We report on the first polarization maintaining single-mode fiber that delivers a flat-top intensity profile at 1050 nm. A high quality fundamental flat mode was obtained. We showed that our fiber can be considered as single-mode in practice with low confinement losses. Its birefringence was measured to be 0.6x10-4, and the PER was measured at more than 20 dB even for a 20 m fiber long. Strategies to enhance this birefringence preserving the flat top profile and the singlemode behaviour as well are also discussed.
Compactness, long term stability and no free-space alignment are important advantages of fiber lasers over bulky systems. These fiber lasers have also demonstrated their capability to deliver high-power pulses and are thus suitable for numerous applications. Nevertheless the intensity profile delivered usually has a Gaussian-like shape, which most of the time is sufficient, but it could be interesting, for many applications (laser-biological tissues interactions, heat treatment, industrial laser processing or for seeding large-scale laser facilities like Laser MegaJoule) to obtain a homogeneous intensity profile at the fiber laser output. Moreover several of these applications required a linearly polarized output beam. In order to achieve all these requirements we have developed and realized a new fiber design. This fiber is the first polarization maintaining single-mode fiber delivering a flat top intensity. A high quality flat mode was obtained at 1.05μm through the use of a well-tailored index profile and single-mode behavior was verified by shifting the injection and using the S² imaging. Moreover, boron Stress Applying Parts (SAPs) including in the cladding led to a birefringence of 0.6x10-4 and a measured PER better than 20dB even for a long fiber length (~20 m). Alongside the fabrication, we developed a simulation code, using Comsol Multiphysics®, to take into account the stress dependency induced by the SAPs. Further modeling allows us to present an effectively single-mode fiber design, delivering a top-hat mode profile and exhibiting a polarizing behavior.
In large scale laser facility dedicated to laser-matter interaction including inertial confinement fusion, such as LMJ or NIF, high-energy main amplifier is injected by a laser source in which the beam parameters must be controlled. For many years, the CEA has developed nano-joule pulses all-fiber front end sources, based on the telecommunications fiber optics technologies. Thanks to these technologies, we have been able to precisely control temporal shaping and phase-modulated pulse. Nowadays, fiber lasers are able to deliver very high power beams and high energy pulses for industrial needs (laser marking, welding,…). Therefore, we have currently developed new nanosecond pulses fibered amplifiers able to increase output pulse energy up to the mJ level. These amplifiers are based on flexible fibers and not on rod type. This allows us to achieve a compact source. Nevertheless the intensity profile of theses fibers usually has a Gaussian shape. To be compatible with main amplifier section injection, the Gaussian intensity profile must then be transformed into ‘top-hat’ profile. To reach the goal, we have recently developed an elegant and efficient solution based on a single-mode fiber which directly delivers a spatially coherent ‘top-hat’ beam. In the conference, we will present this mJ-class top-hat all-fiber laser system, the results and the industrial prototype which can be used as a front-end of high-power lasers or as a seeder for other types of lasers.