There has been a dramatic increase in the commercial, industrial, medical, defence and research applications of lasers in the last 10 to 15 years. During this period, we have not only seen the use of lasers in applications thought of in the early days of lasers getting matured, but we have also witnessed them making their presence felt very effectively in many new areas.
Laser which was rightly called an invention in search of applications or a solution looking for a problem in 70’s is today the most widely used and, more importantly, the most diversely exploited piece of equipment. It is undoubtedly one of the greatest inventions of 20th century along with computers, satellites and integrated circuits.
There are two main factors which have played a major role in enhancing the application potential of lasers. The first of course is the development and subsequent arrival on the commercial scene of a large variety of lasers covering a very large portion of electromagnetic spectrum and producing a wide range of output power levels at an affordable price tag.
Without going back to the days of Maiman and evolution of lasers that followed thereafter, it is important to note that today we have a very large number and types of lasers and hundreds of laser wavelengths discovered all over. Not all of them are such or have reached a stage that their use is commercially viable, yet these numbers do indicate the quantum of research and development that has gone into this field and the promise this magical device holds for us in the future.
The second main reason for the widespread use of lasers is the development that has taken place in the field of electronics that goes along with a laser or a laser-based system. The role of an electronics engineer working in the field of lasers is far more challenging today than it was in the early stages of development of lasers. Laser electronics today involves a large number of complex technologies, some of them specific to lasers.
Laser electronics and related technologies
Electronics plays an important role in the demonstrated capabilities, operational efficiency and achievable system compactness of military lasers and optoelectronics systems. Majority of military laser systems employ one of the three common types, namely, solid-state, semiconductor and gas lasers.
Laser rangefinders and target designators, for instance, are configured around mainly solid-state lasers and in some cases semiconductor diode lasers and gas lasers.
Precision-guided munitions use solid-state laser-based target designators. Electro-optic countermeasure (EOCM) systems are based on high-energy solid-state laser sources. Inertial navigation sensors use an He-Ne ring laser cavity, laser proximity sensors and laser-aiming devices employ semiconductor diode lasers and directed energy laser weapons are projected to be using high-power bulk solid-state or fibre lasers.
On the opto-electronics front, laser sensors, laser seekers, laser detection and ranging (LADAR) sensors and light detection and ranging (LIDAR) sensors constituting bulk of battlefield opto-electronics mainly use silicon, indium gallium arsenide, indium antimonide, mercury cadmium telluride and image intensifier photo sensors.
To summarise, if we were to look at the spectrum of electronics that goes along with military lasers and opto-electronics systems, we would need to consider the electronics of solid-state, semiconductor diode and gas lasers.
When we set out to discuss the role of electronics in lasers and laser-based systems, we usually talk about only those sub-systems that are essential from the operational viewpoint of the laser in question. There is much more than the power supply circuit that is used to operate the laser.
In the following paragraphs we shall take a closer look at the areas of electronics encountered in the case of three most commonly used laser sources, namely, solid-state, gas and semiconductor lasers. Prominent military laser systems configured around these laser sources are also briefly discussed.
Solid-state laser electronics
When we discuss electronics package of a flash-lamp-pumped pulsed solid-state laser (Nd-YAG, Nd-Glass, etc), it is the power supply unit which is needed to charge the energy storage capacitor to store the requisite quantum of energy that is considered representative of the electronics package. It is simply because of the reason that it is not only the most complex of the electronics circuit modules used in the case of flash-lamp-pumped solid-state lasers; its electrical conversion efficiency plays a key role in deciding the wall-plug efficiency and the size/weight of the laser. Capacitor-charging power supplies intended for flash-lamp-pumped solid-state laser applications are commercially available both as benchtop models as well as modular units for OEM applications. Fig. 1 shows photograph of one such capacitor-charging power supply suitable for flash-lamp-pumped solid-state lasers.
These capacitor-charging power supplies are available for a range of input voltage (AC or DC), DC output voltage and charging rate specifications. DC output voltage from a few hundreds of volts to several kilovolts and charging rate in the range of tens of joules per second to thousands of joules per second are common. In addition, these power supplies offer many control and protection features relevant to flash-lamp-pumped laser power supplies. Some of these features include end-of-charge status indication, peak output voltage hold, output voltage monitor, overvoltage and overtemperature protection and so on.
We also talk about, though with less enthusiasm, the other circuit modules such as the simmer power supply which is invariably used in high-repetition rate, flash-lamp-pumped solid-state lasers or the Q-switch driver used in the Q-switched lasers, or even the pulse forming network (PFN) which ensures a critically damped current pulse through the flash lamp when the energy storage capacitor is made to discharge through it.
