Dissertations / Theses on the topic 'Vertical External Cavity Surface-Emitting Laser (VECSEL)'

The extraction of high power with high beam quality from semiconductor lasers has long been a goal of semiconductor laser research. Optically pumped vertical-external-cavity surface-emitting lasers (VECSELs) have already shown the potential for their high power high brightness operation. In addition, the macroscopic nature of the external cavity in these lasers makes intracavity nonlinear frequency conversion quite convenient. High-power high-brightness VECSELs with wavelength flexibility enlarge their applica-tions. The drawbacks of the VECSELs are their poor spectral characteristics, thermal-induced wavelength shift and a few-nm-wide linewidth.The objective of this dissertation is to investigate tunable high-power high-brightness VECSELs with spectral and polarization control. The low gain and microcavity reson-ance of the VECSEL are the major challenges for developing tunable high-power VECSELs with large tunability. To overcome these challenges, the V-shaped cavity, where the anti-reflection coated VECSEL chip serves as a folding mirror, and an extremely low-loss (at tuned wavelength) intracavity birefringent filter at Brewster's angle are employed to achieved the high gain, low-loss wavelength selectivity and the elimination of microcavity. This cavity results in multi-watt TEM00 VECSELs with a wavelength tuning range of 20~30 nm about 975 nm. Also the longitudinal mode discrimination introduced by birefringent filter makes the linewidth narrow down to 0.5 nm. After the tunable linearly polarized fundamental beam is achieved, the tunable blue-green VECSELs are demonstrated by using type I intracavity second-harmonic generation. The spectral control of VECSELs makes it possible to apply them as an efficient pump source for Er/Yb codoped single-mode fiber laser and to realize the spectral beam combining for multi-wavelength high- brightness power scaling.In this dissertation, theory, design, fabrication and characterization are presented. Rigorous microscopic many-body theory of the quantum well gain, based on semiconductor Bloch equations and k.p theory, is introduced. The closed loop design tool based on this theory is not only used to design the VECSEL structure, but also used as a precise on-wafer diagnostics tool by the experiment/theory comparison of the photo-luminescence. The characterization of the wafer shows that the modeling is in good agreement with the measured results.The VECSEL high power high brightness performance relies on the fabrication of the chip. The fabrication method of the VECSEL chip, which provides the optically smooth surface and good heat dissipation, is presented. The anti-reflection coating on the chip surface can significantly improve the slope efficiency of VECSEL when high reflectivity output coupler is used. Over 12-W VECSEL cw output power with 43 % slope efficiency is demonstrated at 0 oC. A beam quality factor (M^2 factor) of 1.75 is obtained at 11 W output power.

Ever since the first laser demonstration in 1960, applications for laser systems have increased to include diverse fields such as: national defense, biology and medicine, entertainment, imaging, and communications. In order to serve the growing demand, a wide range of laser types including solid-state, semiconductor, gas, and dye lasers have been developed. For most applications it is critical to have lasers with both high optical power and excellent beam quality. This has traditionally been difficult to simultaneously achieve in semiconductor lasers. In the mid 1990's, the advent of an optically pumped semiconductor vertical-external-cavity surface-emitting laser (VECSEL) led to the demonstration of high (multi-watt) output power with near diffraction limited (TEM00) beam quality. Since that time VECSELs covering large wavelength regions have been developed. It is the objective of this dissertation to investigate and explore novel cavity designs which can lead to increased functionality in high power, high brightness VECSELs. Optically pumped VECSELs have previously demonstrated their potential for high power, high brightness operation. In addition, the "open" cavity design of this type of laser makes intracavity nonlinear frequency conversion, linewidth narrowing, and spectral tuning very efficient. By altering the external cavity design it is possible to add additional functionality to this already flexible design. In this dissertation, the history, theory, design, and fabrication are first presented as VECSEL performance relies heavily on the design and fabrication of the chip. Basic cavities such as the linear cavity and v-shaped cavity will be discussed, including the role they play in wavelength tuning, transverse mode profile, and mode stability. The development of a VECSEL for use as a sodium guide star laser is presented including the theory and simulation of intracavity frequency generation in a modified v-cavity. The results show agreement with theory and the measurement of the sodium D1 and D2 lines are demonstrated. A discussion of gain coupled VECSELs in which a single pump area accommodates two laser cavities is demonstrated and a description of mode competition and the importance of spontaneous emission in determining the lasing condition is discussed. Finally the T-cavity configuration is presented. This configuration allows for the spatial overlap of two VECSEL cavities operating with orthogonal polarizations. Independent tuning of each cavity is presented as well as the quality of the beam overlap and demonstration of Type II intracavity sum frequency generation. Future applications to this configuration are discussed in the generation of high power, high brightness lasers operating from the UV to far-infrared and even terahertz regimes.

