Selective coupling of HE11 and TM01 modes into microfabricated fully metal-coated quartz probes.

We report computational and experimental investigations on injection and transmission of light in microfabricated fully Aluminum-coated quartz probes. In particular, we show that a selective coupling of either the HE(11) or the TM(01) mode can be carried out by injecting focused linearly or radially polarized beams into the probe. Optical fields, emitted by the probe after a controlled injection, are characterized in intensity and phase with the help of an interferometric technique. With the help of near-field measurement, we finally demonstrate that a longitudinally polarized spot localized at the tip apex is actually produced when the TM(01) mode is coupled into the probe.

Quantitative amplitude and phase measurement by use of a heterodyne scanning near-field optical microscope.

A coherent photon scanning tunneling microscope is presented. The setup employs heterodyne interferometry, allowing both the phase and the amplitude of the optical near field to be measured. Experimental results of measurements on a standing evanescent wave reveal the high resolution that is obtainable with such an approach. In fact we have measured the amplitude and the phase of the near field, with a resolution of 1.6 nm between sample points.

Limitations of interferometry due to the flicker noise of laser diodes

Interferometry with laser diodes is a cost-effective way to perform displacement measurement. The tunability of laser diodes is also of great interest in multiple-wavelength interferometry. However, the additional flicker noise in the frequency-noise spectrum of semiconductor lasers may become a limiting factor. Investigations on the limitations due to the 1/f noise of laser diodes are presented for both classical and multiple-wavelength interferometry. Measurements at the limit of the coherence length of laser diodes with the corresponding phase fluctuations are reported. The theoretical results are verified experimentally.

High-precision velocimetry: optimization of a fabry-perot interferometer.

We present the optimization of a Fabry-Perot velocimeter designed to measure speed at a few millimeters per second with a relative uncertainty of 10(-8). We focus on the accuracy and the optimization of the Fabry-Perot, with a review of the uncertainties related to the geometry, the beam shape, and the Doppler frequency measurement. These errors are quantified to ensure that the required accuracy is reached. We then describe the practical implementation and show the results.

Pure phase correlator with photorefractive filter memory.

We report on the investigation of a new compact configuration of an inverted VanderLugt-type correlator system. The phase of the Fourier transform of the image to be recognized is displayed on a phase-modulating electrically addressed spatial light modulator. This phase display is compared with the phase of the Fourier transforms of a reference library recorded in a photorefractive LiNbO(3) crystal. Angular hologram multiplexing permits fast data access, and the use of the conjugated replica of the stored templates leads to an elimination of phase distortions introduced by the optical system. With such a configuration, the correlator is fully shift invariant in spite of the photorefractive crystal thickness and has good discrimination with sharp correlation peaks.

Dispersive white-light interferometry for absolute distance measurement with dielectric multilayer systems on the target.

We have extended the use of a dispersive white-light interferometer for absolute distance measurement to include effects of dielectric multilayer systems on the target. The phase of the ref lected wave changes as a function of wavelength and layer thickness and causes errors in the interferometric distance measurement. With dispersive white-light interferometry these effects can be measured in situ, and the correct mechanical distance can be determined. The effects of thin films deposited upon the target have been investigated for one and two layers (photoresist and SiO(2) upon Si). Experimental results show that the thicknesses of these layers can also be determined with an accuracy of the order of 10 nm.

Stabilized three-wavelength source calibrated by electronic means for high-accuracy absolute distance measurement.

Multiple-wavelength interferometry offers great f lexibility in working distance and sensitivity by permitting an appropriate choice of the different wavelengths used, and it can be operated on rough surfaces. The accuracy of distance measurement with this approach depends on the stability and the calibration of the different wavelengths. A novel concept of a multiple-wavelength source that uses laser diodes has been developed. It allows one to obtain an absolute calibration of the synthetic wavelengths by the use of electronic beat-frequency measurements. Experimental results show that a calibration of the synthetic wavelength in the millimeter range with an accuracy of better than 10(-5) is feasible.

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.

Reciprocal reflection interferometer for a fiber-optic Faraday current sensor.

A reciprocal fiber-optic reflection interferometer for remote measurement of electrical current through the Faraday effect is described. The effects of polarization cross coupling because of nonideal elements are eliminated with a low-coherence source. Nonreciprocal birefringence phase modulation is employed for detection of the Faraday phase shift. The theoretical predictions are confirmed by measurements with a piece of straight fiber as the sensing element in a 100-turn solenoid. Currents from 0 to 40 A have been measured with a linear response and a noise limit of ~0.015 A/√Hz.

Laser-based refractive-index detection for capillary electrophoresis: ray-tracing interference theory.

The fringe pattern observed in a far field after a laser beam illuminates a fused silica capillary immersed in a refractive-index matching material and filled with an analyte fluid is exploited to develop a sensitive optical detector for capillary chemical analysis. The inner capillary interface splits the laser beam into a reflected beam fan and a refracted beam fan, which, on overlapping in the far field, lead to interferences. The intensity and the position of the fringes for capillaries with 250 microm >/= i.d. (inner diameter) >/= 25 microm are well reproduced by the presented model. The calculation predicts the fringe pattern for various beam/i.d. geometric configurations and is used to optimize the performance of the nanoliter-picoliter refractive-index on-column detection studied. It is found that the best contrast corresponds to a capillary that is illuminated with a beam waist of omega(0) ~ i.d./12, which is off-center focused with an offset of s ~ i.d./2. For a given interference pattern, the fringes that are found to be more sensitive to Deltan are those that appear near the optical axis but still retain high intensity and contrast. The sensitivity increases approximately linearly with the fringe number, and the maximal fringe number increases proportionally with the i.d.

