Fiber Bragg grating (FBG) sensors are widely studied due to their unique advantages of light weight, small size, and sound stability in harsh environments. Most of the conventional demodulation methods for FBG sensing such as the filtering method, wavelength swept laser method, and tunable F-P filter method are performed in the optical domain, and they have the disadvantages of slow demodulation speed and low resolution. Therefore, it is important to develop new demodulation techniques with fast demodulation speed and high resolution. With the development of microwave photonics (MWP), the FBG sensing demodulation techniques based on an optoelectronic oscillator (OEO) have attracted extensive research interest. Compared with optical signals, microwave signals have relatively low frequencies and can be detected more rapidly and accurately. However, most of the current OEO-based FBG sensors are single-loop structures with large frequency fluctuations and a small free spectrum range (FSR) of the microwave signal output from the OEO, which results in a small measurement range and large measurement errors. Most importantly, only single parameter measurement can be realized. To this end, our paper proposes a frequency division multiplexing FBG sensing system based on a dual-loop OEO. We construct an optical dual-loop structure in the OEO loop and then perform frequency division multiplexing by a wavelength division multiplexer (WDM) and two electrical bandpass filters (EBPFs) with different center frequencies. This not only realizes the simultaneous sensing of strain and temperature but also significantly improves the measurement range and sensor stability.

First, we employ two cascaded FBGs as the sensing heads for strain and temperature measurement respectively. Secondly, two optical loops 1 and 2 with great length differences are constructed to form a dual-loop structure to reduce the frequency fluctuation and increase the FSR of the output microwave signals. Then, the optical signal is divided into two paths by WDM and frequency division multiplexed by two EBPFs with different center frequencies to realize the simultaneous sensing. Finally, the wavelength-to-frequency mapping mechanism is adopted to demodulate the strain and temperature. In the experiment, we apply strain to FBG Ⅰ and temperature to FBG Ⅱand gradually increase strain and temperature on FBG with a step of 45 με and 6°C to obtain the sensing sensitivity. The maximum frequency offset of the OEO output is recorded for ten minutes at different strains and temperatures to evaluate the stability of the OEO oscillation frequency. After that, the optical loop 2 is disconnected and the above steps are repeated to measure the sensing sensitivity and the maximum frequency offset of the frequency division multiplexing FBG sensing system based on a single-loop OEO. In addition, the measurement ranges of strain and temperature are estimated for single-loop OEO and dual-loop OEO structures respectively.

By processing and analyzing the experimental data, an FSR of 5.01 MHz is obtained for the dual-loop OEO-based sensor (Fig. 2). The microwave oscillation frequency offset (Δf_dual) from the OEO output has a good linear relationship with the strain and temperature applied to the FBG, and the microwave oscillation frequency (f_dual) shifts to the high frequency region as strain and temperature increase [Figs. 3(a) and 3(c)]. The fitting results show a sensitivity of 0.100 kHz/μ? for strain and 1.135 kHz/℃ for temperature [Figs. 3(b) and 3(d)]. The FSR of the single-loop OEO-based sensor is 177 kHz (Fig. 2), and the sensitivities of strain and temperature are 0.111 kHz/μ? and 1.170 kHz/℃respectively. Generally, the measurement range of the sensor is limited by the FSR of the OEO to avoid frequency ambiguity. Compared with the single-loop structures, the dual-loop structures substantially increase the measurement range of the sensor with little effect on the sensor sensitivity. In addition, the maximum frequency offset of the single-loop structure is 4.637 kHz and 4.420 kHz, corresponding to a measurement error of 42 μ? and 4 ℃ for the sensor [Fig. 4(a)]. The maximum frequency offset of the dual-loop structure is 0.035 kHz and 0.072 kHz respectively, with theoretical measurement errors as low as 0.35 μ? and 0.06 ℃ [Fig. 4(b)]. Therefore, the stability of the dual-loop OEO tracking signals is much higher than that of the single-loop OEO.

We propose and experimentally demonstrate a frequency division multiplexing multifunction sensor based on a dual-loop OEO. By configuring two optical paths with different lengths in the dual-loop OEO structure, the FSR is expanded by about 28 times and the measurement range is improved. Compared with the measurement error of 42 μ? and 4 ℃ based on the single-loop OEO sensor, the theoretical measurement error of the dual-loop OEO is only 0.35 μ? and 0.06 ℃, which provides the sensor with high stability, large measurement range, and small measurement error. Two dual-loop OEO structures are formed by frequency division multiplexing through WDM and two EBPFs with different center frequencies, which allows strain and temperature to be sensed simultaneously. The sensing sensitivities of 0.100 kHz/μ? and 1.135 kHz/℃ are obtained in the experiment. Additionally, if dense WDM and more EBPFs with different center frequencies are applied to our proposed sensors, more parameters can be measured in the form of a group network.