KEYWORDS: Laser applications, Cladding, Semiconductor lasers, Chemical species, Active optics, Wave propagation, Reactive ion etching, New and emerging technologies, Mirrors, Atomic clocks
A GaAs/AlGaAs distributed feedback semiconductor (DFB) laser with a laterally-coupled grating is demonstrated at a wavelength of 780.24 nm with an output power up to 60 mW. A mode expander and aluminum-free active layers have been used in the material epilayer to reduce the linewidth to 612 kHz while maintaining high output power. The fabricated laser demonstrates over 40 dB side-mode suppression ratio with tuning range > 0.3 nm, which is suitable for atom cooling experiments with the D2 87Rb atomic transition and provides substantial potential for the laser to be integrated into miniaturized cold atom systems.
Multiple individually-controllable Q-switched laser outputs from a single diode-pumped Nd:YAG module are presented by using an electrostatic MEMS scanning micromirror array as cavity end-mirror. The gold coated, 700 μm diameter and 25 μm thick, single-crystal silicon micromirrors possess resonant tilt frequencies of ~8 kHz with optical scan angles of up to 78°. Dual laser output resulting from the actuation of two neighboring mirrors was observed resulting in a combined average output power of 125 mW and pulse durations of 30 ns with resulting pulse energies of 7.9 μJ and 7.1 μJ. The output power was limited by thermal effects on the gold-coated mirror surface. Dielectric coatings with increased reflectivity and therefore lower thermal stresses are required to power-scale this technique. An initial SiO2/Nb2O5 test coating was applied to a multi-mirror array with individual optical scan angles of 14° at a resonant tilt frequency of 10.4 kHz. The use of this dielectric coated array inside a 3-mirror Nd:YAG laser cavity led to a single mirror output with average Q-switched output power of 750 mW and pulse durations of 295 ns resulting in pulse energies of 36 μJ.
Optical parametric oscillators (OPOs) offer a route to powerful tunable output in the mid-infrared (mid-IR). Mid-IR OPOs exploit wavelength conversion of near-infrared lasers within non-linear optical materials. A new approach to engineering suitable non-linear OPO materials is being developed as an alternative to conventional chalcopyrite crystals such as ZnGeP2. These new materials use commercially available, high-optical quality gallium arsenide (GaAs) wafers and a novel glass-bonding (GB) process to assemble quasi-phase matched (QPM) multilayer structures. The assembled QPM GaAs stack must have low optical loss and a large useable aperture and needs to be produced reliably with a minimum of 50 layers. Results from a recent sequence of 50-layer GBGaAs stack fabrication will be presented. Of the six stacks successfully bonded two had a useable aperture of approximately 20 mm2 (40% of the maximum available). Of these, one has the lowest absorption and transmission loss per layer (0.07% measured at 2 μm) of any multi-layer glass-bonded QPM GaAs stack produced to date. By adjusting the load distribution at the edges of the stack during bonding the useable optical aperture was increased to nearly 90%. Results from non-linear wavelength conversion experiments into the mid-infrared using multi-layer GBGaAs crystals will be presented.
Non-linear optical (NLO) devices for wavelength conversion of laser sources into the mid-infrared waveband (such as optical parametric oscillators) require the provision of non-linear materials. Quasi-phase matched (QPM) gallium arsenide crystals represent a promising alternative NLO material (high non-linear coefficient, low-optical loss) to conventional birefringent chalcopyrite crystals for use in the mid to far-infrared. To date, several approaches have been investigated to produce QPM GaAs crystals, including diffusion and fusion wafer bonding, orientation patterned growth and total internal reflection techniques. However, these require ultra-clean processing environments, relatively high bonding temperatures or are limited in crystal aperture. We present an approach to developing QPM GaAs crystals based on bonding using an index-matched chalcogenide glass. The glass-bonding (GBGaAs) technique forms low-loss bonds at moderate temperature and has several advantages over existing approaches. In particular, the technique is tolerant to GaAs wafer thickness variations and surface defects, and has the potential to produce large-aperture crystals. The glass-bonding process involves coating individual GaAs wafers with a thin-film of glass, deposited by RF sputtering, and then bonding assembled stacks of coated wafers in a vacuum oven under carefully controlled temperature and pressure conditions to form a single composite structure. To date, GBGaAs crystals consisting of up to 40 layers have been produced and optical losses per layer of less than 0.1% have been achieved. An outline of the production process for manufacturing GBGaAs crystals will be described together with details of optical assessment procedures. The impact of glass purity, sputtering conditions and pressing conditions on optical absorption levels will be reported. Techniques to minimise optical loss in fabricated crystals will be discussed.
