In this work, the operating characteristics of quantum dot (QD) lasers with asymmetric barrier layers (ABLs) are studied. Several different cases are examined, in particular: 1) Effect of excited states on static and dynamic operating characteristics Within QDs, in addition to the lasing ground state, carriers can be captured into excited states, where they then decay into the ground state. This excited-state-mediated capture impacts the operating characteristics, limiting the maximum output power and modulation bandwidth. Three separate cases are considered: only indirect capture with electron-hole symmetry, both direct and indirect capture with electron-hole symmetry, and both direct and indirect capture of electrons but only indirect capture of holes. The impact of different parameters on the operating characteristics is studied, with values for maximizing the output power and modulation bandwidth being found. In addition, it is found that parasitic recombination in the active region in the space between QDs causes the output power to saturate at high injection currents for the cases of indirect capture for both electrons and holes and indirect capture for holes but direct and indirect capture for electrons, although the presence of the ABLs causes it to reach saturation at much lower currents. 2) QD laser with only a single ABL To be effective, the materials for ABLs must be carefully chosen to ensure that the band edges properly align to allow one carrier to enter the active region while preventing the other from overshooting it. Due to this requirement, it may arise that a suitable material only exists for one ABL but not the other. The performance of a QD laser with only a single ABL is considered and compared to a conventional QD laser. Specifically, the output power and characteristic temperature are calculated. While the single ABL laser only offers a negligible increase in output power compared to the conventional laser, it offers a considerable increase in characteristic temperature. 3) Analytical derivation of alpha factor in QD lasers with and without ABLs The alpha factor of a semiconductor laser describes the spectral linewidth broadening that occurs in semiconductor lasers due to changes in the refractive index due to the carrier density. While it has been studied experimentally, there has been little work done on deriving the alpha factor of QD lasers analytically. An expression for the alpha factor is found in this work using the real and imaginary parts of the complex susceptibility. For QD lasers with no inhomogeneous broadening, as well as ones with equilibrium filling of QDs with narrow line of QD size distribution, the alpha factor is independent of carrier density, and is therefore the same for any QD lasers, with or without ABLs. For QD lasers with equilibrium filling without a narrow line of QD size distribution, the alpha factor depends on carrier density, allowing for a potential difference between conventional and ABL QD lasers, however the difference between the two will be lessened. / Doctor of Philosophy / Semiconductor lasers are the most widely used laser, due in part to their ability to be controlled using electricity. Semiconductor lasers are used in a wide variety of consumer electronics, such as optical drives, as well as being used in fiber optic communications, where data is transmitted using the laser's light. Fiber optic communications transmit data by controlling the laser's output, where a high output (brighter light) represents a digital one, and a low output (dimmer light) represents a digital zero. Because semiconductor lasers can be directly controlled by changing the amount of current they receive, their output can easily be changed, allowing fast transfer of data. Despite their benefits, semiconductor lasers suffer from a drawback known as parasitic recombination. Parasitic recombination is a process that makes a significant portion of the current injected to generate useful light go to waste, which negatively impacts the laser's performance. One solution to parasitic recombination is the addition of asymmetric barrier layers (ABLs). By adding ABLs, parasitic recombination can be completely removed. In this work, several different cases of semiconductor quantum dot (QD) laser with ABLs are examined. Starting from a set of equations, the operating characteristics of the lasers in the different cases are found. First, the case of excited states is examined. The presence of excited states in semiconductor lasers impacts the rate that current can be converted to light, lowering their performance. By solving the starting rate equations, which describe the way different values change over time, the performance of the laser can be calculated. Specifically, the impact of several tunable parameters on the output power and modulation bandwidth are examined. The modulation bandwidth is how fast the laser output can be changed, which is equivalent to how fast data can be transmitted. Optimum values for the DC injection current, QD surface density (number of QDs per area), and laser cavity length are found.

In developing a compact electro-optic sampling system for industrial use it is desirable to utilize a semiconductor laser diode as the light source since these devices are compact and economical. This thesis investigates several novel laser driver techniques for generating extremely short optical gain switched pulses from a semiconductor laser diode. These techniques include a novel bias control scheme in which the bias to a semiconductor laser diode, that is being driven with a step recovery diode pulse generator circuit, is turned on and off in order to switch the gain switched optical pulses on and off as desired. The second technique involves a mono-cycle scheme that allows a step recovery diode pulse generator circuit, which is customarily driven by a fixed frequency oscillator, to be driven by a mono-cycle pulse train of variable repetition rate. An equivalent circuit model of a laser diode based on the mono-mode rate equations is discussed and implemented in SPICE for the purpose of studying the interaction of the laser driver circuit electronics and the laser diode. The laser diode equivalent circuit is benchmarked against analytical solutions of the rate equations. A qualitative agreement between the measurements of the laser diode optical and terminal voltage responses and the SPICE simulations of the laser diode equivalent circuit model are demonstrated. / Thesis / Master of Engineering (ME)

This dissertation presents a wide-band lumped element equivalent circuit model and a building block-based scalable circuit model for multiple quantum well laser diodes. The wide-band multiple-resonance model expresses two important laser diode characteristics such as input reflection and electrical-to-optical transmission together. Additionally, it demonstrates good agreements with the measurement results of the selected commercial discrete laser diodes. The proposed building block-based modeling approach proves its validity using a numerically derived scalable rate equation. Since success in a circuit design depends largely on the availability of accurate device models, the practical application of the proposed models provides improved accuracy, simple implementation and a short design time.

