Incoherent Optical Frequency Domain Reflectometry Research Articles

Distributed fiber optic radiation sensors

Abstract. The international research efforts focused on the development of radiation sensors based on optic fibers have their origins in the 1970s (Evans et al., 1978). Generally, the lightweight fiber optic sensors are immune to electromagnetic field interference and high voltages making them deployable in harsh environments at hard to reach areas where conventional sensors usually will not work at all. A further advantage of such radiation sensors is the possibility of remote and real-time monitoring (Huston et al., 2001). In this work, we present our results achieved in several research activities for development of fiber optic dosimeters. The findings show that both the measurement of the radiation-induced attenuation (RIA) along the entire sensing fiber and the accompanying change in the refractive index of the fiber core can be used for distributed radiation monitoring in the kGy and MGy range, respectively. Depending on the fiber type and material the RIA shows varying response to dose rates, environmental temperatures and the wavelength of the laser source used. Thereby, an operation with visible laser light provides most favorable performance in terms of high radiation sensitivity. Operating at these wavelengths, RIA monitoring could yield high-sensitivity dose measurement with sub-gray resolution and accuracy (Stajanca and Krebber, 2017b); however, conventional optical time-domain reflectometry (OTDR) systems for RIA measurements operating in the visible range suffer from low-spatial resolution, long measurement times and poor signal-to-noise (SNR) ratio. The limitations of the OTDR performance can be overcome by the incoherent optical frequency domain reflectometry (I-OFDR) developed by the Federal Institute of Materials Research and Testing (BAM, Liehr et al., 2009) with potential for dynamic real-time measurement. Over the years, several highly radiation sensitive fibers, such as perfluorinated polymer optical fibers (PF-POF, Stajanca and Krebber, 2017a), phosphorous-doped silica optical fibers (SOF, Paul et al., 2009), aluminium-doped SOF (Faustov et al., 2013) and erbium-doped SOF (Wosniok et al., 2016) have been identified and are commercially available. As mentioned before, the radiation-induced RIA increase is associated with an increase in the refractive index leading also to material compaction in the fiber core. The latter two effects can be used for measuring radiation distribution based on Brillouin scattering in the range of high radiation doses of several MGy (Phéron et al., 2012; Wosniok et al., 2016). When using fiber optic sensors for radiation monitoring, the existing post-irradiation annealing behavior of the optical fiber sensors must also be considered.

Simultaneous Temperature Estimation of Multiple Gratings Using a Multi-Layer Neural Network

This paper introduces a method to do simultaneous temperature estimations of multiple gratings in an FBG array. The method involves training a multi-layer neural network using simulated training data. The network is then used to estimate the temperature changes of multiple gratings simultaneously using only the experimentally obtained spectrum of the FBG array. The versatility of the method is seen from the results of three different setups. That is the broadband lightsource with OSA, the broadband lightsource with spectrometer, and incoherent optical frequency domain reflectometry. The method can estimate the temperature changes with high accuracy and low root mean squared error (RMSE) for the setups under consideration. Finally, the method is shown to be capable of simultaneous estimation of temperature changes for 19 FBGs using the BLS setup with a maximum absolute error of $6.64~\mathrm {\textbf {K}}$ and an RMSE of $1.69~\mathrm {\textbf {K}}$ in only 1.74 ms/FBG.

Interrogation of a large-capacity densely spaced fiber Bragg grating array using chaos-based incoherent-optical frequency domain reflectometry.

An interrogation technique for large-capacity densely spaced fiber Bragg grating (FBG) arrays is demonstrated using incoherent optical frequency domain reflectometry (I-OFDR). A distributed-feedback laser diode under chaotic oscillation, modulated by a frequency-swept microwave, is adopted as the optical source. The reflected sensing signals are converted into electrical signals and subsequently mixed with the original microwave. Fast Fourier transform is applied to the generated beat signal to obtain the position of individual FBGs, while thermal tuning of the lasing wavelength leads to the demodulation of wavelength. By using the chaotic source, interference among the reflected sensing signals is reduced, leading to a significantly improved spatial resolution of 10cm, with a multiplexing capacity of 3640 and wavelength demodulation accuracy of 15pm.

Incoherent Optical Frequency-Domain Reflectometry Based on Homodyne Electro-Optic Downconversion for Fiber-Optic Sensor Interrogation.

