Polarization and Spectral Interferometric Techniques for Quantitative Phase Microscopy - Biomedical Optical Phase Microscopy and Nanoscopy - page 272

using quantitative phase measurements. They all share the common aspect of providing
measurements on the nanometer scale, achieved by interferometric techniques which
measure the phase of light. Here, we focus on the extensions of these techniques which
provide additional capabilities and unique information by exploiting other aspects of light,
including its polarization and spectral dependence.
This chapter presents a suite of novel quantitative phase measurement tools designed to
probe the structure and dynamics of living cells by exploiting either the polarization or
spectral dependence of light to increase the sensitivity and stability of these measurements.
These tools share the use of interferometric schemes that measure phase changes in light to
learn about biological cells and tissues. Here, we show how polarization can be used to
improve imaging throughput by providing two phase-shifted images in a single exposure.
We then present a method that uses polarization to generate two spatially displaced beams,
as done in Nomarski microscopy, but the interference that is generated is detected in the
spectral domain, taking advantage of recent instrumental advances in the field of optical
coherence microscopy. Finally, we exploit the rich source of information in the optical
spectrum to obtain unique measurement capabilities, including the simultaneous use of
multiple wavelengths to aid in phase unwrapping and the measurement of dispersion
properties to provide access to spectroscopic features in phase measurements.
14.2 Dual-Interference Channel Quantitative Phase Microscopy
Imaging live cells requires a system that is able to visualize mostly transparent three-
dimensional objects with very little inherent absorption, imparting almost no change to the
light amplitude reflected from or transmitted through them. Conversely, the phase of light
transmitted through these transparent objects can provide information about cell structure
by recording subtle changes in density as variations in optical path delay (OPD). Phase
imaging can be performed by conventional microscopic techniques, such as phase-contrast
microscopy and differential interference contrast microscopy [1] . However, these techniques
do not yield quantitative phase measurements. In addition, they suffer from various artifacts
that make it hard to quantitatively interpret the resulting phase images in terms of OPDs.
Digital holography, on the other hand, yields quantitative measurement of the phase
distribution across a field of view [2 6] . Therefore, it is possible to manipulate the
complex wavefront to conduct quantitative analysis. Digital holography, however, requires
interferometric setups, typically yielding phase images that are vulnerable to phase noise.
Elimination of most phase noise, which can arise from perturbations in the interferometer
arms, can be accomplished by acquiring two or more phase-shifted interferograms of the
same sample [5,6] . However, this approach can be limiting, as certain biological processes,
such as cell membrane fluctuations and neuron activity, occur faster than the acquisition
rates of most optical wide-field interferometric imaging systems.