This thesis is concerned with the non-linear effects observed in the absorption spectra obtained with swept continuous wave quantum cascade laser (cw-QCL) radiation. The slower chirp rate afforded by cw-QCLs, compared with pulsed systems, allows different aspects of the population transfer and polarisation induced in a molecular transition to be elucidated: namely, adiabatic rapid passage and coherent transient effects. The thesis commences with a brief outline of the architecture of QCLs, and an introduction to the non-linear processes inherent in their utilisation. In the following chapter, an outline of the methods required to characterise and drive a QCL chip are described along with the theory and characterisation of a Herriott cell, the latter allowing optically thick but minimally damped samples to be investigated. In chapter 3, a cw-QCL operating at ~10 μm is employed at chirp rates near to the adiabatic limit, in the range 0.1-1.1 MHz ns-1. A ro-vibrational overtone transition of OCS is studied at two path lengths within an astigmatic Herriott cell, 238 and 47 m. The population transfer and non-linear effects induced by the cw-QCL are described and modelled using the Maxwell-Bloch equations, which incorporate different experimental parameters. As such, the maximum population transfer found at a low pressure of 4 mTorr was determined to be 12 %. Additionally, the maximum amplitude of the coherent transient is forced to shift markedly to later times as the sample becomes increasingly optically thick, whilst remaining minimally damped. Intrapulse spectroscopy with a cw-QCL operating at 4.6 μm is investigated in chapter 4. The laser's response to short current pulses, in the range of 20-400 ns, applied in conjunction with a slow ramp, 2.75 kHz ns-1, is characterised. The initial chirp rate during the pulse is found to be ~6 MHz ns-1 by observation of the temporal width of transitions within the pulse. After the pulse a down chirp, at a rate of ~1 MHz ns-1, caused the laser frequency to relax back to the frequency position prior to the pulse. Subsequently, the effect of the pulse on a strongly absorbing N2O transition is monitored. In particular, the pulse position is altered with ii Quantum Cascade Laser Spectroscopy: Non-Linear Optics and Population Transfer respect to the transition line-centre and the coherent transient response of the molecular sample is probed. The free induction decay and RP effects noted are found to have a beat frequency which alters in line with the chirp rate of the laser, and the decay time of the transient signal was found to decrease as the range of velocity groups swept through is increased. The non-linear response of optically thick NH3 is investigated in chapter 5 with a 30 mW laser operating at ~10 μm. The effect of changing the gas pressure, laser intensity, and chirp rate on optically thick transitions is investigated for this molecule, which has a markedly larger dipole moment than OCS, and a comparison between the three molecular systems studied in this thesis is presented. Due to the importance of velocity dephasing in this work, the linewidth of the QCL was measured by Lamb-dip spectroscopy and found to be ~3 MHz. Two noise sources are then employed to alter the linewidth of the QCL: a single frequency modulation and a random white noise source. The noise, applied through the bias-T of the laser, leads to a change in the linewidth and lineshape, and as such, user-selectable linewidths in the range 3-20 MHz can be created. Increasing the laser linewidth has been found to increase the saturation of the sample, and therefore leads to an increase in the population transfer, which is determined to be ~10% at a chirp rate of ~0.1 MHz ns-1. The final chapter introduces the first use of a cw-QCL for population transfer within a molecular beam. The excited state is probed via resonantly enhanced multiphoton ionisation time of flight (REMPI-TOF) spectroscopy and a velocity selected population transfer of ~8 % is achieved. The effects of laser intensity, molecular beam carrier gas and laser linewidth on the population transfer are investigated. This thesis concludes with a discussion of some potential extensions to the work presented in this final chapter.