In optical disk systems, a semiconductor laser is used to read out the data recorded on the disc. Since the laser is the first noise source, there are strictly rated values for laser's noise in various optical disk systems to ensure the normal operation of the system. Among different laser noises, the worst one is the optical feedback noise. The feedback light will disturb the longitudinal modes of the laser and induce huge fluctuation in Laser's output. In our work, we study the effect of optical feedback on Laser's modes and relative intensity noises under various driving current and chop temperature. We try to find out methods that can suppress the Laser's noise and try to give some suggestions for the design of disc driven. Some experiments about a commercialized optical disk driver are also reported as well.

Pages 10, 46, 126, 142 and 146 have been omitted because they were completely blank. / <p> This thesis describes the development of a tunable, ultrashort-pulse semiconductor-based laser system operating in the 1 μm wavelength region. The design of the oscillator is based on a two-contact long-wavelength InGaAs-GaAs quantum-well semiconductor device containing integrated gain and saturable absorber sections. A key design component of the oscillator is the fabrication of a curved ridge-waveguide in the gain section of the device, which allows the laser to be operated in a compact, linear external cavity. Under conditions of passive or hybrid mode-locking, the semiconductor oscillator can generate pulses of 1 to 10 ps in duration, which are tunable from 1030 to 1090 nm. The oscillator is also capable of being passively mode-locked at harmonics of the cavity round-trip frequency, allowing tuning of the pulse repetition rate from 0.5 to over 5 GHz. Noise measurements on two independently hybridly mode-locked semiconductor lasers reveal that the absolute noise of each laser is dominated by phase noise at frequencies below 10^5 Hz, while amplitude noise dominates at higher frequencies.</p> <p>Semiconductor and fibre optical amplifiers are used to scale the average power level of the mode-locked pulses. Semiconductor optical amplifiers consisting of narrow-stripe and flared-waveguide designs have been fabricated using the same material structure as that of the mode-locked semiconductor oscillator. Narrow-stripe devices with a length of 800 μm have produced amplified average signal powers of 13 mW, while 1700-μm-long, 2° flared-waveguide devices have produced amplified average signal powers of 50 mW. A fibre-based system consisting of a single-mode double-clad Yb-doped fibre has been constructed to investigate the suitability of a mode-locked diode laser as a seed-source for a Yb:fibre amplifier. Amplified average signal powers of up to 1.4 W have been obtained at the output of the fibre for a launched pump power of 2.1 W. Compression of the amplified pulses using a modified dual-grating compressor yields pulse durations as low as 500 fs and a peak power of up to 1.5 kW.</p> <p> Preliminary work is reported on the development of a novel dual-wavelength optical source consisting of two synchronized mode-locked diode lasers and a polarization-maintaining Yb:fibre amplifier. Numerical simulations based on a rate-equation model for the amplifier gain are conducted to investigate the performance characteristics of a Yb:fibre amplifier when operated under dual-wavelength signal amplification. The simulations are used to predict and optimize the performance of the fibre amplifier for two mode-locked semiconductor-seed-oscillators operating at wavelengths of 1040 and 1079 nm. Good agreement is obtained between the simulations and experimental results. </p> / Thesis / Doctor of Philosophy (PhD)

More efficient semiconductor lasers will be needed in tomorrow's applications. These lasers can only be realized through the application of new device processing techniques, designed to restrict current, carrier, and/or photon flow through the lasing cavity. This work aims to evaluate a non-conventional stripe laser processing technique which has the potential for effective current and possibly carrier confinement at low cost. This technique, referred to as hydrogen passivation, involves exposing laser material to a low energy hydrogen plasma, causing hydrogen ions to bind to charged acceptor and donor atoms. Such binding compensates the electrical activity of these dopant atoms and thereby increases the resistance of the exposed material. Optical confinement can also be achieved (subsequent to hydrogenation) by using a simple wet-etching process to form a lateral waveguide. Stripe lasers fabricated via hydrogen passivation have been demonstrated previously; however, the benefits of this method have not been fully explored or characterized. Our work aims to quantify the degree of current and carrier confinement provided by this technique. The cleaved cavity method of analysis is used to extract laser parameters via direct measurement. These parameters are then compared against those obtained from more conventional stripe lasers to identify improvements that have accrued from using hydrogen passivation. / Master of Science

