In this article, we propose and experimentally demonstrate a fiber Bragg grating (FBG) sensor interrogation technique based on an optoelectronic oscillator (OEO). The main components of the OEO loop in this proposed scheme contains an electro-optic modulator (EOM), a section of dispersive element, an electric filter, and a photodiode (PD). The reflection signal of the FBG sensor is functioning as the optical source of the OEO. The oscillating frequency of the OEO is determined by the overall time delay of the OEO loop. Due to the dispersive element in the loop, time delay of the OEO loop is a function of the OEO optical source wavelength. As a result, the wavelength change of the FBG can be converted into the oscillating frequency shift of the OEO. A proof-of-concept FBG based axial strain sensing experiment is carried out. The experimental results show that the frequency of the OEO generated microwave signals have a good linear relationship with the axial strain applied to the FBG. The sensitivity is about 58 Hz/με when using dispersion compensation fiber (DCF) with dispersion of −120 ps/(nm*km) as the dispersive medium and tracking the microwave signal with frequency near 2056.4 MHz, which is consistent with the theoretical calculation. The proposed method can also be applied to interrogate optical sensors based on detecting the wavelength change of the optical signals.
© 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
Fiber Bragg grating (FBG) sensors have been widely studied for their unique features such as light, small size, immunity to electronic magnetics . The fundamental principle of the FBG sensors is that the change of parameters under test will lead to either the change of effective index or the pitch of the FBG, and thus the resonance wavelength shift of the FBG. Typically, FBG sensor interrogation is realized through tracking the resonance wavelength of the FBG with the equipment such as OSA, optical edge filter, CCD, unbalanced fiber interferometers . With the development of the microwave photonics technology, FBG sensor interrogation proposals employing microwave photonics links have been proposed and experimental demonstrated [2–4]. The basic concept of the microwave photonic technology-based FBG sensor proposals is mapping the wavelength shift of the FBG sensor into the change of the signals in microwave domain, such as frequency transmission response change of the microwave photonics link [5,6], intensity change  or phase shift  of the microwave signals. Compared with optical signals, microwave signals have a relatively low frequency and are easy to be detected with a faster detecting speed.
Among lots of microwave technology-based optical sensing interrogation systems, optoelectronic oscillator (OEO) based fiber optic sensors have drawn lots of attention for that sensing information can be directly resolved by detecting the frequency of microwave signals generated by the OEO [9,10]; what’s more, OEO is a close hybrid optic-electronics loop which has been widely studied as a promising method to generate microwave signals with good stability and low noise [11–14]. Those features enable the OEO based optical fiber sensing system with advantages such as easy to detect, fast interrogation speed, and high sensitivity.
The fundamental principle of OEO based optic fiber sensors is converting the sensing parameters into the frequency change of the OEO generated microwave signals. As a basic feature of the OEO, its oscillating frequency is determined by the time delay of the whole OEO link. By purposefully design the OEO structure, measurands (such as optical length [15–18], refractive index (RI) , temperature , acoustic , strain ) can directly affect the equivalent optical length as well as the time delay of the OEO loop. As a result, those sensors can be interrogated by detecting the frequency of the microwave signals generated by OEO. Another approach to implement an OEO-based fiber optic sensing system is tuning the passband of the microwave photonics filter (MPF) contained in the OEO loop. The sensor parameters which can lead to the frequency shift of the MPF passband can be resolved by tracking the frequency of OEO generated microwave signals. OEO integrated with a single band pass MPF based fiber M-Z interferometer sensor  and optic integrated RI sensor  have been proposed and demonstrated with good linearity and high sensitivity. OEO containing an equivalent MPF in the link has also been applied to demodulate sensors based on fiber gratings. Phase shifted FBG (PS-FBG) [25–28], FBG based FP interferometer (FBG-FP) sensors [29–31] employing OEO technology have been demonstrated with high sensitivity and fast interrogation speed.
