Multi-wavelength Brillouin erbium-doped fiber laser sensor with high tunable temperature sensing coefficient

Abstract

A multi-wavelength Brillouin erbium-doped fiber laser sensor is proposed and investigated experimentally. It is a linear laser cavity multi-wavelength fiber laser, formed by fixing two high-reflectivity Sangac loop mirrors at both ends. We investigate the laser system output characteristics and test the sensor performance for different temperature. Temperature/humidity and reliable linear-cavity fiber laser sensing operation were successfully achieved. Experimental results show that the temperature sensitivity is $27.15$ MHz/ °C and the measurement error caused by frequency drift is about $\pm 0. 2 8 5$ °C.

1 Introduction

For the practical application of fiber laser sensors, a high tunable sensing coefficient should be emphatically taken into account. Due to the characteristics of strict Brillouin frequency shift (BFS), the linear dependence of BFS on temperature and strain, narrow linewidth, as well as easy compatibility with other fiber optic components, the multi-wavelength fiber lasers (MWFLs) (Yeh et al. 2010; Yuan et al. 2014) based on SBS effect is very suitable for high tunable temperature sensitivity (Xu and Zhang 2017; Iezzi et al. 2014; Liu et al. 2017; Minardo et al. 2017). In spite of these above advantages, Brillouin frequency shift of the first-order Stokes usually used for temperature or strain measurement is very small, causing the temperature and strain sensitivity to be limited to $\sim1$ MHz/ °C and $\sim0.05\;{\text{MHz/}}\mu \varepsilon$, respectively (Xu and Zhang 2017). Such a small frequency shift also restricts the performance of the sensing system. In the sensing principle of the literature (Xu and Zhang 2017), the sensing coefficient of BFS is determined by the highest order of stable Stokes in temperature measurement, which is n times than that of the standard self-released Brillouin frequency shift, where n is the highest order of Stokes waves. When $n \ge 10$, the temperature sensitivity could be raised to the next order of magnitude.

In this paper, a multi-wavelength Brillouin erbium doped fiber laser (MWBEFL) (Hu et al. 2014; Qian et al. 2015; Xie et al. 2015; Motil et al. 2016) has been proposed and utilized as the temperature sensor, and high temperature sensitivity has been realized through combining the linear amplification of erbium-doped fiber with Brillouin nonlinear gain. In addition, only one volume of optical fiber is required as the sensing path to provide input and output signals, greatly reducing the complexity of the sensing configuration (Iezzi et al. 2014). The sensing mechanism and characteristics of the novel optical temperature sensor is studied. The measurement results show that the temperature sensing coefficient can be tunable from $1.086$ MHz/ °C to $27.15$ MHz/ °C. In addition, the error caused by frequency drift and relative humidity are innovatively analyzed, respectively.

2 Experimental structure

The structure of the proposed multi-wavelength erbium-doped fiber laser sensor (MWBEFL sensor) is depicted in Fig. 1. In this configuration, a linear laser cavity is formed by fixing two high-reflectivity Sangac loop mirrors at both ends, so as to improve the multi-wavelength output characteristic (Al-Alimi et al. 2014). The tunable range of the external cavity tunable laser source (TLS) is 110 nm (1470 ~ 1580 nm) with its maximum power of 1.3 mW, which is adopted to provide the seed pump (SP) light for the sensing MWBEFL. A circulator (Cir) is used to control the unidirectional propagation (clockwise) of light before entering the laser cavity. Its port-2 is connected to the 50% port of a 3 dB coupler (OC1) while the port-3 is connected to the optical spectrum analyzer (OSA, AQ6370B). A 980 nm pump laser with its maximum power of 600 mW is employed as the main pump, which constitutes an erbium doped fiber amplifier (EDFA) with a 980/1550 wavelength division multiplexing (WDM) and 6 m long erbium-doped fiber (EDF). The EDFA amplifies SP light at 1550 nm to act as Brillouin pump (BP) light. The Brillouin gain medium is provided by a 25 km long single mode fiber (SMF), whose nonlinear coefficient is equal to $1.1\;{\text{w}}^{ - 1} \;{\text{km}}^{ - 1}$, and effective area is $8 3. 9 7\;\upmu{\text{m}}^{ 2}$. Two 3 dB couplers, OC2 and OC3, separately constitute two optical fiber Sagnac rings (SR1 and SR2), both working as high reflectivity mirrors in the optical path. SMF in the laser cavity is also used as the sensing unit and is placed in the temperature controlling equipment, which is a constant temperature and constant humidity machine (TEMI880) with a resolution of 0.1 °C.

