Long period gratings (LPGs) or so-called transmission gratings are specifically characterized by a large modification period Λ of approximately 100μm to 1000μm. In LPGs a mode coupling between modes travelling into the same direction occurs. The gratings act as an optical notch filter and light at the resonant wavelength is coupled from the core into the cladding (see Fig.1). Different to a fiber Bragg grating, no light is reflected in the core. This basic characteristic among other fundamental properties make LPGs highly interesting for a vast field of applications such as optical notch filters, mode converters and sensor elements [1].

5 Bragg_LPG_01_Principle
Fig.1: Principle of a LPG. (Rights: R.Krämer)

Taking advantage of fs-laser pulses and the line-by-line inscription method (see Fig.2) we are able to implement gratings into various kinds of fibers like standard single mode fibers, large mode area fibers (doped and undoped) [2] or specialty fibers such as fluoride glass fibers [3]. Further, we developed a simulation tool which allows us to tailor the LPGs on demand with high precision and strong selectivity. Our implemented gratings are highly efficient with resonances up to -25dB at design wavelength while simultaneously exhibit low out of band losses of only 0.1dB. Such LPGs are, for example, the ideal candidate to act as fiber integrated filter elements in order to inhibit stimulated Raman scattering in high power fiber lasers [2,4].

 6 Bragg_LPG_02_Inscription
Fig.2: Schematic of the Inscription setup (left) and magnified picture of objective and induced grating
         elements (right). (Rights: R.Krämer)

[1] J. Thomas, C. Voigtländer, R. G. Becker, D. Richter, A. Tünnermann, S. Nolte,
     Femtosecond pulse written fiber gratings: a new avenue to integrated fiber
     technology, Laser & Photon. Rev., 6, 709-723.

[2] M. Heck, R. G. Krämer, D. Richter, T. A. Goebel, C. Matzdorf, T. Schreiber,
     A. Liem, V. Bock, A. Tünnermann, S. Nolte, Mitigation of stimulated Raman
     scattering in high power fiber lasers using transmission gratings, SPIE LASE,
     Fiber Lasers XI, 10512-53 (2018). doi.org/10.1117/12.2289511

[3] M. Heck, S. Nolte, A. Tünnermann, R. Vallée, M. Bernier, Femtosecond-written
     long-period gratings in fluoride fibers, Opt. Lett. 43, 1994-1997 (2018).

[4] T. Schreiber, A. Liem, E. Freier, C. Matzdorf, R. Eberhardt, C. Jauregui, J. Limpert,
     A. Tünnermann, Analysis of stimulated Raman scattering in cw kW fiber
     oscillators, Proc. SPIE 8961, Fiber Lasers XI, 89611T (2014).

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