White-light Interferometric Technique Research Articles

Fabry–Perot Interferometer-Based Absolute Pressure Sensor With Stainless Steel Diaphragm

In this paper, we discuss the design, fabrication, and testing of an external Fabry–Perot interferometer (EFPI)-based absolute type fiber optic pressure sensor of range 0–50 mbar. The FPI has sapphire as the first reflecting surface and stainless steel (SS) sheet (SS316L of thickness 0.18 mm) as the second one. In order to achieve high specular reflectivity, the reflecting surface of sapphire is coated with a broad band thin film coating, while the SS316L sheet is surface finished with the Chemo Mechanical Magneto Rheological finishing process. The parallelism between two reflecting surfaces of FPI and their individual flatness is around $\lambda $ /15, which was achieved by precision machining and assembly with stretched diaphragm. The SS sheet also works as the deflecting diaphragm of the pressure sensor and causes change in the FPI gap with applied pressure. FPI gap is calculated from the reflected spectrum by an improvised yet simple scheme. In this paper, FPI is an enclosed cell with vacuum inside, which acts as the reference pressure of the sensor. Applied pressure of 0–50 mbar changed the FPI gap by around $17~\mu \text{m}$ . This sensor was tested with white light interferometric technique, and the best finesse obtained was 5.5. The proposed device is an optical analogous of a capacitance-based pressure sensor. To the best of our knowledge, FPI-based absolute pressure sensors of sub-atmospheric range with a metal diaphragm have not been reported anywhere.

Spectral-domain measurement of chromatic dispersion difference of polarization modes in polarization-maintaining fibers

A spectral white-light interferometric (WLI) technique for measuring the chromatic dispersion difference of the two polarization eigenmodes in polarization-maintaining fibers (PMFs) is presented. The approach is based on the application of a transverse point-like force on the fiber that causes coupling between the two modes and utilizes their spectral interferogram. A Fourier-transform spectral interferogram (FTSI) and a polynomial curve fitting are applied to obtain the phase function. The chromatic dispersion difference of the long PMFs can be obtained by taking the second derivative of the phase function. The experimental results for two kinds of PMFs of 110 and 750 m length are presented, respectively.

Measurement of chromatic dispersion of polarization modes in optical fibres using white-light spectral interferometry

We report on a white-light interferometric technique for a broad spectral range measurement (e.g. 500–1600 nm) of chromatic dispersion of polarization modes in short-length optical fibres. The technique utilizes an unbalanced Mach–Zehnder interferometer with a fibre under test of known length inserted in one of the interferometer arms and the other arm with adjustable path length. We record a series of spectral interferograms by VIS–NIR and NIR fibre-optic spectrometers to measure the equalization wavelength as a function of the path length difference, or equivalently the differential group index dispersion of one polarization mode. The differential group dispersion of the other polarization mode is obtained from measurement of the group modal birefringence dispersion. We verify the applicability of the method by measuring the chromatic dispersion of polarization modes in a birefringent holey fibre. We apply a five-term power series fit to the measured data and confirm by its differentiation that the chromatic dispersion agrees well with that specified by the manufacturer. We also measure by this technique the chromatic dispersion of polarization modes in an elliptical-core fibre.

Absolute refractive index measurement method over a broad wavelength region based on white-light interferometry

We report a simple method of measuring the absolute values of the phase refractive index of an optical material of a flat plate shape over a wide spectral range at a single measurement run. A white-light interferometric technique with angle rotation of the optical plate sample located in one of the interferometer arms was used in this method. The validity of this method was proved by measuring the absolute phase refractive indices of flat plate samples of fused silica and BK7, and by comparing them with calculated values from their well-known Sellmeier dispersion formulas. The accuracy of this refractive index measurement method was within 0.002, which can be further improved by enhancing the angle measurement accuracy of the angle rotating stage used in this method.

