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Simultaneous temperature and strain sensing is experimentally demonstrated based on erbium-doped fiber laser (EDFL) and Brillouin erbium fiber laser (BEFL) incorporated in a single ring laser cavity. The EDFL can be switched to BEFL by injecting the Brillouin pump into the laser cavity. Longitudinal modes beat frequency and Brillouin frequency shift are monitored to discriminate strain and temperature. The longitudinal modes beat frequency is measured by observing the self-beating signals of the EDFL, while the Brillouin frequency shift is measured by monitoring the heterodyning signal of the BEFL. The simultaneous measurement errors of strain and temperature are within ± 25.8με and ± 0.8°C. The sensor is of simple structure and compact size.
© 2016 Optical Society of America
1. Introduction
Optical fiber sensors have attracted tremendous interest and been widely used to monitor structure health condition and damage identification. So far, optical fiber sensors based on various configurations have been reported, for example, the optical fiber Mach-Zehnder interferometer (MZI), fiber Bragg grating (FBG), high birefringence fiber and photonic crystal fiber based devices [1–6]. These configurations exhibit good performance in the applications of sensing temperature, strain and etc. However, most of the fiber-optic sensors suffer from a problem of cross sensitivity. Thus, simultaneously measurement or discrimination of strain and temperature has become a vital issue that should be solved in practical applications.
Recently, various optical fiber sensors have been reported for simultaneous measurement of strain and temperature. These fiber sensors are usually based on monitoring two physical parameters which have different sensitivities on strain and temperature [7–13]. Wavelength and peak power of fiber lasers are simultaneously monitored to separate strain and temperature [14,15]. But the peak powers of the lasers are usually of low sensitivity to the measured parameters, which will result in large measurement error. In [16], discrimination of strain and temperature is achieved by monitoring the reflective spectrum of a micro-Fabry-Perot cavity and a short fiber Bragg grating which are integrated together. Discriminative measurement is also realized based on cascaded long-period fiber gratings written on few-mode fiber and single mode fiber [17]. Although these sensors are compact and highly sensitive, they are of special structure and difficult to fabricate. Lately, a multimode fiber laser for simultaneous sensing has been proposed and demonstrated by monitoring the frequency variations of longitudinal modes and polarization modes [18]. The sensor is simple and easy to fabricate. However, since the polarization modes are highly sensitive to fiber bending [19], the measurement is vulnerable to ambient disturbance.
In this paper, we propose a combined configuration of erbium-doped fiber laser (EDFL) and Brillouin erbium fiber laser (BEFL) for simultaneous strain and temperature sensing. The EDFL and BEFL share the same laser cavity. The EDFL can be switched to BEFL by injection of the Brillouin pump (BP) into the laser cavity. Two parameters monitored for the discrimination of strain and temperature are longitudinal modes beat frequency (LMBF) and Brillouin frequency shift (BFS). The LMBF is measured by observing the self-beating signals of the EDFL, while the BFS is measured by monitoring the heterodyning signal of the BEFL. Both of the LMBF and BFS are immune to fiber bending, which improves the resistance to ambient disturbance. What’s more, the sensor is of simple structure and compact size.
2. Principle
As illustrated in Fig. 1, the erbium-doped fiber (EDF) used in the laser cavity serves as hybrid gain media, which provides both linear gain and Brillouin gain. Stimulated Brillouin scattering, generated in the EDF by injection of the BP, oscillates in the fiber ring and forms a Brillouin fiber laser. Compared to the BP, the Brillouin laser output has a frequency downshift, which is called as BFS [20]. The frequency shift can be given by
where is the BFS, is the refractive index of the fiber core, is the acoustic velocity in the fiber, and is the wavelength of BP in vacuum. The BFS is generally dependent on applied strain and temperature in the fiber. It has been experimentally demonstrated that the BFS is linearly dependent on the applied temperature and strain change. Thus, by measuring the change of BFS in the fiber, the information of strain or temperature variation can be obtained.In order to realize simultaneous measurement of strain and temperature, another physical parameter is required besides the BFS. In a ring cavity fiber laser, there are numbers of longitudinal modes. The beat frequency between two different modes is given by
where is the LMBF, and are longitudinal mode number, is the light velocity in vacuum, is the effective refractive index, is the whole cavity length. When the strain and temperature applied on the laser cavity varies, the LMBF will change accordingly.Once the BFS and LMBF dependence on the strain and temperature are obtained, variations of strain and temperature can be calculated by
where : applied strain change, : applied temperature change, (): strain (temperature) coefficient of BFS, (): strain (temperature) coefficient of LMBF, : BFS change, and : LMBF change. The coefficient matrix can be achieved by separately measuring the strain and temperature response of the BFS and the LMBF. Then, the strain and temperature can be measured by monitoring the BFS and the LMBF.3. Experiment and results
In the experiment, the fiber laser is constructed in ring cavity configuration. The ring cavity consists of a 3.8 m EDF, a wavelength division multiplexer (WDM), a circulator, a FBG, and two 3 dB coupler (C1 & C2). The EDF is utilized as the linear gain media and the nonlinear Brillouin gain media. The EDF has an ion concentration of , a cut-off wavelength of 912 nm, peak absorption of 6.44 dB per meter near 1530 nm and numerical aperture of 0.228. The WDM is utilized to couple the 980 nm pump light and BP signal into the EDF. The BP is provided by a tunable laser source (TLS, AP3350A). The TLS has a 3 dB linewidth of 10MHz and is tunable in C band. The central wavelength and 3-dB linewidth of the FBG are 1550.2 nm and 0.2 nm, respectively. The laser output is extracted from the 50% leg of C2. The 3.8 m EDF is placed inside an oven for temperature sensing. In addition, a section of the laser cavity of 0.65m single mode fiber (SMF) is loaded with different weights for strain sensing.
