PURPOSE: To make reflected quantity of light always constant irrespective of the kinds of a laser oscillator by letting the light separator which separates part of the laser light having been modulated with a light modulator be a beam splitter by a hologram.

CONSTITUTION: Laser light from an oscillator 1' enters an ultrasonic light modulator 3. On the other hand, the modulating signal is applied from terminal 4a and the feedback signal from a photoelectric multiplier 14 is applied to an amplifier 4. The output from a high frequency oscillator 6 which oscillates the specified high frequency is amplitude-modulated 5 and is applied to the modulator 3, by which the electric signal is converted to mechanical vibrations. These vibrations cause periodical changes in refractive index within an ultrasonic wave medium. The laser light entering the modulator 3 is refracted and receives intensity modulation. When the light passes through the beam splitter 15 by hologram by way of an aperture 8, the greater part of light passes therethrough and is radiated to a film 13. Part of the light diffracted by the splitter 15 is converted to electric signal by the multiplier 14 and is applied to the amplifier 4.