How Does Fourier Domain OCT Extend Interference Beyond Coherence Length?

[Eluri]

Hello,

I don't know if anyone is familiar with OCT, but my question is rather specific and a well explained answer is in my opinion nowhere to be found ...

In OCT, a low coherent light source is used in a simple Michelson interferometer setup. One beam (reference beam) is sent onto a known path length with a mirror on the end, the other beam (sample beam) is reflected by scatterers inside the sample. The two beams come back and they form an interference pattern only if the sample beam has traveled a distance that can only differ from the known reference length no more than the coherence length of the source. By scanning the reference path by moving the mirror, you can scan the whole depth of the sample and get an A-scan.

This setup is called Time Domain OCT.


My question concerns another kind of setup, which is newer: Fourier Domain OCT.
In FD OCT, the reference mirror and thus the reference path length is keeped fixed. It relies on the Wiener-Kintchin relation between the spectral density of the signal and the autocorrelation function (the interference). It basicly measures the present frequency modulations in the spectrum and from this an inverse Fourier Transformation delivers depth information for some magical reason. I don't know why.
I also can't understand how interference can occur between the sample beam and the reference beam over the whole length of the sample (which is millimeters and falls way out of the coherence length of microns). I thought the whole point was just that interference can only occur within the coherence length and therefore interference between two beams that have traveled a different (outside the coherence length) distance can never interfere...

I know this is quite specific, but maybe there is an OCT guru out there, who happens to read this question

Thanks

 

[Valerie Park]

a lot!In Fourier Domain OCT, the reference beam travels a fixed, known path length and does not move like in Time Domain OCT. The sample beam, on the other hand, has an unknown path length and is reflected back by scatterers inside the sample. This creates an interference pattern between the two beams, which depends on the relative path lengths of the two beams. The Wiener-Khinchin theorem states that the power spectrum of a signal is related to its auto-correlation function. In other words, the frequency modulation present in the interference pattern can be used to calculate the depth information of the sample. To do this, an inverse Fourier transformation is performed on the interference pattern to obtain the depth information. This is how Fourier Domain OCT is able to measure depth information over a longer distance than the coherence length of the light source.

 

[astrokreng]

for sharing your question about OCT. As a scientist familiar with this technology, I can provide some insights and clarification on your questions.

First, let's start with a brief explanation of OCT. Optical Coherence Tomography is a non-invasive imaging technique used to obtain high-resolution, cross-sectional images of biological tissues. It works by using low-coherence light, typically in the near-infrared range, to create interference patterns between a reference beam and a sample beam. By measuring the interference patterns, we can obtain depth information about the tissue structure.

Now, to address your specific questions about Time Domain and Fourier Domain OCT. In Time Domain OCT, as you mentioned, the reference mirror is scanned to obtain depth information. This is done by measuring the interference pattern at different reference mirror positions and using this information to reconstruct the depth profile of the sample.

In Fourier Domain OCT, the reference mirror is kept fixed and instead, the spectrum of the interference pattern is measured. This spectrum contains information about the frequency modulations in the signal, which can then be used to obtain depth information using an inverse Fourier transformation.

To understand how interference can occur between the sample beam and the reference beam over the whole length of the sample in Fourier Domain OCT, it's important to note that the coherence length of the light source is not the only factor at play. The coherence length only determines the axial resolution of the system, but in Fourier Domain OCT, we are also utilizing the spectral information to obtain depth information. This allows us to measure interference patterns over a longer distance, beyond the coherence length of the light source.

I hope this helps to clarify your questions about Fourier Domain OCT. It's a complex and constantly evolving technology, so it's understandable that it can be confusing at times. If you have any further questions or would like more information, please don't hesitate to ask.