Fiber-optic Sagnac Interferometer Research Articles

Belt conveyor speed detection based on fiber-optic Sagnac interferometer vibration sensor

With their long transportation distance and high productivity, belt conveyors are characterized as the main equipment for bulk material transportation in coal mines and other key industries. However, its speed detection method has always been an engineering challenge under harsh working conditions. In this work, the elastic-optical effect of the polarization-maintaining fiber is used to design a fiber ring laser vibration sensing system based on a fiber-optic Sagnac interferometer vibration sensor and its packaging structure. The vibration detection is achieved by detecting the change in optical power at the working wavelength, and the belt conveyor speed measurement is completed by detecting the vibration frequency. With such a method, the belt conveyor speed measurement obtained a relative error of less than 1.88%. The proposed method bears the characteristics of anti-electromagnetic interference, intrinsic explosion-proof, and adapts to the harsh smoke and dust working environment, which has a wide range of application prospects in the fields of engineering, machinery, security, etc.

Optimizing the sensitivity of a fiber optic-based Sagnac interferometer for high-performance human voice sensing

This article presents a theoretical and experimental study aimed at enhancing the sensitivity of the fiber optic Sagnac interferometer, specifically designed for efficient detection of human voice signals. The study focuses on optimizing various factors that influence the sensitivity, including the frequency response of the Sagnac interferometer, the structure of the sensing fiber, and the sensor noise. Several fiber sensing structures were investigated both theoretically and experimentally within this framework to enable high-performance human voice sensing. The measurement results validate the theoretical findings, particularly highlighting the directional sensitivity influenced by the geometry of the sensing fiber structure. Notably, we succeeded in recording and intelligible listening to a human conversation at a distance of 5 m using a multi-layer fiber coil with a total length of 50 m. The measured Minimum Detectable Pressure (MDP) is approximately 337 μPa/Hz1/2 at 1 kHz, which can be reduced by increasing the fiber length.

High sensitivity ethanol vapor sensor based on a nematic liquid crystal film embedded optical fiber Sagnac interferometer

Interferometric optical fiber sensors have become the preferred choice for ethanol vapor detection because of their high sensitivity and figure of merit. However, the response time of interferometric fiber optic ethanol vapor sensors is very long. To address this issue, we experimentally investigated an ethanol vapor sensor based on a nematic liquid crystal (NLC) film embedded optical fiber Sagnac interferometer. The high birefringent NLC film, which worked as the sensing media of ethanol vapor for its absorption of ethanol vapor, was penetrated into the Sagnac ring to generate the spectral interference. The results showed that the measurement sensitivity of ethanol gas concentration reached 2.22 pm ppm−1. The detection range was about 1210 ppm–10 000 ppm. Most importantly, the response time of the proposed sensor is only 15 s. The designed sensor, which showed the advantages of fast response, high sensitivity, and stability, could be a competitive candidate for ethanol vapor sensing.

All-optical, non-contact, local measurement of both Young’s modulus and Poisson’s ratio in metals using a combination of Rayleigh and leaky surface acoustic waves

In non-destructive evaluation (NDE), measuring ultrasound (US) longitudinal and shear wave speeds is the main method to determine two independent mechanical moduli (Young’s modulus and Poisson’s ratio). Most US techniques use time-of-flight measurements. However, when the local sample thickness is unknown or the sample geometry is complex, bulk-wave propagation speeds cannot be accurately defined. Here we show that a properly shaped beam of nanosecond laser pulses can be used to efficiently excite (without material ablation) two types of surface acoustic waves. In addition to a conventional surface, or Rayleigh, wave, a leaky surface wave (LSAW) can be launched in the near field of the excitation region. We present the theoretical background, numerical simulations, and experimental results clearly showing that both elastic constants can be reconstructed locally by tracking Rayleigh and LSAW waves. Spatially resolved elastic properties can be obtained using local values of wave speeds obtained by sample scanning. Non-contact optical detection of propagating waves at the sample surface, in which a fiber-optic Sagnac interferometer was used, is a key piece of the method. It does not require acoustic coupling and allows remote measurements in the near field of the laser source with micron-scale resolution.

