High-Speed Interrogation of Low-Finesse Fabry–Perot Sensors Using a Telecom DFB Laser Diode

This paper presents a high-speed high-resolution low-finesse Fabry-Perot sensor interrogation system. The system utilizes a standard telecommunication distributed-feedback laser diode and an all-digital high-speed signal processing based on field-programmable-gate-array. The system can resolve multiple Fabry-Perot Interferometers (FPI) simultaneously, which might include multiple sensors, multiparameter sensors, or combinations of sensor(s) and reference FPIs. This opens up possibilities for adopting different measurement system configurations and compensation schemes. The system exhibits white-noise characteristics for a system bandwidth above 100 Hz, with spectral density of about 2 pm/√Hz when interrogating about 1 mm long low-finesse FPI. At an output sampling rate of 10 Hz, a sensor measurement length resolution of less than 50 pm was demonstrated for about a 600 μm long sensing cavity (corresponding to a strain resolution of about 70 nε). Proper use of simple all-fiber reference also provided low long-term drift. The system provides an up to 40 KHz sampling rate and was used to demonstrate dynamic measurement of strain variation on a piezo-electric stack and for interrogation of in-cylinder pressure variations within a high-rotation-rate miniature diesel engine. Fabry-Perot sensors with typical lengths between a few hundred and a few mm can be read-out with the current version of the proposed interrogator. The proposed system was built out of a small number of highly cost-efficient optoelectronics and electronics components (all used in FTTH) and thus, opens-up potential for new applications where the cost of interrogation is presently limiting.