Two-wavelength Interferometer Research Articles

Simplified dual-channel two-wavelength interferometer using a polarized cube beam splitter.

Two-wavelength interferometers can extend an unambiguous measurement range; however, they suffer from complex optical configurations. To simplify the optical setup for a two-wavelength common-path off-axis interferometer, we propose a dual-channel two-wavelength interferometer using a polarized cube beam splitter. In contrast with the previously presented two-wavelength common-path off-axis interferometer, the proposed method has a simple setup, in which only one polarized cube beam splitter is inserted into the 4f system. With the help of polarization modulation, two single-wavelength interferograms can be captured simultaneously. Several experimental results are presented to demonstrate the advantages and effectiveness of the proposed method.

Two-wavelength interferometer based on sinusoidal phase modulation with an acetylene stabilized laser and a second harmonic generation.

A two-wavelength interferometer (TWI) based on a sinusoidal-phase-modulation method with an acetylene stabilized laser and a second harmonic generation (SHG) was developed. The periodic non-linearity error for the TWI was estimated to be ± 0.1 µm at a dead path of 0.32 m. A long-term measurement showed that the TWI stability was ± 3 × 10(-7) at a dead path of 1.00 m for 12 hours with an ambient pressure variation of 3 hPa under controlled conditions of ambient temperature and humidity. Finally, we confirmed that the TWI has substantially better stability than a single-wavelength interferometer by comparing both interferometers with large temporal and spatial temperature variations.

Two-wavelength sinusoidal phase-modulating interferometer for nanometer accuracy measurement

A two-wavelength sinusoidal phase-modulating (SPM) laser diode (LD) interferometer for nanometer accuracy measurement is proposed. To eliminate the error caused by the intensity modulation, the SPM depth of the interference signal is chosen appropriately by varying the amplitude of the modulation current periodically. Then, the refine theory is induced to the measurement, and the two-wavelength interferometer (TWI) is combined with the single-wavelength LD interferometric technique to realize static displacement measurement with nanometer accuracy. Experimental results indicate that a static displacement measurement accuracy of 5 nm can be achieved over a range of 200 \mu m.

Two-Wavelength Laser Interferometer System Which Reduces the Uncertainty Caused by the Fluctuation of the Refractive Index of Air

Laser interferometers are widely used to measure highly sensitive length and displacement, e.g., in which refractive index fluctuations of air adversely affect measurement accuracy. To compensate for these effects, the two-wavelength interferometer studied has not yet proved practical in industrial use. We studied the interferometer’s performance and practicality, using uncertainty analysis to extract its features. Based on our results, we developed a two-wavelengthMichelson interferometer with wavelengths of 1064 nm and 532 nm. It was calibrated and evaluated using a highprecision laser interferometer whose optical path was in a vacuum. Results confirmed measurement accuracy of 100 nm/50 mm (k=2) under unstable air conditions - superior to a traditional laser interferometer.

Positioning and localization of two-wavelength interferograms for wavefront reconstruction with volume holographic media

This work studies both theoretically and experimentally the formation of the contour interference patterns generated by a two-wavelength real-time holographic interferometer. The resulting contour interference fringes are due to the intersection of the measured surface with parallel, equally spaced planes of constant elevation. The theoretical analysis describes how the spatial frequency of the elevation planes, their angular position, and the localization of the fringes depend on parameters of the optical setup. A theoretical model for fringe localization is developed and confirmed by the experiments, showing a strong dependence of the interferogram position on the slope of the studied surface. Due to the thick Bi(12)TiO(20) crystal employed as the storage medium the Bragg selectivity of the holographic readout is also considered.

Common-path two-wavelength interferometer with submicron precision for profile measurements in on-line applications

We propose a common-path two-wavelength interferometric system based on a single optical element, a Savart plate, that is able to obtain single-shot profile measurements with submicron precision from safe working distances (beyond 100 mm). These characteristics make this sensor ideal for surface inspection in on-line applications. For the illumination branch, two lasers with close wavelengths are combined and then passed through a rotating holographic diffuser for drastic speckle reduction. In the acquisition branch, the interferometric signals of both wavelengths are captured simultaneously by a camera, and their phase signals are combined to extend the measurement range.