In practical engineering applications, optical wavelength encoded FBG sensors are most favorable and widely used fiber grating-based sensors. However, all previously proposed OEO-based interrogation schemes cannot be used to demodulate normal FBG sensors to the best of our knowledge. In this paper, we propose an interrogation scheme for normal FBG sensors, which employs an OEO loop containing a dispersive medium. Optical spectrum reflected by the FBG works as the monitoring signal of the FBG sensor and also the optical source of the OEO. By introducing dispersion into the OEO loop, the overall equivalent optical path as well as the time delay of the OEO is related to the wavelength of the optical source. Therefore, measurand variations which lead to the resonance wavelength shifts of the FBG can also tune the oscillating frequency of the OEO. By tracking the frequency of the microwave signal generated by the OEO, the FBG sensor can be interrogated, and higher order harmonics enjoy higher sensitivity. An FBG based axial strain sensor has been demonstrated in the experiment for verification and the results show that the frequency of the tracking signals has a good linear relationship with the applied strains.
2. Experimental setup and principles
The experimental setup of the proposed FBG sensor interrogation system based on a dispersion link OEO is shown in Fig. 1. An Er-doped fiber based amplifier spontaneous emission source is used as the broadband optical source (BOS). Light emits from the BOS is sent to the FBG sensor through an optical circulator (CIR). Optical source reflected by the FBG is amplified with an Er-doped fiber amplifier (EDFA, Amonics AEDFA-PA-25) in order to compensate the loss and provide sufficient optical power to the OEO system. The amplified signal is transmitted to an electro-optic modulator (EOM, JDSU 20GHz) after a polarization controller (PC) to align light with the EOM. After passing through the EOM, the amplitude modulated optical signal is transmitted and delayed by a dispersion compensation module (DCM, Oclaro). The DCM in this experiment is made up of dispersion compensation fiber (DCF) and serves as the dispersive element. Electrical signals can be recovered when the modulated light reaches the photodiode (PD, Conquer KG-PT 20GHz). The recovered microwave signals are amplified with a microwave amplifier (EA, Conquer KG-RF-10) and part of the microwave signals are sent to the electrical spectrum analyzer (ESA, HP8593E) for detecting via a microwave coupler (MC, Mini-Circuits). The other part of the microwave signals are sent back to the OEO loop again by the EOM. An electrical filter is inserted in the OEO link to suppress extra harmonic signals.
As it is shown in Fig. 1, an FBG is used as an axial strain sensor and the reflection signal of the FBG is working as the OEO optical source. In the experiment, the FBG is glued to a travelable stage and the axial strain can be applied to the FBG sensor by moving the stage. The resonance wavelength of the FBG (defined as) will shift when the axial strain applied to it due to the elastic of the material. Typically, the axial strain () caused wavelength shift () of the FBG can be expressed as Eq. (1) .
According to the working principle of the OEO, the initial oscillating microwave signals of the OEO are originated from noise. When the gain of microwave signals transmitting in the OEO link overcomes the loss, microwave signals start to oscillate and pure microwave signals can be generated. As presented in Fig. 1, the overall time delay () of the OEO loop consists of two parts: electrical link which consists of electrical cables and electrical devices (EA, Filter, MC, PD); optical link includes EOM, DCF and single mode fiber (SMF) pigtails. can be expressed as:
The free spectrum range (FSR) of the microwave harmonic and the frequency () of the th harmonic can be expressed as:
The frequency shift () of induced by optical wavelength shift () due to the dispersion effect of the DCF can be deduced as:Eq. (5). Taking Eq. (1) into account, the relationship between the and can be deduced as:
Equation (5) and Eq. (6) indicate that the frequency shift of each harmonic has a linear relationship with the optical wavelength as well as the axial strain applied to the FBG, and higher order harmonics have higher sensitivities ().
3. Experimental results and discussions
The spectrum of the FBG used in our experiment is measured firstly, which is shown in Fig. 2(a). It has a high reflection (~90%) and resonance wavelength near 1550 nm.
During this test, the axial strain is applied to the FBG with a step of 116 με by moving the translation stage. Figures 3(a) and 3(b) show the reflection spectra of the FBG under different axial strains. As can be seen from the Fig. 3(a), the resonance wavelength of the FBG increases with the axial strain increasing. The resonance wavelength almost returns to the same corresponding position when the axial strain decreases step by step, as indicated in the Fig. 3(b). Figure 3(c) presents the wavelength shift of the FBG resonance wavelength under different axial strains and the linear fitting results.
