ROLAND ROUX
Hydrogen and deuterium halide (HCl, DF, HBr) lasers based on an exothermic chemical reaction initiated either by discharge or by rapid mixing of the reactant gases emit in the 3- to 5-µm wavelength region. Various wavelengths in this spectral range are well suited to long-range beam propagation through the atmosphere because they coincide with atmospheric windows.
Several years ago CILAS (Marcoussis, France) developed a HF-DF laser capable of producing an average power of 600 W at HF wavelengths and of 250 W for DF.1 Long-term operation of a such a laser, however, requires continuous regeneration of the gases being mixed. An alternative approach for experiments that require only low average power is optical pumping of the hydrogen and deuterium halide molecules. The gases used and the pump source selected permit coverage of various spectral regions between 3 and 5 µm. This approach avoids the disadvantages of gas dissociation and regeneration.
More recently, CILAS researchers—supported by DRET (Direction des Recherches, Etudes, et Techniques, Paris, France)—have experimented with a novel low-average-power deuterium fluoride laser.2,3 In this device a short laser pulse stimulates hydrogen or deuterium halide molecules contained in a gas cell, creating a cascaded population inversion. The main difficulty is to find a laser source able to pump the discrete, narrow lines of the various transitions.
Forsterite laser
In the pressure range typically used to make hydrogen or deuterium halide molecules lase, the absorption lines are very narrow (less than 1 GHz). Thus, an efficient pump source must have similar narrow spectral properties and be tuned precisely to an absorption line. A forsterite laser using a series of prisms and two Fabry-Perot devices is able to meet these requirements.
The chromium-doped forsterite (Cr4+:Mg2SiO4) laser is based on a 23-mm-long crystal with a 4 × 6-mm2 cross section and cut at the Brewster angle. A Q-switched Nd:YAG laser longitudinally pumps the forsterite crystal at 1.064 nm. The Nd:YAG laser delivers pulse energies up to 130 mJ with a pulse duration of 10 ns and pulse-repetition frequency of 10 Hz.
The required output wavelength is obtained by passing the beam through three dispersive prisms and rotating the totally reflective mirror. To reduce the spectral bandwidth of the laser emission, two Fabry-Perot devices are placed in the cavity. The forsterite laser is smoothly tunable from 1.16 to 1.33 µm, and the tuning range peak is centered at 1.25 µm. Without the Fabry-Perot devices, the forsterite laser delivers 13-mJ maximum energy per pulse at 1.25 µm with the 130-mJ pump energy; optical conversion efficiency is thus 10%. With the Fabry-Perot devices, at 1.268 µm (the HF line), the pulse energy is 6 mJ, and at 1.193 µm (the DF line), it is 3 mJ. Pulse length is between 50 and 70 ns (FWHM) with a linewidth of 0.06 Å.
The narrow-line output of the forsterite laser is introduced into the gas cell through a curved dichroic cavity mirror that is highly transmissive at the pump wavelength and highly reflective at the HF and DF lasing wavelengths. Stainless-steel cells equipped with calcium fluoride Brewster windows are used; cell lengths are 15 cm for HF and 50 cm for DF.
With this setup pumping the HF cell, the researchers obtained 250-µJ superfluorescence energy (measured without any mirrors) on the transition around 2.8 µm. The HF cell pressure was 30 Torr, and the forsterite pump laser energy was 6 mJ at 1.268 µm with a pulse length of 50 ns. While pumping the DF cell, a cascading laser emission was observed and laser lines between 3.64 and 3.85 µm emitted simultaneously with about 10 µJ of energy. In this case the DF cell pressure was 3 to 6 Torr; pump laser energy was 3 mJ at 1.193 µm.
REFERENCES
1. Henri Brunet, Michel Mabru, and François Voignier, Proc. SPIE 2502, 388 (1994).
2. Pascale Prigent, Geneviève Girard, Annick Lavenant, Henri Brunet, and Pierre Lemaigen, "Mid-IR DF laser pumped by a tunable and narrow-line Cr4+:forsterite laser," submitted to JOSA-B (J. Opt. Soc. Amer.-B).
3. Pascale Prigent et al., GCL/HPI `96, Edinburgh, Scotland (Aug. 1996), poster paper P2-13.