Battlefield Lasers And Opto-electronics Systems
Defence Science Journal, Vol. 60, No. 2, March 2010, pp. 189-196Ó 2010, DESIDOCREVIEW PAPERBattlefield Lasers and Opto-electronics SystemsAnil Kumar MainiLaser Science and Technology Centre, Metcalfe House, Delhi-110 054E-mail: director@lastec.drdo.inABSTRACTDuring the last four decades or so, there has been an explosive growth in commercial, industrial, medical,scientific, technological, and above all, military usage of laser devices and systems. In fact, lasers have influencedevery conceivable area of application during this period. While the expansion of non-military applicationspectrum of lasers is primarily driven by emergence of a large number of laser wavelengths followed by everincreasing power levels and reducing price tags at which those wavelengths could be generated, the militaryapplications of lasers and related electro-optic devices have grown mainly because of technological maturityof the lasers that were born in the late 1960 s and the early 1970 s. Lasers have been used in various militaryapplications since the early days of development that followed the invention of this magical device. Therehas been large scale proliferation of lasers and opto-electronic devices and systems for applications like rangefinding, target designation, target acquisition and tracking, precision guided munitions, etc. during 1970 s and1980 s. These devices continue to improve in performance and find increased acceptance and usage in thecontemporary battle-field scenario. Technological advances in optics, opto-electronics, and electronics, leadingto more rugged, reliable, compact and efficient laser devices are largely responsible for making these indispensablein modern warfare. Past one decade or so has seen emergence of some new potential areas of usage. Some ofthese areas include rapid growth in the usage of lasers and opto-electronics devices and systems for electrooptic countermeasure (EOCM) applications, test and evaluation systems that can perform online functionalitychecks on military opto-electronics systems and also their interoperability. In this paper, an overview of thecurrent and emerging military applications of lasers and opto-electronics systems has been given with anoutline on the likely trends leading to performance enhancement of the existing systems and emergence of newapplication areas. Also, a brief on the developmental activity in the field of laser and opto-electronics devicesand systems at Laser Science and Technology Centre (LASTEC), Delhi has been given.Keywords: Laser, opto-electronic systems, laser devices, military usage, warfare, electro-optic countermeasures1.INTRODUCTIONLaser though confined to premises of prominent researchcentres like Bell Laboratories, Hughes Research Laboratoriesand academic institutions like Columbia University in theearly stages of its development and evolution, it is nolonger so more than four-and-a-half decades after TheodoreMaiman demonstrated the first laser in May 1960 at HughesResearch Laboratories. Laser, an acronym for light amplificationby stimulated emission of radiation, as coined by Gouldin his notebook, is a household name today. It is undoubtedlyone of the greatest inventions of second half of twentiethcentury along with satellites, computers, and integratedcircuits. Laser, which was called an invention in searchof applications in 1960 s, continues to be so even todaydue to the unlimited application potential it holds. Throughthere is an expanded use of lasers and laser technologyin commercial, industrial, medical, scientific and militaryapplications, the areas of its usage are multiplying and soare its applications in each one of those categories. Therehardly has ever been an invention potent with such giganticapplications as the laser has proved to be in the past morethan four decades of its existence.1.1 Expanding Application SpectrumThe growth in the usage of laser devices has beenphenomenal, particularly in the last fifteen to twenty years.During this period, we have not only seen the use of lasersin applications thought of in the early stages of lasersgetting matured, we have also witnessed lasers making theirpresence effectively in many new areas. While on one hand,the systems meant for existing military applications continueto improve in terms of performance specifications and systemengineering, lot of work is being done in technologicallyadvanced countries to exploit the potential of lasers in adomain that is different from that of its existing usage.The application spectrum of lasers is now not onlylimited to range finding, designation and direct energyweapons (DEW) applications. In fact the application spectrumof lasers for military applications is expanding at such afast pace, beyond anyone s imagination. Many new conceptsrelated to the use of lasers for military applications arebeing conceived that will attain maturity status in the nextdecade or so. Lasers are being used for satellite-to-submarinecommunication, for detection of tunnels and other undergroundobjects on one hand to laser designation applications havingReceived 30 September 2009Celebrating Sixty Years of Publication189
