by So-kuen C. Liew. / Thesis (Ph.D.)--Chinese University of Hong Kong, 1995. / Includes bibliographical references (leaves [40-45, 3rd gp.]) and index. / Abstract --- p.ii / Acknowledgments --- p.iv / Table of Contents --- p.v / Chapter 1 --- Introduction --- p.1 / Chapter 1.1 --- Distributed Feedback Lasers --- p.1 / Chapter 1.2 --- Computer Model --- p.6 / Chapter 1.3 --- Analyses --- p.8 / Chapter 1.4 --- Organization of Thesis --- p.11 / Chapter 2 --- Computer Model --- p.13 / Chapter 2.1 --- Comparison of Theoretical Models --- p.15 / Chapter 2.2 --- Assumptions and Approximations --- p.17 / Chapter 2.2.1 --- Longitudinal Spatial Hole Burning --- p.17 / Chapter 2.2.2 --- Spontaneous Emission --- p.18 / Chapter 2.2.3 --- Nonlinear Gain Saturation --- p.19 / Chapter 2.2.4 --- Carrier-Induced Index Change --- p.20 / Chapter 2.2.5 --- Single-mode Operation Assumption --- p.22 / Chapter 2.2.6 --- Otbers --- p.22 / Chapter 2.3 --- Theories and Approaches --- p.25 / Chapter 2.3.1 --- Coupled Wave Theory --- p.25 / Description --- p.25 / Stop-Band --- p.29 / Second-Order DFB Laser --- p.30 / DFB Designs To Improve SMSR --- p.30 / Chapter 2.3.2 --- Transfer Matrix Approach --- p.32 / Chapter 2.4 --- Above-Threshold Model --- p.34 / Chapter 2.4.1 --- Introduction --- p.34 / Chapter 2.4.2 --- Formalism --- p.36 / Facet Output Power and Optical Spectrum --- p.39 / Photon Density Distribution --- p.41 / Variance of Photon Density Distribution --- p.42 / Nearfield Distribution --- p.42 / Surface Emission --- p.43 / Power-Current Characteristics --- p.44 / Optical spectrum --- p.45 / Subthreshold Analysis --- p.47 / Linear Yield Analysis --- p.47 / Chapter 2.4.3 --- Computer Implementation --- p.48 / Flowchart --- p.48 / Subroutines --- p.52 / "Runtime, Numerical Stability" --- p.56 / Chapter 2.5 --- "Discussion,Summary and Future Work" --- p.59 / Chapter 2.5.1 --- Validation of the DFB Model --- p.59 / Chapter 2.5.2 --- Summary --- p.67 / Chapter 2.5.3 --- Topics for Future Work in Theoretical Modeling --- p.68 / Chapter 3 --- Analysis of DFBDBR Laser --- p.72 / Chapter 3.1 --- Introduction --- p.72 / Chapter 3.2 --- Subthreshold Analysis --- p.78 / Chapter 3.2.1 --- Introduction --- p.78 / Chapter 3.2.2 --- Results --- p.81 / Symmetric End-Sections --- p.81 / Asymmetric End-Sections --- p.85 / Chapter 3.3 --- Above-threshold Analysis --- p.88 / Chapter 3.3.1 --- Analysis --- p.89 / Chapter 3.3.2 --- Length Ratio --- p.92 / Chapter 3.3.3 --- Design Plot --- p.99 / Chapter 3.3.4 --- Longitudinal Spatial Hole Burning --- p.102 / Chapter 3.3.5 --- Effective Linewidth Enhancement Factor --- p.104 / Chapter 3.3.6 --- Asymmetric DFBDBR --- p.107 / Chapter 3.4 --- Conclusion --- p.109 / Chapter 4 --- Analysis of Complex-Coupled DFB --- p.110 / Chapter 4.1 --- Introduction --- p.110 / Chapter 4.2 --- Laser Structure --- p.113 / Chapter 4.2.1 --- Grating Layer --- p.114 / Chapter 4.2.2 --- Parameter Values --- p.119 / Chapter 4.3 --- Above-Threshold Analysis of CCDFB --- p.122 / Chapter 4.3.1 --- Threshold Current --- p.122 / Grating Duty Cycle = 0.36 --- p.122 / Grating Duty Cycle = 0.15 --- p.128 / Chapter 4.3.2 --- Power Efficiency --- p.131 / Grating Duty Cycle = 0.36 --- p.131 / Grating Duty Cycle = 0.15 --- p.136 / Chapter 4.3.3 --- Summary --- p.137 / Chapter 4.4 --- Yield Analysis of LCDFB and QWDFB --- p.138 / Chapter 4.4.1 --- Introduction --- p.138 / Chapter 4.4.2 --- Method --- p.140 / Chapter 4.4.3 --- Results --- p.141 / Facet Phase Angle --- p.141 / Quarterwave Phase-Shifted DFB Laser --- p.144 / Loss-Coupled DFB Laser --- p.148 / Chapter 4.5 --- Conclusion --- p.154 / Chapter 5 --- Summary and Conclusion --- p.157 / Chapter 5.1 --- Summary --- p.157 / Chapter 5.1.1 --- Summary of Major Contributions --- p.157 / Chapter 5.1.2 --- Summary of Modeling and Validation --- p.159 / Chapter 5.1.3 --- Summary of Model Applications --- p.160 / DFBDBR Laser --- p.161 / Loss-Coupled DFB Laser --- p.162 / Chapter 5.2 --- Topics for Future Studies --- p.163 / References --- p.R-l / Appendices --- p.A-l / Chapter A. --- Derivations --- p.A-l / Chapter A.1 --- Noise Power --- p.A-l / Chapter A.2 --- Product of Field Vector and Its Adjoint --- p.A-2 / Chapter A.3 --- Gain-Coupling Coefficient --- p.A-5 / Chapter B. --- Subroutines in Computer Program --- p.A-8 / Chapter B.l --- Subroutines in 'drive.f' --- p.A-8 / Chapter B.2 --- Subroutines in 'Core.f' --- p.A-9 / Chapter B.3 --- Subroutines in 'initiaLf' --- p.A-13 / Chapter B.4 --- Subroutines in ´بmisc.f' --- p.A-14 / Chapter C. --- List of Figures --- p.A-17 / Chapter D. --- List of Tables --- p.A-22 / Chapter E. --- List of Abbreviations and Acronyms --- p.A-23 / Chapter F. --- List of Symbols --- p.A-24 / Chapter G. --- List of Publications --- p.A-27 / Index --- p.I-1