Nd-YAG, frequency-shifted Nd-YAG and erbium glass operating at 1064nm, 1540nm and 1550nm, respectively, are the most commonly used laser sources for laser rangefinding, target designation and laser-guided munitions delivery applications. While Nd-YAG is almost invariably used for target designation and laser-guided munitions delivery, lasers operating at 1540/1550nm are preferred for rangefinding applications considering the eye safety of operating personnel.
Semiconductor diode lasers are also used for rangefinding applications both for commercial and military domains. their use is mainly restricted to relatively shorter ranges with maximum measurable range generally not exceeding 5km. On the other hand, portable and handheld Nd-YAG laser-based rangefinders are available for maximum range measuring capability in excess of 25km. Fig. 2 shows the photograph of one such handheld laser rangefinder.
In addition to conventional target rangefinding, there are many related applications where the basic rangefinding concept is put to use in different military laser systems. Some of the prominent ones include proximity sensors, obstacle avoidance sensors and bathymetry for sea-bed mapping. These devices also make use of either solid-state or semiconductor diode lasers.
Semiconductor diode laser electronics
Design of drive and control circuits needed to power semiconductor diode lasers should consider certain handling and protection issues if they were to have the prescribed life and reliability performance. It is more so for semiconductor diode lasers used in military applications. Diode lasers are particularly sensitive to electrostatic discharge, short-duration electric transients such as current spikes, injection current exceeding the prescribed limit and reverse voltage exceeding the breakdown limit.
In order to protect the diode lasers from above failure modes, the driver circuit should be carefully designed and should have all the features recommended by the diode-laser manufacturer. The driver should be a constant current source with in-built features such as soft start, protection against transients, interlock control for the connection cable to the laser and safe adjustable limit for injection current.
In case the laser is to be operated in pulsed mode, the injection current should be pulsed between two values above the lasing threshold rather than between cut-off and lasing mode. A laser diode when used in a laser printer or a laser pointer, or even a compact disk player, may need a conventional constant-current source without too stringent a requirement on the current stabilisation to do the job.
Drive current and diode temperature stabilisation to a high degree become extremely important when the intended application demands a stable output wavelength. One such application area is in laser-based Raman sensor used for detection and identification of chemical warfare agents and explosive materials. The concepts of drive current and diode temperature stabilisation have been put to use very effectively in tuning the diode laser output wavelength, which is a requirement in laser systems designed for detection and identification of chemical warfare agents.
Leading manufacturers of semiconductor diode lasers offer a wide range of current sources for low-, medium- and high-power laser diodes to suit different requirements. Both general-purpose benchtop models and modular units for OEM applications are commercially available from a fairly large number of manufacturers. Fig. 3 shows the photograph of a benchtop precision laser diode driver.
Most of the commercial laser diode drivers offer operation in both constant current and constant power modes and have in-built protection features including adjustable current and voltage limits, intermittent contact protection and so on.
They also offer temperature controllers to stabilise the output wavelength. Temperature controllers for diode laser temperature control are also available in a wide range of performance parameters to suit different requirement specifications. Both benchtop units and OEM modules are commercially available. Fig. 4 shows the photograph of a typical benchtop thermoelectric laser diode temperature controller. Most of these temperature controllers are capable of operating in constant temperature, constant power or constant current modes with temperature stability of better than 0.003°C.
Gas laser electronics
He-Ne and carbon dioxide lasers are the most commonly used gas laser sources. While carbon dioxide lasers are commonly used in some types of laser rangefinders, He-Ne lasers mainly find application in inertial navigation sensor. What most of us know about the He-Ne laser electronics is a high-voltage power supply that initiates and subsequently sustains the plasma. The plasma current stabilisation may not be important if it is to be used for the purpose of alignment. But when it comes to using the same laser in an inertial-grade rotation rate sensor such as a ring laser gyroscope (Fig. 5), the current would need to be stabilised to a level better than 100 ppm.
In addition, you would also need to stabilise its frequency to better than +1MHz on its Doppler broadened gain curve which is about 1400MHz wide for 632.8nm output wavelength. To further add to the design complexity of plasma current initiation and control circuit, the difference between the plasma currents in the two counter propagating laser beams needs to be stabilised to an order of magnitude better current stability than the absolute current stability of each arm.
Frequency stabilisation of gas lasers is a complete field in itself. There are scientists who have worked in this area for decades to discover new methods to stabilise the laser frequency using active means or improve upon those already existing. Different frequency stabilisation techniques include dither stabilisation, optogalvanic stabilisation and Stark-cell stabilisation. It is possible to actively stabilise the frequency of carbon dioxide lasers to better than a few kilohertz using Stark-cell stabilisation by stabilising the frequency to the centre of lamb dip on the gain versus frequency curve.
