A laser light, whose wavelength is to be measured, is introduced into an etalon, a concentric circular interference stripe derived from the etalon is irradiated onto a one-dimensional photodetector array and a diameter of the interference stripe is measured to measure the wavelength of the laser light. Alternatively, if a reference laser light of known wavelength is introduced into the etalon, as described above, a wavelength measurement of extremely high accuracy can be made without being affected by positional deviations of the optical system.
The present invention relates to a wavelength monitoring apparatus capable of measuring both standard light and laser light for semiconductor exposure simultaneously and highly accurately, without a time lag. Entrance-side optical systems 2.sub.1 and 2.sub.2 allow light from a laser 20 for semiconductor exposure and reference light from a He--Ne laser 10 to be incident on different areas of a single etalon 1 in the form of diverging light, converging light or diffused light in such a manner that the respective center axes thereof are displaced relative to each other. Two focusing optical systems 3.sub.1 and 3.sub.2 are provided in approximately coaxial relation to the respective center axes of the laser light and reference light passing through the etalon 1. A one-dimensional array optical sensor 4 is placed in a plane P coincident with the back focal planes of the focusing optical systems 3.sub.1 and 3.sub.2 to receive interference fringes produced by the laser light and the reference light. The positions of the interference fringes on the one-dimensional array optical sensor 4 are detected to calculate the wavelength of the laser light for semiconductor exposure.
An error signal generation system and method for continuous and accurate positioning of a tunable element used in association with a coherent light source. A tunable element is positioned in a coherent light beam with a fixed frequency or wavelength, and a detector is positioned in association with the light beam and tunable element that is capable of generating an error signal indicative of spatial losses associated with the positioning of the tunable element in the light beam. A tuning assembly is operatively coupled to the tunable element and detector and is configured to position the tunable element according to the error signal generated by the detector.
