Micrometric size measurement of biological samples using a simple and non-invasive transmission interferometric set up.

An interferometer with a minimum of optical hardware is employed to measure invasiveness the size of biological samples. Nowadays, there are several techniques in microscopy that render high quality resolved images. For instance, consider optical microscopy that has been around for over a century and has since developed in different configurations such as: bright and dark field, phase contrast, confocal, polarized, and so on. However, only a few of these use interferometry to retrieve not only the sample's amplitude but also its phase. An interesting example of the latter is digital holography which normally uses a Mach Zehnder interferometer setup. In the research work reported here a transmission digital holographic interferometer designed with a simple and minimal optical hardware, that avoids the drawback of the small field of view present in classical optical microscopic systems, is used to measure the microscopic dimensions of pollen grains. This optical configuration can be manipulated to magnify and project the image of a semitransparent sample over a neutral phase screen. The use of a collimated beam through the sample prevents geometrical distortions for high magnification values. The measurements using this novel configuration have been validated using a standard precision pattern displacement specimen with certified dimensions. As proof of principle, microscopically characterized pollen grains are placed in the transmission set up in order to estimate their dimensions from the interferometrically retrieved optical phase. Results match and thus show a relation between the sample's size and the optical phase magnitude.

Overall characterization of assembled optical storage devices with a heterodyne microscope: a qualitative comparison with a confocal microscope.

We present local profile measurements of inner mirrorlike and external front-end polycarbonate surfaces at the same spot of assembled optical storage devices, a CD and a DVD, performed with a heterodyne scanning interferometer that uses Gaussian beams. We show that the heterodyne interferometer can reproduce the profiles of both surfaces with accurate precision. We describe a procedure for calibrating the instrument based on the measurement of reflecting calibrated gratings. To show the advantages that the heterodyne interferometer represents as a valuable tool for the characterization of optical disks, we include a comparison of experimental results obtained with a confocal microscope under similar working conditions.

Fresnel-Gaussian shape invariant for optical ray tracing.

We propose a technique for ray tracing, based in the propagation of a Gaussian shape invariant under the Fresnel diffraction integral. The technique uses two driving independent terms to direct the ray and is based on the fact that at any arbitrary distance, the center of the propagated Gaussian beam corresponds to the geometrical projection of the center of the incident beam. We present computer simulations as examples of the use of the technique consisting in the calculation of rays through lenses and optical media where the index of refraction varies as a function of position.

Inspection of commercial optical devices for data storage using a three Gaussian beam microscope interferometer.

Recently, an interferometric profilometer based on the heterodyning of three Gaussian beams has been reported. This microscope interferometer, called a three Gaussian beam interferometer, has been used to profile high quality optical surfaces that exhibit constant reflectivity with high vertical resolution and lateral resolution near lambda. We report the use of this interferometer to measure the profiles of two commercially available optical surfaces for data storage, namely, the compact disk (CD-R) and the digital versatile disk (DVD-R). We include experimental results from a one-dimensional radial scan of these devices without data marks. The measurements are taken by placing the devices with the polycarbonate surface facing the probe beam of the interferometer. This microscope interferometer is unique when compared with other optical measuring instruments because it uses narrowband detection, filters out undesirable noisy signals, and because the amplitude of the output voltage signal is basically proportional to the local vertical height of the surface under test, thus detecting with high sensitivity. We show that the resulting profiles, measured with this interferometer across the polycarbonate layer, provide valuable information about the track profiles, making this interferometer a suitable tool for quality control of surface storage devices.

Heterodyne two beam Gaussian microscope interferometer.

We present a novel microscope interferometric technique based on the heterodinization of two Gaussian beams for measuring roughness of optical surfaces in microscopic areas. One of the beams is used as a probe beam, focussed and reflected by the surface under test. The second beam interferes with the first beam and introduces a time varying modulating signal. The modulating light beam is obtained from the first diffraction order of a Bragg cell. The two beams are superimposed and added coherently at the sensitive plane of a photodetector that integrates the overall intensity of the beams. We show analytically that it is possible to find appropriate working conditions in which the system has a linear response. Under these conditions, the size of the probe beam at the plane of detection as well as the amplitude of the time varying signal at the output of the photodetector, are both proportional to the local vertical height of the surface under test. As a narrow bandwidth amplifier is used to detect the time varying signal the system exhibits a high signal to noise ratio. We also include experimental results of the measurement of the topography of a sample consisting in a blazed-reflecting grating.