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Among the uses of Laser directed energy weapons (DEW) systems are providing very short-range air defense capability, close-in protection for naval vessels, counter-unmanned aerial vehicle (UAV), and protecting friendly forces from mortar and artillery attack. Laser require a power of the order of 100 kW, to be employed as directed energy weapons, in varieties of missions such as wide-area, ground-based defense against rockets, artillery and mortars; precision strike missions for airborne platforms; and shipboard defense against cruise missiles.
Fiber lasers have emerged most promising technology, for directed energy weapons due to their many advantages like: high electrical to-optical efficiency (40%), high reliability for operation in harsh military environments, and high beam quality near diffraction-limited light output. However, the power of state-of-the-art single-mode fiber lasers is limited by thermal and nonlinear effects like thermal lensing to ∼10kW.
Combining multiple low-power lasers with good beam quality into one high-power beam helps in overcoming power limitations of fiber lasers. There are numerous ways of combining laser beams and some are better suited to a given application than others; e.g. in the trivial case of a couple of beams a simple beam-splitter arrangement may suffice but there are limitations in this approach and it rapidly becomes impractical as the number of constituent beams increases.
By using beam superimposing technology, Rheinmetall has combined the power of single lasers into one multiplied laser beam. This technology not only allows superimposition of multiple lasers on a single gun platform, but also superimposition of multiple gun platforms. This enables an almost unlimited (e.g. 100kW and more) power output in line with the evolving air defence requirement. As a result the high-energy laser gun provides efficient protection against a large spectrum of modern air threats.
Beam combining technologies
The large number of single mode fiber lasers can be combined either coherently, spectrally or incoherently. The simplest approach is incoherent beam combination, which Phillip Sprangle and colleagues at the U.S. Naval Research Laboratory tested by combining beams from four fiber lasers onto a target 3.2 km away. They delivered a total of 5 kW to the target, and concluded that at such distances there “is little difference in the energy on target between coherently and incoherently combined laser beams for multi-kilometer propagation ranges and moderate to high levels of turbulence.”
That inspired the U.S. Office of Naval Research to buy half a dozen 5.5-kW industrial fiber lasers from IPG, and combine their beams incoherently by aiming them with different mirrors through a single telescope toward the same target. Called Navy LaWS, for Laser Weapon System, it was mounted on the USS Ponce when it was deployed to the Persian Gulf. YouTube videos show how the laser destroyed drones and detonated explosives floating on a small boat, although the distances were not identified. The tests were generally encouraging, but showed that thermal blooming, air turbulence and heating of laser optics could make it difficult to deliver continuous powers above 100 kW beyond 5 km.
The incoherent beam combining, which directs many laser beams in the same direction, increases the total power but does not increase the beam brightness. Beams can be combined incoherently by mixing fiber-laser outputs in large-core multimode fibers, producing continuous outputs to 50 kW. However, simply bundling together multiple fibers results in a composite beam with poor beam quality. The laser energy from such a system spreads out quickly as it propagates to a distant target, limiting the effective range of the system.
One long-studied alternative is coherent beam combination, which seeks to carefully match the phases of beams from separate fiber lasers or amplifiers so the light interferes constructively to yield high power and high intensity. Coherent or wavelength beam combining promises the higher brightness and better beam quality needed for many applications. This is analogous to phased-array radar, which matches the phases of radio waves emitted by separate antennas operating at the same frequency to produce a powerful steerable beam.
Coherent beam combining architecture
Coherent combination is the best technology, but requires extremely narrow laser line widths and precise control of the polarization and phase of the individual lasers. Phase matching at optical wavelengths, however, is much harder, because the wavelengths are thousands of times smaller and must be matched with much greater precision.
Phase matching can be eased by dividing light from a single master oscillator among many parallel fiber amplifiers, so in principle the output beams would be coherent with each other.
The laser developed by QinetiQ employs a scalable, coherent beam combining technology to create laser source with a power level of ‘several tens of kilowatts’. The system will be scalable to higher power levels, as required. The coherently combined fibre laser technology developed by QinetiQ associates phase control system that provides a high precision laser source that can be effectively directed at dynamic targets and achieve high power density on target in the presence of turbulence. Beam combining is a technology that is able to achieve enhanced power densities at target, reducing defeat times and increasing engagement range. Therefore, although the system is not of a ‘100 kW’, power level which is considered for weapon grade lasers, the Dragonfire beam director designed by Leonardo optimises the laser beam to optimize to atmospheric conditions that otherwise would dissipate much of the energy.
