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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.

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We propose a new tunable laser source concept for multiple-wavelength interferometry, offering an unprecedented large choice of synthetic wavelengths with a relative uncertainty better than 10 −11 in vacuum. Two lasers are frequency stabilized over a wide range of frequency intervals defined by the frequency comb generated by a mode-locked fiber laser. In addition, we present experimental results demonstrating the generation of a 90 m synthetic wavelength calibrated with an accuracy better than 0.2 parts in 10 6 . With this synthetic wavelength we can resolve one optical wavelength, which opens the way to absolute distance measurement with nanometer accuracy. Two-wavelength interferometry allows absolute distance measurements 1,2 over a range defined by the synthetic wavelength 1 ⌳ = 1 2 / ͑ 1 − 2 ͒ = c / ⌬, where 1 and 2 are the individual wavelengths. Starting from a long synthetic wavelength (e.g., 1 m), a nanometer-level accuracy can be ultimately achieved by gradually decreasing ⌳ until single-wavelength interferometry is performed. This requires the capability to resolve an optical wavelength; thus the accuracy obtained with the smallest synthetic wavelength must be better than / 4. The measurement accuracy at large distances may be limited by the instability and uncertainty of the synthetic and optical wavelengths. We propose a new concept of a tunable twowavelength laser source stabilized with high accuracy and over large frequency ranges that replaces the previously used frequency comb of a Fabry-Perot resonator 3 with the highly accurate frequency comb of a femtosecond (fs) laser. The use of an optical frequency comb has been recently proposed for gauge block length calibration. 4 However, the reported detection technique is based on consecutive measurements at different wavelengths and therefore requires high mechanical stability during the measurement time. We present in this Letter a tunable two-wavelength source with which dynamic distance measurements with nanometer accuracy can be achieved by means of superheterodyne interferometry. 4 An active frequency comb is based on a fs modelocked laser, 5 whose repetition rate, f rep , defines exactly the frequency separation between two adjacent modes of its frequency spectrum. Several cw lasers can be locked to different modes of the comb by beat frequency measurements and electronic phase locked loops. The stability of the laser frequency separation is determined entirely by the relative stability of the frequency reference used to control the repetition rate of the fs laser. To cancel the frequency drift of the comb, either the comb can be self-referenced 6 or one of the lasers can be locked onto a molecular transition and the comb locked to that laser through control of the comb offset.The tunable two-wavelength source (Fig. 1) consists of a Nd:YAG laser ( 1 = 1.319 m, Lightwave Model 125), an external cavity laser diode (ECLD, 2 ϳ 1.3 m, Thorlabs INTUN 1300), and finally a mode-locked fiber laser (Menlo Systems TC-1500). A 10 MHz frequency reference ...

This article presents a new integrated microfluidic/microoptic device designed for basic biochemical analysis. The microfluidic network is wet-etched in a Borofloat 33 (Pyrex) glass wafer and sealed by means of a second wafer. Unlike other similar microfluidic systems, elements of the detection system are realized with the help of microfabrication techniques and directly deposited on both sides of the microchemical chip. The detection system is composed of the combination of refractive circular or elliptical microlens arrays and chromium aperture arrays. The microfluidic channels are 60 µm wide and 25 µm deep. The elliptical microlenses have a major axis of 400 µm and a minor axis of 350 µm. The circular microlens diameters range from 280 µm to 350 µm. The apertures deposited on the outer chip surfaces are etched in a 3000-Å-thick chromium layer. The overall thickness of this microchemical system is <1.6 mm. A limit of detection of 3.3 nM for a Cy5 solution in phosphate buffer (pH 7.4) was demonstrated. The crosstalk signal measured between two adjacent microchannels with 1 mm pitch was <1:5600, meaning that e1.8 × 10 -4 % of the fluorescence light power emitted from one microchannel filled with a 50 µM Cy5 solution reaches the photodetector at the adjacent microchannel. This performance compares very well with that obtainable in microchemical chips using confocal fluorescence systems, taking differences in parameters, such as excitation power into microchannels, data acquisition rates, and signal filtering into account.

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.

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.