Fiber-optics sensors using interrogation based on incoherent optical frequency-domain reflectometry (I-OFDR) offer benefits such as the high stability of interference in the radio-frequency (RF) domain and the high SNR due to narrowband RF detection. One of the main impairments of the technique, however, is the necessity of high-frequency detectors and vector network analyzers (VNA) in systems requiring high resolution. In this paper, we report on two C-band implementations of an I-OFDR architecture based on homodyne electro-optic downconversion enabling detection without VNA and using only low-bandwidth, high-sensitivity receivers, therefore alleviating the requirements of conventional I-OFDR approaches. The systems are based on a pair of modulators that are synchronized to perform modulation and homodyne downconversion at a reference frequency of 25.5 kHz. In the first system, we attain centimeter resolution with a sensitivity down to −90 dB using the modulation frequency range comprised between 3.2 and 14.2 GHz. In the second, we measured, for the first time using this approach, Rayleigh backscattering traces in standard single mode fiber with resolution of 6 m and a sensitivity of −83 dB by use of the 10.1–30.1 MHz range. These results show the feasibility of these simple, homodyne downconversion I-OFDR systems as compact interrogators for distributed or quasi-distributed optical fiber sensors.

Combined distributed Raman and Bragg fiber temperature sensing using incoherent optical frequency domain reflectometry

Abstract. Optical temperature sensors offer unique features which make them indispensable for key industries such as the energy sector. However, commercially available systems are usually designed to perform either distributed or distinct hot spot temperature measurements since they are restricted to one measurement principle. We have combined two concepts, fiber Bragg grating (FBG) temperature sensors and Raman-based distributed temperature sensing (DTS), to overcome these limitations. Using a technique called incoherent optical frequency domain reflectometry (IOFDR), it is possible to cascade several FBGs with the same Bragg wavelength in one fiber and simultaneously perform truly distributed Raman temperature measurements. In our lab we have achieved a standard deviation of 2.5 K or better at a spatial resolution in the order of 1 m with the Raman DTS. We have also carried out a field test in a high-voltage environment with strong magnetic fields where we performed simultaneous Raman and FBG temperature measurements using a single sensor fiber only.

Quasi-Distributed Fiber Bragg Grating Sensing Using Stepped Incoherent Optical Frequency Domain Reflectometry

We examine the quasi-distributed interrogation of fiber Bragg grating arrays (FBGA) with the method of stepped incoherent optical frequency domain reflectometry (IOFDR) combined with wavelength scanning of a tunable laser source. The technique of IOFDR provides some advantages over traditional time division multiplexing schemes, because continuous wave modulation of the light intensity instead of short and low energy pulses is used, maximizing the reflected signal received from the sensor fiber. Moreover, an electrical heterodyne demodulation by a microwave vector network analyzer provides a low-noise detection, while achieving high spatial resolutions down to the centimeter range. A quasi-distributed temperature measurement with ten FBGs of 0.5% reflectivity is demonstrated. In the experiment, a root mean square temperature error of 0.3 K and a maximum error of 1 K is observed, using a fiber-coupled power of only $-$ 12 dBm. A spatial two-point resolution of 2.14 cm is achieved, which enabled a successful interrogation of FBGAs with 20 and 30 cm spacings. The potential multiplexing capability of the given setup could reach more than 50 gratings in a 30 cm spacing at a measurement rate of 50 s/nm.

Interrogation of optical fiber based on the fusion of OFDR and TRA techniques

This paper proposes merging two interrogation techniques, namely, Incoherent Optical Frequency Domain Reflectometry and Transmission-Reflection Analysis. The objective is improving the accuracy for detection and localization of the loss-inducing perturbation along the fiber. The mathematical formulation of our technique was analyzed in terms of the relationship between the continuous-wave and the frequency-modulated light source. Simulations validated the model, which presented increased accuracy in the localization of the loss-induced perturbation, even with the addition of noise in the measurements.

A hybrid sensing system for simultaneous Raman-based distributed and FBG-based quasi-distributed measurements

A hybrid sensing technique, combining the incoherent optical frequency domain reflectometry (IOFDR) based Raman distributed temperature sensor (DTS) with high-reflective fiber Bragg grating (FBG) sensors, is presented for simultaneous distributed and quasi-distributed measurements. By using a laser diode as the common light source, a highly integrated hybrid Raman/FBG sensing system has been developed with a single fiber. The stimulated emission light and the spontaneous emission light of the laser diode are used for DTS and FBG interrogations, respectively. There is no overlap between the spectral range of the Raman backscattered light and the spectral range of the reflected lights from the FBG sensors. The distributed and quasi-distributed measurements can thus be achieved simultaneously by using the wavelength-division multiplexing (WDM) technique. Experimental results show that, both the distributed temperature and the quasi-distributed temperature or dynamic strain can be measured with little interference. Furthermore, the hybrid system shows the capability of high-capacity multiplexing for FBG sensors.