Optically pumped semiconductor vertical external cavity surface emitting lasers (VECSEL) were first demonstrated in the mid 1990's. Due to the unique design properties of extended cavity lasers VECSELs have been able to provide tunable, high-output powers while maintaining excellent beam quality. These features offer a wide range of possible applications in areas such as medicine, spectroscopy, defense, imaging, communications and entertainment. Nowadays, newly developed VECSELs, cover the spectral regions from red (600 nm) to around 5 µm. By taking the advantage of the open cavity design, the emission can be further expanded to UV or THz regions by the means of intracavity nonlinear frequency generation. The objective of this dissertation is to investigate and extend the capabilities of high-power VECSELs by utilizing novel nonlinear conversion techniques. Optically pumped VECSELs based on GaAs semiconductor heterostructures have been demonstrated to provide exceptionally high output powers covering the 900 to 1200 nm spectral region with diffraction limited beam quality. The free space cavity design allows for access to the high intracavity circulating powers where high efficiency nonlinear frequency conversions and wavelength tuning can be obtained. As an introduction, this dissertation consists of a brief history of the development of VECSELs as well as wafer design, chip fabrication and resonator cavity design for optimal frequency conversion. Specifically, the different types of laser cavities such as: linear cavity, V-shaped cavity and patented T-shaped cavity are described, since their optimization is crucial for transverse mode quality, stability, tunability and efficient frequency conversion. All types of nonlinear conversions such as second harmonic, sum frequency and difference frequency generation are discussed in extensive detail. The theoretical simulation and the development of the high-power, tunable blue and green VECSEL by the means of type I second harmonic generation in a V- cavity is presented. Tens of watts of output power for both blue and green wavelengths prove the viability for VECSELs to replace the other types of lasers currently used for applications in laser light shows, for Ti:Sapphire pumping, and for medical applications such as laser skin resurfacing. The novel, recently patented, two-chip T-cavity configuration allowing for spatial overlap of two, separate VECSEL cavities is described in detail. This type of setup is further used to demonstrate type II sum frequency generation to green with multi-watt output, and the full potential of the T-cavity is utilized by achieving type II difference frequency generation to the mid-IR spectral region. The tunable output around 5.4 µm with over 10 mW power is showcased. In the same manner the first attempts to generate THz radiation are discussed. Finally, a slightly modified T-cavity VECSEL is used to reach the UV spectral regions thanks to type I fourth harmonic generation. Over 100 mW at around 265 nm is obtained in a setup which utilizes no stabilization techniques. The dissertation demonstrates the flexibility of the VECSEL in achieving broad spectral coverage and thus its potential for a wide range of applications.

Vertical external-cavity surface-emitting lasers are ideal testbeds for studying the influence of the non-equilibrium many-body dynamics on mode locking. As we will show in this thesis, ultra short pulse generation involves a marked departure from Fermi carrier distributions assumed in prior theoretical studies. A quantitative model of the mode locking dynamics is presented, where the semiconductor Bloch equations with Maxwell’s equation are coupled, in order to study the influences of quantum well carrier scattering on mode locking dynamics. This is the first work where the full model is solved without adiabatically eliminating the microscopic polarizations. In many instances we find that higher order correlation contributions (e.g. polarization dephasing, carrier scattering, and screening) can be represented by rate models, with the effective rates extracted at the level of second Born-Markov approximations. In other circumstances, such as continuous wave multi-wavelength lasing, we are forced to fully include these higher correlation terms. In this thesis we identify the key contributors that control mode locking dynamics, the stability of single pulse mode-locking, and the influence of higher order correlation in sustaining multi-wavelength continuous wave operation.

