^{*}

This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution license (http://creativecommons.org/licenses/by/3.0/).

A distributed optical fiber sensor with the capability of simultaneously measuring temperature and strain is proposed using a large effective area non-zero dispersion shifted fiber (LEAF) with sub-meter spatial resolution. The Brillouin frequency shift is measured using Brillouin optical time-domain analysis (BOTDA) with differential pulse-width pair technique, while the spectrum shift of the Rayleigh backscatter is measured using optical frequency-domain reflectometry (OFDR). These shifts are the functions of both temperature and strain, and can be used as two independent parameters for the discrimination of temperature and strain. A 92 m measurable range with the spatial resolution of 50 cm is demonstrated experimentally, and accuracies of ±1.2 °C in temperature and ±15 με in strain could be achieved.

Optical fiber sensors (OFSs) have attracted intensive research interest around the World for several decades. They have already shown a superior advantage over their conventional electrical counterparts because of their distributed capabilities. A fully distributed OFS is usually operated by measuring the surrounding environment changes along the length of the sensing fiber. Several mechanisms have been successfully applied to fulfill this kind of measurement, such as Brillouin scattering, Raman scattering, as well as Rayleigh scattering. In terms of approaches, optical time-domain reflectometry (OTDR) and optical frequency-domain reflectometry (OFDR) have found their way to meet various practical needs. On the other hand, among a large amount of physical and chemical parameters which OFSs could measure, temperature and strain are the most widely studied, since many applications require accurate measurement of these two parameters, such as monitoring the health of large structures for their conditions, detecting pipeline buckling or leaking, and bridge deformation.

An OFS is sensitive to both strain and temperature, normally making a change in temperature indistinguishable from a change in strain. This cross sensitivity, which exists in almost all the OFSs, including distributed ones, would introduce errors when monitoring strain. In a Brillouin scattering based distributed sensor, as the Brillouin frequency is sensitive to both temperature and strain, and the Brillouin power in single-mode fiber (SMF) is also sensitive to temperature and strain, so Brillouin power and frequency can be used for simultaneous temperature and strain measurement [

For the OFDR technique, temperature and strain discrimination could be achieved in a PMF by measuring the autocorrelation and cross-correlation shifts of the Rayleigh backscatter data [

In this work, we propose to measure temperature and strain simultaneously with stimulated Brillouin scattering and Rayleigh backscatter in a single-mode fiber. Distributed Brillouin frequency shift (BFS) is measured by differential pulse-width pair Brillouin optical time-domain analysis (DPP-BOTDA) [

The experimental setup is shown in

The OFDR configuration consists of a tunable laser source (TLS; Agilent 81980A) operating around 1,550 nm with a continuous sweep mode. Since the polarization state of the scattered signal is arbitrary, we adopt the polarization diversity detection technique. When the laser is tuned, the interference signals obtained from the combination of the Rayleigh scattering signal and the local laser beam is split by a polarization beam splitter (PBS); the resultant “s” and “p” components are then received and digitized as a function of the TLS frequency by a two-channel acquisition. Fourier transform is then used to convert these frequency data into time-domain data; after performing a vector sum of the transformed “s” and “p” components [_{TLS}, in the measurement:
_{g}_{max} is determined by the differential delay in the trigger interferometer using the Nyquist sampling criteria by [_{g}

The two systems are combined together with a 1,310/1,550 nm wavelength division multiplexer 1 (WDM1). WDM2 is used to couple the TLS beam out of the fiber loop after it passes through the FUT; otherwise, the Rayleigh backscatter data might be corrupted by the reflection of the BOTDA components such as the attenuator and the isolator. A small fiber circle is made at the end of 1550-nm port of the WDM2 shown in _{B}_{T}_{,BFS} and _{T}_{,RBSS} are the temperature coefficients of BFS and RBSS respectively, while _{ε}_{,BFS} and _{ε}_{,RBSS} are the corresponding strain coefficients. _{ε}_{,RBSS}_{T}_{,BFS} − _{ε}_{,BFS}_{T}_{,RBSS}. The strain and temperature resolution of the sensor is determined by the condition of the mapping matrix in

Where _{B}

We use a 45/50 ns pulse pair for DPP-BOTDA measurement, determining a 50 cm spatial resolution. The peak power of the optical pulse is approximately 35 mW, and the continuous wave (CW) power from the other source is about 1 mW. The time-domain signals are monitored with a 1 GHz bandwidth AC-coupled PD, and 5,000 averages are taken at each frequency step. The sensing fiber is 92-m long, where near the end of the sensing fiber around 80 m, a section of 0.5 m fiber is used for axial strain measurement by attaching the fiber on a translation stage, and a 0.7 m section is placed in an oven for temperature measurement; the two sections are separated by about 1.5 m as shown in the bottom inset in

For the OFDR measurement, when the TLS is tuned, interference fringes that can be related to the complex reflectivity of the FUT are received at the detectors labeled “s” and “p” via a PBS, triggered by the trigger interferometer. The Rayleigh backscatter as a function of fiber length is obtained through Fourier transform [

Next, we calibrate the temperature and strain coefficients for BFS and RBSS by operating the two systems separately. We measure the BFSs for the first two peaks in LEAF obtaining their strain and temperature coefficients by linearly fitting the experimental data at different strain values and temperatures shown in

Finally, we investigate the distributed capability. We applied strain of 1,640 με and heat the fiber to 53 °C (room temperature of 25 °C) on two fiber sections simultaneously shown as the bottom inset in

In order to calculate the distribution of the temperature and strain, we re-sample the RBSS data by nearest-neighbor interpolation to obtain the same amount data points with those obtained through DPP-BOTDA. Then, substituting the re-sampled RBSS Δ_{B}

A distributed OFS which can simultaneously measure temperature and strain has been demonstrated. BFS distribution is measured by using the DPP-BOTDA approach, while the RBSS is obtained through the OFDR technique. The distributed discrimination for temperature and strain can be achieved provided that a pair of BFS and RBSS along the sensing fiber has been measured. A 92 m long sensing range, a 50 cm spatial resolution with the temperature and strain accuracies of ±1.2 °C and ±15 με is achieved. The spatial resolution and measurable range could in principle be further improved.

This work was supported by Natural Science and Engineering Research Council of Canada (NSERC) Discovery Grants and Canada Research Chair Program. D. P. Zhou would like to thank the Province of Ontario Ministry of Research and Innovation and the University of Ottawa for the financial support of the Vision 2020 Postdoctoral fellowship.

Experiment Setup. OFDR: optical frequency-domain reflectometry; TLS: tunable laser source; DAQ: data acquisition; EOM: electro-optic modulator; OC: optical coupler; PC: polarization controller; PD: photodetector; PBS: polarization beam splitter; WDM: wavelength division multiplexer; FUT: fiber under test. Bottom inset shows the sensing fiber section, where a 0.5 m long fiber could be applied axial strain by attaching the fiber on the translation stage, and a 0.7 m section is placed in the oven for varying temperature; the two sections are separated by about 1.5 m.

The 3-dimensional graphs of the Brillouin gain spectrum using (

Rayleigh backscatter as a function of fiber length.

Calibration of (

Measured (