Patent GB-1402968-A (GB)
Title: Semiconductor laser system
IPC: H01S3/139
Filed: 1973-03-02
Abstract: 1402968 Lasers INTERNATIONAL BUSINESS MACHINES CORP 2 March 1973 [28 June 1972] 10209/73 Heading H1C A semi-conductor junction laser which emits a single transverse mode has at least one external resonator mirror, and a control system is provided to keep the resonator aligned with the junction. Fig. 1 shows the laser body 2 placed between resonator mirrors 10 and 15; lenses 13, 14 and 18, 19 form doublets corrected for spherical abberation and focus the output of body 2 on to the mirrors. Unless the laser is operated near its threshold current opaque plates 11 and 16 having mode control slits 12, 17 are necessary to eliminate all but the TEM 00  mode. The resonator components are mounted on two platforms 21, 22 which can be moved up and down, parallel to the narrow dimension of the laser junction, so that the resonator axis can be made to coincide with the junction. A detector 28 measures the leakage radiation from the almost wholly reflecting mirror 10 and drives control circuit 30 to initially position the resonator such that lasing occurs, then to maximize the output power and finally to maintain the output. The right hand half of Fig. 1 may be replaced with a partially transmitting reflector deposited on the right hand end of the body 2, Fig. 7 (not shown). Control circuit.-This is shown in outline in Fig. 3, and performs a sequence of operations governed by flip-flop 63. When the flip-flop receives a start signal an output Q initiates lasing acquisition by applying a relatively slow triangular waveform from generator 61 to the piezo-crystal 25 carrying resonator platform 21 and a relatively fast sine waveform to the crystal 27 of platform 22. During this process switches 70, 71 are connected to the generators, but as soon as lasing commences, due to near alignment of the resonator as the crystals move the platforms, the detector 28 passes a signal to the flipflop which moves switches 70, 71 to the positions shown in Fig. 3. Steady voltages from sources 66, 67 are then applied to the crystals to hold them in the lasing position, while a signal on flip-flop output Q initiates lasing maximization and tracking. It is assumed that the laser is operated in a pulse mode, at say 1 kHz; a dither generator 75 now applies a slowly varying A.C. signal to the crystal 25 via switch 79. This will cause the output pulses to cyclically increase and decrease in amplitude as the resonator alignment is varied. Fig. 6 shows the pulses as the crystal is dithered, the mean position of the resonator axis having a small offset I from the junction. The pulses from the detector 28 are fed to a peak detector circuit 81, Fig. 3, so that as the crystal is dithered from left to right in Fig. 6 the peak detector circuit receives eleven pulses of increasing amplitude and eight of decreasing amplitude. The circuit causes counter 82 to count eleven, then counter 83 to count eight. These counts are converted to analogue values at 84, 85 and their difference adjusts D.C. source 66 and moves the crystal 25 so that the offset l is reduced. The process repeats until the counts are equal, as detected at 92, which changes over switch 79 so that the dithering is carried out with crystal 27 until it too is positioned for maximum output. If, due to shock or vibration, the lasing ceases the omission of a pulse is also detected at 92 and resets the flip-flop 63 to recommence acquisition. Once the output has been maximized the dither voltage is reduced and switch 79 is periodically changed over so that the maximum output is maintained.