Patent GB-1163314-A (GB)
Title: Dispersive Optical Systems
IPC: G01J9/02
Filed: 1966-09-29
Abstract: 1,163,314. Utilizing radiation detectors. PERKIN-ELMER CORP. Sept.29, 1966 [Oct. 4, 1965], No.43658/66. Heading G1A. [Also in Divisions G2 and H1] A resonant optical circulator employing two spherical mirrors is over illuminated off-axis by a laser output beam so that if the frequency of the laser output changes the mode pattern in the circulator is spatially shifted. A frequency stabilizer arrangement, Fig. 1, of helium neon laser 58 includes an optical resonator 24 constituted of a pair of multi-layer dielectric coated spherical mirrors 20, 22 separated by a distance slightly less than their radius of curvature. The output beam 26 of the laser is focused to an off-axis point 28 on mirror 22 so that the light is reflected back to mirror 20 to line 30 and then to point 28 on mirror 22 where the major portion of the light is reflected back to line 38 to begin another cycle. The output beam 34 is directed on to the apex of a prism 44 acting as a beam splitter. If the beam of light 26 falling on the resonator 24 is of a predetermined frequency half of the beam 34 is reflected to photo-detector 46 and half to photodetector 48 so that the signals on lines 50, 52 applied to differential amplifier 54 will be the same and no signal is applied to line. If the frequency of the laser 58 increases the resonant mode pattern lines 38, 30 move inwardly towards the central axis of the resonator, Fig. 3 (not shown) causing the beam 34 to move to the right so that more light falls on photo-detector 46 than 48 and an appropriate signal is produced on line 56 from amplifier 54. This signal representing the amount of shift is fed to a pile of Piezo-electric discs 116 on which are mounted a laser resonator mirror 114 so that the mirror 114 is displaced to move along the axis of the gas tube 108 to so decrease the frequency of the laser to its original value. Similarly if the frequency decreases more light falls on photo-detector 48 than 46 resulting in the mirror 114 moving to increase the laser output frequency, Fig. 5 (not shown). The resonator 24 also acts a circulator and is enclosed in an environmental control box 36 which may be provided with an aperture closed by a cotton plug, Fig. 10 (not shown) so that only long term changes in the ambient atmosphere affect the resonator 24. In the arrangement of Fig. 8 two lasers 68, 70 are slaved to a common resonator, the output beams of the lasers being orthogonally polarized. Half of the superposed light 142 from beam splitter 140 is used to over illuminate the resonator and half is passed through filter 84 to photo-detector 82 whence it is utilized to monitor the spectrum of beats between the lasers. The light passing through mirror 144 of resonator 24 is spatially separated by calcite prism 72 into its two constituent polarized beam and fluctuations of the angular position of each beam are measured with prisms 148, 150 photo-multipliers 152, 154, 156, 158 and amplifiers 160, 162. Resulting error signals are amplified before driving transducers 78, 80 to tune the lasers to the required frequency. The system may be used as a sensitive microphone by choosing the bandwidth of the servoloops properly i.e. choosing the bandwidth of at least one of the servoloops to be below the audio range and subjecting the resonator 24 to sound waves. A further modification of the apparatus is shown in the lower right hand corner of Fig. 8 in which an resonator 24 is added including an arrangement 169 for varying the path length of the resonator. When a signal is applied to 169 the resonant frequency of the resonator changes and the frequency stabilized beam from laser 70 is deflected on a screen 167. A heterodyne optical communications receiver is shown in Fig. 9. Light from a frequency controlled laser 84 is modulated by modulator 86 and then travels to receiver 88 where it overilluminates resonator 24 together with the light beam from a local oscillator laser 92. The two beams are split by calcite prism 72 and deviations in frequency are detected by detectors 94, 96 and the signals from amplifiers 160, 162 are added in differential amplifier 98 to produce a signal on line 100 to time laser 92 to the frequency of light beam 90. The combined beam is allowed to fall on photo-detector 104 after passing through a polarizer 106 oriented at 45 degrees to the planes of polarization of the light from each of the lasers 84 and 92. The local oscillator laser 92 and the distant modulated laser 84 are locked to the same central frequency by the servoloop and the modulation of the laser 84 is detected when the two signals are combined in the photo-detector 104. In order for the system to act as a super-heterodyne receiver the system is adjusted so that amplifier 98 provides a signal equal to the difference in the two signals supplied to it plus a constant whereby laser 92 operates at a frequency displaced a precise amount from the carrier frequency.