1. Introduction

Fiber Bragg grating (FBG) technology has attracted considerable attention in the field of optical sensors since the sensing probe based on FBG can provide the most simple and attractive means to monitor a variety of external perturbations for example, temperature, strain, and pressure due to its high sensitivity, electro-magnetic immunity, compactness, and ease of fabrication [1–4]. A number of practical FBG based sensor systems have thus been implemented and installed for a range of applications such as temperature sensing, and structural strain monitoring. In such FBG based sensing systems one practical issue is to increase the transmission distance of sensing signals because their maximum transmission distance with a broadband light source is limited up to 25 km mainly due to Rayleigh scattering induced optical noise as well as background signal loss with the transmission fiber [5]. In order to increase the transmission distance and consequently to achieve a long-distance, remote sensing functionality, several novel methods based on Raman amplification in the sensing signal transmission fiber have been suggested. For example, Y. Nakajima et al. proposed a novel concept of the use of distributed Raman amplification in the signal transmission fiber to increase the sensing signal transmission distance over 50 km in a passive FBG sensor system [5]. However, the requirement of two separate light sources of a broadband light source and a Raman pump could be a limiting factor for practical implementation of such systems due to the corresponding higher system cost. Recently, P.-C. Peng et al. proposed an advanced concept of the use of the linear cavity Raman laser configuration based on a FBG and fiber loop mirror for a long-distance strain sensing system [6] to obtain advantages such as high resolution and high optical signal-to-noise ratio (SNR) and our group also demonstrated a simple, all-FBG based long-distance Raman laser temperature sensing system [7]. Although the use of Raman laser configuration eliminates the requirement of an additional broadband light source and improves significantly the sensing signal quality, it would be very hard to achieve the simultaneous measurement of temperature and strain using the previous system configurations.

In this paper, we demonstrate a novel and simple, Raman amplifier-based long-distance sensing system for simultaneous measurement of temperature and strain using a combined sensing probe of an erbium doped fiber and a fiber Bragg grating. Our proposed sensing system has only one pump source for distributed Raman amplification in the transmission fiber without any additional broadband light source and the residual pump power after the transmission fiber is recycled for generation of broadband amplified spontaneous emission (ASE) in the erbium-doped fiber. Using the proposed scheme, we obtain a remote sensing operation of simultaneous temperature and strain measurement at a location of 50 km and deliver the sensing signals through the transmission fiber with distributed Raman amplification. High quality of sensing signals with a ~11 dB SNR is readily achieved even after the 50 km transmission. The temperature sensitivity is measured to be 8.19 pm/°C (wavelength variation) and -0.04 dB/°C (optical power variation), and the strain sensitivity is 1.1 pm/µε (wavelength variation) without output signal power variation.

2. Experiment and results

Figure 1 shows the experimental setup for our proposed long-distance remote sensing system. The Raman pump source consists of two laser diodes operating at 1455 and 1465 nm, respectively. By combining the two pump wavelengths with a passive pump combiner a total pump power of up to 600 mW could be launched into a 50 km long standard single mode fiber (SMF) with a ~0.2 dB/km attenuation via a 1550/1460 nm WDM coupler. At first we measured an on-off Raman gain profile of the SMF using a tunable laser source and the measured Raman gain profile is shown in Fig. 2(a). This level of pump was able to provide an on-off gain of ~12 dB at a 1555 nm band within the SMF that was sufficient to compensate for the total cavity loss including background fiber loss. Then, the residual pump power after the SMF which was measured to be ~23 mW, was launched into a 10 m long EDF via two 1550/1460 nm WDM couplers. The residual Raman pump was reused as a pump source for the broadband ASE generation. An isolator was inserted between the two WDM couplers to suppress undesirable lasing within the sensing signal path. The sensing probe consisted of an EDF and a FBG which were connected directly by fusion splicing. The principle of simultaneous measurement of temperature and strain with a cascading EDF and FBG is well known and the full details can be found in Ref. [8]. The peak absorption coefficient of the EDF at 1530 nm was 6 dB/m. We used the UV beam scanning method with a phase mask to fabricate the FBG in a boron (B)-Germanium (Ge) codoped silica based photosensitive fiber. The center wavelengths of the FBG with a 0.2 nm spectral width was 1555.7 and its measured reflectivity was ~99.6 %. Figure 2(b) shows the optical spectrum of the generated ASE in the EDF after passing through the FBG. It is clearly evident that we could obtain sufficient ASE power for required sensing operation with the residual pump power.