In this dissertation, a theory of modulation response of a semiconductor quantum dot (QD) laser is developed. The effect of the following factors on the modulation bandwidth of a QD laser is studied and the following results are obtained:<br /><br />1) Carrier capture delay from the optical confinement layer into QDs<br /><br />Closed-form analytical expressions are obtained for the modulation bandwidth omega_{-3 dB} of a QD laser in the limiting cases of fast and slow capture into QDs. omega_{-3 dB} is highest in the case of instantaneous capture into QDs, when the cross-section of carrier capture into a QD sigma_n = infinity. With reducing sigma_n, omega_{-3 dB} decreases and becomes zero at a certain non-vanishing sigma_n^{min}. This sigma_n^{min} presents the minimum tolerable capture cross-section for the lasing to occur at a given dc component j_0 of the injection current density. The higher is j_0, the smaller is sigma_n^{min} and hence the direct modulation of the output power is possible at a slower capture. The use of multiple layers with QDs is shown to considerably improve the modulation response of the laser -- the same omega_{-3 dB} is obtained in a multi-layer structure at a much lower j_0 than in a single-layer structure.<br /><br />2) Internal optical loss in the optical confinement layer<br /><br />The internal optical loss, which increases with free-carrier density in the waveguide region, considerably reduces the modulation bandwidth omega_{-3 dB} of a QD laser. With internal loss cross-section sigma_int increasing and approaching its maximum tolerable value, the modulation bandwidth decreases and becomes zero. There exists the optimum cavity length, at which omega_{-3 dB} is highest; the larger is sigma_int, the longer is the optimum cavity.<br /> <br />3) Excited states in QDs<br /><br />Direct and indirect (excited-state-mediated) mechanisms of capture of carriers from the waveguide region into the lasing ground state in QDs are considered, and the modulation response of a laser is calculated. It is shown that, when only indirect capture is involved, the excited-to-ground-state relaxation delay strongly limits the ground-state modulation bandwidth of the laser -- at the longest tolerable relaxation time, the bandwidth becomes zero. When direct capture is also involved, the effect of excited-to-ground-state relaxation is less significant and the modulation bandwidth is considerably higher.<br /> / Ph. D.

This thesis describes the development of a technique for measuring frequency noise of semiconductor lasers. Equivalent laser linewidths were calculated from frequency noise measurements on several InGaAsP lasers with short external cavities to give single mode operation. Conventional 250 um lasers demonstrated linewidths of about 125 MHz-mW, compressively strained quantum well lasers of commensurate length had linewidth of 37 MHz-mW, and 500 um strained quantum well lasers had linewidth of 18 to 28 MHz-mW with an apparent strain dependence. The short external configuration allowed selection of a number of laser modes. Measurement of linewidth variation with laser mode showed a 20% to 40% change over six to eight modes. The system was adapted to make measurements of the optical frequency tuning with fine external cavity length change. This measurement provided a novel means to estimate the linewidth enhancement factor and the reflectivity of the external cavity element. The estimated values of the linewidth enhancement factor for 250 um conventional and quantum well lasers were found to be in the correct ratio to account for the measured difference in linewidth. / Thesis / Master of Engineering (ME)

This work develops a comprehensive theoretical model for a semiconductor laser, which exploits tunneling-injection of electrons and holes into quantum dots (QDs) from two separate quantum wells (QWs). The potential of such a tunneling-injection QD laser for temperature-stable and high-power operation is studied under the realistic conditions of out-tunneling leakage of carriers from QDs (and hence parasitic recombination outside QDs) and the presence of the wetting layer (WL). The following topics are included in the dissertation: 1) Characteristic temperature of a tunneling-injection QD laser The threshold current density jth and the characteristic temperature T0 are mainly controlled by the recombination in the QWs. Even in the presence of out-tunneling from QDs and recombination outside QDs, the tunneling-injection laser shows the potential for significant improvement of temperature stability of jth — the characteristic temperature T0 remains very high (above 300 K at room temperature) and not significantly affected by the QD size fluctuations. 2) Output power of a tunneling-injection QD laser Closed-form expressions for the light-current characteristic (LCC) and carrier population across the layered structure are derived. Even in the presence of out-tunneling leakage from QDs, the intensity of parasitic recombination outside QDs is shown to remain restricted with increasing injection current. As a consequence, the LCC of a tunneling-injection QD laser exhibits a remarkable feature — it becomes increasingly linear, and the slope efficiency grows closer to unity at high injection currents. The linearity is due to the fact that the current paths connecting the opposite sides of the structure lie entirely within QDs — in view of the three-dimensional confinement in QDs, the out-tunneling fluxes of carriers from dots are limited. 3) Effect of the WL on the output power of a tunneling-injection QD laser In the Stranski-Krastanow self-assembling growth mode, a two-dimensional WL is initially grown followed by the formation of QDs. Due to thermal escape of carriers from QDs, there will be bipolar population and hence electron-hole recombination in the WL, even in a tunneling-injection structure. Since the opposite sides of a tunneling-injection structure are only connected by the current paths through QDs, and the WL is located in the n-side of the structure, the only source of holes for the WL is provided by QDs. It is shown that, due to the zero-dimensional nature of QDs, the rate of the hole supply to the WL remains limited with increasing injection current. For this reason, as in the other parts of the structure outside QDs (QWs and optical confinement layer), the parasitic electron-hole recombination remains restricted in the WL. As a result, even in the presence of the WL, the LCC of a tunneling-injection QD laser becomes increasingly linear at high injection currents, which is a further demonstration of the potential of such a laser for high-power operation. / Ph. D.

