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Abstract
We have proposed and experimentally demonstrated a high-precision and strain-insensitive temperature sensor based on an optoelectronic oscillator (OEO). The oscillation frequency of the OEO is determined by the single passband microwave photonic filter (MPF) by using stimulated Brillouin scattering (SBS). The sensing fiber, which acts as the SBS gain medium, is exposed to temperature variations. The Brillouin frequency shift (BFS) changes along with the temperature. Since the central frequency of the MPF is a function of the BFS, the oscillation frequency of the OEO is varied. Besides, due to the mode competition in the OEO, the influence of the strain is eliminated. Thus, the temperature variations can be estimated through measuring the oscillation frequency. We carry out a proof-to-concept experiment. Temperature sensing with a high sensitivity of 1.00745 MHz/°C is achieved. The maximum measurement error of temperature obtained is within ± 0.5 °C. The proposed scheme has merits of simplicity and compact configuration. In addition, the proposed temperature sensor can realize quasi-distributed measurement by utilizing wavelength division multiplexing (WDM).
© 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Fiber-optic sensors have attracted extensive research interest over the past few years due to their immunity to electromagnetic interference, high sensitivity and high stability in harsh environment like chemical erosion [1]. They have been used to measure various physical and chemical parameters. Up to now, various optical fiber sensors with different configurations have been proposed, such as fiber Bragg grating (FBG) sensors [2,3], fiber-optic Mach–Zehnder interferometer (MZI) [4], sensors based on multilongitudinal fiber lasers [5,6], high birefringence fiber and photonic crystal fiber based devices [7]. For FBG sensors, the operation principle is that the effective index or the pitch of the FBG has a relationship with the parameters under test. When these parameters change, the resonance wavelength shifts. Therefore, the variation can be evaluated from the wavelength shift. However, the FBG sensor interrogation is usually done in optical domain by employing optical spectrum analyzer (OSA), optical filter, a tunable laser or an optical interferometer which suffers from poor resolution and low speed [8]. Schemes based on MZI are achieved by analyzing the variations in the interference signal at the output of the MZI [9]. The interrogation is still using OSA, which is again slow and has a poor resolution. Even more, the cross-sensitivity to strain and temperature exists in above two methods. For fiber laser sensor, there are multi longitudinal modes. The change of the parameter can be detected by measuring the beat frequency between any two longitudinal modes. However, these methods are inability to measure high temperature with high accuracy due to the low gain of the active fiber under high temperature [10,11]. And the system sensitivity is heavily dependent on signal to noise ratio of the beat signal.
In 1996, optoelectronic oscillator was first proposed [12]. The original purpose of the OEO is to generate high frequency microwave signal with high spectrum purity and low phase noise which cannot be obtained from electronic oscillators or electronic frequency synthesizers [13]. As a signal generator, OEO has been widely investigated [14–17]. Benefiting from the features, OEO is capable of being an optical sensor with easy detection, high sensitivity, fast interrogation speed and low cost. The fundamental principle of using OEO for optical sensor is to convert sensing parameters, such as temperature, strain, transverse load, to the oscillation frequency shift of the OEO. Since the frequency of the generated electrical signal can be measured by using a high speed digital signal processor, the interrogation speed and resolution are improved greatly [18]. The oscillation frequency of the OEO can be determined through two mechanisms. One is the time delay of the whole OEO link. The optical length, which is equivalent to the time delay, is affected by the sensing parameters directly. Therefore, the oscillation frequency changes. Based on this concept, optical length [19–22], refractive index [23], temperature [8], acoustic [24] and strain [25] have been interrogated successfully. The other is the passband of the MPF. The MPF acts as an oscillation frequency selection element as well as a sensing element. The sensing parameters which cause the change of the MPF response can be interrogated by detecting the frequency of the microwave signals. MPF-based OEOs using phase shifted FBG [26–28], Fabry-Perot FBG [29], MZI [9] have been demonstrated with high sensitivity and fast interrogation speed for optical sensing. However, the cross-sensitivity of strain and temperature is still unsolved in most of the above schemes which limit the practical applications. In Ref. [30], a temperature sensor is realized by using SBS. However, it uses an unmodulated carrier as the pump. High speed and wide bandwidth detectors are needed to process the data.
