As high power lasers have gained more interest from industry for welding and cutting ever thicker metals, the technical advances being made in this field have not gone unnoticed by governments and their military organisations. From chemical to solid state and fibre, laser systems have been investigated for everything from force protection through knocking out mortars and improvised explosive devices (IED) to taking aim at intercontinental ballistic missiles (ICBM).
‘The high power lasers caught the attention of the [Department of Defense] DOD in the US and ministries of defence worldwide,’ says Michael O’Connor, director of advanced applications at fibre laser company IPG Photonics.
Six years ago the US Navy took an interest in high power welding lasers. O’Connor explains that the Navy wanted to know what IPG could deliver in terms of power and beam quality. Since then the DOD, US Army, Navy, Air Force and defence contractors such as Northrop Grumman, Lockheed Martin, Raytheon, Qinetiq and BAE Systems, both its US and UK wings, have worked with IPG on what is referred to as tactical laser applications.
IPG’s US work has seen it support efforts to integrate laser weapons onto the US Army AM General High Mobility Multi-purpose Wheeled Vehicle, better known as Humvees. These Humvee-based lasers were primarily developed as anti-IED systems. According to O’Connor, for anti-IED systems ‘moderate brightness,’ 2-3kW power lasers with a beam quality of M2 at six or seven, is sufficient. The two Humvee-based laser projects were Boeing’s Laser Avenger and Sparta’s Zeus. O’Connor says: ‘Sparta was the first to recognise the fibre laser’s [benefits], after trying out solid state lasers and not finding anything that worked reliably.’ The Zeus was deployed to Iraq to counter IEDs, while the Laser Avenger has also shot down unmanned air vehicles (UAV), also known as drones, under tests.
IPG is continuing this sort of work and developing more powerful lasers. O’Connor says: ‘Right now we’re putting a 10kW single mode laser on to the High Energy Laser Technology Demonstrator (HELTD), for which Boeing has the contract.’ This is a US Army project for a counter-Rocket, Artillery and Mortars (RAM) laser system and Boeing has completed HELTD’s system integration for its key components. The integration included installation of the beam control system and other hardware onto the Oshkosh tactical truck, HELTD’s vehicle platform. O’Connor adds: ‘Originally [HELTD] was going to use a solid state laser and now it is going to use fibre – specifically, our 10kW single mode lasers, 10kW of power with M2 of less than two, offering very high brightness. We’re also working on a 20kW single mode laser.’
O’Connor’s company has also worked with the Canadian Ministry of Defence and what he called ‘Israeli groups’. The work with the Israelis has also been for counter-RAM systems. Israel has already deployed its Rafael manufactured Iron Dome that is a rocket, not laser, system to counter Palestinian militant groups’ rockets. ‘They, the Israelis, have a pressing need against the rockets,’ added O’Connor.
IPG’s work with European companies has also included counter-RAM technology and systems to target UAVs. ‘We’ve done a lot of work with the European companies EADS and Rheinmetall.’ Two IPG 5kW fibre lasers are being used by Rheinmetall for a weapon system that can shoot down drones. With so much interest, IPG is investing in making the fibre lasers more compact and ultra-efficient.
A European company involved in anti-missile systems is laser diode specialist Dilas. While Dilas’ involvement in military lasers goes right back to the company’s founding, with infrared counter measure applications and range finder and target designator pumping, the firm is now involved in laser weapon programmes that have larger targets than counter-RAM systems. Knocking out Inter-Continental Ballistic Missiles is a goal that Dilas is helping its customers achieve in the US and Europe. In the US an anti-ICBM technology demonstrator was Boeing’s Airborne Laser, a converted Boeing 747 that successully hit its distant targets with its chemical laser during testing. Dilas sales and marketing director Dr Jorg Neukum says: ‘Mostly our customers are in the US, but there are also European companies working on these anti-missile systems. Six years ago some of these programmes started and Dilas was involved as a supplier to some of the subcontractors. There are several big contractors in the US working on it and they are supported by the Dilas US facility. Other countries are supported by the German Dilas facility.’
Dilas is providing its customers stacks of diode laser bars. Dilas can deliver up to 70 laser diode bars in a single stack configuration and many stacks are needed to deliver the 3-400kW of pumping light required for the solid state laser to produce the 100kW wanted.
The issue such high power laser programmes have, according to Neukum, is the heat management. Because lasers will lose most of the input power as heat, any high power system has to be able to cope with the heating. ‘All these programmes have to take special care on the thermal management of their crystal,’ explains Neukum.
