Laser rangefinders and laser target designators are the most abundantly used laser systems for tactical battlefield scenarios. Laser rangefinders are used both as standalone devices as well as integral parts of an overall electro-optical fire control system (EOFCS) of armoured fighting platforms.
In the standalone mode, the device is used for determining the range of adversary’s positions by troops in observation and reconnaissance missions. In an EOFCS, the range data produced by the rangefinder is used by a computer to control the gun position for a precise target hit.
The laser source used in such rangefinders is either a Q-switched neodymium-doped yttrium aluminium garnet (QS-Nd:YAG) emitting at 1064nm or an Nd:YAG laser with its output wavelength shifted to an eye-safe wavelength of 1540nm using an optical parametric oscillator. Erbium-doped glass (Er:Glass) lasers are also used to generate an eye-safe wavelength but their use is restricted to rangefinders operating at relatively low repetition rates.
Laser target designators are almost invariably configured around high repetition rate Q-switched Nd:YAG laser sources. These are used in laser-guided munitions delivery applications.
In a typical laser-guided munitions delivery operation, the laser target designator, which could be land based or airborne, irradiates the intended target with laser pulses of a pre-determined pulse repetition frequency (PRF) code, which is also known to the laser seeker head of the guided weapon. The laser seeker head in the laser-guided weapon senses the laser radiation scattered from the target, deciphers the PRF code and then commands the weapon to home on to the source of scatter.
Solid-state laser electronics
When it comes to electronics that go along with laser rangefinders and target designators configured around Q-switched solid-state lasers, we need to discuss the electronics package required for operating the laser source at desired specifications and also the electronics required for range measurement.
Further, pulsed solid-state lasers are either flash-lamp-pumped or pumped by laser diodes. Lasers belonging to 1980s and 1990s were almost exclusively flash pumped. But now they have been largely replaced by laser-diode-pumped versions over the last 10 to 15 years due to much higher optical-to-optical and wall plug efficiency figures.
The following paragraphs describe the electronics packages for flash-lamp-pumped and laser-diode-pumped Q-switched solid-state lasers in general and Nd:YAG lasers in particular. A brief outline on the functions of different modules constituting the overall electronics package, with particular reference to the importance of each module, is presented first, which is followed by a detailed description of each of the important modules along with preferred schematic options and design guidelines.
Electronics for flash-pumped lasers
In the case of flash-pumped Nd:YAG and Nd:Glass lasers, the gain medium is optically pumped by a flash lamp, such as a Xenon or a Krypton flash lamp, whose output optical spectrum matches with the absorption spectrum of the gain medium. There are two possible operational modes in which requisite quantum of energy can be delivered to the flash lamp.
In one of the modes employed in earlier lasers, called non-simmer mode of operation, the electrical energy stored in a capacitor is discharged through the flash lamp by application of a high-voltage trigger pulse to ionise the gas fill and create a low-resistance path.
In the other mode used nowadays, called simmer mode of operation, a low-resistance path is maintained through the flash lamp. The lamp is kept isolated from the energy storage capacitor by an electronic switch, which is triggered to on state by a transistor-transistor logic (TTL) CMOS pulse forcing the stored energy to discharge through the flash lamp.
Fig. 1 shows the detailed block schematic arrangement of electronics package of a flash-lamp-pumped Q-switched solid-state laser. The heart of the system is the main power supply, which is invariably a switched mode one used to charge an energy storage capacitor so as to store the required quantum of energy per pulse to be delivered to the flash lamp.
The main power supply is also called capacitor-charging power supply. The capacitor must charge to the desired voltage in a certain time, which is at the most equal to the reciprocal of the repetition rate of the laser. In practice, it should be slightly less, allowing for some minimum time for flash lamp quenching. The average power that this supply is expected to deliver at its output is the product of the energy per pulse and the repetition rate.
The power supply accounts for more than 90 per cent of the total electrical input to the system. The efficiency of this supply is therefore the prime determinant factor for the overall electrical efficiency of the laser. The conversion efficiency also directly affects the size and weight of the overall system, a parameter particularly important in the military applications of Q-switched, flash-lamp-pumped solid-state lasers.
The simmer module maintains a relatively low-amplitude keep-alive current through the flash lamp at all times, irrespective of whether the lamp is flashing or not. The current varies typically from a few tens of milli-amperes to several hundreds of milli-amperes depending upon the characteristics of the flash lamp. This mode of operation, called the simmer mode, has many advantages.
From the operational viewpoint, it allows one to use a low-voltage (TTL and CMOS) trigger pulse to transfer the energy stored in the capacitor to the flash lamp. Besides, it significantly enhances the flash lamp life, offers tremendous improvement on the pulse-to-pulse jitter and overcomes most of the electromagnetic interference problems present in non-simmer mode of operation.
It may be mentioned here that in the non-simmer mode of operation of the flash lamp, the triggering of the flash lamp is done by applying high-voltage trigger pulses with amplitude of the order of 10kV to 15kV. These pulses appear at a rate equal to the repetition rate of the laser and are the major source of electromagnetic interference.
Operation in simmer mode overcomes this shortcoming. There is a pseudo simmer mode of operation also, which is a slight variation from the traditional simmer mode. In this mode, the simmer current flows for a short time, starting a little ahead of the energy discharge operation. It has all the advantages of simmer mode operation and in addition saves power, but at the expense of added circuit complexity.
Q-switch driver is another important module for solid-state lasers as it generates drive signal for electro-optic Q-switch. The driver needs to generate a high-voltage pulse having the desired amplitude (typically, 2.5kV to 3.5kV), pulse width that could be in the range of 200ns to 500ns and a rise time that should not be more than a few tens of nanoseconds.
