SS-OCT System Base Unit
Chapter 3: Description
Rev A, December 17, 2018
Page 15
3.1.1.
Theory
The interference equation for the cross-correlated interference term is
~2 ∙
∙
∙ cos Δ
With the phase difference
Δ
being a function of the optical path length difference and the wavenumber
Δ
∙ Δz
This optical amplification of a small sample intensity with a strong reference intensity allows the detection of
single photons from the sample and is the key to the outstanding sensitivity of OCT.
Due to the reflective character of the measurement modality the optical path length difference is twice the
distances in the image. The maximum imaging depth, and so twice optical path length difference, is defined by
the wavenumber spacing of the acquisition
δ
.
Δ
1
4 ∙
In Full-Range OCT setups the image depth could be doubled
The signal width has two limits,
One limit for the signal width is given by the spectral distribution of the light source. For a fully used
light source being Gaussian shaped in the equation is:
,
,
2 ∙ ln 2
Δ
with:
Δ
∙
The other limitation for the signal width is given by the sampling. For a rectangular spectrum the FFT
results in a sinc function
,
,
1.21 ∙
with:
2 ∙ Δ
A light source is not shaped in a way that the autocorrelation function is clean normally. Therefore the shape
should be apodized to get a clean point spread function. A good compromise between resolution and side lobe
suppression could be a Hanning window showing a signal width of:
,
2 ∙
In real mesurements the signal width is furthermore limited by noise, dispersion mismatch between sample- and
reference arm of the interferometer, and optical path length distribution of the imaging caused by aberration.