An experimental study of a CW optically pumped far infrared formic acid vapour laser

This paper describes a comprehensive experimental study of the performance of an optically pumped formic acid waveguide laser operating at wavelengths of 394, 419, 433 and 513 μm. The variation of output power with Pyrex waveguide diameter, in the range 30–92 mm, and output coupling fraction, between 5 and 40% per double pass, is studied in detail. The results are fitted to a theoretical model to obtain the gain and saturation intensity as a function of tube diameter. For this 1.5m long laser, pumped by a 33 W CO 2 laser, the optimum diameter and output coupling are found to be ~ 50 mm and ~20% respectively. This laser is capable of giving output powers of the order of 50 mW on all four lines. Output coupling is varied by using a range of aluminium strip gratings on fused quartz substrates with Teflon antireflection coatings on their rear surfaces, for which the transmission properties are accurately calculable. The polarising properties of the strip grating couplers are used to measure the polarisation anisotropy of the laser output. For typical operating conditions the ratio of maximum to minimum power is in the range 1.2–1.9 for the four lines studied. Laser resonator interferometry is used to study the pair of lines with wavelenghts close to 433 μm, which are separated by 56 MHz, and to identify the longer wavelength line as the stronger.