The effect of different factors on the operating characteristics of a semiconductor quantum dot (QD) laser is studied. Specifically, the following topics are included in the dissertation: 1) Effect of carrier-density-dependent internal loss in the optical confinement layer (OCL) on the characteristic temperature. Internal optical loss in a QD laser couples the confined-carrier level occupancy in QDs to the free-carrier density in the OCL. Due to this coupling, which is controlled by the threshold condition, the free-carrier density is increased and more temperature-sensitive, and also the confined-carrier level occupancy becomes temperature-dependent. As a result, the characteristic temperature of a laser is considerably reduced. Carrier-density-dependent internal loss also sets an upper limit for operating temperatures of a QD laser and constrains the shallowest potential well depth and the smallest tolerable size of a QD at which the lasing can be attained. The dependences of the characteristic temperature, maximum operating temperature, and shallowest potential well depth on the parameters of the structure are obtained. At the maximum operating temperature or when any parameter of the structure is equal to its critical tolerable value, the characteristic temperature reduces to zero. 2) Effect of excited-states in QDs on the light-current characteristic (LCC). The carrier capture from the three-dimensional reservoir (optical confinement layer – OCL) into the QD ground-state and escape from the ground-state to the OCL are assumed to occur via the QD excited-state. Such a two-step capture places a fundamental limitation on ground-state lasing—the output power saturates at high injection currents. The saturation power is controlled by the transition time between the excited- and ground-state in a QD. The longest, cut-off transition time exists, beyond which no ground-state lasing is possible. The following characteristics are analyzed versus the injection current density and the transition time: occupancies of the ground- and excited-state, free carrier density in the OCL, threshold current density, number of stimulated photons emitted, output power, internal and external differential quantum efficiencies. 3) Effect of longitudinal spatial hole burning (SHB) and multimode lasing on the LCC. The number of modes is shown to remain limited with increasing injection current. The maximum number of modes that can oscillate in a QD laser is analytically estimated. While this number increases with increasing surface density of QDs or cavity length, it remains limited (first increases and then decreases) with increasing scatter in the QD-size. The critical tolerable values of the structure parameters are derived beyond which higher-order longitudinal modes can not oscillate. It is notable that, in addition to the maximum tolerable scatter, there also exists the minimum scatter in the QD-size for each higher-order mode to start lasing. The threshold currents and output powers of modes are computed numerically. The power of the main mode is reduced due to lasing of higher-order modes and spatially nonuniform carrier distribution. As a new mode turns on, kinks appear in the LCCs of existing modes. SHB reduces the total optical power of a laser and contributes to nonlinearity of the overall LCC. The effect is more significant when any of the structure parameters is close to its critical tolerable value. The LCC becomes more linear with improving QD-size uniformity or increasing surface density of QDs or cavity length. / Ph. D.