From Fig. 3(c), one can see that the resonance wavelength of the FBG sensor vary linearly with the axial strain. Namely, the wavelength of the OEO optical source has a linear relationship with the axial strain. The fitted results show that the FBG based axial strain sensor has a sensitivity about 1.17 pm/με, which is very close to the sensitivity predicted by Eq. (1). In this test, the overall wavelength shift of the FBG is about 3.64 nm, which corresponds to 3132 με axial strain applied to the FBG sensor.
Lastly, we characterize this FBG based axial strain sensor by using the interrogation scheme proposed in Fig. 1. The DCM used in this demonstration contains a DCF with a length of ~700 m. The DCF has a value of −120 ps/nm*km and total dispersion provided by the DCM is about −84 ps/nm (C band). The measured group delay curve of the DCM is shown as Fig. 2(b). The electronic filter used to filter out superfluous harmonics is a bandpass filter with center frequency at 2056 MHz. At the beginning of the test, with the rising of the magnification factor of the EDFA and EA, microwave signals with frequency within the band of the electronic filter are generated. The measured spectrum of the generated harmonics with frequency around 2056 MHz is presented in Fig. 4.
From Fig. 4, one can see that microwave harmonics have equal frequency separation and the measured FSR is about 287 kHz, which agrees with the estimated link length of the OEO loop. In this case, we choose the harmonic with frequency near 2056.4 MHz as the tracking signal.
The relationship between the frequency of the tracking microwave signal and the axial strain applied to the FBG is portrayed in Fig. 5. In this test, the spectra of the microwave signals are recorded when different axial strains are applied to the FBG sensor. To test the hysteresis of the interrogating system, the frequencies of the tracking signal with both increasing and decreasing the axial strain are recorded. From Figs. 5(a) and 5(b), one can see that with the increase of the strain applied to FBG, the frequency of the tracking microwave signal shifts to high frequency range, while the frequency of the tracking signal shifts to the lower frequency range when the strain applied to the FBG decreases. This characteristic is consistent with the analysis presented in section 2.
Results in Fig. 5(c) give the relationship between the frequency shift of the detecting signal and the axial strain applied to the FBG sensor. The measured frequency shifts of the microwave signal have a linear relationship with the axial strain applied to the FBG sensor and the estimated sensitivity of the OEO based FBG axial strain sensor is about 58 Hz/με, which agrees well with the theoretically deduced sensitivity (61 Hz/με, ignoring the effects of the SMF and the electric line).The experimental results indicate that the FBG sensor is successfully interrogated by tracking the frequency of the microwave signal generated by the OEO.
In order to test the stability of the sensor, we recorded the frequency of the tracking signal for 45 minutes with an interval of 1 minute when a constant strain is applied to the FBG sensor, the measured results show that the maximum frequency variation of the tracking signal is around ± 0.3 kHz, which corresponds to ± 5.2 με axial strain measurement error. Part of the frequency variation can be ascribed to the temperature variation of the environment.
According to the Eq. (5), the sensitivity is proportional to the frequency of the tracking signal due to the accumulative effect. Taking into account this principle, the sensitivity can be improved by tracking the harmonic with high frequency. In order to generate high frequency microwave signals, we replace the electronic filter with a bandpass filter whose center frequency is around 3555 MHz. The spectra of the tracking signal under different strains are shown in Figs. 6(a) and 6(b). The sensitivity is estimated to 100 Hz/με when tracking the microwave signal with frequency at 3555 MHz according to the fitted results shown in Fig. 6(c). In this case, by tracking microwave signal with higher frequency, higher sensitivity can be achieved.
It is worth noting that, with the same axial strain applied to the FBG, high frequency signal has a larger frequency shift. In this condition, the frequency shift will exceed the FSR range and bring ambiguity to discriminate the strain value. Figure 6 just presents the results when the strain applied to the FBG in the range of 0-2784 με, which is smaller than the range shown in Fig. 5(c). Therefore, there is a trade-off between the sensitivity and the measurement range.
