Avner Safrani

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.

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.

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.

Spectropolarimetric method for optic axis, retardation, and birefringence dispersion measurement

A new method and algorithm for measuring optical linear birefringence is proposed. The method allows the measurement of both the principal axis orientation angle and the retardation simultaneously by a three-step measurement. The average absolute error of the retardation of an achromatic quarter-wave plate (QWP) is found to be better than 10 -4 parts of the wavelength over the whole spectrum, and its principal axis system orientation is determined with accuracy better than 0.18 deg. In comparison to other methods, the current technique holds several advantages: wavelength independence, low cost, compact setup, ease of alignment, use of a simple algorithm, no polarization reflectance dependence, and possesses high accuracy. The method was applied also to the measurement of an arbitrary retardation of a sapphire plate and to the assessment of the dynamic retardation of a liquid crystal device.

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.

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 tunable filters and polarization controllers for biomedical optical imaging

Liquid crystal (LC) devices exhibit fast and strong tuning and switching capabilities using small voltages and can be miniaturized thus have a great potential to be used with miniature optical imaging systems for biomedical applications. LC devices designed specifically for integration into biomedical optical imaging systems are presented. Using a combination of one or two LC retarders we obtained polarimetric imaging of the skin. LC tunable filters with high dynamic range and large throughput are designed for hyperspectral imaging and for spectral domain optical coherence tomography. The designs are based on several concepts both using the classical stack of retarders and using more modern designs based on single layer in a waveguide or in a Fabry-Perot cavity.

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.

Full-field parallel interferometry coherence probe microscope for high-speed optical metrology.

Parallel detection of several achromatic phase-shifted images is used to obtain a high-speed, high-resolution, full-field, optical coherence probe tomography system based on polarization interferometry. The high enface imaging speed, short coherence gate, and high lateral resolution provided by the system are exploited to determine microbump height uniformity in an integrated semiconductor chip at 50 frames per second. The technique is demonstrated using the Linnik microscope, although it can be implemented on any polarization-based interference microscopy system.

Real time parallel phase shift orthogonal polarization 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 which require high speed and accuracy are demonstrated in 50 volumes per seconds and 2nm height repeatability, dynamic focusing control, fast sub-nm vibrometry, tilt measurement, submicron roughness measurement, 3D profiling of fine structures and micro-bumps height uniformity in an integrated semiconductor chip. Using multiple wavelengths approach we demonstrated phase unwrapped images with topography exceeding few microns.