No description available.

No description available.

No description available.

High pressure and spontaneous emission analysis techniques have been used to probe the recombination mechanisms in mid-infrared semiconductor lasers for the 2-4 mum wavelength range. Analysis of the spontaneous emission collected through a window in the substrate contact of two type-I GalnAsSb / AlGaAsSb, compressively strained, quantum well lasers emitting at lambda ~2.11 mum and lambda ~2.37 mum reveals that the threshold current of both devices is dominated by Auger recombination at room- temperature. Further analysis shows that approximately 80% of the room-temperature threshold current of the 2.37 devices can be attributed to Auger recombination. Comparison with larger band gap near-infrared devices suggests that the CHSH Auger process (involving the generation of hot holes in the spin-orbit split-off band) is suppressed in the 2.37 mum lasers as the spin-orbit splitting energy is greater than the band gap in these structures. However, other types of Auger process, such as CHLH and CHCC, persist and dominate the room-temperature threshold current of these lasers. Hydrostatic pressure measurements on the 2.37 mum devices indicate that as pressure is applied, the band gap increases and approaches the spin-orbit splitting energy and at pressures above 6 kbar the CHSH process becomes important. This is also evident in the larger band gap 2.11mum lasers where the pressure dependence indicates that the CHSH process is important at atmospheric pressure. The loss processes in GaSb-based "W" diode lasers operating at lambda ~ 3.25 /mum (at T = 80 K) are also investigated through analysis of their spontaneous emission characteristics and the pressure dependence of their threshold currents. Spontaneous emission analysis shows that defect/impurity recombination, which had been of concern in these structures, does not have a significant influence on the threshold current. Results suggest that a substantial contribution from Auger recombination exists at 80 K and increases strongly with increasing temperature. It is estimated that Auger recombination accounts for about 87% of the threshold current at 200 K and is the main cause of the poor temperature performance in these lasers.

The drive for low threshold and temperature-stable semiconductor lasers for telecommunication applications has led to a significant interest in quantum dot (QD) lasers emitting in the 1.3 mum and 1.5 mum wavelength range. The literature shows that although low threshold current densities can be achieved, this is usually at the expense of a poor temperature stability. Low-temperature and high-pressure measurements of the threshold current and its radiative component are performed on undoped and p-doped 1.3 mum InAs/GaAs and 1.5 mum InAs/InP (311)B QD lasers. The results show that despite a fairly temperature-stable radiative current around room temperature, undoped QD lasers suffer from a poor temperature stability of their threshold current. This is because there is a large contribution (70% and 90% of the threshold current at room temperature in 1.3 and 1.5 mum lasers, respectively) from a strongly temperature sensitive non-radiative Auger recombination process. Several pieces of evidence are found to explain the observed decrease of the radiative current, explained by an improvement of the carrier distribution with increasing temperature. We find that in p-doped devices the temperature dependence of the radiative component of the threshold current can be modified by the doping. In these devices the radiative current can decrease with increasing temperature around room temperature while the non-radiative current increases. This results in a small range of temperatures over which the threshold current is constant (from ~ 270 to 300 K). This effect is very sensitive to the doping concentration. If the doping concentration is carefully chosen, this can result in high T0 devices but with larger threshold currents than in comparable undoped lasers. Gain measurements reveal that the differential gain of p-doped lasers is less than that of the undoped devices because of the increased non-radiative current and the non-thermal distribution of the carriers induced by the doping. Finally, a new method is demonstrated to measure the band gap dependence of the Auger coefficient, C, using a combination of high hydrostatic pressure measurements coupled with gain calculations.

