Carl C. Aleksoff

Holographic conversion of a Gaussian beam to a near-field uniform beam

A two-element holographic optical system is described for converting an elliptically shaped Gaussian-profile laser beam into a rectangularly shaped beam that is uniform in amplitude and phase in the nearfield. Theoretical analysis, design considerations, and experimental results are presented for a compact converter system of less than 6 cm in length.

Power adjustable visible supercontinuum generation using amplified nanosecond gain-switched laser diode

Supercontinuum covering 0.45-1.2 mum is scaled from 250-740 mW by varying the repetition rate of a frequency doubled telecom laser diode. Efficient SC generation requires minimal non-linearity in the amplifier and fiber dispersion matched to the pump.

Gas lasers as sources for holography.

The quality of a hologram depends on the coherence properties of the source used for recording the hologram. When the source is a laser many of its important properties as a source are determined by the relative intensities and distribution of its modes, both transverse and longitudinal This note describes an experimental demonstration that the reconstruction quality of a hologram can be predicted from a knowledge of the modes, at least for the case presented, namely, a gas laser operating in multilongitudinal but single transverse modes. In holography, a quantity of interest is the ratio of intensity in the reconstruction to that of the object, as a function of the path length difference between the object and reference beams. This ratio, normalized to its value at zero path length difference, is called the reconstruction brightness B(l). The brightness B(l) has a simple relation to fringe visibility V(l) used in interferometry. Fringe visibility V(l) is defined as

Interferometric two-dimensional imaging of rotating objects

It is shown, theoretically and experimentally, that a rotating object can be two-dimensionally imaged by illuminating the object with a sinusoidal interference pattern and then using the temporal modulation of the scattered light as the signal for building up a synthetic aperture. The image is formed in the Fourier-transform plane of the synthetic aperture.

High-precision three-dimensional shape reconstruction via digital refocusing in multi-wavelength digital holography

Three-dimensional (3D) shape reconstructions and metrology measurements are often limited by depth-of-field constraints. Current focus-detection-based techniques are insufficient to profile out-of-focus 3D objects with high axial accuracy. Extended-focus imaging (EFI) techniques can improve the range and precision of such measurements. By incorporating digital refocusing with multiwavelength interferometry, a holographic imaging solution is presented in this paper to accurately measure 3D objects over a large depth range. Accuracy and repeatability of the proposed EFI technique are validated by digital simulations and refocusing experiments. A reconstruction example demonstrates the feasibility of high-precision 3D measurements of objects deeper than the systems classical depth of field.

Grating-based interferometric processor for real-time optical Fourier transformation

A processing approach is introduced that is capable of performing 1-D real-time Fourier transformations on the intensity distribution of an incoherent optical input. The processing approach is based on grating interferometers, and the resulting processors are simple in structure and easily implemented. Possible processor configurations together with experimental results demonstrating the operation of the system are presented. Analyses are given comparing the grating interferometric processor to the Michelson stellar interferometer and the classical coherent optical processor.

Imaging through scattering media by interferometric techniques.

Imaging through scattering media such as fog is a problem with few known viable solutions. Holographic techniques have been demonstrated successfully in the laboratory, but their usefulness is often limited in field conditions by the requirement of a separate coherent reference beam. In this paper, an interferometric imaging technique that utilizes a grating interferometer is presented. Experimental results obtained with this technique show substantial improvement in image contrast over that obtained via direct imaging.

A multiple height-transfer interferometric technique

We propose a multiple height-transfer interferometric technique based on concepts from both multiple wavelength interferometry and wavelength scanning interferometry. Conventional multiple wavelength interferometry requires accurate wavelength information for large step height measurement, while wavelength scanning interferometry is limited by mode-hop-free tuning range. Using the multiple reference heights, it is possible to bypass the wavelength determinations and achieve large step height measurement using relative phase changes. By applying this technique with a proposed multiple height calibration artifact, we experimentally demonstrated accuracy better than 1 micron over 100 mm in a workshop environment.

High resolution line scan interferometer for solder ball inspection using a visible supercontinuum source

A line scan interferometer, which comprises a visible supercontinuum source coupled to Fourier domain Michelson interferometer, is used to obtain 3D images of ~300 μm high solder balls on a semiconductor die with 125 nm axial and 15 μm lateral resolution. The ability to measure curved surfaces enables the determination of solder ball shape defects in addition to ball height. We show that the maximum measurable angular tilt from the sample surface normal for a given source power depends on the surface roughness of the sample. As an example, we demonstrate height measurement over +/-20 degrees from the normal on the solder balls and over +/-60 degrees on a rough steel ball bearing sample.

Discrete step wavemeter

Tunable lasers are used in optical metrology, but their intrinsic tuning accuracy is sometimes inadequate and an external wavemeter is then required to measure the wavelength more accurately. In this paper, we present the design of a discrete step wavemeter to measure the wavelength of the light from a tunable laser during the operation of a multi-wavelength interferometric shape measuring system. This relatively low-cost wavemeter is embedded in the metrology system and consists of a discrete set of small retroreflectors mounted at different ranges on a super-invar base, which eases alignment and allows it to be insensitive to temperature changes. During operation, interference patterns from the retroreflectors are captured by a camera for each phase shift and commanded wavelength and analyzed to determine the actual wavelength. The phase measurement uses a least square fitting algorithm. A Fourier Transform peak finding measurement technique is used for phase unwrapping. Both numerical simulation and experiments indicate improved system performance using this internal wavemeter technique.