Optical Frequency Domain Reflectometry Technique Research Articles

Real time monitoring of water level and temperature in storage fuel pools through optical fibre sensors

We present an innovative architecture of a Rayleigh-based optical fibre sensor for the monitoring of water level and temperature inside storage nuclear fuel pools. This sensor, able to withstand the harsh constraints encountered under accidental conditions such as those pointed-out during the Fukushima-Daiichi event (temperature up to 100 °C and radiation dose level up to ~20 kGy), exploits the Optical Frequency Domain Reflectometry technique to remotely monitor a radiation resistant silica-based optical fibre i.e. its sensing probe. We validate the efficiency and the robustness of water level measurements, which are extrapolated from the temperature profile along the fibre length, in a dedicated test bench allowing the simulation of the environmental operating and accidental conditions. The conceived prototype ensures an easy, practical and no invasive integration into existing nuclear facilities. The obtained results represent a significant breakthrough and comfort the ability of the developed system to overcome both operating and accidental constraints providing the distributed profiles of the water level (0–to–5 m) and temperature (20–to–100 °C) with a resolution that in accidental condition is better than 3 cm and of ~0.5 °C respectively. These new sensors will be able, as safeguards, to contribute and reinforce the safety in existing and future nuclear power plants.

Time-gated digital optical frequency domain reflectometry with 1.6-m spatial resolution over entire 110-km range.

A novel time-gated digital optical frequency domain reflectometry (TGD-OFDR) technique with high spatial resolution over long measurement range is proposed and experimentally demonstrated. To solve the contradictory between the tuning rate of lightwave frequency, which determines the spatial resolution, and the measurable distance range in traditional OFDR, our proposed scheme sweeps the frequency of probe beam only within a time window, while the local reference remains a frequency-stable continuous lightwave. The frequency-to-distance mapping is digitally realized with equivalent references in data domain. In demonstrational experiments, a 1.6-m spatial resolution is obtained over an entire 110-km long fiber link, proving that the phase noises of the laser source as well as environmental perturbations are well suppressed. Meanwhile, the dynamic range was 26 dB with an average of only 373 measurements. The proposed reflectometry provides a simple-structure and high-performance solution for the applications where both high spatial resolution and long distance range are required.

Comparative study of long period and fiber Bragg gratings under gamma irradiation

We evaluated comparatively the radiation sensitivity of two long period gratings (LPGs) manufactured in a SMF28™ optical fiber through the melting-drawing method based on CO2 laser assisted by a micro-flame, of a commercial fiber Bragg grating (FBG), and of a commercially available Draw Tower Grating (DTG), all exposed to gamma radiation. The tests were run on-line using the optical frequency domain reflectometry (OFDR) technique, in order to provide unequal sensitivity, the gratings being wavelength multiplexed. The dose rate dependence of the LPGs response was evaluated for dose rates of 0.37kGy/h and 0.24kGy/h, up to the total dose of (33.9±0.8)kGy and (21.5±0.5) kGy. The FBG and DTG proved to be radiation hardened having a radiation sensitivity of 12pm/kGy, while the LPGs radiation sensitivity was as high as 1.2nm/kGy under the same irradiation conditions. A recovery of the radiation induced wavelength shift for LPGs was noticed at room temperature, at a rate of 2.2pm/h, over a 211h period. The effect of LPG wavelength change after exposure to gamma-ray can be exploited in the development of radiation dosimeters, up to the dose of 10kGy, when saturation appears. The temperature sensitivity of the LPG2 changed from 48pm/°C before the irradiation, to 50 pm/°C after the irradiation for a total dose of 21kGy.

Optical frequency domain reflectometry measurements of spatio-temporal temperature inside catalytic reactors: Applied to study wrong-way behavior

An Optical Backscatter Reflectometer (OBR) with optical frequency domain reflectometry (OFDR) enables measurement of the spatial temporal temperature inside catalytic reactors with unparalleled combined spatial and temporal resolution. It was used to measure the transient temperature inside a single channel of a washcoated monolith reactor in which the Pt-catalyzed oxidation of ammonia was carried out. Measurements of the monolith temperature response to either a rapid step decrease in the feed temperature or an increase of the flow rate were used to test the capability of the OFDR technique and to determine their impact on the transient temperature rise of the monolithic catalyst. The rapid step increase in the flow rate can generate a transient peak temperature of the solid phase higher than that at the steady state. This transient temperature rise was smaller than that caused by a sudden decrease of the feed temperature.

Centimeter-level spatial resolution over 40 km realized by bandwidth-division phase-noise-compensated OFDR

We present a bandwidth-division phase-noise-compensated optical frequency domain reflectometry (PNC-OFDR) technique, which permits a fast sweep of the optical source frequency. This method makes it possible to reduce the influence of environmental perturbation, which is the dominant factor degrading the spatial resolution of frequency-domain reflectometry at a long measurement range after compensation of the optical source phase noise. By using this approach, we realize a sub-cm spatial resolution over 40 km in a normal laboratory environment, and a 5 cm spatial resolution at 39.2 km in a field trial.

