Interferometric Sensor Array Research Articles

Crosstalk suppression of extrinsic Fabry-Perot interferometric sensor array based on five-step phase shift demodulation scheme using multiwavelength averaging.

A five-step phase shift demodulation scheme based on a multiwavelength averaging method is proposed to suppress crosstalk within an extrinsic Fabry-Perot interferometric (EFPI) sensor array. The paper focuses on a two-element sensing system based on the EFPI sensors to investigate the crosstalk in the EFPI sensor array. A detailed theoretical analysis of crosstalk suppression using the proposed demodulation method is presented. Numerical simulations and experiments are put forward to demonstrate the effectiveness of the proposed demodulation scheme in suppressing crosstalk under varying parameters. The results of the multiwavelength demodulation scheme indicate superior crosstalk suppression capability in contrast to the conventional five-step phase shift demodulation scheme based on a single-wavelength demodulation method. Furthermore, the paper reveals the enhanced crosstalk suppression capability of the proposed demodulation scheme when the cavity length difference between elements is not equal to zero. It alleviates the requirement for consistent cavity length among different elements in the sensing array. The proposed demodulation scheme exhibits excellent crosstalk suppression capabilities in optical multiplexing arrays by decreasing the dependency on extinction ratio and could be potentially used in the large-scale optical hydrophone array system.

Analysis and improvement of dynamic range in a time-division-multiplexing interferometric fiber-optic sensor array.

Dynamic range, the ratio of maximum detectable amplitude, and the noise level, are of great importance in characterizing the performance of an interferometric fiber-optic sensor (IFOS). Here we analyze the dependence of dynamic range on carrier frequency and repetition frequency in a time-division-multiplexing (TDM) IFOS array using a heterodyne detection scheme. A novel, to the best of our knowledge, demodulation method with higher carrier frequency and an upgraded algorithm is proposed to improve the dynamic range. In the simulation and experiment, the carrier frequency was increased from 50 kHz to 20 MHz, and the demodulation method was adjusted correspondingly. The maximum detectable phase amplitude is elevated by more than 6 dB as the theoretical expectation, and the demodulated noise floor is proven to be barely influenced, holding at ∼-100 dB ref rad/H z. Experimental results demonstrate that the average dynamic range over 10 Hz to 20 kHz is enhanced by approximately 10.5 dB. The analysis and optimization presented here provide an effective solution to improve the dynamic range of TDM IFOS without any hardware change, which is promising in various applications.

Fast shaping control of x ray beams using a closed-loop adaptive bimorph deformable mirror

High-speed adaptive correction of optics, based on real-time metrology feedback, has benefitted numerous scientific communities for several decades. However, it remains a major technological challenge to extend this concept into the hard x ray regime due to the necessity for active mirrors with single-digit nanometer height errors relative to a range of aspheric forms. We have developed a high-resolution, real-time, closed-loop “adaptive” optical system for synchrotron and x ray free electron laser (XFEL) applications. After calibration of the wavefront using x ray speckle scanning, the wavefront diagnostic was removed from the x ray beam path. Non-invasive control of the size and shape of the reflected x ray beam was then demonstrated by driving a piezoelectric deformable bimorph mirror at ∼ 1 H z . Continuous feedback was provided by a 20 kHz direct measurement of the optical surface with picometer sensitivity using an array of interferometric sensors. This enabled a non-specialist operator to reproduce a series of pre-defined x ray wavefronts, including focused or non-Gaussian profiles, such as flattop intensity or multiple split peaks with controllable separation and relative amplitude. Such changes can be applied in any order and in rapid succession without the need for invasive wavefront diagnostic sensors that block the x ray beam for scientific usage. These innovations have the potential to profoundly change how x ray focusing elements are utilized at synchrotron radiation and XFEL sources and provide unprecedented dynamic control of photon beams to aid scientific discoveries in a wide range of disciplines.

Discriminating Rayleigh backscattering induced false crosstalk in inline interferometric FBG sensor arrays.

