Simplex-coded BOTDA fiber sensor with 1 m spatial resolution over a 50 km range

Abstract

In this Letter, we propose the use of optical pulse coding techniques for long-range distributed sensors based on Brillouin optical time-domain analysis (BOTDA). Compared to conventional BOTDA sensors, optical coding provides a significant sensing-range enhancement, allowing for temperature and strain measurements with 1 m spatial resolution over 50 km of standard single-mode fiber, with an accuracy of 2.2°C/44 , respectively. © 2010 Optical Society of America OCIS codes: 060.2370, 060.4370, 280.4788, 290.5900. In recent years, distributed optical fiber sensors based on stimulated Brillouin scattering (SBS) have attracted a great interest owing to their unique ability to carry out high-performance strain and temperature measurements over long distances [1,2]. In the time-domain approach, the so-called Brillouin optical time-domain analysis (BOTDA) [1-3], a pulsed pump beam and a counterpropagating continuouswave (CW) probe beam, at different frequencies, interact through the intercession of an acoustic wave. Power transfer between both optical beams takes place at any position along the fiber when the frequency offset between them is within the local Brillouin gain spectrum (BGS). The frequency showing maximum gain is called Brillouin frequency shift (BFS) and depends linearly on strain and temperature, allowing us to perform distributed sensing [1]. The best performance reported so far for long-range BOTDA sensors results in 2 m/5 m spatial resolution over 40 km/51 km single-mode fiber [2,3], where pump depletion and modulation instability are the main factors limiting the sensing range [1,4]. In this Letter, we propose, for what we believe to be the first time, the use of optical pulse coding in a BOTDA sensor. We demonstrate that this technique effectively enhances the signal dynamic range, resulting in an extension of the sensing distance in BOTDA-based systems and providing the best performance reported so far, to our knowledge, temperature and strain sensing with 1 m spatial resolution over 50 km of standard single-mode fiber with an accuracy of 2.2°C/44 at the far end of the fiber.In BOTDA sensors, the BGS is reconstructed along the fiber by sweeping the frequency offset ͑⌬͒ between the two counterpropagating optical signals around the BFS. Thus, intensity variations of the probe signal ⌬I CW are measured at the near end of the fiber ͑z =0͒ as a function of time t and ⌬ and can be expressed as [1] ͑1͒where I CWL is the input probe intensity at the far end of the fiber ͑z = L͒, ␣ is the fiber loss, L is the fiber length, v g is the group velocity, ⌬z is the spatial resolution related to the pump pulse duration, and g B ͑ , ⌬͒ and I P ͑ , ⌬͒ are the BGS and the pump intensity at position z = .From Eq. (1) we clearly notice that the CWintensity contrast ͑⌬I CW ͒ mainly depends on the spatial resolution and the pump intensity. Thus, when short spatial resolution is required, the energy transferred to the probe is actually small, reducing ⌬I CW . This feature leads to measurements with low signalto-noise r...

Long-range BOTDA sensing using optical pulse coding and single source bi-directional distributed Raman amplification

“…In normal operating conditions, the energy transfer due to SBS amplification in a BOTDA sensor is very small, and therefore, the intensity variations in the probe signal at the photo-receiver can be linearized [3] as follows…”

“…Equation (2) points out a well-known trade-off between measurement SNR (and hence, measurand resolution) and the maximum sensing distance (considering a given spatial resolution). A significant SNR improvement can be provided by optical pulse coding schemes [3], in which the single-pulsed pump is replaced by a sequence of pulses following i.e. Simplex coding scheme.…”

“…While some techniques have been proposed to achieve high spatial resolution sensing [2], other ones have been focused on extending the sensing range while keeping an accurate temperature/strain resolution, for instance by the use of pulse coding [3] or the use of distributed Raman amplification [4]. Recent studies proposed the combined use of Raman amplification and optical pulse coding to achieve BOTDA with very long measurement ranges and more accurate temperature/strain resolutions [5].…”

BOTDA sensor with 2-m spatial resolution over 120 km distance using bi-directional distributed Raman amplification

“…In particular, Brillouin optical time-domain analysis (BOTDA) sensors are based on the temperature and strain dependence of the Brillouin frequency shift parameter [1][2], allowing for distributed temperature and strain sensing over tens of km of single-mode optical fibers (SMF). Such distributed measurements are of great interest to monitor civil and industrial infrastructures.…”

Long-range Brillouin optical time-domain analysis sensor employing pulse coding techniques

“…In principle, then, by applying pulse coding, the interaction between pump and probe signals in BOTDA sensors can be effectively increased [11,12], thus overcoming the typical trade-off between spatial resolution and sensing range. This possibility has been recently implemented and early results have been obtained in long-range as well as short-range sensors [11][12][13]. In this paper we present a detailed theoretical analysis of optical pulse coding techniques applied to BOTDA sensors, providing a thorough investigation of the underlying assumptions, followed by relevant experimental tests aimed at implementing and validating the proposed technique.…”