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Diode lasers have many advantages such as small size, high efficiency and small angular dispersion of the collimated beam. However they also have some problems like lack of frequency stability. They can not be tuned in wavelength. An external cavity can solve these problems and decrease the laser linewidth. To obtain a tunable, narrow linewidth light source, antireflection coatings were applied to commercial diode lasers which were then tested in an external cavity. Laser characteristics, such as threshold current, spectral behavior, I-V-curves, tuning range, and bandwidth were measured for the original diode laser and then compared to the measurements in an external cavity with and without antireflection (AR) coatings. The tuning range approximately doubled after AR coating. The modal stability was found to be better by a factor of 7 in the external cavity. The power amplification through the external cavity was as high as 50. It was also attempted to process laser diodes from material fabricated at OSU, however the resulting diodes showed a high series resistance and were not usable for this project. Additional work needs to be done in this area. The result of this work is a stable external cavity diode laser tunable over an 18 nm bandwidth which can be used as a tunable source in many applications. / Graduation date: 1995

In this thesis, the development of a femtosecond Cr⁴⁺:forsterite solid-state laser is described where the mode-locking procedure was initiated using two novel saturable absorbers. One was a GaInNAs quantum-well device and the other a quantum-dot-based saturable absorber. These devices had not previously been exploited for the generation of femtosecond pulses from a solid-state laser but in the course of this project, successful mode-locked laser operation in the femtosecond domain was demonstrated for both devices. When the GaInNAs device was incorporated in the Cr⁴⁺:forsterite laser, transform-limited pulses with durations as short as 62fs were obtained. The performance of this femtosecond laser was significantly superior to that for previous quantum-well based saturable absorbers in the 1300nm spectral region. The dynamics of the device were investigated with the aim of refining subsequent devices and to explore the potential to grow future devices for use at longer wavelengths. At the outset of my research work quantum-dot based saturable absorbers had not be used for the mode locking of solid-state lasers in the femtosecond regime. The work presented in this thesis showed that quantum-dot structures could be exploited very effectively for this purpose. This was initially achieved with the quantum-dot element being inclined at an off-normal incidence within the cavity but experimental assessment together with further development of the device allowed for implementation at normal incidence. Reliable operation of the femtosecond laser was demonstrated very convincingly where transform-limited pulses of 160fs duration were generated. Having developed practical femtosecond Cr⁴⁺:forsterite lasers, the final part of the project research was directed towards exemplar applications for a laser operating in the 1300nm spectral region. These were biophotonics experiments in which assessments of both deep tissue penetration and two-photon chromosome cutting were undertaken. This work confirmed the suitability of the 1300nm laser radiation for propagation through substantial thicknesses of biological tissue (~15cm). The demonstration of highly localised two-photon cutting of Muntjac deer chromosomes also represented a novel result because single-photon absorption could be avoided effectively and the temporal broadening of the femtosecond pulses in the delivery optics arising from group velocity dispersion around 1300nm was minimal.

