Call number: LD2668 .T4 1986 F46 / Master of Science / Mechanical and Nuclear Engineering

by Kwok Chi Wing. / Thesis (M.Phil.)--Chinese University of Hong Kong, 1993. / Includes bibliographical references. / Abstract / Acknowledgements / Chapter 1. --- Introduction --- p.1 / Chapter 2. --- Basic Concepts of Lasers and Simple Survey of Laser Theories --- p.4 / Chapter 2.1 --- Introduction --- p.4 / Chapter 2.1.1 --- Basic Structure of a Laser --- p.4 / Chapter 2.1.2 --- "Concepts of "" Mode"" arid "" Mode-Locking""" --- p.6 / Chapter 2.2 --- Brief Review of Laser Theories --- p.9 / Chapter 2.3 --- Other Simple Models --- p.12 / Chapter 2.4 --- Review of the Maxwell-Bloch Equations --- p.17 / Chapter 2.4.1 --- Derivation of Maxwell-Bloch Equations --- p.17 / Chapter 2.4.2 --- Continuous-Wave Operation --- p.23 / Chapter 2.4.3 --- Mean-Field Approximation and Lorenz-Haken Instability --- p.24 / Chapter 2.4.4 --- Adiabatic Elimination of Fast Variables --- p.26 / Chapter 2.4.5 --- Thin-Sheet-Gain Approximation for Multimode Lasers --- p.30 / Chapter 2.4.6 --- Self-Mode-Locking Predicted by Using Maxwell-Bloch Equations --- p.33 / Chapter 2.4.7 --- Hysteresis Phenomena in Switching the Cavity Detuning --- p.35 / Chapter 3. --- "Moving-Mirror ""Mode-Locking""" --- p.41 / Chapter 3.1 --- Conventional Laser Mode-Locking --- p.41 / Chapter 3.1.1 --- Preliminaries： What is Mode-Locking (Conventional) ？ --- p.41 / Chapter 3.1.2 --- Active Mode-Locking and Passive Mode-Locking --- p.43 / Chapter 3.1.3 --- Spectra of Conventional Mode-Locked Lasers --- p.49 / Chapter 3.2 --- Moving-Mirror Mode-Locking --- p.50 / Chapter 3.2.1 --- Historical Notes --- p.50 / Chapter 3.2.2 --- Previously Proposed Explanations --- p.54 / Chapter 3.3 --- MMML Mechanism: our Proposal --- p.59 / Chapter 3.3.1 --- Relation between MMML Lasers and FSFC Lasers --- p.60 / Chapter 3.3.2 --- Concept of Moving Modes --- p.62 / Chapter 3.3.3 --- How are the Moving Modes Locked ？ --- p.64 / Chapter 3.4 --- Numerical Simulations ´ؤ Method and Results --- p.68 / Chapter 3.4.1 --- Description of Our Numerical Model --- p.68 / Chapter 3.4.2 --- Tests on the Simulation Method --- p.71 / Chapter 3.4.3 --- Ultrashort Pulses Generation of a MMML Laser --- p.73 / Chapter 3.4.4 --- Modulation of the Pulses --- p.74 / Chapter 3.4.5 --- Broadband or Discrete Spectra ？ --- p.75 / Chapter 3.4.6 --- Different Operation Regimes in MMML Lasers --- p.79 / Chapter 3.4.7 --- Why Period-T/2 Pulses --- p.84 / Chapter 3.4.8 --- Auto-Correlation Function of the Electric Field --- p.86 / Chapter 3.4.9 --- FSFC Laser with Injection Signal --- p.87 / Chapter 3.4.10 --- MMML in Class C Laser: d = 1.0 --- p.88 / Chapter 3.4.11 --- Exciting the Relaxation Oscillation Resonance --- p.89 / Chapter 4. --- Discussion and Conclusion --- p.92 / Chapter 4.1 --- Limitation of (Conventional) Thin-Sheet-Gain Approximation --- p.92 / Chapter 4.1.1 --- Problem with the Conventional Thin -Sheet-Gain Approximation --- p.92 / Chapter 4.1.2 --- Modified Thin-Sheet-Gain Approximation --- p.93 / Chapter 4.2 --- Concluding Remarks; Possibilities of Further Research --- p.97 / References and Notes / Appendix: Source Codes of the Fortran Program

