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by Hung Wai. / Thesis (M.Phil.)--Chinese University of Hong Kong, 2000. / Includes bibliographical references (leaves 69-[73]). / Abstracts in English and Chinese. / Chapter 1 --- Introduction --- p.1 / Chapter 1.1 --- All Optical Multi-Access Network --- p.1 / Chapter 1.2 --- Multi-access Techniques --- p.2 / Chapter 1.2.1 --- Wavelength-Division Multi-access (WDMA) --- p.2 / Chapter 1.2.2 --- Subcarrier Multi-Access (SCMA) --- p.3 / Chapter 1.2.3 --- Time-Division Multi-Access(TDMA) --- p.3 / Chapter 1.3 --- Numerical Modelling of Semiconductor Mode-locked laser --- p.4 / Chapter 1.4 --- Objective of this Thesis --- p.5 / Chapter 2 --- Optical TDMA networks --- p.7 / Chapter 2.1 --- Introduction --- p.7 / Chapter 2.2 --- OTDM --- p.8 / Chapter 2.3 --- Network Architecture --- p.9 / Chapter 2.3.1 --- Broadcast Networks --- p.9 / Chapter 2.3.2 --- Switch-based networks --- p.10 / Chapter 2.4 --- Key technologies for optical TDMA Network --- p.13 / Chapter 2.4.1 --- High Repetition Rate Short Pulse sources --- p.13 / Chapter 2.4.2 --- Multiplexer and de-multiplexers --- p.15 / Chapter 2.4.3 --- Optical Clock Recovery --- p.17 / Chapter 2.4.4 --- All optical logic gates --- p.18 / Chapter 2.5 --- Summary --- p.19 / Chapter 3 --- A Channel-Tunable Mode-locked Laser Transmitter for OTDM Networks --- p.20 / Chapter 3.1 --- Introduction --- p.20 / Chapter 3.2 --- Principle of Operation --- p.21 / Chapter 3.3 --- Experimental Demonstration --- p.23 / Chapter 3.4 --- The Channel Tuning Transient --- p.25 / Chapter 3.5 --- Experimental Investigation of channel-tuning transient --- p.28 / Chapter 3.6 --- Summary --- p.37 / Chapter 4 --- Modeling of Mode-Locked Semiconductor Laser --- p.38 / Chapter 4.1 --- Introduction --- p.38 / Chapter 4.2 --- Principle of Mode-Locking --- p.39 / Chapter 4.3 --- Simulation Model --- p.41 / Chapter 4.3.1 --- Travelling Wave Rate Equation Analysis --- p.41 / Chapter 4.3.2 --- Large Signal Time Domain Mode-locked Laser Model --- p.42 / Chapter 4.3.3 --- Modeling of Spontaneous Noise --- p.44 / Chapter 4.3.4 --- Modeling of Self-phase Modulation --- p.44 / Chapter 4.3.5 --- Frequency Dependent Gain Profile --- p.45 / Chapter 4.3.6 --- Computation Procedure --- p.45 / Chapter 4.4 --- Device Parameters --- p.47 / Chapter 4.5 --- Simulation Results on Passive Mode-locking --- p.48 / Chapter 4.5.1 --- Pulse Repetition Rate under Passive Mode-locking --- p.48 / Chapter 4.5.2 --- The effect of Differential Gain and Differential Absorption on Mode-locking Regimes --- p.50 / Chapter 4.5.3 --- The Effects of Linewidth Enhancement Factor and Ab- sorber Carrier Lifetime on Mode-locking Pulse Width --- p.53 / Chapter 4.6 --- Simulation Results on Hybrid and Subharmonic Mode-locking --- p.54 / Chapter 4.6.1 --- Modeling the Effect of Modulation on Absorber Section --- p.54 / Chapter 4.6.2 --- Modulation Phase Change Dynamics --- p.55 / Chapter 4.6.3 --- Subharmonc Mode-Locking Induced Amplitude Modulation --- p.62 / Chapter 4.7 --- Summary --- p.64 / Chapter 5 --- Conclusion --- p.66 / Chapter 5.1 --- Summary of the Thesis --- p.66 / Chapter 5.2 --- Future Work --- p.67 / Bibliography --- p.69

Thesis. 1975. Ph.D.--Massachusetts Institute of Technology. Dept. of Electrical Engineering and Computer Science. / Vita. / Includes bibliographical references. / Ph.D.

