Volume-Bragg-gratings (VBGs) are periodic refractive index modulations inside a transparent material. They yield a narrow-banded response with respect to angle and wavelength in transmission as well as reflection (see Fig.1). Based on their unique optical properties they are utilized for a broad range of applications such as beam frequency filters, combiners as well as spectral stabilizers of laser diodes and diode bars.

 7 Bragg_VBG_01_Functionality
Fig.1: Functionality of VBG in transmitting (left) and reflecting (right) configuration. (Rights: D.Richter)

Typically, VBGs are fabricated using interference of UV-laser beams inside photosensitive materials. This limits the range of materials to only a few. Applying ultrashort laser pulses a broad (transparent) material range can be processed and thereby former material restrictions overcome [1]. For directly writing VBGs e.g. in pure fused silica we utilize the phase mask scanning technique (see Fig.2) [2]. The gratings realized intrinsically show very low losses due to absorption and ultra low losses due to stray light. The resulting grating parameters can be tailored on demand from a few percent reflectivity up to far beyond 99% for a large and continuous range of geometries. Even the residual absorption can be adapted to the ultra low state on request.
Applying common VBGs for high power combining of laser beams or stabilization of laser sources a spectral shift of the grating resonance can be observed due to residual absorption. When utilizing ultrashort pulse written volume gratings in fused silica this residual absorption can be significantly reduced and thus the resonance shift [3]. Furthermore, the technique applied can be extended to realize two-dimensional-periodic gratings enabling a much larger set of discrete responses of a single grating compared to the common standard (see Fig.3) [4].

8 Bragg_VBG_02_Inscription
Fig.2: Phase mask inscription setup.  
        (Rights: D.Richter)

1 Bragg_Overview_01_VolumeBraggGrating
Fig.3: Overlayed images of discrete reflection spectrum 
         of a 2D-periodic VBG for different rotation angles.
        (Rights: C.Voigtländer)

[1] D. Richter, C. Voigtländer, R. Becker, J. Thomas, A. Tünnermann, S. Nolte,
     Efficient volume Bragg gratings in various transparent materials induced by
     femtosecond laser pulses, in CLEO/Europe and EQEC 2011 Conference Digest,

[2] C. Voigtländer, D. Richter, J. U. Thomas, A. Tünnermann, S. Nolte ,Inscription
     of high contrast volume Bragg gratings in fused silica with femtosecond laser
     pulses, Appl. Phys. A (2011) 102: 35. doi.org/10.1007/s00339-010-6065-6

[3] D. Richter, M. P. Siems, W. J. Middents, M. Heck, T. A. Goebel, C. Matzdorf,
     R. G. Krämer, A. Tünnermann, S. Nolte, Minimizing residual spectral drift in laser
     diode bars using femtosecond-written volume Bragg gratings in fused silica,
     Opt. Lett. 42, 623-626 (2017). doi.org/10.1364/OL.42.000623

[4] D. Richter, C. Voigtländer, R. G. Krämer, J. U. Thomas, A. Tünnermann, S. Nolte,
     Discrete nonplanar reflections from an ultrashort pulse written volume Bragg
     grating, Opt. Lett. 40, 2766-2769 (2015). doi.org/10.1364/OL.40.002766

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