Survey and technological analysis of laser and its defense applications Syed Affan Ahmed,Mujahid Mohsin*,Syed Muhammad Zubair Ali

Department of Avionics Engineering-College of Aeronautical Engineering(CAE),National University of Sciences and Technology(NUST),Risalpur,KPK,Pakistan

Keywords:Laser systems Laser military applications Directed energy weapons Non-lethal weapons Laser military projects Survey Taxonomy

ABSTRACT The laser technology has made remarkable progress over the past couple of decades.It is being widely employed in diverse domains,such as holography,space sciences,spectroscopy,medical sciences,micro and power electronics,industrial engineering,and most distinctively,as directed energy military weapons.Owing to their active transmissions,laser systems are similar to microwave radars to some extent;however,unlike conventional radars,the laser operates at very high frequencies thus making it a potent enabler of narrow-beam and high energy aerial deployments,both in offensive and defensive roles.In modern avionics systems,laser target indicators and beam riders are the most common devices that are used to direct the Laser Guided Weapons(LGW)accurately to the ground targets.Additionally,compact size and outstanding angular resolution of laser-based systems motivate their use for drones and unmanned aerial applications.Moreover,the narrow-beam divergence of laser emissions offers a low probability of intercept,making it a suitable contender for secure transmissions and safety-critical operations.Furthermore,the developments in space sciences and laser technology have given synergistic potential outcomes to use laser systems in space operations.

1.Introduction

The field of laser has witnessed tremendous scientific and technical developments in the recent past,enabling its deployments for a variety of biological,industrial,commercial and scientific applications.The key applications of laser technology can be divided into the domains of health sciences,engineering,and technology and security and defense,as categorized in Fig.1.One of the rapidly progressing fields in the defense sector is the offensive use of laser for airborne military applications.Laser weapons have many advantages over traditional weapons.First,the transmission at the speed of light allows laser-based weapons to engage distant targets immediately after detection.Second,the directed laser energy provides less collateral damage and low-profile and covert operations capabilities[1].Laser is a surgical weapon of choice,offering precise target-point selection.Initial installation costs are high but after deployment,laser weapons provide cost-effective engagements.Finally,the laser deployments can be flexibly tuned to deliver a gradient effect to tailor the range of results to non-fatal,destructive and disruptive outcomes[2].

Today many types of lasers are available having different power ranges,wavelengths,operating efficacies,spectral bandwidths,and other features.The increase in the maturity of compact optical and laser devices have improved their abilities for military purposes.Consequently,laser technologies have changed the paradigm of modern warfare,by serving in diverse roles,such as indicators &target designators,sensing devices,data relays,active lighting,rangefinders,weather regulators,and directed energy weapons[3].The laser systems deployed as Directed Energy(DE)weapons have the potentials to leave devastating effects on a very large scale.A high energy laser beam can be utilized to destroy moving targets over a thousand miles with optimal accuracy and precision.Huge investment and research are being done in the domain of high energy laser weapons[4].According to United States Department of Defense(DOD)“DE is a general term for technologies that cover the production of concerted electromagnetic(EM)energy and nuclear or atomic particles.DE weapons are those systems that use DE for incapacitating,damaging,disabling,or destroying enemy targets.”Directed Energy Warfare(DEW)is a military operation,which includes the utilization of DE weapons,installation of such weapons,and defensive approach to damage or destruct enemy’s equipment,amenities,or personnel,to identify,reduce,exploit,or avoid hostilities using Electromagnetic Spectrum(EMS);through destruction,disruption,and damage.It also includes acts to protect friendly equipment,amenities or personnel,and to maintain friendly use of EMS.With the maturity of DE technology,weaponized DE systems have become more and more powerful,and have become an important subcategory of the electronic warfare(EW)operational area.Examples of DE include radio frequency(RF)weapons,lasers,active denial technology and DE anti-satellite and High-Power Microwave(HPM)weapon systems[5].

Fig.1.Laser applications categorization.

