We propose a novel scheme to implement tunable multi-tap complex coefficient filters based on optical single sideband modulation and narrow band optical filtering. A four tap filter is experimentally demonstrated to highlight the enhanced tuning performance provided by complex coefficients. Optical processing is performed by the use of a cascade of four phase-shifted fiber Bragg gratings specifically fabricated for this purpose.

Birefringence in fiber Bragg gratings (FBGs) leads to polarization-dependent properties such as polarization-dependent loss (PDL). Although this property is not desired for telecommunications applications, we demonstrate that the PDL generated by uniform FBGs written into standard single-mode fiber can be advantageously used for transverse strain measurements, which is not directly possible through amplitude spectral measurements.

Due to the lateral inscription process, photo-induced birefringence is present in fiber Bragg gratings (FBGs) written into photosensitive single mode fiber. The birefringence value is generally too small to be perceived in the grating spectral response but it can lead to significant polarization dependent loss (PDL) and differential group delay (DGD) evolutions. In this paper, we first theoretically analyze the evolution with wavelength of PDL and DGD as a function of the grating parameters and the birefringence value. We demonstrate that the PDL and DGD evolutions with wavelength can be strongly enhanced by a modification of the grating parameters. Simulations carried out using the coupled mode theory and the Jones formalism are then confirmed by experiments conducted on FBGs written into photosensitive single mode fiber. Our work brings a complete characterization of polarization related phenomena in FBGs and presents a great interest for the evaluation of system performances and the design of gratings for specific applications, either for telecommunications or sensing purposes. In addition, based on the comparison between experimental and simulated evolutions, we are able to verify that the birefringence value is strongly dependent on the fluence of the laser used for the grating inscription.

In this paper, we completely study the wavelength dependency of differential group delay (DGD) in uniform fiber Bragg gratings (FBG) exhibiting birefringence. An analytical expression of DGD is established. We analyze the impact of grating parameters (physical length, index modulation and apodization profile) on the wavelength dependency of DGD. Experimental results complete the paper. A very good agreement between theory and experience is reported.

The design and field test of a novel sensor system based in autonomous wireless sensors to measure the temperature of the heat transfer fluid along a borehole heat exchanger (BHE) is presented. The system, by means of two specials valves, inserts and extracts miniaturized wireless sensors inside the pipes of the borehole, which are carried by the thermal fluid. Each sensor is embedded in a small sphere of just 25 mm diameter and 8 gr weight, containing a transceiver, a microcontroller, a temperature sensor and a power supply. A wireless data processing unit transmits to the sensors the acquisition configuration before the measurements, and also downloads the temperature data measured by the sensor along its way through the BHE U-tube. This sensor system is intended to improve the conventional thermal response test (TRT) and it allows the collection of information about the thermal characteristics of the geological structure of subsurface and its influence in borehole thermal behaviour, which in turn, facilitates the implementation of TRTs in a more cost-effective and reliable way.