Development of an intense optically pumped laser of narrow bandwidth in the far infrared

This thesis describes an experimental study of high intensity, pulsed, optically pumped, far-infrared (FIR) lasers. The work was motivated by the need for a radiation source for the measurement of the ion temperature in magnetically confined, high temperature plasmas (e.g. tokamak plasmas), using Thomson scattering. Constraints imposed by the plasma parameters, the scattering geometry and available detector sensitivities lead to the requirement of a radiation source wavelength between 30andnbsp;andmu;m and 1andnbsp;mm and a source power and#x2273;andnbsp;1 MW in a bandwidth and#x2272;andnbsp;60andnbsp;MHz. Results are presented for a 496andnbsp;andmu;m, 500andnbsp;watt, methyl fluoride (CH3F) cavity laser, with a bandwidth of andlt;andnbsp;30andnbsp;MHz, which was optically pumped by a 9.55andnbsp;andmu;m CO2 laser. Results are also presented for an optically excited mirrorless, super-radiant, CH3F laser, which generated over 0.6andnbsp;MW of FIR radiation within a bandwidth of about 300andnbsp;MHz. The performance of this laser has also been simulated by a computer model, which allows the optimum operating parameters to be predicted. An assembly constructed on the principle of the injection laser, in which low power narrow-band oscillator radiation is used to control the output of a super-radiant system, has been used to generate 250 kW of 496 andmu;m radiation, with a bandwidth of andlt;andnbsp;60andnbsp;MHz. Investigations of the FIR output from heavy water vapour (D2O) in a super-radiant laser assembly, optically excited by several different CO2 laser wavelengths, have resulted in the generation of 60andnbsp;ns (FWHM) pulses of FIR radiation with average powers of 1.3, 9.2 and 15.8andnbsp;MW, at wavelengths of 385, 119 and 66andnbsp;andmu;m, respectively. All these lasers were found to have a higher CO2 to FIR photon conversion efficiency than the 496andnbsp;andmu;m CH3F laser. In addition, the energy level spacing in D2O is such that the molecule can generate narrow bandwidth radiation more readily than the CH3F molecule. From this work it is concluded that an injection laser assembly, similar to that used with CH3F, but containing D2O vapour, optically pumped by a 9.26andnbsp;andmu;m CO2 laser and generating several megawatts of 385andnbsp;andmu;m radiation, would satisfy the source requirements mentioned above.