We demonstrate a practical and simple, all fiber Bragg grating (FBG) based Raman laser sensing probe for long-distance, remote temperature sensing application. Using multiple laser cavities based on FBG’s and a tunable chirped FBG, we obtain the simultaneous multichannel remote temperature sensing operation at a 50 km distance. The temperature sensitivity is measured to be 7.15 pm/°C.
©2004 Optical Society of America
Fiber Bragg grating (FBG) technology in the field of optical sensors has advanced significantly in recent years due to their wavelength selective nature, and resulted in the demonstration and installation of practical FBG based sensor systems for a variety of applications such as temperature sensing, structural strain monitoring [1–4]. The sensing probe based on FBG provides the most simple and attractive method to detect the variation of external perturbations for example, temperature, strain, and pressure due to its high sensitivity, electro-magnetic immunity, compactness, and ease of fabrication. In such FBG based sensing systems one practical issue is to increase the sensing signal transmission distance 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 . In order to increase the transmission distance and consequently to achieve a long-distance, remote sensing functionality, several novel methods have been suggested. For example, Y. Nakajima et al. demonstrated a passive FBG sensor system using distributed Raman amplification along the transmission fiber length, and good operation of this sensor system was achieved over a 50 km distance . An alternative and potentially simple FBG sensor approach is based on fiber laser since the FBG laser sensor provides several advantages like high resolution and high optical signal-to-noise ratio (SNR) . Recently, P.-C. Peng et al. proposed a novel concept of a FBG based linear cavity Raman laser for a long-distance strain sensing system . However, the use of a fiber loop mirror to construct the lasing cavity could be a limiting factor in terms of overall system stability, polarization sensitivity, and simplicity of configuration.
In this paper, we demonstrate a more practical and simple, all FBG based Raman laser sensing probe for long-distance, remote temperature sensing systems. The proposed laser sensor has two independent resonance cavities defined by a combination of two FBG’s and a tunable broadband chirped FBG  with a high reflectivity. Using the proposed method, we obtain simultaneous two-channel remote temperature sensing operation at a location of 50 km. High quality of Raman laser outputs with a ~50 dB SNR is obtained and the temperature sensitivity is measured to be 7.15 pm/°C.
2. Experiment and results
Figure 1 shows the experimental setup for the proposed all FBG based Raman laser temperature sensor system. The Raman laser consists of a 50 km standard single mode fiber (SMF), a tunable chirped FBG for a cavity mirror, and two FBG’s operating as both sensing probes and cavity mirrors. The photosensitive fiber used for our FBG fabrication was boron (B)-Germanium (Ge) codoped silica fiber which was chosen due to its high photosensitivity. We used the UV beam scanning method with a phase mask to fabricate all of the uniform FBG’s used in this experiment. In order to measure the spectral profile of the FBG’s we launched a tunable laser beam with a fixed optical power into the FBG’s via a circulator tuning the operating wavelength over a range of 1540~1570 nm with a 0.1 nm step, and monitored the reflected output using an optical spectrum analyzer (OSA). The resolution bandwidth (RB) of the OSA used in this experiment was always fixed at a value of 0.1 nm. The center wavelengths of two FBG’s with a 0.2 nm spectral width were 1554.2, and 1556.7 nm, each and their measured reflectivity was ~99.6 % as shown in Fig. 2. Two FBG’s were connected to one end of the SMF in a serial form and they were placed into a temperature-heating oven for temperature sensitivity measurement. The temperature-heating oven used in this experiment has a limited temperature tuning range between 30°C and 100°C and a poor temperature stability. The tunable chirped FBG was spliced to the other end of the SMF to form two laser cavities together with the two sensing FBG’s. The tunable chirped FBG was fabricated with the S-bending method using a uniform FBG embedded onto a thin and highly flexible metal plate . Its reflection bandwidth is easily tunable by varying the bending angle of the metal plate. The operating principle and schematics is fully explained in Ref. . In addition, the dynamic range of the proposed sensing system can be enhanced and the cavity structure of multichannel Raman fiber lasers can be simplified since its bandwidth is flexibly controllable and broad. By properly controlling the angle we adjusted the spectral bandwidth of the grating to cover the whole spectral range of the FBG1 and the FBG2 with a sufficient margin as shown in Fig. 2. The tunable chirped FBG showed a better reflectivity at the longer wavelength band (~1558nm) whilst some level of reflection loss was observed at the shorter wavelength band (~1554 nm). The shorter wavelength band showed a reflectivity of ~99.3% whilst a ~99.8 % reflectivity was observed at the longer wavelength band. The reflectivity ripple over the 4 nm bandwidth was ~0.4 %.
The Raman pump source consists of four laser diodes operating at 1425, 1435, 1455, and 1465 nm, respectively. By combining all the pump wavelengths with a passive 14XX/C-band WDM coupler a total pump power of up to 1 W could be launched into the SMF bypassing the chirped FBG. At first we measured an on-off Raman gain profile of the 50 km SMF using a tunable laser source and the measured Raman gain profile is shown in Fig. 3. This level of pump was able to provide an on-off gain at a 1550 nm band within the SMF that was sufficient to compensate for the total cavity loss including background fiber loss. Due to the inhomogeneous nature of gain broadening of Raman amplification we could generate simultaneous two wavelength oscillations in this laser configuration. The laser output through the WDM coupler is monitored by an OSA.
Figure 4 shows the output optical spectrum of the Raman laser. High quality of two independent lasing outputs at different wavelengths is clearly evident with a SNR of more than 50 dB. However, the two lasing outputs show some level of discrepancy in terms of output intensity and spectral linewidth. The intensity difference can be attributed to both non-uniform reflectivity of the chirped FBG (See Fig. 2) and the Raman gain gradient (Fig. 3). The spectral linewidth discrepancy is believed to be associated with the different quality of the two sensing FBG’s. As shown in Fig. 2 the FBG2 has low quality of reflection spectrum and highly undesirable side-lobes which may result in the broad linewidth of the lasing signal from the FBG2.
In order to characterize our Raman laser sensor we measured the center wavelength shift of the laser outputs while changing the temperature on the sensing FBG’s in the range from 30 to 100 °C. Actually we performed the temperature sensitivity measurement only for the FBG1 since we believed that the temperature sensitivity of the FBG2 should be the same to that of the FBG1 due to the same photosensitive fiber used for both of the FBG’s. Figure 5(a) shows the optical output spectrum of the Raman laser output associated with FBG1 when the temperature increases. Linear shift of the center wavelength was clearly observed with the temperature change. The measured data for center wavelength as a function of the applied temperature are summarized in Fig. 5(b). After fitting the measured data with a linear function, we found an equation governing the relationship between laser center wavelength associated with the FBG1 and temperature as follows.
The temperature sensitivity was estimated to be 7.15 pm/°C. The estimated root mean square error (RMSE) of the wavelength measurement is 4.9 pm. 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.
3. Discussion and conclusion
We have demonstrated a simple, all FBG based Raman laser temperature sensor for long-distance, remote sensing applications. Using a simple Raman laser configuration with multiple sensing FBG’s and a tunable chirped FBG, simultaneous multi-channel sensor was readily achieved over a 50 km distance. The use of a tunable chirped FBG as a second reflector can provide simplicity of the whole sensing system. Such a simple all FBG based Raman laser sensors should be able to find their application for a variety of long-distance, remote sensing systems.
References and links
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