 

Fig. 1. Experimental setup for our Raman amplifier-based long-distance remote sensor system using a sensing probe of an EDF and a FBG.

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Fig. 2. (a) Measured Raman gain profile for the 50 km single mode fiber used in this experiment with the two Raman pumps. (b) Measured optical spectrum of the generated ASE in the EDF with the residual pump power after passing through the FBG.

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First we performed temperature sensitivity measurement by placing the sensing probe composed of an EDF and a FBG into a temperature-heating oven. Note that the temperature-heating oven used in this experiment has a limited temperature tuning range between 30 °C and 100 °C. We measured center wavelength shift and output peak power variation of the sensing signal while changing the oven temperature in the range from 30 to 100 °C. Figure 3 shows the output optical spectrum of the sensing signal when the temperature was increased. Linear shift of the center wavelength was clearly observed with temperature change while the output peak power also linearly decreased correspondingly. It is clearly evident that high quality of sensing signal with a signal-to-noise ratio (SNR) of ~11 dB was obtained despite such a long distance transmission of 50 km with distributed Raman amplification. The measured data for center wavelength and output peak power as a function of applied temperature are summarized in Figs. 4(a) and (b), respectively. After fitting the measured data with a linear function, the temperature sensitivity in terms of wavelength variation was estimated to be 8.19 pm/°C with an estimated root mean square error (RMSE) of 11.1 pm, and output peak power variation was also estimated to be -0.04 dB/°C with a RMSE of 0.028 dB. The high RMSE value can be attributed to the poor stability of the temperature-heating oven without a proportional, integral, and derivative (PID) controller. The lower temperature sensitivity can be attributed to the B-Ge codoped photosensitive fiber used for our FBG fabrication. It is well known that the opposite temperature dependence between B ions and Ge ions makes B-Ge codoped fiber less sensitive to temperature change than Ge only doped fiber.

 

Fig. 3. Optical spectrum of the output sensing signal after the 50-km transmission when the temperature was increased

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Fig. 4. (a) Laser center wavelength and (b) output peak power as a function of applied temperature.

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Next we measured the strain sensitivity by applying tensile strain to the FBG at a room temperature (25 °C). The center wavelength increase was observed to be a linear function of applied strain as shown in Fig. 5(a). However, significant output peak power variation of the sensing signal was not observed although a trivial amount of output peak power reduction occurred with higher strain due to uneven flatness of the broadband ASE spectrum (see Fig. 2(b)). The measured data for center wavelength as a function of applied strain are summarized in Fig. 5(b). After fitting the measured data with a linear function, we found the strain sensitivity to be 1.1 pm/µε. The estimated RMSE of the wavelength measurement is 9.5 pm. By subtracting the temperature effect from the strain effect in the data of the sensing signal center-wavelength and peak power we can easily achieve simultaneous measurement of temperature and strain.

 

Fig. 5. (a) Optical spectrum of the output sensing signal after the 50-km transmission when the applied tensile strain was increased. (b) Sensing signal center wavelength as a function of applied tensile strain.

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Using the measured temperature and strain sensitivities, a sensitivity matrix for the simultaneous measurement of temperature and strain can be derived as follows.

where ΔT and Δε are the changes in temperature and strain, respectively. Δλ and ΔP represent the changes in the center wavelength and output peak power, each. By making the inverse sensitivity matrix (K ^-1) and multiplying it to the vector with the measured Δλ and ΔP of the sensing signal, we can thus achieve the simultaneous measurement of temperature and strain. The measurement resolutions of our system for temperature and strain were estimated to be 0.7 °C and 8.64 µε, respectively using the RMSE values of output peak power and wavelength variation.

3. Discussion and conclusion

We have demonstrated a novel, Raman amplifier-based long-distance sensing system for the simultaneous measurement of temperature and strain using a combined sensing probe of an erbium doped fiber and a fiber Bragg grating [8]. By recycling the residual Raman pump power for generation of ASE in the EDF after distributed Raman amplification in the transmission fiber the overall system configuration was significantly simplified without requiring any additional broadband light source. Using the proposed sensing scheme, we achieved a remote sensing operation of simultaneous temperature and strain measurement at a location of 50 km and delivered the sensing signals through the transmission fiber with distributed Raman amplification. The non-flat gain profile of the Raman amplifier and the EDFA in our system could degrade our sensing signal interrogation sensitivities in some degree. However, the incorporation of a gain-flattening filter would be a solution for the problem. Increase of Raman pump power would lead to further increase of both the sensing signal SNR and the transmission distance.