Fig. 1

The schematic diagram of the multi-wavelength Brillouin fiber laser sensor

3 Operational principle

Once turning on TLS, the seed pump signal enters into the optical path though Cir and OC1 sequentially and then is amplified by EDFA for the first time before being injected into SMF. The SBS effect will not be induced when SP power is so weak that the gained BP power is still lower than the stimulated Brillouin threshold (SBST). In contrast, a part of SP power will be absorbed by EDF. Once the power is sufficiently high and BP power reaches SBST, SBS effect occurs in the optical fiber, inducing the first order Stokes (BS1) signal with an obvious frequency shift in the opposite direction to BP.

The unconsumed BP returns to the light path along with BS1 after being reflected by SR1. After experiencing the linear amplification of EDFA for the second time, the signal power is divided into two parts. The signal waveform of half optical signal power can be observed by OSA after coming out from the port-3 of Cir while the remaining optical signal is reflected back to the laser cavity by SR2, which obtains the gain of EDFA to activate the second order Stokes signal (BS2) in SMF. The cascaded Stokes line is continuously generated until the total gain of the laser cavity is less than the cavity loss of the working wavelength. Simultaneously, the power of last order Stokes is lower than the SBST power. Finally, stable output channels including BP and multiple order Stokes lines will be generated in a steady state.

Based on the temperature sensing principle of SBS, the BFS of optical fiber can be seen as a linear function of temperature and strain, which can be described as

$$v_{B} (T,\varepsilon ) = v_{{B,T_{0} ,\varepsilon_{0} }} + C_{T} (T - T_{0} ) + C_{\varepsilon } (\varepsilon - \varepsilon_{0} )$$

(1)

where $C_{T}$ and $C_{\varepsilon }$ is the temperature sensing coefficient and the strain sensing coefficient of BFS, $v_{{B,T_{0} ,\varepsilon_{0} }}$ is the initial Brillouin shift of SMF at the reference temperature $T_{0}$ and the reference strain $\varepsilon_{0}$. If only considering the temperature change of SMF which can influence the effective refractive index and acoustic propagation velocity, $v_{B} (T)$ shows a linear dependence of BFS on temperature. When the temperature is changed from $T_{0}$ to $T$, $v_{B} (T)$ can be expressed as follows:

$$v_{B} (T) = v_{{B,T_{0} }} + C_{T} (T - T_{0} )$$

(2)

When the cascade SBS process produces multiple order Stokes wave and the central frequencies of BP and BS1 are recorded as $f_{BP}$ and $f_{BS1}$, it is obviously that

$$v_{B} = f_{BS1} - f_{BP}$$

(3)

Then the central frequency of the kth-order Stokes wave (BSk), $f_{BSk}$, can be expressed as:

$$f_{BSk} = f_{BP} + kv_{B}$$

(4)

Therefore, when the temperature is changed from $T_{0}$ to $T$, we can obtain

$$f_{BS1,T} - f_{{BS1,T_{0} }} = v_{B,T} - v_{{B,T_{0} }} = C_{T} \Delta T$$

(5)

$$f_{BSk,T} - f_{{BSk,T_{0} }} = k(v_{B,T} - v_{{B,T_{0} }} ) = kC_{T} \Delta T = C_{T}^{k} \Delta T$$

(6)

where $f_{BSk,T}$ and $f_{{BSk,T_{0} }}$ are corresponding to center frequencies of BSk at the temperature of $T$ and $T_{0}$, respectively. It can be seen that the central frequency shift (relative to BP) of BSk is k times than that of BS1, thus the temperature sensitivity coefficient of the former is k times than that of the latter. This means that higher temperature sensitivity coefficient can be achieved by using multi-wavelength Brillouin laser. If 10 or more Stokes wave can be induced in a steady state, the temperature sensitivity coefficient can be increased to the next order of magnitude theoretically.

4 Experimental results and characteristics analysis

When the 980 nm pump laser is tuned at 600 mW and TLS is turned off, SMF does not get any Brillouin pump to induce the SBS effect. The laser resonator can be seen as a bidirectional EDFL at this moment and the output spectrum is shown in Fig. 2. The EDFL performs the self-lasing mode within the range of 1550 ~ 1565 nm and the peak wavelength is 1557.9120 nm. Therefore, the output wavelength of TLS should be set within the above wavelength range.

Fig. 2

Self-lasing mode of MWBEFL

At room temperature (~ 20 °C), fixing the wavelength of TLS at 1550 nm, regulating the output power of TLS at 0.5 mW and the output power of 980 nm pump laser at 33 mW, BP reaches the Brillouin threshold (about − 5.9 dBm), which is injected into SMF, leading to the output of the first order Stokes line (BS1), as shown in Fig. 3. The frequency interval between BS1 and BP is 0.084 nm (i.e. $v_{B} = 0. 084$ nm).