DEF SCI J, VOL. 60, NO. 2, MARCH 2010global reach on the other. Non-lethal laser weapons at onecorner of the globe can be used for dazzling troops at theother corner of the globe. In addition to gas dynamic andchemical lasers, solid state lasers are fast achieving higherpower levels to be worthy of being used for directed energyweapons. An emerging concept in the field of directedenergy weapons is to combine the laser and microwavetechnology to realise a more efficient and compact system.2.DISTINGUISHING FEATURES OF LASERSUnique properties of laser light that distinguish itfrom the conventional light sources include monochromaticity,directionality, coherence and intensity. Monochromaticityrefers to single frequency or wavelength property of theradiation. Laser light is highly directional, which meansthat it spreads very little as it travels through the space.Coherence is indeed the most important of all propertiesthat discriminates laser light from ordinary light fromconventional sources. In a coherent light, all photons havethe same phase and this phase relationship is preservedas a function of time. There is temporal coherence, whichindicates coherence wrt time and spatial coherence, whichis preservation of phase across the width of the beam.Extremely high intensity of laser light, which remains higheven at long distances, is primarily due to low divergenceassociated with it.3.TYPES OF LASERSLooking at the present status of development of lasers,a large number of different types of lasers covering a wavelengthspectra, from shorter than an Angstrom in vacuum ultravioletto several millimeter in the far infrared, have been discoveredby scientists the world over. Tens of thousands of laserwavelengths have been obtained in all kinds of laser mediain solids, liquids, gases, plastics, jet engine exhausts, flames,and so on. Of course, not all of them are potent enoughto be generated cost effectively or matured in to useful laserdevices for any meaningful application. But it does indicatethe amount of interest being taken by researchers and alsothe quantum of funds being spent by various research anddevelopment organisations in the field of lasers. This isevident from their performance records in generating thehighest powers, the shortest pulses, the greatest frequencystability, and the lowest noise figures, and so on.Looking at the laser types that are particularly importantfor battlefield applications, vis-à- vis, the entire gamut ofavailable laser devices, it is a clear tilt towards a specificclass of lasers. Though there are a large variety of solidstate lasers, semiconductor lasers, gas lasers, dye lasers,ion lasers, free electron lasers, x-ray lasers, gas dynamiclasers, chemical lasers, and so on with majority of thesecategories further having a large number of different lasers,it is found that mainly the solid-state lasers, and to someextent that gas lasers, cover almost all military applications.And even amongst the solid-state lasers, it is the neodymiumdoped lasers that rule the supreme as far as better knownmilitary applications are concerned. In the following190paragraphs, at the current status of the battlefield use oflasers are covered and see how neodymium-doped lasershave dominated the scene as far as better known militaryapplications are concerned.In the field of high power lasers of DEW class, thereare gas dynamic lasers (GDL) and the chemical lasers.Under chemical lasers, we have hydrogen fluoride/deuteriumfluoride (HF/DF) lasers and also the upcoming chemicaloxygen iodine laser (COIL). A gas dynamic laser achievespopulation inversion by rapid expansion of high-temperature,high-pressure laser gas mixture to a near vacuum in anadiabatic process through nozzles. Though the expansionreduces the gas temperature, a large number of excitedmolecules are still in the upper laser level. The cavity axisis transverse to the direction of gas flow. The high-temperature,high-pressure gas mixture is created in a combustion reactionand laser is called combustion driven gas dynamic laser(CD-GDL). This type of laser, which is capable of producinghundreds of kW of continuons wave power at 10.6 µmassumed importance as a potential HPL weapon but it hascertain limitations due to heavy absorption of 10.6 µmwavelength by water vapour in the atmosphere. Interestin chemical lasers has assumed significance due to theirpotential as high power/energy laser weapons for battlefieldapplications. There are HF and DF lasers respectively emittingat 2.6 3.0 m and 3.6 4 m bands, respectively and the fastemerging COIL emitting at 1.3 m.3.1 Battlefield LasersBetter known applications of battlefield lasers includerange finding, target designation, target tracking, andguidance. There would hardly be any platform that is notequipped with some kind of laser device. For instance,one can not imagine a modern battlefield tank whose firecontrol system does not utilise the services of a laserrange finder. It is true for