Microgravity Experiments on Phase Change of Self‐Rewetting Fluids

A series of microgravity experiments on self-rewetting fluids has been conducted at the 10-second drop shaft of the Japan Microgravity Center (JAMIC). In all the experiments, 1.5 wt% of 1-butanol aqueous solution were employed as a self-rewetting fluid. The objective of the first experiment was to observe the boiling behavior of two-dimensional adjacent dual vapor bubbles with the aid of a two-wavelength interferometer and tracer particles. A significant difference was observed between a self-rewetting fluid and a normal fluid (CFC-113 in this experiment) in bubble interaction and flow developed along vapor/bubble interface. The second experiment focused on the flow at the bubble/heater contact area and around the three-phase interline, visualized with tracer particles. Differing behavior among three fluids, 1-butanol aqueous solution, CFC-113, and ethanol aqueous solution, was observed. The last microgravity experiment was a demonstration of wickless heat pipes containing three different fluids as a working fluid, 1-butanol aqueous solution, water, and ethanol aqueous solution. The temperature variation of working fluid in the heat pipe was monitored, and the liquid flow returning from the condensation region to the evaporation region was visualized by tracer particles. In addition to microgravity experiments, the performance of conventional heat pipes with 1-butanol aqueous solution was evaluated on the ground, and compared with water heat pipes. Our preliminary results are presented.

Two-wavelength optical coherence tomography

We present the results of studies of the basic principles and describe the design of a low-coherence two-wavelength interferometer based on polarization-maintaining fiber. The interferometer was developed for optical coherence tomography (OCT) imaging of the internal structure of living biological tissue simultaneously at two wavelengths, 830 and 1300 nm. Images of several sites of living biological tissue are presented and analyzed.

Measurement of dye diffusion in agar gel by use of low-coherence interferometry.

We demonstrate low-coherence interferometry for diffusion measurements. We have measured the diffusion coefficient of a phthalocyanine dye in 1.5% agar gel with a two-wavelength interferometer; one wavelength was matched to the absorption peak of the dye at 675 nm, while the other, 805 nm, was not affected by the dye. The diffusion coefficient of the dye was found by fitting a mathematical model for the interferometer signal to the measured low-coherence interferometry amplitude. A 95% confidence interval for the diffusion coefficient was found to be D = (2.5 +/- 0.2) x 10(-10) m2/s. The influence of speckle averaging and experiment time on the determination of the diffusion coefficient has been studied. The presented technique allows in situ characterization of diffusion in semitransparent media.

Phase calculation based on curve fitting with a two-wavelength interferometer

We propose a simple method to calculate the phase measured by a two wavelength interferometer. Our experiment showed that the fitting coefficient with respect to the phase was obtained with a final accuracy of 7.9x10(-5).

Two-wavelength laser diode interferometer with time-sharing sinusoidal phase modulation.

We describe an interferometer system that uses two separate wavelengths to measure step height. The overlapping interference images detected by a CCD camera are easily separated by an ordinary integrating-bucket method and time-sharing sinusoidal phase modulation, in which two laser diodes are alternately modulated with a sinusoidal signal. A phase map is obtained only for the laser diode into which the modulation signal is injected. In this instance, a 1-microm step height was accurately detected.

Real-time displacement measurement with a two-wavelength sinusoidal phase-modulating laser diode interferometer

A two-wavelength interferometer that uses two separate modulating currents with different phases but the same frequencies to detect a greater degree of object displacement in real time is proposed and demonstrated. The measurement error was 57 nm, roughly 1/80 of the synthetic wavelength. We have confirmed that this modulating technique enables us to equip our prototype interferometer with a simple feedback-control system that eliminates external disturbance.

Time-multiplex two-wavelength heterodyne interferometer with frequency-ramped laser diodes

A two-wavelength heterodyne interferometer that is based on a time-multiplexing of interference signals has been constructed by using two frequency-ramped laser diodes. The on/off ramp-current modulation of laser diodes produces a time-multiplexed beat signal. One can measure the tested phase at a synthetic wavelength from the demultiplexed heterodyne signals.

