Patent NL-6403674-A (NL)
IPC: H04B10/50
Filed: 1964-04-07
Abstract: 1,068,052. Modulating laser and maser radiation. SIEMENS A.G. April 13, 1964 [April 11, 1963; April 16, 1963; May 22, 1963], No. 15099/64. Heading H1C. The oscillatory radiation path in a laser or maser extends through a modulator which is controllable so that variable proportions of the laser or maser radiation may be retained in the radiation path and picked off from the path. In Fig. 1, laser element 1 lies in a resonant cavity 2, 2<SP>1</SP>, partial reflector 2 forming with a partial reflector 4 a further cavity containing a control element 5, the refractive index is varied by means of a modulating electric field. The assembly 2, 4, 5 forms a variable reflectivity interference mirror allowing the proportions of radiation picked off and retained to be varied. The assembly is adjusted so that a mean beam intensity is picked off in the absence of a modulating field, or element 5 is D.C. biased. In a push-pull system (Fig. 3, not shown) a similar interference mirror is arranged at the opposite end of the laser element, the control elements of the two assemblies being energized in antiphase. In this arrangement the radiation retained is unchanged by the modulation. In a modification, Fig. 4, control elements 325, 325<SP>1</SP> operated in push-pull vary the plane of polarization of the oscillatory radiation and Nicol prisms 324, 324<SP>1</SP>, 324<SP>11</SP> separate the differently polarized components. Output beams 326, 326<SP>1</SP>, 327, 3271 are obtained. Prism 3241 may be omitted, beam 327 then having an intensity equal to the combined intensities of the original beams 327, 327<SP>1</SP>. If the modulation frequency is low both prisms 324<SP>1</SP> and 324<SP>11</SP> may be omitted. A non-push-pull arrangement is also described (Fig. 2, not shown) in which a single prism and a single control element are used. In Fig. 5 the radiation circulates around a closed path, a unidirectional coupler 336 defining the direction of circulation. Control elements 332, 332<SP>1</SP> are placed in a waveguide (Fig. 6, not shown) and microwave radiation is passed from a source (344) through element 332 (342) is phasereversed and is then passed through element 332<SP>1</SP> (342<SP>1</SP>) to a non-reflective termination (345). The relationship between the time taken for the modulating beam to travel from element 332 to element 332<SP>1</SP> in the waveguide and the time taken for the laser beam to travel between the two members is selected so that operation is push-pull, and the relationship may be such that push-pull operation is achieved without phase reversal of the microwave radiation. For high modulation frequencies elements 332, 332<SP>1</SP> are combined and the relationship between the travel time for the laser beam and the period of the modulating wave is suitably selected. The laser element may be a monocrystalline solid, e.g. ruby, a gaseous medium, a junction semi-conductor crystal (either a PN-junction or a junction between regions having different doping levels) or a semi-conductor crystal without a junction. The control element may be either electrically or magnetically controllable and may be a KDP or ADP crystal or a nitrobenzene cell. In the embodiments of Fig. 4 and 5 the Nicol prisms may be replaced by Wollaston or Rochon prisms. Element 5 in the embodiment of Fig. 1 may consist of a series of dielectric layers of different refractive indices, at least one layer being controllable by the modulating signal. It is also stated that modulation at low frequencies may be achieved by varying the separation between mirrors 2 and 4. In all the embodiments the control electrodes may form the semi-reflecting surfaces of the resonant cavity or cavities, and the various elements may be cemented together.