FPGA-Based High-Speed Optical Fiber Sensor Based on Multitone-Mixing Interferometry

We report a real-time high-speed fiber Bragg grating (FBG) interrogator based on a fiber-optic interferometer. The signal processing is performed by using a low-cost field-programmable gate array (FPGA) system, which is programed to implement a phase-generated carrier (PGC) demodulation algorithm with multitone mixing (MTM) to provide distortion-free signals with high tolerance to modulation depth variations and light intensity fluctuations. The system can stream data at rates up to 1 MS/s and allows multiplexed processing up to two channels. Experimental results show simultaneous measurements of two FBGs, one of which was actuated at frequencies up to 100 kHz. The system features a 3-dB bandwidth of 280 kHz, and a dynamic wavelength resolution of 4.7 fm/Hz ^{mathrm {1/2}}. We also demonstrate a strong reduction of distortion using the MTM approach with respect to the standard technique. Finally, we study the origin of the noise, demonstrating a reduction in common noise sources by using one of the FBGs as a reference. The system can measure FBGs centered at any position within the spectral band of the source, is polarization-independent, and is easily scalable to more than two measurement channels from the same interferometer

Ultracompact microinterferometer-based fiber Bragg grating interrogator on a silicon chip

We report an interferometer-based multiplexed fiber Bragg grating (FBG) interrogator using silicon photonic technology. The photonic-integrated system includes the grating coupler, active and passive interferometers, interferometers, a 12-channel wavelength-division-multiplexing (WDM) filter, and Ge photodiodes, all integrated on a 6x8 mm2 silicon chip. The system also includes optical and electric interfaces to a printed board, which is connected to a real-time electronic board that actively performs the phase demodulation processing using a multitone mixing (MTM) technique. The device with active demodulation, which uses thermally-based phase shifters, features a noise figure of σ  =  0.13 pm at a bandwidth of 700 Hz, which corresponds to a dynamic spectral resolution of 4.9 fm/Hz1/2. On the other hand, the passive version of the system, based on a 90º-hybrid coupler, features a noise figure of σ  =  2.55 pm at a bandwidth of 10 kHz, also showing successful detection of a 42 kHz signal when setting the bandwidth to 50 kHz. These results demonstrate the advantage of integrated photonics, which allows the integration of several systems with different demodulation schemes in the same chip and guarantees easy scalability to a higher number of ports without increasing the dimensions or the cost

Fast FBG interrogation with active sagnac interferometer using off-the-shelf fiber components

We present a polarization-independent fiber Bragg grating (FBG) interrogator based on a rapidly-tunable Sagnac interferometer using a dual-polarization lithium-niobate phase modulator. A bandwidth of 100 kHz along a range beyond a full free-spectral-range is demonstrated

High-Speed FBG Interrogation with Electro-Optically Tunable Sagnac Loops

We report high-speed Fiber Bragg Grating (FBG) interrogation based on fiber interferometry. An electro-optical modulator, used as a tunable retarder, is implemented in a fiber Sagnac loop, enabling fast tuning of the interference fringes. Applying an active modulation scheme, the wavelength of the FBG can be retrieved without any requirement of the spectral position of the FBG with respect to the fringes, with no need of tunable sources or spectral analysis. We report experimental results of wavelength measurements with a bandwidth of 100 kHz (only limited by our data acquisition rate) and a dynamic wavelength resolution of σ = 3.7 fm/√Hz (corresponding to a strain resolution of 3.1 nϵ/√Hz). We also show that the system can track a continuous tunable laser sweep of 40 nm along the C-band. The technique is polarization-independent and allows wavelength division multiplexing schemes using only one interferometer for all the sensing points. The system is fully based on low-cost off-the-shelf fiber-optical components, and the free-spectral-range of the interferometer can be easily modified by just changing the length of the polarization-maintaining fiber segment

Fiber Bragg grating optical sensors for fast dynamic strain measurements in a gasoline direct injector

Control systems for automotive applications have rapidly evolved introducing intelligence to address the increasing demand for pollution and oil consumption reduction. Fiber Bragg Grating (FBG) sensors are used in this work for monitoring Gasoline Direct Injectors (GDI) in order to optimize the engine performance and reduce the emissions. Several fast-Acting solenoid injectors have been instrumented with FBG sensors and mounted in a test bench at the testing department of CPT Italy S.r.l to simulate the injector's behavior during the actuation phase. The FBG sensors, installed on the stem of the GDI, provide dynamic measurement of the strain variation during the injection process, pointing out the unwanted effects of the reopening, leading to injector tip wetting and consequent increased polluting emissions. The acquired data allows one to fully understand the GDI process and to optimize the injector design in order to reduce emissions, as required by recent European directives for the emission standards

Fiber Bragg Grating Sensors for Dynamic Strain Measurements in Gasoline Direct Injectors

The strain in a gasoline direct injector was measured during a single injection process by using a fiber Bragg grating sensor installed along the axis of the injector stem in different operating pressure conditions. A fast FBG interrogator, based on linear transmission filters and direct detection, permits to measure dynamic strain up to 240 MHz frequency with a detection limit of 1.1 μϵ and a dynamic strain resolution of 20.2 nϵ/√Hz. The FBG sensor was first calibrated for an internal pressure ranging from 50 to 250 bar and an energizing duration of 5ms; the FBG sensor allows to measure the static internal pressure of the injector inlet, which is linearly dependent on the FBG static strain, as well as the dynamic strain during the opening/spray/closing processes induced by pressure fluctuations, mechanical vibrations and the impact forces related to the injector needle bounces. The fast FBG interrogator, has been first validated by comparison with commercial interrogators, and then used to detect fast transients. The measured FBG strain variation provides important information on the dynamic behavior of the injector during the spray, confirmed by the analysis of the spray injection performed through a High Speed Video camera. The static and dynamic measurements demonstrate the effectiveness of the FBG sensors technology for monitoring the behavior of the injector and controlling the engine performance and emissions

Integration of Diagnostic Fiber Bragg Grating Sensors in Advanced Carbon Composite Materials

FBG sensors written by femtosecond laser pulses are embedded in carbon composite structures. A comparative analysis, supported by theory, points out the embedding process repeatability and the sensing technology effectiveness for structural health monitoring