Ridge Lasers Research Articles

MEMS-based tunable terahertz wire-laser over 330 GHz

We demonstrate continuous tuning of a terahertz quantum cascade wire laser over a broad range of ∼330 GHz (∼8.6% of the 3.85 THz center frequency) with single-mode operation. Tuning is achieved with a narrow laser ridge (∼λ(o)/8) and a moveable MEMS (microelectromechanical systems) side object (plunger) to manipulate the transverse optical mode. The frictionless MEMS plunger enables continuous and reversible tuning. We demonstrate ∼30 GHz redshift tuning using a silicon plunger and ∼300 GHz blueshift tuning using a metal plunger. The blueshift tuning range is limited by the bandwidth of the gain medium rather than the tuning mechanism.

Single-mode quantum cascade lasers based on a folded Fabry-Perot cavity

We demonstrate single-mode quantum cascade lasers employing a folded Fabry-Perot cavity consisting of two straight sections connected by a semicircular section in a “hairpin” shape. These folded cavity lasers emitting at ∼4.5 μm are fabricated with identical processes as those for plain Fabry-Perot ridge lasers, and show a strong suppression of the comb of Fabry-Perot cavity modes, leading to tunable single-mode emission with up to 27 dB side mode suppression ratio and a single-mode operating current range of up to 60% above the threshold current when operated in pulsed mode; single-mode emission is achieved from 80 to ∼240 K.

Terahertz quantum cascade lasers based on type II InGaAs/GaAsSb/InP

We report the demonstration of a terahertz quantum cascade laser based on the In0.53Ga0.47As/GaAs0.51Sb0.49 type II material system. The combination of low effective electron masses and a moderate conduction band offset makes this material system highly suitable for such devices. The active region is a three-well phonon depopulation design and laser ridges have been processed in a double-metal waveguide configuration. The devices exhibit a threshold current density of 2 kA/cm2, provide peak optical powers of 1.8 mW, and operate up to 102 K. Emission frequencies are in the range between 3.6 and 4.2 THz.

High Characteristic Temperature InGaAsP/InP Tunnel Injection Multiple-Quantum-Well Lasers

We fabricate 1.5 μm InGaAsP/InP tunnel injection multiple-quantum-well (TI-MQW) Fabry—Perot (F-P) ridge lasers. The laser heterostructures, including an inner cladding layer and an InP tunnel barrier layer, are grown by metal-organic chemical-vapor deposition (MOCVD). Characteristic temperature T0 of 160 K at 20°C is obtained for 500-μm-long lasers. T0 is measured as high as 88 K in the temperature range of 15–75°C. Cavity length dependence of T0 is investigated.

High-Performance Visible Semiconductor Lasers Operating at 630 nm

A tensile-strained InGaP quantum-well laser structure operating around 630 nm has been designed. The structure offers low threshold current and beam divergence (19° full-width at half-maximum (FWHM) or 34° at 1/e <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> ) simultaneously by incorporating a V-profile layer within the waveguide. Single-mode lasers (630 nm) were fabricated and tested. The threshold current for coated 1-mm-long and 3-m-wide ridge lasers is only 30 mA, and the slope efficiency is 1.04 W/A. This level of performance makes the lasers ideal for display applications.

High-Performance $\lambda \sim 8.6~\mu {\rm m}$ Quantum Cascade Lasers With Single Phonon-Continuum Depopulation Structures

We report high performance λ~8.6 μm m quantum cascade lasers based on 5 quantum well single phonon-continuum (SPC) depopulation scheme, grown by metal organic vapor-phase epitaxy. Devices with SPC depopulation structures exhibit high device performances in pulsed and continuous wave (CW) operation due to a large dipole matrix element (3.0 nm) and short depopulation time (0.22 ps). A 4 mm-long, 8 μm-wide, high reflective (HR)-coated buried-hetero-structure laser with 33 cascade stages, for active region, demonstrates a high output power of 313 mW, a threshold current density of 1.35 kA/cm2, and slope efficiency of 547 mW/A at 30°C in CW operation. The maximum CW lasing temperature of the device is observed to be 120°C. Furthermore, by using a higher number of cascade stages (N=70) in a 4 mm-long, 22 m-wide, HR-coated ridge laser, a high peak power of 6.3 W is obtained together with a high wall-plug efficiency (WPE) of 10.8%, a slope efficiency of 2.7 W/A, and a threshold current density of 0.76 kA/cm2 in pulsed operation at 300 K. The measured WPE of over 10% at room temperature is close to the theoretical values.

