FIBER BRAGG GRATINGS

Fiber Bragg gratings (FBGs) are based on a refractive index modification with periods within the range of 1µm written directly into the fiber core [1]. They act as narrowband reflectors (see Fig.1). FBGs offer vast application possibilities ranging from robust sensors, dispersive elements, integrated cavity mirrors for fiber lasers to compact tailored wavelength filters.

3 Bragg_FBG_01_Principle
Fig.1: Principle of an FBG. (Rights: R.Krämer)


Using ultrashort pulses, two main approaches for the inscription of FBG are utilized: direct inscription via point-by-point or line-by-line and the phase mask inscription technique (see Fig. 2). The direct inscription offers large flexibility for tailored grating parameters, as the period of the grating and the strength of each layer can be directly addressed [2,3]. Alternatively, the phase mask technique [4] provides high stability and repeatability of the FBG inscription, as the phase mask period defines the grating period inside the fiber, which however can be tailored by applying a deformed wavefront of the inscription laser [5,6] or strain to the fiber [7].
With these techniques, we are able to inscribe FBGs into various fiber types, ranging from pure silica or photosensitive fibers to actively doped laser fibers as well as single mode, large mode area or even multicore fibers. For demanding applications highly robust FBGs are realized by inscribing the refractive index modification through the coating material of the fiber (typical coatings are polymers that are transparent in the VIS to NIR).
As one of our core competences we develop FBGs in large mode area fibers for the application in high power lasers. Those systems have been tested to the multi-kilowatt level revealing not only high output power but also high beam quality and efficiency. Reduced thermal load at high powers could be achieved.
Using the direct inscription technique complex grating designs, such as aperiodic gratings can be realized. Here, one FBG provides several independent reflection wavelengths. Thus, they offer the possibility of extremely compact and highly efficient transmission filter elements e.g. for astrophotonics [8].

4 Bragg_FBG_02_Inscription
Fig.2: Schematic of point-by-point direct writing technique (left) and phase mask inscription technique (right).
        (Rights: R.Krämer)


With these techniques, we are able to inscribe FBGs into various fiber types, ranging from pure silica or photosensitive fibers to actively doped laser fibers as well as single mode, large mode area or even multicore fibers. For demanding applications highly robust FBGs are realized by inscribing the refractive index modification through the coating material of the fiber (typical coatings are polymers that are transparent in the VIS to NIR).
As one of our core competences we develop FBGs in large mode area fibers for the application in high power lasers. Those systems have been tested to the multi-kilowatt level revealing not only high output power but also high beam quality and efficiency. Reduced thermal load at high powers could be achieved.
Using the direct inscription technique complex grating designs, such as aperiodic gratings can be realized. Here, one FBG provides several independent reflection wavelengths. Thus, they offer the possibility of extremely compact and highly efficient transmission filter elements e.g. for astrophotonics [8].


[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. doi.org/0.1002/lpor.201100033

[2] E. Wikszak, J. Burghoff, M. Will, S. Nolte, A. Tünnermann, T. Gabler, Recording
     of fiber Bragg gratings with femtosecond pulses using a "point by point"
     technique," in Conference on Lasers and Electro-Optics/International Quantum
     Electronics Conference and Photonic Applications Systems Technologies,
     Technical Digest (CD) (Optical Society of America, 2004), paper CThM7.

[3] R. J. Williams, R. G. Krämer, S Nolte, M. J. Withford, Femtosecond direct-writing
     of low-loss fiber Bragg gratings using a continuous core-scanning technique,
     Opt. Lett. 38, 1918-1920 (2013). doi.org/10.1364/OL.38.001918

[4] J. Thomas, E. Wikszak, T. Clausnitzer, U. Fuchs, U. Zeitner, S. Nolte,
     A. Tünnermann, Inscription of fiber Bragg gratings with femtosecond pulses
     using a phase mask scanning technique," Appl. Phys. A, 86:153 (2007).
     doi.org/10.1007/s00339-006-3754-2

[5] C. Voigtländer, R. G. Becker, J. Thomas, D. Richter, A. Singh, A. Tünnermann,
     S. Nolte, Ultrashort pulse inscription of tailored fiber Bragg gratings with a
     phase mask and a deformed wavefront [Invited]," Opt. Mater. Express 1,
     633-642 (2011). doi.org/10.1364/OME.1.000633

[6] C. Voigtländer, R. G. Krämer, T. A. Goebel, D. Richter, S. Nolte, Variable
     wavefront tuning with a SLM for tailored femtosecond fiber Bragg grating
     inscription," Opt. Lett.41, 17-20 (2016).
     doi.org/10.1364/OL.41.000017

[7] T. A. Goebel, C. Voigtländer, R. G. Krämer, C. Matzdorf, M. Heck, D. Richter,
     A. Tünnermann, S. Nolte, Wavelength tuning of through-coating-written fiber
     Bragg gratings, in 2017 European Conference on Lasers and Electro-Optics and
     European Quantum Electronics Conference, (Optical Society of America, 2017),
     paper CM_4_5.

[8] T. A. Goebel, G. Bharathan, M. Ams, M. Heck, R. G. Krämer, C. Matzdorf,
     D. Richter, M. P. Siems, A. Fuerbach, S. Nolte, Realization of aperiodic fiber Bragg
     gratings with ultrashort laser pulses and the line-by-line technique,
     Opt. Lett. 43, 3794-3797 (2018). doi.org/10.1364/OL.43.003794



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