From the history of military warfare,almost all weapons rely on the rapid transfer of calamitous energy to targets[6].Technological advances then allow the transfer mechanism to shift from direct hits to impulsion-driven flight and ballistic orbital.The kinetic energy delivered on the target is enhanced by the biochemical energy induced by the explosive nuclear warhead.The basic physical phenomena remain the same.Even the complexity of modern military missiles platform does not rule out the fact that neutral potential mechanisms are still based on physical projectiles to achieve their goals.

This paper reviews and analyzes the applications of laser technology in its earlier and ongoing scientific advancements.The scope of this paper is confined to the offensive use of lasers for airborne military applications,which have revolutionized today’s battlefields.The paper presents a comprehensive taxonomy of such applications while correlating them with historical perspectives as well as recent advancements in the form of government-funded projects.

The rest of the paper is organized as follows.Section 2 presents the background of laser operations and associated physical/environmental factors.Section 3 discusses the historical perspective of early laser deployments and initial applications.Section 4 analyzes the deployment of laser systems for military applications with the help of a novel taxonomy.Finally,Section 5 gives pointers to open problems and future research directions and Section 6 concludes the paper.

2.Background

2.1.System operations

A generic global configuration of the key functional components of any laser system is portrayed in Fig.2.The energy supply and processing blocks generate power for the pump source that generates an active laser beam by using a laser medium and then feed it to the beam control system.The beam control system consists of beam coupler,beam conditioner,and components that align the beam.Afterwards,the beam is directed towards the target of interest.The impact of laser beam over the desired target(s)is normally evaluated through a sensing and control system.Such a system detects turbulences and inaccuracies induced by atmospheric conditions and relative motion or state transitions of the target(s)and implements appropriate control techniques to apply corrections.

Fig.2.Generic functional components of Laser System[7].

2.2.Environmental factors

There are many environmental factors that limits or attenuates the laser propagation when it travels through the atmosphere.The key factors include diffraction,turbulence,scattering,and absorption.Diffractiondefines the bending or spreading of laser beams through edges of micro-atmospheric particles.For lasers,the diffraction can be regarded as an unavoidable behavior due to the spreading nature of light,and even through an ideal vacuum,energy is spread as the beam propagates.For precise laser targeting applications,the inherent diffraction effect asserts physical limits over the level of achievable precision or focus.Turbulenceis much similar to diffraction;however,in addition to spreading,it also induces a random beam walk from the targeted location,thus reducing the effective power delivered to the target.The level of turbulence experienced by a given laser beam depends heavily on altitude,wind speed,and other atmospheric parameters.Scatteringis because the composition of the atmosphere does not transmit the beam perfectly,but changes the path of the beam.Atmospheric particles can contribute towards scattering through refraction,reflection or diffraction of the incident laser.The level of scattering is proportional to the size of particle obstructing the laser’s path.Consequently,lasers with higher wavelengths are less impacted by the scattering effects.Absorptionis an environmental phenomenon that reduces the power delivered by removing the energy from the beam after transforming it into some alternate form,mostly thermal energy.Different atmospheric components exhibit varying absorption characteristics for different wavelengths and altitudes,depending on the weather conditions.All these atmospheric attenuations may significantly influence the performance of laser and therefore,must be carefully considered and appropriately compensated during design,testing and deployment stages of laser targeting,navigation and weapon systems.

