Sagnac Research Articles

Non-Contact Shear Wave Generation and Detection Using High Frequency Air-Coupled Focused Transducer and Fiber Optic Based Sagnac Interferometer for Mechanical Characterization

In shear wave-based material mechanical characterization, the transmit/receiver transducer is generally in contact with the material through a coupling medium. In many applications, especially in biological tissue-related characterization, the application of the coupling medium and the contact method are not ideal, sometimes even unacceptable, due to contamination or stress response concerns. To avoid contact, we developed a 1 MHz air-coupled focused PZT transducer as a moderate pressure generator that could induce a shear wave in soft material and a fiber optic-based Sagnac system for the detection of the propagating shear wave. A calibration indicated that the fabricated air-coupled focused PZT transducer could generate pressure above 1 KPa within its focal range. This pressure is three to five times as much as the pressure generated by a 1 MHz air-coupled transducer currently available on the market. The integrated system was demonstrated through shear wave generation by the fabricated air-coupled PZT transducer and shear wave detection by the fiber optic Sagnac system in a nylon membrane. The results demonstrated the capability of the integrated system in non-contact material mechanical characterization, such as in material modulus measurement.

Partial Discharge Detection and Sensitivity Improvement for Bushing Based on Optical Interference Technique

Bushing plays a significant role in insulation and mechanical support for power transformers. Its working state is closely related to the reliable operation of power transformers. To overcome the electromagnetic interference and security risks during sensor installation, a Sagnac interference optical technique for partial discharge (PD) detection is proposed in this paper. By establishing a simulation model of the sensing structure, it is verified that output signal amplitude varies periodically with the length of sensing and delay fibers. Moreover, the output value decreases with increasing probe radius. The results show that a delay fiber of 1.5 km, a sensing fiber of 3 m, and a bending radius of 5 mm are optimal parameter combinations to detect PD in a 72.5 kV defective bushing. In addition, consistent detection waveforms induced by both internal and external defects are obtained by the optical sensor and high frequency current transformer (HFCT) simultaneously, then the effectiveness of the optical sensor on detecting PD signals for bushing is verified. It is of great help to carry out online monitoring of bushing insulation defects through optical non-destructive installation.

Optimization of VGG16 Algorithm Pattern Recognition for Signals of Michelson–Sagnac Interference Vibration Sensing System

Signal recognition accuracy and recognition time are the two most important parameters of pattern recognition in a fiber optic vibration sensing system. To obtain high recognition accuracy and short recognition time, this paper optimized the VGG16 algorithm and carried out VGG16-1D and VGG16-2D recognition on big data signals generated by a Michelson–Sagnac interferometric vibration sensor system. The results indicate that VGG16-1D has a higher accuracy of 98.44% and a shorter recognition time of 0.03 s. The proposed method is a more accurate and faster pattern recognition method for big data signals from optical fiber vibration sensing systems, which is helpful in promoting further applications of optical fiber sensing systems.

A quest for the origin of the Sagnac effect

In the literature, there is no consensus on the origin of the relativistic Sagnac effect, particularly from the standpoint of the rotating observer. The experiments of Wang et al. (Phys Lett A 312(1–2):7, 2003; Phys Rev Lett 93(14):143901, 2004) has, however, questioned the pivotal role of rotation of the platform in Sagnac effect. Recently, the relative motion between the reflectors which force light to propagate along a closed path and the observer has been ascribed as the cause of the Sagnac effect. Here, we propose a thought experiment on linear Sagnac effect and explore another one proposed earlier to demonstrate that the origin of the Sagnac effect is neither the rotation of frame affecting clock synchronization nor the relative motion between the source and the observer; Sagnac effect originates purely due to asymmetric position of the observer with respect to the light paths. Such a conclusion is validated by analysis of a gedanken Sagnac kind experiment involving rotation.

Testing light speed invariance by measuring the one-way light speed on Earth

We show that the one-way speed of light, along an open contour AB on the uniformly rotating Earth, is observable. Our approach exploits the property of the Sagnac effect that can measure the Earth's local angular velocity and, correspondingly, the peripheral speed of the contour AB relative to Earth Centered Inertial (ECI) frame. Light speed variations, measured on the rotating Earth, may be related to the velocity w of the ECI frame relative to a hypothetical preferred frame. Since it is possible to test Einstein's postulate of a universal light speed, standard special relativity is confirmed to be a viable falsifiable theory.