Fig. 3

The generation of the first order Stokes line

When increasing the output power of 980 nm pump laser, more Stokes waves are generated and eventually 30 Stokes lines are obtained at 980 nm pump power of 600 mW, as shown in Fig. 4. Owing to the power of the highest order Stokes line as Brillouin pump is lower than the SBS threshold, the 31th order Stokes line can not be excited.

Fig. 4

The Stokes lines at SP power = 0.5 mW, 980 nm pomp power = 600 mW

In the temperature sensing experiment, the stability of the sensing MW-BEFL is very important to the performance of the system. Therefore, it is essential to evaluate the system stability of the MBEFL sensor. Fixing the output power of TLS at 0.5 mW, with a scanning frequency of every 10 min in 1 h, then the wavelength stability is represented in Fig. 5. From the experimental data, it is found that the power fluctuations of BS1 ~ BS25 are within 1 dBm, and the obvious wavelength shift is not observed. However, the power fluctuations of BS26 ~ B30 are larger than 1 dBm with poor stability. Because the light power of the signal injected into the SMF is limited by some parameters, such as intra-cavity loss, EDFA gain variation and cascaded loss of all devices, power depletion can occur if the input power is over the Brillouin threshold, especially for fiber optic sensing system based on Brillouin nonlinear effect. Therefore, in the following temperature sensing experiment, BS1 ~ BS25 will be selected for temperature sensing, so as to ensure the stability and accuracy of the sensing configure.

Fig. 5

Stability of BS1 ~ BS30

During the temperature sensing experiment, the output power of TLS is fixed at 0.5 mW at operating wavelength of 1550 nm, 980 nm pump power is set to 600 mW and the temperature (T) is increased from 0 to 100 °C by step of 10 °C. Based on these above conditions, a series of measurements have been carried out. Only the influence of temperature on BFS is considered, but humidity, strain and other physical factors are not taken into account. Thus, in the process of regulating temperature, the relative humidity (RH) coefficients at various temperatures are all set to 0% and any additional strain is not applied on SMF.

In order to observe the frequency shift of wavelength/frequency and to maintain the wavelength stability of the MWBEFL sensor, only 26 stable output channels (i.e. BP and BS1 ~ BS25) are selected for temperature measurement. The scanning range of OSA is kept within 1549.900 ~ 1552.200 nm, corresponding to the frequency range of 193.1404 ~ 193.4270 THz. When the temperature is 0 °C, the output waveform of 26 output channels is shown in Fig. 6.

Fig. 6

The output spectrum (T = 0 °C) of BP, BS1 ~ BS25 for temperature sensing

The SNRs of the selected 25 Stokes waves (BS1 ~ BS25) are all larger than 6 dB, among which the maximum value is about 9.6 dB, as shown in Fig. 7.

Fig. 7

The SNRs of BS1 ~ BS25

Regulating temperature as 0 °C, 50 °C and 100 °C successively, the output waveform data of the MW-BEFL sensor are recorded in each stable temperature condition. Figure 8 shows 26 output waveforms (BP, BS1 ~ B25) at T = 0 °C and T = 50 °C and Fig. 9 shows the output waveforms at T = 0 °C and T = 100 °C. It can be summed up that the greater temperature change ($\Delta T$) increases the larger frequency shift for the same order Stokes wave and the higher order of Stokes wave behaves the larger frequency shift at the same $\Delta T$, thus a higher temperature sensing coefficient $C_{T}^{K}$ can be achieved.

Fig. 8

The output waveforms of BP and BS1 ~ BS25 at T = 0 °C and T = 50 °C

Fig. 9

The output waveforms of BP and BS1 ~ BS25 at T = 0 °C and T = 100 °C

The linear fitting curves of the central frequency shift of BS1 ~ BS25 in the range of 0 ~ 100 °C with interval of 10 °C are drawn in Fig. 10, which contains 25 sub-graphs. In order to directly show the relationship between temperature, the central frequency shift and order of Stokes, taking the center frequency of each order Stokes wave at 0 °C as a reference, the relative central frequency shift of BS1 ~ BS25 in the range of 10 ~ 100 °C is all given in Fig. 11.

