Fiber-optic Interferometry Research Articles

Application of contact laser interferometry in precise displacement measurement

The technology of frequency modulated continuous wave (FMCW) laser interferometer comes from optical interferometry technology, which has the characteristics of less than traditional interferometer devices, high precision, large range and anti-electromagnetic interference. Through in-depth research on the principle of FMCW measurement, taking full advantage of laser interference technology, and combining the features of anti-interference and easy installation of contact sensors, an innovative contact type long-range displacement sensor based on FMCW is proposed. The conversion of non-contact measurement to contact measurement can realize the direct measurement of workpieces with complex surface conditions on the production line, breaking through the limitation of optical measurement and expanding the application of optical fiber interferometry.

High-Accuracy Optical Fiber Transfer Delay Measurement Using Fiber-Optic Microwave Interferometry

Optical fiber transfer delay (OFTD) measurement with high accuracy and stability is an urgent demand for many applications such as fiber-optic sensors and fiber-based distributed systems. In this article, we propose a novel method using fiber-optic microwave interferometry to meet the above practice demand. Two incoherent optical carriers with different wavelengths are coupled into an intensity modulator driven by a microwave signal. The intensity-modulated signal is then divided into two portions through a dense wavelength division multiplexer. One portion directly passes through the reference path while the other undergoes the transfer delay of a fiber under test (FUT). After photo-detection, two probe signals that undergo different delays are recovered and superimposed. By sweeping the microwave frequency, periodic microwave interference fringe is generated. Then, OFTD measurement is achieved by measuring the frequency of the last valley in the interference fringe. Experimental results show that a system stability of ±0.02 ps, an accuracy of ±0.07 ps, and a measurement range of at least 500 m are achieved.

Low-coherent fiber-optic interferometry for in situ monitoring the corrosion-induced expansion of pre-stressed concrete cylinder pipes

The corrosion-induced expansion of pre-stressed concrete cylinder pipes shapes a major determining factor in the durability of most pipeline systems. However, a structure extended in a very large span and accompanied by a slow corrosion-induced structural damage is still a challenge in practice. Here, a low-coherent fiber-optic interferometry is proposed for in situ measuring the corrosion-induced expansion by directly circling the sensing optical fiber on the outer surface of the pipe. Since the accuracy of the absolute deformation measurement by the low-coherent fiber-optic interferometry can reach 3–5 μm, we are able to improve the sensitivity by choosing the length of the sensing fiber. In the experiment, we employ 50-m sensing fiber; therefore, a resolution of about 0.1 microstrains had been approached. We chose 12 places along the pipeline as the corrosion-induced expansion monitoring points. On monitoring for about half a year, a non-uniform interface in structure, such as the repaired places, was found as a key factor of the corrosion-induced expansion development, which could be a reference for the further health care of the pipeline structures.

Ultra-long (290 km) remote interrogation sensor network based on a random distributed feedback fiber laser.

In this work, an interferometric sensor has been interrogated 290 km away from the monitoring station, reaching the longest distance in fiber optic sensing up to date. This has been attained by employing a double-pumped random distributed feedback fiber laser as the light source for a fiber optic low-coherence interferometry scheme. Additionally, the capability of the system to achieve coherence multiplexing for ultra-long range measurements (up to 270 km) has been proved, without presenting crosstalk between the sensors. The use of coherence multiplexing together with a random distributed feedback fiber laser addresses two of the main limitations of long-range sensing setups: their limited multiplexing capability and the need to reach the maximum monitoring distance.

Measurement of deformations of structural elements technical systems of aircraft by fiber optic devices

The article deals with the measurement of deformations of structural elements of technical systems. The analysis of damages and deformations arising in the elements of aircraft structures during operation is carried out. The conducted researches of the fiber-optical sensor (VOD) based on the Fabry-Perot interferometer confirmed its accuracy and reliability of operation, since the interferometer base is significantly less than the length of the fastening element. The influence of the change in ambient temperature on the change in the base is negligible. Thus, the application of the method of fiber-optic low-coherence interferometry makes it possible to provide high accuracy and reliable measurement of the deformations of the structural elements of technical systems in the process of their research, design, manufacture and operation, which makes it possible to recommend the use of the Fabry-Perot interferometer as primary transducers (sensors) as part of measuring systems. The manufacturing technology has been developed and the sensitivity of the VOD temperature sensors made on the basis of the Fabry-Perot interferometer has been tested, which have shown their reliability and future application in automated measuring systems.

