Directed Energy Laser
Demand for high power fiber lasers has increased tremendously in the last 10 years. They are used in an increasing variety of fields such as industrial machines, LIDAR systems, intense laser systems (inertial fusion, plasma), and recently in optical systems that can deliver very high power for defense applications. With the growing presence of drones on the battlefield, the development of offensive laser applications, such as anti-structure laser weapons, has been accelerated.
Until recently, chemical lasers or free-electron lasers, then solid-state lasers, were mainly considered for such applications as Directed Energy Weapons (DEW). However, these technologies are particularly expensive and complex to implement. On the other hand, high power fiber lasers offer many advantages: compactness, lower mass allowing easier deployment, lower production and operating costs, higher reliability over time, resistance to severe and even extreme environments, etc. Thus, they are perfectly suited to operational environments, for example embarked on ships or moving vehicles.
Today, phasing multiple fiber sources of several kilowatt by optical combination can already result in laser architectures delivering powers of up to tens of kilowatts. Ongoing works are carried out to expend this power to several 100 kW which is the estimated power required to disable a drone.
Gaining power with combined fiber lasers
Gaining power with several combined laser beams can be done by superimposing lasers of distinct wavelengths in a uniform laser output. It can be done using the Coherent Beam Combination (CBC) technique, where the combination of several lasers by real-time control of their relative phases allows to permanently maintain constructive interferences and thus guarantee maximum power efficiency during the combination.
Exail developed dedicated low frequency phase modulators, and matching RF amplifiers, for high-power coherent beam combining applications. These modulators are necessary to compensate the dispersion along the optical paths occuring within optical fibers. These phase variations are due to the laser operating environment, for example slow temperature changes. To maintain a constructive power combination of several beams, a low frequency phase modulation up to MHz is necessary. Therefore, Exail has developed low frequency phase modulators for different wavelength bands: around 1060 nm, 1550 nm, and 2000 nm. An RF amplifier is also available in addition to the modulator: it meets both the specifications of the RF signal, and perfectly matches the modulator, both mechanically and electrically.
In addition to the low-frequency phase modulators, Exail also offers optical delay lines (Variable Optical Delay Line – VODL), based on micro-optic assemblies. They are necessary to align the optical paths of the different laser channels. As with phase modulators, the VODL are available in the same wavelength range where lasers are operating.
The Modbox CBC is another optimized multi-channels phase modulation solution for multibeam coherent combination. Such a turnkey laser seeder embeds multi-beams phase modulators arrays, and Variable Optical Delay Line in option. This integrated modulator solution ensures high optical performances thanks to the selection of high grade modulators, ensuring a reduced optical path between the different optical channels.
Coherent Beam Combination
And it can also be done with the Spectral Beam Combination (SBC) technique where different laser beams emitting a continuous signal, centered on distinct wavelengths, are superimposed by adaptive optics. The result of the laser system is a uniform intensity distribution and an optical signal with power proportional to the number of laser beams combined.
Spectral Beam Combination
However, the maximum transmitted optical power in each fiber amplifier is severely constrained by the non-linear Brillouin effect. This effect, also called Stimulated Brillouin Scattering (SBS) can be mitigated by broadening the laser linewidth (to spread the spectral power density some GHz around the central wavelength) by the means of an electro-optical phase modulator. To create these side bands, three different RF source can be combined with the electro-optical phase modulator. A sinusoidal electric signal, a “white noise” or a telecom “PRBS – Pseudo- Random-Bit-Sequence”. With this technique, optical output higher than a kilowatt continuous signal can be reached, spectrally modulated, and combined with other fiber amplifiers to obtain the final and expected laser power level.
Towards the development of Directed Energy Laser using fiber lasers at 2 µm
Today, new applications are rising for fiber lasers in the 2 µm range, which has driven recent innovation in specialty fiber technology. Even if still more expensive than the 1 or 1,5 µm lasers, the 2 µm lasers address has a considerable asset for applications on a battlefield, as it is “eye-safe”, meaning that our eye is sensitive to its beam and instinctively closes itself when touched. That is not the case with thinner laser beams that can reach deep inside our eyes and damage it irreversibly, even after being reflected from a reflective surface. It makes any Directed Energy Weapons (DEW) at less than 2 µm quite dangerous to handle.