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A fourth possibility to achieve these wavelength ranges is to shift the quasi-3-level laser lines (938 nm and 946 nm) of the Nd:YAG laser by replacing aluminium and yttrium by other rare earth elements. Changes of the host lattice characteristics lead to a shift of the upper and lower laser levels. These modified crystals are summarized under the name of "Mixed Garnet" crystals. Only the Mixed Garnet lasers can be pumped directly with diode laser and use a direct approach to generate the required laser pulses without frequency conversion. Therefore no additional non-linear crystals or special pump lasers are needed and a higher electric to optical efficiency is expected as well as single frequency operation using spectral tuning elements like etalons. In a first phase such mixed garnet crystals had been grown and characterised. The outcome was the selection of the gadolinium-scandium garnet for the most suitable laser crystal. During a second phase the complete laser system with output energy about 18 mJ in single 20 ns pulses and up to 8 mJ in free running mode with a combined pulse width of 250 μs at 942 nm have been demonstrated. The results of the first laser operation and the achieved performance parameter are reported. |
1.INTRODUCTIONThe main objective of the WALES mission is to provide essential information on the distribution of atmospheric water vapour and aerosols in the troposphere and lower stratosphere (from 0 to 16 km altitude) for numerical weather prediction, climatology and atmospheric modelling. Absolute quantitative measurements of water vapour will be obtained over a large dynamic range (from 0.01 to 15 g/kg), with a high level of vertical resolution and accuracy not achievable by other systems. The spatial and temporal variation of water vapour will be directly sampled with global coverage, not only above and below optically thin clouds but also above cloud tops and in cloud gaps between dense clouds. The independent set of global water vapour profiles can be used to validate other measuring techniques such as operational passive infrared and microwave. Secondary information can also be derived such as cloud tops and base heights, optical thicknesses, planetary boundary layer height, and surface reflectance/albedo. In the WALES Phase A study three wavelength regions each with a suitable subset of four wavelengths have each been identified. The reselection of these 3 times 4 wavelengths (see Table 1) is a result of a trade-off, which presents the best compromise between scientific requirements and technical reliability. Table 1:Absorption wavelength sets for Water Vapour Dial
The nadir-pointing DIAL instrument performs differential measurements of absorption at four closely separated wavelengths, three of which are ‘online’ measurements at molecular absorption lines of water, and one ‘off-line’ measurement, where absorption is significantly reduced. In this way, the different sub-intervals of the dynamic absorption range are addressed by dedicated on-line wavelengths. The different on-line wavelengths possess different penetration depths, thus allowing measurements over different altitude intervals. The strongly absorbing water vapour lines are used for higher altitudes (low water vapour concentration) and the weakly absorbing lines are used for lower altitudes (high water vapour concentration). The on-line measurements and the separate off-line measurements are used in a cascaded way, i.e. for each on-line measurement, the measurement on the next weaker line serves as its offline measurement. For the development of global climate simulation models the available data about the three-dimensional distribution of water vapor in the atmosphere plays a fundamental role to determine the quality of their parameterization. Only with sufficient and reliable data it will be possible to receive results that describe the real climatic situation. Therefore the knowledge on water vapor distribution in the upper troposphere and lower stratosphere is of high relevance. In various publications it was shown that a differential absorption LIDAR or DIAL system can measure water vapor profiles simultaneously with high resolution and accuracy. The performance of the DIAL system is mainly determined by the parameters of the laser transmitter. The requirements for the laser transmitter are summarized in Table 2. Table 2:Laser transmitter requirements for a water vapor DIAL
The stability and energy requirements for a space borne laser transmitter lead to three different technologies that are currently under development:
In all concepts the exact wavelength match is achieved by injection seeding or amplification of a master laser. 2.ND:YGG AND ND:GSAG GARNET LASERDiode pumped laser materials like Nd:YGG and Nd:GSAG emitting at 935/936 nm and 942/943 nm as suggested by G. Huber and K. Petermann from Hamburg University as well as other garnets can be used for water vapor detection. Both water vapor absorption lines packages can be addressed with garnet lasers as shown in Fig. 1. The wavelength group around 936 nm using Nd:YGG as laser material and the 942 nm wavelength group using Nd:GSAG. Best efficiencies and beam quality can be achieved in a longitudinal pump configuration. The setup shown in Fig. 2 consists of a diode pump laser with 1 kW peak pump power at 808 nm wavelength that is coupled into the resonator longitudinally through the HR mirror. The garnet crystal is mounted in a copper holder with indium between the garnet crystal and the copper holder to improve the thermal contact. The heat removal from the copper holder can be accomplished using either water or Peltier cooling. Q-switching was accomplished by placing a polarizer, a quarter-wave plate and a Pockels cell into the resonator. Fig. 2:Laser setup longitudinally pumped from one side with up to 300 mJ pump energy in 300 µs pulses at 1 kW peak pump power  The presented results apply to measurements with Nd:GSAG. In first experiments without Q-switch a pulse energy of 86 mJ with a conversion efficiency of 33 % at 10 Hz was achieved. That is to our knowledge the highest pulse energy obtained from a Nd:GSAG laser in the 942 nm wavelength region. For Q-switch operation a change of the resonator length from 3 cm to 30 cm was necessary and resulted in Q-switched pulses with 25 ns duration and pulse energies of 18 mJ compared to 30 mJ for the same setup but in free running mode without active Q-switch. The optimum pump pulse duration was 300 µs. With output energies of 1-10 mJ at 942 nm wavelength the beam quality was in the range of M2 = 1.5. The Q-switched pulse energy is presently limited by laser-induced damage of the optical components, which can be overcome by advanced resonator design. In addition, frequency stabilisation of the Nd:GSAG-laser has to be developed to meet the spectral properties as given in table 2. 3.CONCLUSIONAn optical efficiency of 20 % with a compact design should be possible with a Garnet laser. This transmitter technology is however presently only usable for water vapor DIAL but additional wavelength regions can be accessed in the future by using other garnet materials. The properties of the garnet laser, especially the high overall conversion efficiency, make it the most suitable concept for DIAL measurements of water vapor concentrations. |