3.Historical perspective

Ever since the primary demonstration of lasers in 1960,when a light was flashed through a crystal structure,at California’s Hughes Research lab,it has almost taken fifty years to bring lasers for effective use[8].Laser technology has established groundbreaking engineering enhancements and scientific developments that have been built for numerous scientific,industrial,military,medical and commercial applications.Laser technology has brought great enhancements in surgery,data storage,photography,holography,spectroscopy and numerous other fields[9].The coherency,high mono-chromaticity,and ability to achieve extraordinarily high powers are few of the unique laser characteristics,which facilitate its deployments for these specialized applications.Laser technology uses the process of excited emission,which helps in generating a coherent optical beam with highly directed energy.Many kinds of lasers are obtainable these days with totally different power levels,spectral bandwidth,wavelengths,temporal characteristics,and operative efficiencies.These advancements in compact optical systems and the maturity of lasers have also increased military operational capabilities.Ever since the early stages of development of the very first operational laser being invented,the military has always undoubtedly been interested in laser technology.Especially,since it was certain that these laser devices will bring revolution in technological warfare,once used as sensors,rangefinders,weather modifiers,directed energy weapons,data relay devices,target designation,active illumination,and several other applications.During the 1970s-1990s,Radio Frequency(RF)coupled with Digital Signal Processing(DSP)proved to be the most useful application for military warfare.Though,due to the constantly increasing demand for data capacity in electronic warfare,wider spectrum communications and broadband radar systems are needed.Radio Frequency isn’t capable of handling such high demands.For all such military applications,lasers are thought of as a decent alternative over Radio Frequency signals.Laser technology can handle the desired large capabilities and real-time dynamic range processing[10].Over the years,the sufficiently matured laser technology has managed to supply cost-efficient,energy-economical,wavelengthflexible and high-speed systems that can assist a diverse range of military operations like remote sensors,directed energy weapons and communication,etc.Another major advantage is that the laser technology is capable to deliver very high data transmission rates,at par with the fiber-optic links,and that too without any constraints of physical media as required by the fiber optics.Also,if the carrier wavelength of the laser beam is selected in the invisible band,the ability to detect,observe or intercept becomes even tougher unless there’s any type of electro-optic system which is placed in between the route of laser transceivers.Further,the ability to provide larger gains through optical carrier wavelengths,the laser-based communication systems outperform the radio frequency systems with regards to lower Size,Weight,Power and Cost(SWaP-C)requirements.

The favorable characteristics of laser-based systems,as discussed above,have attracted governments and militaries from developed nations to use them as a potent force multiplier in the form of Directed Energy Weapons(DEWs).DEWs have demonstrated capabilities to damage physical targets over the area of several miles with high precision and accuracy.Over the past few years in the field of High Energy Laser weapons,several funded research projects,prototype developments,and studies have been conducted to study the power requirements and the amount of destruction caused by these weapons[3,11].U.S.Defense advanced analysis agency and Air Force science lab developed a high power airborne laser platform for laser experiments and trials in the air[12,13].Other nations like China Republic,Soviet Union,Russia,Japan,and Germany are also conducting intensive research and developments towards offensive use of laser technology.The government-level funded project is being supported by leading share-holders of military avionics systems such as Lockheed Martin,Northrop Grumman,Raytheon,BAE Systems and Boeing[14].Besides High Energy Laser(HEL)weapons,other military applications of lasers include active imaging devices,illuminators,data relaying,remote sensing and range finding,to name a few.The development of several offensive laser-based systems is under progress;however,none has been openly declared as cleared for battlefield-deployments,to date[15,16].Fig.3 demonstrates a timeline of various military laser projects during the past few years.A brief introduction of these projects,including their scope,purpose and sponsoring organization,is summarized in Table 1.

Fig.3.Timeline of military laser projects over the past few years.

4.Laser based military applications

The laser technology has numerous military applications.We divide these applications into three major categories or domains as Communication,Destructive Systems,and Navigation,Guidance&Control.Each of the domains is further sub-divided into various sub-categories to form a taxonomy of laser applications,as shown in Fig.4.This section discusses the details of military application domains while covering some of the major contributions and ongoing projects in each of the defined sectors.

4.1.Laser communication

With the recent advancements in communication infrastructure and protocols,offering high-speed,long-distance and low-latency information exchange,the communication warfare has itself evolved into a potent force multiplier on the battlefield.However,with the ever-increasing military demands of communication bandwidth,speed and security,linked for example with High definition airborne video surveillance,real-time and context-aware data analytics and sensitive electronic warfare operations,the existing wireless links operating in the RF range are falling short of meeting such requirements[28].Even when using data compression techniques,the requirement to send an increasingly amount of tactical intelligence between airborne sensors and data processing units further highlights the limitations of present aerial data links.Data communication using short wavelength lasers have several benefits over other traditional methods operating within the RFspectrum.These advantages include low power utilization,higher signal strength,more compact equipment,and high interference impermeability.Therefore,the use of laser communication,also known as Free-Space Optics(FSO),is deemed to offer promising solutions to military communication challenges.FSO has higher speed and bandwidth,ultra-low latency,no compression constraints,high carrier frequency and immunity to EM radiation[29,30].The probability of being detected of a laser depends on the frequency spectrum emitted and the beam divergence.Concealed military operations require operation near the infrared band using narrow beam divergence and spurious emissions or minimal spill,such as side lobes.