Performance of the fiber-optic low-coherent ground settlement sensor: From lab to field.

A fiber-optic low-coherent interferometry sensor was developed to measure the ground settlement (GS) in an accuracy of the micrometer. The sensor combined optical techniques with liquid-contained chambers that were hydraulically connected together at the bottom by using a water-filled tube. The liquid surface inside each chamber was at the same level initially. The optical interferometry was employed to read out the liquid level changes, which following the GS happened at the place where the chamber was put on and, thereby, the GS information was calculated. The laboratory effort had demonstrated its potential in the practical application. Here, the denoising algorithms on the measurement signal were carried out based on the specific environment to ensure the accuracy and stability of the system in field applications. After that, we extended this technique to the high-speed railway. The 5-days continuous measurement proved that the designed system could be applied to monitor the GS of the high-speed railway piers and approached an accuracy of ±70 μm in the field situation with a reference compensation sensor. So the performance of the sensor was suitable to the GS monitoring problem in the high-speed railway. There, the difficulties were to meet the monitoring requirement of both a large span in space and its quite tiny and slow changes.

Mid-infrared fiber-optic photothermal interferometry.

We demonstrate mid-infrared photothermal interferometry (PTI) in a hollow-core fiber (HCF) for trace gas detection. Compared with free-space PTI, the optical intensity could be increased by orders of magnitude by confining the pump and probe lights in the hollow core. We coupled the pump light at 4.46μm from a quantum cascade laser and the 1555.14nm probe light into the HCF with a bore size of 200μm. The HCF was filled with nitrous oxide (N2O) which has strong absorption at the pump wavelength. The probe light detects the N2O absorption-induced phase change in the HCF via a fiber-optic Mach-Zehnder interferometer. With a 25cm fiber length and 6mW of pump power, we have achieved a minimum detection limit of 0.8ppm for N2O, corresponding to a normalized noise equivalent absorption coefficient of 4.9×10-7 cm-1 WHz-1/2.

Gravitationally induced phase shift on a single photon

The effect of the Earth’s gravitational potential on a quantum wave function has only been observed for massive particles. In this paper we present a scheme to measure a gravitationally induced phase shift on a single photon traveling in a coherent superposition along different paths of an optical fiber interferometer. To create a measurable signal for the interaction between the static gravitational potential and the wave function of the photon, we propose a variant of a conventional Mach–Zehnder interferometer. We show that the predicted relative phase difference of 10−5 rad is measurable even in the presence of fiber noise, provided additional stabilization techniques are implemented for each arm of a large-scale fiber interferometer. Effects arising from the rotation of the Earth and the material properties of the fibers are analysed. We conclude that optical fiber interferometry is a feasible way to measure the gravitationally induced phase shift on a single-photon wave function, and thus provides a means to corroborate the equivalence of the energy of the photon and its effective gravitational mass.

Modeling of Distributed Sensing of Elastic Waves by Fiber-Optic Interferometry.

This paper deals with the transduction of strain accompanying elastic waves in solids by firmly attached optical fibers. Stretching sections of optical fibers changes the time required by guided light to pass such sections. Exploiting interferometric techniques, highly sensitive fiber-optic strain transducers are feasible based on this fiber-intrinsic effect. The impact on the actual strain conversion of the fiber segment’s shape and size, as well as its inclination to the elastic wavefront is studied. FEM analyses show that severe distortions of the interferometric response occur when the attached fiber length spans a noticeable fraction of the elastic wavelength. Analytical models of strain transduction are presented for typical transducer shapes. They are used to compute input-output relationships for the transduction of narrow-band strain pulses as a function of the mechanical wavelength. The described approach applies to many transducers depending on the distributed interaction with the investigated object.