A reflective Tiered Fresnel Zone Plate, herein called a Tiered Fresnel Mirror TFM, with a focal length on the order of a meter is studied for use as the mirror(s) in a Fabry-Perot interferometer type of laser. The relative phase transition within the individual zones (ideally smooth from zero to pi ) is stair-stepped or tiered in the longitudinal direction of the mirror. Within an individual zone the step height is constrained to a constant whereas the width of the tiers are monotonically decreased when traversing radially outward so that the overall profile follows the ideal smooth curve. The effectiveness of the number of tiers per zone, measured by the loss per pass or round-trip, varies from a Plane Mirror (zero tiers per zone) to a Spherical Mirror (an infinite number of zones per tier). The Fox and Li iterative method of determining the E-Field as the beam propagates back and forth is applied to an empty cavity resonator to determine the diffraction loss. A computer program is written to investigate the diffraction loss of various mirror configurations. The performance of the TFM is found to be not as efficient as the Spherical Mirror (the number of tiers per zone is shown to be a major variable) but may be tolerable under applications of a moderately high gain laser medium. The Gaussian Fundamental mode is easier to maintain since the higher order modes have a higher loss per round trip. The manufacture of the TFM can be incorporated easily into an IC process thereby making the cost of the novel mirror relatively cheap when produced in quantities. A major cost variable is again the number of tiers per zone which is proportional to the number of processing steps. The TFM's performance with respect to the etch depth of the steps in the mirror's stair-stepped profile is simulated and found to be a very doable etch with the current plasma etch technology.

Thesis (M.S.)--Massachusetts Institute of Technology, Dept. of Aeronautics and Astronautics, 1982. / Microfiche copy available in Archives and Barker. / Includes bibliographical references. / by Peter David McQuade. / M.S.

Laser Doppler Vibrometry (LDV) is a non-contact technique for sensing surface vibrations. Traditionally, LDV uses one or more fixed beams to measure the vibrational velocity of specific points and orientations. In order to measure an angular velocity at least two laser beams are required. Instead, this research proposes to develop a Continuous Scanning Laser Doppler Vibrometer (CSLDV) technique, based on a single laser beam continuously sweeping the area of interest using a scanning mirror. Linear scans allow the measurement of normal and angular velocity while circular scans allow the measurement of normal velocity and two angular velocities. The first part of the study analyzes the performance of rigid body models of both the short line and circular scans (< 1 cm) for measuring low broadband frequency vibrations of gel samples. This thesis focused on low frequency broadband vibration since natural human body vibrations (such as tremor or breathing) are typically below a few hundred hertz. Results for normal and angular velocity measurements are validated against conventional method of using two fixed LDVs. The second part of this research investigates the CSLDV technique for longer scans (< 5 cm). These long scans will be used to act as an array of virtual transducers at multiple points along the scanning path of the single laser beam; thus yielding similar information obtained using an array of several real fixed LDVs. A practical challenge encountered when using CSLDV is speckle noise, that is generated when a coherent light source is reflected back from an optically rough surface. The effect of speckle noise will be quantified by varying different parameters such as scan lengths, scanning frequency, target to sensor distance and the amplitude of excitation. These parameters will be optimized in order to reduce the error of vibration measurements obtained from the CSLDV. Such systems will be used to monitor multiple degrees of freedom of human skeletal muscle vibrations for elastography purposes. The forced vibration of human muscles will be analyzed using these CSLDV techniques. Overall contributions of this work include: (1) Validation of rigid body models of both short line and circular scans CSLDV for broadband low frequency linear and angular velocity measurements; (2) application to sensing natural human body vibrations (e.g., hand tremors); (3) replacement of an array of vibration sensors by a single long line scan CSLDV. (4) development of a dynamic elastography technique for skeletal muscles using CSLDV.