A primarily software based Fourier Deep Level Transient Spectroscope (FDLTS) is built. The raw capacitance transient is acquired and digitized using capacitance meter HP4280A whereas the signal analysis is done using a customized software module. The software module calculates both the conventional DLTS spectrum and the Fourier DLTS spectrum. This home-made FDLTS set up was compared to a commercial conventional box-car DLTS system (Sula Technology's DLTS) as well as to a commercial Fourier DLTS system (Bio-rad) and it was found to be either equivalent to the commercial systems or even better in some respects. In one case, Fourier analysis using the home-made setup, led to the detection of a trap completely undetected by the commercial conventional DLTS. The FDLTS system together with the commercial conventional DLTS were used to study possible gold contamination in an industrial process. The study was accomplished by comparing conventional and Fourier DLTS spectra and corresponding calculated trap properties using Schottky barrier diodes fabricated on the suspect wafers and an intentionally gold diffused reference sample wafer. During the investigation minority carrier emission in DLTS using Schottky barrier diodes was observed. The study revealed the presence of some possible gold-like contamination which trapped minority carriers (i.e. electrons) in p type silicon. / Graduation date: 2003

We present a comparative theoretical study of the transient grating coherent effects in resonant picosecond excitation-probe experiments. Signals in both the probe and conjugate directions are discussed. The effects of recombination, non-radiative scattering and spatial and orientational diffusion are included. The analysis is applied to both a molecular and to a semiconductor model. Signal contributions from concentration and orientational gratings are distinguished and their temporal natures discussed. The theory is used to explain our recent observations in germanium. Included are discussions of picosecond transient grating self-diffraction measurements that can be understood in terms of an orientational grating produced by anisotropic (in k-space) state-filling. Though there have been predictions and indirect experimental evidence for isotropic state-filling in germanium, this is the first direct experimental indication of anisotropic state-filling in a semiconductor.

This thesis presents the development of III-nitride materials for deep-ultraviolet (DUV) light emitting devices. The goal of this research is to develop a DUV laser diode (LD) operating at room temperature. Epitaxial structures for these devices are grown by metalorganic chemical vapor deposition (MOCVD) and several material analysis techniques were employed to characterize these structures such as atomic force microscopy, electroluminescence, Hall-effect measurement, photoluminescence, secondary ion mass spectrometry, transmission electron microscopy, transmission line measurement, and X-ray diffraction. Each of these will be discussed in detail. The active regions of III-nitride based UV emitters are composed of AlxGa1-xN alloys, the bandgap of which can be tuned from 3.4 eV to 6.2 eV, which allows us to attain the desired wavelength in the DUV by engineering the molar fraction of aluminum and gallium. In order to emit photons in the DUV wavelength range (> 4.1 eV), high aluminum molar fraction AlxGa1-xN alloys are required. Since aluminum has very low ad-atom mobility on the growth surface, a very low group V to group III precursor ratio (known as V/III ratio), high growth temperature, and low growth pressure is required to form a smooth surface and subsequently abrupt heterointerfaces. The first part of this work focuses on developing high-quality multi-quantum well structures using high aluminum molar fraction ([Al] > 60%) AlxGa1-xN alloys. Optically pumped DUV lasers were demonstrated with threshold power density as low as 250 kW/cm² for the emission wavelength as short as 248.3 nm. Transverse electric (TE) -like emission dominates when the lasers were operating above threshold power density, which suggests the diode design requires the active region to be fully strained to promote better confinement of the optical mode in transverse direction. The second phase of this project is to achieve an electrically driven injection diode laser. Owing to their large bandgap, low intrinsic carrier concentration, and relatively high dopant activation energy, the nature of these high aluminum molar fraction materials are highly insulating; therefore, efficiently transport carriers into active region is one of the main challenges. Highly conducting p-type material is especially difficult to achieve because the activation energy for magnesium, a typical dopant, is relatively large and some of the acceptors are compensated by the hydrogen during the growth. Furthermore, due to the lack of a large work function material to form a p-type ohmic contact, the p-contact layer design is limited to low aluminum molar fraction material or gallium nitride. Besides the fabrication challenges, these low aluminum molar fraction materials are not transparent to the laser wavelength causing relatively high internal loss (αi). In this work, an inverse tapered p-waveguide design is employed to transport holes to active region efficiently while the graded-index separate-confinement heterostructure (GRINSCH) is employed for the active region design. Together, a multi-quantum well (MQW) ultraviolet emitter was demonstrated.