Laser-based data communication can be used on both types of platforms,static and mobile;for air/space,underwater or ground[31].In the past few decades,many defense organizations have witnessed various developments in laser-based communications.In order to support the requirements of the commercial and

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defense departments,various experimental studies have been conducted on the terrestrial laser communication link,in-space laser link,land to air & air to land laser link,sea to air & air to sea laser link.,as illustrated in Fig.5.Table 2 demonstrates the development of various laser military applications in the field of communication during the last ten years.

Table 1Overview of military laser projects since 2010.

Fig.4.Military laser applications taxonomy.

Fig.5.Diverse laser communication scenarios.

Table 2Demonstration of various laser-based communication applications[18,26,27,32].

In the course of recent years,a few techniques have been created to enhance the airborne laser interchanges as far as execution.A space-based laser communication framework including an inter satellite network has been developed in the past few years.The proposed“Internet in the Sky requires Low Earth Orbit(LEO)and Geo Synchronous Orbit(GEO)satellite space optical communication”[21].Various provincial projects comprising laser communication frameworks have been created,which also includes the improvement of satellite cross-link communications.

4.2.Navigation,guidance & control

In the course of history,navigation,guidance&control played a vital role to ensure safety on the battlefield.In modern warfare where time to identify and detect the incoming threat is the key to eliminate the threat.Some of these defensive applications based on laser technology are discussed below.Moreover,scanning the earth through satellite or airborne vehicle i.e.remote sensing is also desirable before carrying out any mission.Although remote sensing is a very wide research area and beyond the scope of the paper but some key points are briefly discussed.

4.2.1.Target tracking

The key element of modern warfare is the time to identify,track and take down the target to guarantee success.This can be achieved by improving the target pinpoint accuracy and designating capabilities.Laser target designators(LTD)and range finders(LRF’s)use high resolution scanning and staring techniques to track down the targets that are located outside the point blank ranges.LRF’s are used for position control and 3D vision,providing measuring ranges from a few meters up to tens of kilometers.LRF’s use low pulse repetitions rates(PRF’s)and such visual wavelengths that give minimum atmospheric losses.LRF apparatus is equipped with cooling systems and thermal equalizers which can compensate temperature over wide functional ranges while functioning.Furthermost the developed LRFs and LTDs that are presently in use today are based on Neodymium-doped Yttrium Aluminum Garnet(Nd:YAG),which uses short wavelengths around 1.065μm and emit short coded pulses to decrease the risk of spoofing and jamming[33].The short pulse lengths can be achieved by using solid state lasers when Q-switching where their power levels can be substantially increased.The laser beam emitted from the Nd:YAG is invisible to the human eye,the frequency of the lasers can be increased by bringing the beam towards the visible region i.e.green at 0.532μm or the frequency can be further divided down around 3 to 4 times to remove the spectrum from the Infrared(IR)range to Ultraviolet(UV)range.A laser designator has been developed by Lockheed Martin,the“low altitude navigation and targeting infrared for night”known as LANTIRN.It produces short width pulses of extra high power,nearly up to 10,000 KW,it is currently being used by the United States Air Force(USAF).To protect the eyes of soldiers in the battleground,the wavelength of laser with 1.4μm or greater is favored because the radiations emitted get absorbed and cannot reach the retina of a human eye that can cause damage.Consequently,eye harmless lasers i.e.solid state Er.Glass laser operating at wavelength 1.5μm or CO2operating at wavelength 10.6μm having a pulse energy of less than 10 mJ,thus these are preferred choices for day or night time operations[34].Other eye safe lasers types include ER:fiber laser type and Raman-shifted Nd:YAG laser type as given in Table 3.After CO2emission and Nd:YAG laser,the paradigm has been shifted to fiber laser.Today,the development in blue laser technology will be the next major shift for the military as well as industrial applications[35].

