THORLABS CAL110, Руководство пользователя

Страница: 21 / 68

SD-OCT Base Unit Chapter 3: Description
Rev E, November 7, 2018 Page 17
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
Gaussian-shaped light source in -space, 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 realistic light source does not necessarily have a mathematically ideal shape best-suited for the
autocorrelation function. Therefore, it is recommend that the signal be apodized to obtain a smooth, 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 measurements, 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.