This paper, “ALADIN: an atmospheric laser Doppler wind lidar instrument for wind velocity measurements from space," was presented as part of International Conference on Space Optics—ICSO 1997, held in Toulouse, France.
The remote detection and identification of liquid chemical contamination is a difficult problem for which no satisfactory solution has yet been found. We have investigated a new technique, pulsed indirect photoacoustic spectroscopy (PIPAS), and made an assessment of its potential for operation at stand-off ranges of order 10m. The method involves optical excitation of the liquid surface with a pulsed laser operating in the 9-11μm region. Pulse lengths are of order 3μs, with energy ~300μJ and repetition rates ~200Hz. Rapid heating of the liquid by the laser pulse produces acoustic emission at the surface, and this is detected by a sensitive directional microphone to increase the signal-to-noise ratio and reduce background clutter. The acoustic pulse strength is related to the liquid's absorption coefficient at the laser wavelength; tuning allows spectroscopic investigation and a means of chemical identification. Maximum coverage rates have been examined, and further experiments have examined the specificity of the technique, allowing a preliminary assessment of false-alarm and missed-signal rates. The practical aspects of applying the technique in a field environment have been assessed.
Zinc germanium phosphide (ZGP) is well suited to use in optical parametric oscillators (OPOs) for conversion of near-infrared laser output into the mid-infrared waveband (3 to 5 μm). Typical OPO applications seek to exploit pump wavelengths close to 2 μm so that both the output wavelengths fall within the mid-infrared waveband. However, the material typically suffers optical loss arising from growth defects that becomes significant at wavelengths below about 2.5 μm. We report the results of calorimetric studies that show that the loss can comprise both absorptive and scattering components. We have assessed the affect of loss at the pump wavelength on the conversion efficiency of a high pulse repetition frequency, doubly-resonant, ZGP OPO pumped with 2.094 μm radiation generated by a wavelength doubled Nd:YLF laser. The OPO used crystals having loss coefficients in the range 0.03 cm-1 to 0.3 cm-1. The reduction in slope efficiency for the conversion process was evaluated over a range of pump beam diameters (1/e2 intensity) from 0.12 mm to 0.30 mm. For the largest beam diameter a slope efficiency of 57% was measured for a ZGP OPO crystal having a loss coefficient of 0.03 cm-1. The slope efficiency reduced to about 30% when the loss coefficient was increased to 0.3 cm-1.
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 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.
The phenomenon of thermal emission form non-volatile liquid surface coatings following pulsed laser heating has been experimentally and theoretically studied with a view to developing a differential thermal imaging scheme for the remote detection of contaminated surfaces. Pulsed UV and IR laser sources have been used to generate radiance profiles from contaminants which are correlated with their characteristic spectra. Data from experiments and numerical simulations are compared and a reasonable level of agreement is demonstrated.
We present results obtained from a new class of self- switching, high repetition rate, HF (DF) laser. The laser utilizes a magnetically stabilized longitudinal discharge, transverse to a high velocity gas flow. The gas mixture is pre-mixed, and consists of He, SF6 and H2(D2) in the ratio 1000:9:2 at a total pressure of around 52 torr. A centrifugal fan recirculates the gas and provides a linear flow velocity of 80 ms-1 in the gain region. Permanent magnets provide the stabilizing magnetic field of approximately 1400 Gauss. This magnetic field ensures that the discharge and optic axes are co-linear. The discharge length is 30 cm, and the gas flow channel 0.5 cm in height. Conventional stable resonators were used to extract the laser energy. We show that the self-switching behavior is a result of the negative I-V characteristic in the positive column of a constricted SF6 discharge, coupled with the current limitations imposed by the external electrical circuit. It is found that the switching frequency, and therefore lasing repetition rate, can be controlled via either the applied discharge current or the RC time constant for the external circuit. Higher discharge currents and shorter time constants both result in higher pulse repetition frequencies. We have demonstrated self-switched lasing at repetition rates from 400 Hz up to 17 kHz.
We describe the design and performance of a closed cycle, high pulse repetition frequency HF(DF) laser. A short duration, glow discharge is formed in a 10 SF6:1 H2(D2) gas mixture at a total pressure of approximately 110 torr. A pair of profiled electrodes define a 15 X 0.5 X 0.5 cm3 discharge volume through which gas flow is forced in the direction transverse to the optical axis. A centrifugal fan provides adequate gas flow to enable operation up to 3 kHz repetition frequency. The fan also passes the gas through a scrubber cell in which ground state HF(DF) is eliminated from the gas stream. An automated gas make-up system replenishes the spent fuel gases removed by the scrubber. Total gas admission is regulated by monitoring the system pressure, whilst the correct fuel balance is maintained through measurement of the discharge voltage. The HF(DF) generation rate is determined to be close to 5 X 1019 molecules per second per watt of laser output. Typical mean laser output powers of up to 3 watts can be delivered for extended periods of time. The primary limitation to life is found to be the discharge pre- ionization system. A distributed resistance corona pre- ionizer is shown to be advantageous when compared with an alternative arc array scheme.