Ultra-high sensitivity gas pressure sensor based on cascaded Mach-Zehnder interferometer and Sagnac interferometer

A highly sensitive optical fiber gas pressure sensor based on Vernier effect of cascaded optical fiber Sagnac interferometer (SI) and Mach Zehnder interferometer (MZI) is proposed, and the sensing characteristics are verified. The SI consists of a 3 dB coupler and a section of panda fiber, and SI is used as a filter. As the sensing part, MZI has high sensitivity and good stability. It is composed of single-mode fiber, multimode fiber and quartz capillary. By controlling the length of two interferometers, their free spectrum ranges (FSR) are similar but not equal. Therefore, the cascade structure produces the Vernier effect, which can amplify the gas pressure sensitivity of the sensor. The experimental results show that the gas pressure sensitivity of a single MZI is only − 7.75 nm/MPa. The cascade structure can amplify the sensitivity to − 211.9 nm/MPa, and the magnification factor is 27.3, which is consistent with the theoretical analysis results. In addition, the proposed sensor has excellent repeatability and stability.

Angular accelerometer based on a dual-polarization fiber-optic Sagnac interferometer.

High-performance angular accelerometers are essential for precise dynamics control of aircraft, satellites, etc. Here, we propose, for the first time to the best of our knowledge, an angular accelerometer based on a dual-polarization fiber-optic Sagnac interferometer, which exhibits relatively high sensitivity and a broad bandwidth. The experimental results show that the angular accelerometer achieves a flat frequency response in the bandwidth range of 0.01-100 Hz. The sensitivity reaches 6.6 × 10-8 rad/s2/Hz. In addition, the proposed fiber-optic angular accelerometer does not rely on any mechanical structure and has strong environmental adaptability. This research provides a feasible solution for the design and implementation of new high-performance angular accelerometers, which contributes to their development in the fields of inertial navigation and rotational seismology.

Sagnac interferometer-based optical fiber strain sensor with exceeding free spectral measurement range and high sensitivity

Sagnac interferometer-based sensors have shown high sensitivity, but limited measurement range due to the free spectral range (FSR) limitation especially the higher the sensitivity. In this paper, we designed an optical fiber Sagnac interferometer with a polarization maintaining fiber (PMF) and a fiber Bragg grating (FBG) in the loop. The strain sensing performance was experimentally conducted. The PMF in the Sagnac interferometer showed a high strain measurement sensitivity with a limited measurement range due to the FSR. On the contrary, FBG in the Sagnac interferometer showed a low strain sensitivity but a large measurement range. By combining the PMF and FBG in a Sagnac interferometer, a high strain sensitivity of about 17.08 pm/µε with a large measurement range of 0–5187 µε was experimentally achieved. The method used in this paper can also be utilized to expand the sensing range in other interferometers. The proposed PMF and FBG constructed Sagnac interferometer shows the merits of simple structure, high sensitivity, and large measurement range, which make it a competitive candidate in strain monitoring.

Type of non-reciprocity in a fiber Sagnac interferometer induced by geometric phases.

The non-reciprocity of a Sagnac interferometer provides ultra-high sensitivity for parameter estimation and offers a wide range of applications, especially for optical fiber sensing. In this work, we study a new type of non-reciprocity existing in an optical fiber Sagnac interferometer where the polarization dependent loss is taken into consideration. In particular, this non-reciprocity is irrelevant to the physical effects that have been considered in previous studies, which originates from the geometric phases induced by a continuous-weak-measurement. Thus, it has a unique phenomenon of sudden phase transition, which may open a new way for the future design of high precision optical fiber sensors.

A highly sensitive optical fiber temperature sensor based on the enhanced Vernier effect

A highly sensitive optical fiber temperature sensor based on enhanced Vernier effect is proposed in this paper. The sensor is a cascaded structure which is composed of fiber optic Sagnac interferometer (FSI) and intermodal Mach–Zehnder interferometer (IMZI). Because the interference fringes of the IMZI and the FSI shift to opposite directions as ambient temperature varies, the enhanced Vernier effect is obtained. Compared to the temperature sensor based on the traditional Vernier effect, the proposed sensor realizes the high sensitivity temperature measurement and doesn’t need any special sensitizing materials, complicated manufacture process or a reference interferometer which need to be desensitized. Experimental results show that the sensitivity of the sensor is 20.86 nm/°C. A good linear responses can be obtained in temperature measurement range from 25 °C to 33 °C.