In this paper, we proposed and experimentally demonstrated a high-precision and strain-insensitive temperature sensor based on an optoelectronic oscillator. The key device in the OEO-based sensor is the single passband MPF which is constructed by SBS effect. Due to the narrow bandwidth of the SBS gain/loss spectrum, the phase modulated signal is converted to a single-sideband intensity-modulated signal. The BFS, which is affected by the temperature and strain, determines the central frequency of the MPF. Due to the mode competition, the effect induced by strain is eliminated. The microwave frequency change of the OEO is only affected by temperature. Thus, the cross-sensitivity of strain and temperature is solved. We can estimate the temperature by measuring the microwave frequency change. We carry out a proof-to-concept experiment. High sensitivity temperature sensing with a sensitivity of 1.00745 MHz/°C is achieved. The maximum measurement error of temperature obtained is within ± 0.5 °C. The proposed scheme has merits of simplicity, compact configuration and strain independent. It can realize high accuracy measurement without high speed and wide bandwidth detectors. Furthermore, the scheme can implement quasi-distributed measurement by using WDM. It has the potential applications in the structural health monitoring, pipeline security monitoring, ocean-bottom seismic system and so on.
2. Principle
Figure 1(a) schematically illustrates the proposed OEO-based temperature sensor. The system comprises of a laser diode (LD), an optical couple (OC), a Mach–Zehnder modulator (MZM), a phase modulator (PM), an erbium-doped fiber amplifier, a circulator (CIR), a length of single mode fiber (SMF), a photodetector (PD), an electrical amplifier (EA), a power divider and an electrical spectrum analyzer (ESA).
Fig. 1. (a) Schematic of the proposed OEO-based temperature sensor; (b) Optical spectra after PM and MZM; (c) Optical spectra after SBS; (d) Frequency response of the MPF.
The optical carrier from LD is divided into two paths by an OC. The upper branch is modulated by a MZM. After amplified by the EDFA, the intensity modulated signal is sent to port 1 of the CIR and goes into SMF from port 2 to act as the pump of SBS. The lower branch is modulated by a PM. The phase modulated signal is injected to the SMF as the probe of SBS. Then, the probe signal from port 3 of CIR is converted into an electrical signal through the PD. The electrical signal is divided into two parts by a power divider. One portion is amplified by an EA and fed back to the PM to form the OEO loop. And other is monitored by an ESA. In order to selectively amplify and attenuate the ${\pm} $ 1st order sidebands of the phase modulated signal, the driving frequency of MZM should be larger than at least a BFS ${f_{RF}} > {f_B}$. As shown in Fig. 1(b), the + 1st order sideband of the phase modulated light is amplified by the SBS gain and the −1st order sideband is attenuated by the SBS loss. As we know, the ${\pm} $ 1st order sidebands of the phase modulated signals are out of phase with same amplitude. No electrical signal generates if they are fed into the PD directly. In the proposed scheme, SBS effect breaks the amplitude balance of the phase modulated sidebands as shown in Fig. 1(c). The unbalanced ${\pm} $ 1st order sideband at this frequency is converted into electrical signal whose frequency is ${f_{MPF}} = |{{f_{ {\pm} 1}} - {f_0}} |= {f_{RF}} - {f_B}$. The SBS converts phase-modulation to intensity-modulation and implements a photonic MPF. When the sensing parameters change, the central frequency of MPF changes as well as the BFS ${f_B}$ as shown in Fig. 1(d). The generated microwave signal, whose frequency is ${f_{MPF}}$, is sent back to the OEO loop and drives the PM to generate ${\pm} $ 1st order sidebands at frequency ${f_0} \pm {f_{MPF}}$. After experiencing SBS, the electrical signal at ${f_{MPF}}$ is generated by a PD again. These two processes interact each other until the OEO is stable.
The BFS is influenced by the temperature and strain. If the temperature and strain change simultaneously, the BFS at different location is different. Seen from Fig. 2, in the temperature region, the ${\pm} $ 1st order sidebands at red line will be continuously amplified and attenuated respectively. After passing through the whole fiber, the ${\pm} $ 1st order sidebands are converted into electric signal. In a similar way, the ${\pm} $ 1st order sidebands at green line, whose BFS is affected by the strain, experience SBS effect and are transformed into microwave signal. Thus, the amplitude balance of red lines and green lines are broken. And both of them can be converted into electric signals which are fed back to the PM. Due to the proportion of the SMF affect by temperature are larger than that affected by strain, the magnification and attenuation multiple of red lines is larger than the green lines. As a consequence, the power of electrical signal generated from red lines is higher than that from green lines in one loop. In a closed OEO loop, competition exists in different modes. If the power of microwave signal from green lines do not reach the threshold value to oscillate, only one signal exist which is affected by temperature in the OEO output. Therefore, the cross-sensitivity of strain and temperature is solved. We can estimate the temperature by measuring the microwave frequency change.
