Fabrication of 2 kW-Level Chirped and Tilted Fiber Bragg Gratings and Mitigating Stimulated Raman Scattering in Long-Distance Delivery of High-Power Fiber Laser

Chirped and tilted fiber Bragg gratings (CTFBGs) have attracted a lot of attention in stimulated Raman scattering (SRS) suppression of high-power fiber lasers. However, the laser power handling capacity seriously limits their applications. In this paper, by optimizing the inscription parameters and post-processing strategy, we fabricate a large-mode-area double-cladding CTFBG with a thermal slope of ~0.015 °C/W due to the low insertion loss of about 0.15 dB, which make it possible for direct kilowatt-level application. A 2 kW-level fiber laser oscillator is employed to test the CTFBG, and a series of experiments have been carried out to compare the effect of SRS mitigation in high-power fiber laser long-distance delivery. In addition, the influence of CTFBGs on laser beam quality is studied for the first time. Experimental results indicated that the CTFBG could effectively mitigate SRS and has no obvious influence on laser beam quality. This work opens a new opportunity for further power scaling and the delivery of high-power fiber lasers over longer distances.

Effect of Turbulence on the Wavefront of an Ultrahigh Intensity Laser Beam

Ultrahigh intensity lasers face thermal management issues that limit their repetition rates. The key challenge is to efficiently evacuate the heat deposited in the amplifier by the optical pumping without impacting the output laser beam quality. The amplifier can have a multislab geometry where the laser beam crosses successive amplifying slabs and the cooling channels that separate them. This work investigates numerically how a cryogenic cooling of the amplifier by turbulent channel flows may affect the wavefront of the laser beam. To this end, large eddy simulations (LESs) representative of the amplifier cooling are performed using TrioCFD, a code developed by the CEA. First, validation simulations are carried out for heated channel flows, allowing comparisons to direct numerical simulation (DNS) results from the literature. Then, LESs of an open turbulent channel flow cooling two slabs are conducted using conjugated heat transfer between the solid and the fluid. The phase distortions, mean and fluctuations, induced by the inhomogeneous and turbulent temperature field are computed directly from the LES. A moderate although non-negligible effect of the turbulence on the laser wavefront was found. This optical effect increases when the slab heating increases. A comparison to the Sutton model, widely used in aero-optic studies, was performed, and its applicability was found limited for this problem. For the first time, TrioCFD is used to address the question of the beam impact of the cooling of laser amplifiers, and it has proven to be a valuable tool for such application.

M6 formalism – generalization of the laser beam quality factor M2 to the 3D domain

Here we define a theoretical basis for the generalization of the beam quality factor M2 to three-dimensional (3D) space, which we call M6 formalism. The formalism is established through the use of examples of multifocal and Axicon optical systems to illustrate discrete and continuous axial beam shaping, respectively. For the continuous case, we expand the definition of the Rayleigh range to incorporate a quality factor having both axial and transverse components $M_{{\rm{add}}}^2$ and M2. Using geometrical ray tracing simulations, a proportion factor C is found to empirically describe the axial quality factor $M_z^2$ of an optical setup including an Axicon and a paraxial focusing lens with a Gaussian single mode input beam. Using our M6 formalism depth of focus (DOF) ranges are calculated for higher M2 beams, and are shown to be in good agreement with the simulated DOF range, demonstrating the usefulness of the M6 formalism for the design of real optical systems.