In the firing line

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Greg Blackman looks at uses of photonics for military applications, from infrared countermeasures to target designators

Modern warfare is very different from that of, say, the First and Second World Wars. Weaponry has advanced and one major difference is the distance at which soldiers can engage the enemy. Wars are no longer fought at point-blank range and the ability to fire on targets from greater distances gives massive advantages, especially if the enemy cannot match those distances. Guided missiles, early versions of which date back to World War II, now have a range of thousands of kilometres and can therefore be fired at a target from relatively safe distances – from battleships or submarines, for instance.

Photonics is used heavily within military equipment and applications vary from guidance systems, such as gyroscopes, to range finders and countermeasure devices. Laser-based weapons are also being developed and global security company Northrop Grumman, as part of the US military’s Joint High Power Solid State Laser programme, has produced a light ray created by a solid-state laser measuring 105kW. This is a major milestone as 100kW is traditionally viewed as the power threshold for ‘weapons grade’ lasers although lower power systems can be used, depending on the application. The system utilises laser amplifier chains, each producing 15kW of power output with high beam quality, which allows the weapon to be scalable in power depending on the threat. Seven laser chains were combined to produce the 105kW beam and potential applications for the weapons system include force protection and precision strike missions.

Military research, typically, is concerned with very high power lasers, as, according to Graham Catley, EMEA sales director at Gooch and Housego, this increases the effective range of the devices. ‘In terms of using high-power lasers for R&D in industrial applications, the maximum power output tends to be in the tens or hundreds of watts range, whereas for military research kilowatt laser systems are used,’ he comments. Gooch and Housego provides optical components and sub-systems for a number of different industries including military and defence.

Beam combination

High power, however, is only one variable and often the wavelength of the beam is vital for certain applications. Pranalytica, a company based in Santa Monica, California, producing quantum cascade lasers (QCL), is supplying its laser technology as part of a project for the US Army Aviation and Missile Command (AMCOM). The project, which began in early January 2009 and which is led by the Defense Advanced Research Projects Agency (DARPA) Small Business Innovation Research (SBIR) programme, is designed to increase the power output of a laser source by combining the beams of a number of high-power QCLs. The aim is to develop more effective laser sources in the mid- (MWIR) and long-wave infrared (LWIR) for directional infrared countermeasures (DIRCM), advanced stand-off chemical sensors used to detect explosives or chemical warfare agents (CWA), and laser radar (LADAR).

Phase I of the project looks at combining an array of 200mW average power, thermoelectrically cooled QCLs into a 1W module. Pranalytica’s QCL modules, which in themselves were developed under the Efficient Mid-Infrared Laser (EMIL) programme from DARPA, can produce more than 2W at 4.6μm (MWIR) from a single emitter. Therefore, the power output required to satisfy Phase I of the beam combining project could be generated using one QCL system. However, the project aims to increase the power output, with Phase II extending the approach to produce more than 5W in the MWIR, and Phase III taking that further to exceed 10W.

Two watts of output power is high for a single QCL system and while certain military research programmes are using kilowatt lasers, the wavelength requirements for these applications are very specific and are met by QCL modules. Dr C Kumar Patel, president and CEO of Pranalytica, explains that QCLs are beneficial for DIRCM and explosives detection (either in situ or remote), because the laser output wavelengths can be tailored for the specific needs. DIRCM applications require wavelengths in the 4-5μm MWIR region, while for explosives detection, wavelengths in the 8-12μm LWIR region are optimal.

‘Five QCL modules could potentially be combined to produce 10W of power, however, the challenge is not only to combine the beams, but to maintain the beam quality,’ explains Patel. Individual emitters produce a beam that is close to a Gaussian profile and as the beams are combined this profile would have to be maintained to allow the beam to be effective over long distances. ‘The ultimate goal for the project is to combine any number of laser sources, not simply five, to get an arbitrarily high power output while maintaining beam quality.’

Missile deactivation

The high power output would increase the capabilities of DIRCM systems, advanced stand-off chemical sensors and LADAR systems, used by the US military. DIRCM systems are designed to protect military aircraft from heat-seeking missiles by deactivating their guidance systems. The missile locks onto the aircraft’s heat signature from its exhausts, which is at an effective temperature of approximately 700°K and has its peak emission wavelength in the MWIR region, and hones in on this to target the aircraft. As a countermeasure, a DIRCM device is attached to the underside of the aircraft and is primed to recognise a missile. The device then directs a MWIR laser beam at the missile of the same wavelength range onto which the missile locks. The beam output is a series of pulses designed to disable the missile’s guidance system causing it to lose its target.

‘Currently, several aerospace and defence companies are using 2W lasers in DIRCM systems, but the higher the power output the greater the effective range of the device and the further from the aircraft the missile can be deactivated,’ says Patel.

QCL systems, operating in the 8-12μm wavelength region, have also been used to provide accurate local detection of explosive and chemical warfare agents. Pranalytica’s L-PAS (laser photoacoustic) sensor, being developed under DARPA’s L-PAS project and that uses LWIR QCLs, to detect and identify CWAs and explosives by analysing air samples drawn into the sensor. A further DARPA programme is being conducted using CO2 lasers for explosive detection, which are expected to provide a range of approximately 200m. QCL modules provide greater laser wavelength tuneability than CO2 lasers, which allows the system to detect a wider range of explosives. However, to match the stand-off distance of 200m, the QCLs will need to match the power output of the CO2 lasers, which is approximately 5W. Patel explains that by combining output power from a number of QCLs, each tuneable in wavelength, the project aims to provide a powerful and flexible system that can be tuned to identify specific explosive substances at large stand-off distances.

The military also use laser technology to illuminate specific targets. Laser designators shine a series of coded pulses onto a target, which scatter into the atmosphere and are picked up by the seeker of a guided missile. These laser pulses guide the missile onto the target.

Target designators operate over several kilometres and emit a beam that is in either in the MWIR or LWIR. The regions of 4-5μm and 8-12μm are critical because, at these wavelengths, there is relatively low optical absorption arising from carbon dioxide and water vapour. Therefore, these regions, which are known as the first and the second infrared windows, allow long distance propagation of the beam through the atmosphere.

Pranalytica’s PoyntIR device emits at wavelengths of 4.6μm or 9.6μm and can be used in battle zones to illuminate targets for ground-based troops and mobile combat vehicles at distances greater than 1km and for combat aircraft at up to 10km. Lasers can also be used for so-called IFF (identify friend or foe) applications, whereby, as well as target designation, the technology can summon help or alert friendly forces to the soldier’s location.

There are numerous other areas within the military and defence sector where photonics plays a role. Catley of Gooch and Housego notes that communication channels within military vehicles and equipment are being upgraded to fibre optic methods of delivery. ‘In place of copper wiring as the traditional communication medium, photonics is being adapted as the technology of choice,’ he says. ‘The cabling enabling a control panel to talk to a laser system when modulating the beam output is now typically fibre optic, and projects such as FONDA (Future Optical Network Distribution for Aerospace) have been instrumental in developing fibre optic communication networks for aerospace and defence products.’ Fibre optic cabling provides a high bandwidth for fast data transport and has the security benefit of the data being encrypted when sent.