other forms of armoured combatvehicles too. Interestingly enough, these devices are inuse even in conjunction with squad weapons like assaultrifles, light machine guns (LMG), and so on 1. Small lowcost, low-power semiconductor diode laser modules withprovision of precise X-Y movement are finding widespreaduse on squad weapons for target aiming and pointing,particularly during night operations. This increases weapon seffectiveness by improving the single shot, target hitprobability, and reducing the collateral damage. LASTEChas also developed a laser aiming aid for a similar role,which is currently in production at Opto-electronics Factory,Dehradun, under the trade name of INSA-LAKSHYA.3.1.1 Laser Range Finders and Target DesignatorsThe laser range finders used on land-based platforms,either as a stand-alone system or as a part of the integratedfire control system are usually low-repetition rate systemsproducing laser pulses in the range of 5 pulses/min to 30pulses/min. Due to advances in electronics and electrooptic technologies, these are available as compact handheld devices similar in appearance to binoculars (Fig. 1).Celebrating Sixty Years of Publication
MAINI: BATTLEFIELD LASERS AND OPTO-ELECTRONICS SYSTEMSFigure 1. Laser range finders.Another class of range finders that combine the functionsof target designation and range finding for munitions guidance,or those which form the sub-system of an overall lasertracker, has relatively much higher repetition rates in therange of 10 pulses/s to 50 pulses/s. Figure 2 shows onesuch portable ground-based laser designator. These lasersare not only becoming smaller in size, these are now availablewith additional features to give them in many built countercountermeasure capability against similar systems developedby the adversary in an electro-optic countermeasure role.1540 nm. Low repetition rate lasers usually employ erbiumdoped glass as the active medium. However, erbium-dopedglass is not suitable for high repetition rate lasers due topoor thermal conductivity of the glass host. Raman-shiftedYAG and OPO-based YAG lasers have emerged as verystrong contenders for building high repetition rate lasersources for target designation applications.Diode-pumped solid state lasers, due to higher efficiencyand consequent smaller size and lower weight have alreadyfound their place in the inventory of Armed Forces. It willnot be long before the diode-pumped versions will completelyreplace the flash-pumped lasers for battlefield applications.Figure 3 shows a semiconductor diode-based laser rangefinder.Figure 3. Semiconductor diode-based laser range finder.Figure 2. Portable ground-based laser designator.4. EMERGING CONCEPTS4.1 Laser Target DesignatorsAn emerging concept, which will become a reality inthe next decade or so, is the use of laser target designatorsto illuminate targets located few hundreds of kilometerfrom them. Such a system makes use of a space-basedoptical reflector. One such system is shown in Fig. 4. Thesystem employs a optically flat reflector in MEO orbit andan image sensor on ground. The target area is identifiedand is imaged using the space reflector as mirror. Whenthe exact target location is determined, the laser beam fromthe designator is reflected from the same reflector andplaced on the target.Another topology can be to have the image sensoralso positioned in the space along with the reflector asshown in Fig. 5.As the neodymium-doped YAG and glass range findersemit at 1064 nm, which has a serious eye hazard, the personnelusing these always run the risk of being exposed to thishazardous radiation. The eye protection for the friendlytroops comes in the form of safety goggles, which attenuatethe harmful laser radiation to an absolutely safe level. Thishas also led to the development of eye-safe laser rangefinders and target designators. The low repetition rateclass of range finders discussed earlier is being graduallyreplaced by eye-safe versions. Eye safe lasers emit at4.2 Detection of Mines, Tunnels, FacilitiesOne of the other potent uses of lasers can be fordetection of buried mines, tunnels and other undergroundfacilities and activities. The laser system in this case isdeployed on an unmanned aircraft for aerial surveillance.The principle of operation of such a system will be thatthe laser beam reflected from a vibrating object will befrequency shifted due to Doppler s effect. By measuringthe frequency shift, the vibration frequency of the targetcan be determined, and hence, the target can be identified.Celebrating Sixty Years of Publication191