The crosstalk between channels is a main factor restricting the performance of interferometric fiber Bragg grating (FBG) time-division and wavelength-division hybrid multiplexing arrays. The time-division crosstalk caused by multiple reflections and the wavelength-division crosstalk due to insufficient isolation are the main crosstalk problems in the Fabry-Perot (F-P) structure, seriously limiting the number of multiplexing devices and the system's applications. Many theoretical research and suppression scheme designs have been done to solve these problems. We have previously found a new crosstalk phenomenon called the false crosstalk in hybrid multiplexing arrays. This paper focuses on this phenomenon and constructs a theoretical model to analyze its causes and influencing factors. The model demonstrates the influences of Rayleigh backscattering (RB) noise and sensor position on the false crosstalk. Both theoretical and experimental results show that the false crosstalk is induced by parasitic interference in the leading fiber and changes with the leading fiber length. This study quantitatively reveals the crosstalk performance degradation with the change of sensor position in the hybrid multiplexing array and provides key support for optimizing the system array design and expanding the large-scale multiplexing capacity.

Analysis and Suppression of Aliased Noises in Time-Division-Multiplexing Interferometric Fiber-Optic Sensor Array

Aliased noises elevate the ultimate noise level of a pulse-triggered time-division-multiplexing (TDM) interferometric fiber-optic sensor array. Here we for the first time present an analytical model of the aliasing factor based on noise power spectral density analysis of the interferometric fiber sensor array. In contrast to the previous reports, the aliasing factor is found not only dependent on the pulse width and the repetition frequency, but also on the rising time of the pulse when matched filters are applied to suppress the noise components out of the main lobe. The model also indicates the explicit scaling rule between the aliasing factor and the number of TDM channels. The model are experimentally validated by measuring the noise floors under different combinations of pulse parameters. Based on the model, an ultra-low noise floor of -114 dB ref rad/ <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$\sqrt{\mathbf {Hz}}$</tex-math></inline-formula> (i.e. 2 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$\boldsymbol{\mu}$</tex-math></inline-formula> rad/ <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$\sqrt{\mathbf {Hz}}$</tex-math></inline-formula> ) is achieved and up to 6 interferometric sensors can be multiplexed, which, to the best of knowledge, represents the lowest noise level to date for the TDM interferometric fiber-optic sensor array. Our results reveal the prominent influence of the pulse rising time on the aliased noises and provides a practical approach to suppress noise floor of TDM sensor array.

Crosstalk interference synthesis and polarization switching hybrid interrogating method for inline interferometric fiber Bragg grating sensor arrays

We presented a crosstalk interference synthesis and polarization switching (CIS-PS) hybrid interrogating method designed for inline interferometric fiber Bragg grating (FBG) sensor arrays, targeting the multi-reflection (MR)-induced time division multiplexing (TDM) crosstalk and polarization-induced signal fading simultaneously. The inline interferometric FBG sensor array, which has found wide applications in underwater sensing, generally consists of a series of FBGs written on a commercial single mode fiber (SMF), where the coupling of TDM crosstalk and polarization-induced signal fading often makes signal extraction difficult. In this paper, this coupling was modeled, and the principle of the CIS-PS hybrid method was demonstrated. Experiments showed that even though the FBG reflectivities were all larger than 5%, the measured MR-induced TDM crosstalk reached −50 dB, which was the lowest to the best of our knowledge. The background phase noises were lowered and polarization-independent due to efficient control of polarization-induced signal fading. The demodulation stability of the sensing signal was enhanced from ±2.25 dB to ±0.15 dB.

High-performance railway perimeter security system based on the inline time-division multiplexing fiber Fabry-Perot interferometric sensor array

This paper proposes a high-performance railway perimeter security system based on the inline time-division multiplexing fiber Fabry-Perot interferometric (TDM-FFPI) sensor array. A rectangular-pulse binary phase modulation (RPBPM) method is used for phase demodulation and a field programmable gate array (FPGA)-based acquisition system is adopted for receiving and processing the signals with the advantages of low complexity and high real-time capability in this system. Besides, a three-polarizer polarization diversity receiver (tr-PDR) is used to overcome the polarization-induced fading elimination. For the demodulated phase signals, a real-time and accurate two-level pattern recognition method based on the support vector machines (SVM) is employed for the recognition of different events (non-intrusion events including environment, rain and wind, and four intrusion events including shaking, cutting, climbing and knocking). The feasibility of the present system is verified experimentally. The average recognition accuracy is up to about 94.5% with the integrating time less than 0.2 s, which satisfies the high accuracy and high real-time requirements.

High dynamic range distributed acoustic sensing based on dual-wavelength fiber Bragg grating pairs.