Gain analysis of blue nitride-based lasers by small signal modulation

With a small signal frequency-modulation of the driving current, the resonance frequency and the damping factor of the optical output power response of blue nitride-based ridge lasers grown on [0001]-plane gallium-nitride substrates were investigated with a network analyzer setup. From the linear dependence of the squared resonance frequency on the driving current, the gain coefficients of the logarithmic gain model could be extracted being 7680 cm−1 for blue nitride-based lasers. For this purpose, additional parameters such as the carrier density and the confinement factor were assigned by carrier lifetime and quantum efficiency measurements and one dimensional transfer matrix simulations, respectively.

Tunable terahertz quantum cascade lasers with external gratings

We demonstrate a frequency tunable external cavity terahertz quantum cascade laser using an abutted antireflection-coated silicon lens to reduce facet reflection and as a beam-forming element, with an external grating providing frequency selective optical feedback. Angle tuning of the grating allows a single longitudinal mode of the laser ridge to be selected, resulting in discontinuous tuning over a 165 GHz range around a center frequency of 4.4 THz. Another device exhibited 145 GHz of total tuning with 9 GHz of continuous tuning near the longitudinal modes of the laser.

Systematic study on the confinement structure design of 1.5-μm InGaAlAs/InP multiple-quantum-well lasers

An in-dept analysis on the separate confinement heterostructure (SCH) design parameters of 1.5-μm InGaAlAs/InP multiple quantum-well (MQW) lasers is reported. Theoretical calculations show a drastic enhancement on threshold current and slope efficiency from step-index SCH (STEP-SCH) to graded-index SCH (GRIN-SCH) design, but the effect ceases beyond a critical number of grading steps. This finding implies ease of the growth process and reduction in cost. The overall GRIN-SCH’s thickness is found to exert great influence over the achievable laser’s threshold current and slope efficiency. An average of 27 mA threshold current reduction and more than 32% of slope efficiency increment were achieved by optimizing the GRIN-SCH thickness. Increasing the grading energy range of the GRIN-SCH decreases the slope efficiency, but is found to effectively reduce carrier leakage at elevated temperature leading to a less temperature-sensitive threshold current MQW ridge lasers were fabricated and characterized out of two laser materials, one with a reference STEP-SCH and another with a proposed optimized GRIN-SCH profile. The laser with optimised SCH design has shown a 36% reduction in room temperature threshold current as compared to that with the STEP-SCH design, which is in good agreement to the theoretical simulation. In addition, a record high characteristic temperature (T0) of 105 K was obtained on the GRIN-SCH laser structure, which is more than three fold increment as compared to the STEP-SCH design.

Phase-locked arrays of surface-emitting terahertz quantum-cascade lasers

We report the demonstration of phase-locked arrays of surface-emitting distributed-feedback (DFB) terahertz quantum-cascade lasers with single-mode operations. Carefully designed “phase sector” locks several surface-emitting DFB laser ridges in-phase, creating tighter beam-patterns along the phased-array direction with full width at half maximum (FWHM)≈10°. In addition, the phase sector can be individually biased to provide a mechanism of frequency tuning through gain-induced optical index change, without significantly affecting the output power levels. A tuning range of 1.5 GHz around 3.9 THz was achieved. This fine tunability could be utilized to frequency- or phase-lock the DFB array to an external reference.

Strain-compensated GaInAs/AlInAs/InP quantum cascade laser materials

Strain-compensated (SC) GaInAs/AlInAs/InP multiple-quantum-well structures and quantum cascade lasers (QCLs) with strain levels of 1% and as high as 1.5% were grown by organometallic vapor phase epitaxy (OMVPE). The structures were characterized by high-resolution X-ray (HRXRD) diffraction and atomic force microscopy (AFM), and narrow-ridge QCL devices were fabricated. HRXRD and AFM results indicate very high quality materials with narrow satellite peaks, well-defined interference fringes, and a step-flow growth mode for 1% SC materials. A marginal broadening of satellite peaks is measured for 1.5% SC structures, but step-flow growth is maintained. QCLs based on a conventional four-quantum-well double-phonon resonant active region design with nominal 1% SC were grown with doping concentration varied from 1 to 4×10 17 cm −3 in the active region. The performance of ridge lasers under pulsed conditions is comparable to state-of-the-art results for 4.8 μm devices. QCLs with a novel injectorless four-quantum well QCL design and 1.5% SC operated in pulsed mode at room temperature at 5.5 μm.

