Broad-area Semiconductor Lasers Research Articles

Photonic microstructures as laser mirrors

Deeply etched 1-D third-order Bragg reflectors have been used as mirrors for broad-area semiconductor lasers operating at 975-nm wavelength. From a threshold and efficiency analysis, we determine the mirror reflectivity to be approximately 95%. The design of the GaAs-based laser structure features three InGaAs quantum wells placed close (0.5 μm) to the surface in order to reduce the required etch depth and facilitate high-quality etching. Despite the shallow design and the proximity of the guided mode to the metal contact, the threshold current density (J_(th) = 220 A/cm^2 for infinite cavity length) and internal loss (α_i = 9±1 cm^(−1)) are very low.

Nonequilibrium spatiotemporal dynamics of the Wigner distributions in broad-area semiconductor lasers

We investigate the coupled microscopic and macroscopic nonequilibrium spatiotemporal dynamics of broad-area semiconductor lasers. Characteristic nonequilibrium Wigner distributions of the charge carriers and the interband polarization reveal spatially inhomogeneous field-induced heating and cooling as a consequence of the coupling of the optical field to the carrier and phononic systems. The numerical simulations are performed on the basis of microscopic spatially resolved Maxwell-Bloch equations in which the spatiotemporal variation of thermal properties of the active laser medium are self-consistently included. In particular, the spatiotemporal light-matter interactions are determined by the temporal and spatial transport of the electron-hole plasma, its dependence on carrier-carrier, carrier-phonon, and phonon-phonon scattering processes as well as on the optical properties and macroscopic boundary conditions. In resonantly and nonresonantly excited broad-area semiconductor lasers, the microscopically computed Wigner distributions of the charge carriers and the interband polarization reveal the spatiospectral dependence of the laser gain, the induced refractive index, as well as the electron and hole plasma temperatures. The spatiotemporal nature of these processes explains macroscopic effects such as dynamic thermal lensing and formation of spatiospectral optical structures.

Spatio-temporal characteristics of filamentation in broad-area semiconductor lasers: experimental results

We experimentally study the spatial and temporal characteristics of filamentation in broad-area semiconductor lasers. Near-field measurements show that the sparing between filaments decreases as either the pumping level or the linewidth enhancement factor is increased. Spectral measurements reveal that spatial filaments are accompanied by periodic or chaotic temporal variations. Our experimental results show excellent qualitative agreement with a theory based on a linear stability analysis of the rate equations for an infinitely wide stripe laser.

High-power single-mode emission from a broad-area semiconductor laser with a pseudoexternal cavity and a Fabry–Perot etalon

A simple pseudoexternal cavity with a commerically available broad-area laser diode, standard optics, and a thin FabryPerot etalon provides single-mode emission with 1-W optical power and a tuning range of ~0.5nm . The linewidth is estimated at 200MHz. Such high-power semiconductor laser systems are promising light sources for atomic physics, in which narrow-band high-power light is necessary, for example, for optical pumping of alkali metal vapor. Methods for expanding the tuning range and achieving higher narrow-band optical power are discussed.

Apodizing holographic gratings for the modal control of semiconductor lasers

We first present the fabrication technique of apodizing holographic gratings. Gratings with a spatially variable reflectivity profile were obtained by the interference of two Gaussian beams on a glass plate covered with a photoresist. When the exposure time was short enough to avoid saturation of the photoresist, gratings with a quasi-Gaussian reflectivity profile for the beam reflected in the -1 order were produced; the reflectivity at the center could be as high as 71%, and the half-width of the reflectivity profile at the e(-1) position could be as small as 180 mum. Apodizing gratings were used as the end mirror of the external cavity of a broad-area semiconductor laser. Single longitudinal- and lateral-mode operation was observed over the full range of allowed injection currents.

Broad-area semiconductor lasers with diffraction-grating mode control

Broad-area semiconductor lasers with stripe patterns formed on their front mirrors and the properties of such lasers are described. The stripes are of alternately different reflectivity and therefore the pat- tern acts as a low frequency grating. Such a grating significantly affects the shape of the laser output beam. The envelope of its intensity profile becomes almost Gaussian and the beam cross section is much closer to a circle than in the case of standard lasers. © 1997 Society of Photo-Optical Instrumentation Engineers. (S0091-3286(97)01207-5) Subject terms: semiconductor lasers; broad-area lasers; diffraction gratings; spa- tially modulated reflectivity; mode control.

