In recent years, the demand for high-power, polarization maintaining single mode lasers operating in the 1.55 µm wavelength band significantly increased due to technological advances in free space communication, coherent LiDAR, quantum computing, and remote sensing. These applications benefit from the compactness, robustness, and efficiency of diode-pumped Erbium Ytterbium-doped fiber amplifiers (EYDFAs) and triggered the development of EYDFs with enhanced performance. In this work, we demonstrate a new single-mode PM EYDF with robust single-mode operation beyond 20 W output power and discuss the remaining challenges to scale the power further and how we plan to mitigate those.
Continued recent developments in Thulium- (Tm) doped silica fiber design have enabled average power scaling of 2 µm fiber laser system beyond the kW-level. One approach to furthering this development is to maximize the slope efficiency of Tm-doped fiber lasers by selecting highly-doped double-clad fibers (TDF’s) so as to promote the cross-relaxation process. The success of this approach was exemplified by Tumminelli et al. who employed an all-halide vapor-phase fabrication process to produce a single-mode (SM) fiber with a Tm concentration of 8.5 wt% and demonstrated ~70 % slope efficiency. In this work, we report what we believe to be the first high-concentration (8 wt% Tm), double-clad (DC) large-mode area (LMA) Tm-doped fiber (TDF), which was manufactured by the solution-doping MCVD process. Critical performance such as slope efficiency and lasing wavelength are characterized and compared to legacy LMA-TDF-25P/400 fiber.
Novel extra-large mode area active and passive fibers deigned to achieve up to 100 mJ and > 1 kW output power in industrial fiber laser systems will be presented. Three Yb-doped XLMA fiber sizes will be proposed, able to offer high extraction efficiency, MW peak power handling, manageable thermal load, excellent spliceability and stable beam profile. Fiber performance capabilities will be discussed with numerical simulation results. Active and passive fiber prototypes will be manufactured and tested. Key design aspects and performances including efficiency, beam profile, cladding absorption and photodarkening will be reported.
One of the current challenges towards the development of ultrafast 2 microns all-fiber laser systems delivering transform-limited pulses is to manage dispersion and nonlinearities which are well-known limiting factors in fiber-based systems due to their negative impact on pulse duration and shape.
Here, we present what we believe to be, to the best of our knowledge, the first all-solid step-index dispersion tailored fiber designed with anomalous dispersion around 2 microns. This all-solid, step-index ultra-high numerical aperture (UHNA) fiber offers an efficient and simple alternative compared to existing approaches such as free-space optical systems or micro-structured fibers that are complex to manufacture and handle. The combination of highly Ge-doped core with a small core diameter allows tailoring the material and waveguide components of the dispersion to reach the anomalous dispersion required by the application.
In this work, details will be provided using experimental and calculated values via the example of a non-PM UHNA fiber with 2.45 microns core and 0.34 NA. This fiber was designed to achieve anomalous dispersion of -45 ps/(nm.km) at 2 microns. It will be shown that the UHNA fiber design can be further tuned to achieve specific values of anomalous dispersion and dispersion slope. The fiber performances were confirmed using a 2 microns chirp-pulsed fiber amplifier where the pulse duration was measured at 24 ps and 4.3 ps without and with the UHNA fiber respectively. A PM-UHNA fiber design is currently being developed and will be characterized and tested following a similar fashion.
Large-mode area (LMA) thulium-doped fibers (TDF) are one of the key components when designing 2μm laser and amplifier systems aiming to further scale deliverable output powers. Current design limitations of LMA TDF’s affecting optical-to-optical efficiency and output beam quality are well-understood. In the present work, design optimizations focused on the core and pedestal waveguides of the active fiber are proposed. Using experimental and numerical tools, the effect of splice-induced heat on the refractive index profile of the active fiber is investigated. We demonstrate that fibers designed with larger pedestal-to-core ratios suffer less index distortions during splicing allowing the end-user to achieve high coupling efficiencies and high beam qualities in a reliable fashion.
Beam delivery fibers have been used widely for transporting the optical beams from the laser to the subject of irradiation in a variety of markets including industrial, medical and defense applications. Standard beam delivery fibers range from 50 to 1500 μm core diameter and are used to guide CW or pulsed laser light, generated by solid state, fiber or diode lasers. Here, we introduce a novel fiber technology capable of simultaneously controlling the beam profile and the angular divergence of single-mode (SM) and multi-mode (MM) beams using a single-optical fiber. Results of beam transformation from a SM to a MM beam with flat-top intensity profile are presented in the case of a controlled BPP at 3.8 mm*mrad. The scaling capabilities of this flat-top fiber design to achieve a range of BPP values while ensuring a flat-top beam profile are discussed. In addition, we demonstrate, for the first time to the best of our knowledge, the homogenizer capabilities of this novel technology, able to transform random MM beams into uniform flat-top beam profiles with very limited impact on the beam brightness. This study is concluded with a discussion on the scalability of this fiber technology to fit from 50 up to 1500 μm core fibers and its potential for a broader range of applications.
Single-mode (SM) kW-class fiber lasers are the tools of choice for material processing applications such as sheet metal cutting and welding. However, application requirements include a flat-top intensity profile and specific beam parameter product (BPP). Here, Nufern introduces a novel specialty fiber technology capable of converting a SM laser beam into a flat-top beam suited for these applications. The performances are demonstrated using a specialty fiber with 100 μm pure silica core, 0.22 NA surrounded by a 120 μm fluorine-doped layer and a 360 μm pure silica cladding, which was designed to match the conventional beam delivery fibers. A SM fiber laser operating at a wavelength of 1.07 μm and terminated with a large-mode area (LMA) fiber with 20 μm core and 0.06 NA was directly coupled in the core of the flat-top specialty fiber using conventional splicing technique. The output beam profile and BPP were characterized first with a low-power source and confirmed using a 2 kW laser and we report a beam transformation from a SM beam into a flat-top intensity profile beam with a 3.8 mm*mrad BPP. This is, to the best of our knowledge, the first successful beam transformation from SM to MM flat-top with controlled BPP in a single fiber integrated in a multi-kW all-fiber system architecture.
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.
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.
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.
Optical fibers that support single mode operation while achieving large mode areas are key elements for scaling up
optical powers and pulse energies of fiber lasers. Here we report on a study of the modal properties of a new-generation
of polarization maintaining large-mode-area photonic crystal fibers based on the spectrally and spatially resolved (S2)
imaging technique. A fiber designed for Tm fiber laser system single mode operation in the 2μm spectral range is
demonstrated for coiling diameters smaller than 40cm. At shorter wavelengths in the 1.3μm range, efficient higher order
mode suppression requires tide coiling to about 20cm diameters.
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