We have proposed a novel FBG sensor interrogation method based on an OEO with a section of DCF in the loop. An FBG based axial strain sensor is experimentally demonstrated for verification. The experimental results indicate that the frequency of the tracking microwave signal has a good linear relationship with the FBG resonance wavelength as well as the axial strain applied to the FBG. The sensitivity of the axial strain is estimated to about 58 Hz/με when tracking the signal with frequency around 2056.4 MHz. The sensitivity can be further increased by employing DCF with larger dispersion coefficient or devices with larger dispersion value like chirped FBG. Since the basic concept of this method is converting the wavelength change of the FBG sensor into the frequency shift of the generated microwave signals of the OEO loop, the proposed method can also be applied to other FBG based sensors such as FBG based temperature, transverse load sensors. Besides FBG sensor demodulation, this method is also applicable for interrogating other wavelength-encoded fiber sensors.
Natural Science Foundation of China (NSFC) (61775074). 111 Project (B07038).
1. A. D. Kersey, M. A. Davis, H. J. Patrick, M. LeBlanc, K. P. Koo, C. G. Askins, M. A. Putnam, and E. J. Friebele, “Fiber grating sensors,” J. Lit. Technol. 15(8), 1442–1463 (1997). [CrossRef]
2. J. Hervas, A. L. Ricchiuti, W. Li, N. H. Zhu, C. R. Fernandez-Pousa, S. Sales, M. Li, and J. Capmany, “Microwave Photonics for Optical Sensors,” IEEE J. Sel. Top. Quantum Electron. 23(2), 327–339 (2017). [CrossRef]
3. J. P. Yao, “Microwave Photonics for High-Resolution and High-Speed Interrogation of Fiber Bragg Grating Sensors,” Fiber Integr. Opt. 34(4), 204–216 (2015). [CrossRef]
4. L. R. Chen, M.-I. Comanici, P. Moslemi, J. J. Hu, and P. Kung, “A Review of Recent Results on Simultaneous Interrogation of Multiple Fiber Bragg Grating-Based Sensors Using Microwave Photonics,” Appl. Sci. (Basel) 9(2), 298 (2019). [CrossRef]
5. A. L. Ricchiuti, J. Hervás, and S. Sales, “Cascade FBGs distributed sensors interrogation using microwave photonics filtering techniques,” Opt. Laser Technol. 77, 144–150 (2016). [CrossRef]
6. R. Cheng, L. Xia, J. Yan, J. Zhou, Y. Wen, and J. Rohollahnejad, “Radio Frequency FBG-Based Interferometer for Remote Adaptive Strain Monitoring,” IEEE Photonics Technol. Lett. 27(15), 1577–1580 (2015). [CrossRef]
7. J. Zhou, L. Xia, R. Cheng, Y. Wen, and J. Rohollahnejad, “Radio-frequency unbalanced M-Z interferometer for wavelength interrogation of fiber Bragg grating sensors,” Opt. Lett. 41(2), 313–316 (2016). [CrossRef] [PubMed]
9. X. H. Zou, X. K. Liu, W. Z. Li, P. X. Li, W. Pan, L. S. Yan, and L. Y. Shao, “Optoelectronic Oscillators (OEOs) to sensing, measurement, and detection,” IEEE J. Quantum Electron. 52(1), 1–16 (2016). [CrossRef]
10. J. P. Yao, “Optoelectronic Oscillators for High Speed and High Resolution Optical Sensing,” J. Lightwave Technol. 35(16), 3489–3497 (2017). [CrossRef]
11. X. S. Yao and L. Maleki, “Optoelectronic microwave oscillator,” J. Opt. Soc. Am. B 13(8), 1725–1735 (1996). [CrossRef]
12. L. Maleki, “The optoelectronic oscillator,” Nat. Photonics 5(12), 728–730 (2011). [CrossRef]
