DTA

Dottorato

ELASKAR PLAZAS, JAVIER DARIO

etd-09062024-113019

Photonic Circuit Integration for Interferometric FBG Interrogation

ING-IND/12

Istituto di Tecnologie della Comunicazione, dell'Informazione e della Percezione - PHD IN EMERGING DIGITAL TECHNOLOGIES

relatore Prof. OTON NIETO, CLAUDIO JOSE

• Fiber Bragg Grating
• Fiber optical sensing
• Integrated photonics
• High-speed vibration sensing

04/12/2024;

completa

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

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