A high dynamic range distributed acoustic sensing (DAS) system is demonstrated based on a dual-wavelength (2λ) interferometric acoustic sensor array, constructed using 2λ fiber Bragg grating pairs (FBGPs). Each FBGP is composed of two weak FBGs presenting -30-dB reflectivity and center wavelengths of 1535 nm and 1565 nm. With the phase demodulation results from the two wavelengths, 2λ linear regression phase unwrapping is implemented, which significantly enhances the dynamic range of the DAS system. By examining the four time-division-multiplexed sensors, an average background noise level of -105.2dBrad/Hz at 1 kHz and a cross talk level lower than -70dB at 1 kHz are achieved.

Analysis on leading-fiber-induced Doppler noise in interferometric FBG sensor arrays using polarization switching and PGC hybrid processing method.

The random disturbance in the leading fiber is considered as a vital noise source in the practical interferometric fiber Bragg grating (FBG) sensor array, which is usually interrogated by periodic laser pulse pair. As the two interrogation laser pluses propagate through the leading fiber in a time-sharing manner, the leading fiber disturbance could cause undesired demodulated phase noises to both the polarization state and the pulse-interval, which are summarized as the polarization fading induced noise and the Doppler noise, respectively. This paper focused on the Doppler noise under the demodulation scheme of polarization switching (PS) and phase generated carrier (PGC) hybrid processing method. A model describing the transformation from arbitrary leading fiber stretching to sensor phase background was presented. The complexity was that the Doppler noise was coupled with the birefringence states, as verified by both simulation and experiment. In response to this issue, a two-stage Doppler noise suppression method was proposed, which is based on the PS and PGC hybrid processing and a reference sensor. A processing procedure was presented where the polarization synthesis must be performed before and the reference sensor was considered. Otherwise, the suppression algorithm will be completely invalid due to the mutual coupling of the Doppler noise and the birefringence. Experimental results showed that only after the first stage of polarization synthesis, identical Doppler noise in the two TDM channels could be obtained, with an amplitude error of 0.02 dB. The second stage involved non-sensitive reference sensor subtraction, which achieved a maximum suppression of about 30 dB, which was the highest to be best of our knowledge. The two-stage Doppler noise suppression method was tested for sinusoidal and wideband leading fiber disturbances, providing a solution for practical interferometric FBG array applications.

A low noise 8-TDM inline FBG interferometric sensor array for low frequency sensing

An inline time division multiplexed (TDM) fiber Bragg grating (FBG) interferometric sensor array for low frequency bands was presented. The length of the sensing fiber for each element was 58.78 m and the TDM number was 8, both of which are the largest to the best of our knowledge. Besides, in order to pick up phases stably in this ultra-long array with low background noise, a polarization switching (PS) and crosstalk suppressing method was adopted in which a hybrid interference complex synthesis and phase generated carrier (ICS-PGC) algorithm played an important role. A unified demonstration of the ICS-PGC method suitable for 8 or more TDM channels was presented. Experiments showed that even when the PGC modulation frequency was limited to be 6 kHz, the measured noise level could achieve −92 dB@100 Hz, making the system attractive for low frequency sensing applications.

Analysis on real-time phase delay in an interferometric FBG sensor array using polarization switching and the PGC hybrid processing method.

The polarization switching and phase generated carrier (PS-PGC) hybrid method is typically adopted to control signal fading induced by fiber birefringence and to precisely demodulate signals in interferometric fiber Bragg grating (FBG) sensor arrays. Unlike simple PGC demodulation, the real-time phase delay between the detected interference and the PGC carrier in the hybrid method has more adverse effects as both demodulation accuracy and background noise can be deteriorated, which may invalidate the polarization switching (PS) method. Aiming at this issue, the real-time phase delay and its compensation method were analyzed in detail in this paper. The features of the real-time phase delay in the PS-PGC hybrid method were summarized and the differences among polarization channels were investigated. Theoretical analysis indicated that the real-time phase delay mainly affected the interference complex synthesis (ICS) procedure, ultimately bringing about errors in the PS-PGC algorithm. The method for demodulating the real-time phase delay from sampled interference was presented, which provided the key compensation parameter. Experimental results showed that the compensation method could greatly improve the stability of the demodulated signal and suppress the sensor background phase noise. The amplitude of the demodulated signal was stabilized with a fluctuation less than ± 0.75dB and a noise suppression of 5dB. The acceptable compensation error was also analyzed.