Detection of disbonds in adhesively bonded aluminum plates using laser-generated shear acoustic waves

Adhesively bonded metals are increasingly used in many industries. Inspecting these parts remains challenging for modern non-destructive testing techniques. Laser ultrasound (LU) has shown great potential in high-resolution imaging of carbon-reinforced composites. For metals, excitation of longitudinal waves is inefficient without surface ablation. However, shear waves can be efficiently generated in the thermo-elastic regime and used to image defects in metallic structures. Here we present a compact LU system consisting of a high repetition rate diode-pumped laser to excite shear waves and noncontact detection with a highly sensitive fiber optic Sagnac interferometer to inspect adhesively bonded aluminum plates. Multiphysics finite difference simulations are performed to optimize the measurement configuration. Damage detection is performed for a structure consisting of three aluminum plates bonded with an epoxy film. Defects are simulated by a thin Teflon film. It is shown that the proposed technique can efficiently localize defects in both adhesion layers.

The improved denoising algorithm of acoustic sensor based on linear optical fiber Sagnac interferometer

Abstract This paper is devoted to the study of an acoustic sensor system based on linear fiber optic Sagnac interferometer. An improved threshold denoising algorithm combined with the wavelet theory is proposed. Furthermore, a hybrid scheme based on the improved algorithm with bandpass filter is explored, and experimental verification of theoretical investigation is accomplished as well. Taking the measured speech signals as examples, the Allan variance of the signals can be reduced, while the class spacing criterion can be increased. It is of great significance for promoting the application of the fiber optic acoustic sensor system.

Evaluation of disbonds at various interfaces of adhesively bonded aluminum plates using all-optical excitation and detection of zero-group velocity Lamb waves

Inspection of adhesively bonded metallic plates, commonly used in aircraft structures, remains challenging for modern non-destructive testing (NDT) techniques. When a probing ultrasound (US) wave interacts with the plate boundaries, it produces multiple propagating guided waves (or Lamb modes). Analysis of these waves can be complicated due to a strong geometrical dispersion. However, recent studies showed that for specific frequencies zero-group velocity (ZGV) modes exist. Any changes in a bounded structure, like delaminations, drastically alter the conditions for zero-group velocity waves, thus making this method highly sensitive for NDT applications. Laser ultrasound (LU) provides a very broad bandwidth of the generated waves, thus it is a feasible tool for a spectroscopy-based investigation of the ZGV modes. In this paper, we demonstrate an application of high-repetition rate LU with a non-contact fiber-optic Sagnac interferometer on receive for high resolution imaging of delaminations in a structure consisting of 3 epoxy-bonded aluminum plates. The investigation is supported by numerical analysis of Lamb waves existing in the structure to determine ZGV modes sensitive to delaminations at particular bonding interfaces. Tracking the selected modes permits imaging and identification of defects. We also show that mean frequency estimation of the ZGV modes can improve the contrast-to-noise ratio compared to an amplitude of the single ZGV frequency.

A comparative study and experimental observations of optical fiber sagnac interferometric based strain sensor by using different fibers

Abstract Designing of strain monitoring system is a vibrant topic for the researchers. Strain monitoring systems are required in various fields such as mechanics, aeronautics, metallurgy and health monitoring of complex structures etc. Researchers designed different strain monitoring systems based on requirements. In this paper, we have reported a comparative study of Optical Fiber Sagnac Interferometer (OFSI) based strain sensors using three different types of fiber, i.e Polarization Maintaining Fiber (PMF), Standard Photonic Crystal Fiber (PCF) and High Birefringence Photonic Crystal Fiber (Hi-Bi PCF). The transmission spectrums of the sensors were recorded using Optical Spectrum Analyzer (OSA). We have experimentally analyzed the response and sensitivity of the sensors at different applied strains. The experimental results show that the optical interferometric strain sensor using PMF is having highest sensitivity in comparison to other sensors.

Partial Discharge Ultrasound Detection Using the Sagnac Interferometer System.

