In this concluding part of the article, let us wrap up by learning some more about high-energy laser weapons.
Beam combination of multiple lasers
Power scale-up to ten to hundreds of kilowatts in a single laser, maintaining the desired beam quality, has many a technological challenges to overcome. Developments in the field of beam-combination techniques have opened new avenues of building higher-power lasers than could be combined in a single laser. Combining laser output from multiple fibre lasers is an important area of relevance to high-power directed energy laser weapon systems, as it allows achieving higher output power from relatively low-power individual lasers.
Direct diode lasers, bulk solid-state lasers and fibre lasers have been experimented with in recent years to generate higher powers by combining outputs of multiple lasers of a given type. Power enhancement and beam quality issues have been studied over propagation distances in kilometre range. Three common laser beam combining techniques include spectral beam combining, coherent beam combining and incoherent beam combining.
In the case of spectral beam combining, multiple laser beams with non-overlapping optical spectra are combined by using a wavelength-sensitive beam combiner such as diffraction grating and prisms. Optical components with wavelength-sensitive transmission characteristics such as volume Bragg gratings and dichroic mirrors can also be used.
In the case of coherent beam combining, multiple lasers are combined to generate higher output powers with more or less the same beam quality as that of individual lasers. Coherent combining also preserves the spectral bandwidth.
In one method of incoherent beam combination, multiple laser beams are combined by overlapping individual laser beams on the target with a beam director consisting of independently-controlled steering mirrors. Adaptive optics may be used to compensate for the distortions caused by atmospheric turbulence. This technique is relatively much simpler than other beam-combining techniques including spectral and coherent beam combining. It does not require phase locking or polarisation locking of individual lasers and its power can be readily scaled up for directed energy weapon applications.
Representative directed energy laser weapon systems
Presently, a large number of directed energy laser weapon systems are reported to be under development and upgradation. Some of these are experimental, some technology demonstrators, while others are being upgraded and ruggedised to become realistic battlefield weapon systems in the near future.
Some of the more talked about laser weapon systems include high-energy laser system developed by Diehl and LFK of Germany, general-area defence integrated anti-missile laser system from TRW, mid infrared chemical laser from TRW, high-energy laser weapon system from TRW, Boeing’s airborne laser and advanced tactical laser, tactical high-energy laser, mobile tactical high-energy laser, Raytheon’s Laser Phalanx and laser weapon system.
High-energy laser, jointly developed by two German companies Diehl and LFK, is a short-range tracked vehicle-mounted system for use as an air-defence system against low-flying, high-performance aircraft, missiles and attack helicopters. The system is configured around a gas dynamic CO2 laser emitting at 10.6 microns. It has associated target-acquisition and tracking sensors.
Another air-defence laser system is TRW’s general-area defence integrated anti-missile laser system. This system, too, is a short-range complement to surface-to-air missile defence designed to engage discrete ballistic threats at longer ranges. The system generates a 400kW laser beam, which is delivered through a 0.7-metre beam pointer/tracker.
One of the very early directed energy laser systems, a technology demonstrator, is TRW’s mid infrared chemical laser. It is a 2.2MW CW deuterium-fluoride laser with a maximum lasing duration of 70 seconds. It uses a 1.8-metre Sealite pointing and tracking device. The system was reported to carry out trials against different types of targets at White Sands Missile Range in New Mexico. In one of the tests conducted in 1996, a small fraction of laser power was used to destroy a 122mm short-range artillery rocket in flight. The laser beam was locked to the target for 15 seconds. It is also reported to have been tested against sea-skimming missiles.
Yet another recent system from TRW is high-energy laser weapon system, which is again a chemical laser using ethylene, hydrogen and fluorinated nitrogen as the active medium. The system is based on their earlier experience gained from building mid-infrared chemical laser. The system has an integral electro-optic tracker.
Another laser based directed energy weapon system in serious contention till recent past, particularly for operation from an aerial platform, is the famous airborne laser. It uses a 1.2-megawatt chemical oxy-iodine laser emitting at 1.315 microns, generated by six chemical oxy-iodine laser modules of 200 kilowatts each. It is configured on Boeing 747-400 freighter aircraft (Fig. 10).
Airborne laser is capable of destroying a ballistic missile in boost phase. In operation, the aircraft patrols friendly air space. After an enemy missile launch is detected by a variety of sensors, this information is relayed to the aircraft configured as high-power laser system for further action.
Advance tactical laser uses 80kW chemical oxy-iodine laser and is mounted on a modified Boeing C-130H Hercules aircraft, with the most obvious visual difference being a rotating turret protruding from the aircraft’s underside through a hole. This chemical laser is similar to the one developed for Airborne Laser programme with much lower output power.