Development of high-energy lasers with military potential is leading to the production of beam weapons that transfer coherent light energy to the intended target to cause structural damage. In the coming years, monopoly of chemically-propelled projectiles may give way to dynamic co-existence and competition with directed energy weapons (DEWs) as the next-generation weapons for both tactical and strategic missions.
Introduction of high-power lasers (HPLs) has enabled execution of many new missions hitherto extremely complex to realise with conventional kinetic energy weapons (KEWs). These include ground based laser for disabling low-Earth orbit satellites, airborne laser for destroying ballistic missiles and space based laser for negation of theatre and inter-continental ballistic missiles. The capability demonstrated by HPLs, particularly in the last 10 to 15 years, to destroy various types of fast-moving aerial targets such as unmanned aerial vehicles has established high-energy laser weapons as the weapons for the 21st century.
Primary advantages of laser based DEWs include speed-of-light delivery, near-zero collateral damage, multiple target engagement and rapid re-targeting capability, immunity to electromagnetic interference and no influence of gravity. Deep magazine and low cost per shot are the other advantages.
Laser weapons engage targets at speed of light with essentially no time-of-flight as compared to conventional KEWs that require a finite travel time. As an example, one of the world’s fastest cruise missiles, BrahMos, with a supersonic speed of 2.8Mach to 3Mach, would take about five minutes to reach its target located at its maximum operational range of 300km.
On the other hand, the same target when targeted by a laser DEW would be hit in a millisecond. This makes these particularly suitable for engaging fast-moving targets.
Multiple target engagement and rapid retargeting features of laser based DEWs is attributed to their being powered by rechargeable chemical energy (in the case of chemical lasers) or electrical energy (in the case of solid-state and fibre lasers). Shifting from one target to another involves only repointing and refocusing of the beam-directing optical system.
The processes of generation and transfer of lethal laser power to the target are purely in the optical spectrum and, hence, are immune to any electromagnetic interference and jamming. Laser pointing is practically without any inertia, and light bullet has no mass and therefore is not influenced by gravity. As a result, it does not require any mid-course correction.
The total number of shots a laser can fire is limited only by the amount of chemical fuel in the case in chemical lasers or electrical power for solid-state and fibre lasers.
Cost per shot in the case of laser based DEWs is much lower than in the case of conventional KEWs. Projectile weapon systems, guided missiles in particular, expend a lot of expensive hardware such as rocket motors, guidance systems, avionics, seekers, airframes and the like, every time these are fired. In case of laser weapons, cost of each laser firing is essentially the cost of chemical fuel or electrical power consumed.
As an example, a fourth-generation shoulder-fired surface-to-air missile of FIM-92 Stinger series costs about US$ 40,000. This missile can be used against an aircraft to carry out one such mission. A similar mission from a land based laser DEW system can be carried out with 50kW to 100kW solid-state laser by firing the laser beam for a dwell time of about five seconds. Going by present-day technology level, this laser system would draw about 400kW to 500kW of electrical power for a period of five seconds, which is the same as the electrical power consumed by a 100W bulb in seven hours.
Laser based DEWs also have some limitations. Some of these include their line-of-sight dependence, requirement of finite dwell time, problems due to atmospheric attenuation and turbulence, and ineffectiveness against hardened structures.
Laser weapons require direct line-of-sight to engage a target. Their effectiveness is reduced or neutralised by the presence of an object or structure in front of the target that cannot be burned through.
Unlike projectile weapons that instantly destroy the target upon impact, laser weapons require a minimum dwell time of the order of three to five seconds to deposit sufficient energy for target destruction.
Effectiveness of the laser weapon is adversely affected by atmospheric conditions. The laser beam suffers attenuation due to absorption and scattering by airborne particles and gas molecules, deterioration of beam quality in the form of deformation of the laser beam wave front and increase in the laser beam spot size at the target.
Laser weapons are not very effective against hardened structures. However, equipment such as antennae, sensors and external fuel stores mounted on these structures can be targeted effectively.
The operational scenario of directed-energy laser weapons is broadly categorised as short- and medium-range tactical missions and long-range strategic missions. Some of the important application areas of tactical-class laser weapons include stand-off neutralisation of ordnances such as mines, unexploded ordnances and improvised explosive devices, ground based defence against rockets, artillery and mortars (Fig. 1), ground based capability to destroy unmanned aerial vehicles (UAVs) of the adversary, airborne defence of aircraft against man-portable air defence systems such as shoulder-fired surface-to-air missiles (Fig. 2), ship defence against maneuvering cruise missiles and tactical ballistic missiles (Fig. 3).
Stand-off neutralisation of ordnances requires laser power in the range of 1kW to 2kW for operational ranges up to 300 metres. Solid-state or fibre-laser sources operating around 1.0µm are used for the purpose. Avenger and Zeus of the USA and Israeli Thor are laser ordnance systems with comparable specifications.
Anti-UAV operations up to a range of eight to ten kilemetres require about 100kW of laser power. Operational ranges of five to six kilometres are possible with 50kW laser systems. Again, preferred lasers are solid-state and fibre lasers.
Chemical oxy-iodine laser may also be used. For applications such as air defence against rocket, artillery, mortar targets, rocket-propelled grenades, battlefield missiles, laser-guided munitions and so on, typically, 100kW of laser power is needed for operational ranges of five to ten kilometres.
Long-range strategic applications of laser based DEW systems mainly include ballistic missile defence (Fig. 4), space control such as Space based lasers and anti-satellite applications (Fig. 5). In all these applications, operational ranges are generally in hundreds to thousands of kilometres, and required power levels are of the order of 1MW to 20MW depending upon the actual mission.
Space-control applications such as anti-satellite applications require relatively much higher power than that needed for ballistic missile defence.
Components of directed-energy laser weapon system
Unlike conventional military applications of laser systems such as laser range finders and target designators, which primarily comprise a laser source of desired specifications, laser based DEWs are much more than an HPL source as is evident from the block schematic of Fig. 6.
The sub-systems other than the HPL source are required for the purpose of directing the laser beam to the intended point on the target, keeping it there for the desired dwell time and producing the desired value of fluence on the target. Laser based DEWs essentially comprise two major sub-systems, namely HPL source and beam-control system.