Flexible tuning of femtosecond written fiber Bragg gratings

Fiber Bragg gratings (FBGs) are used more and more in sensor applications for measuring e.g. strain or temperature. A key element are sensor arrays, consisting of multiple FBGs with different grating periods. Usually, such sensors are fabricated with well-established UV-laser inscription techniques, which however are limited to photosensitive fibers and offer relatively low temperature stabilities. These drawbacks can be overcome by using ultrashort laser pulses. The inscription process is not limited to photosensitive materials and the femtosecond inscribed FBGs offer higher temperature stabilities [1]. However, reliable inscription techniques with flexible period tuning are required for the realization of sensor arrays. An option is the point-by-point technique, where the easily adjustable translation speed of the fiber during the inscription process defines the grating period. But point-by-point gratings suffer from unwanted cladding modes and high scattering losses [2], which are unsuitable for sensor applications. An alternative is the phase mask technique, where the refractive index modulation is generated by an interference pattern from an overlap of the ±1st diffraction order underneath the mask (see Figure 1(a)) [3]. The period of the resulting grating is defined by the period of the phase mask. Therefore, this technique has a high period stability, however, offering no option of tuning the grating period.