependent on gain requirement, or cavity length. While carrier localisation in quantum dots minimises degradation, an increase in the number of defects in the early stages of ageing can increase the internal optical-loss which, can initiate rapid degradation of laser performance due to the rise in threshold carrier density. Population of the 2-D states is considered the major factor for determining the rate of degradation, which can be significant for lasers requiring high threshold carrier densities. This is demonstrated by operating lasers of different cavity lengths with a constant current and measuring the change in threshold current at regular intervals. A segmented-contact device, which can be used to measure the modal absorption and also operate as a laser, is used to determine how the internal optical-loss changes in the early stages of degradation. Structures grown on silicon show an increase in internal optical-loss whereas the same structure grown on GaAs show no signs of increase in internal optical-loss when operated under the same conditions

InAsP quantum dot lasers grown by MOVPE

We report on InAsP quantum dot lasers grown by MOVPE for 730-780 nm wavelength emission and compare performance with InP dot samples grown under similar conditions and with similar structures. 1-4 mm long, uncoated facet InAsP dot lasers emit between 760 and 775 nm and 2 mm long lasers with uncoated facets have threshold current density of 260 Acm−2, compared with 150 Acm−2 for InP quantum dot samples, which emit at shorter wavelengths, 715-725 nm. Pulsed lasing is demonstrated for InAsP dots up to 380 K with up to 200 mW output power. Measured absorption spectra indicate the addition of Arsenic to the dots has shifted the available transitions to longer wavelengths but also results in a much larger degree of spectral broadening. These spectra and transmission electron microscopy images indicate that the InAsP dots have a much larger degree of inhomogeneous broadening due to dot size variation, both from layer to layer and within a layer

12.5-GHz InP quantum dot monolithically mode-locked lasers emitting at 740 nm

Monolithic InP/GaInP quantum dot passively mode-locked lasers, designed using gain and absorption measurements, are realised for the first time, emitting at 740 nm with 12.5 GHz repetition frequency