Laser-interrogated optical-fiber mechanicaland chemical sensorsG. Gagliardi, M. Salza, P. Ferraro, P. De NataleCNR - Istituto Nazionale di Ottica Applicata (INOA), sezione di NapoliComprensorio A. Olivetti, via Campi Flegrei 34, 80078 Pozzuoli (NA) G. Gagliardi, M. Salza, P. Ferraro, P. De Natale,"Fiber Bragg-grating strain sensor interrogation using laser radio-frequency modulation," Opt. Express 13, 2377-2384 (2005)J. Barnes, B. Carver, J. Fraser, G. Gagliardi, H.-P. Loock, Z. Tian, M. Wilson, S.Yam, O. Yastrubshak, Loss determination in microsphere resonators by phase-shift cavity ring-down measurements, Opt. Express 16, 13158-13167 (2008). K. Bescherer, S. Dias, G. Gagliardi, H.P. Loock, N.R. Trefiak, H. Waechter, S. Yam, Measurement of Multi-Exponential Decays by Phase-Shift Cavity Ring-Down, Appl. Phys. B 96, 390-397 (2009)Different systems using fiber-optic structures as chemical and mechanical sensors are shown. The interrogation approach is based on laser-frequency modulation techniques with single fiber Bragg-gratings (FBGs), or resonators made from high-reflectivity FBGs, and fiber rings. In the case of fiber resonators the laser is frequency locked to the cavity resonance by an in-fiber Pound-Drever-Hall (PDH) scheme. For strain detection in static and dynamic regime, the locking-loop error and correction signals fed back to the laser both serve as strain monitor with sub-picostrain sensitivity. On the other hand, the resonators fiber optical field may be evanescently coupled to the external environment, providing information on the external medium refractive index and/or concentration. Minimally-invasive, selective chemical sensing can be thus carried out on liquid and gas species absorbing in the lasers wavelength range.Here, we show our experimental work based on a frequency-modulation spectroscopy scheme. The idea is to superimpose sidebands to the laser carrier to obtain three different frequencies interrogating an FBG structure. Using this method, both static and dynamic strain measurements can be performed, with a sensitivity of the order of 150 ne/Hz in the quasi-static domain (2Hz) and 1.6 ne/Hz at higher frequencies (1 kHz). A further improvement of the first apparatus relies on resonant in-fiber optical resonators, such as -shifted FBGs or FBG Fabry-Prot. The basic principle is to interrogate it by a frequency-locked laser using Pound-Drever-Hall (PDH) technique (at 2-10 MHz). The reflected signal from the cavity is demodulated by a mixer that gives a dispersive-like signal containing information about the strain. The idea is that the laser follows any shift of the resonance eventually caused by changes of the intra-cavity optical pathlength. Test strain signals can be applied to the fiber in a controlled manner via a piezo actuator. Similarly, acceleration measurements can be performed by a proper mechanical mount.Two different systems devoted to chemical sensing in liquids are presented. A first set-up exploits the sensitivity enhancement of a whispering-gallery microsphere resonator made from the tip of a common silica fiber. Another one relies on evanescent-wave spectroscopy in a fiber-loop resonator. The detection principle is to probe the resonant absorption due to an external medium that induces extra losses in the cavities. This can be accomplished measuring the change in the intrinsic cavity-decay time by either phase-shift ring-down spectroscopy (PS CRDS) or direct mode-locked cavity-enhanced spectroscopy (CEAS). Cavity-based strain sensor G. Gagliardi, M. Salza, P. Ferraro, P. De Natale, Interrogation of FBG-based strain sensors by means of laser radio-frequency modulation techniquesJ. Opt. A: Pure Appl. Opt. 8 (2006) S507S513G. Gagliardi, P. Ferraro, S. De Nicola, P. De Natale Interrogation of fiber Bragg-grating resonators by polarization-spectroscopy laser-frequency locking, Opt. Express 15, 3715-3728 (2007). G. Gagliardi, M. Salza, P. Ferraro, P. De Natale, A. Di Maio, S. Carlino , G. De Natale and E. Boschi, Design and test of a laser-based optical-fiber Bragg-grating accelerometer for seismic applications, Meas. Sci. Technol. 19 (2008) 085306 Optical-frequency combfor lasers noise reduction-shifted-FBG 3D optical accelerometerEDA = 1.5 1015 cm-2 -Department of Chemistry, Queen's University, Kingston, ON (Canada)Centre for Photonics and Optical Engineering, Optical Sensors Group, Cranfield University, Bedford (UK)-Fibre Optics Laboratory, Central Glass & Ceramic Research Institute (CSIR), Calcutta (India)-Centre for Gravitational Wave, Australian National University, Canberra (AU)International collaborationsRelated publicationsFiber-loop evanescent-wave sensorWhispering-gallery-mode spherical resonator sensorTransmission and backscattered spectra of a silica microsphere300-micron Silica sphereEvanescent-wave ring-down spectroscopy of ETDAfrom the spheres backscattered field:Phase-shift spectraFrom Rayleigh scatteringA 1560-nm diode laser is locked on thefiber ring resonance:The transmission spectrum contains informationon the RI and loss change caused by the sample (EDA)Applications:Real-time analysis of biological compoundsEnvironmental monitoringCavitys transmission decrease and shift due to evanescent-wave absorption by liquid EDAQuasi-static sensingAcoustic sensing3-axis simultaneous acceleration-shifted FBGSensors performance:3D sensing1.5-kHz bandwidth20 g/Hz sensitivity100 g dynamic rangeApplications:Seismic monitoringGravimetryGiroscopic sensingtowards the 10-14 levelPatentsIT patent on fiber cavity laser locking. RM2006A000279. Inventors: G. Gagliardi, P. Ferraro, S. De Nicola, P. De Natale (2007).IT patent on Fiber-optic seismic sensor. RM2007A000589. Inventors (CNR-INOA): G. Gagliardi, M. Salza, A. Di Maio, P. Ferraro, P. De Natale. (2007)EU patent on fiber cavity laser locking. PCT/IT2007/000365. Inventors: G. Gagliardi, P. Ferraro, S. De Nicola, P. De Natale (2008).

CantileversexcitationStrain-to-accelerationFrequency locking