This research investigated the spectra of two different semiconductor lasers subjected to optical feedback. For the vertical -cavity surface-emitting laser (VCSEL),a wavelength selective element (Etalon) was used to feedback one of the transverse modes into the laser cavity. The experimental results showed that the multi-transverse modes could be suppressed to yield the fundamental mode output or a higher-order transverse mode output, in which the side-mode suppression ratio (SMSR) where 14.8 dB and 6.4 dB respectively. For the external cavity AR-coated laser, we studied the feasibility of generating mode-locking pulses by the temporal modulation of orthogonal-polarization optical feedback (OPF). The RF spectrum analyzer was employed to analyze the interaction. The observation clearly presented the spectra of mode- locking-like pulses in the laser¡¦s output.

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The use of optical feedback for improving response linearity of electrooptic light modulators has been investigated. The modulator is configured as a straight channel waveguide flanked by electrodes in a lithium niobate (LiNbO3) substrate. Light is coupled into the waveguide in both TE and TM polarizations, and a voltage applied across electrodes causes a relative phase shift between two polarization components. An output analyzer converts the phase modulation to intensity modulation. Optical feedback of light in both polarization modes results from reflection of light at the polished edges of the substrate. Channel waveguides supporting a single guided mode for TE and TM polarizations were fabricated in x-cut LiNbO3 substrates using titanium-indiffusion technique. The waveguides and modulators were characterized at a wavelength of 1.55 ??m using a distributed feedback laser. The modulators were driven with a sinusoidal voltage waveform. To minimize harmonics of the modulating frequency in the intensity output, the magnitude of the optical feedback and the substrate temperature were adjusted. The feedback level was altered by applying refractive index-matching liquid to one or both ends of the waveguide at the edges of the crystal. It was found that a high degree of response linearity in the presence of feedback was achievable at certain substrate temperatures. The spurious-free dynamic range (SFDR) relative to the noise floor was measured at different feedback levels and substrate temperatures in an effort to maximize the modulator response linearity. An SFDR of 68.04 dB, limited by third-order nonlinearity, was achieved by applying index-matching fluid to the input end of the substrate. This compares with an SFDR of 64.84 dB limited by second-order nonlinearity when index-matching fluid was applied at both ends of the substrate. By changing the temperature of the same substrate to adjust the phase shifts experienced by TE and TM polarizations, the SFDR with index-matching fluid at the input end increased to 71.83 dB, limited by third-order nonlinearity. In tests at constant modulation depth, an improvement of as much as 9.6 dB in SFDR vs. the theoretical value for an interferometric modulator without feedback was achieved.

The coherence of a semiconductor laser subjected to a coherent optical feedback of greater than -30 dB will collapse, causing the laser¡¦s performance in many applications to be severely degraded. This research investigated the feasibility of suppression the coherence collapse with orthogonal-polarization optical feedback. In the experiments, we first studied the characteristics of coherence collapse state of semiconductor laser. Under an operational condition of 63.9 mA, 21.9¢J and -11 dB of coherent optical feedback, a Hitachi HLP 7806G semiconductor laser would be conducted from its solitary single-mode state into a multimode oscillation state with a very broad linewidth. At the same time, the relative intensity noise measured from the RF spectrum analyzer drastically raised about 15 dB. When a -16 dB and a -23 dB orthogonal-polarization optical feedback were fed back into the laser¡¦s cavity. The coherence collapse could be effectively suppressed, while the laser was conducted back to its solitary single-mode state stably. This result will greatly increase the application of the laser. Some experiments were also reported to explore the interaction between the laser and the orthogonal-polarization feedback light.

This work investigated the characteristics of transverse modes and polarization of a VCSEL and attempted to control its transverse mode with optical feedback. The fundamental studies indicated that this solitary VCSEL's output consisted of two mutually orthogonal linear polarizations with different threshold currents and different transverse mode distributions. Furthermore, the temperature dependence of the transverse modes of the VCSEL revealed a better stability than that of edge-emitted lasers. In the mode controlling experiments, an iris was employed to select a specific transverse mode for feedback, resulting in an enhancement in the power of the feedback mode. A significant improvement was accomplished in the feedback of the fundamental mode of the major polarization component: the laser oscillated in a single mode with a side mode suppression ratio greater than 19.8 dB and with a depression of the relative intensity noise more than 20 dB/Hz. Nevertheless, a week feedback in the minor polarization component would stimulate more competition among transverse modes and degraded the performance of the VCSEL. Finally, no effect was observed in the orthogonal-polarization optical feedback experiment due to the excessively small of feedback ratio.

The use of optical feedback for improving response linearity of electrooptic light modulators has been investigated. The modulator is configured as a straight channel waveguide flanked by electrodes in a lithium niobate (LiNbO3) substrate. Light is coupled into the waveguide in both TE and TM polarizations, and a voltage applied across electrodes causes a relative phase shift between two polarization components. An output analyzer converts the phase modulation to intensity modulation. Optical feedback of light in both polarization modes results from reflection of light at the polished edges of the substrate. Channel waveguides supporting a single guided mode for TE and TM polarizations were fabricated in x-cut LiNbO3 substrates using titanium-indiffusion technique. The waveguides and modulators were characterized at a wavelength of 1.55 ??m using a distributed feedback laser. The modulators were driven with a sinusoidal voltage waveform. To minimize harmonics of the modulating frequency in the intensity output, the magnitude of the optical feedback and the substrate temperature were adjusted. The feedback level was altered by applying refractive index-matching liquid to one or both ends of the waveguide at the edges of the crystal. It was found that a high degree of response linearity in the presence of feedback was achievable at certain substrate temperatures. The spurious-free dynamic range (SFDR) relative to the noise floor was measured at different feedback levels and substrate temperatures in an effort to maximize the modulator response linearity. An SFDR of 68.04 dB, limited by third-order nonlinearity, was achieved by applying index-matching fluid to the input end of the substrate. This compares with an SFDR of 64.84 dB limited by second-order nonlinearity when index-matching fluid was applied at both ends of the substrate. By changing the temperature of the same substrate to adjust the phase shifts experienced by TE and TM polarizations, the SFDR with index-matching fluid at the input end increased to 71.83 dB, limited by third-order nonlinearity. In tests at constant modulation depth, an improvement of as much as 9.6 dB in SFDR vs. the theoretical value for an interferometric modulator without feedback was achieved.

