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.
Until recently, chemical lasers or free-electron lasers, then solid-state lasers, were mainly considered for such applications as Directed Energy Lasers (DEL). 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.
Coherent Beam Combination
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 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 Laser (DEL) system at less than 2 µm quite dangerous to handle.
Exail leverages 20 years of experience in manufacturing doped fibers. Its portfolio counts a range of Tm & Ho doped fibers for amplifiers and fiber lasers addressing the 2 µm market, as well as Fiber Bragg Gratings (FGBs), for the design of fiber laser cavities in specific wavelengths. They are key components for the development of laser architecture in the 2 µm range that are as efficient as possible, and that can deliver high power (around 200 W).
C. Louot and al. IEEE Photonics (2023)
In recent years, fibered components such as doped fibers and FBGs have become the most adapted technologies to develop more powerful laser systems. FBGs in particular have been improved considerably to adapt to the high-power laser applications, in the 2 µm range but also in other wavelengths.