Partial discharge detection is crucial for electrical cable safety evaluation. The ultrasonic signals frequently generated in the partial discharge process contains important characteristic information. However, traditional ultrasonic transducers are easily subject to strong electromagnetic interference in environments with high voltages and strong magnetic fields. In order to overcome this problem, an optical fiber Sagnac interferometer system is proposed for partial discharge ultrasound detection. Optical fiber sensing and time-frequency analysis of the ultrasonic signals excited by the piezoelectric ultrasonic transducer is realized for the first time. The effective frequency band of the Sagnac interferometer system was up to 175 kHz with the help of a designed 10 kV partial discharge simulator device. Using the cumulative histogram method, the characteristic ultrasonic frequency band of the partial discharges was between 28.9 kHz and 57.6 kHz for this optical fiber partial discharge detection system. This new ultrasound sensor can be used as an ideal ultrasonic source for the intrinsically safe detection of partial discharges in an explosive environment.

Fibre-Optic Sagnac Interferometer in a FOG Minimum Configuration as Instrumental Challenge for Rotational Seismology

Rotational seismology caused high interest in the investigation on rotational movements generated by earthquakes, mines, and existing in engineering structures. The most oppressive aspects of the research in this field are technical requirements for sensors. They have to be extremely sensitive, as well as possess an extremely wide dynamic range of operation from 0.01 to 100 Hz. This paper points out that solutions basing on the optical technology are the most promising devices in comparison to existing mechanical and electromechanical ones. Especially, instruments basing on the von Laue–Sagnac effect seem to be the most appropriate to investigate rotational phenomena due to the fact that they are entirely insensitive to a translational motion, operate without inertia mass, and are able to measure rotation rate in wide frequency and amplitude band. The paper presents a new device named FOSREM that, based on the FOG minimum configuration, possesses special solutions that makes it perfect, in our knowledge, for any rotation sensing. It enables to measure only a rotational component in a wide range of signal amplitude of 2·10–8–10 rad/s, as well as in a wide frequency band from DC to 328.12 Hz. The presented performance characteristics, as well as a laboratory investigation, confirm FOSREM's opportunity to measure rotation both in seismological observatories and engineering constructions.

A partial discharge detection and localization system for high voltage cable based on long-tailed Sagnac interferometric fiber optic sensor

Abstract A partial discharge detection and localization system for high voltage cable based on a long‐tailed optical fiber Sagnac interferometer has been proposed and experimentally demonstrated. In the system, optical fiber is used as the sensing arm and a piezoelectric transducer is used to...

Sagnac interferometer hydrogen sensor based on panda fiber with Pt-loaded WO3/SiO2 coating.

A highly sensitive optical fiber Sagnac interferometer hydrogen sensor is proposed and demonstrated. The device is fabricated by inserting a segment of panda fiber coated with Pt-loaded WO3/SiO2 into a Sagnac interferometer loop. When Pt/WO3 film is exposed to hydrogen, the exothermic reaction raises the temperature of the panda fiber, resulting in the resonant wavelength shift of the interferometer, and the resonant dip obtained has a large extinction ratio of ∼25 dB and a narrow linewidth of 2.5 nm. Such a device responds fast to hydrogen, exhibits a high sensitivity of -7.877 nm/% (vol. %) within the range of 0%-1.0% and is robust, low cost, and easy to fabricate.

Multi-resolution intrusion localization algorithm through cepstrum in distributed fiber optic Sagnac interferometer

