Time-Domain Optical Coherence Tomography (OCT)

Optical coherence tomography (OCT) is an emerging non-contact imaging modality for micrometer scale sub-surface imaging of biological tissue.1 OCT has found its main application in ophthalmic imaging, but this technology is also being used in small animal imaging as well as imaging of embryonic structures in developmental biology. A simplified optical setup for a fiber based OCT system utilizing a low coherence source and a Michelson-type interferometer is illustrated in Figure 1, below.
Cross sectional data along an axial line through the sample, called an A-scan, is acquired by axially scanning the position of the reference arm. Interference fringes are acquired at the photodiode detector when the round trip distance from the sample reflection matches that of the reference reflection. The magnitude of the observed fringes is proportional to the reflectivity of the scatterer. A two dimensional profile, or B-scan, is generated by scanning the interrogating beam laterally across the sample and acquiring an axial scan at each lateral location. Subtle differences in adjacent layers are visualized as differences in scattering intensities.
For the case of a point reflector in the sample, i.e. a mirror, the extent of the fringes, representing the axial resolution of the measurement, is determined by the coherence length of the source spectrum,lc, given by:
where lo is the center wavelength of the spectrum and dl is the full width half max (FWHM) spectral bandwidth, assuming a Gaussian spectral profile. In OCT, the axial resolution is thus decoupled from the transverse resolution, which is dependent on the optical setup, in particular on the numerical aperture of the objective lens.

Figure 1: Simplified OCT system setup using a free space Michelson interferometer.
Continuing with the case of a single reflector in the sample, and for a system without dispersion, the photocurrent at the detector, ID, can be written as a function of Dx, the difference in reference and sample reflection positions,
where Rs and Rr represent the reflectivity of the sample and reference object, S(x) is the Fourier transform of the source spectrum, and ko is the center wavenumber of the source bandwidth. The first term on the right hand side represents a DC offset due to non-interfering components, and the second term describes the overlying interferometric fringes. The reference arm power is typically much larger than the sample arm, and determines the shot noise limit in the system sensitivity.
  1. D. Huang, E. A. Swanson, C. P. Lin, J. S. Schuman, W. G. Stinson, W. Chang, M. R. Hee, T. Flotte, K. Gregory, C. A. Puliafito, and J. G. Fujimoto, "Optical Coherence Tomography," Science 254, 1178 (1991).