Patent GB-1178023-A (GB)
Title: Improvements in or relating to Lasers
Inventor: DILLON JOSEPH FRANCIS JR
IPC: H01S3/16
Filed: 1967-03-08
Abstract: 1,178,023. Lasers. WESTERN ELECTRIC Co. Inc. 8 March, 1967 [23 March, 1966], No. 10886/67. Heading H 1C. A laser active medium consists of a ferrimagnetic host composition having a garnet structure and containing emitting ions. Suitable garnets include yttrium-iron garnet, the rareearth iron garnets of erbium, thulium, lutecium and ytterbium, modifications of these garnets in which the iron is partially replaced by aluminium, gallium or vanadium, modifications in which the rare-earth site is partially occupied by bismuth or a compensating ion such as calcium, and combinations of such garnets. The iron may also be substituted by germanium or silicon. Suitable emitting ions are holmium, thulium or erbium, singly or in combination, with the possible addition of ytterbium as an energy transferring ion. In one example thulium and erbium are provided to absorb energy from the pump, while holmium emits. Ytterbium is then provided to extend the absorption range. Such a device can operate at liquid nitrogen temperature, whereas at higher temperatures thulium is the emitting ion. Magnetic properties.-The net magnetization of yttrium-iron garnet may be reduced by replacing part of the yttrium with rare earth elements such as erbium, ytterbium and thulium. The net magnetization is then temperature dependent, the magnetic moment becoming zero at a temperature depending upon the substituting ion. Substitution of the iron with aluminium also reduces the net magnetization, the former acting mainly as a magnetization diluent, whereas the latter reduces the magnetization without line broadening. Increasing the concentration of gallium initially causes a reduction in the magnetic moment to a zero point and then an increase of opposite sigh. Output control using an axial field.-Application of a magnetic field to a ferrimagnetic garnet laser element parallel to the output beam gives rise to rotation of the plane of polarization of the beam. The rotation is non- reciprocal so that if the device acts as an amplifier it operates as its own isolator preventing energy being transmitted backwards to the input end. In an oscillator this results in cancellation of the forward and backward waves at points where the waves are nearly 180. degrees out of phase. The amplitude of the output may therefore be controlled, preferably in a digital fashion. The magnetic field may be produced by a coil energized with A.C.,.D.C. or intermittent current. If the laser output is not already plane polarized due to the crystal properties it may be made so by providing at one or each end of the resonant cavity a polarizing element. A polarizer at the output end of an oscillator may serve as an analyser. When the magnetic field is removed the magnetization of the crystal breaks up into random domains producing little or no rotation. Application of the field may quench the laser output either by the forward wavebackward wave cancellation or by the provision of an analyser. The easy axis of magnetization of the crystal preferably lies along the beam direction so that small applied fields may be used. Output control using a transverse field.- Application of a transverse field gives rise to Zeeman splitting, the different lasing thresholds with and without the applied field or with the field acting in different directions in a plane normal to the laser axis making possible either Q-switching or amplitude modulation. The frequency may also be shifted by such line splitting, either for frequency modulation or fortuning.  If a high frequency magnetic field is to be applied the laser may be placed in a microwave cavity. The pump preferably consists of a tungsten filament source, the operating temperature of which may be varied so that its emission spectrum matches the absorption characteristics of the garnet .selected. The pump and laser element may be placed in an elliptical cavity.