Fast Incoherent OFDR Interrogation of FBG Arrays Using Sparse Radio Frequency Responses

Abstract

We present two implementations of fast, discrete incoherent optical frequency-domain reflectometers (I-OFDR) for the interrogation of equally-spaced fiber Bragg grating (FBG) arrays, based on the determination of the array's radiofrequency (RF) response at a sparse number of frequencies. FBG reflectivities are determined by use of the inverse discrete Fourier transform (IDFT) of the sparse RF response, in a dynamic range limited by crosstalk induced by FBG positioning errors. The first implementation employs the complete, vector RF response at a number of frequencies equal to the number N of FBGs in the array. In the second, the introduction of a reference reflector allows for an interrogation using the power (phaseless) RF response in 4N − 1 frequencies. Demodulation based on IDFT leads to total interrogation times determined by the network analyzer scan time, which can be as low as 10 µs per FBG. Depending on the interrogation technique, electrical bandwidth requirements are 1-2 GHz in our array with 10cm separation. We implemented both techniques in a N = 10 array, inducing decays in reflectivity by 10 dB in one or several FBGs. Unambiguous detection of FBG decays was obtained in both interrogation methods. Additional tests performed on the measured reflectivities also show that measurement linearity is preserved in the 10-dB decay range. As discrete I-OFDR systems, the proposed techniques show the possibility to reach compromises between interrogation time and dynamic range or accuracy in reflectivity measurements, using the number of interrogation frequencies and the sensor topology.

Time-resolution enhanced multi-path OTD measurement using an adaptive filter based incoherent OFDR

“…Multi-path and high-accuracy optical transfer delay (OTD) measurement is fundamental to many applications, including distributed and quasi-distributed optical sensing [1] , distributed coherent aperture radars [2,3] , and optical true-time-delaybased phase arrays [4,5] . Commonly applied multi-path OTD measurements are realized based on time-multiplexed [6][7][8] and frequency-multiplexed [9][10][11][12][13][14][15][16][17] techniques. Time-multiplexed measurement systems, taking optical time-domain reflectometry (OTDR) as an example, can achieve long-range OTD measurement.…”

“…Benefitting from the fine and stable frequency scanning of the microwave source, these methods generally have a high accuracy (sub-picosecond level) [15] . Previously, researchers developed various algorithms to optimize the I-OFDR system's performance [14][15][16][17][18] , such as accuracy and speed. However, the time resolution of the I-OFDR is limited by the reciprocal of the bandwidth [16] , which can be hardly improved by other methods.…”

Distributed Acoustic Sensing Based on Coherent Microwave Photonics Interferometry

“…Frequency modulation in the gigaherz bandwidth enables spatial resolution in the centimeter range [27]. Distributed systems based on this approach have been developed for interrogating Rayleigh scattering [28], fiber Bragg grating (FBG) arrays [29][30][31], and interferometers [24,27]. Recently, coherence-length-gated microwave photonics interferometry (CMPI) was developed to improve strain sensitivity using cascaded interferometers [24,32].…”

“…However, most of the above-mentioned systems require scanning through the entire microwave band to acquire one frame of distributed strain information, and this limits the measurement bandwidth to less than 100 Hz. Recent investigations show that microwave multitone modulation [35] or sparse frequency measurements [30] can significantly improve the measurement rate, resulting in tens of kilohertz of sensing bandwidth. However, either the sensing range or the number of sensing units is limited by those approaches.…”

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

“…Wavelength-resolved reflectograms have been obtained by sweeping the wavelength of a narrowband optical probe wave [11] or filtering the backreflected light under broadband illumination [22,23], and the addition of dispersive elements has been used to map Bragg wavelength shifts into RF time delays directly measurable by the VNA [24,25,26]. In addition, specific techniques for discrete narrowband reflectors, typically FBG, have been adapted to I-OFDR systems, such as slope-assisted dual-wavelength [27] or dual-filter [28] interrogation, or fast techniques relying on model-based search [29] or equally-spaced arrays of reflectors [30]. In a different context, I-OFDR techniques have also proven its viability under a different set of stringent requirements, as embedded systems occupying a single RF channel for in-line monitoring of sub-carrier multiplexed fiber lines using Rayleigh backscattering (RBS) [31,32].…”