Without BP launched, the laser performs as a free-running EDFL at the EDF peak gain in the passband of FBG. The EDFL output is sent to a PD from one port of C2. The LMBF is generated at the PD and measured with an electrical spectrum analyzer (ESA).
When BP is injected into the EDF through the isolator and C1, SBS is excited in the EDF. It is amplified by both Brillouin gain from the SBS process and linear gain from the EDF. If the total gain exceeds the peak EDF gain in the passband of FBG, free-running modes will be suppressed. Once the cavity loss is overcome, the laser performs as a BEFL. The BFS is obtained by heterodyning the output of the BEFL with the amplified BP modulated by an electro-optic modulator (EOM) with 10 GHz microwave signal.
When the applied strain and temperature change, the BFS and LMBF will change accordingly. To measure the LMBF variation, we turn the laser at EDFL condition. To measure the variation of BFS, we turn the laser at BEFL condition by injecting BP into the EDF.
With 76 mW 980 nm pump power, an EDFL is produced as depicted in Fig. 2(a). The spectrum of the beating frequency signal is given in Fig. 2(b). It is found that there are many LMBFs. The frequency space between the two adjacent longitudinal modes is 16.07 MHz, which indicates that the cavity length is 12.9 m. According to Ref [21, 22], the high-frequency LMBFs have higher sensitivities but lower signal-to-noise ratio (SNR), while the low-frequency LMBFs have higher SNR but lower sensitivities. In order to balance the two parameters, the LMBF of 2397 MHz is chosen as the sensing signal with SNR as high as 55 dB.
With 76 mW 980 nm pump power and 8 mW BP power, the laser performs as a BEFL as shown in Fig. 3(a). The TLS wavelength is set at 1550.12 nm. The Brillouin wavelength space is 0.088 nm. The Brillouin laser has an optical signal-to-noise ratio (OSNR) higher than 30 dB with peak power as high as 1 mW. The heterodyning signal between BEFL and amplified BP after modulation are depicted in Fig. 3(b). The black solid line represents the spectrum of the measured signal after 100 times average. The red solid line represents the Lorentz fitting spectrum of the averaged spectrum. The BFS is deduced from the central frequency of the Lorentz fitting spectrum. In Fig. 3(b), the BFS is 11.231 GHz according to the Lorentz fitting spectrum.
First, we measure the temperature dependence of the BFS and LMBF by heating the EDF with an oven. In order to eliminate the temperature fluctuation of the oven, the temperature is kept at a fixed value for fifteen minutes before each measuring. The temperature is controlled from 50 up to 110°C, and a group of heterodyning signal spectra and LMBFs are recorded at each applied temperature. With increasing temperature, the heterodyning signal frequency shifts toward higher frequency as shown in Fig. 4(a). A Lorentzian fitting is utilized to derive the BFS from the beat spectrum. On the contrary, the LMBF move towards lower frequency since the cavity length expands with increasing temperature.
The responses of BFS and LMBF to temperature are shown in Fig. 5. The temperature sensitivities of BFS and LMBF are 0.88 MHz/°C and5.56 kHz/°C, respectively.
Then, the response of BFS and LMBF to strain are measured. The strain applied on the laser cavity for strain sensing is changed from 869.33με to 3477.32με. The response of LMBF to strain is given in Fig. 6. The strain sensitivity of LMBF is -0.12 kHz/με. Since the SBS process takes place in the EDF, BFS keeps unchanged in spite of the strain variation applied on the SMF. Thus, in the experiment, the response of BFS to the applied strain is 0.
