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Femtosecond pulse inscription of a selective mode filter in large mode area fibers

: Krämer, Ria G.; Voigtländer, Christian; Freier, Erik; Liem, Andreas; Thomas, Jens U.; Richter, Daniel; Schreiber, Thomas; Tünnermann, Andreas; Nolte, Stefan


Hendow, S.T. (Ed.) ; Society of Photo-Optical Instrumentation Engineers -SPIE-, Bellingham/Wash.:
Fiber Lasers X. Technology, Systems, and Applications : San Francisco, California, USA; February 02, 2013
Bellingham, WA: SPIE, 2013 (Proceedings of SPIE 8601)
ISBN: 978-0-8194-9370-5
Paper 86010S
Conference "Fiber Lasers - Technology, Systems, and Applications" <10, 2013, San Francisco/Calif.>
Conference Paper
Fraunhofer IOF ()
fiber Bragg grating; fiber optics components; ultrafast processes in condensed matter; fiber laser

We present a selective mode filter inscribed with ultrashort pulses directly into a few mode large mode area (LMA) fiber. The mode filter consists of two refractive index modifications alongside the fiber core in the cladding. The refractive index modifications, which were of approximately the same order of magnitude as the refractive index difference between core and cladding have been inscribed by nonlinear absorption of femtosecond laser pulses (800 nm wavelength, 120 fs pulse duration). If light is guided in the core, it will interact with the inscribed modifications causing modes to be coupled out of the core. In order to characterize the mode filter, we used a femtosecond inscribed fiber Bragg grating (FBG), which acts as a wavelength and therefore mode selective element in the LMA fiber. Since each mode has different Bragg reflection wavelengths, an FBG in a multimode fiber will exhibit multiple Bragg reflection peaks. In our experiments, we first inscribed the FBG using the phase mask scanning technique. Then the mode filter was inscribed. The reflection spectrum of the FBG was measured in situ during the inscription process using a supercontinuum source. The reflectivities of the LP01 and LP11 modes show a dependency on the length of the mode filter. Two stages of the filter were obtained: one, in which the LP11 mode was reduced by 60% and one where the LP01 mode was reduced by 80%. The other mode respectively showed almost no losses. In conclusion, we could selectively filter either the fundamental or higher order modes.