Dissertations / Theses on the topic 'Broad area laser'

Semiconductor lasers in high-power regime typically manifest a poor beam quality due to its asymmetric beam divergence and its large beam quality factor (M2). The emission along one of the transverse directions (slow axis) is typically multimode while being single mode in the perpendicular direction (fast axis). Due to the multimode emission and the absence of any intrinsic mode selection mechanism, the beam quality of broad area semiconductor (BAS) laser is degraded. In this work, we demonstrate that, by means of spatial filtering technique, the beam quality may be improved along the lateral direction (slow axis). For the filtering purpose, we propose the use of photonic crystal (PhC) as the spatial filtering element. The PhC offers an advantage in terms of its compact size, which allows miniaturization of the device. The successful implementation of this scheme could lead to monolithic integration, with the PhC directly integrated between the front facet of the semiconductor material and the laser cavity mirror. The PhCs were fabricated on N-BK7 glass substrate using tightly focused femtosecond Bessel beam. Two different PhC with different parameters were used in the experiment. The filtering of the PhC appears due to the deflection of selected components of the beam out of the propagation direction, using the Laue configuration. Since the PhC operates only in the near field plane, the access to this plane for the experimental testing of the idea introduces a technological difficulty in such small devices.We followed a simplified approach using an extended cavity configuration, which mimics the action of the compact cavity. The advantage of the extended cavity setup is that it allows testingdifferent filtering geometries in the same setup. The extended cavity was built using AR coated single emitter BAS laser, fast axis collimator, a double 4f lens system, and an external mirror with reflectivity of 4%. This extended cavity allows implementing two different techniques, i.e. an intracavity slit and a photonic crystal in the same setup. The cavity was characterized by measuring the output power, beam quality factor (M2), spectrum, and near/far field laser profiles. Before testing the action of PhC, we tested the conventional spatial filtering using intracavity slit placed in the far field and acting on the beam in the lateral direction by blocking the higher angular components of the beam. The output beam from the external cavity was characterized by measuring the M2 along the slow axis and calculating the brightness of the beam. We demonstrate an enhancement in the brightness by factor of two compared to the unfiltered beam situation. We further evaluated the effect of the reflectivity of the feedback mirror on the output power and spatial profile of the beam. The filtering action of the PhCs was characterized using the same extended cavity configuration. In this configuration an enhancement of the brightness by a factor between 1.3 and 1.5 was demonstrated for the different crystals. In addition, we used another emitter with longer cavity length and transverse width and with 4 % coating on the front facets. The experimental results obtained using intracavity slit and PhC were compared with the numerical results obtained froma numerical model of broad area laser created to simulate the action of spatial filtering using either the intracavity slit or the PhC. Both results showed a good agreement between the experiments and the numerical results.The numerical code was used to further optimize the brightness enhancement by simulating different filtering geometries.<br>Los láseres de semiconductor en régimen de alta potencia suelen manifestar una mala calidad del haz debido a su divergencia asimétrica del haz y a su elevado factor de calidad del haz (M2). La emisión a lo largo de una de las direcciones transversales (eje lento) es típicamente multi-modo mientras que es mono-modo en la dirección perpendicular (eje rápido). Debido a la emisión multi-modo y a la ausencia de cualquier mecanismo intrínseco de selección de modo, la calidad del haz del láser semiconductor de área amplia (BAS) se degrada. En este trabajo demostramos que, mediante la técnica de filtrado espacial, la calidad del haz puede mejorarse en la dirección lateral (eje lento). Para filtrar el haz, proponemos el uso de cristal fotónico (PhC) como elemento de filtrado espacial. El PhC ofrece una ventaja en términos de su tamaño compacto, que permite la miniaturización del dispositivo. La demostración de la posibilidad de usar este esquema podría conducir a una integración monolítica, con el PhC directamente integrado entre la cara frontal del material semiconductor y el espejo de la cavidad del láser. Los PhC se fabricaron en un sustrato de vidrio N-BK7 usando un haz de Bessel de femtosegundos fuertemente focalizado. En el experimento se utilizaron dos PhC diferentes con diferentes parámetros. El filtrado de los PhC aparece debido a la desviación de componentes seleccionados del haz fuera de la dirección de propagación, usando la configuración Laue. Dado que el PhC opera sólo en el plano del campo cercano, el acceso a este plano para el ensayo experimental de la idea introduce una dificultad tecnológica en dispositivos tan pequeños. Hemos seguido un enfoque simplificado utilizando una configuración de cavidad extendida, que imita la acción de la cavidad compacta. La ventaja de la configuración de la cavidad extendida es que permite probar diferentes geometrías de filtrado en la misma configuración. La cavidad extendida se construyó utilizando un láser BAS de un solo emisor con recubrimiento anti-reflectante, un colimador de eje rápido, un sistema de lentes dobles 4f y un espejo externo con una reflectividad del 4%. Esta cavidad extendida permite la implementación de dos técnicas diferentes, una rendija intra-cavidad y un cristal fotónico, en la misma configuración. La cavidad se caracterizó midiendo la potencia de salida, el factor de calidad del haz (M2), el espectro y los perfiles del láser de campo cercano y lejano. Antes de probar la acción del PhC, probamos el filtrado espacial convencional utilizando una rendija intra-cavidad situada en el campo lejano y actuando sobre el haz en dirección lateral bloqueando los componentes angulares superiores del haz. El haz de salida de la cavidad externa se caracterizó midiendo el M2 a lo largo del eje lento y calculando el brillo del haz. Demostramos un aumento del brillo por un factor de dos en comparación con la situación del haz sin filtrar. Evaluamos además el efecto de la reflectividad del espejo de retroalimentación en la potencia de salida y el perfil espacial del haz. La acción de filtrado de los PhC se caracterizó utilizando la misma configuración de cavidad extendida. En esta configuración se demostró un aumento del brillo en un factor entre 1,3 y 1,5 para los diferentes cristales. Además, se utilizó otro emisor con una longitud de cavidad más larga y una anchura transversal y con un recubrimiento del 4 % en las caras frontales. Los resultados experimentales obtenidos mediante la rendija intra-cavidad y el PhC se compararon con los resultados numéricos obtenidos a partir de un modelo numérico de láser de área amplia creado para simular la acción del filtrado espacial utilizando la rendija intra-cavidad o el PhC. Ambos resultados mostraron una buena concordancia entre los experimentos y los resultados numéricos. El código numérico se utilizó para optimizar aún más el aumento del brillo simulando diferentes geometrías de filtrado

