Patent GB-1235905-A (GB)
Title: Improvements in or relating to distance measurement systems
IPC: G01S17/32
Filed: 1968-11-28
Abstract: 1,235,905. Lasers. SIEMENS A.G. 28 Nov., 1968 [1 Dec., 1967], No. 56505/68. Heading H1C. [Also in Division H4] Specification describes a method of distance measurement by the transmission and reflection of a signal modulated on a light beam, in which a fine measurement is affected by measuring the phase difference between the transmitted modulation signal and the reflected modulation signal, to give an ambiguous measurement of relatively high accuracy, and in which a coarse measurement is affected by measuring the transit time of a pulse of said light beam to give an unambiguous measurement of low accuracy. The phase and transit time measurement are carried out by means of a pulse counter, the transmitted and reflected modulation signals being heterodyned down prior to measurement whereby the high accuracy of the counter which would have been required for the direct measurement on the modulation signals is reduced to that accuracy required by the transit time measurement. The light source 1, Fig. 3, may comprise a luminescent diode and/or a laser diode or, as in the described embodiment, a gas laser. The modulation signal for the laser at frequency f 1  is produced by generator 5 and is applied via switch 4. During the high accuracy/ambiguous mode, switch 4 is closed and switches 16, 16<SP>1</SP> are open. Closure of switch 17 for the coarse/ unambiguous mode opens switch 4 and closes switches 16 and 16<SP>1</SP>. In the high accuracy/ ambiguous mode the amplitude modulated laser light is directed to a remote reflector by optical means 10. A sample of the light is diverted by beam splitter 9 to a photodiode 12<SP>1</SP>, the output of which, after amplification at 12<SP>11</SP> is applied to mixer 13<SP>1</SP>. The reflected light is detected, via optics 11, by photomultiplier 12 and the output thereof is applied to mixer 13. A generator 19 supplies a heterodyne signal to the mixers 13, 13<SP>1</SP> at a frequency f 2 # 1  (1-α) = 10-<SP>3</SP> is described embodiment, whence the frequency conversion ratio is :- Pulse shapers 14, 14<SP>1</SP> produce a pulse when the outputs of the respective mixers pass through a positive going zero. One of these pulses from shaper 14<SP>1</SP> is chosen by the closure of switch 15 and closes switch 6 whereby clock pulses from generator 5 pass to counter 7. The next pulse from shaper 14 opens switch 15 and stops the count, the counter then registering the fine but ambiguous range measurement. In the course/ unambiguous mode the now unmodulated laser is pulsed by the momentary closure of switch 2. The reflected laser pulse and the diverted portion of the transmitted pulse by-pass the heterodyning stages via switches 16 and 16<SP>1</SP> and operate directly on switch 6. Counter 7 now registers the coarse range measurement. The two range measurement combine to produce a fine unambiguous range indication at 8. The gas laser, Fig. 4, comprises a glass tube 21 containing the gas and formed with Brewster angled end faces. The gas is laterally pumped (see three arrows) and the resonant cavity is defined by mirrors 20 and 20<SP>1</SP> to have an average length L. Mirror 20 is supported by an electromechanical body 26 which is energized at the modulation frequency f 1  whereby it flexes of said frequency and modulates the length of the cavity and said frequency. By choosing I and f 1  such that the laser output is concentrated into pulses, Fig. 5. The pulsed laser beam passes through a polarization filter 22 and a Kerr cell 23. With no voltage applied to the cell 23, the beam polarization is unaffected and passes in toto back through filter 22. For the fine/ambiguous mode however a D.C. potential is applied by means 24, whereby cell 23 causes a change of polarization and the laser beam is partially diverted out of the cavity by filter 22 for utilization. In the coarse/unambiguous mode the cell is pulsed (dotted line, Fig. 5) by means of means 24 synchronized by f 1  generator 27, whereby only one pulse of the laser output is diverted.