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Abstract
A high-resolution and large-dynamic-range temperature sensor adopting a pair of fiber Bragg grating as Fabry–Pérot cavity (FBG-FP) and laser frequency dither locking method is proposed and experimentally demonstrated. This sensor exhibits a temperature resolution of 7×10−4 °C and a dynamic range of ∼46 °C. It is especially useful for applications where very small temperature changes need to be detected, such as deep ocean temperature measurement.
© 2021 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Temperature is an important parameter in a variety of applications, such as food inspection, pharmacy, high-voltage transformers, and oceanography [1]. In recent years, fiber-optic temperature sensors have attracted extensive attention due to a series of advantages, such as free of electromagnetic interference, small size, light weight, capability of multiplexing, harsh environment resistant, remote sensing. Up to now, different sensor structures and architectures, including fiber Bragg gratings (FBGs) [2–4], long-period fiber gratings (LPFGs) [5], Photonic Crystal Fibers (PCF) [6], Mach-Zehnder interferometers [7], Michelson interferometers and Fabry-Pérot (FP) interferometers [8–10], have been developed and studied for temperature measurement. The temperature induced spectral shift is on the order of ∼10 pm/°C for FBGs [2], which is low comparing to other types of sensors. Interferometric based sensors have achieved sensitivities in the range of ∼10 to 1000 pm/°C range [8,10]. With some special designs, such as the ethanol-filled structures, its sensitivity up to a few nm/°C could be achieved [7].
Usually, the temperature detection is based on resolving the wavelength position in a spectral feature (i.e., a fringe valley or a fringe peak), so the temperature resolution is limited by the wavelength resolution of the costly and bulky optical spectrum analyzer (OSA). As a result, a wavelength resolution of 1 pm can only distinguish a temperature change of ∼ 0.1 °C for a FBG [2]. High resolution requires high sensitivity sensors or high precision demodulation systems [11]. A micro-fabricated fiber-optic Fabry-Pérot (FP) sensor has been developed to achieve a temperature resolution of 6×10−4 °C by the average wavelength tracking method [10]. But the fabrication technique of the FP cavity in the micron dimension, in addition to complicated data processing, constrains its practical application.
In this paper, we propose and demonstrate a high-resolution, large-dynamic-range fiber-optic temperature sensor with a fiber Bragg grating based Fabry-Pérot cavity (FBG-FP) as the sensing resonator [12,13]. This sensor uses laser frequency dither locking scheme to convert the direct measurement of resonance frequency shift into the measurement of feedback voltage, with advantages of high resolution, rapid response and large dynamic range. Our results show that the sensor features a temperature resolution of 7×10−4 °C and a dynamic range of ∼46 °C.
When a pair of FBGs inscribed in an optical fiber is close to each other, the reflection spectrum of the FBGs can form a Fabry-Pérot interferometer, as shown in Fig. 1(a). FBG-FP sensor has the advantages of both FBG and FP sensor. FBG-FP has a series of resonances much narrower than that of FBG’s, which could be used for high-resolution temperature sensing by tracking changes of resonance frequency.
Fig. 1. Overview of laser frequency dither locked to a FBG-FP. (a) FBG-FP structure. (b). Schematic diagram of the sensing control system. Optical paths are shown by solid lines, electrical signals are shown by dashed lines. (c) Conceptual basis for temperature measurement. The laser frequency (orange solid line) is locked to the resonance frequency v0 of the FBG-FP (blue line). When the temperature changes, the resonance frequency of the FBG-FP shifts to a new location (green dashed line). The feedback voltage signal that is needed to keep the laser locked to the new resonance frequency v1 (orange dashed line) is measured.
Compared to Pound-Drever-Hall frequency (PDH) locking technology [4,14–16], laser frequency dither locking scheme [17,18] offers a significant cost saving than using an external phase modulator.
A 1550-nm distributed feedback (DFB) laser could be used in the dither locking scheme because it has relatively high output power and small size than that of the ECDL (extended cavity diode laser). Its continuous mod-hop-free tuning range is also many times larger than that of typical ECDLs [19].
2. Sensor system and temperature measurement principle
The experimental setup adopting laser frequency dither locking scheme is illustrated in Fig. 1(b). A tunable DFB laser was directly modulated by a sinusoidal driving current at dither frequency of about 10 kHz. The modulated laser beam was coupled into a fiber optical circulator, then sent into a FBG-FP. The reflected light by the FBG-FP was focused onto a 150-MHz bandwidth photodetector (Thorlabs, PDA05CF2) via the same optical circulator and demodulated with a lock-in amplifier (LIA) that generated an error signal whose amplitude is proportional to the difference between the laser frequency and the resonance frequency of the FBG-FP. The error signal was processed by a proportional-integral-derivative (PID) controller. The PID controller output was fed back to the driving current of the laser that finely tuned the laser frequency and continuously locked the laser frequency to the resonance frequency of the FBG-FP.
In a dither locking scheme, the system signal is extracted by monitoring the feedback voltage required to maintain locking. Temperature drift would alter the resonance condition of the FBG-FP, as shown in Fig. 1(c). This results in the change of feedback voltage, which is proportional to the temperature change applied to the FBG-FP. The feedback voltage was recorded using a 16-bit data acquisition unit.
The DFB laser is custom made with a continuous mod-hop-free tuning range of ∼60 GHz, and a frequency tuning rate of 36 MHz/mV.
