IOP PUBLISHING JOURNAL OF OPTICS A: PURE AND APPLIED OPTICS J. Opt. A: Pure Appl. Opt. 9 (2007) 958–962 doi:10.1088/1464-4258/9/10/029 Fiber laser strain sensor device N Beverini 1,2,3 , E Maccioni 1,2,3,5 , M Morganti 1,2 , F Stefani 2 , R Falciai 4 and C Trono 4 1 Dipartimento di Fisica ‘E Fermi’, Universit` a di Pisa, Largo B Pontecorvo 2, 56127 Pisa, Italy 2 Istituto Nazionale di Fisica Nucleare, Largo B Pontecorvo 2, 56127 Pisa, Italy 3 Consorzio Nazionale Interuniversitario per le Scienze Fisiche e della Materia (CNISM) sezione di Pisa, Largo B Pontecorvo 2, 56127 Pisa, Italy 4 Istituto di Fisica Applicata ‘N Carrara’, IFAC-CNR, Via Madonna del Piano 2, 50019 Sesto Fiorentino, Firenze, Italy E-mail: maccioni@df.unipi.it Received 11 May 2007, accepted for publication 7 August 2007 Published 18 September 2007 Online at stacks.iop.org/JOptA/9/958 Abstract We present a fiber laser strain sensor (FLSS) with noise-equivalent sensitivity equal to or better than 80 pε rms (Hz) −1/2 at very low frequencies, from 100 mHz to several hundreds of hertz. The strain affects the fiber laser emission wavelength, and an imbalanced Mach–Zender interferometer (MZI) converts wavelength variations into phase-amplitude variations. The sensor has been also tested in the time domain by applying sinusoidal strain bursts: the device also shows a good signal-to-noise ratio at the lowest burst frequencies. Keywords: fiber laser sensor, strain measurement, strain sensor 1. Introduction Sensors based on fiber Bragg gratings (FBGs) offer many possible applications, concerning pressure, temperature and strain measurements [1–3]. FBGs present advantages in comparison with other sensors: they have small physical dimensions, can be used in hostile environmental conditions, and several FBGs can be implemented on the same optical fiber and interrogated by a broad-band light source to form a multiplexed sensor [4]. When an FBG is subjected to external perturbations, in terms of pressure, temperature or strain variations, its elasto-optics and thermo-optics properties are changed and quantitative information is encoded on the reflected wavelength [1, 2, 5]. Another family of remote- interrogated passive sensor rests on using in-fiber Fabry–Perot (FFP) resonators, formed by a pair of separated FBGs. A FFP shows improved sensitivity, gaining advantage from its reduced line-width which makes it a frequency discriminator better than a simple FBG sensor. For a lot of applications (monitoring of civil structures, rock deformation probing, seismic and geodynamical monitoring) there is a growing interest in ‘quasi- static’ strain sensing (0.01–100 Hz). In this regime, low- frequency acoustic noise, local temperature fluctuations and air 5 Author to whom any correspondence should be addressed. movements are the limiting factors to the attainable sensitivity. A sensitivity of 1.2nε(Hz) −1/2 was reached at 1.5 Hz by remote interrogation of an FBG through an external-cavity diode laser [6]. In another work [7, 8], a highly sophisticated stabilized laser system, using a Pound–Drever–Hall frequency locking scheme, was employed in the remote interrogation of an FFP. With this technique, a sensitivity of better than 1pε(Hz) −1/2 beyond 100 Hz, and of 6 pε(Hz) −1/2 around 100 Hz, was attained. By radio-frequency modulation of a commercial diode laser, a sensitivity of 150 nε(Hz) −1/2 at 2 Hz and of 1.6nε(Hz) −1/2 at 1 kHz was achieved with an FBG sensor [9]. The same scheme, applied to an FFP sensor, reached a sensitivity of 20 pε(Hz) −1/2 at 1.3 kHz, and a sensitivity better than 20 nε(Hz) −1/2 was estimated around 1 Hz [10]. All the above-described systems cannot be implemented easily in multiplexed strain sensor arrays. In fact, these devices use the resonance condition between the radiation emitted by the laser source and the reflection peak of the grating or of the Fabry–Perot cavity, and only one sensor at a time can be interrogated. Fiber Bragg lasers (FBLs) have demonstrated sensitivity at the level of few tens of femto-strain for signals in the kilohertz bandwidth [11], and their potentiality as acoustic sensors in water has already been exploited up to 100 kHz [3, 12, 13]. 1464-4258/07/100958+05$30.00 © 2007 IOP Publishing Ltd Printed in the UK 958