Cette thèse est consacrée à l'analyse expérimentale de la dynamique spatio-temporelle d'un laser à grand rapport d'aspect (GRA) fonctionnant dans le régime de verrouillage de mode localisé dans le temps. Dans ce régime, le laser émet un ensemble périodique d'impulsions et chaque impulsion de cet ensemble peut être adressée individuellement par une perturbation externe. Par conséquent, ces impulsions sont des structures localisées dans le temps (SLT), l'équivalent, dans le domaine temporel, des structures localisées spatiales qui ont été observée dans une grande variété de systèmes dissipatifs. Le système que nous étudions est un laser à émission surfacique à cavité externe (VECSEL) où la cavité externe est délimitée par un miroir à gain et par un miroir absorbant saturable à semi-conducteur (SESAM). Ce dispositif a été conçu et développé en collaboration avec l'Institut d'Electronique et des Systèmes de Montpellier pour satisfaire aux conditions d'existence des SLT. Il a été réalisé par la centrale technologique Renatech au Centre de Nanosciences et de Nanotechnologies de Paris. Dans ce travail j'étudie la possibilité de créer des SLT dans un VECSEL à GRA, où la localisation spatiale et temporelle de la lumière deviendrait possible. Les structures localisées spatio-temporelles dans les systèmes dissipatifs, également appelées Balle de Lumière, sont très attractives car elles peuvent être utilisées pour encoder des bits d'information dans les trois dimensions du résonateur laser. Pour réaliser un VECSEL à GRA il faut que la cavité externe soit dans la condition auto-imageante (CAI) et, en même temps, qu'une large section du miroir à gain soit pompée. J'ai identifié un protocole d'observation pour atteindre la CAI de la cavité externe en présence de la lentille thermoélectronique induite par la pompe. Lorsque l'on opère le VECSEL près du CAI des motifs spatiaux complexes apparaissent. Leurs caractéristiques dépendent des coefficients de la matrice ABCD d'aller-retour de propagation dans la cavité externe. Alors que B contrôle le coefficient de diffraction du second ordre, C introduit un potentiel de champ proche qui produit un effet de focalisation (C>0) ou défocalisation (C<0). Pour C<0, les motifs observés consistent en une combinaison d'une onde plane axiale avec un ensemble d'ondes plane inclinées ayant une disposition presque hexagonale dans l'espace de Fourier. Ces ondes planes sont verrouillées en phase et leur interférence donne naissance à un profil en nid d'abeille en champ proche. Pour C>0, les motifs consistent en un ensemble de couples d'ondes plane inclinées ayants des vecteurs d'onde transversaux opposés. Différents couples présentent des vecteurs d'onde transversaux orientés différemment mais ayant le même module, ce qui dessine un cercle dans le profil de l'émission en champ lointain. Lorsque la symétrie rotationnelle est brisée par une anisotropie dans la cavité, seuls deux points sont observés dans le champ lointain et un rouleau apparaît dans le champ proche. Dans le domaine temporel, ces motifs correspondent à des impulsions picosecondes qui peuvent être adressées individuellement ; il s'agit donc de motifs localisés dans le temps. Dans chaque situation, l'ensemble du motif pulse de manière synchrone et tous les points du profil de champ proche sont corrélés. Même si le système n'émet pas spontanément de manière décorrélée, on peut l'induire à le faire en superposant à la pompe principale de faisceaux de pompage de taille faible qui permettent de contrôler le gain localement dans la section du VECSEL. Ainsi, j'ai pu créer des points sources émettant de motifs dont la pulsation est contrôlée individuellement et elle peut être asynchrone. Ces « points chauds » ont également été imprimé directement sur la facette supérieure du miroir de gain par le dépôt d'un masque absorbant en chrome. Ainsi, j'ai réalisé une preuve de principe pour la réalisation de sources multiplexées de motifs SLT dans le même VECSEL
This thesis is devoted to the experimental analysis of spatio-temporal dynamics in a large aspect-ratio semiconductor laser operated in the regime temporally localized mode-locking. In this regime the laser emits a periodic set of pulses and each pulse in this set can be individually addressed by an external perturbation. Hence, these pulses are Temporally-Localized Structures (TLS), the equivalent in time domain of the more famous Spatially-Localized Structure, which appear in many different dissipative systems in nature ranging from biology to physics. The system we study is a Vertical External-Cavity Surface-Emitting Laser (VECSEL) where the external cavity is delimited by a gain mirror and by a Semiconductor Saturable Absorber Mirror (SESAM). This device was developed to match the requirements for hosting TLS in collaboration with the Institut d'Electronique et des Systèmes of Montpellier. The devices were realized by the technological facility Renatech at Centre de Nanosciences et de Nanotechnologies de Paris. Preliminary work has shown that this system can host TLS with a single transverse mode profile.In this work I address the possibility of implementing TLS in a large-aspect ratio VECSEL, where both spatial and temporal localization of light may be possible. Spatio-temporal localized structures, also called Light Bullets (LB), in dissipative system are very attractive for their application to information processing because they can be used to encode information bits in the three