Patent GB-1377924-A (GB)
Title: Laser apparatus
IPC: H01S3/136
Filed: 1973-03-19
Abstract: 1377924 Lasers PHILCO-FORD CORP 19 March 1973 [22 March 1972] 13091/73 Heading H1C A laser emits two adjacent longitudinal modes having orthogonal polarizations by providing a birefringent element 7 within its resonator 3, 5, Fig. 1, coupling between the modes being minimized, (to prevent one mode from suppressing the other), by including in the resonator a birefringent-producing electro-optic modulating element 8 excited at a frequency equal to the frequency difference between the two adjacent modes. Carbon dioxide pumped by gas discharge is used as the active medium in the arrangement shown, the gas discharge tube 1 being located in a confocal resonator and emitting radiation in the range 9-11 microns. A separate birefringent element 7 may be used, as shown, or the window 6 of the gas discharge tube may be birefringent. The laser output 10 is obtained from partially-transmitting resonator mirror 5, and in an application to ranging is applied to a polarization sensitive beam splitter which passes one longitudinal mode 10a towards a moving target 13 and reflects the other, orthogonally polarized, longitudinal mode 10b along a path which includes a mirror 15, a polarizer angled at 45 degrees with respect to the two orthogonal polarizations, and a photodetector 14. The mode 10a, after reflection at the moving target, is deflected by a beam splitter 12 towards the photo-detector, the polarizer 16 mixing the two modes 10a, 10b so that a beat frequency is obtained at a demodulator 17 which increases if the target recedes and decreases if the target approaches the laser. A frequency stabilizing arrangement is shown in Fig. 2, the output beam 10 being monitored by using a beam splitter 18 to deflect a part of the output towards a polarization-sensitive beam splitter 19, such as a Wollaston prism, which separates the two longitudinal modes so that they pass to respective photo-detectors 20, 21. A differential amplifier 22 compares the amplitudes of the two photo-detector outputs and adjusts the position of mirror 5 through a piezo-electric transducer 23 to maintain equality of the two longitudinal modes. A frequencymodulated communications output 10 is obtained in the Fig. 3 arrangement, the birefringent element 24 being electrically varied by an amplitude modulated signal from a modulator 25 superposed on a D.C. bias so as to correspondingly vary the frequency separation of the two longitudinal modes. A portion of the frequency-modulated output beam 10 is diverted by a beam splitter 17 through a 45 degrees polarizer 16 to a photo-detector 14 which provides an electrical output signal to an amplifier 26 having an A.C. component equal to the frequency difference of the two modes. The amplifier output is applied to an electro-optic modulating element 8 in the resonator to maintain the two separate longitudinal output modes. A keyed communication system is shown in Fig. 4 in which the modulating electro-optic element 8 is excited by the keyed R.F. output from a driver 27. Two separate and adjacent longitudinal modes having orthogonal polarizations are then obtained only when the R.F., of frequency equal to that of the mode separation, is applied to element 8. At the receiver the presence of mode separation is detected by the combination of a 45 degrees polarizer 16, a photo-detector 14 and a bandpass filter 28 tuned to the mode frequency separation, the filter output passing to a demodulator 29. Other suitable lasers are YAG and ion.