Fig. 10

The linear fitting curve of center frequency shift and temperature sensing of BS1 ~ BS25 at various temperature (T Temperature, f frequency)

Fig. 11

The linear fitting curve of relative center frequency shift of BS1 ~ BS25 at various temperature against 0 °C (T Temperature, f frequency)

Obviously, the central frequency shift corresponding to BS1 (i.e. BFS) has linear change with the increase of temperature. The central frequency shifts of BS2 ~ BS25 also show good linear dependence on temperature, and the higher order Stokes behaves a larger central frequency offset. In each sub graph, the temperature sensitivities of BS1 ~ BS25 which are obtained from all the linear fitting slopes are represented in the range from $1.086$ to $27.15$ MHz/ °C. This means that the temperature sensing coefficient of BS25 is approximately 25 times than that of BS1. The fitting curve of temperature sensitivity of each order Stokes is drawn in Fig. 12 by comparing experimental value and theoretical value. It is intuitively shown that the temperature sensitivities obtained in this experiment is basically consistent with the theoretical ones.

Fig. 12

Comparison between experimental and theoretical values of temperature sensitivity of 0 BS1 ~ BS25

Figure 13 provides the fluctuation power and center frequency drift of BS25 within 1 h at 40 °C. The maximum fluctuation of center frequency shift is 15.56 MHz and the power fluctuation is less than 1dBm, which show that the measurement error is about $\pm 0. 2 8 5$ °C when using BS25 for temperature sensing.

Fig. 13

The fluctuation power and center frequency offset of BS25 within 1 h at T = 40 °C

In addition, the interaction between Brillouin scattering and light field can be described by the steady-state coupling strength equation. The scattering coefficient is defined by these coupled equations, which depends on the physical parameters of materials, such as density, refractive index, longitudinal elastic light coefficient and so on (Lalam et al. 2015). Therefore, the physical phenomenon of the interaction between the optical field and the optical fiber may be affected by the ambient humidity and then the possible error effect of humidity on the proposed MWBEFL temperature sensor is analyzed as follows.

When fixing the temperature of FUT at 20 °C, 40 °C and 80 °C respectively, then adjusting the relative humidity (RH) from 20 to 98% at various temperatures, the corresponding spectrum waveforms are shown in Fig. 14a–c. Apparently, the change of RH at the same temperature has little influence on the output waveform of MWBEFL sensor.

Fig. 14

The output waveform of RH = 20% and RH = 98% at T = (a) 20 °C, (b) 40 °C, (c) 80 °C

Based on the data analysis, when adjusting the relative humidity from 20 to 98% at the temperature of 20 °C, 40 °C, 80 °C, the center frequency shift of BS25 changes 4.7 MHz, 4.6 MHz, 4.62 MHz respectively, resulting in the measurement error of $\pm 0. 0 8 6 6$ °C, $\pm 0. 0 8 4 7$ °C, $\pm 0. 0 8 5 1$ °C. In general, the measurement error caused by humidity is around $\pm 0. 0 8 5$ °C, which is much smaller than the measurement error of the sensor caused by the frequency drift. Normally, the influence of relative humidity on measurement results can be ignored.

For the proposed fiber sensor, the sensitivity coefficient of different order can be selected according to the practical sensing application. Besides, the multi-wavelength signals are used in the optical fiber sensing system as the detection light, the efficiency of the sensing optical fiber can be improved, and the measurement performance of the sensing system can be improved by signal processing. Besides, the proposed temperature sensing system only uses the ordinary single mode fiber as the sensing element, and it will be suitable for the high temperature sensing application. Its sensing temperature can reach the melting temperature ~ 1000 °C that the quartz fiber can bear. Due to the limit of temperature controller, our sensing experiment is carried out in the temperature range of 0 ~ 100 °C. However, because of the dense channel with small frequency interval and high light power density, the FWM phenomenon is remarkable, resulting in the decline of the system signal power and the rise of the crosstalk or error rate, which makes the performance of the system worse.

5 Conclusions

In this paper, a novel temperature sensor by utilizing a multi-wavelength fiber laser which combines erbium amplification with Brillouin gain is proposed and experimentally studied. The proposed MWBEFL sensor realizes high temperature sensitivity. Firstly, some basic characteristics of the MWBEFL sensor as multi-wavelength laser are analyzed. The range of self-lasing mode range is ~ 15 nm. The MWBEFL generates 31 output channels at most, and the waveform stability in 1 h is generally well. Secondly, SNRs of BS1 ~ BS25 are greater than 6 dB which are all selected for temperature sensing. Thirdly, the temperature sensing results show that the temperature sensitivity of $27.15$ MHz/ °C can be achieved, and the measurement error caused by frequency drift is about $\pm 0. 2 8 5$ °C. Finally, the influence of humidity on the temperature sensor is analyzed. The measurement error caused by the change of relative humidity from 20 to 98% at the same temperature is about $\pm 0. 0 8 5$ °C.