GB1178862A - Device for Modulating a Beam of Radiation - Google Patents

Abstract

1,178,862. Laser. PHILIPS ELECTRONIC & ASSOCIATED INDUSTRIES Ltd. 23 Feb., 1967 [25 Feb., 1966], No. 8627/67. Heading H1C. [Also in Division H4] To modulate a beam of electromagnetic radiation it is directed to a semi-conductor crystal having contacts, the crystal being such that when an electric field is applied across it, i.e. between the contacts, a zone travelling from one contact to the other is produced having a higher resistivity and a higher potential gradient than other parts of the crystal and thus optical properties which differ from those of the other parts of the crystal. These zones appear as a result of the Gunn effect or as a result of a piezo-electric interaction of the electrons passing through a homogeneous semiconductor body and the lattice vibrations of the semi-conductor. The semi-conductor crystal may comprise a gallium arsenide crystal cut in the (110) direction and used at the ambient temperature or at 77‹ K., or a cadmium sulphide, cadmium telluride, gallium antinonide or tellurium crystal used at room temperature. The Gunn effect appears at about 1500 V/cm. whereas the piezo-electric effect appears at about 300V/cm. at a temperature of 77‹ K. With the Gunn effect the speed of travel of the zone is about 10<SP>7</SP> cm./sec., whereas with the piezo-electric effect it is about 3À5 x 10<SP>5</SP> cm./sec. in GaAs, i.e. the speed of propagation of a shear wave in the crystal. The optical properties utilized to modulate the radiation include the displacement of the absorption threshold due to the strong electric field in the zone (Franz-Keldysh effect), the effect of the field being equivalent to a small reduction of the width of the energy gap in the semi-conductor, i.e. a rise of the threshold wavelength at which strong absorption occurs. The presence of a zone of strong absorption may also be used to produce a surface having reflecting properties. Another optical property which may be utilized is double refraction which causes a change in the state of polarization of incident plane polarized light. In Fig. 2, when the voltage from generator 13 exceeds the threshold value a zone 34 appears in the vicinity of contact 31 and travels through the semi-conductor crystal 30 towards transparent contact 32. A visible or non-visible radiation beam 36 which is preferably mono-chromatic passes through a variable thickness 38 of semi-conductor crystal before it is reflected on the front 39 of the zone 34 of the steep potential gradient. Modulation of the emerging beam 40 is brought about by the variable absorption due to the variable thickness 38 of the semi-conductor. Laser application.-The arrangement of Fig. 2 may be employed for controlling and checking the radiation emission of a laser, if the transparent ohmic contact 32 forms one of the end faces of an interferometer, e.g. a Fabry-Perot type, containing a laser-active medium, in which the stimulated emission starts when the front 39 reaches the transparent contact 32.