Patent GB-1170835-A (GB)
Title: Improvements in or relating to Laser Systems
Inventor: KERENYI IMRE
IPC: G01S17/10
Filed: 1967-07-12
Abstract: 1,170,835. Laser radar. G. & E. BRADLEY Ltd. 27 June, 1968 [12 July, 1967], No. 32042/67. Heading H4D. [Also in Divisions G1 and H1] The Specification describes a laser radar wherein a control member 13, Fig. 1, in a first position directs pulses from the laser to the remote target and in a second position directs light reflected by the target on to a photodector 35. The laser comprises a ruby rod 14 at one focus of an elliptical housing and a pump flash tube 17 at the other focus. The resonant path of the ruby comprises prisms 19, a Lummar-Gehreke plate 22 (a highly selective interference filter) and a mirror 21. Surface 18 of the ruby is semi-silvered and is the energy exit port. The control member 13 comprises a shutter disc rotated about axis 24 by motor 23 and having an aperture 15, Fig. 2, for passing the transmitted laser beam and a reflective coating 32 for directing the reflected light to the photomultiplier 35. To ensure that the laser only pulses when the aperture is in line with the beam 34, mirror 20 is rotated about axis 21 in synchronism with disc 13. Scattered light from the transmitted pulse is picked up by photodiode 27 and used to trigger the output from a bi-stable 28. The output of 28 opens a gate 29 to clock pulses from 30. The reflected laser pulse is reflected by surface 32 which has an area increasing as the square of the angle turned from the open (transmit) position, thus compensating for the fall of the intensity of the reflected light as the inverse square of the target range. A filter 36 stops stray light from falling on the photomultiplier 35, and the reflected pulse after energizing threshold means 37 stops the output of bistable 28. The clock pulse count in 31 thus indicates target range. An oscilloscope 40 may be used to give an A type indication of the range. A shield 38 fixed to disc 13 shields the photomultiplier during the pulse transmission. The laser pulse is only produced when the rotation of the disc 13 and mirror 21 is such that the p.r.p. thereof will be a standard value. D.C. motor 23 is thus started and builds up speed. Pulses proportioned to its speed (and that of disc 13 &c.) and fed to control circuit 25. When circuit 25 detects that the required speed has been reached motor 23 is switched off and a signal is sent to power supply 26 whereby the flash tube 17 is energized and the laser rod 14 emits a pulse. The pulses from motor 23 are passed to an AND gate both directly an via a 1 m.sec. delay, whereby the gate gives an output pulse at the attainment of said required speed. The supply 26 includes a first D.C. EHT source feeding a bank of capacitors and a resistor in series. A surge limiting inductor, the secondary winding of a transformer, and the flash tube 17 are connected in series are connected in parallel with the bank of capacitors. A resistor and a silicon controlled rectifier in series are connected across a second D.C. EHT source. A capacitor and the primary winding of the said transformer in series are connected in parallel with the silicon controlled rectifier. The gate of the silicon controlled rectifier is coupled to the output of said AND gate. In operation, the voltage applied by the first EHT source to the flash tube 17 is not sufficient to fire the flash tube. When a pulse is provided by the output of the AND gate, the silicon controlled rectifier conducts such that the capacitor connected thereto discharges through the primary winding of the transformer. The voltage induced in the secondary winding of the transformer is large enough to initiate the firing of the flash tube 17, and the bank of capacitors connected to the flash tube discharges through the flash tube, the secondary winding, and the surge-limiting inductor. The control circuit 25 includes a transistor connected in series with the motor 23. The base of the transistor is connected to the common junction of a resistor and a further silicon controlled rectifier which are connected in series with one another and in parallel with the motor and the transistor. A uni-junction transistor is connected in parallel with the further silicon controlled rectifier and has its emitter coupled to the output of the AND gate. The base of the silicon controlled rectifier is connected to the common junction of a capacitor and a further resistor connected in series, the further resistor being connected between the base and the cathode of the silicon controlled rectifier. A manually-operable momentarycontact control switch is arranged to cause, when actuated, a starting pulse to be applied to the base of the silicon controlled rectifier by way of the capacitor. When the silicon controlled rectifier is conducting, the transistor is fully conductive and the motor 23 is energized. The provision of a pulse by the output of the AND gate causes the uni-junction transistor to conduct momentarily, thereby shortcircuiting the silicon controlled rectifier which consequently ceases to conduct. The transistor thereupon also ceases to conduct and the motor 23 is de-energized and comes to rest. The invention may alternatively be used to determine the visibility of the medium through which the laser pulse passes. For this, the laser becomes a CW gas laser and the pulsing is caused by a disc being provided with a plurality of apertures. The reflective coating 32 has uniform area. Working at a known range from the target the visibility is determined by comparing the amplitudes of the transmitted and reflected pulses.