This work presents a new theoretical framework that extends the standard theory of quasi continuous-wave (CW) rare-earth-doped fibres to the ultrashort pulsed regime. We derive a time dependent, spectrally-resolved, nonlinear propagation equation for the coupled spectral components of the amplified pulse, within the rate-equation approximation. Our approach combines in a unified manner the effects of gain saturation, gain dispersion, fibre dispersion, fibre nonlinearity, stimulated Raman scattering, spontaneous Raman emission (with its temperature dependence) and amplified spontaneous emission (ASE). We also present theoretical models for rare-earth-codoped fibre lasers and amplifiers, in which the donor ions absorb a significant fraction of the pump power, delivering it to the acceptor ions through- an efficient energy transfer mechanism. In particular, we consider the rate-equation model for the Er3+-Yb3+ codoped system with and without Er3+ clustering, which takes into account energy transfer between the (donor) Yb3+ and the (acceptor) Er3+ ions. We also derive rate-equations for the Nd3+-Yb3+ codoped system that includes energy transfer between Nd3+ and Yb3+ ions, interactions among Nd3+ ions, single-ion transitions, and ASE due to Nd3+ and Yb3+ ions. An approximate, quasi-analytical solution to the model equations is derived for CW lasers. Previous theoretical works on short pulse amplification in doped fibres were derived mainly from the generalized nonlinear Schroedinger equation in the time domain. Furthermore, works on high-power rare-earth-codoped fibres have been limited mainly to CW Er3+-Yb3+ codoped fibre lasers and amplifiers. Using our new formulation, we extend previous works on short pulse amplification in doped fibres to the mixed spectrotemporal domain, allowing the full spectral resolution of both the incident pulse and the fibre parameters. We also apply our theoretical models to extend previous works on short length Er3+-Yb3+ codoped fibre amplifiers (EYDFA) to the quasi-CW regime and to study CW Nd3+-Yb3+ codoped fibre lasers (NYDFL). To this end, we solve numerically the rate-propagation equations and investigate various system design parameters of practical interest. The effects of the most important pulse and fibre parameters (e.g., incident pulse wavelength, pulse peak power, pulse chirp, fibre length and pump power) on the spectral and temporal characteristics of short amplified pulses are studied, both in the anomalous and in the normal fibre dispersion regimes. We analyze transient dynamic effects (e.g., amplifier stability) in short length, high-concentration, all-optical gain-clamped EYDFAs, in response to input signal modulations. We also study the effects of key parameters (e.g., fibre length, pump and laser wavelengths, output mirror reflectivity and dopant concentrations) on the output power, efficiency and optimal design (e.g., optimal fibre length and optimum dopant concentrations) of high-power CW NYDFLs. Key words: Optical fibre theory, optical fibre amplifiers, optical fibre dispersion, optical pulse amplifiers, Raman scattering, gain clamping, wavelength-division-multiplexing (WDM), optical fibre lasers, Erbium (Er), Ytterbium (Yb), Neodymium (Nd).

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No description available.

This thesis is concerned with quantum well semiconductor lasers that operated at optical communication wavelengths around 1550nm. It concentrates on lasers that are made from the latticed matched InGaAs/InAlGaAs material which have less temperature dependence of their threshold current in comparison to the established phosphorous quaternary semiconductor material. In particular, multiple width quantum wells (which is also referred to as asymmetric wells) are employed in the active region to broaden the gain spectrum. A conventional identical width quantum well structure is also employed in order to assess the advantages of including multiple width wells in the active region. The comparison between the multiple and identical well structures included carrying out spectral TE and TM gain and internal optical loss measurements. Spectral absorption measurements are also presented for both multiple and identical width quantum well devices. This was carried out in order to study the quantum well band edge shift with increasing reverse biased electric field applied perpendicular to the quantum well layers, a process commonly known as quantum confined Stark effect. First colliding pulse mode-locked operation in lasers made from the multiple width quantum well material is also demonstrated. The electric field auto-correlation experiment has been carried out, using both the multiple and identical width quantum well lasers, in order to investigate the influence of the gain broadened material on the pulse width obtained from the mode-locked operation.

In this thesis, theoretical and experimental studies on hollow optical fibers (HOFs) and depressed clad hollow optical fibers (DCHOFs) for cladding-pumped high power fiber laser sources, operating at a wavelength range 900 - 1100 nm, are reported. In addition, the suppression of the stimulated Raman scattering (SRS) in a single-mode high power fiber-based master oscillator power amplifier (MOPA) source, using W-type Yb The suppression of the stimulated Raman scattering (SRS) in a small and single-mode core becomes an issue when considering the power scaling of a fiber laser. Here a W-type fiber was designed for SRS suppression with the fundamental mode cut-off set, by design, between the signal and the 1st order Raman Stokes wavelengths. SRS suppression was demonstrated in a high peak power fiber MOPA source with a single-mode output. The maximum peak power achieved was 13 kW with a small, and single-mode, core fiber without any evidence of SRS occurring.