GB1257990A - - Google Patents

Abstract

1,257,990. Light modulators. UNION CARBIDE CORP. 6 May, 1969, No. 23075/69. Heading H4F. [Also in Division G2] The intensity of frequency doubled radiation derived by passing radiation of a fundamental frequency from a laser 10 through a crystal 13 is sensitive to mismatching of the refractive indices exhibited by the crystal to the ordinary fundamental frequency rays and the extraordinary doubled frequency rays, due to imperfect temperature control or variations in other parameters affecting the optical properties of the crystal. In the present invention an electric field provided across the crystal normal to the optical path may be varied to vary the refractive indices of the crystal and so modulate the doubled frequency output. To produce maximum output, a probe signal f p from 25 is modulated on a bias voltage from 18 and applied to electrodes 14, 15, and the modulated light output reflected from a beam splitter 22 is detected at a photo-tube 23. A detector 24 compares the probe signal f p and the output of tube 23 and provides a feedback signal of polarity and magnitude indicative of any shift in the intensity of the radiation output which controls the bias source 18 to compensate therefor. By adding a D.C. component to the feedback signal any desired operating point on the radiation outputapplied voltage curve may be attained, Fig. 2 (not shown). An information signal f i from 19, of substantially greater frequency than signal f p , may also be supplied to electrodes 14, 15, in which case the operating point is preferably a point of inflection on the output-voltage curve for maximum modulation efficiency. Alternatively, simple on-off modulation of the output -signal may be effected. In the example given, crystal 13 is of lithium niobate and laser 10 is a neodymium doped Y1G laser with a fundamental wavelength of 1À06 Á.