
New 2 µm fiber laser sources for defense
For many years now, the French-German research institute of Saint-Louis (ISL) has been a partner of Exail in the rising field of 2 µm fiber lasers. Exail supports the emergence of innovations in this field with its extensive experience and portfolio.
The ISL is a bi-national institute jointly operated by the French
Republic and the Federal Republic of Germany. The mission of the ISL is
to develop technical innovations in the fields of defense and security,
from basic research to the development of preindustrial prototypes that
can be integrated in operative equipment (TRL from 1 to 6). A large
proportion of its research contracts are concluded with the DGA (French Defense Procurement Agency) and the BAAINBw (Federal Office of Bundeswehr
Equipment, Information Technology and In-Service Support) as well as
with industry at national and international levels.
Among other activities, the Institute focuses on “Controlling the
effects of projected energy and increasing its precision”. It includes
the development of new laser sources emitting in the wavelength range
from 2 µm – 5 µm for defense applications, based on new lasing materials
such as silica doped fibres (thulium – Tm, holmium – Ho). The ISL
obtained in 2021 some very interesting results using an Exail optic
fiber: a monolithic laser source with Tm-Ho codoped fiber in single
oscillator emitting 195 W at 2090 nm in continuous mode (A. Motard and
al. Optics Express, 2021). This result obtained at the occasion of a
research program with the DGA opened new horizons for the development of
Directed Energy Laser (DEL) in the 2 µm range using laser architecture
based on Tm/Ho doped fibers.

Exail leverages 20 years of experience in manufacturing doped fibers at
its Lannion site. Its portfolio counts a range of Tm & Ho doped
fibers for amplifiers and fiber lasers addressing the 2 µm market, but
also high-performance modulation solutions (LiNbO3 modulators and
ModBox) and integrated micro-optics benches adapted to this wavelength
range.
The main products delivered by Exail on which the ISL relies to
develop its research are the Tm doped fibers, the Tm-Ho doped fibers and
the 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). Fibered laser systems
have plenty of advantages for defense applications: they are compact and
robust, and resist to vibrations, which is key for embedded
applications. In the past five years, fibered components such as doped
fibers and FBGs have become the most adapted technologies to develop
more powerful laser systems.
High energy laser emitting in the sub-2 µm range (between 3 and 5 µm when combined and converted) also find application in the Directional Infrared Counter Measures (DIRCM) field, which consists in targeting surface-to-air missiles optoelectronic system with a high efficiency and highpower laser beam. A relatively small device, such as a compact and stable fiber laser system installed on an aircraft, can disrupt the missile seeker by focusing all of its infrared output on it. This effective method of jamming infrared missile seekers through the sensor confuses the missile guidance system with pulsing flashes of infrared energy. It prevents the tracking of the targeted aircraft which rely on the accurate tracking of the infrared emission coming from the thermic signature of the target.
In the air, the water molecules will absorb infrared light in one specific wavelength at 1940 nm. It is thus important to ensure that any laser source developed for such applications in free-space emits over 2000 nm. In Spring 2023, new results were published by Exail and the ISL, this time with a Single-Oscillator Monolithic Thulium-Doped Fiber Laser, allowing highefficiency emission at 2.09 µm, reaching 193 W (C. Louot and al. IEEE Photonics 2023).
"It is the first time that a laser architecture only based on thulium doped fibers show such an efficient emission, opening the way for developments relying only on thulium ions" adds Anne Dhollande.
Exail and the ISL are already working on a new type of fiber laser architecture aiming at reaching powerful emission for DEL applications, in the framework of a research project with French ONERA and funded by DGA. The laser architecture is based on a Tm doped fiber monolithic continuous source at 1940 nm, pumping a holmium-doped triple clad fiber emitting at 2.12 µm (delivered by Exail). Such a specific architecture should allow to increase the laser power while avoiding heat in the second inner cladding, and thus protecting the acrylate cladding.