DEF SCI J, VOL. 60, NO. 2, MARCH 2010Figure 4. Long-range laser target designator with remote sensoron ground.Figure 6. Satellite-to-submarine communication using lasers.lasers of different types in particular stems form the factthat there is a large-scale use of electro-optic devices suchas sighting and observation devices and laser systemssuch as those used for target range finding and designationin the contemporary battlefield today. Laser technologyis quite mature today, which is leading to induction of laserdevices in the Armed Forces for many new applicationsnot thought of some time ago. This has led to a plannedand concerted effort on the part of developed countriesto develop and deploy EOCM systems to enhance thesurvivability quotient of the Armed Forces equipped withsuch a capability.Figure 5. Long-range laser target designator with remote sensorin space.4.3 Satellite-to-Submarine CommunicationYet another futuristic battlefield use of lasers can befor satellite-to-submarine communication applications (Fig.6). The system uses blue-green laser systems placed onthe satellite and large number of laser beams are directedat random places so as not to divulge the location of theships. An optical sensor is placed on the ship, viewingin the upward direction which is used for receiving thesignals from the satellite.5.ELECTRO-OPTIC COUNTERMEASURESOne of the newer types of laser systems that havefound recognition and wide acceptance by the Armed Forcesin recent times is the laser systems that can offer effectivecountermeasures against those laser systems already inuse. The relevance and importance of electro-opticcountermeasure (EOCM) equipment in general and EOCM1925.1 Types of EOCM EquipmentThere are two broad categories of EOCM equipmentincluding the EOCM class of lasers such as anti-sensorlasers and laser dazzlers, and the support devices suchas laser threat detection systems. In the category of antisensor systems, the EOCM systems that are capable ofcausing only a temporary disability of electro-optic devicesand opto-electronic sensors deployed by the adversaryand also the systems that are capable of inflicting a permanentdamage. In both cases, the target is the front-end opticsand opto-electronic sensors. These are also sometimesreferred to as soft-kill EOCM systems.LASTEC has developed a dual role EOCM systemcapable of operating in anti-sensor and dazzling roles andhaving maximum operational range of 2.5 km. These systemsby no means have the capability of inflicting a physicalor structural damage to the platform carrying those systems.There are hard-kill EOCM systems that are capable of inflictinga physical damage to the front-end optics of any electroopticalsystem. These systems are usually vehicle-mounted andare much larger in size and weight than their soft-kill counterparts.The pulse energy levels in such lasers is of the order ofseveral kilo-joules as compared to a few Joules in case ofsoft-kill systems.Celebrating Sixty Years of Publication
MAINI: BATTLEFIELD LASERS AND OPTO-ELECTRONICS SYSTEMS5.1.1 Laser DazzlerLaser dazzler is the other popular device that has beengrouped under the EOCM lasers with the difference thata dazzler works on the biological sensor, i.e., human eye,rather than the opto-electronic sensors. These devices arenon-lethal and eye-safe, which implies that such deviceswould cause only a temporary dazzling effect and not alasting injury of any kind to the unaided eyes. Laser dazzleris very effective, both in the battlefield as well as forcounter-insurgency operations due to its inherent surpriseelement and the least collateral damage associated withit. These features also make it the preferred choice whenthe Armed Forces are out on a peacekeeping or humanitarianmission. LASTEC has also developed a short-range handheld laser dazzler with maximum operational range of50 m (Fig. 7). The technology of the device has beentransferred to production agency.Figure 8. Precision laser warning sensor for AFVs developedat LASTEC.Figure 7. Hand-held short-range laser dazzler developed atLASTEC.5.1.2 Laser Warning SystemLaser warning system is invariably an integral part ofany anti-sensor EOCM system. For example, knowledgeof the type of laser threat and the angle-of-arrival of thelaser beam may be used to trigger a cluster of aerosol/smoke grenades to block laser radiation. This allows oneto take an evasive action during those 50 s to 60 s forwhich smoke/aerosol screen remains effective. This typeof defensive countermeasure action is particularly effectiveagainst laser-guided bombs (LGB). LASTEC has developeda precision laser warning sensor having a spectral coveragefrom 700 nm to 1600 nm and an angle-of-arrival accuracyof 3 , as shown in Fig. 8. In the case of active countermeasuressystem equipped with suitable laser emitters, the informationon the incoming laser threat is fed to the servo controlsystem, which in turn re-orients the EOCM laser in theprecise direction of the incoming laser radiation. Such asystem is usually deployed against incoming anti-tank guidedmissiles (ATGMs).6.EMERGING CONCEPTSA laser dazzler system having a global coverage canbe designed based on the concept highlighted in Fig. 9.The system includes a reflecting sphere
checks on military opto-electronics systems and also their interoperability. In this paper, an overview of the current and emerging military applications of lasers and opto-electronics systems has been given with an outline on the likely trends leading to performance enhancement of the
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