Thesis (M.S.)--Miami University, Dept. of Physics, 2005.<br>Title from first page of PDF document. Document formatted into pages; contains [1], ix, 93 p. : ill. Includes bibliographical references (p. 88-93).

A wide range of applications currently utilize conventional optical elements to individually transform the phase, polarization, and spectral transmission/reflection of the incident radiation to realize the desired system level function. The material properties and the feasibility of fabrication primarily impact the device and system functionality that can be realized. With the advancement in micro/nano patterning, growth, deposition and etching technology, devices with novel and multiplexed optical functionalities have become feasible. As a result, it has become possible to engineer the device response in the near and far field by controlling the phase, polarization or spectral response at the micro scale. One of the methods that have been explored to realize unique optical functionalities is by varying the structural properties of the device as a function of spatial location at the sub-micron scale across the device aperture. Spatially varying the structural parameters of these devices is analogous to local modifications of the material properties. In this dissertation, the optical response of interference transmission filters, guided mode resonance reflection filters, and diffraction gratings operated in Littrow condition with strategically introduced spatial variation have been investigated. Spatial variations in optical interference filters were used to demonstrate wavelength tunable spatial filters. The effect was realized by integrating diffractive and continuous phase functions on the defect layer of a one-dimensional photonic crystal structure. Guided mode resonance filters are free space optical filters that provide narrow spectral reflection by combining grating and waveguide dispersion effects. Frequency dependent spatial reflection profiles were achieved by spatially varying the grating fill fraction in designed contours. Diffraction gratings with space variant fill fractions operating in Littrow condition were used to provide graded feedback profiles to improve the beam quality and spatial brightness of broad area diode lasers. The fabrication of space variant structures is challenging and has been accomplished primarily by techniques such as ruling, electron beam writing or complex deposition methods. In order to vary the desired structural parameter in a designed manner, a novel technique for the fabrication of space variant structures using projection lithography with a fidelity that rivals any of the current technologies was also developed as a part of this work. The devices exhibit wavelength dependent beam shaping properties in addition to spatial and spectral filtering and have potential applications in advanced imaging systems, graded reflectivity laser mirrors, and engineered illumination. The design, modeling, microfabrication and experimental characterization of space variant micro optical elements with novel optical functionalities are presented.<br>Ph.D.<br>Optics and Photonics<br>Optics and Photonics<br>Optics PhD