Distributed fiber optic sensors are studied extensively, for monitoring abnormal events in continuous space, due to the advantages of immunity to electrical interference, non-conductivity and light weight. Moreover, the position of abnormal events, such as intrusions, could be determined directly without additional measurements. Among the various techniques, Sagnac interferometers prove to be promising for providing high sensitivity and large dynamic range in detecting intrusions. Two interference light beams are used which are naturally of equal optical path length in static status. When an intrusion occurs along the sensing fiber, the two light beams arrive at the intrusion position in different time and thus cause different phase changes induced by the intrusion. Analysis of the phase difference signal can predict the intrusion position, as well as the existence of the intrusion. As a Faraday rotator mirror (FRM) connected in the far-end of sensing fiber, both beams travel twice to the intrusion position after being reflected by the FRM. The propagation time interval T between the two interactions corresponds to the distance between the intrusion position and the far-end of sensing fiber Lx, which is further extracted as the localization of intrusion. Previously, the auto-correlation algorithm deals with the phase difference signal in the time domain and the null-frequency algorithm is used in frequency domain to calculate the distance. However the poor localization performance usually can not meet the requirement in high-quality monitoring applications. To determine the position of an intrusion effectively and accurately, the localization algorithm which deals with the phase difference signal in cepstrum domain is proposed in this article. Inspired by the research on the pitch examination we first introduce the algorithm for intrusion localization. Through theoretical analysis, the phase difference signal can be regarded as the convolution of the original waveform of intrusion and the T-related transform function. By applying the fast Fourier transform to the logarithmic spectrum, the phase difference signal is changed into the cepstrum domain, where the original waveform of intrusion and the transform function behave differently and are separated. The propagation time interval T, as well as the distance Lx, can be directly acquired from the peak produced by the transform function. In addition, to overcome the roughness in localization resolution brought by down-sampling of the phase difference signal, the decimator factor is scanned from 30 to 50 for multi-resolution localization at an original sampling rate 4 million/s-1. Besides the basic peak, high order peaks also emerge in the cepstrum in high signal-noise-ratio condition, which can also be used for localization. Since the localizations from different decimator factor and different peaks spread around the actual distance, an average of all reasonable localizations is calculated as the ultimate localization result for the intrusion. Firstly in experiments, intrusions occurring at a position 40.498 km are produced for the verification of the algorithm. The localizations are 40.489, 40.515 and 40.487 km, with localization errors as small as 9, 17 and 11 m respectively. Intrusions at different positions are tested and also correctly localized. For comparison, the standard deviations of localization error are respectively 695 m and 118 m for the auto-correlation algorithm and the null frequency algorithm, which are 58 times and 10 times of the result 12 m, which is obtained by the proposed cepstrum algorithm. The performance suggests promise to achieve better localization in practical applications.

Modelling of Extrinsic Fiber Optic Sagnac Ultrasound Interferometer Used for Displacement Measurements

Ultrasonic waves are used extensively in nondestructive testing both for characterization of material properties, in this paper, we describe a fiber optic sensor suitable for detection of ultrasonic waves. This sensor is based on an extrinsic fiber optic sagnac interferometer. The proposed sensor model can act as a conventional in-phase detector or as a narrowband detector. In this study we use methods interference of ultrasonic waves between the source of ultrasonic waves and the object under investigation is exploited. The main advantages of the proposed sensor are the ability to detect ultrasonic waves on the surface; this sensor possesses higher sensitivity and accuracy than the pulse method. The cavity resonator was very successfully used for measurement of small ultrasound velocity changes. The ultrasonic interferometric technique based on phase-locked loop is the most suitable for measurements of small displacements. This method ensures the highest sensitivity and accuracy. Keywords : Laser-based ultrasonic, fiber optic sensor, Sagnac interferometer.

A 1 kHz A-scan rate pump-probe laser-ultrasound system for robust inspection of composites.

We recently built a fiber-optic laser-ultrasound (LU) scanner for nondestructive evaluation (NDE) of aircraft composites and demonstrated its greatly improved sensitivity and stability compared with current noncontact systems. It is also very attractive in terms of cost, stability to environmental noise and surface roughness, simplicity in adjustment, footprint, and flexibility. A new type of a balanced fiber-optic Sagnac interferometer is a key component of this all-optical LU pump-probe system. Very high A-scan rates can be achieved because no reference arm or stabilization feedback are needed. Here, we demonstrate LU system performance at 1000 A-scans/s combined with a fast 2-D translator operating at a scanning speed of 100 mm/s with a peak acceleration of 10 m/s(2) in both lateral directions to produce parallel B-scans at high rates. The fast scanning strategy is described in detail. The sensitivity of this system, in terms of noise equivalent pressure, was further improved to be only 8.3 dB above the Nyquist thermal noise limit. To our knowledge, this is the best reported sensitivity for a noncontact ultrasonic detector of this dimension used to inspect aircraft composites.