A theory of broad-area semiconductor lasers with modal reflectors

This paper provides a theoretical analysis of guided lateral modes in broad-area (BA) semiconductor lasers with modal reflectors formed by patterning the reflectivity of the front facet. The analysis has been performed by using the model based on the effective-index method and the concept of the effective facet reflectivity. The numerical results include mode threshold conditions and far-field patterns of the lasing modes for various reflector configurations.

Modified substrate spontaneous emission in broad area semiconductor lasers

We observe modification of the spontaneous emission spectrum emitted through the substrate of a quantum well semiconductor laser. The effect is driven by the proximity of the quantum well active region to the p-side electrical contact of the device, representing an excellent example of cavity quantum electrodynamics. Modification of the spontaneous emission rate and spectrum can be substantial; it must be accounted for in order to infer modal gain or carrier heating phenomena in the device correctly.

Numerical simulation of broad-area high-power semiconductor laser amplifiers

A comprehensive model is presented to study the electrical, thermal, and optical behavior of broad-area traveling-wave InGaAs-AlGaAs semiconductor amplifiers. Finite-element thermal analysis and carrier transport mechanisms are integrated into the beam propagation method to include thermal lensing and optically induced nonlinearities. A self-consistent iteration is developed to simulate the beam filamentation in broad-area semiconductor amplifiers with residual facet reflectivities. The experimentally observed periodic filamentation with intensity minima close to zero is investigated numerically.

Nonlinear mechanisms of filamentation in broad-area semiconductor lasers

There are three nonlinear mechanisms that can lead to filamentation in broad-area semiconductor lasers: gain-saturation-induced changes in the refractive index through the linewidth-enhancement factor, self-focusing due to heat-induced index changes, and self-defocusing through intensity-dependent index changes in the cladding layer. We present a theoretical model to analyze these mechanisms and their relative roles in destabilizing the laser output. We find that there exists a critical value for the linewidth-enhancement factor below which broad area lasers are stable for wide stripe widths (as wide as 250 /spl mu/m) and high pumping levels (as high as 20 times threshold). We also find that broad-area lasers are less susceptible to filamentation through self-defocusing and show how an intensity-dependent index in the cladding layer may be used to suppress filamentation caused by the linewidth-enhancement factor.

Broad-area semiconductor lasers with spatially modulatedreflectivity of the mirrors

Emission properties of broad-area semiconductor lasers with stripes etched on one of the laser mirrors, perpendicular to the p-n junction plane, are described. Spatial modulation of the mirror reflectivity induced lasing of individual, phase coupled filaments and significantly affected the shape of the laser output beam. The cross-section of the beam became almost circular and had an intensity distibution along its horizontal and vertical axes that could be approximated by a Gaussian function.

Lateral spatial effects of feedback in gain-guided and broad-area semiconductor lasers

Starting from Fresnel diffraction theory, we derive analytic expressions for the lateral spatial dependence of feedback fields for the cases of conventional and phase-conjugate optical feedback. By using numerical simulations we show that for narrow-stripe gain-guided lasers, both types of optical feedback from an external cavity can convert the twin-lobed far fields into a nearly single-lobed far field. We also find that conventional and phase-conjugate feedbacks in broad-area lasers induce a spatial modulation of the lateral field that increases the tendency for filamentation at moderate (-30 dB) feedback levels.

Second-moment approach to the time-averaged spatial characterization of multiple-transverse-mode laser beams

Analytical expressions are derived for the time-averaged propagation characteristics of multimode light beams made up of a superposition of paraxial transverse modes. The transverse-mode distributions are not restricted to the usual Hermite–Gaussian or Laguerre–Gaussian functions and may thus have arbitrary characteristics. It is shown that the propagation characteristics of multimode beams can be expressed in simple form in terms of those of the constituent transverse modes, which can be obtained directly from the modal complex-amplitude profiles in any given transverse plane. These expressions may simplify considerably the calculation of the second-moment characteristics of multimode beams because the numerical propagation of each modal field in free space is no longer required. The expressions are used to investigate numerically the degradation of the spatial quality of the output beam radiated by a broad-area semiconductor laser as its fundamental-lateral-mode operation ceases.

