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Laser technology provides major advantages for military applications over kinetic weapons due to High precision and rapid on-target effect, precise and scalable effects, avoidance of collateral damage caused by fragmenting ammunition, Low logistics overhead, and minimum costs per firing.
The proliferation of small, low-cost Unmanned Aircraft Systems (UAS) on the battlefield requires
a layered defense that includes low-cost Directed Energy. The deep magazines of Laser Weapon
Systems (LWS) are ideally suited to counter swarms of hostile UAS.
A variety of other compelling Service LWS applications have been identified, particularly since LWS have the potential to achieve very low operational cost assuming low production costs can be achieved. The development of laser weapons requires many critical technologies, first is development of lasers capable of generating powers in kilowatts to megawatts range to be able to produce useful damage effects on the target.
“The market for counter UAS and other applications encompasses HEL sources with a broad range of power levels from a few kilowatts to megawatts. Today, however, each new LWS demands a high level of design and engineering,” says DARPA.
While high-power, high-brightness lasers have come a long way in the past couple of decades, the highest power output of a single-mode laser still reaches up to “only” around 10 kW. To achieve higher powers with single-mode beam quality for purposes such as military weapons and some types of materials processing, the only way to go higher is to use beam combining.
Today’s Laser Weapon Systems (LWS) are not scalable across the full mission space due to the
use of multiple beam-combined high-power fiber amplifiers as the HEL sources and large complex optical subsystems needed to condition and project the laser beam. Alternatively, coherent beam combined tiled array HEL sources are scalable by eliminating the need for these large subsystems.
Coherent beam combining (CBC), which theoretically allows any number of lasers, all coherent with respect to each other, to be combined into a lone single-mode beam. In actuality, light from a single seed laser is split and sent into a number of amplifiers to create numerous beams that are all coherent with respect to each other before combining. Existing ways to combine the beams are of two general types: tiled aperture, and filled aperture.
In tiled aperture, the near-field laser beams are all placed next to each other, creating, in the far field, a single beam (although usually with unwanted diffraction orders) with real-life efficiencies of up to about 50%. In filled aperture, two beams are combined coherently with a beamsplitter; the resulting beam can be then combined with another pre-combined beam, and so on, with efficiencies of up to 90% or more—with the disadvantage of requiring many precisely aligned concatenated optics to achieve the combination.
Coherently beam combined tiled arrays offer a path to better HEL sources because of (1) the ability to generate and project the LWS beam directly without bulk optics, (2) the intrinsic scalability of a tiled array with no inherent limits, (3) the ability to perform non-mechanical beam steering for beam jitter corrections, and (4) the ability to apply complex phase corrections to compensate for atmospheric disturbances.
MELT program
The goal of this program is to develop a mass-producible, low Size, Weight, and Power (SWaP),
scalable laser source. This will require the development of a new type of HEL source, as current
HEL technologies are very complex, have high part counts, and require skilled labor to
manufacture and assemble. In addition, due to the use of brightness converters, the potential for
further SWaP reductions of current HEL technologies beyond SoA levels is very limited. The
MELT program is thus interested only in semiconductor diode-based laser technologies that do
not include optically-pumped brightness converters.
To maintain the SWAP, high beam quality, and scalability goals, beam combination is expected
to be performed coherently, rather than spectrally or incoherently. Active coherent beam
combination allows for advanced features which passive coherent, spectral, and incoherent
combining cannot perform, such as non-mechanical beam steering and atmospheric turbulence
compensation. Therefore, passive coherent, spectral, and incoherent beam combination are not
within the scope of this solicitation.
MELT seeks to develop a laser tile as the building block for compact, scalable, panelized HEL
sources. It is envisioned that the laser tiles will be integrated into planar arrays for scalable HEL
sources with comparable or better performance than current HEL sources.
The MELT program envisions the building block of the scalable laser technology to be a single
tile, which is composed of many laser emitters. These tiles shall be four-side-abuttable, which
allows an array of tiles to be created in any planar configuration (e.g., MxN).
Necessary support functions (e.g., power delivery/conversion, thermal dissipation, computing, phase sensing and control, and external connections) should be integrated in the tile and contained within the tile footprint to allow scalability. While any backplane used for mechanical integration of the 3×3 panelized array is not included in the mass and volume metrics, it is desirable for the backplane to be of minimal thickness. Electrical and coolant leads leading to the tiled array will also not be included in these metrics; however, inter-tile electrical, coolant and data connections are included.
