Paul D. Mason

Chalcogenide glass films for the bonding of GaAs optical parametric oscillator elements

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.

Spectral line narrowing in PPLN OPO devices for 1-μm wavelength doubling

One route to generating mid-infrared (mid-IR) radiation is through a two-stage non-linear conversion process from the near-IR, exploiting powerful neodymium lasers operating at wavelengths close to 1 μm. In the first stage of this process non-linear conversion within a degenerate optical parametric oscillator (OPO) is used to double the wavelength of the 1 μm laser. The resultant 2 μm radiation is then used to pump a second OPO, based on a material such as ZGP, for conversion into the 3 to 5 μm mid-IR waveband. Periodically poled lithium niobate (PPLN) is a useful material for conversion from 1 to 2 μm due to its high non-linear coefficient (deff ~ 16 pm/V) and the long crystal lengths available (up to 50 mm). Slope efficiencies in excess of 40% have readily been achieved using a simple plane-plane resonator when pumped at 10 kHz with 3.5 mJ pulses from a 1.047 μm Nd:YLF laser. However, the OPO output was spectrally broad at degeneracy with a measured full-width-half-maximum (FWHM) linewidth of approximately 65 nm. This output linewidth is significantly broader than the spectral acceptance bandwidth of ZGP for conversion into the mid-IR. In this paper techniques for spectral narrowing the output from a degenerate PPLN OPO are investigated using two passive elements, a diffraction grating and an air spaced etalon. Slope efficiencies approaching 20% have been obtained using the grating in a dog-leg cavity configuration producing spectrally narrow 2 μm output with linewidths as low as 2 nm. A grating-narrowed degenerate PPLN OPO has been successfully used to pump a ZGP OPO.

A high-repetition-rate PPLN mid-infrared optical parametric oscillator source

High average power sources operating in the 3 to 5 μm mid-infrared waveband are of interest for a wide variety of applications. We present design and performance results for a high-power engineered breadboard mid-IR source based on near-infrared pumped periodically-poled lithium niobate (PPLN) optical parametric oscillator (OPO) technology. The source design utilises a pair of singly-resonant PPLN OPOs pumped by a commercial 40 Watt, Q-switched, diode-pumped Nd:YLF laser. The mid-IR outputs from each OPO are polarisation recombined into a single output beam. A twin OPO design was chosen to minimise the effect of optical absorption, reduce thermal loading within each PPLN crystal and provide additional flexibility by offering the option for dual-wavelength mid-IR operation. An average output power approaching 4 Watts has been obtained with a corresponding slope efficiency of 15%. The mid-infrared beam is 6 times diffraction limited. Laser operation is controlled by a remote PC link and power, spectral and temporal pulse diagnostics are included within the system.

Review of the development of nonlinear materials for mid-IR generation

A review of the development of nonlinear materials suitable for use in the mid-IR (3 to 5 μm) is presented. This examines the properties, performance, limitations and availability of a range of materials, including birefringently phasematched crystals and engineered quasi-phasematched materials. Higher-order nonlinear processes in alternative materials are also considered and a discussion on material suitability for down-conversion of near-IR lasers into the mid-IR to meet various application requirements is also presented.

Improved multi-layer glass-bonded QPM GaAs crystals for non-linear wavelength conversion into the mid-infrared

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.

Laser calorimetry as a tool for the optimization of mid-infrared OPO materials

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 samples 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.

Assessment of diffusion-bonded KTP crystals for efficient, low pulse energy conversion from 1 to 2 μm

Diffusion bonded (DB) walk-off compensated KTP crystals offer an alternative nonlinear medium for efficient 1 to 2 microm conversion within optical parametric oscillators (OPOs) at low pulse energies. Spatial variations in optical absorption and transmission values measured at 2 mum are reported for two DB-KTP crystals. Finally, a comparison is made between the conversion efficiency obtained from a degenerate 1 microm pumped OPO using a single 20 mm KTP crystal and an equivalent length DB-KTP crystal consisting of two bonded 10 mm crystals.

Optical parametric amplification of mid-infrared radiation using multi-layer glass-bonded QPM GaAs crystals

Non-linear optical wavelength conversion of near-infrared lasers within optical parametric oscillators (OPOs) offers a route to powerful tunable sources in the mid-infrared (mid-IR). Engineered quasi-phasematched (QPM) non-linear optical materials based on gallium arsenide (GaAs) offer an alternative to conventional birefringently phasematched single-crystal materials such as ZnGeP2, which are currently used in mid-IR OPOs. QPM GaAs crystals have been assembled from commercially available, high-optical quality 100-micron thickness gallium arsenide (GaAs) wafers using a novel glass-bonding (GB) process. This uses thin layers of an infrared transmitting glass (refractive index matched to GaAs) deposited onto each GaAs wafer, which, when heated under pressure, fuse the wafers together to form a monolithic structure. By varying the thickness of the deposited glass layers, the dispersion in the glass can be used to compensate for variations in GaAs wafer thickness and to fine tune the phasematching wavelengths of the QPM crystal. GBGaAs crystals with up to 100 layers have been designed and built for wavelength conversion from 2 &mgr;m into the mid-IR. We report the performance of these crystals used as optical parametric amplifiers (OPAs) in the mid-IR, when pumped by a 2.094 &mgr;m source, and compare these results to measurements for a ZGP OPA. In addition, the dependence of conversion within GBGaAs crystals on the polarisation state of the amplifier seed beam has been investigated along with the temperature dependence of the optimum operating wavelength. Good agreement between experimental results and performance predictions obtained from a numerical model is observed.

CW and temporal theoretical model predictions and experimental results for Tm:YAG and Ho:YAG lasers

We describe the development of a time dependent thulium laser model. The model is used to predict both the CW and temporal behaviour of a Tm:YAG laser. Experimental results from a diode-pumped Tm:YAG laser are obtained and the model is used to obtain good agreement with these observations for both the CW and temporal behaviour of the laser. Particular results relate to switch-on time delays and the effect of pump diode modulation on Tm laser efficiency. The laser model has been extended to the case of the Ho:YAG laser where other important effects due to ground state depletion and self re-absorption must be taken into account. The holmium laser model has recently been used to predict reported experimental results from a thulium fibre laser pumped Ho:YAG laser.

Novel approach to realizing quasi-phase-matched gallium arsenide optical parametric oscillators for use in mid-IR laser systems

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.