In this thesis, our primary objective was to theoretically analyze the real world modulation bandwidth of a DTI QD laser and this was done by analyzing the effect of out-tunneling leakage of carriers from QDs, and by analyzing the effect of electron-hole asymmetry on the device characteristics. We are confronted with the following results: 1) Effect of Out-Tunneling Leakage on Modulation Bandwidth in Double Tunneling Injection Quantum Dot Lasers To purely focus on this effect, the conditions of instantaneous carrier exchange between the OCL and QW (on each side of the structure) and tunneling injection into QDs are assumed and closed-form analytical expressions for modulation bandwidth are obtained. The relative decrease in modulation bandwidth, due to this effect, in a DTI QD laser (from plots of modulation bandwidth vs j on increasing wout) is then shown to be small, and at ranges of injection currents of operational interest, nearly negligible. Consequently, it is shown that the DTI laser is a robust device in terms of sensitivity to out-tunneling leakage i.e. much effort need not be paid in suppressing this phenomenon. 2) Effect of Electron-Hole Asymmetry on Modulation Bandwidth of Double Tunneling Injection Quantum Dot Lasers On analyzing the effect of electron-hole asymmetry on the device characteristics of a DTI QD laser, it can be noted (from plots of modulation bandwidth vs injection current) that there is no reduction in the maximum modulation bandwidth i.e. electron-hole asymmetry does not indicate a reduction in the effectiveness of such a DTI design. This is shown to occur as the maximum modulation bandwidth depends on both, the effective differential non-stimulated recombination time as well the photon lifetime in the optical cavity. The photon lifetime being much smaller than the former acts as the dominating factor, and hence we see no appreciable change in the maximum modulation bandwidth. In the course of this analysis, we also see that the actual condition i.e. that of electron hole asymmetry is closer, among the cases of symmetry, to symmetry assuming hole parameters rather than electron parameters. As such, in cases where electron-hole symmetry must be used (in order to facilitate numerical simplifications), a recommendation of this study is to use hole parameters instead. / Master of Science / In this age of internet and optical communications, semiconductor lasers have a profound impact on the way we interact with our world. They act as intermediaries converting digital signals into optical pulses (in order to be transmitted) and then back into digital code. Understandably, the maximum speed at which these lasers can encode and decode information limits the speed of this entire communication network. This speed can be defined as the modulation bandwidth. A new design, the double tunneling-injection (DTI) quantum dot (QD) laser shows considerable promise, however its modulation bandwidth under real world operating conditions was yet to be analyzed. The aim of this thesis was to then theoretically analyze the real world modulation bandwidth of this new semiconductor laser design. This was done by analyzing the effect of unwanted leakage of carriers (out-tunneling) from the active region (Quantum Dots), and by analyzing the effect of electron-hole asymmetry on the device characteristics. The relative decrease in modulation bandwidth, due to leakage of carriers, in a DTI QD laser is then shown to be nearly negligible. Consequently, it is shown that the DTI QD laser is a robust device in terms of sensitivity to out-tunneling leakage, i.e., much effort need not be paid in suppressing this phenomenon. On analyzing the effect of electron-hole asymmetry on the device characteristics of a DTI QD laser, it is shown that there is no reduction in the maximum modulation bandwidth, i.e., electron-hole asymmetry does not indicate a reduction in the effectiveness of such a design. Thus, this analysis reiterates the fact that DTI QD lasers indeed show incredible potential to drastically improve modulation bandwidth and must be investigated further.