A hybrid fiber-optic sensing system for down-hole pressure and distributed temperature measurements

Abstract A hybrid fiber-optic sensing technique, combining the extrinsic Fabry–Perot interferometer (EFPI) based pressure sensor with the incoherent optical frequency domain reflectometry (IOFDR) based distributed temperature sensor (DTS), is presented for down-hole measurements. By using a laser diode as the common light source, a highly integrated hybrid EFPI/DTS sensing system has been developed with a single fiber. With the injection current of the laser diode below lasing threshold, the broadband spontaneous emission light is used for EFPI based pressure sensing; while with the injection current above the threshold, the stimulated emission light is used for Raman based distributed temperature sensing. There is no overlap between the spectral range of the reflected light from the EFPI sensor and the spectral range of the Raman scattered light. Pressure and distributed temperature can thus be measured by using wavelength-division multiplexing (WDM) technology. Experimental results show that both the pressure and the distributed temperature are measured with little interference. Furthermore, the pressure measurement can be compensated by the measured temperature values.

OFDR based distributed temperature sensor using the three-channel simultaneous radio-frequency lock-in technique

Weak signal detection for single-mode fiber-optic distributed temperature sensor (DTS) is a key technology to achieve better performance. A hybrid technique combining the incoherent optical frequency domain reflectometry (IOFDR) and the three-channel simultaneous radio-frequency (RF) lock-in amplifier (LIA) is presented to improve the signal-to-noise ratio (SNR) of the measured spontaneous Raman backscattered light. The field programmable gate array (FPGA) based RF-LIA is designed with a novel and simple structure. The measurement frequency range is achieved from 1 kHz to 100 MHz. Experimental results show that the backscattered light signal of picowatt level can be detected with high SNR. With a 2.5 km single-mode fiber, a 1064 nm laser source, and the measurement time of 500 s, this sensing system can reach a spatial resolution of 0.93 m and a temperature resolution of about 0.2°C.

Optical frequency domain reflectometry based single-mode fiber-optic distributed temperature sensor using synchronous polarization scrambling technique

In order to reduce the polarization-induced impact on the incoherent optical frequency domain reflectometry based single-mode fiber-optic distributed temperature sensor, a synchronous polarization scrambling technique with a low-speed electrically driven polarization controller (EPC) is presented. The polarization-induced error is derived by error analysis. By simulating the distribution of the states of polarization on the Poincare sphere, optimized EPC driven parameters are selected. The polarization scrambling process is synchronous with the frequency response measurement of the Raman backscattered light. Additionally, the scrambling period is set to be equal to the measurement time of each frequency response. Experimental results show that the polarization-induced error is ∼±3°C, and it is basically in accord with the result of a theoretical error analysis. By using the synchronous polarization scrambling technique, the polarization-induced fluctuation of the measured temperature distribution has been almost eliminated.

Incoherent optical frequency domain reflectometry based on a Kerr phase-interrogator.

We present a novel approach for incoherent optical frequency-domain reflectometry based on a frequency-swept sinusoidal optical signal and a Kerr phase-interrogator. The novel approach eliminates dependence on the laser coherence-length allowing for long-range operation. Long-range detection of reflection points as far as 151 km at a spatial-resolution of 11.2 cm is experimentally demonstrated.

Application of Quasi-Distributed and Dynamic Length and Power Change Measurement Using Optical Frequency Domain Reflectometry

Application results of a dynamic technique for simultaneous measurement of length changes and optical power changes between multiple reflection points in an optical fiber are presented. The technique is based on incoherent optical frequency domain reflectometry (I-OFDR) and allows for measuring for example length changes and optical power changes quasi-distributed at repetition rates up to 2 kHz. Precise measurement with length change resolutions in the -range can be conducted using standard single-mode or multi-mode fibers. Previous results of dynamic refractive index change measurement and the use of polymer optical fibers for high-strain measurement are concluded and possible sources of measurement inaccuracies are discussed. Field test results with sensors installed on a masonry building during a seismic shake test are presented. The versatility and simplicity of this technique makes it potentially interesting for application in the structural health monitoring sector and chemical process control.

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.

A novel quasi-distributed fibre optic displacement sensor for dynamic measurement

We present a novel technique based on incoherent optical frequency domain reflectometry (OFDR) to measure length changes quasi-distributed between reflection points in optical fibres. The technique enables length changes to be measured with a resolution better than 1 µm and allows for static and dynamic measurement capabilities up to 2 kHz. We demonstrate that dynamic measurements of multiple fibre sections can be conducted independently from each other with high precision. Due to the precise and dynamic measurement capabilities, the proposed sensor system is expected to open new fields of application, especially in the structural-health-monitoring sector. Possible applications are discussed in the paper.

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.