High sensitive temperature sensing system based on multi-wavelength fiber laser with dispersion shifted fiber

Bowen Chen; Tianshu Wang; Qingsong Jia; Zhen Wang; Desheng Zhao

doi:10.1364/OSAC.1.000401

1.Introduction

With rapid improving of fiber technology, fiber sensing system has been widely applied in many fields because of its advantages such as anti-electromagnetic interference, corrosion resistance, low price compared to traditional electrical sensing system [1,2]. Fiber sensing system based on laser beat frequency technology can be applied in transverse force sensing [3], vibration sensing [4], strain sensing [5,6]. In addition, high sensitive temperature sensing system based on fiber laser by using beat frequency technology has been improved in recent years, it has mainly three types including single-longitudinal mode fiber laser sensing system [7], multi-longitudinal mode fiber laser sensing system [8] and multi-wavelength fiber laser (MWFL) sensing system [9].

Because Brillouin frequency shift of fiber can change with temperature changing [10], temperature sensing system based on MBFL has been proposed [11–13]. Temperature sensing system based on MBFL has high temperature sensitivity [12,13], besides it has also stable Brillouin frequency shift and narrow linewidth and is easy to connect with other optical fiber devices [14–17].

Yang et al. proposed a temperature sensing system based on beat frequency technology by putting two fibers with different Brillouin frequency shift in a ring cavity laser [11], temperature information can be demodulated according to frequency variation of the beat frequency microwave signal, and temperature sensitivity measured is 1.015 MHz/°C. Iezzi proposed a high sensitivity temperature sensing system based on MBFL with double Brillouin frequency shift spacing based on a segment of single mode fiber (SMF) as nonlinear gain medium [12], and temperature sensitivity can be 6.92 MHz/°C by using 6th order Stokes light pair. Xu et al. demonstrated a temperature sensing system based on MBFL with single Brillouin frequency shift spacing using 25km long SMF [13], and temperature sensitivity can be 13.08 MHz/°C by using 12th order Stokes light pair.

We tested the number of the Brillouin multi-wavelength by using three kinds of gain fiber, they are SMF, dispersion compensation fiber (DCF) and the dispersion shifted fiber (DSF). We found that more Brillouin Stokes signals can be generated by using DSF at the same pump power. In this paper, a temperature sensing system based on MBFL is proposed and demonstrated. The structure is simpler than that in reference [12]. We used a 3.2km long DSF to produce double Brillouin frequency shift spacing. The diameter of mode field is 8.25 μm which leads to the effective area (A_eff) is about 53.456 μm², attenuation (A_tt) at the wavelength of 1550 nm is 0.206 dB/km, which leads to the effective length (L_eff) is about 4.854 km. With maximum pump power of 1.5 W, both MBFL can output more than 20 Stokes light wavelengths. By using the 18th order Stokes light pair, temperature sensitivity can be 19.93 MHz/°C. The uncertainty of the temperature measurement is ± 0.125°C .

2. Experimental setup and principle

Figure 1

Fig. 1 Experimental setup of the temperature sensing system. BP: Brillouin pump. EDFA1 and EDFA2: erbium-doped fiber amplifier. ISO: isolator. OC1~OC4: optical coupler. TOF: tunable optical filter. PD: photo-detector. ESA: electrical spectrum analyzer.

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shows the experimental setup of the temperature sensing system. Brillouin pump (BP) can be amplified by an erbium-doped fiber amplifier (EDFA1) of which the amplifying power is set to 1.5 W, then amplified BP injects into MBFL1 and MBFL2 respectively by a 3 dB coupler (OC1). Optical isolator (ISO) can protect EDFA1 from damaging caused by reflective light. In MBFL1, BP can inject into OC2 with the coupling ration 30/70 through port1 and can be divided into two beams. Once power injected into the DSF through port 3 with 70 percent ration exceeds Brillouin threshold of DSF, the first order Stokes light can generate in DSF and propagate backwards. Then the first order Stokes light enter into DSF again through port 2, the second order Stokes light can generate and propagate backwards when the power of the first order Stokes light exceeds Brillouin threshold. Then is divided into two parts by OC2, one injects into DSF through port 3 to repeat the process above, the other injects into OC4. This cascaded process continues until the next order Stokes threshold condition can’t be met. MBFL2 has the same structure as MBFL1 and the coupling ration of OC3 is the same as OC2, therefore, it can produce the same multi-wavelength laser. In the experiment, DSF of MBFL1 is used as sensing fiber and DSF of MBFL2 is reference fiber. Each even order Stokes light from two MBFLs respectively can be coupled by 3 dB OC4 and be filtered by TOF with bandwidth of 0.08 nm. The selected Stokes light pair can be amplified by EDFA2 with low noise and converted to beat frequency microwave signal by a photo-detector (PD). At last, frequency variation of beat frequency microwave signals can be observed on an electrical spectrum analyzer (ESA, Agilent CSA Spectrum Analyzer N1996A) when the temperature of the sensing fiber changes.