Besides, the proposed scheme can realize quasi-distributed temperature sensing by utilizing WDM [25]. Figure 3 presents the schematic structure of the OEO based quasi-distributed temperature sensor system. The optical carrier is provided by a tunable light source (TLS). Each sensing unit comprises a length of SMF and a pair of WDMs. When the carrier wavelength is ${\lambda _1}$, the light only passes through SMF1 because of the WDM1. Thus, the collected data only reflects the temperature in sensing unit 1. By that analogy, each sensing area and wavelength is one to one correspondence. We can measure each sensing unit in sequence by changing the output wavelength of the TLS.
3. Experimental results and discussions
We carry out an experiment to verify the effectiveness of the technique and to study the system performance. The wavelength and power of the optical carrier is 1550 nm and 10 dBm which is provided by APEX Technologies (AP 1000–8). The MZM has a 45 GHz electrical bandwidth and half-wave voltage of 4.7 V. The bandwidth of PM is 40 GHz. A 2.5 km SMF is used to generate SBS. The Brillouin frequency shift ${f_B}$ of SMF is about 10.8 GHz.
We first measure the spectrum of the pump light. The ${\pm} $ 1st order sidebands of modulated signal play as the pump and the carrier does not influence the SBS. Thus, there is no need to bias the MZM at carrier-suppressed modulation. Figure 4 is the optical spectrum after MZM.
Then we close the OEO loop. By controlling the gains of the EDFA and EA to make the OEO net loop gain higher than 0 dB, the OEO will start to oscillate. We measure the frequency response of the OEO at room temperature. The driving frequency of the MZM is set at 10.8, 10.9, 11.0, 11.1 and 11.2 GHz, respectively. Figure 5(a) depicts the measured frequency response of the OEO. The pump frequency has a frequency difference of 100 MHz and ${f_{MPF}} = |{{f_{ {\pm} 1}} - {f_0}} |= {f_{RF}} - {f_B}$, so each oscillation frequency spaces about 100 MHz apart. We can infer that the Brillouin frequency shift ${f_B}$ is 10.715 GHz. In order to investigate the stability of the OEO, we measure the OEO output when the temperature is 61 °C. The electrical spectra of generated signals are shown in Fig. 5(b). As can be seen, signals with equal frequency interval are generated. The frequency space is still 100 MHz. However, the oscillation frequency changes because the temperature varies. The BFS ${f_B}$ is 10.75 GHz at 61 °C. The temperature variation induces frequency shift of 35 MHz. Therefore, the proposed scheme operates steadily at different temperature.
Fig. 5. Electrical spectra of generated signals for different driving frequency at (a) room temperature; (b) 61 °C.
Figure 6(a) shows the spectrum of the generated 284.5 MHz microwave signal at a room temperature when the pump frequency is 11 GHz. The zoom-in-view of the spectrum of the signal is demonstrated in Fig. 6(b). As can be seen, there are several peaks which are the side modes of the OEO. They are caused by the bandwidth of the SBS gain/loss spectrum. Although the SBS-MPF cannot realize single-mode oscillation, the main oscillation mode is dominate, which is reach to the requirement for accurate sensing.
Fig. 6. (a) Electrical spectrum of the generated 284.5 MHz microwave signal at room temperature, the resolution bandwidth (RBW) is 4 MHz; (b) Zoom-in view of the signal (the RBW is 1 MHz).
We explore the performance of the OEO-based temperature sensor system. In the experiment, the driving frequency is 11 GHz and the temperature is set at 40, 60, 81, 91 and 101 °C. The measured frequency responses of the OEO at different temperature are demonstrated in Fig. 7(a). With increasing temperature from 40 to 101 °C, the oscillation frequency shifts toward lower frequency from 272 to 211 MHz. This is because of that when the temperature gets higher, the BFS ${f_B}$ becomes larger and the ${f_{MPF}}$ gets smaller as shown in Figs. 1(b)–1(d). Figure 7(b) depicts the measured oscillation frequency as a function of the applied temperature on the sensing fiber from 40 to 101 °C. The sensitivity by linearly fitting the measured data in Fig. 7(b) is 1.00745 MHz/°C. The ${R^2}$ coefficient of determination is 0.999, indicating the regression line fits the data well.
Fig. 7. (a) Measured electrical spectra of the generated microwave signal at different temperatures; (b) Measured oscillation frequency as a function of the applied temperature to the sensing fiber from 40 to 101 °C when the driving frequency is 11 GHz.
In order to investigate the frequency stability of the system, we set the driving frequency at 11.5 GHz and the temperature at 40, 60, 81, 91, 101 °C. The measured frequency responses of the OEO at different temperature are demonstrated in Fig. 8(a). And the relationship between oscillation frequency and temperature is shown in Fig. 8(b). As can be seen, the tendency is same with Fig. 7. When the temperature gets higher, the oscillation frequency shifts toward lower frequency. Thus, the proposed scheme operates stable as long as the driving frequency is larger than the Brillouin frequency shift ${f_B}$.
