Optical Coherence Tomography: Technical Aspects - Biomedical Optical Imaging Technologies: Design and Applications - page 171

Fig. 5.5 Axial resolution vs. bandwidth of light sources for center wavelengths of 800, 1,100,
1,300, and 1,500 nm. Micrometer-scale axial resolution requires extremely broad optical band-
widths, and bandwidth requirements increase dramatically for longer wavelengths

coherence length. Apart from the wide linewidth, the sources for OCT have to
exhibit a smooth Gaussian spectrum profile [ 19 , 20 ]. Figure 5.5 shows a plot of
axial resolution vs. bandwidth for light sources at different wavelengths. Since the
axial resolution is inversely proportional to the bandwidth of the light source, broad-
bandwidth light sources are required to achieve high axial resolution.

5.4.2 Lateral Resolution

As in conventional microscopy, the lateral resolution that may be achieved with
OCT imaging is determined by the focus spot of the light beam and is completely
decoupled from the axial resolution. Therefore, the optical design of the lateral
scanning does not correlate with the axial resolution. The lateral resolution x can
be written as

f
d

;

4

x D (5.25)

where f is the focal length of the objective lens and d is the 1=e 2 Gaussian beam
waist at the objective lens. It can be seen from the above equation that high lateral
resolution can be achieved by using a large numerical aperture (NA) to decrease the
spot size. However, for the selection of optics, there is a trade-off between the lateral