Hence, with the obtained experimental results, four coefficients in Eq. (3) are as follows:
The temperature and strain measurement matrix can be given by:
According to this matrix, the temperature and strain can be calculated out after measuring the variations of BFS and LMBF. To evaluate the sensor response, a simultaneous temperature and strain test is implemented. The temperature applied on the fiber for temperature sensing is increased from 50 to 110°C, and the strain applied on the fiber for strain sensing is varied randomly from 869.33με to 3477.32με. Figure 7 shows the comparison between the measured and applied parameters. The maximum errors of strain and temperature obtained are within ± 25.8με and ± 0.8°C over the range of 869.33-3477.32με and 50-110°C.
Fig. 7 Comparison between simultaneously applied strain-temperature and measured strain-temperature.
Finally, the impact of FBG wavelength shift induced by ambient temperature variation on the sensor performance is analyzed. In the experiments, the FBG is a key element for the generation of both EDFL and BEFL. It determines the EDFL wavelength and tunable range of the BEFL. The central wavelength of the FBG varies as the ambient temperature changes. In order to measure its temperature sensitivity, the FBG is heated with an oven from 40°C to 90°C. The EDFL output under different temperature is recorded in Fig. 8(a). The temperature sensitivity of the FBG is calculated to be 11.13 pm/°C.
The EDFL wavelength changes with the central wavelength of the FBG. The EDFL is used for generation of the LMBF. The LMBF will vary with the EDFL wavelength due to the fact that the effective refractive index which is inversely proportional to the LMBF is wavelength dependent [23]. However, since the refractive index changes very slowly around 1550nm, as long as the ambient temperature doesn’t change dramatically, the LMBF variation is quite small. Therefore, stable EDFL can be realized under large ambient temperature change without giving rise to significant measurement error.
For the BEFL, if the variation of central wavelength of the FBG exceeds its tunable range, the Brillouin laser cannot be excited. A wide tunable range indicates that the Brillouin laser can work effectively under large ambient temperature change. In order to measure the tunable range of the BEFL, the TLS wavelength is gradually tuned. Figure 9 shows the tunable range of the BEFL. In the passband of the FBG, stable BEFL can be generated within 0.18 nm. Considering that the temperature sensitivity of the FBG is 11.13 pm/°C, the temperature variation range for stable Brillouin oscillation is calculated to be 16.2°C. Although the Brillouin laser cannot oscillate in the laser cavity as the ambient temperature changes over 16.2°C, by tuning the TLS wavelength to track the central wavelength of the FBG, the Brillouin laser will oscillate again in the cavity at another wavelength. Then, the operating temperature range of the BEFL increases. Therefore, the sensor based on combination of EDFL and BEFL can function well with the variation range of ambient temperature much larger than 16.2°C.
It can be deduced that in order to further improve the sensor’s resistance to ambient temperature change, a possible solution is to utilize FBG which has wider linewidth or which is less sensitive to temperature. The chirp grating has the potential to support the sensor to operate under large temperature variation due to its wider bandwidth compared to the uniform FBG [24]. Several temperature-insensitive FBG have been reported in previous research [25,26], which can be substitution for the temperature-sensitive FBG used in the experiment. Also, the FBG can be specially packaged to reduce the perturbation of outside temperature. Besides, thin-film based filter is a potential choice due to its high temperature stability [27].
4. Conclusion
In conclusion, we propose and experimentally demonstrate an incorporated configuration of EDFL and BEFL for simultaneous strain and temperature sensing. The EDFL and BEFL share the same laser cavity. By injecting the BP into the laser cavity, The EDFL can be switched to BEFL. LMBF and BFS are monitored to calculate out the variations of strain and temperature. The temperature sensitivities of BFS and LMBF are 0.88 MHz/°C and-5.56 kHz/°C, respectively. The strain sensitivity of LMBF and BFS is -0.12 kHz/με and 0, respectively. The experimental measurement errors are within ± 25.8με and ± 0.8°C. The advantage of the sensor is of simple structure and compact size and immune to fiber bending. Although the FBG used in the experiments is sensitive to temperature, the sensor can work effectively under large ambient temperature change. To further improve the sensor’s resistance to ambient temperature change, a possible solution is to utilize thin-film based filter or FBG with wider linewidth and lower sensitivity to temperature. Also, the FBG can be specially packaged to reduce the perturbation of outside temperature.
Acknowledgement
This work is supported by the National Natural Science Foundation of China (NSFC) under Grant No. 61377074 and Research Fund for the Doctoral Program of High Education of China under Grant 20120142130004.
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