Clemence Jollivet

Detailed Investigation of Mode-Field Adapters Utilizing Multimode-Interference in Graded Index Fibers

We present a detailed study of mode-field adapters (MFA) based on multimode interference in graded index multimode fibers. We have fabricated and characterized MFAs from a selection of commercially available single-mode and graded index fibers. Compared to existing techniques, the presented MFAs can be fabricated very quickly and are not limited to certain fiber types. Insertion losses of <; 0.5 dB over a spectral range of several hundred nanometers have been obtained, which is comparable or better than the industry standard.

Comparison of higher-order mode suppression and Q-switched laser performance in thulium-doped large mode area and photonic crystal fibers

We report the influence of higher order modes (HOMs) in large mode fibers operation in Q-switched oscillator configurations at ~2 μm wavelength. S(2) measurements confirm guiding of LP(11) and LP(02) fiber modes in a large mode area (LMA) step-index fiber, whereas a prototype photonic crystal fiber (PCF) provides nearly single-mode performance with a small portion of light in the LP(11) mode. The difference in HOM content leads to a significant difference in Q-switched oscillator performance. In the step-index fiber, the percentage of cladding light increases by 20% to >40% with increasing pulse energy to ~250 µJ. We accredit this degradation to saturation of the gain in the fundamental mode leading to more light generated in the HOMs, which is eventually converted into cladding light. No such degradation is seen in PCF laser system for >400 µJ energies.

Mode-resolved gain analysis and lasing in multi-supermode multi-core fiber laser

Multi-core fibers (MCFs) with coupled-cores are attractive large-mode area (LMA) specialty fiber designs that support the propagation of a few transverse modes often called supermodes (SMs). Compared to other LMA fibers, the uniqueness of MCF arises from the higher degrees of design space offered by a multitude of core-array geometries, resulting in extended flexibility to tailor SM properties. To date, the use of MCF as gain media has focused on lasers that operate in only one selected SM, typically the lowest order in-phase SM, which considerably limited the potential of these multi-core structures. Here, we expand the potential of MCF lasers by investigating multi-SM amplification and lasing schemes. Amplifier and laser systems using a 7 coupled-cores Yb-doped MCF as gain medium were successfully designed and assembled. Individual SM could be decomposed using the correlation filter technique mode analysis and the modal amplification factors (γi) were recorded. With access to amplification characteristics of individual transverse modes, a monolithic MCF laser was demonstrated that operates simultaneously on the two SMs carrying the highest optical gain.

Comparative study of light propagation and single-mode operation in large-mode area fibers designed for 2-μm laser applications

Abstract. Output performances of fiber-based optical systems, in particular fiber lasers and amplifiers, can be controlled using tailored fiber designs, gain profiles, and pump light overlap with the gain medium. Here, the performances of 2-μm light, propagating in three large-mode area fibers, a step-index fiber, a photonic crystal fiber (PCF), and a leakage channel fiber (LCF), designed to deliver a single-mode (SM) beam at this wavelength, were compared. Using the S2 imaging technique, the transverse mode content has been decomposed, and propagation losses, SM purity, and mode-field area (MFA) were measured for various input mode overlap and coiling diameters. It was experimentally demonstrated that coiling the PCF and LCF to 40 and 20 cm in diameter, respectively, resulted in efficient higher-order mode suppression, pure SM beam delivery, moderate (∼1  dB) coil-induced losses in the fundamental mode, and nondistorted, large MFA (∼1600  μm2) beam delivery.

Detailed Characterization of Optical Fibers by Combining

Spatially and spectrally resolved imaging (S2 imaging) and correlation filter technique (CFT) are two very different, widespread fiber mode analysis techniques. Both techniques have been successfully employed to decompose few-modes and multimode beams respectively. In this study, we present a novel experimental tool combining S2 imaging and CFT mode analyses in a unique system. We demonstrate that both methods are complementary with the ability to fully resolve scalar and vector-valued transverse modal fields. Using results from the combined experiment, mode powers (ρ2) evaluated from CFT analysis and S2 imaging are directly compared for a wide range of fiber beams (from single- to multi-mode). As a result, we experimentally identify the mode detection limit of each mode analysis and prove that S2 imaging accuracy range can be considerably increased employing an analytical mode evaluation method. The conclusion contains a table summarizing the expertise of each mode analysis.

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Novel monolithic fiber laser architectures utilizing large mode area (LMA) photonic crystal fiber (PCF) and fiber Bragg gratings (FBG) in conventional single-mode fibers (SMF) are presented. The main challenge is to address high cavity losses arising from the intrinsic 18-fold mode-field mismatch between the SMF and the active LMA PCF. Employing an all-fiber, robust and reproducible mode-field matching approach based on graded-index multimode fibers, we numerically and experimentally demonstrate that the SMF-to-LMA PCF coupling can be more than three-fold improved. This MFA approach is further implemented in monolithic fiber laser cavities combining FBGs in SMF and active LMA PCF. We demonstrate that cavity losses can be significantly mitigated when using appropriate MFAs resulting in a substantial increase of the laser output performances.

Imaging With Correlation Filter Mode Analysis

Abstract. We present here a method to create spectrally addressable phase masks by encoding phase profiles into volume Bragg gratings, allowing these holographic elements to be used as phase masks at any wavelength capable of satisfying the Bragg condition of the hologram. Moreover, this approach enables the capability to encode and multiplex several phase masks into a single holographic element without cross-talk while maintaining a high diffraction efficiency. As examples, we demonstrate fiber mode conversion with near-theoretical conversion efficiency as well as simultaneous mode conversion and beam combining at wavelengths far from the original hologram recording wavelength.

Monolithic Fiber Lasers Combining Active PCF With Bragg Gratings in Conventional Single-Mode Fibers

Recent design of large-mode-area leakage channel fiber is measured with low-attenuation and bend-induced single-mode propagation between 1 μm and 2 μm. We demonstrate remarkable low-loss, diffraction-limited output at 2 um for coiling radii <;30 cm.

Holographically encoded volume phase masks

Lasing operation of a tunable fiber laser employing a flat cleaved 7-core gain fiber fusion spliced with a standard fiber Bragg grating is investigated. We present the first modal decomposition during multicore laser operation.

Low-loss, single-mode propagation in large-mode-area leakage channel fiber from 1 to 2 μm

To scale to power levels of up to tens of kW with a few fiber lasers, the best candidates are large core fibers guiding a few large-area higher order modes with the outputs of these fibers combined into a single beam. However, in many applications it is desirable to convert these higher order modes into a Gaussian profile. Here, we propose a method to accomplish this task via single volume phase element. This element accepts multiple higher order mode beams and simultaneously converts and combines them to a single Gaussian profile in the far field.