ABSTRACT
We report here on an experimental investigation of the temporal behavior of transverse mode instabilities into "fully aperiodic large-pitch fibers" (FA-LPFs) operated in high-power continuous-wave laser configuration. To ensure an effective transverse single-mode emission into FA-LPFs, a perfect index matching between the active core and the background cladding materials (Δn=0) is required. The original design of such fibers enables an effective transverse single-mode emission by strengthening the higher-order mode delocalization out of the gain region, even for high heat load levels, consequently leading to the improvement of the beam spatial quality. The study was conducted over fibers of various gain region diameters, from 58 to 100 µm, for a refractive index mismatch Δn of about +8×10-5. The emitted beam is characterized using both M2 measurements and time traces to study the changeover of a stable temporal behavior to an unstable one.
ABSTRACT
Two evolutions of fully aperiodic large-pitch fiber designs employing few stress-applying parts are presented. The induced elasto-optic stress discriminates the two orthogonal polarization modes (LP01x and LP01y) of the fundamental mode, selectively delocalizing one of them into the cladding via a suitable coupling to one or several cladding modes. This ensures the propagation of a single linear polarization mode. For the largest core dimensions, however, the applied stress can strongly influence the intensity distributions of core modes, and a tailored design process must thwart this. The polarization properties are investigated experimentally with core scalability over a large spectral bandwidth into passive structures, leading to the evidencing of a single-mode single polarization over a large span from 1 to 1.6 µm with a core dimension of 80 µm and, notably, at 1400 nm for a core dimension of 140 µm. The polarization extinction ratio is also determined.