MILITARY AND DEFENCE
The Boeing Laser Avenger uses IPG's laser technology.
Throughout the turbulent economic period, the military and defence market has remained stable. Warren Clark asks if the sector will stay profitable in the future
When the industrial sector for photonics stepped off a cliff at the beginning of 2009, the military and defence sector, along with the medical sector, was a saving grace for many photonics companies. Were it not for government funding of defence projects, many photonics companies may not have made it through the recession.
The opportunities for photonics companies within military and defence include applications such as imaging and target designation, through to the more ‘science fiction’ excitement of ‘directed energy’ – lasers as weapons themselves.
‘There is now significant research in the use of high-power lasers for directed energy,’ says Michael O’Connor, of IPG Photonics. ‘There is potential for them to replace or augment kinetic methods, such as bullets, bombs and missiles.’
Research in this area has been going on since the 1970s, but it’s only now that prototypes are emerging.
‘In particular, huge strides have been made in the past five years in the use of lasers in tactical applications at distances of a few hundred metres up to about 10km,’ enthuses O’Connor. ‘A lot of this is down to the improvements and availability of fibre lasers, which have qualities that are attractive to the military – they are rugged by nature, very efficient, and do not represent an environmental hazard as certain chemical lasers do.’
One specific application that is genuinely in the prototype stage – and could even be introduced in the field within a year or so – is the use of lasers for counter-IED (improvised explosive devices) or counter-UXO (unexploded ordinance). Here, the laser is used to heat up the device in question and cause it to detonate. It causes a lower-order detonation than with bullets etc, thereby causing less collateral damage. ‘Such devices,’ says O’Connor, ‘including the Boeing Laser Avenger, would be mounted on a humvee or a Cougar, and operate at a distance of around 200 to 300m.’
Other possibilities are counter-RAM (rockets, artilleries and mortars) uses, where the laser would be used for area defence or ship defence. Operating at a range of a few kilometres, the laser would be shot at incoming projectiles. Technology exists to allow the laser to lock on to its target and destroy it before it reaches its destination.
‘In a similar vein, lasers could be used to defend against UAVs (unmanned aerial vehicles), which offer a threat to a ship or area, particularly if launched en masse,’ adds O’Connor. ‘There is already kinetic weaponry designed to counter this threat, but it uses huge amounts of ammunition. Laser power, provided enough electricity is available, offers a “bottomless magazine”.
‘One of the obstacles to fielding a laser in combat is that the current lasers are usually not eyesafe. The wavelengths in question are around 1,060 to 1,070nm, which is invisible to the naked eye, but has the potential to cause retinal damage. There is some concern over backscatter and the damage this could cause to the operators or innocent people nearby. And, although the laser achieves its goal at the speed of light, there are drawbacks in that it does not operate well in bad weather, such as fog.’
Mark Gitin, director of marketing and business development at Coherent, adds: ‘There are a number of different approaches to these directed energy weapons, and some rely on very high power, diode-pumped solid-state lasers. Specifically, weapons’ lasers that produce from 25kW to 100kW are already in the testing phase.
‘Coherent supplies its high power Onyx Micro Channel Cooled Package (MCCP) diode-laser bars to developers of directed energy-laser weapons. These water-cooled Onyx lasers deliver power levels as high as 150W per bar. These can then be combined into horizontal and vertical (two dimensional) arrays to achieve total output powers as high as 65kW.
‘Achieving these levels of output with the kind of reliability needed for real-world military environments places stringent demands on pump-diode performance and reliability. One key way that Coherent is achieving the required reliability in its Onyx products is through the use of hardsolder construction. Traditionally, high-power laser bars are mounted to a copper heat sink using indium. The ductile indium allows for differential thermal expansion between the GaAs laser and the copper heat sink. However, at high current densities the indium can migrate causing voids, shorts or catastrophic optical damage, leading to device failure. The result of Coherent’s hard solder approach is an order of magnitude improvement in reliability under stressful operating conditions.’
‘Fibre laser-based systems are making significant inroads into the military sector,’ says Gary Catella, VP of technology at Gooch and Housego. ‘Fibre lasers do have the potential to be a “game changer” for the military sector, particularly in the way that systems are built.’
Photonic technologies, as applied to the defence sector, have long played a part in target designation and night vision applications.
‘The primary defence-related business for our lasers is in the night vision illumination and targeting application,’ says Andre Wong, product line manager for communications and commercial optical products at JDSU. ‘Last year, the market for lasers in this application was robust.’
Laser-based rangefinders and target designator systems typically utilise beam delivery optics in the form of a beam expanding telescope to deliver a small, well-defined laser spot at the target. This laser spot can be used to enable a weapons system to illuminate and lock on to the target, or for purposes of establishing target distance. Often, these systems utilise multiple laser lines in the near infrared for illumination, targeting and rangefinding purposes. This multispectral operational requirement, together with the relatively long distances over which the system must operate, combine to place very stringent requirements on the performance of the beam delivery optics.
Now, precision optical solutions provider REO has developed a new assembly technique that dramatically lowers the cost of manufacturing ultra-precision beam delivery optics for target designators and rangefinders.
