Patent GB-1089386-A (GB)
Title: Devices for modifying the characteristics of laser or maser beams
IPC: G02F1/015
Filed: 1966-04-28
Abstract: 1,089,386. Modulating and deflecting light; television. COMPAGNIE GENERALE DE TELEGRAPHIE SANS FIL. April 28, 1966 [May 4, 1965], No. 18621/66. Heading H4F. A beam of coherent waves, such as a laser or maser beam, is amplitude or frequency modulated or deflected, by varying the plasma frequency of a semi-conductor positioned in the path of the beam. As shown, Fig. 1, a coherent light beam b from a laser L is passed through a semi-conductor S which has electrodes A and B connected to a D.C. voltage source V so that charge carriers may be injected into the semiconductor along the direction of the electric field, i.e. substantially parallel to beam b. Variation of voltage V varies the concentration (density) of the charge carriers in the semiconductor which varies the plasma frequency # p  of the semi-conductor. If the frequency# of beam b is equal to or higher than # p , the beam emerges with a phase shift depending on the ratio # p /#. Thus if the semi-conductor has a concentration such that # p <# and voltage V is varied the output beam b<SP>1</SP> is phase modulated, i.e. frequency modulated. When # p >#, for V= 0 the beam is totally reflected, but by applying voltages V of different amplitudes, the output beam b<SP>1</SP> emerges with densities depending on the values of V. Amplitude modulation may thus be obtained, accompanied by phase (or frequency) modulation. In a second embodiment, Fig. 2, not shown, a doped semi-conductor, in which the carrier concentration varies transversely to the laser beam, is used and an electric field is applied perpendicularly to the beam. If # p ># all over the semi-conductor, it presents to the beam a medium with a variable refractive index in the direction perpendicular to the beam, and thus the beam is refracted and by varying voltage V the beam is caused to scan. In a third embodiment, Fig. 3, the semiconductor S is inclined at an angle to the beam b and doped in such a way as to present a high gradient of carrier density in the longitudinal direction (surfaces of equal density are represented by dotted lines). One of these surfaces, namely # p  is arranged to have a plasma frequency higher than that of the laser and consequently beam b is totally reflected on that surface and turns back along b<SP>1</SP>. By modulating the voltage V the surface # p  moves back and forth in the direction of the beam and the reflected beam is thus frequency modulated. An auxiliary beam of light projected on to the semi-conductor and intensity modulated may be used to modulate the plasma frequency of the doped semi-conductor instead of employing an electric voltage. A train of travelling waves of frequency N substantially lower than that of the laser may be excited in the semiconductor which produces zones in which # p ># alternating with zones in which # p >#, and propagating along the semi-conductor at a phase velocity depending on the frequency N. The beam is reflected by the first zone encounted with # p ># and as this zone moves the reflected beam is frequency modulated as in Fig. 3. "'Helicon" " type waves may be excited in the semi-conductor (see R. Bowers and M. C. Steele in " Proceedings of the I.E.E.E.", October 1964, pages 1107-1108) by placing the semi-conductor in a magnetic field parallel to the axis of the semi-conductor. The helicon waves have a frequency lower than the plasma frequency and produce concentrations of carriers moving at the phase velocity of the waves. The light beam is thus frequency modulated as in Fig. 3 due to these density variations which accompany the helicon waves. In a television receiver three such semiconductor devices may be used, one to intensity modulate a laser beam and the other two to deflect the beam in two orthogonal directions across a receiver screen. For a colour television picture a fourth semi-conductor device may be used to frequency modulate the laser beam in accordance with chrominance signals.