Small-signal dynamic response of semiconductor quantum dot (QD) lasers with asymmetric barrier layers was studied. Semiconductor lasers are used in many communication systems. Fiber optic communication systems use semiconductor lasers in order to transmit information. DVD and Blu-ray disk players feature semiconductor lasers as their readout source. Barcode readers and laser pointers also use semiconductor lasers. A medical application of semiconductor lasers is for minor soft tissue procedures. Semiconductor lasers are also used to pump solid-state and fiber lasers. Semiconductor lasers are able to transmit telephone, internet, and television signals through fiber optic cables over long distances. The amount of information able to be transferred is directly related to the bandwidth of the laser. By introducing asymmetric barrier layers, the modulation bandwidth of the laser will improve, allowing for more information to be transferred. Also, by introducing asymmetric barrier layers, the output power will be unrestricted, meaning as more current is applied to the system, the laser will get more powerful. An optimum pumping current was found which maximized modulation bandwidth at -3dB, and is lower in QD lasers with asymmetric barrier layers (ABL) as opposed to conventional QD lasers. Modulation bandwidth was found to increase with cross section of carrier capture before reaching an asymptote. Both surface density of QDs and cavity length had optimum values which maximized modulation bandwidth. Relative QD size fluctuation was considered in order to see how variation in QD sizes effects the modulation bandwidth of the semiconductor QD laser with ABLs. These calculations give a good starting point for fabricating semiconductor QD lasers with ABLs featuring the largest modulation bandwidth possible for fiber optic communication systems. In semiconductor QD lasers, the electrons and holes may be captured into excited states within the QDs, rather than the ground state. The particles may also jump from the ground state up to an excited state, or drop from the excited state to the ground state. Recombination of electron-hole pairs can occur from the ground state to the ground state or from an excited state to an excited state. In the situation if the capture of charge carriers into the ground state in QDs takes place via the excited-state, then this two-step capture process makes the output power from ground-state lasing to saturate in conventional QD lasers. By using ABLs in the QD laser, it is predicted that the output power of ground-state lasing will continue to rise with applied current, as the ABLs will stop the electrons and holes from recombining in the optical confinement layer. Thus, ABL QD lasers will be able to be used in applications that require large energy outputs. / Master of Science / Semiconductor lasers (also known as diode lasers) have been used in numerous applications ranging from communication to medical applications. Among all applications of diode lasers, of particular importance is their use for high speed transmission of information and data in fiber optic communication systems. This is accomplished by direct conversion of the diode laser input (electrical current) to its output (optical power). Direct modulation of the laser optical output through varying electrical current helps cut costs by not requiring other expensive equipment in order to perform modulation. The performance of conventional semiconductor lasers suffers from parasitic recombination outside of the active region – an unwanted process that consumes a considerable fraction of the laser input (injection current) while not contributing to the useful output and thus damaging its performance. Asymmetric barrier layers were proposed as a way to suppress parasitic recombination in semiconductor lasers. In this study, the optimal conditions for semiconductor quantum dot lasers with asymmetric barrier layers were calculated in order to maximize their modulation bandwidth – the parameter that determines the highest speed of efficient information transmission. This includes finding the optimal values of the dc component of the pump current, quantum dot surface density and size fluctuations, and cavity length. As compared to conventional quantum dot lasers, the optimal dc current maximizing the modulation bandwidth is shown to be considerably lower in quantum dot lasers with asymmetric barrier layers thus proving their outperforming efficiency. In the presence of extra states in quantum dots in conventional lasers, the optical output of needed ground-state lasing may be heavily impacted – it may remain almost unchanged with increasing the laser input current. As opposed to conventional lasers, the output power of ground-state lasing in devices with asymmetric barrier layers will continue growing as more input current is applied to the system.

GaAsBi is a relatively unexplored alloy with interesting features such as a large bandgap reduction for a given lattice mismatch with GaAs substrates and good photoluminescence which make it promising for long wavelength light detection and emission applications. In this research, the molecular beam epitaxy (MBE) method was used to grow epi-layers and hetero-structures. A Vertical-external-cavity surface-emitting-laser (VECSEL) was grown as a part of collaboration with Tampere University in Finland. The process of laser growth promoted the writer’s skills in the growth of hetero-structures and led into an investigation of the effect of growth conditions on GaAsBi optical properties with important results. For instance, when the substrate temperature during growth was reduced from 400°C to 300°C and all other growth conditions were fixed, the Bi concentration in the deposited films increased from 1% to 5% and the photoluminescence (PL) intensity decreased by more than a factor of 1000. This is an indication of the importance of growth temperature in GaAsBi crystal quality. n+/p junctions were grown for the deep level transient spectroscopy (DLTS) experiments in collaboration with Simon Fraser University. The DLTS measurements showed that lowering the GaAsBi growth temperature increases the deep level density by a factor of 10. These deep levels are the source of non-radiative recombination and decrease the PL intensity. The structural properties of GaAsBi were investigated by high resolution x-ray diffraction and polarized PL and revealed long distance atomic arrangement (Cu-Pt ordering) in GaAsBi. The measurements showed that the ordering is more probable at high growth temperature. This can be due to the larger mobility of the atoms on the surface at high growth temperatures that allows them to find the ordered low energy sites. / Graduate

