A Long-Distance Remote Sensing Technique Using a Multiwavelength Raman Fiber Laser Based on Fiber Bragg Gratings Embedded in a Quartz Tube

A long-distance remote sensing technique using a multiwavelength Raman fiber laser with fiber Bragg gratings (FBGs) embedded in a quartz tube is proposed. A Raman linear cavity for multiwavelength generation was configured by using a long length of single mode fiber and FBGs. To discriminate concurrent sensitivities, such as temperature and strain sensitivities, FBG2 was etched to reduce the cladding diameter. Then, two FBGs with different cladding diameters put in a quartz capillary tube and FBG1 with a large diameter (125 μm) was firmly attached in a quartz capillary tube with UV curable epoxy. Since the two FBGs have identical material composition, the multiwavelength Raman fiber laser outputs have the same temperature sensitivities. However, since the FBG1 with a diameter of 125 μm was substantially fixed in a quartz capillary tube, the strain-induced wavelength shift was effectively suppressed. Since the FBG2 with a small cladding diameter of 60 μm only responds to the external strain change, the multiwavelength Raman fiber laser outputs have different strain sensitivities. Therefore, it is possible to simultaneously measure temperature and strain over a long distance of more than 75 km by using the proposed multiwavelength Raman fiber laser without additional input sources.