OCTG Series Scanner
Chapter 5: Description
Rev C, April 06, 2016
Page 9
Chapter 5
Description
5.1.
Theory
5.1.1. Signal Generation
Spectral domain optical coherence tomography (SD-OCT) generates cross-sectional images up to several
millimeters deep into tissue. The images are assembled by performing a series of scans at adjacent,
increasing depths, allowing 2D and 3D reconstruction of the specimen.
Therefore the interference between the light coming back from the specimen and from a reference is sampled
for a broad range of different wavelengths. This is performed either by using a broadband light source divided
into separate wavelength using a spectrometer or by a laser with small bandwidth quickly tuned over a broad
wavelength range.
The phase delay of the back-reflected and back-scattered light (with respect to the stationary reference) is
recorded as a function of wavenumber, and a Fast Fourier Transform (FFT) yields the cross-sectional images
as a function of sample depth.
5.1.2. Limitations
The spatial resolution and sensitivity of the OCTG scanner depends on several parameters, including the
following:
Wavelength Range:
The optical components within the OCTG are optimized for a specific
wavelength range, depending upon the model. For the usable wavelength range please refer to Table
1.
Optical Power:
The sensitivity of the OCTG is directly related to the intensity of the light returning
from the sample. Factors that can reduce the collected light intensity from the sample fiber include:
dirty fibers, blocked/cropped beams, and condensed water in the environment.
Physical Movements:
OCT systems use a camera to detect the phase relation of the light returning
from the sample. Even small movements of the specimen in relation to the optical reference arm can
”wash out” the wavenumber-resolved phase contrast, affecting the image.
Imaging:
In a fiber-based OCT setup, the light returning from the sample is focused into the core of
an optical fiber. Hence, the fiber can be thought of as a spatial filter for the light. This filter has an
effective diameter,
referred to as the “mode field diameter. For single mode propagation, mode field
diameter is larger than the fiber core diameter. Poor focusing, caused by optical aberration or
misalignment, therefore leads to loss of contrast and sensitivity.