Development of the passive vibroacoustic isolation system for the path matched differential interferometry based fiber-optic sensors

Fiber-optic interferometric sensor arrays are very attractive for many applications. One of the common used interrogation approaches for fiber-optic sensor arrays is the path matched differential interferometry (PMDI). It allows to interrogate a large number of multiplexed fiber-optic sensors by using an auxiliary compensation interferometer (CIF). This approach with using the phase modulator allows to implement the phase-generated carrier demodulation for multiplexed fiber-optic sensors. However, this approach has the significant disadvantage, which is that environmental vibroacoustic impacts on the CIF produce undesirable phase signals from the sensor array. It leads to increasing the noise floor level of the measuring system. In this paper the development methodology and results of the experimental investigation of the passive vibroacoustic isolation system for the CIF in the PMDI-based fiber-optic interferometric sensor are presented. The proposed isolation system is implemented by using acoustic absorbing layers and the special mechanical suspension system for the CIF mounted in the single rack unit. It is shown that the proposed approach for the CIF isolation allows to reduce its sensitivity to vibroacoustic impacts by up to 30 dB in the frequency range up to 500 Hz. It is also demonstrated that potting of the CIF fiber-optic components reduces its isolation efficiency by an average value of 8.3 dB in the considered frequency range, that is not applicable for interferometric sensors. The presented methodology and obtained results might be used in PMDI-based fiber-optic interferometric sensors to improve their performance characteristics.

Minimization of noise floor level of fiber-optic interferometric sensor array by adjustment of operational amplifier cascade parameters

Minimization of noise floor level of fiber-optic interferometric sensor array by adjustment of operational amplifier cascade parameters

FPGA-based interrogation controller with optimized pipeline architecture for very large-scale fiber-optic interferometric sensor arrays

Fiber-optic sensor arrays are always organized utilizing hybrid time division multiplexing (TDM)/wavelength division multiplexing (WDM) techniques to share some common optical components and reduce the cost. Modern sensor arrays can support up to several thousands of sensors by employing distributed amplification technique. They generate huge amount of data and pose other big challenge on the interrogation controller of the sensor system, such as long demodulation time and heavy computational burden. Aiming to solve these problems, we present a complete design of the interrogation controller based on a field programmable gate array (FPGA) by lending its powerful parallelism potential. The interrogation controller adopts a hardware-implementation-friendly modulation method called the rectangular-pulse binary (RPB) modulation. The functional modules in the FPGA are properly organized according to the working principle of the RPB method and carefully designed by optimizing their pipeline architecture to minimize the output latency, which is essential to increase the maximal supportable number of sensors. By analyzing the resource budget and the actual resource consumption result, it demonstrates that the controller can support up to 1000+ sensors in real time by using a single middle-end Stratix III FPGA chip. A 8-sensors array prototype is constructed to validate its functionality. This work can greatly reduce the cost of the sensor system and push it closer to the practical applications.

In situ metrology for adaptive x-ray optics with an absolute distance measuring sensor array.

Adaptive x-ray mirrors are emerging as one of the primary solutions for meeting the performance needs of the next generation of x-ray light sources. Currently, these mirrors operate open loop with intermittent feedback from invasive sensors that measure the beam quality. This paper outlines a novel design for real-time in situ metrology of the shape of these mirrors using an array of interferometric sensors that does not interrupt the x-ray beam. We describe a proof-of-principle demonstration which shows sub-nm agreement over a range of mirror deflection magnitudes and shapes as compared to simultaneous measurements by using a large-aperture Fizeau interferometer.

Efficient and Compact Inline Interferometric Fiber Sensor Array Based on Fiber Bragg Grating and Rectangular-Pulse Binary Phase Modulation

In this paper, an efficient and compact inline interferometric fiber sensor array based on Fiber Bragg Grating (FBG) and rectangular-pulse binary phase modulation is proposed. FBGs are made inline on the fiber to act as the reflection mirrors to construct the interferometer working as the sensing area. Polarizer-based polarization diversity receiver method is used to overcome the polarization-induced signal fading. This sensor configuration makes the whole system efficient, compact, and cost-effective while maintaining satisfactory performance. With binary phase modulation imposed on the specific location of the pulsed light source with a rectangular pulse, three phase-shifting steps of ${\pi }/ {2}$ , 0, and − ${\pi } / {2}$ , radians are generated and the phase shifts that carry the vibration signals are demodulated with an orthogonal demodulation algorithm. The method has the advantages of low complexity, low cost, and high real-time capability. The techniques are theoretically analyzed and experimentally demonstrated using a two-channel time-division multiplexing array. The obtained resolution of the sensor system can reach $3\times 10^{-4}\,\,{\mathrm {rad}} /{{\surd \text {Hz}}}$ . The efficiency and compactness of the proposed sensor array further narrow down the gap between the laboratory demonstration and practical use of practical fiber-optic sensors.

