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

Thesis



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.

Authors: Taylor, G

• Publication date: 1977
• Type of award: DPhil
• Level of award: Doctoral
• Awarding institution: University of Oxford
• Language: English
• Subjects: Vacuum ultraviolet spectroscopy; Far infrared lasers; Tokamaks