SD-OCT Base Unit
Chapter 4: System Operation
Page 34
MTN020880-D02
Imaging through Refractive Media
To acquire OCT images, the optical path lengths of the sample arm and reference arm need to be matched.
Imaging through refractive media:
increases the optical path length of the sample arm, which can be compensated for in the reference
arm (for more details see chapter 5.8).
changes the amount of dispersion, which can be compensated using the dispersion compensation
control in the ThorImage
®
OCT software application.
Every time the amount of refractive media is changed, this procedure should be repeated in order to achieve a
good image quality.
Figure 37 Step-by-Step Adjustments when Imaging through Refractive Media
Instead of adjusting focus and reference arm length at the same time, it is often helpful to first find the focus
and then adjust the reference arm length. Figure 37 shows this procedure step by step, starting with a sample
that is out of focus and with a misaligned reference arm, so that no sample can be seen in the B-scan (Figure
37A). After finding the focus, autocorrelation terms will appear in the upper part of the image even without any
reflecting surface in the field of view of the B-scan. When these artifact signals are the strongest, stop focusing
and adjust the reference arm length so that the image appears in the top half of the window.
Furthermore, the refractive media will introduce dispersion effects, i.e. washing out due to the dependence of
the phase velocity on the wavelength. This can be compensated using the ThorImage
®
OCT software (for more
details, see the ThorImage
®
OCT software manual). Note that the amount of dispersion depends on the
thickness of the refractive media. Hence, the dispersion compensation needs to be adjusted every time the
thickness of the refractive media changes.
Note:
If you are able to identify autocorrelation signals but are not able to move the OCT image into the
observable window, the optical path length of the sample arm has likely increased to an extent that cannot be
compensated by the reference arm anymore. Thorlabs offers a special reference arm adapter to further increase
the reference arm length; please contact Thorlabs’ OCT support (see Chapter 13) for more information.
Autocorrelation
Signals
A
B
C
D