Fig. 8. (a) Measured electrical spectra of the generated microwave signal at different temperatures; (b) Measured oscillation frequency as a function of the applied temperature to the sensing fiber from 40–101 °C when the driving frequency is 11.5 GHz.
We also study the resolution and the accuracy of the OEO-based temperature sensor system. In the experiment, the driving frequency is 11 GHz. The resolution of the oven used is 1 °C, so the temperature range is 60–70 °C with a step of 1 °C. The measured electrical spectra of the generated microwave signal are shown Fig. 9(a). The oscillation frequency varies with the temperature. Figure 9(b) illustrates the measured temperature as a function of the applied temperature and the measured errors, respectively. The maximum error of temperature obtained is within ± 0.5 °C. Therefore, the proposed temperature sensor has high sensitivity and high accuracy.
Fig. 9. (a) Measured electrical spectra of the generated microwave signal at different temperatures; (b) Measured temperature as a function of the applied temperature and Measured errors.
In order to verify the scheme is insensitive to strain, we apply strain to the sensing fiber. At the end of fiber (about 5 m), different values of strain are applied. The whole fiber is in the oven under 60 $^oC$ temperature. We compare the frequency responses of the OEO without and with strain. We found the oscillation frequency is same. This is not to say that the strain does not induce the change of BFS. As shown in Fig. 2, the BFSs induced by temperature (red line) and strain (green line) are different. The sidebands at these two locations experience gain and loss. Due to the SBS affected by temperature has longer interaction length than that by strain, the sidebands at red line will get more power. After conversion by PD, the power of generated microwave signal at ${f_{\textrm{Tem}}}$ is larger than that at ${f_{\textrm{Str}}}$. The generated signal is fed back to the PM and experience the same process. The signal at ${f_{\textrm{Tem}}}$ gets more and more power until the OEO is stable. As we know, there exists mode competition in a closed loop. In our scheme, most power concentrates in the signal at ${f_{\textrm{Tem}}}$ which leads to that the signal at ${f_{\textrm{Str}}}$ has not enough power to oscillate. As a result, there is only one signal which is affected by temperature in the OEO output. The cross-sensitivity of strain and temperature is solved. We can estimate the temperature by measuring the microwave frequency change.
To investigate the stability, the proposed temperature sensor is allowed to operate at room temperature for a period of 30 minutes. The oscillation frequency is recorded for 30 min with a step of 2 min which is shown in Fig. 10. The max frequency drift of the oscillation signal is within 500 KHz, which corresponds to a temperature measurement accuracy of 0.5 °C.
The resolution and accuracy of the proposed temperature sensor is limited by the quality of the generated signal. In our scheme, we use SBS effect to act an ultra-narrow filter. Although the generated signal has pure spectrum, the OEO is still under multiple longitudinal mode operation. If we apply a single longitudinal mode OEO to act a sensor, the resolution and accuracy will be improved greatly. In addition, the proposed scheme can realize a Fourier domain mode-locked OEO. The single passband MPF is implemented by using SBS. The pump light is a modulated signal whose frequency is determined by the driving frequency of the MZM. When adjusting the driving frequency, the MPF passband changes. If we set the driving signal at sweep mode, the MPF becomes a frequency scanning filter. By tuning the scanning period to synchronize with the cavity round-trip time, a mode-locked OEO is achieved.
4. Conclusion
In conclusion, we have proposed a high-precision and strain-insensitive temperature sensor based on an OEO. Utilizing SBS as a narrow filter, a single passband MPF whose center frequency is a function of the BFS is built. The oscillation frequency of the OEO is determined by the MPF. The variation of BFS induced by temperature maps to the change of the oscillation frequency. Thus, the temperature variations can be estimated through measuring the oscillation frequency. Due to mode competition in a closed OEO loop, the influence from strain is eliminated. We carry out a proof-to-concept experiment. Results show that a high sensitivity of 1.00745 MHz/°C is achieved. The maximum measurement error of temperature obtained is within ± 0.5 °C. The proposed scheme has merits of simple structure, low cost, high temperature sensitivity and strain-insensitive. Besides, the scheme can implement quasi-distributed sensing by using WDM. It has the potential applications in the structural health monitoring, pipeline security monitoring, ocean-bottom seismic system and so on.
Funding
National Natural Science Foundation of China (61405020, 61475029, 61635004, 61775023, 61905029, 61905030); Ministry of Science and Technology of the People's Republic of China (2016YFC0801202); China National Funds for Distinguished Young Scientists (61825501); Fundamental Research Funds for the Central Universities (2019CDXYGD0028).
Disclosures
The authors declare no conflicts of interest.
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