By program end, MELT seeks to demonstrate a 3×3 panelized array of laser tiles with excellent beam quality (BQ) as a scalable HEL source.
The mass, volume, and size goals for the laser tiles and panelized array of laser tiles include the
semiconductor amplifier emitters, optics, phase sensing and control, power delivery/conversion,
thermal dissipation, computing, external connections, inter-tile electrical, coolant, and data
connections. Excluded from the mass, volume, and size goals are the seed laser, chiller, backplane
for array of tiles, and electrical and coolant leads leading to the array of tiles.
There are at least two different laser diode technologies that can generate high optical power with
excellent beam quality: (1) vertical-cavity surface-emitting laser (VCSEL) diodes, and (2) edge-emitting laser diodes. Both technologies are limited to watt-class output power per emitter and
therefore a large number of emitters need to be combined to realize a high energy laser weapon
system. The challenge in adapting these emitter technologies to directed energy applications is
maintaining excellent beam quality while scaling power, which requires coherent beam combining (CBC) of the multiple individual emitters.
Single-mode VCSEL devices (500 mW) and two-dimensional (2D) arrays of single-mode VCSEL
devices (100 watt-class CW power) have been grown in III-V wafers to form laser oscillators,
consisting of an active region with a relatively low-gain section sandwiched between highly reflective Bragg mirrors. Since 2D VCSEL arrays can be fabricated and tested at the wafer level, they can be manufactured cheaply, with efficient use of semiconductor material. However, demonstrations of passive CBC of VCSEL laser oscillators configured in a common, coupled-cavity geometry have shown the combining efficiency suffers severe degradation when element counts become larger than ~10. Furthermore, demonstrations using VCSEL 2D arrays have been limited to low power with poor prospects for scalability.
Arrays of single-mode edge emitters have been demonstrated at 100 watt-class continuous wave
(CW) power from a monolithic 2D stack. By forming a waveguide in the wafer plane, edge emitters can achieve long interaction lengths for high gain as an optical amplifier. Operating edge-emitter devices as amplifiers with a common master oscillator has been shown to maintain high combining efficiency and high output power with element counts greater than 200. However, the semiconductor materials used to generate these amplifiers suffer from high optical loss and are poorly suited for direct photonic integration. Packaging edge-emitting devices into onedimensional arrays is also labor intensive, with poor scaling to 2D.
To realize HEL power levels and excellent beam quality, the MELT program envisions combining
the favorable attributes of both semiconductor laser technologies. Each MELT tile will contain a
2D array of laser emitters whose phase can be continuously sensed and controlled to achieve
coherent beam combination. For scalable output power, several to several hundred of these tiles
may be arranged as a panelized, gimbal-mounted laser weapon source that produces a directly
usable output beam. The arbitrary phase control necessary to implement CBC on the panelized
array can be leveraged for fine pointing and wavefront correction.
The actual size of a tile will be the result of trades made by the performer. Larger size allows for
fewer piece parts and a greater footprint for support functions but comes at the cost of
manufacturing yield for both the semiconductor wafer and micro-optics. The number of emitters
on each tile is not defined by the program; however, DARPA is particularly interested in solutions
that meet the program goals for power density, beam quality, and non-mechanical beam steering.
A smaller emitter pitch allows for maintaining higher beam quality across a larger steering angle,
but presents challenges for packaging and thermal management. Performers may find that low
power emitters of higher beam quality and smaller pitch may provide a better solution than fewer
emitters operating at or near the power limit.
The need for support functions to reside within the footprint of each tile may require performers
to exploit manufacturing and packaging techniques which take advantage of the third dimension (i.e., through the side opposite of the emitting surface). Novel methods of waste heat dissipation
will also need to be developed, as emitter spacing and tile packaging will prevent diffusion in the
plane of the tiles.
Awards
The Defense Advanced Research Projects Agency has awarded Northrop Grumman a $7.8 million contract to work on a scalable laser weapon system under the Modular Efficient Laser Technology program. The aerospace and defense company bested eight other bidders for the award, the Department of Defense said in Oct 2022.
The MELT program aims to build a modular laser tile to serve as a foundation for compact, extensible and panelized high-energy laser sources by utilizing technologies such as semiconductor fabrication techniques, beam combining, photonic integration, as well as 3D integration and packaging. DARPA targets integrating MELT tiles into planar arrays equipped with upgraded high-energy laser technology.