Optical feedback interferometry is a sensing technique with relative recent implementation for the interrogation of fluidic systems. The sensing principle is based on the perturbation of the laser emission parameters induced by the reinjection in the laser cavity of light back-scattered from a distant target. The technique allows for the development of compact and noninvasive sensors that measure various parameters related to the motion of moving targets. In particular, optical feedback interferometers take advantage of the Doppler effect to measure the velocity of tracers in flowing liquids. These important features of the optical feedback interferometry technique make it wellsuited for a variety of applications in chemical engineering and biomedical fields, where accurate monitoring of the flows is needed. This thesis presents the implementation of optical feedback interferometry based sensors in multiple fluidic systems where local velocity or flow rate are directly measured. We present an application-centered study of the optical feedback sensing technique used for flow measurement at the microscale with focus on the reliability of the signal processing methods for flows in the single and the multiple scattering regimes. Further, we present experimental results of ex vivo measurements where the optical feedback sensor is proposed as an alternative system for myography. In addition we present a real-time implementation for the assessment of non-steady flows in a millifluidic configuration. A semi-automatized system for single particle detection in a microchannel is proposed and demonstrated. Finally, an optical feedback based laser sensor is implemented for the characterization of the interactions between two immiscible liquid-liquid flowing at the microscale, and the measurement is compared to a theoretical model developed to describe the hydrodynamics of both fluids in a chemical microreactor. The present manuscript describes an important contribution to the implementation of optical feedback sensors for fluidic and microfluidic applications. It also presents remarkable experimental results that open new horizons to the optical feedback interferometry.

This research accomplished a series of investigations on the polarization switching of Vertical-Cavity Surface-Emitting Lasers (VCSEL). The research was preceded experimentally and theoretically to observe the variation of the VCSEL¡¦s polarization switching under various polarization optical feedback and various feedback ratios. A VCSEL with significant polarization-switching hysteresis loop was employed in the experiments to study the interaction between the optical feedback and polarization switching. The experimental results matched the theoretical simulations very well. These results could be interpreted with the graphic analysis of the linear gain model of the VCSEL. In the investigation of the polarization-switching hysteresis loop, experiments found that, inside the loop and at some constant current, the polarization switching would be conducted with a switching delay time. The switching delay time was shortened under stronger optical feedback, revealing an exponential dependence between the switching delay time and optical feedback ratio. In the experiments of small current modulation to continuously switch the laser¡¦s polarization, optical feedback could improve the success of the current-driven polarization switching, significantly enhancing its bandwidth. For a large-range tuning of the laser¡¦s current, as the current modulation frequency increased, the polarization-switching current in the increasing-current process would significantly shift toward the high current end, while the polarization-switching current in the decreasing-current process was affected less. These results greatly contribute to the understanding of the VCSEL¡¦s polarization switching.

Microstructure manipulation is a fundamental process to the study of biology and medicine, as well as to advance micro- and nano-system applications. Manipulation of microstructures has been achieved through various microgripper devices developed recently, which lead to advances in micromachine assembly, and single cell manipulation, among others. Only two kinds of integrated feedback have been demonstrated so far, force sensing and optical binary feedback. As a result, the physical, mechanical, optical, and chemical information about the microstructure under study must be extracted from macroscopic instrumentation, such as confocal fluorescence microscopy and Raman spectroscopy. In this research work, novel Micro-Opto-Electro-Mechanical-System (MOEMS) microgrippers are presented. These devices utilize flexible optical waveguides as gripping arms, which provide the physical means for grasping a microobject, while simultaneously enabling light to be delivered and collected. This unique capability allows extensive optical characterization of the structure being held such as transmission, reflection, or fluorescence. The microgrippers require external actuation which was accomplished by two methods: initially with a micrometer screw, and later with a piezoelectric actuator. Thanks to a novel actuation mechanism, the “fishbone”, the gripping facets remain parallel within 1 degree. The design, simulation, fabrication, and characterization are systematically presented. The devices mechanical operation was verified by means of 3D finite element analysis simulations. Also, the optical performance and losses were simulated by the 3D-to-2D effective index (finite difference time domain FDTD) method as well as 3D Beam Propagation Method (3D-BPM). The microgrippers were designed to manipulate structures from submicron dimensions up to approximately 100 µm. The devices were implemented in SU-8 due to its suitable optical and mechanical properties. This work demonstrates two practical applications: the manipulation of single SKOV-3 human ovarian carcinoma cells, and the detection and identification of microparts tagged with a fluorescent “barcode” implemented with quantum dots. The novel devices presented open up new possibilities in the field of micromanipulation at the microscale, scalable to the nano-domain.