Displacement measurement by synthesized light source based on fiber Bragg gratings

This work presents a novel two-wavelength interferometer based on fiber Bragg gratings. Two light sources with different wavelengths are generated from separate fiber Bragg gratings. The high wavelength accuracy of fiber Bragg gratings allows us to enhance the measured accuracy of the interferometer. In addition, a Michelson interferometer is used to reliably measure the path-length difference signal, thereby demonstrating the proposed interferometer's effectiveness.

Camera influence on the phase-measurement accuracy of a phase-shifting speckle interferometer

In a real phase-shifting interferometer the camera (i.e., a photodetector plus an analog-to-digital converter) cuts off intensities above some saturation level and provides a limited number of digitization steps. Owing to the intensity statistics of speckle fields, this might severely influence the accuracy of the calculated speckle phase. The optimum beam ratio and the modulation of the camera are computed. To calculate the standard deviation of the phase difference, first, we derive a relation that shows that the variances of the two measured phase frames are equal and that they must be added with the decorrelation-dependent variance. To obtain the minimum phase-measurement error of 25.1 mrad, it is found that the mean speckle intensity ought to be adjusted to be 0.058 times the saturation intensity of the camera and that the beam ratio is to be 5.7. The results are confirmed by computer simulation of a two-wavelength speckle interferometer.

Absolute distance measurement using wavelength‐multiplexed phase‐locked laser diode interferometry

A new type of two-wavelength interferometer which applies a phase-lock technique is described. The wavelength tunability of a laser diode (LD) is used for phase modulation and phase control in this inter- ferometer. Since the two wavelengths are derived from a single LD in our system, the optical system is simple and there is no need to consider the fluctuation of relative wavelength between the two LD's in the conven- tional two-wavelength interferometers. The equivalent wavelength L is 15 mm and the measurement accuracy reaches ;L/400. The error sources in the system and the error analysis are also described. © 1996 Society of Photo-Optical Instrumentation Engineers. Subject terms: two-wavelength interferometers; laser diodes; phase lock.

Overcoming coherence length limitation in two wavelength interferometry — an experimental verification

An experimental verification of the possibility of interferometric measurements at distances at which the optical path difference exceeds the coherence length of the laser source will be presented. A two-wavelength heterodyne interferometer setup using one diode laser source and an acousto-optic modulator for the generation of the second wavelength is described and measurements at a distance of 110 m are shown.

Two-wavelength laser-diode heterodyne interferometry with one phasemeter

A two-wavelength laser-diode interferometer that is based on heterodyne detection with one phasemeter has been constructed. Two laser diodes are frequency modulated by mutually inverted sawtooth currents on an unbalanced interferometer. One can measure the tested phase at a synthetic wavelength from the sum of the interference beat signals by synchronizing them with the modulation frequency. The experimental result presented shows a phase-measurement range with a 4.7-μm synthetic wavelength.

Improvement of speckle statistics in double-wavelength superheterodyne interferometry

In this paper we analyze the probability density function of the superheterodyne signal obtained in a two-wavelength interferometer from the beat of a local oscillator laser beam with a speckled return beam from a rough target. Theoretical investigation shows that, by using an increased number of spatially separated detectors, one can improve noticeably the detection probability of the superheterodyne signal. Experimental results obtained with a four-quadrant detector are in good agreement with theory.

Two-wavelength laser-diode interferometer with fractional fringe techniques

A two-wavelength interferometer with a fractional fringe technique (the method of coincidence) has been constructed by using dual frequency-ramped laser diodes. The respective wavelengths of two optical phases were measured by the heterodyne technique. The detected two phases are employed with real-time electronic processing to produce two signals that correspond to the integer and the fractional fringe numbers at a single wavelength. These summed signals can yield a synthetic phase having a single-wavelength resolution. The upper limits for the measurement accuracy are theoretically analyzed.