Breitstreifenlaser haben eine breite Emissionsapertur, die es ermöglicht eine hohe Ausgangsleistung zu erreichen. Gleichzeitig führt sie jedoch zu einer Verringerung der lateralen Strahlqualität und zu ihrem nicht-stationären Verhalten. Forschung in diesem Gebiet ist anwendungsgetrieben und somit ist das Hauptziel eine Erhöhung der Brillanz, die sowohl die Ausgangsleistung als auch die laterale Strahlqualität beinhaltet. Um die zugrunde liegenden raumzeitlichen Phänomene zu verstehen und dieses Wissen zu nutzen, um die Kosten der Brillanz-Optimierung zu minimieren, ist ein selbst-konsistentes Simulationstool notwendig, welches die wichtigsten Prozesse beinhaltet. Zunächst wird in dieser Arbeit ein quasi-dreidimensionales elektro-optisch-thermisches Model präsentiert, welches wesentliche qualitative Eigenschaften von realen Bauteilen gut beschreibt. Zeitabhängige Wanderwellen-Gleichungen werden genutzt, um die inhärent nicht-stationären optischen Felder zu beschreiben, welche an eine Ratengleichung für die Überschussladungsträger in der aktiven Zone gekoppelt sind. Das Model wird in dieser Arbeit um eine Injektionsstromdichte erweitert, die laterale Stromspreizung und räumliches Lochbrennen korrekt beschreibt. Des Weiteren wird ein Temperaturmodel präsentiert, das kurzzeitige lokale Aufheizungen in der Nähe der aktiven Zone und die Formierung einer stationären Temperaturverteilung beinhalten. Im zweiten Teil wird das beschriebene Modell genutzt, um die Gründe von Brillanz-Degradierung, das heißt sowohl die Ursprünge der Leistungssättigung als auch des nicht diffraktionslimitierten Fernfeldes zu untersuchen. Abschließend werden im letzten Teil Laserentwürfe besprochen, welche die laterale Brillanz verbessern. Hierzu gehört ein neuartiges “Schachbrettlaser” Design, bei dem longitudinal-laterale Gewinn-Verlust-Modulation mit zusätzlicher Phasenanpassung ausgenutzt wird, um eine sehr geringe Fernfeld-Divergenz zu erhalten.<br>Broad-area lasers are edge-emitting semiconductor lasers with a wide lateral emission aperture that enables high output powers, but also diminishes the lateral beam quality and results in their inherently non-stationary behavior. Research in the area is driven by application and the main objective is to increase the brightness which includes both the output power and lateral beam quality. To understand the underlying spatio-temporal phenomena and to apply this knowledge in order to reduce costs for brightness optimization, a self-consistent simulation tool taking into account all essential processes is vital. Firstly, in this work a quasi-three-dimensional opto-electronic and thermal model is presented, that describes well essential qualitative characteristics of real devices. Time-dependent traveling-wave equations are utilized to describe the inherently non-stationary optical fields, which are coupled to dynamic rate equations for the excess carriers in the active region. This model is extended by an injection current density model to accurately include lateral current spreading and spatial hole burning. Furthermore a temperature model is presented that includes short-time local heating near the active region as well as the formation of a stationary temperature profile. Secondly, the reasons of brightness degradation, i.e. the origins of power saturation and the spatially modulated field profile are investigated and lastly, designs that mitigate those effects that limit the lateral brightness under pulsed and continuous-wave operation are discussed. Amongst those designs a novel “chessboard laser” is presented that utilizes longitudinal-lateral gain-loss modulation and an additional phase tailoring to obtain a very low far-field divergence.