dimensions of a laser cavity. Large aspect-ratio VECSEL requires a self-imaging external cavity and, at the same time, a broad pumped area on the gain section. I have identified an observational protocol to achieve self-imaging condition (SIC) of the external cavity taking account the parasitic thermo-electronic lens induced by the pump.When operating the VECSEL close to SIC, complex patterns appear due to degeneracy and complex non-linear light matter interaction. Their characteristics depends on the coefficients of the ABCD roundtrip matrix describing the ideal propagation in the external cavity. While B controls the second order diffraction coefficient, C introduces a near field potential which may produce a focusing effect (C>0) or a defocusing effect (C<0). For C<0 and normal diffraction regime (B>0) the non-linear patterns observed consist of a combination of an axial Gaussian spherical plane-wave with a set of tilted waves having a nearly hexagonal arrangement in the Fourier space. These plane waves are locked in phase and their interference gives birth to a honeycomb profile in near-field. For C>0 the patterns observed consist of a set of counterpropagating tilted waves with opposite transverse wavevectors, carrying a gaussian beam. These wavevectors share the same modulus and they draw a circle in the far field profile. When the rotational symmetry is broken by some anisotropy in the cavity, only two spots are observed in the far field and a roll pattern appears in the near-field. In the time domain these patterns correspond to picosecond pulses which can be individually addressed; hence they are temporally localized patterns. In each situation the entire pattern is pulsating synchronously, and all the points of the near-field profile are correlated.Even if the system does not emit spatially in a decorrelated way spontaneously, I have induced it by overlapping to the main pump small pump beams that create hot/multiplex spots on the section of the gain mirror. By controlling the gain level on each of these hot spots, I have induced gain-pinned temporally-localized pattern which are pulsing asynchronously. These hot spots have been also engineered directly onto the top facet of the gain mirror through integration of sub-lambda non diffractive Chromium absorptive mask. Accordingly, I have realized a proof of principle for the realization of multiplexed sources of TLS patterns into the same VECSEL

This dissertation has three parts which are distinctive from the perspective of their frequency regime of operation and from the nature of their contributions to the science and engineering communities. The first part describes work that was conducted on a vertical-external-cavity surface emitting-laser (VECSEL) in the optical frequency regime. We designed, fabricated, and tested a hybrid distributed Bragg reflector (DBR) mirror for a VECSEL sub-cavity operating at the laser emission wavelength of 1057 nm. The DBR mirror was terminated with a highly reflecting gold surface and integrated with an engineered pattern of titanium. This hybrid mirror achieved a reduction in half of the number of DBR layer pairs in comparison to a previously reported, successful VECSEL chip. Moreover, the output power of our VECSEL chip was measured to be beyond 4.0Wwith an optical-to-optical efficiency of 19.4%. Excellent power output stability was demonstrated; a steady 1.0 W output at 15.0 W pump power was measured for over an hour. The second part reports on an ultrafast in situ pump-probing of the nonequlibrium dynamics of the gain medium of a VECSEL under mode-locked conditions. We proposed and successfully tested a novel approach to measure the response of the inverted carriers in the active region of a VECSEL device while it was operating under passively mode-locked conditions. We employed the dual-frequency-comb spectroscopy (DFCS) technique using an asynchronous optical sampling (ASOPS) method based on modified time-domain spectroscopy (TDS) to measure the nonequilibrium dynamics of the gain medium of a phase-locked VECSEL that we designed and fabricated to operate at the1030 nm emission wavelength. Our spectroscopic studies used a probe pulse of 100 fs and an in situ pump pulse of 13 ps. We probed the gain medium of the VECSEL and recorded a depletion time of 13 ps, a fast recovery period of 17 ps, and 110 ps for the slow recovery time. Our scans thus demonstrated a 140 ps full depletion-recovery cycle in the nonequilibrium state. The third part discusses work in the microwave and millimeter wave frequency regimes. A new method to fabricate Luneburg lenses was proposed and demonstrated. This type of lens is well known; it is versatile and has been used for many applications, including high power radars, satellite communications, and remote sensing systems. Because the fabrication of such a lens requires intricate and time consuming processes, we demonstrated the design, fabrication and testing of a Luneburg lens prototype using a 3-D printing rapid prototyping technique both at the X and Ka-V frequency bands. The measured results were in very good agreement with their simulated values. The fabricated X-band lens had a 12 cm diameter and produced a beam having a maximum gain of 20 dB and a beam directivity (half-power beam width (HPBW)) ranging from 12° to 19°). The corresponding Ka-V band lens had a 7 cm diameter; it produced a beam with a HPBW about the same as the X-band lens, but with a maximum gain of more than 20 dB.

