Three wavelengths parallel phase-shift interferometry for real-time focus tracking and vibration measurement.

Instantaneous high-resolution, wide-range focus tracking and a vibrometry system based on three-wavelength (3λ) parallel phase-shift polarization interferometry using three detectors per wavelength is presented. The system, implementing 3λ in-parallel three-phase-shift-interferometry channels for the first time, to the best of our knowledge, allows single-shot position tracking of motion profiles with extremely high velocities and vibration rates, long inter-step heights, and sub-nanometer scale accuracy. The system's simple design and algorithm presented here do not rely on active optical components, making its performance limited only by the detectors' bandwidths and allowing the setting up of a very high-performance low-cost vibrometry system.

High-speed 3D imaging using two-wavelength parallel-phase-shift interferometry.

High-speed three dimensional imaging based on two-wavelength parallel-phase-shift interferometry is presented. The technique is demonstrated using a high-resolution polarization-based Linnik interferometer operating with three high-speed phase-masked CCD cameras and two quasi-monochromatic modulated light sources. The two light sources allow for phase unwrapping the single source wrapped phase so that relatively high step profiles having heights as large as 3.7 µm can be imaged in video rate with ±2 nm accuracy and repeatability. The technique is validated using a certified very large scale integration (VLSI) step standard followed by a demonstration from the semiconductor industry showing an integrated chip with 2.75 µm height copper micro pillars at different packing densities.

Real-time phase shift interference microscopy.

A real-time phase shift interference microscopy system is presented using a polarization-based Linnik interferometer operating with three synchronized, phase-masked, parallel detectors. Using this method, several important applications that require high speed and accuracy, such as dynamic focusing control, tilt measurement, submicrometer roughness measurement, and 3D profiling of fine structures, are demonstrated in 50 volumes per second and with 2 nm height repeatability.

Ultrahigh-resolution full-field optical coherence tomography using spatial coherence gating and quasi-monochromatic illumination.

We developed an ultrahigh-resolution full-field optical coherence tomography (FF-OCT) microscope that is based on the spatial, rather than the temporal, coherence gating. The microscope is capable of observing three-dimensional microbiological structures as small as 0.4 µm × 0.4 µm × 1.0 µm (xyz) using quasi-monochromatic light and a liquid crystal retarder. Unlike traditional FF-OCT systems, this microscope can be operated in high resolution for any preferable wavelength with minimized defocusing and dispersion effects. High-resolution images of an onion cell are presented.

Spatial coherence effect on layer thickness determination in narrowband full-field optical coherence tomography.

Longitudinal spatial coherence (LSC) is determined by the spatial frequency content of an optical beam. The use of lenses with a high numerical aperture (NA) in full-field optical coherence tomography and a narrowband light source makes the LSC length much shorter than the temporal coherence length, hence suggesting that high-resolution 3D images of biological and multilayered samples can be obtained based on the low LSC. A simplified model is derived, supported by experimental results, which describes the expected interference output signal of multilayered samples when high-NA lenses are used together with a narrowband light source. An expression for the correction factor for the layer thickness determination is found valid for high-NA objectives. Additionally, the method was applied to a strongly scattering layer, demonstrating the potential of this method for high-resolution imaging of scattering media.

Skin biomedical optical imaging system using dual-wavelength polarimetric control with liquid crystals.

Spectropolarimetric skin imaging is becoming an attractive technique for early detection of skin cancer. Using two liquid crystal retarders in combination with a dual-band passive spectral filter and two linear polarizers, we demonstrate the spectral and polarimetric imaging of skin tissue in the near infrared. Based on this concept, a compact prototype module has been built and is being used for clinical evaluation.

Liquid-crystal polarization rotator and a tunable polarizer.

A liquid-crystal (LC) voltage-controlled linear polarizer is demonstrated using two LC retarders stacked with two quarter-wave plates and an intermediate linear absorptive polarizer. The device was examined experimentally using unpolarized light and was found to be in accordance with the theoretical prediction. Under certain conditions the device acts as a polarization rotator with possibility for simultaneous amplitude modulation. Hence it has a potential application in high-dynamic-range polarimetric imaging.