A new fully integrated wavelength stabilization scheme based on grating-coupled surface-emitting lasers is explored. This wavelength stabilization scheme relies on two gratings. The first grating is fabricated on the p-side of the semiconductor laser in close proximity to the laser waveguide such that it couples light out of the guided mode of the waveguide into a propagating mode in the substrate; this grating is known as the grating coupler. The second grating is fabricated on the n-side of the substrate such that for the stabilization wavelength, this second grating operates in the Littrow condition and is known as the feedback grating. Furthermore with the proper design of the two gratings, the feedback grating will operate under total internal reflection conditions allowing a near unity retro-reflection of the light of the stabilization wavelength. The grating coupler and feedback grating together comprise a dual grating reflector (DGR). The DGR wavelength stabilization scheme is investigated both theoretically by means of numerical modeling and experimentally by integration of a DGR as a wavelength selective reflector into a single quantum well semiconductor laser with a gain peak centered at 975nm. Numerical modeling predicts a peak reflection of approximately 70% including losses and a spectral width of 0.3nm. The integration of a DGR into a semiconductor laser proved both the efficacy of the scheme and also allowed us to experimentally determine the effective reflectivity to be on the order of 62%; the spectral width of light output from these devices is typically on the order of 0.2nm. Furthermore, these devices had light-current characteristic slopes greater than 0.84W/A operating under continuous wave conditions. The DGR was then modified to provide a reflection with two spectral peaks. A semiconductor device incorporating this dual wavelength DGR was fabricated and tested. These devices showed a peak optical power of in excess of 5.5W and a light-current characteristic slope of 0.86W/A in quasi continuous wave operation; these devices also exhibit a large operating current range in which both wavelengths have comparable output powers. Another modified DGR design was investigated for the purpose of providing an even narrower spectral reflection. Devices incorporating this modified design provided an output with a spectral width as narrow as 0.06nm. DGRs were also integrated into an extremely broad area device of an unorthodox geometry; square devices that lase in two orthogonal directions were fabricated and tested. The last idea investigated was combining a DGR wavelength stabilized laser with a tapered semiconductor optical amplifier into a master oscillator power amplifier device, with the optical coupling between the two components provided by identical grating couplers disposed on the p-side surfaces of each of the devices. These master oscillator power amplifiers provide a peak power of 32W when operating under quasi continuous wave operation.<br>Ph.D.<br>Other<br>Optics and Photonics<br>Optics

碩士<br>國立交通大學<br>光電工程研究所<br>84<br>In this thesis, we report the fabrication of broad area high power 0.85 um single quantum well AlGaAs/GaAs graded index separate confinement (GRINSCH SQW) lasers by proton implantation and molecular beam epitaxy techniques.Lasers withand without anti-reflection and high-reflection coating on the two facets arecompared. Low threshold current density of 230 A/cm2 and threshold current of230 A are obtained for a 100 um wide and 1 mm long cavity. At 2.5 A injection, laser output reaches 2 W at room temperature CW condition.The characteristic temperature T0 is 139K (5~35C), slope effiency is 1.06W/A , wall plug efficiency is 38% and the internal loss is 3.585cm-1. The thermal resistance ofpackaged device is 9.6 K/W. The performance of our lasers is comparable to thestate of art devices that is commerically available.

博士<br>國立交通大學<br>光電工程所<br>91<br>We present experimental studies on the transverse modal behaviour of oxide-confined Vertical-Cavity Surface-Emitting Laser (VCSEL). The near- and far-field transverse mode patterns are investigated under CW pumping condition. The VCSEL with 20 μm diameter of the oxide apertures exhibits the complex flower-like and y-junction structured patterns in far-field images. A comparison between a 20 μm diameter aperture oxide-confined VCSEL and a proton-implanted VCSEL enables us to clearly distinguish the influence of different guiding mechanisms on the transverse modal behaviour. From the experimental results, we conclude that the VCSEL cavities can usually support many transverse modes, especially in large-diameter index-guided structures due to nonuniform carrier-concentration profile and temperature distribution. Furthermore, the boundary effect induced by oxidized layers strong influence the transverse modal behaviour of oxide-confined VCSELs. The combination of these mechanisms causes a strong tendency towards the emission of multi-high-order transverse mode which causes the formation of complex flower-like and y-junction structured transverse mode patterns.