In this thesis design, development and realisation of a substrate emission electrically pumped vertical external cavity surface emitting lasers (EP-VECSELs) emitting in the 980 nm wavelength range is discussed. Chapter 1 provides a literature review of the relevant VCSEL and (OP-VECSEL) technology required for the design of an EP-VECSEL. In chapter 2, different areas of the device design are highlighted, including electrical and optical performance of the distributed Bragg reflectors (DBRs), active region design, device detuning and antirefiective coating design. Chapter 3 provides a description of the method used to fabricate EP-VECSEL devices and focuses on optimisation of different process steps, namely the trench etch profile and depth, as well as the contact metalisation. A method for characterising the detuning of a wafer is also presented. In chapter 4 measurements of fabricated EP-VECSEL are presented, with a method for the characterisation of the EP-VECSEL material by modulating the output coupler mirror reflectivity demonstrated. This method is then used to examine the affect of different substrate dopings on device performance. Data is also presented on beam quality, power scaling and thermal properties. Chapter 5 investigates methods for improving electrical aspects of device operation, with improved nand p DBR designs proposed. In addition, analysis of SIMS data for an EP-VECSEL and n-DBR are presented, along with an investigation of the top contact geometry. In chapter 6 a discussion of the QW active region is provided, first by analysing the epitaxial material used in chapter 4 and then proposing improvements to the growth process. A comparison of a 3, 6 and 9 QW active region is then presented, where the trade offs in the optimum number of QWs are examined. Finally, this thesis is summarised and a new device design is proposed from the findings.

Les travaux de thèse présentés en ce mémoire ont comme objectif principal le développement des sources lasers à semi- conducteurs en cavité verticale sur substrat InP, intègrent des régions actives à nanostructure quantiques, et émettent à des longueurs d’onde “télécom” (1550-1600 nm). Le développement d’un nouveau procédé technologique pour la réalisation de composants VCSEL compactes est détaillé. Ce procédé (nommé TSHEC) a été utilisé pour réaliser des émetteurs VCSELs en pompage optique sur plateforme hôte Si, ayant des performances très satisfaisantes. Ce même procédé a été adapté à la réalisation de VCSELs en pompage électrique, avec une étude préliminaire de la section de confinement électrique basée sur une BTJ en InGaAs, et le développement d’un nouveau jeu de masque dédié. Grace à la mise au point de la technologie des μ-cellules à cristaux liquides réalisé en partenariat avec LAAS, IMT Atlantique et C2N, on a pu adapter le procédé TSHEC pour la réalisation de dispositifs accordables. Une photodiode accordable autour de 1.55 μm a été réalisée, et des émetteurs VCSELs accordables basés sur la même technologie sont actuellement en cours de développement. Dans ces travaux on a également abordé le développement des VECSELs à base de bâtonnets quantiques InAs et émettent à 1.6 μm. Un premier dispositif a été réalisé et caractérisé en régime multimode et mono-fréquence. Finalement, la réalisation d’un banc expérimental pour la mesure directe de la constante de couplage dans des VECSELs bi-fréquence a été détaillée. Ce banc a permis de quantifier précisément le couplage existant entre deux états propres orthogonaux d’un VECSEL à puits quantiques émettent à 1.54 μm, et prochainement permettra la même étude dans des structures anisotropes, tels quels les bâtonnets quantiques ou le boites quantiques, dans le but d’investiguer l’effet de l’élargissement inhomogène présenté par ces milieux à gain en termes de couplage entre modes propres
The work presented in this dissertation focus on the development of InP-based semiconductor vertical-cavity lasers, based on quantum nanostructures and emitting at the telecom wavelengths (1550-1600 nm). A new technological process for the realization of compact VCSELs is described. This process (named TSHEC) has been employed to realize optically-pumped VCSELs, integrated onto a host Silicon platform, with good performances. The same process has been adapted to develop an electrically-driven version of VCSELs: a preliminary study of the confinement section based on a InGaAs-BTJ is presented, together with the development of a mask set. Thanks to the development of the liquid crystals μ-cell technology (in collaboration with LAAS, IMT Atlantique et C2N), we realized a tunable photodiode at 1.55 μm, and a tunable VCSEL is currently under development. This work also presents the first realization of a 1.6 μm- emitting optically-pumped quantum dashes-based VECSELs, and its characterization in multi-mode and single-frequency regime. Finally, the realization of an experimental setup for the investigation of the coupling between two orthogonal eigenstates of a bi- frequency 1.54 μm-emitting SQW-VECSEL has been conceived and realized. This setup, which allowed the direct quantification of the coupling constant on such a device, in the near future will allow performing the same study on anisotropic structures like quantum dashes or quantum dots, with the objective of studying the inhomogeneous broadening effect observed in these gain regions