This invention relates to an optical heterodyne detector for detecting the frequency difference between light beams which are greatly different from each other in frequency.
An optical heterodyne detector for detecting the modulation frequency of frequency-modulated light beam has been well known in the art.
The arrangement of the conventional optical heterodyne detector is outlined in FIG. 5. In FIG. 5, reference numeral 1 designates a beam splitter for splitting a laser beam applied thereto; 2 and 3, mirrors; 4, a frequency-modulating element; 5, a half mirror; 6, a high-speed photodetector comprising a PIN junction photodiode or the like; and 7, an oscilloscope or spectrum analyzer.
In the optical heterodyne detector shown in FIG. 5, the input laser beam is split into two light beams by the beam splitter 1. One of the two light beams is applied directly to the mirror 2, where it is reflected at right angles. The other light beam is reflected at right angles by the mirror 3 and applied to the frequency-modulating element 4 such as an acousto-optical element. In the frequency-modulating element 4, the input light beam is frequency-modulated. The output light beam of the frequency-modulating element 4 is applied to the half mirror 5. In the half mirror 5, the light beam not modulated which is applied thereto by means of the mirror 2 is combined with the light beam frequency-modulated which is applied thereto by means of the frequency-modulating element 4. The output light beam of the half mirror 5 is applied to the high-speed photodetector 6. In the photodetector 6, modulation frequency given to the frequency-modulated element 4 is detected as a beat frequency by interference. The signal thus detected is applied to the oscilloscope or spectrum analyzer 7 for observation of the modulation frequency.
In the above-described conventional optical heterodyne detector, the high-speed photodetector is combined with the oscilloscope or spectrum analyzer for detection and observation of the modulation frequency. Therefore, it is impossible for the detector to detect a high modulation frequency. Furthermore, with the optical heterodyne detector, the variation in intensity of a laser beam at a spatial point can be measured, but, in the case where components having different frequencies in space are provided in a one-dimensional direction, such one-dimensional spatial data cannot be measured with the optical heterodyne detector.