One solution which has been proposed a while ago by US researchers is the use of a gaseous laser medium. Big pumps are used to exchange this gas and heat will be less of an issue. Some of the gas vapours of interest are the alkali metals. According to Neukum, lithium, rubidium and cesium are all candidates. The alkali metals vapour is in a transparent cell, with some buffer gas and is pumped. ‘The pumping is done at very particular wavelengths. Dilas has succeeded in developing and delivering stacks working at such wavelengths,’ says Neukum.
While gases provide a solution to the heat problem, they require very small pump light line widths, even when there is a high pressure buffer gas used in addition. This means the pumping laser also has to meet such narrow line width requirements. ‘Such requirements will be solved by using volume holographic gratings for the line narrowing and we are now working on that with our customers,’ explains Neukum.
Despite these issues the lithium, rubidium and cesium alkaline lasers have been demonstrated in laboratories but none to the power levels achieved by solid state lasers, around 100kW. But for alkaline lasers this is the goal.
At what point these laser systems will have technology mature enough for deployable systems is open to question. ‘The question is “when will the military fund this?”,’ says Neukum. He points out that it is not enough for the technology to work; it must be reliable and it must be able to react in the window of time defenders have before it is too late.
In developing the pumping laser for such alkaline lasers, Dilas can use its scalable stacking technology to deliver multiple kilowatts from a single stack of laser bars with 100W of pump power per bar. But for the 100kW laser weapons far more is needed. Despite the distance to go, Neukum believes that good progress has been made because of the spectral power density that has been achieved. He says: ‘We have been able to produce this power [in kilowatts] for years, but to do it within a very small spectrum, to do 766nm plus or minus half a nanometre, that is the achievement.’
At the other end of the spectrum and at a much lower level of power output are the range finders and target designators, both for handheld devices and the pods installed under an aircraft’s wing. Laser Components’ Stuart Nunn sees the trends for rangefinders shifting the wavelength used to 1550nm, from 905nm. This is because it is eye safe, being a wavelength absorbed by the cornea and not the retina. The wavelength also delivers the accuracy over the distance required. Another preference for the higher cost rangefinders is the use of Indium Gallium Arsenide for bandpass filters. Bandpass filters are placed in-front of the camera sensor so it transmits the light at a wavelength similar to the laser wavelength reflected by the target and blocks a significant proportion of the ambient light that it would otherwise pick up. That stops saturation of the sensor making it more effective at detecting incoming light.
According to Nunn, 1550nm is also an attractive wavelength, because the cameras used to detect it are less readily available. ‘It’s important the enemy can’t detect the rangefinder,’ says Nunn. ‘Another trend is greater accuracy and greater distance over which you can measure distances. You can do that with more power or by using greater collimation.’ To meet this trend, Laser Components manufactures and supplies pulsed laser diodes. Pulsed lasers with high brightness will provide good collimation.
‘We anticipated the direction of the market, so we produced pulsed lasers with high brightness for better collimation to meet that demand,’ says Nunn. Whatever the laser type, Nunn points out that customers are always interested in power, efficiency, lifetime and heat loss; the heat that has to be dissipated. Customers can also asked for reliability data. ‘Price always comes into consideration; some require low cost and others prioritise performance. The lower end users want 905nm lasers.’ Another area that Laser Components is illumination, using infrared light to illuminate surroundings so only night vision goggles can see. At US company Reo, target designators and range finders are also the focus. The trend is for lighter equipment. Legacy rangefinders and designators were 60lb devices sitting on tripods, but now they are like heavy duty binoculars.
One way the technology has been reduced in size is the use of aspheric optics. Another is to shrink beam diameters and, hence, lens sizes. But this raises power density, which means that optics have to be defect-free to avoid laser damage. Ion beam sputtering is used to make films on the optics surfaces with increased damage threshold. System weight can also be reduced by using thinner optics but coating stress can distort the shape of these thin lenses. Mechanical stress can pull the optic out of shape and it won’t deliver the wavefront required. Sometimes coatings on both sides will be applied to counteract each coating’s tendencies for deformation.
Coatings can also be affected by heat and humidity. Reo sometimes carries out annealing to remove mechanical stress. Whether it is range finders for binoculars or anti-improvised explosive device technology, photonic technology is set to have a role in military affairs, from targeting to neutralising greater threats, for some time to come.