Compact high power fiber lasers and the vertical-external-cavity surface-emitting lasers (VECSELs) are promising candidates for high power laser sources with diffraction-limited beam quality and are currently the subject of intensive research and development. Here three large mode area fiber lasers, namely, the photonic crystal fiber (PCF) laser, the multicore fiber (MCF) laser, and the multimode interference (MMI) fiber laser, as well as the VECSEL are modeled and designed.For the PCF laser, the effective refractive index and the effective core radius of the PCF are investigated using vectorial approaches and reformulated. Then, the classical step-index fiber theory is extended to PCFs, resulting in a highly efficient vectorial effective-index method for the design and analysis of PCFs. The new approach is employed to analyze the modal properties of the PCF lasers with depressed-index cores and to effectively estimate the number of guided modes for PCFs.The MCF laser, consisting of an active MCF and a passive coreless fiber, is modeled using the vectorial mode expansion method developed in this work. The results illustrate that the mode selection in the MCF laser by the coreless fiber section is determined by the MMI effect, not the Talbot effect. Based on the MMI and self-imaging in multimode fibers, the vectorial mode expansion approach is employed to design the first MMI fiber laser demonstrated experimentally.For the design and modeling of VECSELs, the optical, thermal, and structural properties of common material systems are investigated and the most reliable material models are summarized. The nanoscale heat transport theory is applied for the first time, to the best of my knowledge, to design and model VECSELs. In addition, the most accurate strain compensation approach is selected for VECSELs incorporating strained quantum wells to maintain structural stability. The design principles for the VECSEL subcavity are elaborated and applied to design a 1040nm VECSEL subcavity that has been demonstrated for high power operation of VECSELs where near diffraction-limited output over 20 W is obtained. Physical modeling of the VECSEL is also discussed and used to compare VECSEL subcavity designs on the laser level.

Vertical-cavity surface-emitting lasers (VCSELs) areattractive as potential inexpensive high-performance emittersfor fibre-optical communication systems. Their surface-normalemission together with the small dimensions are beneficial forlow-cost fabrication since it allows on-wafer testing,simplified packaging and effective fibre-coupling. Forhigh-speed data transmission up to hundreds of metres, 850-nmVCSELs are today the technology of choice. For higher bandwidthand longer distance networks, emission at long-wavelength(1.3-1.55 µm) is required. Long-wavelength VCSELs are,however, not available since no materials system offershigh-index-contrast distributed Bragg reflectors (DBRs) as wellas high-gain active regions at such wavelengths.High-performance DBRs may be built up from AlGaAs/GaAsmultilayers, but long wavelength quantum wells (QWs) are onlywell established in the InP system. Therefore, the bestperforming devices have relied on wafer-fusion betweenInP-based QWs and AlGaAs-DBRs. More recently, however, the mainefforts have been shifted towards all-epitaxial GaAs-baseddevices, employing 1.3-µm GaInNAs QWs. In this thesis, different processing technologies forlong-wavelength VCSELs are described. This includes a thoroughinvestigation of wafer-fusion between InP and GaAs regardingelectro-optical as well as metallurgical properties, and thedevelopment of a stable low-pressure process for the selectiveoxidation of AlAs. Optimised AlGaAs/GaAs DBRs were designed andfabricated. An important and striking observation from thatstudy is that n-type doping potentially is much moredetrimental to device performance than previously anticipated.These investigations were exploited in the realisation of twonew VCSEL designs. Near-room-temperature continuous-waveoperation of a single-fused 1.55-µm VCSEL was obtained.This demonstrated the potential of InGaAsP/InP DBRs inhigh-performance VCSELs, but also revealed a high sensitivityto self-heating. Further efforts were therefore directedtowards all-epitaxial GaAs-based structures. This resulted in ahigh-performance 1215-nm VCSEL with a highly strained InGaAssingle QW. This can be viewed as a basis for longer-wavelengthVCSELs, i.e., with an emission wavelength approaching 1300 nm,either by an extensive device detuning or with GaInNAs QWs. <b>Keywords</b>: VCSEL, vertical cavity laser, semiconductorlaser, long-wavelength, DBR, oxidation, wafer fusion, InGaAs,semiconductor processing