‘The key to the technique is the use of an air-bearing spindle, together with a laser-based system for measuring lens centring within the mechanical assembly, and an automated actuator for adjusting component position,’ says Mark Damery, vice president and general manager of worldwide sales at REO. ‘In operation, the lens housing is mounted to the air bearing, which delivers much greater rotational accuracy than a mechanical bearing. A lens component is then placed into the housing and the assembly is spun; a laser is reflected off the component to determine the separation of the optical centre of the lens from the mechanical centre of the lens housing. The rotation is stopped, and, a micro-actuator is used to push the lens into centre. This process may be iterated several times to achieve near perfect centring. Lens position is fixed with epoxy, which is dispensed automatically. This ensures that enough adhesive is used so that the component will not subsequently move; it also keeps too much adhesive from being dispensed, which would then necessitate its removal and cleaning of the len’s surface. The entire process is repeated for each element of the assembly.
‘The most important benefit of this approach is that it eliminates the need to hold tight mechanical tolerances at the component level; specifically, the mechanical precision required in the centration of the len’s elements and the lens barrel drops by several orders of magnitude, from a few microns to a few hundred microns, which is extremely easy to achieve. The end result is a cost reduction.’
‘Diode-pumped Nd:YAG lasers are frequently used in target designators and range finders,’ says Coherent’s Gitin. ‘The output wavelength of the pump diodes is temperature dependent, so these are typically actively temperature controlled using thermoelectric cooling in order to ensure that the diode output wavelength matches the relatively narrow absorption band of the Nd:YAG crystal. Now, Coherent has introduced a “Rainbow stack” version of its G-stack series of conduction-cooled diode laser arrays. The Rainbow stack produces a broader range of output wavelengths than traditional pump diodes, allowing it to effectively pump a Nd:YAG crystal even under large swings in ambient temperature.
‘Rainbow stack lasers are available in power levels exceeding 3kW, yet are small enough to fit in the palm of a hand. This small size, together with the elimination of active temperature control circuitry, significantly reduces the size, weight, complexity, power demands and cost of target designator and range-finder lasers.’
Infrared countermeasures are another important military laser application. These are aircraft-mounted lasers with output typically in the 3μm to 5μm range, which are used to disable the tracking systems of enemy missiles. These lasers typically utilise a diode-pumped solid-state laser, whose output is shifted farther into the infrared through the use of an OPO.
‘Because these systems are on aircraft, it is desirable to minimise size and weight,’ continues Coherent’s Gitin. ‘One approach is by minimising temperature control systems. One way to achieve this is by operating lasers at constant elevated temperatures. This requires a unique combination of high efficiency and high reliability at elevated operating temperature.
Sofradir is a developer of advanced infrared detectors for the military and defence sector. It has been developing multi-linear IR arrays (detectors), such as dual-band, which are just beginning to become available after a long period of materials development in several countries. Dual-band IR detectors can operate in the midwave and longwave bandwidths, allowing users to switch spectral bands depending on the particular object to be identified or surveyed. The longwave bandwidth can optimise detection at cooler temperatures, and be beneficial on a battlefield, in the presence of dust, smoke or fog. The midwave bandwidth will enhance performance in high temperature and high humidity. The cameras will also provide for efficient image fusion between the two bands, as the images will be naturally registered.
‘In general, it is a slow period in the defence market, in spite of what you might see or hear in the media,’ says Gooch and Housego’s Catella. ‘It’s stable, for sure, and it’s good business to have in these tough economic times as the projects span normal business cycles. However, defence contracts usually mean fairly low profit margins.
‘The ongoing conflicts overseas mean that portions of any government defence funds that exist are being diverted to support these frontline operations, rather than being invested in R&D, and the majority of laser-based applications are still very much dependent on the latter. Compare this to 40 or 50 years ago, where those heading research programmes in any area of defence were given blank cheques to go and do blue sky research. Now, the funding is about specific requirements for specific programmes.’ In terms of technology, many photonics companies are working on new developments that could have a major impact on the defence sector.
‘This year we are working on the patterning of optical filters by lithography methods enabling small-feature, pixel-sizes and patterned structures,’ says Nada O’Brien, product line manager of custom optics in the AOT (Advanced Optical Technologies) business segment of JDSU. ‘The patterned filters are used with detectors for multi-spectral imaging, or integrated with sensors at the wafer level to reduce cost and physical height of the device. We are also taking infrared linear variable filters, and integrating them with a linear detector array to enable their use as a portable spectrometer at a fraction of the cost of conventional IR spectrometers, like FTIR for example. These devices are used for chemical identification of hazardous materials as part of bio-security and bio-safety. And last, but not least, we have developed products based on birefringent liquid-crystal polymer and dielectric-thin films to manage the polarisation or phase properties of light. Adding polarisation imaging to spectral imaging, one can see objects not easily seen through spectral means alone, such as in heavy fog and smoke conditions.’
Gooch and Housego’s Catella concludes: ‘Photonic technologies are present throughout every area and at every level of the defence sector – from optical communications and remote sensing to satellite imagery and beyond. The advance in photonic technologies is one reason why conflicts in Iraq and Afganistan are being fought on very different terms compared to the 1991 war.’