The design and performance of a closed cycle high repetition rate HF laser is described. A short pulse, glow discharge is formed in a 10 SF6:1 H2 gas mixture at a total pressure of approximately 110 torr within a 15 by 0.5 by 0.5 cm3 volume. Transverse, recirculated gas flow adequate to enable repetitive operation up to 3 kHz is imposed by a centrifugal fan. The fan also forces the gas through a scrubber cell to eliminate ground state HF from the gas stream. An automated gas make-up system replenishes spent gas removed by the scrubber. Typical mean laser output powers up to 3 W can be maintained for extended periods of operation.
AEROSPATIALE, leading a European team, has just conducted a successful study, under ESA contract, to demonstrate the feasibility of a spaceborne Doppler wind lidar instrument meeting the scientific requirements of wind velocity measurements from space with high spatial resolution. A first parametric investigation, based upon the initial set of mission requirements, and supported by dedicated models and detailed trade-off studies, took account of capabilities of most promising signal processing algorithms and calibration/validation constraints: it yielded a large conically scanned instrument deemed technologically risky. A risk analysis was then carried out to propose a less challenging instrument meeting most key mission requirements. The fixed line-of-sight concept with return signal accumulation appeared as most attractive. A second set of requirements agreed upon by scientific users was therefore issued, with relaxed constraints mainly on horizontal resolution, keeping roughly the same level of wind velocity measurement accuracy. A second instrument and subsystem trade-off was then performed to eventually produce an attractive instrument concept based upon a pair of small diameter telescopes each one associated to one scanning mirror rotating stepwise around the telescope axis, which drastically reduces the detection bandwidth. Following the main contract, studies of accommodation on the International Space Station have been performed, confirming the interest of such an instrument for wind measurements from space.
AEROSPATIALE, leading a European team, has just conducted a successful study, under ESA contract, to demonstrate the feasibility of a spaceborne Doppler wind lidar instrument meeting the scientific requirements of wind velocity measurements from space with high spatial resolution. A first parametric investigation, based upon the initial set of mission requirements, and supported by dedicated models and detailed trade-off studies, took account of capabilities of the most promising signal processing algorithms and calibration/validation constrains: it yielded a large conically scanned instrument deemed technologically risky. A risk analysis was then carried out to propose a less challenging instrument meeting most key mission requirements. The fixed line-of-sight concept with return signal accumulation appeared as most attractive. A second set of requirements agreed upon by scientific users was therefore issued, with relaxed constraints mainly on horizontal resolution, keeping roughly the same level of wind velocity measurement accuracy. A second instrument and subsystem trade- off was then performed to eventually produce an attractive instrument concept based upon a pair of small diameter telescopes each one associated to one scanning mirror rotating stepwise around the telescope axis, which drastically reduces the detection bandwidth. Following the main contract, studies of accommodation on the International Space Station have been performed, confirming the interest of such an instrument for wind measurements from space.
The transmitter laser is recognised to be one of the most critical technologies for space-based Doppler windlidar [1].
We present initial evaluation of the performance of an e-beam sustained device in the 1OJ, 10 Hz class. Lifetime issues
are addressed in a subsidiary paper. We describe the design of the device and the results of a number of characterisation studies:
1) General nonoptical tests of gas circulation and heat exchanger efficiency. 2) Performance optimisation to maximise multimode efficiency as a function of energy loading, main discharge
E/N and gas composition, all tests allowed for optimisation of cavity extraction. 3) Characterisation of the novel plasma anode electron gun with respect to beam uniformity, secondary electron
concentration, and current constancy. 4) Optical characterisation to examine operating wavelength, pulse shape, beam profile in the near and far-field,
output energy and electrical to optical conversion efficiency, and frequency behaviour during the pulse.
The European Space Agency program for Development of a CO2 Laser for Spaceborne Doppler Wind Lidar Applications addresses both performance and lifetime aspects. Lifetime issues are of particular importance due to the 109 pulse life requirement for a spaceborne laser operating continuously at 10 Hz for a period of three years. Particularly critical lifetime issues for an e-beam sustained laser have been identified as the electron transmitting metal foil separating the electron gun and the laser, and the gas life. Four areas of study have been undertaken to address the foil and gas lifetime issues: Parametric Study of Gaseous Catalysis to determine the range of operating conditions under which oxidation of CO by high energy electrons can be expected to offset dissociation of CO2, thus eliminating the need for solid catalyst. Extended Sealed Runs to demonstrate long life in a representative laser system of the actual size required. Several runs of 107 pulses, and one run of 6.5 X 107 pulses, have been performed. The Foil Thermal Profile has been monitored using a pyroelectric vidicon camera to determine the maximum temperature reached by different candidate foil materials under representative conditions. High Temperature Foil Fatigue tests of 109 pulses have been carried out to simulate the effect of the laser pressure pulse, by performing fatigue tests on foil materials at high temperature.
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