Table 3Specifications of eye safe lasers[3].

4.2.2.Remote sensing

Laser Remote Sensing(LRS)is a generic term used to describe a procedure for obtaining the system’s physical information from a remote location with the help of a laser.This technique is also known as LIDAR:Light Detection and Ranging.The principle is:light emitted from the laser hits the target and the reflected light is then detected by a telescope and analyzed.The most common LRS is used for environmental and atmospheric studies from aircraft,ground vehicles or satellite systems,measuring pollutant concentrations,mapping of cloud formation and monitoring the agricultural fields.It also includes studies of aerosol science,fluorescence spectroscopy,Doppler measurement of atmospheric wind and turbulent motions,and retrieval of CO2concentration levels over local and global scale[36].Recently a remote sensing platform is developed to probe the CO2emission[37].Laser remote sensing offers more accurate strategic information and bomb damage valuation.This will provide significantly better utilization of air assets,enhancing the capabilities of any air force[38].Moreover,remote sensing can further be divided into sub categories depending on its type and utilization.

4.3.Weapon systems

The devastating power punch,high precision and reachability offered by a focused beam of laser energy can also be employed as a weapon system.We classify laser weapons as non-lethal(incapacitating)and destructive systems based on their impact,as discussed in ensuing paragraphs.

4.3.1.Non-lethal(incapacitating)weapons

Non-lethal weapons,also known as non-deadly weapons,are designed to disable or immobilize a living target.These weapons are often used to infuse controlled(often reversible)damage to aggressive individuals or groups,such as rioters,mobs and advancing soldiers.Tactical non-lethal laser weapons can cause pain and temporary damage or disability to the victims,thus forcing incapacitation-driven distraction,diversion or retreat.Such laser weapons can also be used in conjunction with or before the use of lethal firepower.Many experts believe that the victims to such weapons are more likely to suffer from irreversible loss or injuries,thus making them controversial to achieve the intended objectives[39].Some non-lethal eye safe lasers technologies have already been discussed in Table 3.The degree of damage done by these lasers varies significantly with exposure time,incident angle,laser power and wavelength.Gunther[40]proposed the design of a non-lethal laser weapon system for causing transient blindness to a large group of people.Similarly,Eisenberg et al.[41,42]presented a long-range and high-intensity spotlight based on the incoherent collimated light source for temporary and reversible incapacitation of humans.Moreover,Rubtsov[43]suggested the use of visual light strobing(a combination of temporal flashing and spatial scanning)with a specific pattern to cause human impairment or distraction,thus compromising the victim’s combat effectiveness.Recently,US Joint Non-Lethal Weapons Development Program(JNLWD)also introduced Non-Lethal Laser Induced Plasma Effects(NL LIPE)system to create an ear-splitting burst of sound energy[44].

4.3.2.Directed energy weapons

Laser weaponries are an effective,precise and influential antidote against any type of threat,which includes air as well as ground threats.Lasers have multiple benefits over traditional weapons systems.As the propagation speed of a laser beam is equivalent to the speed of light,it also offers real time data transmission when the target is spotted[45].The laser beam is so coherent such that it provides extremely focused energy by converting laser into thermal energy,causes physical damage to the structure or material.Since these active devices are continuously powered by electrical or chemical energy,thus they are capable of engaging numerous targets with less moving mechanical components.Mostly laser weaponries are categorized over the concept of power/energy levels:low energy,medium energy or high energy weapons.According to some research efforts,laser weapons can also be categorized based on their operational impact as described in Fig.6.Low energy laser is used for jamming or physical destruction of the sensor system.When the target is a human being in the battlefield,low energy laser can be used to target the radiation sensitive parts of the body such as eyes.If the target is to cause burn injuries to the soldier medium energy laser can be used.However,medium energy laser can also be used to destroy opto-electronic devices.While high energy laser is used when the aim is to neutralize helicopters,missiles or any other airborne or ground vehicle[46].Moreover,the power needed for Directed energy weapons can be increased by combining various lasers together[47].