Optimized kinoform structures for highly efficient fan-out elements.

We discuss the realization of highly efficient fan-out elements. Laser-beam writing lithography is available now for fabricating smooth surface relief microstructures. We develop several methods for optimizing microstructure profiles. Only a small number of parameters in the object plane are necessary for determining the kinoform. This simplifies the calculation of M x N arrays also for large M and N. Experimental results for a 9-beam fan-out element are presented.

Fan-out elements recorded as volume holograms: optimized recording conditions.

The recording of efficient fan-out elements as volume holograms is investigated by using the coupled-wave theory. In contrast to the results published in the standard literature, we find that the efficiency and the uniformity of regular fan-out elements depend strongly on the relative phases of the object waves, at least, if the thickness of the hologram is less than ~50 wavelengths. High efficiency and uniformity can be achieved by optimized recording conditions. At the same time, the required dynamic range of the holographic material becomes minimum.

Electronically scanned white-light interferometry: a novel noise-resistant signal processing.

We describe a signal-processing method for determining the center position of a white-light fringe signal. This is achieved in two steps. First, the center of gravity of the signal power is calculated to better than half a fringe period. Second, a synchronous sampling with four samples per fringe period is used to calculate the phase of the zero fringe. The theoretical analysis and experimental results show that the proposed signal processing is simple to operate, fast, accurate, and extremely noise resistant.

Polarimetric fiber optical sensor with high sensitivity using a Fabry-Perot structure.

A novel polarimetric Fabry-Perot sensor concept, based on the phase detection of the transmitted light, is presented in detail. This concept has been successfully applied to measure static force by stress induced birefringence in an optical fiber with high sensitivity. The detection scheme consists of locking the optical frequency of a laser diode to a resonance peak, where the sensitivity is highest, and using heterodyne detection to measure the phase difference between the eigenpolarizations.

Heterodyne interferometer for submicroscopic vibration measurements in the inner ear.

Conditions in the inner ear for interferometric measurements are quite different from those encountered in other mechanical systems: (i) The inner ear is not mechanically stable, due to blood pulsations and breathing artifacts; (ii) access to the inner ear is limited by anatomical constraints that make it difficult to visualize the structures of interest; (iii) vibration amplitudes to be measured in the inner ear are very low; (iv) the structures in the inner ear are nearly transparent; therefore, the reflectivity is low and attempts to change this reflectivity artificially usually alter the response characteristics; (v) cells are subject to light damage if the incident light intensity is too high, which limits the laser power that can be utilized in the interferometer. A heterodyne interferometer specially designed to measure vibrations in the living inner ear is described. Theoretical and experimental characteristics of this instrument are discussed in detail. In contrast to the homodyne system, the measurement accuracy of this interferometer is not affected by the low-frequency animal movements. This system does not require attachment of a reference mirror to the animal, thereby providing an unobstructed view of the structure to be measured. It has a high linearity and dynamic range. Its vibration sensitivity is high (2.8 X 10(-13) m for 1-Hz bandwidth) even under the condition of low light reflectivity (0.02%), with 0.5-mW incident laser power.

Two-wavelength laser interferometry using superheterodyne detection.

In two-wavelength interferometry, synthetic wavelengths are generated in order to reduce the sensitivity or to extend the range of unambiguity for interferometric measurements. Here a novel optoelectronic technique, called superheterodyne detection, is presented, which permits measurement of the phase difference of two optical frequencies that cannot be resolved by direct optoelectronic heterodyne detection. This technique offers the possibility for operation of two-wavelength interferometry in real time with arbitrary synthetic wavelengths from micrometers to meters in length. Preliminary experimental results are reported. An optical arrangement for absolute range-finding applications using tunable-laser sources (e.g., semiconductor lasers) is proposed.

Holographic optical scanning elements with minimum aberrations.

An analytical method to design holographic optical elements for focusing laser scanners, especially disk scanners, with minimum aberrations and optimum scan line definition is reported. The results reveal that the focused spot constraint to a straight line is always astigmatic. However, by accepting small deviations from the straight line, the astigmatism can be eliminated. The second-order analytical solutions are examined with the help of geometrical ray tracing and compared with experimental results. By extending the method to higher-order approximations, it was found that the correction of the aberrations is essentially limited to the direction perpendicular to the scan line.

Strain measurement by heterodyne holographic interferometry.

Heterodyne holographic interferometry provides automated interference phase measurement with high resolution and allows the quantitative evaluation of 3-D displacement and strain on solid objects. The fundamentals of heterodyning in double-exposure holography are reviewed and its possibilities and limitations are discussed in detail. Experimental results of strain measurement on a curved object surface are reported. Three double-exposure hologram recordings with different illuminations are used to get the vector displacement. Based on locally calculated derivatives of the displacement field, a sensitivity for the strain components of 1 microm/m with a spatial resolution of a few millimeters has been achieved. The features of heterodyne holographic interferometry are compared with those of quasiheterodyne (phase stepping) fringe interpolation.