Optical Interferometric System Research Articles

Lithographically fabricated gratings for the interferometric measurement of material shear moduli under extreme conditions

Electron beam lithography and photolithography were used to fabricate diffraction gratings on targets for laser-driven shock-wave experiments. This target design was used with an optical interferometric system to measure transverse wave motion of the target during dynamic (shock-wave) compression. A wedged-shaped diamond substrate and reflective grating on the sample's surface allowed detection of transverse motion. Proof of principle tests on single-crystal 〈100〉 Si samples gave a transverse wave speed of 5.9 km/s at 5 GPa and a shear modulus of 81 GPa. This experimental design has tremendous potential, including the possibility of measuring the shear properties of pure iron at Earth core conditions.

Tomographic and volumetric reconstruction of composite materials using full-field swept-source optical coherence tomography

We demonstrate tomographic and volumetric reconstruction of composite material using a full-field swept-source optical coherence tomographic (FF-SS-OCT) system. The FF-SS-OCT system is based on a co-axial and common-path optical interferometric system based on Mirau objective lens. By means of sweeping the frequency of the broad-band light source, multiple spectral interferograms are recorded and stacked in the X–Y–λ axes and processed in a computer. Optically sectioned images of the composite material at different depths are reconstructed. From the spectral interference fringe signal, the phase maps at various depths are also reconstructed. Due to its common path configuration, the present FF-SS-OCT system is highly stable and less sensitive to external vibration and does not require lateral B-scan leading to a large area measurement of the sample.

Development of a Hybrid Atomic Force Microscopic Measurement System Combined with White Light Scanning Interferometry

A hybrid atomic force microscopic (AFM) measurement system combined with white light scanning interferometry for micro/nanometer dimensional measurement is developed. The system is based on a high precision large-range positioning platform with nanometer accuracy on which a white light scanning interferometric module and an AFM head are built. A compact AFM head is developed using a self-sensing tuning fork probe. The head need no external optical sensors to detect the deflection of the cantilever, which saves room on the head, and it can be directly fixed under an optical microscopic interferometric system. To enhance the system’s dynamic response, the frequency modulation (FM) mode is adopted for the AFM head. The measuring data can be traceable through three laser interferometers in the system. The lateral scanning range can reach 25 mm × 25 mm by using a large-range positioning platform. A hybrid method combining AFM and white light scanning interferometry is proposed to improve the AFM measurement efficiency. In this method, the sample is measured firstly by white light scanning interferometry to get an overall coarse morphology, and then, further measured with higher resolution by AFM. Several measuring experiments on standard samples demonstrate the system’s good measurement performance and feasibility of the hybrid measurement method.

Improvement of the oblique-incidence optical interferometric system to measure tooth flanks of involute helical gears

We put forward a plan of improving the oblique-incidence optical interferometric system applied in the measurement of tooth flanks of an involute spur gear in order to expand its capability to measure an involute helical gear. On the basis of the features of an involute helical tooth flank, we discuss how to realize the parallelism between the optical axis of the object arm of the optical system and the straight lines constructing the involute helical tooth flank. This parallelism helps the optical system produce an interference fringe pattern as clear as the one of an involute spur gear [Appl. Opt.49, 6409 (2010).]. A numerical simulation is then performed to examine the correctness of the improvement. During simulating, we unify the equation of difference tooth flanks by means of importing two parameters in relation to the left or right side of a tooth flank and the helical direction of teeth, respectively. Finally, the actual experiment is fulfilled through the real optical system built on an optical table. The simulation and experiment results verify the correctness and feasibility of the proposed improvement.

基于单个光纤布拉格光栅反射镜的迈克尔逊光纤干涉型传感系统

A sensing system, with Michelson-type fiber optical interferometer based on single fiber Bragg grating (FBG) as the reflector, is demonstrated. The system used a frequency-matched ring fiber optical laser as the source. The closed Michelson-type fiber optical interferometer system will be helpful in simplifying the developed interferometric sensor by replacing the double reflectors with only one FBG reflecting the double-side light. The basic sensing properties of the system are demonstrated, with a fiber optic piezoelectric ceramic transducer embedded in the arm of the interferometer simulating the sensing signal.