Burn-in mechanism of 450nm InGaN ridge laser test structures

We investigated the short term stability of the optical output power of 450nm InGaN test lasers. The short term degradation strongly depended on ridge width. It was mainly caused by an increase in threshold current. From measurements of subthreshold wave-length blueshift, carrier lifetime, and output power, we found a decrease in carrier density after 15h of aging. We show a direct correlation of the short term aging with current spreading effects.

Directional emission and universal far-field behavior from semiconductor lasers with limaçon-shaped microcavity

We report experimental demonstration of directional light emission from limaçon-shaped microcavity semiconductor lasers. Quantum cascade lasers (QCLs) emitting at λ≈10 μm are used as a model system. Both ray optics and wave simulations show that for deformations in the range 0.37&amp;lt;ε&amp;lt;0.43, these microcavities support high quality-factor whispering gallerylike modes while having a directional far-field profile with a beam divergence θ∥≈30° in the plane of the cavity. The measured far-field profiles are in good agreement with simulations. While the measured spectra show a transition from whispering gallerylike modes to a more complex mode structure at higher pumping currents, the far field is insensitive to the pumping current demonstrating the predicted “universal far-field behavior” of this class of chaotic resonators. Due to their relatively high quality factor, our microcavity lasers display reduced threshold current densities compared to conventional ridge lasers with millimeter-long cavities. The performance of the limaçon-shaped QCLs is robust with respect to variations of the deformation near its optimum value of ε=0.40.

Thermal analysis of buried heterostructure quantum cascade lasers for long‐wavelength infrared emission using 2D anisotropic heat‐dissipation model

AbstractWe have theoretically investigated and compared the thermal characteristics of γ ∼ 10.6 μm InGaAs/InAlAs/InP buried heterostructure (BH) quantum cascade lasers (QCLs) with different heat‐sinking configurations by a steady‐state heat‐transfer analysis. The heat‐source densities were obtained from laser threshold power densities measured experimentally under room‐temperature continuous‐wave mode. The two‐dimensional anisotropic heat‐dissipation model was used to calculate the temperature distribution, heat flux, and thermal conductance (Gth) inside the device. For good thermal characteristics, the QCLs in the long‐wavelength infrared region require the relatively narrow BH structure in combination with epilayer‐down bonding due to thick active core/cladding layers and high insulator losses. The single‐ridge BH structure results in slightly higher thermal conductance by ∼2–4% than the double‐channel (DC) ridge BH structure. For W = 12 μm with 5 μm thick electroplated Au, the single‐ridge BH laser with epilayer‐down bonding exhibited the highest Gth value of 201.9 W/K cm2, i.e. increased by nearly 36% with respect to the epilayer‐up bonded DC ridge waveguide laser. This value is improved by ∼50% and ∼62% with respect to the single‐ridge BH laser and DC ridge waveguide laser with W = 20 μm in the epilayer‐up bonding scheme, respectively. (© 2009 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

High-Brightness and Ultranarrow-Beam 850-nm GaAs/AlGaAs Photonic Band Crystal Lasers and Single-Mode Arrays

Lasers with a waveguide based on a longitudinal photonic band crystal designed for 850 nm emission are investigated. They demonstrate ultranarrow vertical beam divergence of about 7deg full-width at half maximum without using corrective optics. A high internal efficiency of 93% is achieved. Broad-area 50-mum-wide stripe lasers with unpassivated facets show a high total output power of about 20 W in pulsed mode with far-field divergences of 9.5deg and 11.3deg of the slow and fast axis, respectively, equivalent to an ultrahigh brightness of 3t108 Wmiddotcm-2 middotsr-1 and a low aspect ratio of only 1.2. Narrow ridge lasers with 5 mum stripes demonstrate more than 1.5 W maximum output power in continuous-wave operation. Large arrays with up to 256 uncoupled single-mode laser diodes demonstrate low threshold currents of about 70 mA per laser, independent of the number of lasers in the array.

