UBAPHODESA project is granted by the Industrial Training Networks FP7-PEOPLE-2013-ITN European
Union with the grant number 607627
Collaboration between the University of Kent, School of Physical Sciences, Applied Optics Group (United
Kingdom) and NKT Photonics A/S (Denmark)
Project title: Supercontinuum generation in specific windows versatile for optical coherence tomography
investigations and spectroscopic optical coherence tomography
A more detailed description of the project and current results can be found below
Name: Felix Fleischhauer
European Early Stage Researcher Industrial Training Networks PhD Student / European Industrial Doctorate
Supercontinuum generation in specific windows versatile for optical coherence
tomography investigations and spectroscopic optical coherence tomography
In this project the possibility of using a supercontinuum in the visible wavelength range for spectroscopic
analysis in combination with imaging, so called spectroscopic optical coherence tomography is investigated.
Spectroscopic optical coherence tomography combines imaging of optical coherence tomography with
localized absorption, which is used as functional information. These functional information are asked for from
several medical doctors to improve diagnostics and screening.
By applying a different processing of the spectrometer data compared to conventional optical coherence
tomography (OCT) spectral and structural information can be linked. Measurements show that by using
several samples, such as blood or laser dyes their absorption spectrum can be reconstructed and that it is
influenced by the used concentration. These data are measured in combination with high resolved structural
information due to the wide bandwidth of the supercontinuum source and the broadband spectrometer. The
measurements are done to possibly investigate the absorption of oxygen by comparing oxygenated and
deoxygenated haemoglobin. This can be used in applications, such as retinal venous and arteriolar
occlusions, diabetic retinopathy, glaucoma or assessment of central venous and arteriolar oxygen saturation.
The used OCT system shown is in Figure 1 a) and b) and consists of a customized supercontinuum light
source, a Michelson interferometer and a commercially available spectrometer. The light from the
supercontinuum light source is split by a fibre-coupler into the two interferometer arms. The light returned by
the retroreflectors is combined by the fibre coupler and directed into the spectrometer; see Figure 1 a). In
further measurements the cuvettes are removed and the retrorefector in the sample arm is replaced with an
objective and a sample realising position sensitive SOCT, as can be seen in Figure 1 b).
Figure 1: Sketch of the OCT system for a) cuvette measurements and b) thin glass capillaries. FC: Wideband
fibre coupler, L: Collimation lenses, R: Retroreflector, Q: Quartz cuvettes, DC: Dispersion compensation, Obj.:
Objective, C: Thin glass capillary
The combination of spectrometer and broadband light source enables detection of light in the range of 480 to
730 nm on 1700 pixels. The set-up has an axial resolution of 1.15 µm, which is slightly larger than the
theoretical value of 0.65 µm, calculated with a Gaussian spectral shape of a 250 nm full-width at half
maximum. The sensitivity is 60 dB and has a roll-off of -16 dB/mm. The usable imaging depth is limited to
around 500 µm.
Molar extinction
coefficient [cm^-1/M]
As the laser dye rhodamine B has a similar absorption peak as oxygenated haemoglobin, as can be seen in
Figure 2, it is used to evaluate our set-up and its performance.
120000
Haemoglobin
100000
Rhodamine B
80000
60000
40000
20000
0
490
540
590
640
690
Wavelength [nm]
Figure 2: Absorption spectra of haemoglobin and rhodamine B. Rhodamine B and haemoglobin have a similar
absorption peak around 542 nm.
100
90
80
70
60
50
40
30
20
10
0
1:1000
1:2000
1:4000
Transmission [%]
Absorption [%]
In a first measurement rhodamine B is filled into a 10 mm quartz cuvette and inserted into one interferometer
arm and the absorption is measured. By varying the dye concentration (1:1000; 1:2000; 1:4000; 1:5000 and
1:1000000 [dye in ml: water in ml]) the influence of concentration variation on the spectroscopic OCT signal
is investigated, which can be seen in Figure 3. Plotting the transmission as function of concentration, the
data can be fitted by an exponential function, which describes the absorption as function of concentration
(comes from the Beer-Lambert law).
1:5000
1:1000000
490
540
590
640
Wavelength [nm]
690
Figure 3: With spectroscopic OCT reconstructed
absorption spectra of laser dye rhodamine B.
Different concentration show different absorption
amplitude.
100
90
80
70
60
50
40
30
20
10
0
1:1000
1:2000
1:4000
1:5000
1:1000000
Exponential fit
0
0.0005
0.001
Concentration [Dye : ethanol]
0.0015
Figure 4: Fitted absorption of rhodamine B to
exponential fit, with which transmission can be
described.
In a next experiment the spectral information are combined with structural information. For this a small glass
capillary is filled with rhodamine B and the localized absorption between the two glass-dye surfaces is
reconstructed. A total of eight spectral bands are used to reconstruct the absorption spectrum. Each band
has a different central wavelength, but a constant spectral width in wavenumber, which they cover.
In Figure 5 the structural information can be seen, while in Figure 6 the reconstructed absorption spectra of
different dye concentrations (not diluted; 1:10; 1:50 [dye in ml: ethanol in ml]) of rhodamine B in between the
two glass-dye surfaces are shown.
70
Dye
140
High
resolution
A-scan
Amplitude [dB]
120
A-scan
window
542 nm
100
80
60
40
Pure
Attenuation [dB]
160
60
1:10
50
1:50
40
30
20
10
20
0
0
0
200
Depth [µm]
400
600
Figure 5: A-scans of small capillary with inserted laser
dye rhodamine B in different concentrations.
490
540
590
640
Wavelength [nm]
690
Figure 6: With spectroscopic OCT reconstructed
absorption spectra of different rhodamine B
concentrations in the capillary as shown in Figure 5.
Conclusion
We have shown that SOCT in the visible spectral range is suitable for measuring absorption of rhodamine B,
which has similar absorption properties as proteins in the globin group. These measurements are verified in
cuvette measurements and in localised reflection samples. Both samples are based on reflection, but we
expect to be able to reconstruct similar results for a scattering sample and detect different absorption
amplitude due to concentration changes. A phantom which combines absorption with scattering that imitates
biological samples better than the cuvette and capillary measurements is considered for further optimisation.