Multimodal Biomedical Imaging Systems - Biomedical Optical Imaging Technologies: Design and Applications - page 328

and the angle of incidence without consideration of diattenuation and retardance.
In many situations, even for polarization imaging systems, this decoupled process,
where the lens and the coating are designed independently, works well. However,
for some polarizationsensitive imaging systems, it is necessary to analyze the
polarization effects from optical coatings.
During the optical design phase, we need to determine the forms of polar-
ization effects (diattenuation or retardance or both) and identify surfaces with
large polarization effects. Low polarization design requires that the optical system
designer consider surface geometries, optical materials, and coatings that can cause
polarization aberrations and end-to-end polarization aberration analysis of the entire
optical system. We also need to understand how the polarization aberrations add or
cancel between surfaces. Measurements of coatings or dielectric samples during the
design phase allow models to better simulate system performance.
A general guide in designing a polarization imaging system is to place the
polarizer as the last element in the illumination path and to place the analyzer as
the first element in the imaging path. With the polarizer as the last element in
the illumination path, undesired polarization effects induced by elements in the
illumination path are minimized. Similarly, with the polarizer as the first element,
no special consideration is necessary for optical elements in the imaging path.
However, this guideline is not suitable for some applications, for example, when
the illumination path and imaging path share some common components.

9.3.4 Design of OCT Imaging Systems

Chapter 5 discusses the different configurations of OCT systems. Three different
OCT techniques have been developed: time-domain, spectrometer-based Fourier-
domain, and swept-source Fourier-domain techniques. While the configurations
for the sample arm are the same, each technique has different configurations in
reference arm and signal detection. For time-domain OCT (TD-OCT), a high-
speed scanning optical delay line (ODL) is needed for real-time imaging. The most
commonly used optical delay line for real-time time-domain OCT is Fourier-domain
rapid-scanning ODL (RSOD) [ 35 ]. In spectrometer-based Fourier-domain OCT
(FD-OCT), a spectrometer based on a linear detector array samples the spectrum
from the reference arm and sample arm. The design of the spectrometer should
consider the frequency resolution, the spectral response, and the spot size of the
beam. These parameters determine the falloff characteristic and axial resolution of
the image. Considering the limited number of pixels in the detector array, a trade-off
is required between the imaging range and the axial resolution in the spectrometer
design. In swept-source OCT (SS-OCT), both the reference arm and detection arm
are relatively simple. The optical path length in the reference arm is fixed, and the
light from the reference arm and sample arm is delivered to the photodiode directly
without additional optics.