A Sagnac Interferometer Ultraviolet Sensor Based on ZnO-Coated No-Core Fiber

A Sagnac interferometer based optical fiber ultraviolet (UV) sensor is theoretically proposed and experimentally demonstrated in this work. A layer of ZnO film was deposited upon the surface of a silica no core fiber (NCF) using the magnetron sputtering method. Since the ZnO film was sensitive to UV, the ZnO-coated NCF was introduced into the Sagnac interferometer to help convert changes of the UV irradiance to the wavelength shifts of the interference spectrum. In comparison with the preparation of ZnO nanostructures, the ZnO film had the advantage of simple operation, high deposition rate, good adhesion and stable structure. The influence of the ZnO film thickness on the UV response was investigated, which proved that with the increase of the film thickness, the UV sensitivity decreased gradually. When the ZnO film thickness was 30 nm and the UV irradiance varied from 0 mW/cm² to 19.95 mW/cm², the designed sensor had the highest sensitivity of −124.9 pm/(mW/cm²) (R² = 0.9854) and a detection accuracy of 0.4 mW/cm². The proposed sensor has competitive sensitivity, high linearity, satisfactory repeatability and stability, which can be expected to conduct high-precision UV sensing in industrial, biomedical and military regions.

Bi-directional superluminal ring lasers without crosstalk and gain competition

In this paper, we present the experimental observation of simultaneous bi-directional superluminal lasing in a triangular ring cavity without gain competition and crosstalk as needed for realizing a gyroscope based on the Sagnac effect. The gain spectrum for each of the lasers is tailored to be a narrow dip on top of a broad gain using two stable isotopes of Rb. Specifically, we make use of 85Rb to produce a broad gain spectrum via the optically pumped Raman gain process and 87Rb to produce a narrow absorption spectrum via the optically pump Raman depletion process. A separate gain cell is used for the laser in each direction. Inferred from the simulation, the spectral sensitivity enhancements of the clock-wise and counter-clock-wise superluminal ring lasers are ∼362 and ∼505, respectively, with the imbalance attributed to differences in pump powers.

High precision temperature and strain discrimination using TCF–TPMF fiber structure based Sagnac interferometer

We propose a fiber Sagnac interferometer (FSI) for precise temperature and strain discrimination. A thin core fiber (TCF) and a thin polarization maintaining fiber (TPMF) are firstly fusion spliced with two single mode fibers of a 3 dB coupler, respectively, and then fusion spliced with each other by core offset splicing to fabricate the FSI. Cladding modes are introduced to affect the temperature and strain sensitivities. The spectra composed of polarization modes and dominant cladding modes are extracted from the transmission spectra by a fast Fourier transform (FFT) filter. Temperature and strain discrimination is realized by building a simultaneous measurement matrix. Experimental results show that the temperature and strain resolutions of the proposed FSI can reach 0.0076 °C and 0.018 με, and the maximum measurement errors of temperature and strain variations are ±0.062°C and ±4.48με, respectively. The proposed sensor has promising application prospects in safety monitoring of engineering structures due to the advantages of high precision, ease of fabrication, intrinsic safety and low cost.

Lateral Force Sensing Based on Sagnac Interferometry Realized by a High-Birefringence Suspended-Core Fiber

In this paper, a directional lateral force sensor realized with a high-birefringence suspended-core fiber (HB-SCF)) based on a Sagnac interferometer (SI) is proposed and demonstrated experimentally. The two ends of a 40-cm long HB-SCF are spliced to the two arms of a 3-dB coupler to form the Sagnac interferometer. The lateral force leads to a change in refractive index, thus altering the birefringence of the HB-SCF. The output spectra show a wavelength shift which has a linear relationship with the lateral force. The relationship between the lateral force direction and the force sensitivity was investigated. A finite element analysis (FEA) is conducted to simulate the lateral force responses of the sensor, and the force sensitivities of various force directions are calculated theoretically. The results of both simulation and sensing experiment show that the lateral force sensitivity varied with the force-applied direction in a period of π, and the maximum lateral force sensitivity of 19.032 nm/(N/mm) was achieved. Such lateral force sensor has a good repeatability and low temperature sensitivity.Due to its simple fabrication, low cost, and high sensitivity, it is expected to be a competitive candidate in force sensing applications.

Magneto-optical measurements of mesoscopic Nb superconducting structures using a ferromagnetic metal indicator layer

Imaging local magnetic fields produced by nano- and micrometer-scale superconductors has become a vital tool that can not only reveal crucial information on the vortex dynamics and order parameters of the superconducting materials but also visualize the working mechanism of superconducting devices made for quantum information. Here, we performed measurements of the magnetic field distributions of mesoscopic superconducting structures with various geometries by combining a thin ferromagnetic metal layer as a magneto-optical sensing element that responds to the environmental magnetic fields and a custom-made sensitive Sagnac interferometer. The sensitivity of the technique enables the observation of magnetic flux jumps due to individual vortex entries into nanostructured superconductors. In addition, with the control of incident power at a focused laser spot, we induce thermally driven movement of vortices that leaves a trace of a microscopic optical heating pattern.