Distributed strain measurement in perfluorinated polymer optical fibres using optical frequency domain reflectometry

We present the latest advances in distributed strain measurement in perfluorinated polymer optical fibres (POFs) using backscatter techniques. Compared to previously introduced poly(methyl methacrylate) POFs, the measurement length can be extended to more than 500 m at improved spatial resolution of a few centimetres. It is shown that strain in a perfluorinated POF can be measured up to 100%. In parallel to these investigations, the incoherent optical frequency domain reflectometry (OFDR) technique is introduced to detect strained fibre sections and to measure distributed length change along the fibre with sub-millimetre resolution by applying a cross-correlation algorithm to the backscatter signal. The overall superior performance of the OFDR technique compared to the optical time domain reflectometry in terms of accuracy, dynamic range, spatial resolution and measurement speed is presented. The proposed sensor system is a promising technique for use in structural health monitoring applications where the precise detection of high strain is required.

Study of dynamic chirp in direct modulated DFB laser for C-OFDR application

A description of the chirp induced by direct modulated DFB laser is presented. Two approaches are considered: the first one is based on a resolution of laser rate equations; the second, on a simulation with a commercial software. We compare results of the two approaches, we demonstrate that the optical frequency can be controlled via the injected current. We also characterize the linear variation of the optical frequency in time (for triangular and sawtooth modulation), in order to choose the appropriate values of laser and modulation parameters for a perfect linearity of the chirp in time. This study will be very helpful to validate the use of direct linear modulated DFB laser as a tunable source in Coherent Optical Frequency Domain Reflectometry technique C-OFDR. We present a simulation result of Mach–Zehnder delay time measurement based on C-OFDR system using a direct modulated semiconductor laser source. The obtained results are very important because it depicts the beat frequencies relating to each delay time, with respect to the modulation format used. This is very encouraging for the implementation of an experimental C-OFDR.

Incoherent optical frequency domain reflectometry and distributed strain detection in polymer optical fibers

We present, to our knowledge for the first time, the possibility of measuring the backscatter signal of perfluorinated polymer optical fibers (POF) using an incoherent optical frequency domain reflectometry (OFDR) technique. The OFDR setup is described and it is shown that the dynamic range and measurement speed are superior to standard OTDR systems. It is shown for the first time that distributed detection of strain in POF is possible using the OFDR technique.

Soil-embedded optical fiber sensing cable interrogated by Brillouin optical time-domain reflectometry (B-OTDR) and optical frequency-domain reflectometry (OFDR) for embedded cavity detection and sinkhole warning system

A soil-embedded optical fiber sensing cable is evaluated for an embedded cavity detection and sinkhole warning system in railway tunnels. Tests were performed on a decametric structure equipped with an embedded 110 m long fiber optic cable. Both Brillouin optical time-domain reflectometry (B-OTDR) and optical frequency-domain reflectometry (OFDR) sensing techniques were used for cable interrogation, yielding results that were in good qualitative agreement with finite-element calculations. Theoretical and experimental comparison enabled physical interpretation of the influence of ground properties, and the analysis of embedded cavity size and position. A 5 mm embedded cavity located 2 m away from the sensing cable was detected. The commercially available sensing cable remained intact after soil collapse. Specificities of each technique are analyzed in view of the application requirements. For tunnel monitoring, the OFDR technique was determined to be more viable than the B-OTDR due to higher spatial resolution, resulting in better detection and size determination of the embedded cavities. Conclusions of this investigation gave outlines for future field use of distributed strain-sensing methods under railways and more precisely enabled designing a warning system suited to the Ebersviller tunnel specificities.

Multiplexing Scheme of Long-Period Grating Sensors Based on a Modified Optical Frequency Domain Reflectometry

We propose a novel quasi-real-time sensing scheme by employing a modified optical frequency domain reflectometry technique to multiplex long-period grating (LPG) sensors of transmission mode. Different sensing signals are demodulated by different beat signals through presetting different values for the optical path differences between the measuring arms and a static reference arm. The amplitude of each beat signal is associated with the transmissivity of a sensing LPG. The present multiplexing scheme has the advantages of real-time, high speed, high sensitivity, large capacity, and high resolution over an intermediate length. A measurement of fiber bending curvature is demonstrated to verify good performances of the present LPG sensing system.

Theoretical suppression of cavity interferences in a fiber Bragg grating array interrogatedwith coherent optical frequency domain reflectometry

A theoretical method for the suppression of cavity interference is described that suppressesthe cavity interference in a fiber Bragg grating array (FBGA) interrogated using thecoherent optical frequency domain reflectometry (C-OFDR) technique. In C-OFDR, atunable laser source is used to scan an interferometer, composed of a FBGA and abroadband reference reflector, as a function of wavelength. The interference between theFBGA and the reference reflector results in a composite modulated interferometric signal.A Fourier transform of this signal is then made to spatially separate out each fiber Bragggrating (FBG) in the array. In this work, the undesirable cavity interference component ofthis signal is simultaneously suppressed by directly subtracting the cavity interferencesignal from the overall interference signal. This method is simulated with polarizationoptics and a circularly polarized tunable laser source. The cavity suppression methodallows the C-OFDR/FBGA to operate at half the bandwidth of the downshifting method,potentially saving optical fiber material, and at a higher return signal than the FBGreflectance reduction method. In addition, the detection system is polarizationinsensitive.

