Method and apparatus for aligning a diffraction grating for tuning the

Abstract

A method and improved apparatus for aligning a diffraction grating, as, for example, one used in tuning the output of a laser. The total angular misalignment .epsilon. between the incident and diffracted beams is a function of three angles: the tuning angle .theta.16, the angle of tilt .beta. between the plane of the grating 15 and the tuning rotational axis, and the angle .alpha. between the grating lines and the projection of the turning rotational axis in the plane of the grating 15. The present invention provides a method of aligning the diffraction grating so that the residual alignment error is very small over the desired range of tuning angle .theta. 16, by adjusting only .alpha. or only .beta. to zero at two reference tuning angles .theta..sub.0 and .theta..sub.1. Knowledge of the value of either .alpha. or .beta. is not necessary. For a CO.sub.2 laser tuned over the wavelength range 9.11 to 10.88 .mu.m, calculations show that the present method grating alignment results in a residual angular misalignment error .epsilon. of less than 60 .mu.rads, when .alpha. is close to 12,000 .mu.rads and .beta. is equal to 10,000 .mu.rads. Since adjustment of only .alpha. or .beta. is necessary in minimizing the total misalignment, an improved apparatus for aligning diffraction gratings consists of providing means for adjusting only one of the two angles .alpha. and .beta. in addition to the tuning angle .theta..