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Single-mode mid-infrared lasers for gas sensing in the 2–4um range

Single-mode mid-infrared lasers for gas sensing in the 2–4um range,10.1109/ICO-IP.2011.5953725,J. A. Gupta,P. J. Barrios,A. Bezinger,P. Waldron,B. F.

Single-mode mid-infrared lasers for gas sensing in the 2–4um range  
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Type-I interband laser diodes were developed for trace gas sensing applications in the 2-4um wavelength range. The devices were grown by molecular beam epitaxy on GaSb substrates using InGaAsSb/Al(In)GaAsSb active regions. Tunable, single-mode lasers were produced using distributed feedback grating processing or by incorporating Fabry-Perot lasers in an external cavity configuration. Sensitive gas detection was demonstrated using these lasers in tunable-diode laser absorption spectroscopy. I. INTRODUCTION There are many applications for gas sensing systems based on mid-infrared laser diodes. Trace gas sensing with a single- mode laser can be accomplished using tunable diode laser absorption spectroscopy (TDLAS), in which the wavelength of the laser is modulated through a strong absorption feature of the gas of interest, in a wavelength range free of interferences from other species. Laser diodes in the 2-3um range can be readily produced on GaSb substrates using compressively- strained InGaAsSb type-I quantum wells surrounded by AlGaAsSb waveguide and cladding layers which are lattice- matched to the substrate. Molecular beam epitaxy (MBE) is a preferred technique for the growth of these structures because of its excellent thickness and compositional control. These properties are even more important for the development of devices with wavelengths beyond 3um. In this case, it has been demonstrated that laser performance can be improved through the use of quinary AlInGaAsSb barrier layers, which simultaneously increase the hole confinement and decrease the conduction band offset (1). This more favourable band offset prevents hole leakage and improves the homogeneity of electron injection into multiple quantum well active regions. In this work we have developed single-mode lasers using quantum well active regions designed for specific operating wavelengths of 2476nm and 3240m. The shorter wavelength, single-mode devices were fabricated using a regrowth-free distributed feedback (DFB) process(2) involving laterally- coupled etched gratings to provide continuous single-mode tuning with current and temperature through absorption features of HF gas. Single-mode operation at longer wavelengths was achieved by controlled adjustment of a diffraction grating in an external cavity configuration. The resulting single-mode lasers operate at wavelengths around 3240nm, which is extremely important for the detection of methane and other hydrocarbons. II. EXPERIMENT The laser structures were grown on (100) GaSb:Te substrates in a V90 MBE system using conventional group-III effusion cells and valved cracker cells for As2 and Sb2. The structures consist of Te- and Be-doped Al0.6Ga0.4As0.052Sb0.948 cladding layers lattice-matched to the GaSb substrate (thicknesses in the range 1.5-3.0um). The composition and thickness of the quantum-well active region for each laser was carefully designed for the intended application. For the laser structure with target wavelength of 2476nm, the active region contains three 10.9nm In0.43Ga0.57As0.15Sb0.86 quantum wells separated by 30nm, with Al0.24Ga0.76As0.02Sb0.98 barrier and waveguide layers. Single-mode DFBs at 2476nm were produced using a two step inductively-coupled plasma reactive ion etching (ICP-RIE) procedure. In the first step, a narrow ridge waveguide structure was etched in the semiconductor. In the second step, first- order lateral gratings were etched on either side of the ridge to provide evanescent coupling to the optical mode (grating pitch λ=349.12nm, 50% duty cycle, thickness 330nm). E-beam lithography with fine-pitch control was used to write the lateral gratings using ZEP resist. Standard TiPtAu and NiGeAu metallization was used for the p- and n-contacts, respectively. After cleaving, the front output facet of the lasers was coated with a single layer of Al2O3 using ion-beam sputter deposition to provide a reflectivity of approximately 7%. The back facet of the laser was coated with a similar Al2O3 layer, followed by a multilayer stack of SiO2/TiO2 to provide a reflectivity of 90%. With these coatings, approximately 97% of the laser light should be emitted from the front facet.
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