FEATURE
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Perfect beam, perfect defence

A coherent strategy is not the only way to stop your enemy, as Rob Coppinger finds that directed energy needs good beam quality to defend its military users

Whether it is detecting explosives or heating up incoming mortars or rocket warheads so they destroy themselves, lasers are progressing for defensive applications to defend homelands against terrorists and forward operating bases from insurgents’ attacks.

‘Directed energy, I think, is the biggest area in terms of dollars spent worldwide for defence, but there is a fair amount in Lidar as well,’ IPG Photonics’ director of advanced applications Michael O’Connor tells Electro Optics. ‘Our biggest activity is in directed energy; we do Lidar as well.’ O’Connor explains that most of the directed energy work is for area defence, ‘the idea being someone is firing rockets or mortars or artillery at you’. The response is to shoot it down with lasers before it lands and injures or kills people. O’Connor points to Israel’s Iron Doe system as something that could be augmented with lasers.

‘Their system is very effective, but they are using very expensive missiles to shoot down relatively inexpensive rockets [fired by militant Palestinian groups]. Though it is effective from a military point of view, it is ineffective economically,’ says O’Connor. ‘The cost is something like $25,000 to $50,000 per missile, whereas with a laser to augment the missile system, for those targets you can affect with a laser, the cost would be pennies per shot. It is the cost of the diesel fuel to generate the energy. The per-shot cost is dramatically lower.’

Another possible use is the shooting down of remotely piloted aircraft, known as drones or by their old acronym of Unmanned Aerial Vehicles (UAVs), says O’Connor. ‘The US Navy has LAWS, the laser weapon system, and they have demonstrated a 30kW system for shooting down UAVs and they are upgrading those systems.’

They need to upgrade those systems – the military really wants 100kW lasers that have very good beam quality, meaning a small spot on the target for maximum power density. Whether it is area defence or shooting down surveillance aircraft such as UAVs, targets need to be destroyed at a distance, and only highly coherent powerful lasers will do the job. 

‘We aren’t able to provide a bright enough laser; people want 100kW or more with a very good beam, like practically perfect quality,’ says O’Connor. ‘As you propagate over distances of kilometres, if you have a beam that’s not high quality it diverges very rapidly and you end up with very low power density.’

According to O’Connor, beam quality and beam power are the key ingredients; a perfect beam with a few watts ‘would not do any damage’, he adds. ‘If you had 100kW single mode perfect beam laser today it would be highly desirable. We don’t; instead we have 10kW single mode at IPG and we are working on 20kW single mode.’ It is this lack of brightness that means IPG, like other companies, are opting for building systems with multiple lasers that, with their beams combined, deliver tens of kilowatt power levels to the target. But 100kW still evades them.  ‘You need fibre lasers that are coherent, where the spectrum of the light is narrow so they can be phased together,’ explains O’Connor. ‘Multiple beams in phase give you constructive interference and you get increased brightness; you can go from 1kW to 10kW but it would appear like a  perfect beam because they are all phased up.’

According to O’Connor, there are programmes funded by the US Department of Defense’s Joint Technology Office and the Defense Advanced Research Projects Agency. ‘One is called RELI, which stands for Robust Electric Lasers Initiative. In the RELI programme, two of the five contestants are fibre laser systems that are either being coherently combined or spectrally combined,’ says O’Connor. ‘The idea is to get to, in the near term, a 25kW laser with a perfect beam and ultimately 100kW with a perfect beam.’ He added, ‘IPG can deliver a 100kW fibre laser but it is going to have an M2 of 30 or 50 or something like that, a very divergent beam.’

O’Connor explains that the effectiveness of the laser against a target is proportional to the power of the beam laser and also to the beam quality squared. O’Connor points to the many demonstrations that have taken place with fibre lasers, such as shooting down missiles, drones, and mortars that have been conducted by Raytheon, Boeing, Lockheed, Rheinmetall and others. But those demonstrations belie fundamental shortcomings in what can be deployed to the field.

