Patent GB-1178022-A (GB)
Title: Improvements in or relating to Solid State Lasers
Inventor: DILLON JOSEPH FRANCIS JR
IPC: H01S3/102
Filed: 1967-03-08
Abstract: 1,178,022. Lasers. WESTERN ELECTRIC CO. Inc. 8 March, 1967 [23 March, 1966], No. 10885/67. Heading H1C. A laser includes a crystalline saturable magnetic host material, with means for applying a saturating field to produce a magnetization direction having a component in the direction of a permitted laser beam. Suitable host materials include those disclosed in Specification 1,178,023. In one arrangement a transverse saturating magnetic field is applied continuously to a laser rod having reflective ends and a variable longitudinal field is then applied to produce a resultant field having a variable orientation. If the laser output beam is plane polarized, either due to the crystal structure or due to polarizing elements placed at one or each end of the active medium, the plane of polarization is rotated to an extent dependent upon the longitudinal field component. The rotation is non-reciprocal so the forward and reflected beams interact destructively at certain regions within the active medium so preventing stimulation. Variation of the longitudinal field varies the amplitude of the laser output beam. A polarizer may be provided at the output end of the active medium to act as an analyser in a Q-spoiling arrangement. The amplitude of the output beam may be varied either digitally or in analogue fashion, or may be continuously modulated. For digital operation using a YIG crystal the easy magnetization directions [111] are aligned with the transverse magnetic field and the desired resultant field direction, while for analogue operation a hard direction [100] should lie midway between the transverse and resultant field directions. Single field devices -If the crystal is arranged to have an easy magnetization direction at an acute angle to the output beam direction a unidirectional magnetic field of varying amplitude is applied to rotate the magnetization direction to vary the rotation of the plane of polarization. The changes in the Zeeman splitting of the emitting energy levels due to changes in the field may also be utilized. The single field device depends upon the anisotropy field of the crystal and this may be increased by reducing the overall saturation magnetization of the crystal, e.g. by compensation with gallium in the case of YIG. Frequency limitations.-When the applied normal magnetic field corresponding to an unmodulated condition of the output beam, is just sufficient to cause saturation, the magnetization direction will not follow the modulating field at frequencies approaching the natural precessional resonance frequency. By increasing the normal field the frequency limit may be increased. Devices using magnetostriction.-The length of the laser rod may be varied magnetostrictively by varying the direction of an applied magnetic field. The variation may be made manually or by means of a feedback circuit arranged to maintain a desired operating condition, e.g. for adjusting the frequency or optimizing a desired mode. Directional control.-The device shown in Fig. 2 may be subjected to magnetic fields in any of the directions H1, H2 and H3. Opposed reflecting surfaces 11-12, 13-14 and 15-16 define separate resonant cavities in a single crystal 10. An output beam is produced in a direction for which there is no axial magnetic field component, since the presence of such a component results in rotation of the polarization plane and cancellation of the forward and backward waves. The beam direction normal to the applied field thus defines the preferred mode. The crystal orientation is preferably such that the field directions H1, H2, H3 coincide with easy magnetization directions. If this is impossible the sector between the normal and most remote field directions should include a hard direction of magnetization. Microwave control.-In the arrangement of Fig. 3 (not shown), the laser rod is disposed within a microwave cavity and a transverse magnetic field is applied, which has a magnitude which results in a desired resonance frequency. Microwave energy at this frequency is coupled into the cavity to cause a precessional resonance resulting in amplitude variation of the laser beam at twice the microwave frequency. If the steady magnetic field is canted so that precession is between a normal and an extreme position the amplitude variation frequency will equal the microwave frequency.  Phase modulation.-Interaction of a transverse field and a parallel field changes the velocity of one circularly polarized component of a laser beam relative to the other. Selection of either component results in an effective change in the length of the resonant cavity, thus changing the phase of the output.