Method and apparatus for aligning a diffraction grating for tuning the output of a laser.

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 0 and theta 1. Knowledge of the value of either alpha or beta is not necessary. For a CO2 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 .