Through-the-coating femtosecond laser inscription of very short fiber Bragg gratings for acoustic and high temperature sensing applications

Abstract

Very short Type I and Type II Bragg gratings, on the order of 100 µm in length, are written through the protective polyimide coating of high NA and standard single mode silica optical fibers with infrared femtosecond pulses and a phase mask. By exploiting the transverse walk-off of apertured diffracted beams produced by the phase mask and a slit placed proximate the mask, complex grating structures are fabricated and characterized. These gratings are suitable for structural health monitoring based on acoustic measurements or localized high-temperature measurements.

High-temperature stable π-phase-shifted fiber Bragg gratings inscribed using infrared femtosecond pulses and a phase mask

“…We also assume that the width of A, which is denoted by 2w, is significantly smaller than the beam width along the x-axis at the 1/e 2 intensity level. If the distance from A to M along the z-axis is not too large compared with 2w, diffraction effects at A can be neglected [28] and the light intensity distribution at M (and F) along the x-axis becomes quasi-flat-top, as schematically presented in red in Fig. 3(b).…”

“…Interference patterns produced with fs-pulses and a uniform phase mask (i.e., phase diffraction grating) are well-studied both theoretically and experimentally [28,38,39]. Consider an ideal situation when the mask (M in Fig.…”

“…In reality, the 0th order always contains a certain portion of the incident laser power, albeit it can be as low as 1% for state-of-the-art holographic masks. As a consequence, the contrast of the Talbot pattern generated by a 1storder mask depends on the ratio of laser powers diffracted into the 1st and 0th order and the longitudinal walk-off of the orders [28,38,39]. This power ratio can be varied within a broad range by selecting the mask design, but for a given design it will also be affected by the pulse polarization.…”

“…IR fs-laser technology has a great advantage over its UV counterpart as Bragg gratings can now be inscribed in almost any optical fiber without the need to photosensitize the fiber core [21]. Moreover, if the IR fs-pulses are focused sufficiently tightly, inscription through the protective coating ('trans-jacket inscription') becomes possible [22][23][24][25][26][27][28][29]. This allows one to avoid the removal of the protective coating and, if it is needed, its reapplication after grating inscription.…”

“…The presence of the V-shaped region causes a pronounced asymmetry in the spectra of FBGs written with the 1st-order mask. We show that this asymmetry can be corrected by covering the π-phase-shift region of the mask with a rectangular beam stop and choosing the correct mask-to-fiber distance based on the transverse and longitudinal walk-off of the diffracted fs-beams [28,38,39].…”