Extrinsic Fabry–Perot interferometric optical fiber hydrogen detection system

We present a new hydrogen sensor system based on the use of an optical fiber extrinsic Fabry-Perot interferometer coated with palladium silver (Pd-Ag) film. The sensing mechanism of such a sensor is based on the mechanical stress that is induced in the Pd-Ag film when it absorbs hydrogen. To obtain the absolute length of the air gap, a modified cross-correlation signal processing method is introduced. Results are presented for a comparison between two Pd-Ag films of different thicknesses. A temperature compensation method based on an optical switch is presented. The sensor is suitable for monitoring concentrations of hydrogen below the lower explosive limit.

Elimination of drift in a fiber-Bragg-grating-based multiplexed Michelson interferometer measurement system

Random phase drift in single-mode optical fiber interferometers used with measurement systems, which is resulted from various types of environmental disturbances, should be eliminated in order to obtain high measurement precision. We propose an optical fiber interferometric measurement system which has the function of self-eliminating the random phase drift and is stable and robust enough for real-time precision measurement. By employing the characteristics of fiber Bragg gratings, the system interleaves two fiber Michelson interferometers together that share the common-interferometric-optical path. The signal of one of the interferometers is used to stabilize the system while the signal of the other interferometer is used for measurement. An electronic feedback loop for the stabilizing action is designed. The bandwidth of the feedback loop is 5 kHz, sufficiently wide to eliminate random phase drift resulted from various environmental disturbances. The system is endowed with high stability and therefore suitable for real-time precision measurement. By means of an active phase tracking technique to measure displacement, the linear regression coefficient of the displacement measurement results is 0.9998.

Spatial Heterodyne Spectro-Polarimetry Systems for Imaging Key Plasma Parameters in Fusion Devices

Imaging diagnostics systems are very important to aid the understanding of core and edge confinement in 3D helical magnetic devices. In this paper we consider recent developments in optical coherence interferometric systems that open new diagnostic capabilities for next generation devices, with particular focus on Motional Stark Effect (MSE) imaging. We present preliminary results obtained using a hybrid spatio-temporal heterodyne snapshot imaging polarimeter-interferometer for motional Stark effect imaging of the q-profile in the TEXTOR tokamak.

Polarization insensitive homodyne detection of optical phase modulation by two-wave mixing in saturable Er-doped fiber

We report linear polarization-insensitive detection of optical phase modulation in an adaptive interferometer based on transient two-wave mixing via dynamic population gratings in saturable Er-doped fiber (EDF). The proposed simple and robust configuration has an all-fiber design based on commercially available elements which makes it promising for applications in optical fiber interferometric sensors and coherent optical communication systems. The experimentally investigated single-mode polarization-maintaining EDF arrangement operated at 1492 nm and demonstrated efficient compensation of the polarization drifts below 100 Hz.

Microprocessor-Based Temperature Monitoring System Using Optical Fibers

This paper describes a microprocessor-based temperature monitoring instrumentation system based on phase modulation principle using interferometric optical sensors system, where the phase of a beam through a temperature sensing fiber (placed in a hot chamber) is compared with that of a beam from a reference fiber placed out side the hot chamber. The sensor system consists of a He-Ne laser source, two beam splitters, two single-mode optical fibers, and two light dependent resistor (LDR)-based timer circuits as detectors. The frequencies of the timer circuits change according to the change in temperature of the hot chamber. A 8085 microprocessor-based system is used to sample the frequencies of the timer circuits.

