Fiber-based laser and amplifier systems provide high output power, excellent beam quality and easy handling by utilizing a setup consisting of fiber components like signal-pump combiners and cladding-light strippers. Large-mode-area fibers are used in these systems to improve the performance while maintaining single-mode operation. Compared to pure single-mode fibers, these fibers always guide higher order modes, which can have an impact on the system’s performance. An instrument was developed based on the S2-method, which is able to perform high-speed, in-line measurements of the modal composition and polarization extinction ratio. This device is used to monitor the fiber component manufacturing process using large-mode-area fibers in real-time, enabling us to improve critical process steps and, thereby, optimize the manufactured components, namely signal-pump combiners and fiber endcaps.
We present the development of a fiber-based high-efficiency laser amplifier (HELA) as a demonstrator for future high throughput satellite communication systems operating in the 1µm wavelength range. The work is conducted in the context of ESA’s ARTES ScyLight framework programme (SeCure and Laser Communication Technology) and more specifically within its HydRON project. Combining the experiences of in-house manufactured optical fiber components and of laser developments for space applications, an all-fiber amplifier solution was designed. The realized laboratory testbed simultaneously amplifies a 10-channel seed input from the mW-level up to a total output power of 100W. Leveraging the benefits of advanced fiber technology, an enhanced wall-plug efficiency of about 29% has been achieved.
Fiber-based laser systems enable high output power in combination with diffraction limited beam quality. Their output power is generally limited by the onset of nonlinear effects. The chirally coupled core (CCC) fiber provides a large mode field diameter while also suppressing higher-order-modes. This is needed to further increase a laser’s output power and maintaining single-mode operation. However, the integration of specialty fibers in an all-fiber laser setup is in most cases not possible because suitable fiber components are not available. We report on the development of a cladding light stripper and a signal-pump combiner with integrated 34/250-µm CCC fibers which allow for the development of spliceless all-fiber amplifier systems. The cladding light stripper is manufactured by structuring the CCC-fiber’s cladding using a CO2-laser to interrupt pump light guiding within the cladding. The cladding light stripper enables a stripping efficiency of 19 dB and was tested up to a stripped optical power of 100W, which is sufficient to enable kW-class amplifier systems. The signal-pump combiner relies on a side-pumped design with four pump input fibers. Its characterization reveals a pump-to-signal fiber coupling efficiency of 90% and a signal-to-pump isolation of 30 dB. Component stability was tested at a pump input power of 500W. An S2 -measurement confirmed that the spatial mode content of the signal light propagating through the CCC-fiber-based signal-pump combiner remains unaffected. Furthermore, a signal-pump combiner was subjected to temperature cycles between -5 and 85 °C over a time period of <1000 h and showed no degradation.
Current research focuses on very-large-mode-area fibers (core diameters of 34 μm and above) and all-fiber laser systems to deliver high output power with linear polarization, low noise properties, narrow-band linewidth and high fundamental mode content. All-fiber systems have the advantage of low-maintenance and alignment-free operation. Because power scaling in fiber-based amplifiers is limited by non-linear effects like stimulated Brillouin scattering, very-large-mode-area fibers such as specialty fiber designs like the chirally coupled core (CCC) fiber are investigated. This fiber type offers a signal core of 34 μm or larger while also providing a near single-mode output beam quality. It thereby enables further power scaling in systems that are limited by such nonlinear effects. However, efficient components such as signal-pump-combiners (SPC) with this fiber type need to be developed. The SPC couples the required pump light of multiple high power laser diodes into the gain fiber of the laser system. We report on the development of a SPC with an integrated 34=250 μm feed-through CCCfiber with a pump-to-signal fiber coupling efficiency of 90% and three input pump fibers with a signal-to-pump isolation of 30 dB. The device is tested with an input power of up to 380 W. In addition, different experiments for monolithic implementation of the CCC-fiber type into systems that rely on standard polarization-maintaining (PM) fibers are conducted. We show the polarization maintaining behavior (polarization extinction ratio (PER) < 19 dB over several hours) of the fiber by imprinting externally induced birefringence on the fiber. Experiments with all-fiber setups using the CCC-fiber and a step-index PM-fiber show a PER of < 15 dB with reduced long-term stability.
