Effects of Output-coupling Apertures in a Resonator on Spatial Coherence of CO 2 Laser

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Abstract
The degree of spatial coherence S n12 (0) S has been measured for a CO2 laser beam of radiation emerging from a central output-coupling hole, or four output-coupling holes in the centre-symmetric annular region, in a plane mirror of the Fabry-Perot resonator. The effects of these two kinds of output-coupling apertures on the spatial coherence are studied in association with mode structure in the resonator. The value of S n12 (0) S is always above 0·9 for two points in any location in the beams of radiation emerging from the four output-coupling holes, while it varies with the time in the range 0·5 to 1·0 in a beam of radiation emerging from the central output-coupling hole. Some analytical considerations are given, according to which the value of S n12 (0) S should be approximately unity over the cross-sectional area of a beam of radiation emerging from an output-coupling hole of 6 mm in diameter. This is partly in agreement with the results of the measurements. For the central coupling hole, the fact that S n12 (0) S varies with time is explained qualitatively to be due to the intensity fluctuation in every operating mode, competing with each other in the active resonator. For the four coupling holes a particular mode may be made to survive due to relatively strong mode competition arising from the mode selective effects of the off-centred four coupling holes. On this account a high value of S n12 (0) S may be obtained at two points in the beam separated by 25 mm.