Position Measurement over Wide Range by Simultaneous Use of Low and High Coherence Light Source

In this paper we propose and experimentally demonstrate a new technique for position/displacement measurement using fiber optic interferometry. We simultaneously employ a low- and a high- coherence light source in the same opto-mechanical configuration. The low coherence signal was obtained using a Fizeau receiving interferometer, implemented as an optical glass wedge, accompanied by a linear CCD array. Combining low- and high-coherence parts was possible because we showed that the accuracy of low-coherence based position measurement was better than 50 nm, less than the high coherence signal periodicity. We verify the method in the dynamic range of low-coherence measurement of about 100 m, while the accuracy of the high-coherence part was about ±1 nm. Thus, the method provides more than 100,000 measuring points. The advantages of this sensing technique include absolute position/displacement measurement in a large dynamic range with nanometer accuracy.

A numerical algorithm to determine straightness error, surface roughness, and waviness measured using a fiber optic interferometer

Fiber optic interferometry has been used to detect small displacements in diverse applications. Counting the number of fringes in fiber-optic interferometry is challenging due to the external effects induced in dynamic systems. In this paper, a novel interference fringe counting technique is developed to convert the intensity of interference data into displacements in the range of micrometers to millimeters while simultaneously resolving external dynamic effects. This technique consists of filtering the rough experimental data, converting filtered optical interference data into displacements, and resolving dynamic effects of the experimental system. Filtering the rough data is performed in time by using the moving average method with a window size of 400 data points. Filtered optical data is further converted into displacement by calculating relative phase differences of each data point compared to local maximum and local minimum points. Next, a linear curve-fit is subtracted from the calculated displacement curve to reveal dynamic effects. Straightness error of the lead screw driven stage, dynamics of the stepper motor, and profile of the reflective surfaces are investigated as the external dynamic effects. Straightness error is characterized by a 9th order polynomial function, and the effect of the dynamics of the stepper motor is fitted using a sinusoidal function. The remaining part of the measurement is the effect of roughness and waviness of the reflective surfaces. As explained in the experimental setup part, two fiber-optic probes detect the vertical relative displacements in the range of 1-50µm, and the encoder probe detects 13.5mm horizontal displacement. Thus, this technique can detect three order of magnitude different dynamic displacements with sub-micrometer resolution. The current methodology can be utilized in different applications which require measuring straightness error of lead-screw driven stages, large area surface profile of specimens, and vibration of actuators such as stepper motors.

Construction of a ³He magnetic force microscope with a vector magnet.

We constructed a (3)He magnetic force microscope operating at the base temperature of 300 mK under a vector magnetic field of 2-2-9 T in the x-y-z direction. Fiber optic interferometry as a detection scheme is employed in which two home-built fiber walkers are used for the alignment between the cantilever and the optical fiber. The noise level of the laser interferometer is close to its thermodynamic limit. The capabilities of the sub-Kelvin and vector field are demonstrated by imaging the coexistence of magnetism and superconductivity in a ferromagnetic superconductor (ErNi2B2C) at T = 500 mK and by probing a dipole shape of a single Abrikosov vortex with an in-plane tip magnetization.

Characterizing tissue stiffness at the tip of a rigid needle using an opto-mechanical force sensor.

We present a novel device that allows the user to measure the Young Modulus of a material at the opening of a 5 mm diameter needle. The device relies on a miniaturized cantilever spring mounted at the end of the needle and interrogated via Fabry-Pérot optical fiber interferometry. The probe is repetitively brought in and out of contact with the sample at the end of the needle by means of a steel cable that is controlled via a piezoelectric actuator located at the proximal end. We demonstrate the ability of our device to detect and quantify layers of varying stiffness during needle insertion in a gelatin phantom and to successfully locate tissue boundaries in bovine liver tissue embedded in gelatin.

