An astigmatic wavelength meter

An astigmatic scheme of a laser wavelength meter based on a single air-gap Fizeau interferometer is described. For a multimode laser, the accuracy in determining the center of gravity of a spectrum is within 1 GHz. Two complementary testing techniques are proposed for the instrument. By using them, it was shown for the first time that, for this type of meters, a systematic error arises and increases with a decrease in the radiation-spectrum width. The effect is periodic in the lasing frequency and results from a weak beam that is brought about by a reflection from the front surface of the interferometer. Moreover, in the previously designed optical schemes, this effect is so strong that unambiguous determination of the wavelength of a single-frequency radiation is impossible. The use of an astigmatic scheme helps additionally attenuate the influence of the third beam, thus eliminating the ambiguity in the results and reducing the absolute error to a value of +/-1.5 GHz.