Feedback effects in tapered broad-area semiconductor lasers and amplifiers

A detailed experimental investigation of the effect of optical feedback upon the operation of high-power broad area InGaAs strained quantum well semiconductor lasers is presented. In particular, we examine the effect optical feedback has upon the beam quality of high-brightness lasers employing a tapered electrical contact. The far-field of such lasers is shown to degrade rapidly with power feedback ratios exceeding -30 dB, leading to a significantly reduced power in the central diffraction-limited far-field lobe. Far-field degradation is accompanied by dynamic instability and spectral broadening. Good beam quality and low intensity noise can be recovered, however, by rotating the feedback polarization. >

Numerical procedure for the lateral-mode analysis of broad-area semiconductor lasers with an external cavity

A numerical procedure for the investigation of the lateral modes of semiconductor lasers with an external cavity is described. The propagation of the optical field inside the semiconductor laser is carried out via a standard beam-propagation scheme, while the method of coordinate scaling with the generalized Huygens-Fresnel integral allows for a computationally efficient propagation of the field in the external cavity. Using an eigenfunction solver based on the Prony method, it is shown that a simple external-cavity configuration comprising a broad area laser, a collimation lens, and a uniform end reflector can exhibit a complex modal behavior. Compared to a solitary broad area laser, optimized external cavities can yield enhanced modal discriminations, resulting in a single-lateral-mode operation with output power in excess of 100 mW along with a clean, single-lobed far-field pattern. The spatial-filtering action of the broad-area laser is highlighted, and we also discuss of the role played by the wavefront curvature of the beam incident upon the broad-area laser for achieving an efficient suppression of the higher order modes along with minimum increase of the threshold current. >

Filamentation and beam propagation in broad-area semiconductor lasers

The spatio-temporal dynamics of broad-area semiconductor lasers is studied theoretically by numerically solving space-dependent coupled partial differential equations for the (complex) optical fields, the interband polarization and the density of charge carriers. The formation and longitudinal propagation of unstable transverse optical filamentary structures is analyzed in a model configuration for typical double-heterostructure broad-area lasers. Spectral and spatial holeburning in the carrier distributions is observed as a consequence of selective density depletion by the transverse laser modes. The transverse holeburning leads to complex spatio-temporal patterns. >

Spatio-temporal complexity in multi-stripe and broad-area semiconductor lasers

The spatio-temporal dynamics of multi-stripe and broad-area semiconductor lasers is studied theoretically. The dynamics is governed by coupled equations for the (complex) optical fields, the polarization and the density of charge carriers. The (partial differential) model equations are solved numerically. Complex spatio-temporal patterns are observed in multi-stripe lasers. In broad-areas lasers, several transverse modes are exited, forming spatially unstable optical filaments. The dynamic behaviour and the complexity in space and time are analysed by various theoretical tools. Transversely dependent cumulants of the bit-number and a characteristic cross-correlation function are used for the quantification of the spatio-temporal behaviour. The transverse generic eigenmodes of the broad-area laser are determined.

Mode locking of a broad-area semiconductor laser with a multiple-quantum-well saturable absorber

Hybrid mode locking of a broad-area semiconductor laser with a multiple-quantum-well saturable absorber in an external cavity is demonstrated. A novel method for mode control of the broad-area laser output, based on patterning of the multiple quantum well absorber into a microdot mirror structure, is presented. Pulses as short as 15 ps, at a repetition rate of 593 MHz, with an average power of 9 mW and a peak power of 1 W have been achieved.

Multiple-quantum-well GaInAs/GaInAsP tapered broad-area amplifiers with monolithically integrated waveguide lens for high-power applications

A 1.48- mu m tapered broad-area semiconductor laser amplifier with a monolithically integrated waveguide lens as demonstrated. The gain saturation characteristics of the tapered amplifier were examined. A maximum output power of 300 mW and a 3-dB saturation power of 200 mW under quasi-CW conditions were obtained from the amplifier without the waveguide lens. Output power of 200 mW was obtained with a broad emission spectrum of approximately 30 nm when the device was used as a superluminescent diode. The amplified output was focused to a single lobe by the monolithically integrated aspheric waveguide lens, which may be useful for efficient coupling of the output into a single-mode fiber.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Directionality in photoluminescence from λ/2 periodic quantum wells

Our investigations on the photoluminescence from InGaAs/AlGaAs half-wave periodic quantum wells show an enhanced directivity, and suppression of transverse amplified spontaneous emission. These properties are explained by a model that treats the quantum wells as periodic arrays of radiating dipoles, like ‘‘directional antennas’’ for the micro- and radio frequency regimes. Such quantum optic structures have potential for efficient optical amplifiers and external cavity broad area semiconductor lasers.