Modal Interferometer Based on ARROW Fiber for Strain and Temperature Measurement

In this letter, interferometric sensors based on antiresonance reflecting optical waveguide (ARROW) fibers were developed, and used to sense strain and temperature. Two types of solid-core ARROW fibers were considered and signal demodulation was achieved by using the white light interferometric technique. The ARROW fibers have two rings of high index rods arranged in a hexagonal structure with a lattice constant of ap6 mum. The different sizes of the rods cause different measurand sensitivities for the two fibers. Resolutions of plusmn1.1 muepsiv and plusmn0.07degC were achieved for strain and temperature, respectively.

Modal interferometer based on hollow-core photonic crystal fiber for strain and temperature measurement

In this work, sensitivity to strain and temperature of a sensor relying on modal interferometry in hollow-core photonic crystal fibers is studied. The sensing structure is simply a piece of hollow-core fiber connected in both ends to standard single mode fiber. An interference pattern that is associated to the interference of light that propagates in the hollow core fundamental mode with light that propagates in other modes is observed. The phase of this interference pattern changes with the measurand interaction, which is the basis for considering this structure for sensing. The phase recovery is performed using a white light interferometric technique. Resolutions of +/- 1.4 microepsilon and +/- 0.2 degrees C were achieved for strain and temperature, respectively. It was also found that the fiber structure is not sensitive to curvature.

Group index dispersion of holey fibres measured by a white-light spectral interferometric technique

We present a new white-light interferometric technique to measure the group index of holey fibres over a wide wavelength range. The technique utilizes an unbalanced Mach–Zehnder interferometer with a fibre under test of known length placed in one of the interferometer arms and the other arm with adjustable path length. In a first step, the differential group index of the fibre is measured over a wide wavelength range. In a second step, the fibre is replaced by the reference sample of known thickness and group dispersion to determine precisely the group index of the fibre at one specific wavelength. The group index as a function of wavelength is measured for two different holey fibres, one made of pure silica glass and the other made of SK222 glass. For both fibres, the wavelength dependence of the group index of the outer cladding and modes supported by the fibre is measured.

Continuous wavelet transform for micro-component profile measurement using vertical scanning interferometry

White-light interferometric techniques have been widely used in three-dimensional (3D) profiling. This paper presents a new method based on vertical scanning interferometry (VSI) for the 3D profile measurement of a micro-component that contains sharp steps. The use of a white-light source in the system overcomes the phase ambiguity problem often encountered in monochromatic interferometry and also reduces speckle noises. A new algorithm based on the continuous wavelet transform (CWT) is used to retrieve the phase of an interferogram. The algorithm accurately determines local fringe peak and improves the vertical resolution of the measurement. The proposed method is highly resistant to noise and is able to achieve high accuracy. A micro-component (lamellar grating) fabricated by sacrificial etching technique is used as a test specimen to verify the proposed method. The measurement uncertainty of the experimental results is discussed.

Absolute Measurement of Aspheric Surfaces Using White-Light Interferometry

White-light interferometric technique has been widely applied in the measurement of three-dimensional profiles and roughness with high-precision. Based on the characteristic of interferometric technique, a new method combined with image location and a three-dimensional stage is proposed to achieve the non-contact absolute shape measurement for aspheric and spherical surface in a slarge range. The interference fringes vary with the horizontal displacement of the measured surface, the surface information was obtained by locating the transformation of the maximal intensity in the interferograms. Two main influence factors are discussed; they are performance of the inerferimetric microscope and the stage. Since the performance of the stage directly determines the measurement precision, a three-dimensional displacement stage with a large range and a high precision was developed. Some experiments were carried out to verify the performance of the three-dimensional displacement stage and the validity of the new measurement method with satisfactory results.

Differential group refractive index dispersion of glasses of optical fibres measured by a white-light spectral interferometric technique

We report on a white-light interferometric technique employing a low-resolution spectrometer to measure the differential group refractive index of glasses of optical fibres over a wide wavelength range. The technique utilizes an unbalanced Mach–Zehnder interferometer with a fibre under test of known length inserted in one of the interferometer arms and the other arm with adjustable path length. We record a series of spectral interferograms to measure the equalization wavelength as a function of the path length difference, or equivalently the group dispersion. Subtracting the group dispersion of the optical components present in the interferometer along with the fibre, we measure the wavelength dependence of differential group refractive index for pure silica and SK222 glasses. We confirm that the differential group dispersion measured for pure silica glass agrees well with that described by the dispersion equation.

