A TTL/CMOS pulse applied to the gate of SCR switches it on, thereby producing high-voltage trigger pulse across the secondary winding of trigger transformer. High-voltage trigger produces required pre-ionisation, forcing simmer current to flow through it.
This circuit, however, has a drawback that if simmer current stops due to some reason, there is no in-built mechanism to restore it. This shortcoming is overcome in the circuit schematic of Fig. 5. In this case, if the simmer current stops, an astable multivibrator controlled by a comparator restores normal operation. The astable multivibrator operates typically at 20Hz to 30Hz. Simmer power supply modules like many other laser electronics subsystems, such as capacitor-charging power supplies and flash lamp trigger circuits, are also commercially available for OEM manufacturers. One such module is shown in Fig. 6.
The pulse-forming network (PFN) produces a critically damped current pulse through the flash lamp when the energy storage capacitor is discharged through it. This is the most efficient way of energy transfer, which also minimises reverse voltage appearing across the capacitor.
Fig. 7 shows single-stage PFN. The value of energy storage capacitor depends upon stored energy (E0), desired pulse width (tP) and flash lamp impedance parameter (K0), and is given by C=(0.09E0tP2K0–4)1/3. Also, tP=√LC and L=tP2/9C.
Flash lamp trigger circuit
Flash lamp trigger circuit is required only in case of non-simmer mode of operation. Common modes of flash lamp triggering include overvoltage triggering, external triggering, series triggering and parallel triggering. Overvoltage and parallel triggering schemes are less popular. External and series triggering circuits are more common and are shown in Figs. 8 and 9, respectively.
The main advantage of external triggering is that it does not interfere with the main energy discharge circuit. The disadvantage is that high-voltage trigger point is exposed and therefore needs to be properly isolated from the environment, lest it causes problems in high altitude or humid conditions.
External triggering is recommended for low repetition rate, low energy systems where the flash lamp is air cooled.
In the case of series triggering circuit, the series trigger transformer is designed in a way that the transformer core saturates and the saturated secondary winding inductance serves the purpose of the PFN inductor also. Series triggering offers the advantages of reliable and reproducible triggering.
Receiver electronics of laser rangefinder
Fig. 10 shows the block schematic arrangement of different building blocks of receiver electronics of a typical laser rangefinder. Optoelectronics front-end circuit is one of the most critical building blocks of the receiver section. It is supposed to transform the received laser pulse, which could be anywhere in the range of 10ns to 20ns, to an equivalent electrical signal.
The peak power of the received laser pulse could be as low as a few tens of nano-watts when ranging a far-off target and as high as a few tens of milli-watts when the target is close by. This implies that the amplifier portion of the front-end needs to have a dynamic range as high as 100dB to 110dB. This is usually achieved partly in the avalanche photodiode by controlling the responsivity of the device through its reverse-bias variation and partly in the gain-controlled amplifier stage.
Laser pulse width decides the bandwidth of the front-end and is given by bandwidth = 350/tR where tR is rise time of the laser pulse in nanoseconds. Range counter clock frequency determines the range measurement accuracy and is given by ±(c/2fCLK) where c=3×108 m/s. A large number of manufacturers offer different modules of laser rangefinder electronics including photodetector amplifiers, fast pulse peak stretchers and range counters (Fig. 11).
Present-day laser rangefinders and target designators are largely configured around diode-pumped pulsed solid-state lasers. In that case, the transmitter electronics is nothing but the drive and control circuitry required for laser diodes used to optically pump the gain medium. Laser diode drive and control electronics mainly include current source to drive the laser diodes and temperature controller to operate the laser diodes at the desired temperature. Different building blocks of laser diode electronics are discussed in the following parts of the article.
To be continued in the third part
Dr Anil Kumar Maini is a senior scientist, currently the director of Laser Science and Technology Centre, a premier laser and optoelectronics research and development laboratory of Defence Research and Development Organisation of Ministry of Defence. Nakul Maini is a technical editor with Wiley India Pvt Ltd