Fig.6.Laser weapons categorization[3].

These energy lasers have some special requirements for their effective operation,i.e.laser fuel/power requirements,cooling/thermal requirements,tracking and pointing requirements,personal and environmental safety requirements.Since DE weapons require a lot of power.To place the DE weapons on the airborne platform,it is desirable to reduce the size of the DE weapon systems to place them on smaller aircraft or even on space based platforms.Moreover,cooling requirements are essential for such types of lasers to overcome a large amount of thermal energy evolved when the laser is excited.If there is no such cooling system to maintain the temperature of the laser beam generator,it results in widening the laser beam,causing difficulties for a laser beam to get align with the object[48].

Laser weapons can be categorized as space based or ground based.Ground stationed laser weapons use different relay mirrors present on high altitude air-ships in space to eliminate the threat of ballistic missiles.The purpose of these high altitude relay mirrors is to increase the transmission and detection range of High Energy Laser(HEL)weapons as they used to compensate the limitations caused by atmospheric turbulence,absorption,and curvature of the Globe.Interestingly,the operating cost of HEL is almost zero,and the flight time of the laser beam is instantaneous that leaves the target with no space to avoid the first attack[49].The airborne laser system has two major sections,the ground-based section having all the heavier components while the space-based or airborne section having relatively lighter components[7],as depicted in Fig.7.Based on functional components some of the key laser weapons system specifications used for military applications are given in Table 4.

Fig.7.Division of functional parts of Laser System[7].

Table 4Key military laser system specifications[3].

5.Future research directions

Design and testing of high energy laser weapons require the generation of an adequate amount of power,preferably in megawatts,to cause considerable damage to the distant targets.Such high-power transmissions pose serious safety challenges during lab/field testing,where the performance of such systems is to be evaluated under a controlled environment.As this domain is gaining maturity,development,and configuration of safe,costeffective and comprehensive testing facilities for high-energy laser-experiments has emerged as an important research challenge.

As already discussed,high-energy laser generation systems also require efficient cooling mechanisms,having capabilities to maintain stable temperatures during transmission cycles.Moreover,enhancing the accuracy of laser systems under intentional(jamming)or naturally-encountered unfavorable weather conditions,such as due to heavy smoke,humidity or dust,is another important research direction.Another important consideration while dealing with high-power laser transmission is to minimize collateral damage and threats to nearby friendly sensors and equipment.Research in this domain involves exploring potent methods to electronically and physically harden friendly electronics to ensure their optimal functionality and reliability.

Table 5 presents a brief summary of some of the major ongoing projects involving R&D efforts in laser technologies.As evident from the listed projects,laser-deployments in space is emerging as a key research domain,which includes space communications,space wreckage cleaning and laser-propelled interstellar detectors.

Table 5Future laser-based military projects.

6.Conclusion

This paper presented a review of state-of-the-art laser technologies and discussed their deployment avenues in different application domains,particularly in the military sector.The paper also covered a holistic overview of diverse laser-based projects of the last decade,besides proposing an application-oriented taxonomy for laser-based systems,coupled with future research directions and corresponding military initiatives to bridge these research gaps.Laser technology has shown strong potentials to revolutionize modern-day battlefield scenarios,through multifaceted supportive,defensive and offensive/destructive applications.In a supportive role,the lasers serve as a very powerful device for the warfighters when employed as range finders & target designators,power beamers,LIDARs and long range,high speed,and secure communication systems.In the domain of defensive countermeasures,laser technology offers large bandwidth and good angular resolution,required for countering laser-jamming or deceiving laser-navigators/disruptors.The offensive use of laser technology has introduced a wide range of airborne and groundbased weapons with capabilities to precisely deliver large-scale(yet controlled)damage to electronic systems,combat troops,optical devices,high-speed approaching missiles and even physical installations,thus revolutionizing the whole paradigm of destructive weapons.