We present an efficient way to remove unwanted Amplified Stimulated Emission (ASE) in high-power fiber lasers and amplifiers using intracavity Chirped Tilted Fiber Bragg Grating (CTFBG) filters. The grating is written with tilted fringes so that the unwanted ASE is reflected into the fiber cladding where it is no longer amplified. Depending on the desired emission wavelength and active fiber, one or several filters are spliced within the active fiber to suppress ASE before it reaches a detrimental power. Numerical simulations clearly show that adding the filters allows amplification in configurations that would just be impossible due to the onset of ASE. The filter bandwidth and extinction, and the maximum allowed active fiber length between each filter are also computed depending on the core/cladding diameter ratio of the active fiber used and the targeted emission wavelength. As an example, a fiber laser at 1018 nm is assembled in a 20/400μm core/cladding diameter ytterbium fiber that is cladding pumped at 976 nm. Two CTFBGs with 20 dB attenuation from 1025 nm to 1070 nm are spliced within the 6-meter-long ytterbium fiber. 432 W of laser emission at 1018 nm is efficiently achieved at 77% slope efficiency with respect to the absorbed pump power. The extinction between the 1018 nm signal and the ASE is greater than 50 dB. Removing the ASE filters from the cavity clearly leads to only self-pulsation of the ASE between 1030 nm and 1050 nm, no generation of 1018 nm light was possible. The measured thermal slope of the filters shows scalability above the kW level. Demonstration at 1908 nm with a 25/400 core/cladding diameter thulium doped fiber is also done. Tests were done to inscribe the CTFBG directly in an ytterbium fiber for simpler implementation and avoid additional splicing.
Double-clad fibers (DCF) found in kilowatt-class fiber lasers typically have a second cladding made of fluoroacrylate. At high power, thermal damage or accelerated aging of this material becomes an issue. The operating temperature of the fluoroacrylate coating is found to be highly sensitive to the numerical aperture (NA) distribution of the pump light flowing through the fiber. Characterization of this effect with an optical loss measurement is impractical as this loss remains typically very low. Measurement of the coating temperature for a given input power and far-field distribution is much more sensitive. Furthermore, it directly gives the parameters that are key to the design of a high-power fiber laser. A system for the measurement of the thermal slope of DCF fibers and high-power fiber components has been built and tested. This system allows varying the input power and the source NA under high power with a unique splice to the device being tested. To achieve this, different types of fiber-coupled pump diodes are spliced to the inputs of a pump combiner. Fiber tapers are used to fine tune the sources’ NA. By turning on different diodes, the NA of the injected pump light can be varied. The thermal slope for a given NA can then be measured with a thermal camera and a power meter. Measurements show differing thermal slopes of DCF measured before and after a damp heat tests. These thermal slope variations are stronger when operating at a high numerical aperture.
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