Time-frequency analysis of long fiber Bragg gratings with low reflectivity

A new technique to investigate the spatial distribution of the reflection spectrum along fabricated long weak fiber Bragg gratings (FBG) is experimentally demonstrated, together with its potential applications for distributed fiber sensing and broadband signal processing. A short pulsed coherent light signal is launched into a FBG and the signal frequency is scanned through the FBG reflection spectrum. When the pulse duration is set much shorter than the transit time through the grating a time-resolved reflected signal can be obtained for each signal frequency. It informs about the distribution of the refractive index periodic perturbation along the entire FBG length, hence the uniformity or frequency chirp information of the fabricated FBG. This technique has been implemented to demonstrate a distributed temperature sensing system with high spatial resolution and to also realize a robust all-fiber tunable delay line for broadband signals.The authors wish to acknowledge the financial support of the European Commission through the COST Action TD1001 "OFSeSa"; Infraestructura FEDER UPVOV08-3E-008, FEDER UPVOV10-3E-492, Ministerio de Ciencia e Innovacion through the project TEC2011-29120C05-05, the Swiss National Science Foundation through project 200021-134546 and the EPFL Space Center.Sancho Durá, J.; Chin, S.; Barrera Vilar, D.; Sales Maicas, S.; Thevenaz, L. (2013). Time-frequency analysis of long fiber Bragg gratings with low reflectivity. Optics Express. 21(6):7171-7179. https://doi.org/10.1364/OE.21.00717171717179216Kersey, A. D., Davis, M. A., Patrick, H. J., LeBlanc, M., Koo, K. P., Askins, C. G., … Friebele, E. J. (1997). Fiber grating sensors. Journal of Lightwave Technology, 15(8), 1442-1463. doi:10.1109/50.618377Liu, W.-F., Liu, I.-M., Chung, L.-W., Huang, D.-W., & Yang, C. C. (2000). Acoustic-induced switching of the reflection wavelength in a fiber Bragg grating. Optics Letters, 25(18), 1319. doi:10.1364/ol.25.001319Azaña, J. (2007). Proposal of a uniform fiber Bragg grating as an ultrafast all-optical integrator. Optics Letters, 33(1), 4. doi:10.1364/ol.33.000004Littler, I. C. M., Rochette, M., & Eggleton, B. J. (2005). Adjustable bandwidth dispersionless bandpass FBG optical filter. Optics Express, 13(9), 3397. doi:10.1364/opex.13.003397Volanthen, M., Geiger, H., & Dakin, J. P. (1997). Distributed grating sensors using low-coherence reflectometry. Journal of Lightwave Technology, 15(11), 2076-2082. doi:10.1109/50.641525Hotate, K., & Kajiwara, K. (2008). Proposal and experimental verification of Bragg wavelength distribution measurement within a long-length FBG by synthesis of optical coherence function. Optics Express, 16(11), 7881. doi:10.1364/oe.16.007881Chen, L. R., Benjamin, S. D., Smith, P. W. E., & Sipe, J. E. (1997). Ultrashort pulse reflection from fiber gratings: a numerical investigation. Journal of Lightwave Technology, 15(8), 1503-1512. doi:10.1109/50.618383Azana, J., & Muriel, M. A. (2003). Study of optical pulses - Fiber gratings interaction by means of joint time-frequency signal representations. Journal of Lightwave Technology, 21(11), 2931-2941. doi:10.1109/jlt.2003.819864Doran, N. J., & Wood, D. (1988). Nonlinear-optical loop mirror. Optics Letters, 13(1), 56. doi:10.1364/ol.13.000056Sanghoon Chin, Primerov, N., & Thevenaz, L. (2012). Sub-Centimeter Spatial Resolution in Distributed Fiber Sensing Based on Dynamic Brillouin Grating in Optical Fibers. IEEE Sensors Journal, 12(1), 189-194. doi:10.1109/jsen.2011.2126568Song, K. Y., He, Z., & Hotate, K. (2006). Distributed strain measurement with millimeter-order spatial resolution based on Brillouin optical correlation domain analysis. Optics Letters, 31(17), 2526. doi:10.1364/ol.31.002526Lancry, M., & Poumellec, B. (2013). UV laser processing and multiphoton absorption processes in optical telecommunication fiber materials. Physics Reports, 523(4), 207-229. doi:10.1016/j.physrep.2012.09.00