Photonic Circuit Integration for Interferometric FBG Interrogation

Most current commercial FBG interrogators are based on discrete optical components. This makes them expensive, bulky, and less reliable. Shifting towards interrogators with integrated photonics would overcome these problems. In addition, commercial and reported state-of-the-art interrogators have limited detection bandwidths of up to a few tens of kHz, for both integrated and non-integrated technologies. Detection bandwidths in the order of hundreds of kHz, would enable the usage of the FBG interrogators for new applications that require high-speed detection, such as vibrations in rotatory equipment. This thesis reports the results of three FBG interrogators, the first one is discrete-component-based, with a large bandwidth, the second is integrated with a limited bandwidth, and the third is integrated and with a large bandwidth. The three of them used interferometry techniques for wavelength shift detection, as this technique has a high sensitivity. The responsivity of the interferometer can be selected by design with the free-spectral-range of the device. However, interferometers have responsivity fading problems when the tracked wavelength moves away from the quadrature points. To overcome this problem a modulation-demodulation technique known as multi-tone-mixing was used. It is an improved variant of the known phase-generated-carrier technique. An FPGA was used in all interrogators to perform the multi-tone-mixing technique in real-time and stream the data to the PC. The first interrogator was based on discrete components with a Sagnac interferometer and a fast Lithium-niobate phase modulator. It has a 280 kHz bandwidth, dynamic wavelength resolution of 4.7 fm/Hz1/2 , and 4 channel detection. It was packaged in a case for out-of-the-lab experiments. The second interrogator was integrated into a silicon photonic chip, with grating couplers, active and passive Mach-Zehnder interferometers, heaters for phase modulation, arrayed waveguide gratings for multi-channel detection, and Ge photodiodes. The passive interferometer could detect signals of up to 42 kHz, but with high noise levels. There were two interferometers for active detection that used the multi-tone-mixing technique. One has a free-spectral-range of 1.5 nm and a 4.9 fm/Hz1/2 dynamic wavelength resolution. The other one has a free-spectral-range of 6 nm and 17.8 fm/Hz1/2 dynamic wavelength resolution. A lower free-spectral-range improves the responsivity but sets the limit for absolute wavelength tracking within the fringe. Both active interrogators had 700 Hz bandwidth and could detect up to 12 simultaneous FBG channels. The third interrogator was also integrated on a photonic chip, with grating couplers, an active Mach-Zehnder interferometer, and a silicon-doped modulator for fast phase modulation. It has a 150 kHz bandwidth and 12.3 fm/Hz1/2 dynamic wavelength resolution