The FBG-FP is acted as a temperature sensing element that is consisted of a pair of nominally matched Bragg gratings (R≈ 99%) spaced 10 mm apart written in a Corning SMF-28 Ultra optical fiber. The FBG-FP has the same temperature/strain sensitivity as the FBG [12,13], so the shift Δν in resonance frequency ν of the FBG-FP with temperature can then be expressed as:
where αn, αf and ΔT are the thermo-optic coefficient of the silica fiber, the thermal expansion coefficient of the fiber, and the temperature variation, respectively. For FBG-FP, the normalized thermal responsivity at constant strain [2] is3. Results and analysis
3.1 Characterization of FBG-FP by laser frequency scanning and frequency locked operation
A voltage of triangle wave with frequency of 400 mHz and amplitude of 100 mVp-p was input to the laser to sweep the laser frequency. The reflected signal of the FBG-FP, as measured by a photodetector, and the corresponding error signal were recorded using a digital oscilloscope while the laser frequency was scanned. They are displayed in Figs. 2(a), 2(b).
Fig. 2. (a) Reflected signal by laser frequency scanning. (b) Resonance frequency locking process for reflected and error signal.
To improve the sensitivity, we chose the resonance with the narrowest linewidth. The full-width half maximum (FWHM) bandwidth for the voltage scan in Fig. 2(a) corresponds to 5 mV on the laser voltage. Because the laser voltage input provided 36 MHz/mV of tuning, the FWHM bandwidth of this resonance can be determined to be 180 MHz, corresponding to the quality factor Q of 1.1×106.
When the laser is near the resonance of the FBG-FP, the reflected intensity approaches its minimum, and the feedback loop is engaged to the acquired lock. This process was recorded by the digital oscilloscope traces shown in Fig. 2(b).
3.2 Temperature measurement results
In order to characterize the general behavior of the sensor, we need to calibrate the feedback voltage with a reference thermometer, for which we use a calibrated platinum resistance thermometer (uncertainty: 2×10−3 °C) located near the FBG-FP.
The FBG-FP was mounted on a thermoelectric cooler (TEC) with a temperature stability of 0.03 °C. It was carefully sandwiched between the TEC and a copper block with a groove, and was surrounded by thermally conducting silicone gel to ensure good thermal contact and stress free between the FBG-FP and its surrounding. The temperature was regulated by a temperature controller. The TEC was placed inside an incubator which was used to provide a close external temperature environment.
To verify the correlation between the feedback voltage and temperature, a wide temperature scan (-5 °C ∼ 37 °C) was performed, as shown in Fig. 3(a). Every temperature setting was held for 5 minutes to guarantee that the system has reached its steady state. Figure 3(b) is a magnified view from 75 s to 170 s. It is worth noting that the sensor is capable of following the mini temperature variations of the TEC. The feedback voltage shows more oscillations than the temperature because the sampling rate of temperature data points was 1 Hz while the sampling rate of feedback voltage data points was 10 Hz. The linear fit and quadratic fit are shown in Fig. 3(c) and Fig. 3(d). A quadratic function can better fit the feedback voltage shift with the temperature because a simple linear temperature coefficient is insufficient to represent the shifts of the resonance frequency of the FBG-FP over the temperature range from -5 °C to 37 °C [20]. The linear fit sensitivity was found to be 35 mV/°C, which is close to the theoretical value.
Fig. 3. (a) Time dependent feedback voltage response at different temperatures. (b) A magnified view from 75 s to 170 s. (c) Linear fit between temperature and feedback voltage. (d) Quadratic fit between temperature and feedback voltage.
Since the laser frequency tracks the resonance frequency of the FBG-FP, the temperature dynamic range is ∼46 °C, which is limited by the laser tuning range. Note that the maximum temperature and the minimum temperature can be tailored by changing the operating frequency of the laser or the resonance frequency of the FBG-FP as required. For example, the temperature range can be adjusted from -5 °C ∼ 37 °C to 6 °C ∼ 50 °C for different applications.
The sensor stability and noise feature have been analyzed by Allan deviation (AD). By placing the FBG-FP into a gallium-melting-point (GaMP) cell stabilized at 29.7646 °C (uncertainty: 2.5×10−4 °C) and recording the feedback voltage with a sampling frequency of 10 Hz, the calculated AD is shown in Fig. 4. For short averaging times (below ∼10 s), the decreasing green dashed line τ-1/2 represents that the sensor noise is dominated by white noise. A temperature resolution of 7×10−4 °C can be achieved at an averaging time of 10 s.
Fig. 4. The Allan deviation plot. The τ-1/2 (green dashed line) indicates that the Allan deviation is only affected by white noise.
Table 1 presents the performance comparisons of fiber-optic temperature sensors with different interrogation schemes. Compared to other temperature sensors, our proposed sensor has the advantages of high resolution and large dynamic range which are achieved by simple setup, commercially available sensor head, and simple data processing. Furthermore, the temperature resolution of our proposed sensor can be further improved by improving the sensitivity of the sensor head.
Table 1. Performance comparison of temperature sensors
4. Conclusion
This paper demonstrates a fiber-optic temperature sensor using FBG-FP with a temperature resolution of 7×10−4 °C, a dynamic range of ∼46 °C. Through laser frequency dither locking scheme, the feedback voltage is used to detect the resonance frequency shift of FBG-FP due to temperature changes. It is worth noting that the laser frequency dither locking scheme is versatile and can also be used to high-resolution sensing on other parameters, such as strain, refractive index and pressure, with a proper design to the sensor head. When a whispering-gallery mode (WGM) resonator is used as the sensor head, our proposed sensor can measure small temperature fluctuations on the submicron scale, such as in microcircuits and intracellular liquids [23].
These efforts pave the way for future optoelectronic hybrid integration and miniaturization to achieve in-situ measurement [15].
Funding
Ministry of Science and Technology of the People's Republic of China (2019YFC1408600); Department of Science and Technology of Shandong Province (2018YFJH0702, 2019JZZY0207, S190401010001).
Disclosures
The authors declare no conflicts of interest.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
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