A Gaussian beam weakly diffracted by a circular aperture can be approximated in the far field by another Gaussian beam with slightly different characteristics. Equations giving the intensity, the divergence, and the radius of the modified beam are derived in simple practical form for experimentalists. These approximated formulas show that, even in the case of negligible power losses through the aperture, the diffracted beam characteristics may appreciably differ from those of the incident beam. In a first approximation, diffraction effects may be ignored only if the ratio a/r(0) of the aperture radius a to the l/e intensity beam radius r(0) in the aperture plane is larger than 3.

Describes the first continuous submillimetre waveguide laser operating on the 337 mu m HCN line, and presents the results of the first comprehensive experimental study of its low-loss modes. The laser cavity is a 5 cm diameter, 2.4 m long Pyrex tube with plane reflectors (one of them is a metal mesh) against its ends. Eight waveguide modes were identified. Intensity distributions in the beam cross section and mode polarization were studied. The results agree with theoretical predictions for dielectric waveguide modes. The propagation of the EH11 mode in free space was also studied. The beam propagates as a gaussian beam at moderate and large distances from the mesh coupler. The maximum output power of the laser is of the order of 100 mW, representing the highest output power per unit volume so far reported for a continuous HCN laser.

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The gain and the saturation intensity of optimized cw 337-microm lasers are shown empirically to vary inversely as tube diameter. An expression is then derived for the power of a waveguide laser as a function of geometrical parameters, losses, and coupling. For optimized coupling the power is a strong function of tube diameter with a well defined maximum. Optimum diameter depends on tube length and losses only. The results agree well with the measured powers of three waveguide lasers delivering 30 mW, 100 mW, and 170 mW from discharges of 1-m, 2-m, and 3-m lengths. Such lasers are competitive with optically pumped submillimeter lasers.