A weak double-peak fiber Bragg grating (FBG) temperature sensor is proposed and demonstrated. Wavelength-swept tunable laser is regarded as one of the most popular demodulation methods for fiber Bragg grating (FBG) sensors. However, due to the limitations of the existing tunable laser technologies, a fast, compact, stable and low-cost tunable laser for FBG sensors is still unavailable, which will become one of the major barriers for more widespread applications of FBG sensors. To further improve the efficiency and accuracy of the FBG interrogation system, a FBG temperature sensor is proposed and demonstrated by using tunable laser and a weak double-peak FBG. Since the reflection of the weak double-peak FBG has two main reflection peaks and relatively wide bandwidth, it is convenient to track the two characteristic peaks to accurately obtain the wavelength shift during the alteration of ambience temperature. A proof-ofconcept experiment is also conducted to verify the theory. By demodulating a weak double-peak FBG in the temperature experiment, a sensor sensitivity of 10.17 pm/ °C is measured for the proposed interrogation system.
A multi-longitudinal mode (MLM) laser sensor system based on software radio demodulation is proposed, which realizes the simultaneous measurement of vibration and temperature. The software demodulation is realized through the SDR (Software Defined Radio) technique due to the applied vibration can be analyzed as the superposition of a series of frequency modulation (FM) signals. Furthermore, the absorption of erbium-doped fiber (EDF) decreases with the increase of temperature, so the output power of the laser can be measured as another sensing signal. As a result, by demodulating these two sensing signals, the vibration from 10 Hz to 2 kHz can be successfully measured and the sensitivity of the temperature is around 0.011 dBm/°C.
An innovative demodulation system for multilongitudinal mode fiber laser sensor has been proposed. By using a bandpass filter and a low-speed analog-to-digital converter (ADC), the high-frequency sensing signal can be downconverted and sampled simultaneously when the unaliasing condition is satisfied. Since the MLM fiber laser sensor could generate a wideband electrical signal after optical-to-electrical conversion, it is convenient to filter the signal to meet the unaliasing condition by a tunable bandpass filter while keeping the sample rate unchanged. Moreover, each tone of the beating frequency signal has the full information for demodulation of measurand. The demodulation system only needs a bandpass filter and a low-speed ADC which reduces the cost of the system and make the system more stable. A proof-of-concept experiment is conducted to verify the proposed scheme. Eventually by demodulating a beat frequency in 1.625GHz, a sensitivity of -5.87kHz/°C is achieved in a fiber laser sensing system with a sample rate of 500MHz.