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by Lee Yip-Chi. / Thesis (M.Phil.)--Chinese University of Hong Kong, 1995. / Includes bibliographical references (leaves [129]-[134]). / Acknowledgments / Abstract / Chapter Chapter 1. --- Introduction --- p.1-1 / Chapter 1.1) --- Recent approaches for wavelength-tunable optical pulse generation --- p.1 -2 / Chapter 1.2) --- Self-seeding a gain-switched Fabry-Perot laser diode --- p.1 -5 / Chapter 1.3) --- About this project --- p.1-8 / Chapter Chapter 2. --- Basic theory --- p.2-1 / Chapter 2.1) --- Basic mechanism of gain-switching --- p.2-1 / Chapter 2.2) --- Mechanism of self-seeding --- p.2-5 / Chapter 2.2.1) --- General principle --- p.2-5 / Chapter 2.2.2) --- Dynamics of singlemode formation --- p.2-7 / Chapter 2.2.3) --- Different cases of modal selection --- p.2-8 / Chapter 2.2.4) --- Reduction of turn-on delay time jitter of optical output --- p.2-10 / Chapter Chapter 3. --- Instrumentation --- p.3-1 / Chapter 3.1) --- Second harmonic autocorrelator --- p.3-1 / Chapter 3.1.1) --- Principle --- p.3-1 / Chapter 3.1.2) --- Description of the 2nd harmonic autocorrelator system --- p.3-3 / Chapter 3.1.3) --- Data acquisition --- p.3-4 / Chapter 3.1.4) --- Alignment and Measurement procedures --- p.3-5 / Chapter 3.1.5) --- Pulsewidth determination by curve fitting --- p.3-7 / Chapter 3.2) --- Optical pulse detection by high speed photodetector --- p.3-9 / Chapter 3.2.1) --- High speed photodetectors --- p.3-9 / Chapter 3.2.2) --- Data acquisition --- p.3-10 / Chapter 3.2.3) --- Deconvolution of the measured optical pulsewidth --- p.3-11 / Chapter Chapter 4 --- Self-seeding 830 nm laser diode using conventional grating method --- p.4-1 / Chapter 4.1) --- Introduction --- p.4-1 / Chapter 4.2) --- Design parameters --- p.4-2 / Chapter 4.2.1) --- External cavity length --- p.4-2 / Chapter 4.2.2) --- Grating orientation --- p.4-3 / Chapter 4.3) --- Experiment --- p.4-4 / Chapter 4.3.1) --- Experimental setup --- p.4-4 / Chapter 4.3.2) --- Equipment Description --- p.4-5 / Chapter 4.4) --- Results and discussion --- p.4-6 / Chapter Chapter 5. --- Self-seeding 1.3 μm LD using fiber-optic configuration --- p.5-1 / Chapter 5.1) --- Optimized operation of self-seeded laser diode --- p.5-1 / Chapter 5.1.1) --- General Description --- p.5-1 / Chapter 5.1.2) --- Components --- p.5-1 / Chapter 5.1.3) --- Experimental setup --- p.5-6 / Chapter 5.1.4) --- Feedback rate measurement --- p.5-8 / Chapter 5.1.5) --- Results and discussion --- p.5-9 / Chapter 5.2) --- Electrical bias dependence on the self-seeded LD --- p.5-15 / Chapter 5.3) --- An efficient scheme to improve tuning range and provide continuous tuning --- p.5-20 / Chapter 5.3.1) --- General Description --- p.5-20 / Chapter 5.3.2) --- Principle of thermal control scheme --- p.5-20 / Chapter 5.3.3) --- Experimental setup --- p.5-22 / Chapter 5.3.4) --- Results and Discussions --- p.5-23 / Chapter Chapter 6. --- A novel self-seeding configuration --- p.6-1 / Chapter 6.1) --- Principle --- p.6-1 / Chapter 6.2) --- Highly dispersion-shifted fiber --- p.6-2 / Chapter 6.3) --- Optical fiber-mirror --- p.6-3 / Chapter 6.3.1 --- Fabrication --- p.6-4 / Chapter 6.3.2) --- Characterization: --- p.6-6 / Chapter 6.4) --- Experiment --- p.6-10 / Chapter 6.5) --- Results --- p.6-12 / Chapter 6.6) --- Discussions --- p.6-27 / Chapter 6.6.1) --- Electrical tuning characteristic --- p.6-27 / Chapter 6.6.2) --- Sidemode supression ratio characteristics --- p.6-30 / Chapter 6.6.3) --- Thermal tuning characteristics --- p.6-33 / Chapter 6.7) --- Summary --- p.6-36 / Chapter Chapter 7. --- Half-period delayed dual-wavelength picosecond optical pulse generation using a self-seeded laser diode --- p.7-1 / Chapter 7.1) --- Introduction --- p.7-1 / Chapter 7.2) --- Principle --- p.7-2 / Chapter 7.3) --- Experiment --- p.7-4 / Chapter 7.4) --- Results and discussions --- p.7-5 / Chapter Chapter 8. --- A proposed self-seeding configuration for the programmable multi- wavelength optical pulse generation --- p.8-1 / Chapter Chapter 9. --- Conclusion --- p.9-1 / References / Appendix / List of accepted and submitted publications