Vibration-displacement measurements with a highly stabilised optical fiber Michelson interferometer system

A highly stabilised vibration-displacement measurement system, which employs fiber Bragg gratings (FBGs) to interleave two fiber Michelson interferometers that share the common-interferometric-optical path, is presented. The phase change in the interferometric signals of the two fiber Michelson interferometers have been tracked, respectively, with two electronic feedback loops. One of the fiber interferometers is used to stabilise the system by the use of an electronic feedback loop to compensate the environmental disturbances. The second fiber interferometer is used to perform the measurement task and employs another electronic feedback loop to track the phase change in the interferometric signal. The measurement system is able to measure vibration-displacement and provide the sense of direction of the displacement. The frequency range of the measured vibration-displacement is from 0.1 to 200 Hz and the measurement resolution is 10 nm.

Optical Feedback Self-Mixing Interferometry With a Large Feedback Factor $C$: Behavior Studies

This paper studies the behavior of optical feedback self-mixing interferometric (OFSMI) systems, where the semiconductor lasers operate at a single mode (perturbed external cavity mode) with a large optical feedback factor C. Based on analysis of the spectral linewidth associated with all the possible lasing modes at different C values, a set of mode jumping rules are proposed following the minimum linewidth mode competition principle proposed in . According to the rules, the C factor can be classified into different regions, on which an OFSMI system will exhibit distinct phenomena. In particular, for the same amount of displacement associated with the external cavity, the fringe number reduction on the OFSMI signal should be observed when C increases from one region to the next. An experimental setup with a laser diode HL7851G was implemented and employed to verify the proposed rules. The behavior of the OFSMI predicted by the paper has been confirmed by the experiments with C value up to 8.0.

Design and analysis of a highly stabilised optical fiber Michelson interferometer measurement system

Single-mode optical fiber interferometers used with measurement or sensing systems should be stabilised to achieve high measurement accuracy and linearity in the presence of differential phase drift in their arms resulting from temperature fluctuations and other types of environmental disturbances. We propose an optical fiber interferometric measurement system which has the function of self-compensating the influences induced by the environmental disturbances and is stable and robust enough for on-line precision measurement or sensing. By employing the characteristics of fiber Bragg gratings (FBGs), the system interleaves two fiber Michelson interferometers together that share the common-interferometric-optical path. One of the fiber interferometers is used to stabilise the system while the other one is used to perform the measurement or sensing task. An electronic feedback loop for stabilising the measurement system is designed. The bandwidth of the feedback loop is 5 kHz, sufficiently wide to compensate various disturbances from environment. By these means, the system is endowed with high stability in various environments and suitability for on-line precision measurement or sensing.

Amplification of Optical Disk Readout Signals by Homodyne Detection

We experimentally demonstrated the amplification of optical disk readout signals by homodyne detection. This technique uses optical interference to amplify the signals. We further applied phase-diversity detection to reliably obtain the amplified readout signal. The optical system was carefully designed so that a sufficiently amplified readout signal can be obtained. In particular, we applied a corner cube prism as a reflection mirror to achieve sufficient stability of the interferometric optical system. We experimentally demonstrated a 3.6 times amplification of a Blu-ray Disc readout signal. The estimated signal-to-noise ratio (SNR) improvement for an assumed eight-layer optical disk readout signals by applying homodyne detection on the basis of the observed amplification was +7.9 dB, which significantly enables reliable readout of recorded signals. The present technique will be essential for the real commercialization of next-generation multilayer optical disk because of its outstanding ability of SNR improvement.

Lissajous figures in the application of micro-vibration measurement

Based on Michelson interferometer and phase generated carrier (PGC) homodyne demodulation technique, an optical interferometer system is built, and a novel method using the central angles of Lissajous figures to measure micro-vibration displacement is proposed. The Lissajous figures are obtained by synthesizing two interferometric signals that their vibration directions are orthogonal and their intensities are equal. Through theoretical analysis, software simulation and experimental test, it is concluded that the central angles of Lissajous figures have a linear relationship with the phase modulation coefficients and the micro-vibration displacements when the phase modulation coefficients are small. So the micro-vibration displacements can be directly obtained by measuring the central angles of Lissajous figures. The method is simple and convenient. The experimental results indicate that the micro-vibration displacements measuring range can reach 5–775 nm, with a resolution of about 5 nm.