Thesis (MSc (Physics))--University of Stellenbosch, 2006. / External cavity diode lasers are used increasingly as sources of light in applications ranging from industrial photonic systems to basic laboratory research on the interaction of light and atoms. External cavity diode lasers offer more stable output frequency and narrow spectral bandwidth than the typical free-running diode lasers. These characteristics are achieved by exploiting the sensitivity of diode lasers to external optical feedback. In this study the design and development of an external cavity diode laser system for future applications in spectroscopy and laser cooling of rubidium atoms is presented. The external cavity diode laser including mechanical components and control electronics of the system is developed from basic components. The system uses frequency selective optical feedback from a diffraction grating in a Littrow configuration to provide collimated, narrow-band, frequency tunable light near 780 nm. The external cavity diode laser is designed to increase the mode-hop-free frequency tuning range, and allow accurate frequency tuning and stabilisation. A low-noise current source and a temperature controller for thermal stability were developed as part of the system since the output frequency changes with temperature and current. The temperature controller is optimised experimentally for the thermal characteristics of the external cavity. An electronic sidelock servo circuit for frequency locking of the external cavity diode laser to an external reference for long term frequency stabilisation is proposed and discussed. The servo circuit electrically controls the grating tilt and the current through the diode laser in order to lock the frequency of the diode laser. The external cavity diode laser is optimised and characterised near 780 nm. Results obtained in this study indicate that the external cavity diode laser is suitable for future applications in spectroscopy and laser cooling of neutral rubidium atoms.

Indiana University-Purdue University Indianapolis (IUPUI) / This thesis proposes an automated grid-based alignment system utilizing lasers and an array of light-detecting photodiodes. The intent is to create an inexpensive and scalable alignment system for pick-and-place robotic systems. The system utilizes the transformation matrix, geometry, and trigonometry to determine the movements to align the robot with a grid-based array of photodiodes. The alignment system consists of a sending unit utilizing lasers, a receiving module consisting of photodiodes, a data acquisition unit, a computer-based control system, and the robot being aligned. The control system computes the robot movements needed to position the lasers based on the laser positions detected by the photodiodes. A transformation matrix converts movements from the coordinate system of the grid formed by the photodiodes to the coordinate system of the robot. The photodiode grid can detect a single laser spot and move it to any part of the grid, or it can detect up to four laser spots and use their relative positions to determine rotational misalignment of the robot. Testing the alignment consists of detecting the position of a single laser at individual points in a distinct pattern on the grid array of photodiodes, and running the entire alignment process multiple times starting with different misalignment cases. The first test provides a measure of the position detection accuracy of the system, while the second test demonstrates the alignment accuracy and repeatability of the system. The system detects the position of a single laser or multiple lasers by using a method similar to a center-of-gravity calculation. The intensity of each photodiode is multiplied by the X-position of that photodiode. The summed result from each photodiode intensity and position product is divided by the summed value of all of the photodiode intensities to get the X-position of the laser. The same thing is done with the Y-values to get the Y-position of the laser. Results show that with this method the system can read a single laser position value with a resolution of 0.1mm, and with a maximum X-error of 2.9mm and Y-error of 2.0mm. It takes approximately 1.5 seconds to process the reading. The alignment procedure calculates the initial misalignment between the robot and the grid of photodiodes by moving the robot to two distinct points along the robot’s X-axis so that only one laser is over the grid. Using these two detected points, a movement trajectory is generated to move that laser to the X = 0, Y = 0 position on the grid. In the process, this moves the other three lasers over the grid, allowing the system to detect the positions of four lasers and uses the positions to determine the rotational and translational offset needed to align the lasers to the grid of photodiodes. This step is run in a feedback loop to update the adjustment until it is within a permissible error value. The desired result for the complete alignment is a robot manipulator positioning within ±0.5mm along the X and Y-axes. The system shows a maximum error of 0.2mm in the X-direction and 0.5mm in the Y-direction with a run-time of approximately 4 to 5 minutes per alignment. If the permissible error value of the final alignment is tripled the alignment time goes down to 1 to 1.5 minutes and the maximum error goes up to 1.4mm in both the X and Y-directions. The run time of the alignment decreases because the system runs fewer alignment iterations.