Brillouin frequency shift in fiber can be expressed as follows.

v_{B} = 2 n_{e} V_{A} / λ_{B P}

where n_e is effective refractive index of fiber, V_A is longitudinal phonon velocity of fiber, λ_B_P is central wavelength of BP. V_A can change as temperature of fiber change, then it can result in variation of v_B.

Frequency of the first order Stokes light in fiber can be shown as follows.

f_{B S}^{1} = f_{B P} - v_{B}

where

f_{B S}^{1}

is frequency of the first order Stokes light, f_BP is central frequency of BP. Frequency equation of the kth order Stokes light is as follows according to the cascaded process of SBS.

f_{B S}^{k} = f_{B P} - k v_{B}

The relationship between Brillouin frequency shift v_B of the sensing fiber and the change of temperature ∆T is as shown below

v_{B} (T, 0) = v_{B} (T_{0}, 0) (1 + C_{T}) Δ T

where C_T is temperature coefficient of the first order Stokes light and T₀ is the ambient temperature of reference fiber. The frequency equation of the beat frequency microwave signal produced by the kth order Stokes light pair can be expressed as follows in combination with Eq. (4)

f^{k} = k C_{T} Δ T = C_{T}^{k} Δ T

where

C_{T}^{k}

is temperature coefficient of kth order Stokes light. It can be seen from Eq. (5) there exists a linear relationship between frequency of microwave signal and temperature change of sensing fiber, and the temperature coefficient of the frequency of microwave signal by using kth order Stokes light pair is the k times as much as that by using the first order Stokes light pair, so high temperature sensitivity can be achieved by using high order Stokes light pair.

3. Results and discussion

Figure 2

Fig. 2 The output spectrum of MBFL 1 and 2.

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shows multi-wavelength spectrums of MBFL1 and MBFL2 measured by an optical spectrum analyzer (YOKOGAWA, AQ6370D), both MBFLs have more than 20 orders Stokes light wavelength. However, Stokes light more than 20th order are all too low to be used as beat frequency lasers. Intensity of each order Stokes light has a little difference between two MBFLs due to technology limit of fiber devices.

Figure 3

Fig. 3 (a): Each order Stokes light pair after TOF (b): Each order Stokes light pair amplified by EDFA 2

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shows multi-wavelength spectrum of 18 orders Stokes light. Obviously, intensity of each order Stokes light pair can be effectively enhanced by EDFA2 as shown in (b). The optical signal-to-noise ratio of each order Stokes light pair output from EDFA 2 is 6.2 dB lower than that of the input. Because the use of EDFA2 increase the optical noise. We just test the beat frequency microwave signals produced by 2nd~18th order Stokes light pair, because power of 20th and higher order Stokes light pair is too low after TOF to be amplified by EDFA 2.

In the experiment, Brillouin Stokes light pairs from 2nd order to 18th order are selected by tuning center wavelength of TOF. Figure 4

Fig. 4 Variation situation of central frequency of beat microwave signal by using each order Stokes light pair at different test temperatures.