The pulse-forming network (PFN) ensures that the discharge current pulse through the flash lamp has the desired pulse width and is critically damped, thus giving the most optimum energy transfer.
The command module generates flash lamp firing command pulses and also the delayed trigger pulses for the Q-switch driver module. It may be mentioned that Q-switch drive pulse is applied after a certain known time delay from the application of flash lamp trigger command pulse to allow for the population inversion to build up to its peak value.
The flash lamp command pulses in the case of simmer mode operation are low-voltage pulses (TTL and CMOS) and high-voltage trigger pulses in the case of non-simmer mode of operation. The delayed trigger pulses for the Q-switch driver are always low-voltage pulses and the Q-switch driver produces the desired high-voltage pulses for the electro-optic Q-switch. In addition, there is an auxiliary module that generates the regulated low-voltage DC power supplies from the input source of power for the operation of different circuit modules.
In the case of non-simmer mode of operation, electronics is similar to the one for simmer mode of operation, except that there are no electronic switch and simmer power supply and that the command module feeds a high-voltage trigger generator circuit.
Capacitor-charging power supply
The capacitor-charging power supply is the most important of all the modules for reasons outlined earlier. The power supply output needs to charge a high-value capacitor, typically 20µF to 50µF in case of designators and rangefinders and as high as thousands of microfarads in high-power pulsed lasers, producing laser pulse energies of several kilojoules meant for electro-optical countermeasures and laser weapon applications.
Switched-mode concept is invariably used for the design of capacitor-charging power supply. Externally driven flyback converter is the preferred topology. Its design, however, is not as straightforward as it would be in the case of a resistive load. The reason is as follows.
In the case of capacitive load, energy storage and energy transfer mechanisms are relatively more complex. Each time an energy packet is stored in the primary of the transformer and subsequently transferred to the capacitor, the time needed to transfer the packet of energy depends upon the quantum of voltage it would impart to the capacitor.
As a result, for the same energy quantum, time required for transfer continuously reduces as the voltage builds up across the capacitor from zero to the final value due to diminishing voltage quantum. Therefore it is not advisable to use a fixed frequency or fixed off-time switching supply. The drive waveform needs to be a variable frequency one with the off-time periods governed by the charge status of the energy-storage capacitor.
It can be mathematically proved that the size of voltage packet varies from V when the capacitor is fully discharged to [√N–√(N–1)]×V, where N is the number of packets required to charge the capacitor to the final value. If the off-time of the drive waveform could be varied or decreased to be more precise in accordance with the voltage build up across the capacitor, the power supply would operate at the highest possible conversion efficiency.
Fig. 2 shows the preferred block schematic arrangement of a capacitor-charging power supply configured around an externally-driven flyback converter.
The circuit is divided into two major parts, namely, the drive circuit and the feedback circuit. The drive circuit comprises a cascaded arrangement of a voltage-controlled oscillator (VCO), a monoshot circuit and a drive circuit. Output of VCO feeds the trigger input of the monoshot. The pulse width of the monoshot is chosen to be equal to the desired on-time of the switching device. The frequency of monoshot output and hence the off-time of the waveform is governed by the frequency of the VCO output, which in turn depends upon the voltage applied to its control input.
The drive circuit provides the required drive current and/or voltage depending upon the type of switching device used. The feedback circuit VCO output frequency is configured around a comparator and a subtractor. The comparator circuit is used to reset the monoshot circuit and therefore withdraw drive current from switching device as and when the output voltage reaches the desired output voltage.
The subtractor output controls the frequency of the VCO and therefore the off-time of the drive waveform to the switching device. The subtractor output in turn depends upon the voltage across the energy storage capacitor decreasing with increase in voltage.
Decrease in control voltage to VCO increases the output frequency, thereby reducing the off-time at the output of monoshot. What is important here is that the pattern of reduction of off-time is linearly related to the reduction of control voltage, which in turn is linearly related to increase in voltage across the capacitor. Thus off-time reduces in accordance with increase in the capacitor voltage. This design yields the most optimum results.
Compact capacitor-charging power supplies covering a range of input-output voltage and power output specifications are available from different manufacturers of laser electronics. One such unit intended for OEM applications is shown in Fig. 3.
Simmer power supply
The simmer power supply maintains a steady-state partial ionisation of the flash lamp during the time the lamp is not flashing by maintaining a low keep-alive current though it. Simmer power supply must be designed with due consideration to I-V characteristics of the flash lamp. The simmer power supply is a high-voltage DC power supply producing an output voltage in the range of 800V to 1500V depending upon the characteristics of the flash lamp and the required magnitude of simmer current. A high-voltage trigger pulse, typically 10kV to 15kV, creates pre-ionisation before the simmer power supply can take over and deliver the keep-alive current through the flash lamp.
The output of the simmer power supply is applied to the flash lamp through a series resistor called ballast resistor. The magnitude of the simmer current therefore depends upon the difference between the simmer supply output voltage and voltage across the flash lamp in the simmer mode, and the value of the ballast resistance. The value of the ballast resistance should be slightly higher than the negative impedance offered by the flash lamp in the simmer regime.
Simmer power supply is generally designed around an externally-driven flyback converter topology with output power delivery capability equal to product of required open circuit output voltage and magnitude desired simmer current. Open circuit voltage is further equal to sum of voltage across the lamp in simmering condition and voltage drop across the ballast resistor. Fig. 4 shows simmer power supply interface with the flash lamp.
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
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