Romain Dauliat

Inner cladding microstructuration based on symmetry reduction for improvement of singlemode robustness in VLMA fiber

Very large mode area, active optical fibers with a low high order mode content in the actively doped core region were designed by removing the inner cladding symmetry. The relevance of the numerical approach is demonstrated here by the investigation of a standard air-silica Large Pitch Fiber, used as a reference. A detailed study of all-solid structures is also performed. Finally, we propose new kinds of geometry for 50 μm core, all-solid microstructured fibers enabling a robust singlemode laser emission from 400 nm to 2200 nm.

Optical fiber microstructuration for strengthening single-mode laser operation in high power regime

Abstract. We propose an in-depth investigation of all-solid microstructured optical fibers for the development of very large mode area (VLMA) fiber lasers. The inner cladding microstructure of these VLMA fibers is carefully optimized in order to get a robust single-mode laser operation in the high power regime. We describe the numerical approach used to devise a novel kind of fiber structures, the core of which should be larger than 50 μm while showing an improved single-mode emission compared to that of the state-of-the-art large pitch fibers. With the aim of overpassing the limitations of chemical vapor deposition techniques, we opted for a manufacturing process called Repusil, based on the sintering and vitrification of doped powders. Then, our opto-geometrical considerations result from the optical properties offered by this method and the use of the stack and draw. Finally, we present our very first fabrication for the proposed all-solid microstructured fibers in which a laser emission of 52 W in a continuous wave regime was obtained.

Broadband single-mode single-polarization passive fully aperiodic large-pitch fibers

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.

140 μm single-polarization passive fully aperiodic large-pitch fibers operating near 2 μm

In this paper, we demonstrate a single-polarization feature out of passive very-large-mode-area fully aperiodic large-pitch fibers. It has been previously shown theoretically that one of the two polarizations of the fundamental mode is selectively coupled to a cladding mode. This coupling does not require fiber bending, which permits us to avoid any decrease in mode effective area. The coupling is achieved owing to boron-doped silica inclusions implemented into the microstructured cladding and acting as stress-applying parts. This mechanism has been assessed experimentally in this work using fibers of two different core diameters: 60 μm and 140 μm, providing mode field areas of 3637u2009u2009μm2 and 14,590u2009u2009μm2, respectively, at 1942 nm. The tested fibers have a length of 45 cm and an outer diameter exceeding 1 mm, yielding rod-type fibers. Each sample has been characterized using an unpolarized laser source emitting at 1942 nm. This laser, based on a thulium-doped large-mode-area step-index fiber, has a spectral bandwidth of about 0.5 nm. After passing through a piece of the passive fiber, a polarization extinction ratio higher than 16 dB has been achieved.

High-power passively mode-locked dissipative soliton fiber laser featuring cladding-pumped non-CVD thulium-doped fiber

We are reporting on the characterization of a thulium-doped fiber laser applying new powder technology in the mode-locking regime. A high average output power of 185 mW at a repetition rate of 9 MHz was achieved directly from the oscillator, which resulted in 21 nJ of pulse energy. The single-pulse operation regime was confirmed by careful numerical modeling of the laser cavity.

Single-polarization large-mode-area fibre at 1030nm and 1550nm

High-power fibre laser systems generally rely on microstructured fibres combining an effective single-mode emission and a very large-mode area (V-LMA, Dcore > 50μm). Most of time, the targeted applications (one can cite for instance the frequency conversion [1]) require a crucial control of the state of polarization (SOP) of the emitted beam, leading to the development of polarization-maintaining (PM) or single-polarization (PZ) fibres owing to the use of stress applying parts (SAP) [2, 3]. The current study has been focusing on judiciously introducing SAP into VLMA fibres to control the SOP of the fundamental mode (FM). Fully-aperiodic large-pitch fibres (FA-LPF) which have proved their potential to delocalize efficiently the HOM out of the gain region [4] are used here as basic concept. First, a numerical approach has been led consisting in adding SAP in the fibre structure in order to obtain enough birefringence while maintaining the effective single-mode operation. A single-polarization is then obtained due to a selective modal coupling of one SOP of the FM with a cladding HOM as shown on Fig. 1(a-d). In a second time, the designed fibre has been fabricated. Several samples have been drawn with core diameter from 40μm to 140μm.

Experimental study of the mode instability onset threshold in high-power FA-LPF lasers

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 100xa0μm, 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.

Precompensation of the thermal-induced refractive index changes into a fully aperiodic LPF for heat load resilience

In this paper, a strategy consisting of precompensating the thermal-induced transverse refractive index changes is undertaken to push further the appearance threshold of a multimode regime. First, a standard air-silica large pitch fiber (LPF) and a fully aperiodic large pitch fiber are confronted in regard to their heat load resilience and capabilities for single-mode emission. Thereafter, slight refractive index depressions are judiciously introduced into the active core area. This approach enhances the delocalization of the high-order modes even under severe heat load levels. This combination of aperiodic cladding microstructuration and index-precompensation theoretically increases the multimode regime threshold while preserving large mode field areas. This investigation is performed at 1.03 and 2 μm operating wavelengths.

Ultra Large Mode Area Fibres with Aperiodic Cladding Structure for High Power Fibre Lasers

This communication presents the latest designs, fabrication steps and first results of large mode area fibres with aperiodic cladding structure for high power singlemode emission. Pre-compensation of thermal loading and first laser emission are detailed.