关于光纤低相干测厚系统中光束耦合效率的研究

关于光纤低相干测厚系统中光束耦合效率的研究

Corrosion monitoring of rock bolt by using a low coherent fiber-optic interferometry

Abstract Corrosion of rock bolts is a major cause for deterioration of the anchor-reinforced concrete slopes structures. In order to evaluate this corrosion-based deterioration in an early stage, a nondestructive technique was required. However, until now, there are no commercialized solutions that are straightforwardly available. Here, a low-coherent fiber-optic sensing technique was developed. This method can carry out the monitoring of the corrosion-caused expansion at the accuracy of sub-microstrains by circled the sensing optical fiber in two ways. One was wound the fiber on the surface of steel rock bolt directly, and thereby generated a nonuniformity in the interface of cement with rock bolt. The other was circled the fiber on a cement mortar cushion without destroying the interface any way. The sensing fiber was configured as one arm of the fiber-optic Michelson interferometer. The acceleration corrosion experiments demonstrated that a uniform interface between cement and rock bolt determined the progress of corrosion development. An early stage evaluation of the corrosion development in rock bolts was monitored.

Extraordinary Dynamic Mechanical Response of Vanadium Dioxide Nanowires around the Insulator to Metal Phase Transition

Nanomechanical resonators provide a compelling platform to investigate and exploit phase transitions coupled to mechanical degrees of freedom because resonator frequencies and quality factors are exquisitely sensitive to changes in state, particularly for discontinuous changes accompanying a first-order phase transition. Correlated scanning fiber-optic interferometry and dual-beam Raman spectroscopy were used to investigate mechanical fluctuations of vanadium dioxide (VO2) nanowires across the first order insulator to metal transition. Unusually large and controllable changes in resonator frequency were observed due to the influences of domain wall motion and anomalous phonon softening on the effective modulus. In addition, extraordinary static and dynamic displacements were generated by local strain gradients, suggesting new classes of sensors and nanoelectromechanical devices with programmable discrete outputs as a function of continuous inputs.

Adaptive filter-based interrogation of high-sensitivity fiber optic Fabry-Perot interferometry sensors

A novel method for interrogation of fiber-optic Fabry-Perot interferometric (FPI) sensors arranged in a white-light setup is presented. The proposed approach is based on a recursive least square (RLS) adaptive filtering to estimate the length of the Fabry-Perot cavity. Applied to an extrinsic FPI sensor with 1.6nm/kPa sensitivity, the interrogation method achieves pressure accuracy of 6.1Pa (0.045mmHg), with an improvement of 8.7 times over standard Q-point tracking method at no computational expense. The RLS-based algorithm also exhibits better resilience to low signal-to-noise ratio (SNR) conditions, achieving 0.87mmHg accuracy for SNR=−5.0dB. The proposed approach finds its best application in medical pressure sensors, for sub-mmHg in vivo pressure detection, and is based on a biocompatible FPI design.

A fiber-optic unit for measuring absolute distances and displacements with a nanometer resolution

A fiber-optic unit for measuring small absolute distances, namely, the base of the Fabry-Perot interferometer, formed by the fiber end and the reflecting surface from 30 to 250 μm with a resolution of no worse than 50 nm is described. The foundation for operation of the unit is the spectral method of the fiberoptic low-coherence interferometry.

Fiber-optic project-fringe interferometry with sinusoidal phase modulating system

A fiber-optic sinusoidal phase-modulating (SPM) interferometer for fringe projection is presented. The system is based on the SPM technique and makes use of the Mach–Zehnder interferometer structure and Young’s double pinhole interference principle to achieve interference fringe projection. A Michelson interferometer, which contains the detection of Fresnel reflection on its fiber end face and interference at one input port of a 3 dB coupler, is utilized to achieve feedback precise control of the fringe phase, which is sensitive to phase drifting produced by the nature of the fiber. The phase diversity for the closed-loop SPM system can be real-time measured with a precision of 3 mrad. External disturbances mainly caused by temperature fluctuations can be reduced to 57 mrad for the fringe map. The experimental results have shown the usefulness of the system.

Collecting optical coherence elastography depth profiles with a micromachined cantilever probe

We present an experimental setup that combines optical coherence elastography depth sensing with atomic force microscope indentation. The instrument relies on a miniaturized cantilever probe that compresses a sample with a small footprint force and simultaneously collects an optical coherence tomography (OCT) depth profile underneath the indenting point. The deflection of the cantilever can be monitored via optical fiber interferometry with a resolution of 2 nm. The OCT readout then provides depth profiles of the subsurface layer deformation with 15 nm resolution and depth range of a few millimeters.