Precisely Automatic Time Window Locating for an Interferometric Fiber-Optic Sensor Array Based on a TDM Scheme.

Interferometric fiber-optic sensors are often organized in the form of large-scale arrays by lending the technique of time division multiplexing (TDM) to reduce the system cost. Discriminating the time windows for different sensor units is the prerequisite to successfully demodulate the sensing message, but it traditionally calls for a very time-consuming manual calibration process. To combat this problem, a novel automatic time window locating method is proposed in this paper. It introduces the concept of shape function and carries out the cross-correlation operation between the shape function and the sensor signal. The shape function is defined as the function whose curve profile reflects the main data characteristics of the sensor signal. The time window information is then extracted from the correlation result. This whole process is carried out automatically by the interrogation controller of the sensor system without any manual intervene. Experiments are conducted to validate this method. The proposed method can greatly reduce the complexity of locating time windows in large-scale TDM sensor arrays, and make the practical use of the TDM scheme much more convenient.

Analysis and mitigation of the carrier phase delay effect of the digital phase generated carrier algorithm.

We present an improved digital phase generated carrier algorithm based on the synchronous carrier restoration (SCR) method to mitigate the carrier phase delay effect. The most distinguishing feature of this method is that it directly picks up the carrier signal information (frequency and phase) from the interference signal and synchronically accomplishes the processing of carrier signal restoration. In comparison to the traditional method, which adopts the initial carrier signal, total-harmonic-distortion with SCR is only 0.091%, lower than the traditional SCR of 18.38%, and the signal-to-noise ratio increases by 29 dB. Further, we derived the analytic expression of the distortion component and verified it by experiments. This technique may be potentially applied to a long-distance large-scale distributed fiber-optic interferometric sensor array.

Low-Crosstalk and Polarization-Independent Inline Interferometric Fiber Sensor Array Based on Fiber Bragg Gratings

In this paper, a low-crosstalk and polarization-independent inline interferometric optical fiber sensor array with four time division multiplexing (TDM) channels was demonstrated. The low level of the multireflection (MR) induced crosstalk was realized by the ultra-weak ( $ 1%) fiber Bragg gratings (FBGs). The FBGs were fabricated in a single-mode fiber without any fusion points with a pulling force and a phase mask matching method. A polarization switching (PS) method based on the phase-generated carrier (PGC) scheme was proposed to eliminate the polarization-induced signal fading and the input-polarization-induced phase noise simultaneously. The PGC-based PS method was validated and the MR-induced crosstalk was tested in the experiment. The equivalent visibilities of the 4-TDM sensors separately range from 0.9 to 0.92 for sensor 1, from 0.95 to 0.96 for sensor 2, from 0.97 to 0.98 for sensor 3, and from 0.87 to 0.88 for sensor 4 for multiple measurements and the phase noise induced by the polarization fluctuation of the leading fiber was eliminated effectively. The background noise could reach $-$ 97 dB/ $\sqrt{\text{Hz}}$ at 1 kHz. The crosstalk level lower than $-$ 45 dB was achieved for the 4-TDM sensor array. The PGC demodulation scheme is first combined with the PS method using a complexing-exponent synthesis skill which can be used in the heterodyne interrogation scheme as well. Besides, though there are only four TDM channels in the study, the low crosstalk about $-$ 40 dB can be achieved if the TDM channels are expanded to five using the ultra-weak FBGs. The ultra-weak FBGs to reduce the crosstalk is especially suitable for the high-resolution applications of short-distance fiber sensing system such as the towed-array.

High performance architecture design for large scale fibre-optic sensor arrays using distributed EDFAs and hybrid TDM/DWDM

A distributed amplified dense wavelength division multiplexing (DWDM) array architecture is presented for interferometric fibre-optic sensor array systems. This architecture employs a distributed erbium-doped fibre amplifier (EDFA) scheme to decrease the array insertion loss, and employs time division multiplexing (TDM) at each wavelength to increase the number of sensors that can be supported. The first experimental demonstration of this system is reported including results which show the potential for multiplexing and interrogating up to 4096 sensors using a single telemetry fibre pair with good system performance. The number can be increased to 8192 by using dual pump sources.