Optical fiber multiplexing interferometer system with a single laser diode and its application to online displacement measurement

A multiplexed optical fiber Michelson interferometer system that is self-referenced with a stabilizing feedback loop is presented. This system employs fiber Bragg gratings and wavelength division multiplexing technique to combine two optical fiber interferometers that share the same optical path in the main part of the optical system. When one Michelson interferometer, which uses the fiber Bragg gratings as reflective mirrors and is used as reference interferometer, is stabilized by an electric feedback loop, the other interferometer, which is used for the measurement, is also stabilized. This system is therefore suitable for online precision measurement. An active phase-tracking technique is applied for signal processing to achieve high resolution.

Application of an optical interferometer for measuring the surface contour of micro-components

The application of an optical interferometric system using a Mireau objective to measure the surface profile of micro-components is described. The proposed system produces a uniform monochromatic illumination over the test area and introduces an interference fringe pattern localized near the test surface. Both the interference fringes and the 2D image of the test surface can be focused by an infinity microscope system consisting of a Mireau objective and a tube lens. A piezoelectric transducer (PZT) attached to the Mireau objective can move precisely along the optical axis of the objective. This enables the implementation of phase-shifting interferometry without changing the focus of a CCD sensor as the combination of the Mireau objective and the tube lens provides a depth of focus which is deep in comparison to the phase-shifting step. Experimental results from surface profilometry of the protrusion/undercut of a polished fibre within an optical connector and of the curved surface of a micromirror demonstrate that features in the order of nanometres are measurable. Measurements on standard blocks also show that the accuracy of the proposed system is comparable to an existing commercial white-light interferometer and a stylus profilometer.

Visualization of domain inversion region characteristics in RuO2:LiNbO3 crystal by digital holographic interferometry

In order to obtain the two dimensional distribution of the refractive index changes due to the domain inversion in the RuO2:LiNbO3 crystal, we make use of the Mach-Zehnder interferometer and 4f optical lens system to record holograms, with the object light transmitting through the poled RuO2:LiNbO3 crystal, and then filter the spectrum in the frequency domain to get the object light spectrum for numerically reconstructing the object wavefront. The results confirm that the electro-chromic effect region well coincides with the domain inversion region in RuO2:LiNbO3 crystal. Digital holographic interferometry has the merits of being nondestructive and it provides a prospective way to in-situ visualization, monitoring and analysis of the domain inversion in LiNbO3 crystal.

6-H single-crystal silicon carbide thermo-optic coefficient measurements for ultrahigh temperatures up to 1273 K in the telecommunications infrared band

6H single-crystal silicon carbide (SiC) is an excellent optical material for extremely high temperature applications. Furthermore, the telecommunication infrared band (e.g., 1500–1600 nm) is an eye safe and high commercial maturity optical technology. With this motivation, the thermo-optic coefficient ∂n∕∂T for 6H single-crystal SiC is experimentally measured and analyzed from near room temperature to a high temperature of 1273 K with data taken at the 1550 nm wavelength. Specifically, the natural étalon behavior of 6-H single-crystal SiC is exploited within a simple polarization-insensitive hybrid fiber-free-space optical interferometric system to take accurate and rapid optical power measurements leading to ∂n∕∂T data. The reported results are in agreement with the previously reported research at the lower <600K temperatures.

Estimating the parameters of semiconductor lasers based on weak optical feedback self-mixing interferometry

The paper presents a practical approach for measuring the linewidth enhancement factor /spl alpha/ of semiconductor lasers and the optical feedback level factor C in a semiconductor laser with an external cavity. The proposed approach is based on the analysis of the signals observed in an optical feedback self-mixing interferometric system. The parameters /spl alpha/ and C are estimated using a gradient-based optimization algorithm that achieves best data-to-theoretical model match. The effectiveness and accuracy of the method has been confirmed and tested by computer simulations and experiments, which show that the proposed approach is able to estimate /spl alpha/ and C with an accuracy of 6.7% and 4.63%, respectively.