Coherent optical frequency domain reflectometry for interrogation of bend-based fiber optic hydrocarbon sensors

The paper presents distributed fiber optic bending sensor for petroleum hydrocarbon leak detection based on coherent optical frequency domain reflectometry (C-OFDR) technique. In order to introduce bending losses a sensitive polymer, which reversibly swells under hydrocarbon influence is employed. In this work we used lumped reflectors, namely fiber Bragg gratings, placed between distributed sensitive elements. Design of proposed sensor utilizes the principle of distributed detection with section localization (DDSL) of perturbation. Analysis of signal-to-noise ratio (SNR) for multi-reflectors system was performed. We have demonstrated that the C-OFDR technique with bending sensor is capable to detect hydrocarbon presence within a few minutes for 20-cm perturbation-length with spatial resolution about 0.5 m for strong perturbation.

Comparison of distributed gain in two dual-wavelength pumping schemes for thulium-doped fibre amplifiers

Using the coherent optical frequency domain reflectometry technique, a non-invasive study has been made of the distributed gain in two pumping schemes for thulium-doped fibre amplifiers. The compared schemes employed 800+1050 nm or 800+1400 nm pump wavelengths, and it was concluded that the former is advantageous over the latter primarily due to the ground-state absorption and pumping mechanism.

Distributed-gain measurements in S-band TDFA by coherent optical frequency domain reflectometry

The implementation of the optical frequency domain reflectometry technique to characterise S-band Tm-doped fibre amplifiers (TDFA) is described. Using this non-destructive method, the gain-length product for a TDFA can be optimised against pump power. Insight into the distributed gain dynamics in the TDFA is also obtained.

Novel PMD measurement method based on OFDR using a frequency-shifted feedback fiber laser

We propose a new method to measure polarization mode dispersion (PMD) in optical fibers based on optical frequency domain reflectometry technique. In this method the PMD is directly determined from the beat frequency generated by interference between lights from the Fresnel reflection at the end of the device under test, which makes the measurement at single end of the device possible. An automated PMD measurement system is demonstrated on polarization maintaining fibers with a frequency-shifted feedback fiber laser as a light source.

Local birefringence measurements in single-mode fibers with coherent optical frequency-domain reflectometry

Measurements of intrinsic and induced birefringence of optical fibers are performed at 1550 nm using the optical frequency-domain reflectometry technique. The experiment confirms the theoretical analysis, which predicts the appearance of oscillations on the detected Rayleigh backscattering intensity, with periods equal to the polarization beat length L/sub b/ and to L/sub b//2. Polarization mode-coupling length values are obtained from local birefringence and polarization mode dispersion measurements.

–152.5 dB sensitivity high dynamic-range optical frequency-domain reflectometry

Optical frequency domain reflectometry (OFDR) measurements with 60 dB dynamics on the 1.55 /spl mu/m Rayleigh backscattering signal with 2 cm spatial resolution and 10 m maximum range are presented. A sensitivity of -152.5 dB is demonstrated, representing the highest sensitivity reported to date using the OFDR technique. This sensitivity level was achieved with <3 min total acquisition time.

New method of in-service fault localisation in passive optical subscriber loops

A new method of detecting fibre breaks in a passive optical subscriber network without interruption to nonaffected customers is demonstrated. For these live measurements, an optical frequency domain reflectometry technique (OFDR) is used. No separate wavelength or additional components in the network are required. An integration facility on the AM/VSB TV-transmission system, without affecting TV-picture quality, is realised.

Modified optical frequency domain reflectometry with high spatial resolution for components of integrated optic systems

An apparatus which allows detection of scatterers and faults as well as measurement of reflections in fiber or integrated optic devices and systems with a spatial resolution in the region of 10-100 mu m is discussed. The dynamic range is sufficient to detect reflection with a reflection coefficient down to 10/sup -10/. The system uses a modified optical frequency domain reflectometry (OFDR) technique whereby signal light and local oscillator light are coupled into the waveguide under test from opposite directions. The measurement principle requires the wavelength of the light source to be swept continuously up or down. It is shown that the experimental relationship between frequency shift and waveguide length is in fairly good agreement with the theoretical estimate. Polarization-sensitive experimental results are given for reflection factors in short, side-polished polarization-maintaining fibers. >

Fault location in optical fibers using optical frequency domain reflectometry

This paper describes the experimental and analytical investigations performed on another backscattering technique named Optical Frequency Domain Reflectometry (OFDR). In this approach the temporal signatures of graded-index multimode fibers are obtained as the inverse Fourier transform of the OFDR data. In the employed OFDR system the entire process is automated, and the modulation frequency is changed stepwise to obtain an exact inverse Fourier transform. The effects of starting frequency, frequency increment, and highest modulation frequency on OFDR temporal response are considered. The experimental results are in good agreement with those obtained by computer simulations. Also, a synchronous detection type of filtering process is suggested for signal-to-noise ratio improvement in OFDR technique.