Ultrafast Gyroscopic Measurements in a Passive All‐Fiber Mach–Zehnder Interferometer via Time‐Stretch Technique

Almost all inertial navigation systems rely on optical gyroscopes, operating on the Sagnac effect. Laser gyroscopes demonstrate high precision in demanding applications such as seismology and geodesy. Passive optical gyroscopes, typically fiber‐optic gyroscopes (FOGs), are of particular interest due to the lack of the “lock‐in” effect, which is the most detrimental effect in active laser systems. Still, the current data acquisition rate of modern FOGs cannot satisfy emerging applications, particularly for autonomous navigation. Herein, a novel interferometric FOG, based on the measurements of ultrashort pulse phase via the dispersive Fourier transformation, is presented, demonstrating the highest up‐to‐date acquisition rate of 15 MHz. This setup is insensitive to the timing jitter and the fluctuations of the carrier‐envelope phase of the pulses. The single‐shot resolution of the phase retrieval is 7.3 mrad, which corresponds to a time shift of 8.7 attoseconds. As a confirmation of the high‐speed performance, movements of a stepper motor are recorded with an angular velocity resolution of 0.33 mdeg s−1 and a bias instability of 0.06 deg h−1 at acquisition time of 17.07 μs. The proposed method can facilitate various phase measurements at a high repetition rate and is not limited only to gyroscopic applications.

Accurate measurement of the Sagnac effect for matter waves.

A rotating interferometer with paths that enclose a physical area exhibits a phase shift proportional to this area and to the rotation rate of the frame. Understanding the origin of this so-called Sagnac effect has played a key role in the establishment of the theory of relativity and has pushed for the development of precision optical interferometers. The fundamental importance of the Sagnac effect motivated the realization of experiments to test its validity for waves beyond optical, but precision measurements remained a challenge. Here, we report the accurate test of the Sagnac effect for matter waves, by using a Cesium atom interferometer featuring a geometrical area of 11 cm2 and two sensitive axes of measurements. We measure the phase shift induced by Earth’s rotation and find agreement with the theoretical prediction at an accuracy level of 25 parts per million. Beyond the importance for fundamental physics, our work opens practical applications in seismology and geodesy.

Compact Mach-Zehnder Interferometer for Practical Vernier Effect Sensing System With High Extinction Ratio

Recently, Vernier effect is introduced into fiber optic interferometric sensors to enhance the sensitivity. To achieve a high-quality of the Vernier spectrum, the match of the free spectral range (FSR) and the extinction ratio (ER) in two arms is required. Furthermore, the reference arm should be designed to be different from the sensing arm to avoid crosstalk. In this work, a compact in-line Mach-Zehnder interferometer (MZI) by sandwiching a hollow capillary fiber (HCF) between two sections of no-core fiber (NCF) is proposed for the reference arm in Vernier effect based sensing system. FSR of the in-line MZI can be tuned by modulating the length of the HCF, while the ER can remain high up to ∼20 dB, which is equivalent to that of the polarization-maintaining fiber (PMF) based interferometers. Experimental results show that the MZI exhibits low sensitivity to temperature, strain, refractive index (RI) and torsion. When employed as the reference arm cascaded with PMF Sagnac interferometer (SI), the upper and lower envelope are with high extinction ratio (ER). The proposed MZI is a promising candidate for the reference arm in Vernier effect based sensing system with the merits of compact size, good spectral characteristics and insensitivity to ambient environment.

Effect of relativity and vacuum fluctuations on quantum measurement

Vacuum fluctuations can obscure the detection signal of the measurement of the smallest quantum objects like single particles seemingly implying a fundamental limit to measurement accuracy. However, as we show relativistic invariance implies the disappearance of fluctuations for the space-like spectrum of an observable at zero temperature. This complete absence of noise can be harnessed to perform noiseless measurement of single particles, as we illustrate for electrons or photons. We outline a general scheme to illustrate the noiseless measurement involving the space-like spectrum of observables based on the self-interference of counter-propagating paths of a single particle in a triangular Sagnac interferometer.