Quantum cascade lasers with integrated plasmonic antenna-array collimators

We demonstrated in simulations and experiments that by defining a properly designed two-dimensional metallic aperture-grating structure on the facet of quantum cascade lasers, a small beam divergence angle can be achieved in directions both perpendicular and parallel to the laser waveguide layers (denoted as theta perpendicular and theta parallel, respectively). Beam divergence angles as small as theta perpendicular=2.7 degrees and theta parallel=3.7 degrees have been demonstrated. This is a reduction by a factor of approximately 30 and approximately 10, respectively, compared to those of the original lasers emitting at a wavelength of 8.06 microm. The devices preserve good room temperature performance with output power as high as approximately 55% of that of the original unpatterned lasers. We studied in detail the trade-off between beam divergence and power throughput for the fabricated devices. We demonstrated plasmonic collimation for buried heterostructure lasers and ridge lasers; devices with different waveguide structures but with the same plasmonic collimator design showed similar performance. We also studied a device patterned with a "spider's web" pattern, which gives us insight into the distribution of surface plasmons on the laser facet.

High-Performance Quantum Cascade Lasers Grown by Metal-Organic Vapor Phase Epitaxy and Their Applications to Trace Gas Sensing

We present an overview of our results on the design, material growth, device characterization, and spectroscopic applications of MOVPE-grown quantum cascade lasers (QCLs). These devices are capable of room-temperature (RT) continuous-wave operation and high power emission. The first section focuses on growth of laser material, device fabrication, and quantum design The second section discusses RT pulsed operation, in particular the doping dependence of laser performance and broadband emission. Near-field measurements performed on the devices' facets correlating lateral modes to device size are also discussed. Section III deals with continuous-wave high-temperature operation from lasers with different active region designs, including their spectral characteristics and in the emergence of coherent phenomena at high power levels. Section IV analyses the devices thermal dissipation capabilities, while in Section V we report reliability data. The final section focuses on spectroscopic applications and tunability. Optofluidic narrow ridge lasers and their application to chemical sensing are reported along with recent data on a broadband on chip spectrometer consisting of individually addressable distributed feedback QCLs. Various spectroscopic techniques and in particular quartz-enhanced photoacoustic absorption spectroscopy and its use in gas sensing systems are discussed. Finally, optofluidic narrow ridge lasers and their applications to fluid sensing are presented.

Thermal analysis of InP-based quantum cascade lasers for efficient heat dissipation

We have theoretically investigated the thermal characteristics of double-channel ridge–waveguide InGaAs/InAlAs/InP quantum cascade lasers (QCLs) using a two-dimensional heat dissipation model. The temperature distribution, heat flow, and thermal conductance (Gth) of QCLs were obtained through the thermal simulation. A thick electroplated Au around the laser ridges helps to improve the heat dissipation from devices, being good enough to substitute the buried heterostructure (BH) by InP regrowth for epilayer-up bonded lasers. The effects of the device geometry (i.e., ridge width and cavity length) on the Gth of QCLs were investigated. With 5 μm thick electroplated Au, the Gth is increased with the decrease of ridge width, indicating an improvement from Gth=177 W/K⋅cm2 at W=40 μm to Gth=301 W/K⋅cm2 at W=9 μm for 2 mm long lasers. For the 9 μm×2 mm epilayer-down bonded laser with 5 μm thick electroplated Au, the use of InP contact layer leads to a further improvement of 13% in Gth, and it was totally raised by 45% corresponding to 436 W/K⋅cm2 compared to the epilayer-up bonded laser with InGaAs contact layer. It is found that the epilayer-down bonded 9 μm wide BH laser with InP contact layer leads to the highest Gth=449 W/K⋅cm2. The theoretical results were also compared with available obtained experimentally data.

Surface-emitting terahertz quantum cascade laser source based on intracavity difference-frequency generation

We report a surface-emitting terahertz source based on intracavity difference-frequency generation in dual-wavelength midinfrared quantum cascade lasers with integrated giant second-order nonlinear susceptibility. The terahertz light is coupled out of the waveguide by a second-order grating etched into the laser ridges. In contrast to sources where the difference-frequency radiation is extracted from the facet, this approach enables extraction of the terahertz emission from the whole length of the device even when the coherence length is small.

Thermoreflectance imaging of laser diodes and VCSELs along and perpendicular to the emission direction

Thermal characterization of semiconductor lasers is an important issue for optoelectronics. This paper presents our thermoreflectance measurements on two different types of laser diodes: classical ridge laser diodes and vertical cavity surface emitting lasers (VCSELs). We first studied the external temperature increase in ridge diodes in order to determine inhomogeneity. Then, we tried to determine the inner temperature of the VCSELs.