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shows variation situation of central frequency of beat microwave signal by using each order Stokes light pair at different temperature change that are 10°C, 19.4°C, 28.8°C, 39.9°C, 50.5°C and 61°C respectively. Sensing fiber is in a temperature control box, and temperature of the fiber can be adjusted. In addition, temperature of reference fiber can keep room temperature of 22.5°C. It can be seen from Fig. 4 that the peak position of each order beat frequency signal in frequency spectrum is clear, therefore, the exact temperature information can be obtained. Central frequency of beat frequency microwave signal by the same order Stokes light increases as temperature of sensing fiber increases. Central frequency shift of beat frequency microwave signal increases with order enhancing of Stokes light pair when temperature of sensing fiber changes. Change of temperature has little effect on power and 3 dB width of beat frequency microwave signal produced by the same order Stokes light pair. The average 3 dB width of each order beat frequency signal are 26.34 MHz, 27.28 MHz, 28.13 MHz, 29.92 MHz, 30.24 MHz, 30.87 MHz, 31.17 MHz, 31.68 MHz, 32.35 MHz respectively. The MBFLs are multi-longitudinal-mode fiber lasers, this can broaden the Brillouin gain spectra. Besides, stimulated Brillouin scattering process weakens as the order of Brillouin Stokes light increases and spontaneous scattering becomes dominant, with the effect of thermal noise, high order beat frequency signal has larger width [12]. Intensity of beat frequency signal decreases with order enhancing of Brillouin Stokes pair. The reason is that the optical intensity of higher-order Brillouin Stokes light is relatively lower than lower-order shown as Fig. 2. Peak at vertical axis is DC noise produced by ESA.

Figure 5

Fig. 5 The relationship between the variation of the temperature change of sensing fiber and the central frequency of the beat frequency microwave signals produced by each order Stokes light pair.

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shows relationship between central frequency of beat frequency microwave signals produced by 18 orders Stokes light pair and temperature change of sensing fiber. Experimental results agree with Eq. (4) well. A linear relationship can be seen between frequency variation of beat frequency microwave signal and temperature change of sensing fiber. Temperature sensitivities of beat frequency microwave signals produced by 2nd~18th order Stokes light pair can be 2.20 MHz/°C, 4.56 MHz/°C, 6.53 MHz/°C, 8.94 MHz/°C, 11.04 MHz/°C, 13.27 MH/°C, 15.65 MHz/°C, 17.82 MHz/°C, and 19.93 MHz/°C respectively. Therefore, temperature sensitivity can increase by using higher order Stokes light pair and also can be selected according to different application requirements.

Stability of sensing signal is especially important for temperature sensing system. Under testing temperature of 62.4°C, we measured power jitter and frequency drift of beat frequency microwave signal generated by 18th order Stokes light pair every 5 minutes in half an hour. As in Fig. 6

Fig. 6 The power jitter and central frequency drift of the beat frequency microwave signal produced by 18th order Stokes light pair.

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, power fluctuation range is within 2 dB and drift range of central frequency is within 2.5 MHz. The frequency drift can result a uncertainty of temperature measurement of ± 0.125°C according to Eq. (5). Because the MBFLs are multi-longitudinal-mode fiber lasers, each longitudinal mode competes with each other, which can result the drift of the central wavelength of Brillouin Stokes light at peak power. Therefore, the central frequency of beat frequency signal produced by Brillouin Stokes light pair can generate drift. And the little shake of testing temperature observed on the viewing screen on temperature control box can also contribute to the drift of beat signal according to Eq. (5). It can be overcome by adding a filter before OC4 to reduce the number of longitudinal mode and using a temperature control box with more stable performance.

4. Conclusion

A simple temperature sensing system based on MBFL with double Brillouin frequency shift spacing is proposed and demonstrated in this paper. Highest temperature sensitivity can be 19.93 MHz/°C by using 18th order Stokes light pair. Under 62.4 °C, the maximum power jitter is less than 2 dB, frequency drift is within 2.5 MHz and the uncertainty of temperature measurement is ± 0.125°C.

Funding

National Natural Science Foundation of China (NSFC) (60907020); Science and Technology Project of Jilin Province (20170414041GH);Research Project of Jilin Provincial Education Department (JJKH20181090KJ).

Acknowledgments

This work was supported by National Natural Science Foundation of China (NSFC) (Grant No. 60907020), Science and Technology Project of Jilin Province (No. 20170414041GH) and Research Project of Jilin Provincial Education Department (No. JJKH20181090KJ).

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High sensitive temperature sensing system based on multi-wavelength fiber laser with dispersion shifted fiber

Abstract

1.Introduction

2. Experimental setup and principle

3. Results and discussion

4. Conclusion

Funding

Acknowledgments

References

Cited By

Figures (6)

Equations (5)

OSA Continuum