Single-crystal Sapphire Based Optical Polarimetric Sensor for High Temperature Measurement

Optical sensors have been investigated and widely deployed in industrial andscientific measurement and control processes, mainly due to their accuracy, high sensitivityand immunity to electromagnetic interference and other unique characteristics. They areespecially suited for harsh environments applications, where no commercial electricalsensors are available for long-term stable operations. This paper reports a novel contactoptical high temperature sensor targeting at harsh environments. Utilizing birefringentsingle crystal sapphire as the sensing element and white light interferometric signalprocessing techniques, an optical birefringence based temperature sensor was developed.With a simple mechanically structured sensing probe, and an optical spectrum-codedinterferometric signal processor, it has been tested to measure temperature up to 1600 °Cwith high accuracy, high resolution, and long-term measurement stability.

Dispersive white-light spectral interferometry including the effect of thin-film for distance measurement

A spectral-domain white-light interferometric technique is used for measuring distances in a Michelson interferometer with a mirror represented by a thin-film structure (TFS) on a substrate. A fibre-optic spectrometer is employed for recording spectral interferograms that include wide wavelength range effects of dispersion in a cube beam splitter and multiple reflection within the TFS. Knowing the effective thickness of the beam splitter, its dispersion and parameters of the TFS and substrate, the positions of the second interferometer mirror are determined precisely by a least-squares fitting of the theoretical spectral interferograms to the recorded ones. We apply the technique to the beam splitter made of BK7 optical glass and to a uniform SiO 2 thin film on a silicon wafer. We compare the results of the processing that include and do not include the effect of the TFS.

High-precision non-contact profile measurement for spherical surfaces and aspherical surfaces

Using the advantage of white-light interferometric technique, a new method for the optical non-contact profile measurements of spherical surface and aspherical surface based on image orientation technology is presented in this paper. In this method, a new system uses the grating sensors and piezoelectric transducer to control the motion of the objects, and utilizes the vertical scanning interferometric techniques to achieve the large-range profile measurement for spherical surface and aspherical surfaces.

Measurements of birefringence dispersion and intermodal dispersion in a two-mode elliptical-core optical fibre using an interferometric method

Summary A spectral-domain white-light interferometric technique is used in measurement of the dispersion of the group modal birefringences for the LP01 and LP11 spatial modes in an elliptical-core optical fibre. The dispersion of the intermodal group optical path differences (OPDs) between the LP01 and LP11 modes of each polarization is also measured. The technique, which employs a low-resolution spectrometer at the output of a tandem configuration of a Michelson interferometer and a two-mode optical fibre, utilizes the resolving of so-called equalization wavelengths at which the OPD in the interferometer is the same as the differential or intermodal group OPD. The measured data are fitted to polynomials to obtain the dispersion of both the phase modal birefringences and the differences between propagation constants of two LP modes. The results obtained are compared with the results of an adequate theoretical analysis.

Recent progress in bidirectional interrogation techniques for enhancing multiplexing capability of fiber optic white light interferometric sensors