Coherent arrays of fiber lasers appear scalable to weapons-class (100kW) powers with near-perfect beam quality in rugged and compact packages. Gregory Goodno, from Northrop Grumman Aerospace Systems, describe one coherent beam combining architecture, wherein a master oscillator is split to seed N fiber channels, followed by individual phase, polarization, and path length (P3) components , followed by a kilowatt-class fiber amplifier chain.
To launch light into free space, kilowatt fiber tips were mounted into a close-packed, thermally stable array. A spherical mirror simultaneously collimates and images the array onto a reflective diffractive optical element (DOE). In this case, the DOE—which has essentially the same power handling as a typical high-energy laser mirror—is a multi-port beamsplitter with a lithographically-etched, periodic-surface-relief, low-angle structure.
When illuminated by N phase-locked input beams at angles matching the DOE diffractive orders, the DOE can function as a beam combiner. “To enable servo locking of the fiber phases, polarizations, and path lengths, we sent a single beam sample of the combined output to our P3 controller. The use of coherent detection enables scaling of the P3 controller to hundreds of fiber-laser channels with high control bandwidths, allowing for the rejection of vibrational disturbances that may be coupled from the environment or “platform of operation.”
In summary, our results show that the coherent combining of fiber lasers is an efficient scaling path toward 100kW-class DE lasers with diffraction-limited beam quality. By using only two common-path optics and a replaceable-element fiber array, our system architecture is suitable for harsh, non-laboratory environments. In principle, CBC has no intrinsic scaling limit on channel counts, but in practice, scaling will be dictated by engineering constraints of cost, complexity, and size. In this regard, further increases in coherent fiber powers combined with a 2D array and DOE geometry8 will enable scaling well beyond 100kW in compact packages.”
So far, experimental results have remained modest, but new approaches are being studied, such as coherently combining pairs of polarized beams, creating a new beam that in turn can be combined with other combined, polarized beams to produce even higher powers.
Spectral beam combining
An alternate approach employs spectral beam combination, in which each fiber is operated at a slightly different wavelength and
So far, the most successful approach to producing weapon-grade fiber lasers has been spectral beam combination—essentially a military-strength version of dense wavelength-division multiplexing. It uses many separate single-mode fiber lasers to generate beams at a series of closely spaced wavelengths and using dispersive optics to multiplex together efficiently into a high-power beam delivered by a fiber. This avoids the complexities of phase matching but produces wider-band laser emission. Although the resulting beam can be nearly diffraction limited,power scaling is constrained by the available source spectral brightness and laser gain bandwidth.
Lockheed Martin demonstrated that approach by building 96 ytterbium-doped fiber lasers, each emitting 300 W at a different wavelength in the ytterbium band. At the 2017 CLEO meeting, Eric Honea of Lockheed reported that the spectral beam combination generated a 30-kW beam with more than 95 percent combination efficiency.
Powers now are reaching the 100-kW class. Last year Lockheed delivered a 60-kW version to the Army Space and Missile Defense Systems Command in Huntsville that has wall-plug efficiency of 35 to 40 percent. This year, Lockheed received a U.S. Navy contract to build a pair of spectral beam combination fiber lasers emitting 60–150 kW—one for tests at sea, the other for use on land. And the DoD’s Missile Defense Agency says spectral beam combination of fiber lasers could allow scaling to a proposed future 300-kW generation of laser weapons within five or six years.
At the University of Central Florida (Orlando, FL), Leonid Glebov has used volume Bragg gratings to coherently combine two fiber-laser beams with 99% efficiency to generate 282 W. Spectral beam combining is commonly implemented using either surface diffraction gratings or volume Bragg gratings (VBGs). VBGs have the advantage of being modular and can be easily scaled when high channel numbers are desired. The focus of the research presented in this paper is on spectral beam combining by volume Bragg gratings recorded in photo-thermo-refractive glass.
“To make systems such as the one just described compact, lower their complexity and minimize the induced thermal distortions, we propose the use of special volume Bragg elements which have several Bragg gratings written inside as combining optical components. The properties of the PTR glass allow the recording of several VBGs in a single piece and therefore the complexity of the setup could be decreased proportional to the number of gratings recorded together. “
Comparison of Coherent (CBC) and wavelength beam combining (WBC)
Coherent beam combining (CBC) and wavelength beam combining (WBC) are two techniques used for combining multiple laser beams into a single beam. Both techniques can improve the output power, beam quality, and efficiency of laser systems, but they differ in their operating principles and performance characteristics.
