Inscription of fiber Bragg gratings with femtosecond pulses using a phase mask scanning technique

Abstract

We report on the fabrication of highly efficient fiber Bragg gratings (FBG) in non-photosensitive fibers based on nonlinear absorption of fs laser light. Up to 40 mm long gratings with a transmission of T = -25 dB at the Bragg reflection wavelength were obtained and their coupling constant determined by spectral analysis. Therefore, a phase mask scanning technique with appropriate control of the focus was established

Femtosecond pulse written fiber gratings: a new avenue to integrated fiber technology

“…In such a setup, the length of the FBG is limited by the beam diameter, the diffraction angle of the phase mask and the distance of the fiber from the phase mask. The first inscription of femtosecond FBG by side illumination was achieved in this way [44] and it is now a common approach [44,46,52,[70][71][72][73][74]. It turns out that this approach actually benefits from the short coherence length of the femtosecond laser because of the order-walk-off effect firstly observed and numerically calculated by Smelser et al [75,76].…”

“…In most cases, a chirped or regeneratively amplified Ti:Sapphire laser system is used that typically provides 50-150 fs pulses at a repetition rate of 1 kHz at 800 nm (e. g. [43][44][45][46][47][48]). Frequency doubling or quadrupling is employed, whereas inscription wavelengths in the VIS or UV are desired to realize shorter periods [40,[49][50][51][52][53].…”

“…Thus, with increasing s 0 the order-walk-off Δs becomes larger than the coherence length and different diffraction orders do no longer interfere [46]. The result is a pure two-beam interference, which can be exploited to inscribe transversally homogeneous FBGs.…”

“…Stitch-free FBGs with lengths only limited by the physical size of the phase mask can be inscribed by moving both phase mask and fiber under the beam along the fiber axis (along the z-axis as sketched in Fig. 3) -so called scanned phase mask interferometer [10] or phase mask scanning [46].…”

“…In IR-femtosecond pulse written FBGs, significantly strong reflections resulting from Δn 2 up to Δn 4 could be measured [46,82,125]. With longer periods, Fourier orders from Δn 7 to Δn 12 could be exploited for single mode reflection [95].…”