Coherent beam combination for Directed Energy Laser

Published on October 20, 2023 – News

Exail offers an optimized solution for the accurate, adjustable, and reliable phaselock modulation between several laser beams, using the coherent beam combining technique: the ModBox CBC.

The development of Directed Energy Lasers (DEL) for defense applications is accelerating tremendously. High power fiber lasers have many advantages over chemical or solid-state lasers for such applications: compactness, lower mass for easier deployment, lower production and operating costs, higher reliability over time, resistance to severe and even extreme environments, etc. 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 for defense applications.

The Coherent Beam Combining (CBC) technique is the most efficient solution to gain power with several combined laser beams as it ensures the combination of several lasers by real-time control of their relative phases. It thus allows to permanently maintain constructive interferences and thus guarantee maximum power efficiency during the combination, a crucial requirement to develop lasers powerful enough to disrupt a drone for example.

An integrated solution to combine several beams with individual phase adjustments

The Modbox CBC-1064 nm is an optimized multi-channels phase modulation solution for multibeam coherent combination. The ModBox CBC integrates up to 8 independent channels with phase and delay tuning capabilities. Each channel allows to adjust the temporal phase for synchronization of the different laser beams. The design integrates – in each channel – Exail proprietary low frequency phase modulator with its matching RF electronic (Exail driver DR-VE) as well as a tunable optical delay line that is selected for high accuracy and a wide delay adjustment range. Special care is taken for the assembly of the ModBox: the Exail electro-optical phase modulators integrated into each system are screened from the company regular production to ensure very low insertion loss, high polarization extinction ratio, low residual amplitude modulation, and high phase modulation stability. The selection of high-grade modulators also secures a reduced optical path between the different optical channels, thus ensuring high optical performances. The phase modulators are also locked in temperature, to ensure a lower drift over time and to limit the photorefractive effect.

Additionally, Exail phase modulator is well known to be the best planar phase modulator in the NIR featuring the highest optical input power handling capability. The NIR-MPX-LN (Near InfraRed – Modulator Phase with X-cut, on LiNbO3) is widely used and represents a commercial success for such applications.

Exail’s Modbox CBC integrates the optical delay lines (Variable Optical Delay Line – VODL) as an option. It is a patented technology based on micro-optic assemblies used to align the optical paths of the different laser channels, thanks to a very precise and stable control of the optical delay (from 0 ps to 600 ps, with a resolution of a few femtoseconds). The optimal optical delay between channels (only tenths of picoseconds of variation) is also ensured by a fine tuning of the fibers lengths of each channel within the system. The ModBox-CBC is coming with a Graphical User Interface (GUI) allowing to control Optical Delay Lines delays. Each channel can be adjusted individually.

Companies manufacturing laser architectures for applications in DEL can rely on Exail ModBox CBC as a proven and reliable solution to develop and test their first prototypes. They can then integrate the sub-components, the phase modulator, the RF driver and the tunable optical delay line, into their own system that can count up to hundreds of channels to reach the kilowatts of power required. “Already a number of companies chose Exail modulation solutions to develop their DEL laser applications.” highlights Nicolas Bourriot, product line manager at Exail.

Modbox CBC-2000 nm for the development of Directed Energy Laser using fiber lasers at 2 μm

Today, new applications are rising for high power fiber lasers in the 2 μm range, which has driven recent innovations in specialty fibers technology. Even if still more expensive than the 1 or 1.5 μm lasers, the 2 μm lasers offer a considerable asset for applications in Directed Energy Laser. This wavelenght 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 2 μm less dangerous to handle. The ModBox CBC is available in the 2000nm range.

Exail also offers a wide range of specialty fibers for the development of 2 μm fiber lasers for DEL. They are co-doped fibers with thulium (Tm), holmium (Ho), or both (Tm/Ho), with on demand customization: any core size (4 μm to 25 μm), one-two or three claddings, any kind of coating (including innovative ones for high temperature or harsh environment) and any kind of doping level. The fiber can also contain a polarization-maintaining (PM). Matching passive fibers are available too.