Coherent beam combining involves phase-locking multiple laser beams so that they all have the same phase and combine constructively to form a single beam. This technique requires active feedback control of the laser beams’ phases, and typically, all the lasers have the same wavelength. The combined output beam has a narrow spectral linewidth and high beam quality. CBC can be used to achieve high output powers, but it is typically limited to a few lasers due to the need for precise phase control.
Wavelength beam combining, on the other hand, involves combining laser beams of different wavelengths to form a single beam with a broader spectral range. This technique does not require phase control, but rather relies on the spectral properties of the individual lasers and their output couplers. WBC is useful for combining many lasers, as each laser can operate independently, and the combined beam has a wider spectral bandwidth. However, the output beam quality is lower than CBC due to the difference in beam properties between the individual lasers.
In summary, CBC and WBC are two different techniques used for combining multiple laser beams. CBC relies on precise phase control and can achieve high beam quality and high output powers, but is limited to a few lasers. WBC does not require phase control and can combine many lasers, but the output beam quality is lower than CBC due to the differences in beam properties between the individual lasers. The choice of technique depends on the specific requirements of the laser system.
Another difference between CBC and WBC is the complexity of their optical setups. CBC requires active feedback control of the laser beams’ phases, which can be challenging to implement in practice. The optical setup for CBC typically includes phase modulators, feedback loops, and beam combiners. WBC, on the other hand, is simpler and more straightforward since it only requires passive optical elements such as mirrors, filters, or gratings to combine the different wavelengths.
Another advantage of WBC is that it is more tolerant to variations in laser output power, wavelength, and beam quality than CBC. In WBC, each laser operates independently, so the overall output power is the sum of the individual lasers’ power. In CBC, the phase relationship between the lasers must be maintained, so any variation in the output power, wavelength, or beam quality of the individual lasers can degrade the overall performance of the combined beam.
Finally, the choice between CBC and WBC depends on the application requirements. CBC is suitable for applications where high beam quality and narrow spectral linewidth are critical, such as in high-power laser systems for scientific or industrial applications. WBC, on the other hand, is suitable for applications where a broader spectral range is required, such as in telecommunications, sensing, or medical applications.
In summary, CBC and WBC are two different techniques used for combining multiple laser beams, each with its advantages and disadvantages. CBC requires active phase control, has a high beam quality, and is suitable for applications where narrow spectral linewidth is critical. WBC is simpler, more tolerant to variations in laser output, has a broader spectral range, and is suitable for applications where a broader spectral range is required.
A team from Lincoln Labs and Northrop Grumman Aerospace Systems (Redondo Beach, CA) have compared coherent and wavelength beam combination with commercial 500 W ytterbium-fiber lasers. Coherent combination was harder to implement because it requires phase control to within a small fraction of a wavelength, close path-length matching, narrow laser linewidth, and uniform polarization, they found, but it offers high spectral brightness and allows phase adjustments for beam steering or atmospheric compensation. Wavelength beam control is simpler to implement but produces a broader spectrum, so it is best suited for applications requiring raw power delivery where spectral brightness is not important.
Lasers combining into a diamond crystal
Instead of laser beams joining into a single one in space though, researchers from Macquarie University in Australia were able to combine several laser beams into one by placing an extremely pure diamond crystal at the point of convergence. The crystal allows the power to be directed in a specific direction without beam distortions.
“This discovery is technologically important as laser researchers are struggling with increasing power beyond a certain level due to the large challenges in handling the large heat build-up, and combining beams from multiple lasers is one of the most promising ways to substantially raise the power barrier,” lead author Dr Aaron McKay said in a statement. The physical mechanism behind this technology is known as Raman scattering. While this is exhibited in many materials, it’s particularly strong in diamonds. Diamonds also dissipate heat well and change the color of the beam, which is important for safety reasons.
Multiplexing several lasers using beam combination represents a method for surpassing the power barriers of single lasers. Authors propose and demonstrate a novel approach to beam combination and power scaling based on Raman conversion in diamond. Power from multiple non-collinear pump beams is efficiently transferred onto a single Stokes beam in a single-pass amplifier. Using three mutually-independent nanosecond pulsed beams from a free-running-linewidth 1064 nm laser, 69% of the total peak pump power of 6.7 kW was transferred onto a TEM00 Stokes seed pulse at 1240 nm in a 9.5 mm long diamond crystal. Compared to other beam combination techniques, diamond beam combination has advantages of relaxed constraints on pump beam mutual coherence, while enabling narrowband output.
“The particular wavelength of the directed energy beam is critical to the efficient transmission through the atmosphere and to reduce the eye hazard for people, or indeed animals, who may be in the vicinity of the beam,” added co-author Professor Rich Mildren.