Deep-learning-based recognition of fractional C -point indices in polarization singularities

Polarization singularities are ubiquitous in most of the vector beams and can be divided into different morphologies as characterized by the polarization singularity index. A precise identification of different polarization topologies may provide abundant polarization morphology for singular optics and increase the information-carrying capacity with a vector beam. However, discrimination of the fractional polarization singularity indices is still an untapped study area. Here, based on deep learning of the Stokes polarimetry, an efficient method is proposed to accurately identify the fractional $C$-point indices in polarization singularity for the vector light fields. Various vector light fields carrying different fractional $C$-point indices are prepared by a polarization-controlled Sagnac interferometer with an embedded spatial light modulator. Then, by analyzing the measured Stokes parameters with a training set of the residual convolutional neural network, the identification of a fractional polarization singularity index with an accuracy up to 0.01 is achieved. The method applies to all $C$-point degenerate beams and may hold promise for increasing the information channel capacity with vector light.

Measuring optical vortices in a speckle pattern by means of dual shearing-type Sagnac interferometers

Measuring the positions of the singularities of the optical vortices is an essential part in the research of speckles and adaptive optics. The measurement accuracy is restricted by the performance of optical devices and the properties of optical vortices, such as density and size. In order to achieve highly accuracy and wide range of application, the dual shearing-type Sagnac interferometers were proposed using two shearing plates to adjust the precision of optical vortices measurement. The shearing displacements are able to balance the measuring precision and the value of the intensity ratio point to provide optimal measurement performance. And the optimal shearing displacement is about a tenth of the speckle size. This method realizes measuring the positions of the singularities of the optical vortices in a speckle pattern in one measurement. And it is useful for the observation of optical vortices with different sizes and densities in a speckle pattern, especially for the high density condition.

Temperature compensated fiber optic magnetic sensor based on the combination interference principle.

In this paper, a highly sensitive temperature compensated fiber optic magnetic field sensor by Sagnac and Mach-Zehnder combination interference (SMZI) is proposed and verified. The sensing structure relies on microstructured exposed core fiber (ECF) filled with ethanol and magnetic fluid (MF). The refractive index of MF and ethanol is affected by the magnetic field and temperature (MFT). SMZI is based on the multimode and birefringence characteristics of ECF. The measurement principle is that the spectra of Sagnac interference and Mach-Zehnder interference have respective sensitivities to the MFT. The magnetic sensitivity can reach 1.17 nm/mT, and the temperature sensitivity is up to -1.93 nm/°C. At the same time, the sensor has good repeatability and low detection limits of 0.41 mT and 0.25°C, respectively. It not only solves the cross-influence of temperature but also makes the spectral analysis more intuitive. The sensor has a broad development prospect in the application of MFT detection.

Compact instantaneous phase-shifting Sagnac interferometer for nanoscale tilt measurement

There is a large inter disciplinary need for high precision angular sensors in state-of-the-art technologies. This particularly applies to optical metrology in the field of Extreme Ultraviolet (EUV) or X-ray where the optics are commonly characterized in terms of slope errors at a nanoradian scale level. This paper presents the in-lab development of an angular sensor based on a single reflection compact Sagnac interferometer using instantaneous phase shifting. Our automated prototype and first results show less than 15 nrad RMS repeatability, at least 200 nrad sensitivity and 280 nrad reproducibility over 100 μrad angular range.

A tunable quantum random number generator based on a fiber-optical Sagnac interferometer

Quantum random number generators (QRNGs) are based on naturally random measurement results performed on individual quantum systems. Here, we demonstrate a branching-path photonic QRNG implemented using a Sagnac interferometer with a tunable splitting ratio. The fine-tuning of the splitting ratio allows us to maximize the entropy of the generated sequence of random numbers and effectively compensate for tolerances in the components. By producing single-photons from attenuated telecom laser pulses, and employing commercially-available components we are able to generate a sequence of more than 2 gigabytes of random numbers with an average entropy of 7.99 bits/byte directly from the raw measured data. Furthermore, our sequence passes randomness tests from both the NIST and Dieharder statistical test suites, thus certifying its randomness. Our scheme shows an alternative design of QRNGs based on the dynamic adjustment of the uniformity of the produced random sequence, which is relevant for the construction of modern generators that rely on independent real-time testing of its performance.

On some applications of the Sagnac effect

Considering exact spacetimes representing rotating black holes and naked singularities, we study the possibility that the Sagnac effect detects i) higher dimensions, ii) rotation of black holes in higher dimension, and also, iii) distinguishes black holes from naked singularities. The results indicate that the Sagnac time delay gets affected by the presence of extra-dimension or its associated angular momentum. This time delay is also different in the spacetime of a naked singularity compared to that of a black hole. Hence, the Sagnac effect may be used as an experiment for better understanding of spacetimes with higher dimensions or those that admit naked singularities.