碩士<br>國立交通大學<br>光電工程研究所<br>84<br>The primary focus of this thesis is the investigation of the output linewidth and the feasibility of the frequency stabilization of a tunable broad- area laser diode in an external cavity . The external cavity consists of a grazing- incidence grating , lens , and strip-mirror . By using a V-shape strip mirror , the laser could be operated in a dual- wavelength mode . Frequency stabilization of the laser output is achieved by feedback control of the external cavity length . A Fabry-Perot interferometer was utilized as the frequency discriminator . The laser linewidths are lessthen 94 MHz (instrument-limited) for both single- wavelength and dual-wavelength operation . With the servo loop operating , the peak- to-peak frequency fluctuation of the tunable single wavelength external cavity laser system can be reduced from 35 MHz down to 0.9 MHz over a period of 4600 sec. . This correspond to an Allan variance of {符號 115 \f "Symbol" \s 14|} =1.22{符號 180 \f "Symbol" \s 14|}10-9 for an integrating time-constant of {符號 116 \f "Symbol" \s 14|} =1sec . For the dual-wavelength operation , the peak-to-peak frequency fluctuation decreased from 43 MHz to 4.2 MHz , or {符號 115 \f "Symbol" \s 14|} =1.14{符號 180 \f "Symbol" \s 14|}10-8 at {符號 116 \f "Symbol" \s 14|} =2 sec. To our knowledge , frequency stability of the dual-wavelength laser diode in an external cavity has been demonstrate for the first time in this work .

Fiber based ultrashort pulse laser sources are desirable for many applications; however generating high peak powers in fiber lasers is primarily limited by the onset of nonlinear effects such as self-phase modulation, stimulated Raman scattering, and self-focusing. Increasing the fiber core diameter mitigates the onset of these nonlinear effects, but also allows unwanted higher-order transverse spatial modes to propagate. Both large core diameters and single-mode propagation can be simultaneously attained using photonic crystal fibers. Thulium-doped fiber lasers are attractive for high peak power ultrashort pulse systems. They offer a broad gain bandwidth, capable of amplifying sub-100 femtosecond pulses. The longer center wavelength at 2 ?m theoretically enables higher peak powers relative to 1 [micro]m systems since nonlinear effects inversely scale with wavelength. Also, the 2 [micro]m emission is desirable to support applications reaching further into the mid-IR. This work evaluates the performance of a novel all-fiber pump combiner that incorporates a thulium-doped photonic crystal fiber. This fully integrated amplifier is characterized and possesses a large gain bandwidth, essentially single-mode propagation, and high degree of polarization. This innovative all-fiber, thulium-doped photonic crystal fiber amplifier has great potential for enabling high peak powers in 2 [micro]m fiber systems; however the current optical-to-optical efficiency is low relative to similar free-space amplifiers. Further development and device optimization will lead to higher efficiencies and improved performance.<br>M.S.<br>Masters<br>Optics and Photonics<br>Optics and Photonics<br>Optics and Photonics

碩士<br>國立臺灣大學<br>物理研究所<br>92<br>High power and good beam quality are desired for semiconductor lasers in many applications such as spectroscopy, device and material characterization, laser and amplifier pumping, and nonlinear wavelength conversion. We propose a new type of broad area laser diodes that is capable of emitting good beam quality, high power and broadband tuning. The new type of laser diodes is fabricated with a broad-area waveguide that is oriented at an angle from the facet normal. This device does not require the DFB structure, so the fabrication is much simpler. The L-I curves, spectra, near-field patterns and far-field patterns of the angled broad-area waveguide laser diode are measured. The direction of the far field pattern along the facet normal for the device operated above the threshold current indicates that the light path is not along the waveguide direction. With external cavity by grating, the laser diode is tunable from 1275nm to 1310nm with output power up to 1 watt at 6Amp. The beam quality is good and the near field is avoid of filamentation.

