Abstract

A model of an intracavity 16 micron Raman laser developed¹ which predicted that 16 micron output could be obtained if an atmospheric CO₂ laser is used to pump an intracavity parahydrogen cell. With this system line tunability was predicted. If a high pressure CO₂ laser is used as a pump source continuous tunability in the 16 micron region could be expected. The proposed laser consists of a high pressure CO₂ laser pumping an intracavity parahydrogen Raman cell. The coupled mode equations describing the first order Stokes and anti-Stokes radiation and the laser rate equations were solved simultaneously. Figure 1 shows typical calculated laser, Stokes and anti-Stokes pulses for a system with a 1 meter long CO₂ gain section and a 0.5 m long parahydrogen gain cell. The Stokes output energy predicted was 54 mJ and the anti-Stokes energy 2.7 mJ. The pump pulse shown is that inside the resonator and the Stokes and anti-Stokes outside the resonator. The pump pulse builds up until the Raman threshold is reached, at which stage all the pump energy is converted to either Stokes or anti-Stokes radiation. The dependence of the output Stokes energy on the small signal gain of the CO₂ laser was calculated and it was found that the threshold value is approximately 1.1%/cm. This corresponds to an intracavity intensity of approximately 11 MW/cm². Tunable 16 micron radiation could thus be expected as long as the small signal CO₂ gain is kept above 1.1%/cm. The calculated tuning curve of a 10 atmosphere CO₂ laser, operating on the 10 micron R-branch, is shown in Figure 2. Tunable output from 614 cm⁻¹ to 624 cm⁻¹ is predicted. It was assumed that the peak CO₂ gain was 3.5%/cm. If a higher gain value is used tunability over a larger range will be predicted. The optimum Stokes output coupler reflectivity was found to be dependent on the intracavity losses. For typical loss factors the optimum output coupling was found to be between 10% and 25%.

© 1995 IEEE