Most of the applications that require frequency agile solid state laser systems for use in the mid-infrared are centred on the development of optical parametric oscillators. These exploit the non-linear optical characteristics of non-centrosymmetric materials, in particular the chalcopyrite class of materials that includes AgGaSe2 and ZnGeP2. Whilst such materials are generally difficult to produce, major strides have been made in recent years to optimise crystal growth processes which have enabled the generation of moderate laser output powers. Other approaches have been centred on the use of periodically poled lithium niobate and diffusion bonded gallium arsenide. The latter system is particularly attractive because it exploits a readily available crystalline material, but its implementation is difficult because of the need for an ultra-clean processing environment and relatively high bonding temperatures. This paper describes progress in the development of a new, low-temperature approach for achieving quasi-phase matched gallium arsenide by bonding with an index-matched chalcogenide glass. A major advantage of this approach is the tolerance to GaAs wafer thickness variations and to defects at the surface of the GaAs wafers. Several glass compositions in the germanium-arsenic-selenium-tellurium system have the desired refractive indices, but only some provide the characteristics necessary to ensure the formation of stable low-loss bonds. The glass bonding process begins by RF sputtering films of the glass from pre-manufactured targets onto each side of individual GaAs substrates. These coated substrates are then assembled in a vacuum oven and uniaxially pressed under carefully controlled conditions until a single composite assembly is formed. Issues such as glass purity, the integrity of the sputtering process and choice of pressing conditions are important in ensuring that a high quality non-linear crystal is produced.
A new laser calorimetric technique has been developed to enable absorption, transmission and heat capacity measurements to be made on arbitrarily shaped crystals and other optical materials. Samples are mounted inside a unique cradle device, which ensures minimal heat exchange with the sample's surroundings. A transmission map of the sample is formed by moving the sample, under computer control, through a fixed laser beam. The absorption of the sample at specific points is obtained by recording the temperature rise of the sample due to heating by the laser beam. Spatially resolved measurements are reported for a number of materials including ZnGeP2 and quasi-phase matched GaAs, and correlated with transmission characteristics obtained using a mid-IR band InSb camera.
There are many applications driving the need for frequency agile solid state laser systems for use in the mid-infrared. Most of these are centred on the development of optical parametric oscillators (OPOs), which exploit the non-linear optical characteristics of non-centrosymmetric materials. In a new approach highlighted in a companion paper, OPO elements are formed by bonding gallium arsenide wafers precoated with RF sputtered films of a quaternary chalcogenide glass. The conditions used for sputtering the glass films are critical in ensuring the realisation of reliable bonds, where the glass is required to be index matched to the GaAs within very close tolerances. Issues such as glass composition, purity, porosity, devitrification and optical absorption are all key factors in determining the success of the approach. This paper describes a summary of some of the results achieved, emphasising the degree of control necessary for both the sputtering process and the preparation of the sputtering targets. Composition changes on sputtering can influence the refractive index of the glass and can easily introduce levels of insertion loss that are unacceptable by the time that stacks containing 50 or more individual phase-matched GaAs elements have been produced. Oxygen-related impurities are also easily introduced from a variety of sources and can degrade performance levels further. Such difficulties have been overcome and a reproducible technique for fabricating glass-bonded GaAs crystals has been developed. Optimised conditions for thermal bonding pairs of glass coated GaAs wafers are also reported.
A technique for bonding semiconductor optics is described. A thin film of chalcogenide glass is sputtered onto each surface to be bonded. The sputtered films are then placed in close contact and heated at low temperature under pressure to cause them to fuse. With careful choice of materials the resulting interface is virtually invisible. The technique has been demonstrated with gallium arsenide plates. A quaternary chalcogenide glass has been developed with a refractive index within 3% of that of gallium arsenide (3.34 @ 2.07μm). The glass sputters with no change in composition onto the surfaces of the plates to be bonded. Heat treatment at less than 200°C results in an interface with an optical absorption of less than 0.1% measured using a laser calorimeter operating at 2.07μm. The absorption of the structure was similar to that of an equivalent single piece of gallium arsenide.
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