In smart structure applications where fiber sensors are embedded within structural materials, multiple lead in/out fibers are preferred for redundancy and improving reliability. The use of only one lead/out fiber is not optimal because the breakage of a fiber at one location due to, for example, local structural damage, would cause the failure of the whole sensing system. The multiplexing and networking techniques suitable for such applications have attracted considerable research recently. In this article, based on the bidirectional interrogation technique for white light interferometric sensors arrays, a multiplexed fiber optic deformation sensor loop network suitable for smart structure applications has been designed and demonstrated. Loop-network sensor systems are based on the white light interferometric technique. Michelson and Mach–Zehnder optical path interrogators have been developed and demonstrated, respectively. For the usually used one direction interrogate sensing system, it is clear that multiplexed sensor arrays suffer from relatively large fiber segment-induced optical reflective and excess insertion loss that generally limit the total number of sensors that can be accommodated in this configuration. This loop-network bidirection interrogating technique greatly extended the multiplexing capacity of fiber optic white light interferometric sensors system. A practical implementation of this technique is presented which makes use of a popular light emitting diode, superluminescent diode, or amplified spontaneous emission optical light source and standard single mode fiber, which are commonly used in the communication industry. The sensor loop topology is completely passive and absolute length measurements can be obtained for each sensing fiber segment so that it can be used to measure quasidistribution strain or temperature. For large-scale smart structures, this technique not only extends the multiplexing potential, but also provides a redundancy for the sensing system. It means that the sensor loop permits one point breakdown, because the sensing system will still work whenever the embedded sensor loop breaks somewhere.

Single-crystal sapphire fiber-based strain sensor for high-temperature applications

Single-crystal sapphire fibers have a very high melting point (up to 2050°C), which renders them a very good candidate for sensing applications at a very high temperature. We present in this paper the recent work of developing single-crystal sapphire fiber extrinsic Fabry-Perot interferometric strain sensors based on the white-light interferometric spectrum demodulation technique. Prototype sapphire strain sensors were fabricated and tested at high temperatures up to 1004°C. The preliminary experimental results indicate that the sensors are promising to be used under high-temperature environments for making strain measurements with strain measurement resolution of 0.2-μ strain.

Measurements of intermodal dispersionin few-mode optical fibres using a spectral-domain white-light interferometricmethod

A spectral-domain white-light interferometric technique employing a low-resolutionspectrometer is used in measurement of the intermodal dispersion in optical fibres.The technique utilizes a tandem configuration of a compensated Michelsoninterferometer and a few-mode optical fibre and the resolving of so-calledequalization wavelengths at which the optical path difference (OPD) in theinterferometer is the same as the intermodal group OPDs. The intermodaldispersion in three different optical fibres was measured and the results fortwo of them were compared with the results of an adequate theoreticalanalysis using a model of a weakly guiding, step-index optical fibre.

Time-domain intermodal interference including higherorder dispersion effects

The intermodal interference is analysed theoretically at the output of both a few-mode optical fibre alone and a tandem configuration of a Michelson interferometer with an optical fibre when higher-order intermodal dispersion effects are taken into account. It is revealed theoretically that in a special case of a two-mode optical fibre with a quadratic spectral dependence of the difference of propagation constants, which includes the equalization frequency with zero intermodal group delay, and fibre excitation by a broad-band light source, the time-domain visibility functions differ substantially from those corresponding to fibre excitation by a low-coherence light source. Moreover, measuring a two-mode optical fibre using time-domain and spectral-domain white-light interferometric techniques, some theoretical results are confirmed experimentally.

Design of a fiber-optic quasi-distributed strain sensors ring network based on a white-light interferometric multiplexing technique.

A fiber-optic quasi-distributed strain sensors ring network has been designed based on a Mach-Zehnder optical paths interrogator. The optical paths matching for each sensor are discussed, and the optical power budgetary analysis is performed. The relation between the number of sensors and the intensity of the signals of the ring network is given for evaluation of the multiplexing capacity. Experimentally, a seven-sensor array ring network was realized under the condition of light source power 35 microW at 1310 nm, and the distribution strain test was also demonstrated.

High-precision shape measurement by white-light interferometry with real-time scanner error correction.

White-light interferometric techniques allow high-precision shape measurement of objects with discontinuous structures by detecting the peak of the coherence envelope. These techniques assume a specific change in the optical path difference (OPD) between the interfering beams; however, the scanning device effecting that change often introduces OPD errors that are carried over to the measurements. We present a technique for measuring OPD changes from the collected interference fringes during each measurement. Information about the scan is directly fed into the algorithm, which compensates for the errors, resulting in improved measurement accuracy. The method corrects not only the scanner errors but also slowly varying vibrations. In addition, this technique can be easily adapted to any existing low-coherence interferometer because no large data storage or postprocessing is required.