As O’Connor explains, his company’s 25kW perfect beam, ‘would be an effective laser at a short distance of a few hundred metres, but at many kilometres it is not very effective’. And it is the longer, kilometre-plus range that is necessary.

Rheinmetall, the  German defence company gave a presentation on its efforts at the IPQC Directed Energy Systems 2013 conference, held in London from 5 to 7 March. At the event Rheinmetall staff explained their work to date. The company carried out a first demonstration in 2005 with a single telescope linked laser system that did not move. Targets were static and a flown target flew past the fixed laser.

 In 2012 Rheinmetall demonstrated a 30kW laser that could track targets from mortars to drones. At the conference Rheinmetall showed videos of the tracking and shooting of the mortars and drone. A static mortar target with a highly reflective surface was also lased and exploded, to demonstrate that reflective surfaces were not a defence against lasers. Rotating reflective surfaces were also stated to be no defence against lasers. The company’s plans for 2013 are to demonstrate a six laser system that operates across two platforms to defend against multiple incoming targets.

Photonics technology for defence does not just have to be directed energy. A four-year European Union project has led to a prototype remote sensing explosives detector being evaluated by the Spanish police force, the Guardia Civil. Spectrometer, light source, fibre optic and software provider Avantes participated in the project; CEO Benno Oderkerk says. ‘It has been tested at the FOI, a Swedish federal defence agency for research. It is being tested by the Guardia civil in Spain, that is the final stage,’ he said. ‘We have something to demonstrate and it actually works.’

The sensor uses Raman spectroscopy and the laser induced breakdown of the target material.

Oderkerk says: ‘You have to identify that there are explosives in the container, but also what kind of explosives. And it has to make a full identification in 30 seconds. That’s relatively rapid.’ He adds: ‘Raman spectroscopy is about the vibration of molecules, mostly in liquids, but it works well with explosives, and the other technology used is laser-induced breakdown spectroscopy.

‘You shoot a laser at a fingerprint size amount of material that vaporises and as it is heated and cools down, there is light emission. That light gives you information about what the material is.’ The sensor is able to measure the explosives directly and indirectly from residual explosive material that would have contaminated the surfaces of whatever contains the explosive.

‘Explosives are sticky stuff, so while the explosives are inside a container you will have contamination outside – and that is what is detected,’ explains Oderkerk. The spectroscopic sensor has a range of 20 metres.  This is to allow security forces to detect the presence of explosives in a container at an airport or in someone’s bag in the street or in a car at a checkpoint. The technology was developed under the European Union’s Seventh Framework Programme, an EU wide multi-year research and development effort that spans many technologies and has a budget of billions of euros. Of the explosives detector, Oderkerk says: ‘The next stage is to see if we can commercialise this prototype. It had to be transportable; it has been developed to test left behind luggage and packages from a distance of 20 metres.’

This sort of mobile application for photonics in defence is not lost on optics and sub-assembly manufacturer Swiss Optic. Its management has found that there is a demand for hand-held optical systems but they have to be lighter, less complex and more rugged.

It is these key aspects that defence customers demand and Swiss Optic has worked with its customers to deliver that. One technological change that has led to size and weight improvements is the reduction of optical channels by using broad band optical systems.  ‘Aspheric lenses and the use of novel materials have also enabled Swiss Optic to deliver lighter, rigged, and simpler systems,’ according to Rob Roach, sales director of Armstrong Optical and the UK sales representatives of Swiss Optic.

Wherever future conflicts may occur or whoever the combatants are, one aspect of warfare is universal – the side with the fastest sensor-to-shooter time is going to win.

Whether that is a sensor for detecting an improvised explosive device, in the ground or vehicle-borne to explode it at a safe distance, or the tracking and destruction of surface-to-surface or surface-to-air munitions, photonic sensors have the speed necessary to counter any threat, and lasers have the destructive power to stop the enemy.