The power consumption of the laser systems is an important aspect of optical satellite communication technology. We present an optical amplifier for a WDM optical communication system with a simultaneous multichannel amplification in a single fiber in the 1 μm wavelength range. The desired enhanced wall-plug efficiency of ∼30% can only be achieved through fiber technology. Combining the experiences of in-house manufactured optical fiber components and of laser developments for space applications, an all-fiber amplifier solution was designed. Each of the 10 channels can be efficiently amplified up to a total power level of 100W.
Specialty fibers such as chirally-coupled-core fibers (3C®-fiber) show a high potential for further power scaling of single-frequency fiber amplifiers. Especially, the application of gravitational wave detectors requires a high optical output power at low noise characteristics. The output power of fiber-based single-frequency amplifiers is typically limited by nonlinear effects (e.g. stimulated Brillouin scattering). In general, to reduce the impact of nonlinearities, the mode area of the fiber core is enlarged and 3C®-fibers have been specifically designed to enable single-mode operation with a large mode area core. This fiber type consists of a step-index fiber structure, whose signal core is additionally chirally surrounded by one or more satellite cores. Because of the phase matching and the helical geometry, the higher order modes are pulled out of the signal core, which allows a high-purity modal content in the core. The development of compact all-fiber lasers in conjunction with specialty fibers combines the advantages of both techniques. For the first time, we demonstrate a spliceless all-fiber amplifier, where all optical components are directly integrated in a single Yb3+-doped 3C®-fiber. Such a spliceless laser design allows a compact and robust architecture using specialty fibers, while maintaining excellent beam properties. At an output power of 336 W, a fundamental mode content of 90.4% was demonstrated. This work emphasizes the suitability of 3C®-fibers in high-power laser and amplifier systems and the potential as laser sources for the next generation of gravitational wave detectors.
We present the development of an all-fiber side-fused signal-pump-combiner based on an integrated 3C R feedthrough fiber. This specialty fiber uses a 34 μm single-mode core and shows great potential to enable further output power scaling while maintaining high beam quality. The side-fusing technique has the advantage of an uninterrupted signal core and can be used in co- and counter-pumped fiber lasers and amplifiers. The signal-pump-combiner was operated up to an input power of 600W from four pump fibers and coupling efficiencies of 79% were achieved. The component was additionally investigated by computer tomography imaging, which revealed that the cladding structure of this specialty fiber prevented the required level of glass fusion of the 3C R fiber with the pump fibers. The investigation will help to further increase the pump coupling efficiency of the signal-pump-combiners. This represents the first step of developing all-fiber and high power capable laser systems based on the 3C R fiber.
The output power of fiber-based single-frequency amplifiers, e.g. for gravitational wave detectors, is typically limited by nonlinear effects (e.g. stimulated Brillouin scattering). In addition to a high output power, long-term stable and less complex laser systems are required. It has been shown that all-fiber amplifier systems can be a suitable option to avoid power scaling problems of single-frequency solid-state lasers with injection locking. Chirally-coupled-core (3C®) fibers have been specifically designed to enable single-mode operation with a large mode area core to overcome these limitations. 3C®-fibers consist of a step-index fiber structure, whose signal core is additionally chirally surrounded by one or more satellite cores. For this purpose, the all-solid design of 3C®-fibers allows a manufacturing process of fiber-based components. We present various optical components based on 3C®-fibers for the realization of a single-frequency all-fiber amplifier. These amplifiers typically consist of a mode field adapter (MFA), cladding light stripper (CLS) and pump combiner (PC) to minimize the excitation of higher order modes, remove residual pump light and optimize the coupling efficiency of the pump light in the 3C®-fibers. The components have been specifically designed for the first time with 3C®-fibers and tested according to their performance. As a first prototype, a robust and monolithic fiber amplifier with an ytterbiumdoped 3C®-fiber in combination with commercially available standard fibers was developed. Overall, the fiber amplifier achieves an optical output power of 165W in a linearly polarized TEM00-mode. This work emphasizes the high potential of amplifiers based on 3C®-fibers as laser sources for the next generation of gravitational wave detectors and demonstrates that compact and robust amplifiers can be realized using 3C®-fibers.
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