碩士<br>國立交通大學<br>電子工程學系<br>85<br>Filamentation commonly occurs in broad area semiconductor laser due to two intinsic mechanisms: carrier-induced index change and intensity-induced index change. In this thesis, we investigate the effects of thesetwo mechanisms to the formation of filaments in semiconductor lasers theoretically. The theoretical model is derived thoroughly and two numericalmethods are used to calculate it, and they are 2-D Beam Propagation Method and 1-D Nonlinear Eigen-Equation method. The results show that when the magnitude of the carrier density and electric field intensity existing inside the laser cavity are higher, smaller stripe width is needed for the lasers to avoid occurring filamentation. Furthermore, unsymmetry laserstructure will be lower down the critical stripe width of the lasers. Finally, our results suggest the possibility of obtaining large stripe width semiconductor laser diodes (50um) without filamentation by using compensation between these two mechanisms.

博士<br>國立交通大學<br>電子物理系所<br>100<br>Basic properties as well as applications in physics research of broad-area VCSELs from below to far above threshold are thoroughly investigated. Firstly, the high-resolution measurement demonstrates that over a thousand cavity modes with a narrow linewidth can be perfectly exhibited in the subthreshold emission spectra. Further analyses of the obtained spectra confirm that the subthreshold emission spectra of broad-area VCSELs can be exploited to analogously investigate the energy spectra of the 2D quantum billiards. In addition, due the analogy between Helmholtz equation and time-independent Schrödinger equation, transverse wave patterns of oxide-confined VCSELs enable us to visualize the wave functions of the 2D quantum billiards with the same lateral shapes. Therefore, typical lasing modes for VCSELs with various cavity shapes are explored. Furthermore, since paraxial wave equation and time-dependent Schrödinger equation are highly equivalent in mathematics, free space propagation of lasing modes for VCSELs can be use to analogously study diffraction in time effect which is one of the most relevant quantum transient phenomena in matter waves. Mode selection mechanism for stadium-shaped VCSELs is also experimentally studied. We then perform statistical analysis to quantitatively understand characteristics of experimental chaotic and scarred wave patterns. Finally, polarization dynamics of broad-area VCSELs far above threshold are considered. Experimental results reveal that cavity geometry plays a significant role for polarization stability of the lasing mode that has large frequency detuning.

博士<br>國立交通大學<br>電子物理系所<br>97<br>The analogies between paraxial optics and two-dimensional (2-D) quantum mechanics lie in the heart of this thesis. Based on the similarity between Helmholtz equation and time-independent Schrödinger equation, mesoscopic wave functions of quantum billiards are used to interpret the high-order transverse modes of broad-area Vertical-Cavity Surface-Emitting Lasers (VCSELs). Reversely, chaotic-shaped VCSELs can be employed to analogously observe the wave functions of non-integrable billiards which have no analytic solutions. In addition, the free-time evolution of coherent waves suddenly released from quantum billiards can be analogously observed from the free-space propagation of lasing modes emitted from VCSELs. Furthermore, the analogies are not restricted to quantum wave functions and optical transverse modes, the orbital angular momentum (OAM) density carried by a light beam emitted from VCSELs can be analogously analyzed by calculating the OAM of coherent waves abruptly set free from quantum billiards. More interestingly, the lasing modes of VCSELs are not only linearly-polarized but can form the vector fields, in which the polarization is spatially dependent. Since the polarization of light corresponds to the spin of quantum wave, the analyses of the vector fields in VCSELs can provide important information for quantum-billiard systems (such as ballistic quantum dots) with consideration on electronic spin.

碩士<br>國立成功大學<br>微電子工程研究所碩博士班<br>96<br>In the thesis, the performances and characteristics of broad area semiconductor lasers were simulated using the carrier distribution equation and the FFT-BPM method. The physics mechanisms such as hole burning resulting from the interaction between carriers and electromagnetic field, and self-focusing caused by the nonlinear Kerr effect were taken into account in simulation. A broad-area semiconductor laser has a broad area of pump stripe for current injection that contributes high output power but also gives rise to bad beam profiles in near and far fields called filamentation. Without reducing the output power, the thesis succeeded in minimizing the filamentation in near and far fields using the genetic algorithm by modulating the shape of the laser pump stripe.