In the category of carbon dioxide lasers, we have both DC-excited as well as RF-excited lasers. While the design of power supply for a DC-excited carbon dioxide laser is similar in concept to the one used in the case of He-Ne lasers except of current and voltage levels. in the case of RF-excited lasers, typical excitation source comprises an RF source operating in the frequency range of 50-150MHz and an impedance matching network. The RF source may further be split up into an RF oscillator and a cascade arrangement of RF amplifiers depending upon the output power delivering capability. RF power is fed to the laser cavity through impedance matching network.
In a large number of opto-electronic systems, in particular the battlefield opto-electronic systems, the primary function is detection of laser radiation with or without its important parameters depending upon the intended application. Some such systems include laser warning sensors, laser position sensors and laser seekers. Laser warning sensor system is an essential constituent of both passive and active EOCM systems.
In the simplest form, a laser warning sensor may be used to detect the existence of a laser threat without giving any information on the direction of arrival of the threat.
In another case, it may also have the direction-sensing feature with the resolution of direction sensing varying with the intended application from a coarse-sensing capability in the range of ±5° to ±45° to high-resolution sensing with direction-of-arrival sensing capability in the range of ±0.1° to ±1.0°.
In addition, the operational wavelength band may also vary from one application to another. More complex laser warning systems indicate the type of laser threat, the wavelength and the direction of arrival of the threat. A typical high-end laser warning sensor is capable of characterising threats from laser rangefinders, laser target designators and laser beam riders, and these are available for mounting on helicopters, main battle tanks and light-armoured vehicles.
While one of the most common military applications of a laser warning sensor is as a sub-system in an integrated EOCM system for armoured fighting platforms, there are many other application scenarios where laser sensors are deployed.
Emerging trends indicate the use of an array of laser sensors interfaced with a high-energy laser to protect critical and strategic assets such as aircraft shelters, ammunition depots, strategic buildings, naval vessels and so on from laser-guided munitions attack. Arrays of laser sensors in this case detect laser threat and decode its parameters. These parameters are used to control the operation of a high-energy laser source. The laser source in turn illuminates a dummy target to misguide the incoming laser-guided munitions towards the dummy target.
Laser-guided munitions use a laser sensor called laser seeker, which is also a kind of position sensor. Laser seeker is the heart of the guidance system of a laser-guided weapon such as laser-guided bomb or missile.
Fig. 6 shows the laser-guided bomb integrated with laser seeker head. A typical laser seeker employs a quadrant sensor for determination of direction of arrival of laser radiation scattered off the intended target when illuminated by a laser target designator. The laser target designator and the laser seeker work in harmony. Both operate on the same pulse repetition frequency (PRF) code, which allows the bomb to home on to the source of laser scatter.
LADAR sensor (Fig. 7) is one of the most contemporary forms of laser seekers usually used for ultra-high precision hitting of strategic targets. It is mainly used in conjunction with other guidance systems on strategic payloads for intended target discrimination from advanced decoys and aim point selection. It is also well suited for combat identification, navigation of autonomous vehicles and topography.
LADAR is also suitable in finding targets hidden by camouflage nets and foliage. LADAR seeker can detect and identify specific features of the target with very high definition up to the resolution of a few centimetres from the distance of a few kilometres.
LADAR sensors are usually employed on loitering systems that look at the target from different angles, verify target’s identity and select the best attack position for desired results. The sensor in essence generates a 3D image of the intended target. The 3D image is compared with various 3D templates stored in weapon’s memory before the mission and it facilitates identification of target and selection of aim point.
Test and evaluation of lasers
Measurement of laser power, energy, pulse width, etc is yet another area that is electronic or more precisely opto-electronic in nature. Today, we have all kinds of lasers producing CW or pulsed or Q-switched pulsed laser outputs. While we would be mainly interested in the output power in the CW lasers such as gas lasers, it is the energy and the pulse width that would be of interest in case of pulsed lasers. In case of Q-switched pulsed lasers, such as solid-state lasers, we would like to measure energy per pulse, average power and also the peak power. Equipment that are capable of measuring one or more of these parameters are commercially available. Fig. 8 shows one such commercial meter capable of measuring laser power and pulse energy over a wide range when used in conjunction with suitable sensor heads.
While commercially available test and measurement equipment can be used for carrying measurement of important laser parameters and therefore be useful to the engineers responsible for maintaining military laser equipment, there are cases where dedicated test systems are needed to perform health checks. Sometimes, these quick health checks, also called serviceability checks, are needed to be performed on the systems integrated on the platform.
The point that we are trying to drive home is that while talking or writing about laser electronics, it would be far from being justified to confine the discussion to merely power supplies for different types of lasers, because then we would probably be covering not more than 30 per cent of the electronics that concerns contemporary lasers and laser-based systems.
In the present article, we have presented different areas of usage of laser and opto-electronics systems in defence, and we have briefly touched upon the role of electronics in each of those areas. Details will be presented in the following parts of the article.
To be continued in second 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