14. A. Liu, Y. Yang, R. Song, J. Liu, J. Dai, Z. Tian, and K. Xu, “High-performance millimeter-wave synergetic optoelectronic oscillator with regenerative frequency-dividing oscillation technique,” Opt. Express 27(7), 9848–9856 (2019). [CrossRef] [PubMed]
15. X. Zou, M. Li, W. Pan, B. Luo, L. Yan, and L. Shao, “Optical length change measurement via RF frequency shift analysis of incoherent light source based optoelectronic oscillator,” Opt. Express 22(9), 11129–11139 (2014). [CrossRef] [PubMed]
16. J. Lee, S. Park, D. H. Seo, S. H. Yim, S. Yoon, and D. Cho, “Displacement measurement using an optoelectronic oscillator with an intra-loop Michelson interferometer,” Opt. Express 24(19), 21910–21920 (2016). [CrossRef] [PubMed]
17. P. Cui, L. Yang, Y. Guo, J. Lin, Y. Liu, and J. Zhu, “Absolute Distance Measurement Using an Optical Comb and an Optoelectronic Oscillator,” IEEE Photonics Technol. Lett. 30(8), 744–747 (2018). [CrossRef]
18. J. Wang, J. Yu, W. Miao, B. Sun, S. Jia, W. Wang, and Q. Wu, “Long-range, high-precision absolute distance measurement based on two optoelectronic oscillators,” Opt. Lett. 39(15), 4412–4415 (2014). [CrossRef] [PubMed]
19. L. D. Nguyen, K. Nakatani, and B. Journet, “Refractive Index Measurement by using an optoelectronic oscillator,” IEEE Photonics Technol. Lett. 22(12), 857–859 (2010). [CrossRef]
23. Y. Wang, J. Zhang, and J. Yao, “An Optoelectronic Oscillator for High Sensitivity Temperature Sensing,” IEEE Photonics Technol. Lett. 28(13), 1458–1461 (2016). [CrossRef]
24. S. X. Chew, X. Yi, W. Yang, C. Wu, L. Li, L. Nguyen, and R. Minasian, “Optoelectronic oscillator based sensor using an on-chip sensing probe,” IEEE Photonics J. 9(2), 1–9 (2017). [CrossRef]
26. F. Kong, B. Romeira, J. Zhang, W. Li, and J. Yao, “A dual-wavelength fiber ring laser incorporating an injection-coupled optoelectronic oscillator and its application to transverse load sensing,” J. Lightwave Technol. 32(9), 1784–1793 (2014). [CrossRef]
27. Q. Shi, Y. Wang, Y. Cui, W. Xia, D. Guo, and M. Wang, “Resolution-enhanced fiber grating refractive index sensor based on an optoelectronic oscillator,” IEEE Sens. J. 18(23), 9562–9567 (2018). [CrossRef]
28. J. Liu, M. Wang, Y. Tang, Y. Yang, Y. Wu, W. Jin, and S. Jian, “Switchable Optoelectronic Oscillator Using an FM-PS-FBG for Strain and Temperature Sensing,” IEEE Photonics Technol. Lett. 29(23), 2008–2011 (2017). [CrossRef]
29. Y. Yang, M. Wang, Y. Shen, T. Yu, Z. Jing, W. Yue, S. Xiao, J. Liu, B. Wei, and D. Qi, “Refractive Index and Temperature Sensing based on an optoelectronic oscillator incorporating a Fabry-Perot fiber Bragg grating,” IEEE Photonics J. 10(1), 1–9 (2017).
30. B. Yin, M. Wang, S. Wu, Y. Tang, S. Feng, and H. Zhang, “High sensitivity axial strain and temperature sensor based on dual-frequency optoelectronic oscillator using PMFBG Fabry-Perot filter,” Opt. Express 25(13), 14106–14113 (2017). [CrossRef] [PubMed]
31. B. Wu, M. Wang, Y. Dong, Y. Tang, H. Mu, H. Li, B. Yin, F. Yan, and Z. Han, “Magnetic field sensor based on a dual-frequency optoelectronic oscillator using cascaded magnetostrictive alloy-fiber Bragg grating-Fabry Perot and fiber Bragg grating-Fabry Perot filters,” Opt. Express 26(21), 27628–27638 (2018). [CrossRef] [PubMed]