Femtosecond laser point-by-point writing is a commonly used method for fabrication of Fiber Bragg Grating sensors dedicated to harsh environments, such as high temperature or irradiation. In addition, a femtosecond laser platform allows for inscription of compact fiber optic diffraction gratings that consist of micro-voids or filaments formed into the fiber core and cladding by focusing laser pulses using microscope objectives. Light propagating in the fiber is coupled to radiation modes due to Mie scattering, thus providing wavelength dispersion in free space. Chirping the grating period further allows focusing of the outcoupled light in a given plane. Such an all-fiber focusing grating forms a compact photonic device permitting its use for FBG sensor interrogation. In this paper, fabrication of such spectrometers operating at 850 and 1550 nanometers is described. A characterization setup allowing measurements of spectra of FBGs at those wavelength bands is presented, and results corresponding to various focusing distances, grating lengths and chosen microscope objectives are exposed. Azimuthal distribution of scattered light is discussed, as well as focusing distance versus grating period chirp and spectrometer resolution versus grating length. Finally, spectra reflected by pointby- point FBG sensors are presented, thus demonstrating the great potential interest of such gratings for FBG interrogation.
Fiber Bragg Grating (FBG) sensors offer multiple benefits in comparison with electronic sensors due to their compactness, electromagnetic immunity as well as their resistance to harsh environments and their multiplexing capabilities. Structural Health Monitoring (SHM) is one of the various potential industrial applications that could take full advantage of those sensors. However, there is a need for a low size, weight, power and cost interrogation unit for certain application areas such as aerospace or aeronautics. That is the reason why recent efforts have been made to use integrated components and circuits for interrogation of FBGs. Among different techniques, interrogation with a swept laser source is of high interest since it has a high multiplexing capability and could reach a high level of integration using other integrated components such as photodetectors, grating couplers or directional couplers to form a compact interrogation unit. In this paper, we present characterization results of a fully-packaged hybrid III-V on silicon tunable laser diode operating in the C and L bands. Wavelength maps are produced and analyzed and modulation of emitted wavelength is discussed. Preliminary results corresponding to a moderate frequency (10-Hz sweep rate) were obtained and FBG reflection spectra acquired with a broadband source (BBS) and a swept laser diode are compared. Finally, we discuss potential design improvements in order to reach high scan rates (< 10 kHz) and a large tuning range
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