Nano-emitters are the new generation of laser devices. A photonic-crystal cavity, which highly confines light in small volumes, in combination with quantum-dots can enhance the efficiency and lower the threshold of this device. The practical realisation of a reliable, electrically pumped photonic-crystal laser at room-temperature is, however, challenging. In this project, a design for such a laser was established. Its properties are split up into electrical, optical and thermal tasks that are individually investigated via various device simulations. The resulting device performance showed that with our design the quantum-dots can be pumped in order to provide gain and to overcome the loss of the system. Threshold currents can be as low as 10’s of μA and Q-factors in the range of 1000’s. Gallium arsenide wafers were grown according to our specifications and their diode behaviour confirmed. Photonic-crystal cavities were fabricated through a newly developed process based on a TiOₓ hard-mask. Beside membraned cavities, also cavities on oxidised AlGaAs were fabricated with help to a unique hard-mask removal method. The cavities were measured with a self-made micro-photoluminescence setup with the highest Q-factor of 4000 for the membrane cavity and a remarkable 2200 for the oxide cavity. The fabrication steps, regarding the electrically pumped photonic-crystal laser, were developed and it was shown that this device can be fabricated. During this project, a novel type of gentle confinement cavity was developed, based on the adaption of the dispersion curve (DA cavity) of a photonic-crystal waveguide. Q-factors of as high as 600.000 were measured for these cavities made in Silicon.

This thesis presents a study of lasing properties and optical amplification in semiconducting conjugated polymers and dendrimers. Configured as surface-emitting distributed feedback lasers, the effect of incorporating wavelength-scale microstructure on the output of the devices is examined along with the ability to create such structures using simplified fabrication processes such as soft lithography. Conjugated materials have received a great deal of interest due to their broad spectral absorption, emission, ability to exhibit gain and ease of processing from solution. As a result, they show great potential for a variety of applications such as photovoltaics, displays, amplifiers and lasers. To date however, there has only been one demonstration of a polymer optical amplifier. A broadband, solution based polymer amplifier is presented where the gain overlaps with the transmission window of polymer optical fibres. The effect of transitions that reduce the availability of gain in conjugated polymers is also examined by studying saturation of absorption in thin films. Producing wavelength scale microstructure is traditionally a slow, expensive technique. Here, solvent assisted micromoulding is used to pattern polymer films in less than two minutes. The effect of the variations in the pattern transfer on the laser characteristics is examined. The micromoulding technique is then applied to fabricating novel device types such as circular gratings and flexible plastic lasers. Encapsulation of the micromoulded laser is then shown to improve the lifetime of the device by over three orders of magnitude. The degradation effects witnessed during this extended operation are characterised quantitatively, an area of study where little data exists in the literature. A novel class of branched dendrimer materials whose properties can be independently tuned due to their modular architecture are configured as blue-emitting distributed feedback lasers. The ability to tune the emission wavelength by varying the film thickness is demonstrated. By changing the chemical groups contained within the molecule, further tuning of the emission can be obtained along with the demonstration of a highly efficient blue-emitting dendrimer laser. Chemosensing using dendrimer lasers is presented by demonstrating the incredibly sensitive response of the laser device to trace vapours of nitro-benzene compounds. The future application of which could be highly beneficial in the detection of explosives.