Patent GB-1100506-A (GB)
Title: High speed laser frequency selector
IPC: H01S3/107
Filed: 1966-11-02
Abstract: 1,100,506. Lasers. INTERNATIONAL BUSINESS MACHINES CORPORATION. 2 Nov., 1966 [16 Dec., 1965], No. 49048/66. Heading H1C. In order to select a particular frequency or frequencies of operation in a laser, a frequency selector including an electrooptical crystal is inserted in the laser cavity. In the embodiment of Fig. 2 a helium-neon laser tube 21 with Brewster angle windows is inserted in cavity 25, 26 together with the frequency selector 27. The latter cavity comprises a quartz crystal 28, an electro-optical crystal 29 controlled by a variable D.C. voltage 31, and a quarter-wave plate 30 and is enclosed by Brewster angle windows 34, 35 and filled with an index matching fluid. Because the Brewster angle windows linearly polarize the laser beam parallel to the plane of incidence only light with the correct polarization will leave and re-enter the frequency selective cavity with minimum loss. The quartz crystal 28 rotates the plane of polarization of light by amounts depending on the frequency, Figs. 3, 4 (not shown), and in operation the D.C. source 31 is varied so that the electro-optic rotator rotates the selected frequencies polarization by an amount so that the polarization of the light emerging from the quartz rotator 28 is again in the plane of incidence. The selected frequency passes through cavity 27 without loss, whereas adjacent frequencies are not rotated by the rotator 29 by the correct amount to compensate for the rotation by crystal 28 and their losses are too great for lasing to occur at these frequencies. In a second arrangement, Fig. 6, the linearly polarized beam is passed through a phase plate whose phase shift is a function of the light frequency. The argon or other ionized gas laser tube 40 is arranged in the optical cavity 45, 46 together with a gypsum phase plate 47, an electro-optic phase crystal 48 and a polarizing calcite crystal 49. The linearly polarized light from laser tube 41 is converted into elliptically polarized light by phase plate 47 which ellipticity is a function of light wavelength which light will also suffer losses in passing through the Brewster windows and will not lase. The electro-optical crystal 48 imparts substantially the same phase shift to all wavelengths of light but the amount of shift is a function of the electric field across the plate. The wavelength to lase is selected by varying the electric field across plate 48 so that the phase shift introduced thereby compensates for the phase shift of the selected wavelength produced by plate 47 and this wavelength will pass through polarizer 49 without loss. However, all other lines will remain elliptically polarized and will suffer losses passing through 49 and will not lase. Polarizer 49 may be omitted in certain cases. Plates 47, 48, 49 may be replaced by a single electro-optic phase plate. Two laser lines may be selected by using two phase plates and two electro-optic phase plates in addition to a polarizing crystal which are inserted in the laser cavity, Fig. 8 (not shown). The first wavelength is selected by arranging that the two phase plates cause a resultant phase shift of zero and applying a zero electric field to the electro-optic phase plates and a second frequency is selected by applying proper voltages to the electro-optic phase plates so that the resultant phase shift of all four plates is zero for the second selected wavelength. Two or more wavelengths may be made to lase simultaneously with this arrangement.