Optics & Photonics News - November 2009

Spectral domain low-coherence interferometry is a powerful way to measure the magnitude and echo time delay of backscattered light with very high sensitivity.

resolution for the light’s echo time delays. In low-coherence interferometry, axial resolution is given by the width of the field autocorrelation function, which is inversely proportional to the bandwidth of the light source.

Transverse resolution in OCT is the same as in optical microscopy, and it is determined by the diffraction-limited spot size of the focused optical beam. The diffraction-limited minimum spot size is inversely proportional to the numerical aperture or the focusing angle of the beam. In OCT, the coherence-gated information about the elementary volume of the scatterers within the obscuring scattering specimen can be obtained from either the time-domain measurement ( TD-OCT) or the frequency domain measurement.

In the TD-OCT system, the reference mirror is scanned to match the optical path from reflections within the sample. First-generation commercial OCT instruments have been developed for ophthalmology based on the TD-OCT configuration. However, the main drawback is that the depth scanning is realized by scanning the reference arm axially. Moreover, this mechanical scanning has limited repeatability and gives rise to motion artifacts due to mechanical jitter.

These effects deteriorate imaging quality, especially at high speeds. FD-OCT is a variant interferometric imaging modality; it has widely attracted interest in the field of biomedical imaging because of its higher sensitivity and imaging speed compared to its time domain counterpart. The principle of FD-OCT relies on the transformation of the OCT time-varying signal along the optical axis—termed the A-scan—into the frequency domain.

FD-OCT has the advantage of not requiring any moving parts in order to obtain an axial scan. In FD-OCT, a spectral fringe pattern is recorded either by a spectrometer with a line-scan camera (spectral domain OCT, SD-OCT) or by a single detector with a rapidly tunable laser as a light source (swept source OCT, SS-OCT). The figure on the facing page shows a simple free-space implementation of the SD-OCT system based on a Michelson interferometer; it illustrates how the system reconstructs the depth-resolved information from the anterior chamber of an eye.

In SD-OCT, researchers acquire a broadband interference signal with spectrally separated detectors with a dispersive optical element such as a grating and a linear detector array. To suppress autocorrelation, self-cross correlation and camera noise artifacts, investigators ensemble-average all the spectral interferograms in each slice along the x-direction at each wavelength to obtain a reference spectrum. This background spectrum is then subtracted from each A-scan.

Researchers then remap the subtracted spectral interferograms from -space to k-space by using the spline interpolation method. Due to the Fourier relation (Wiener-Khintchine

theorem between the auto correlation and the spectral power density), the depth-resolved information can be immediately reconstructed without moving the reference arm, by using a Fourier-transformation from the remapped spectra.

Optical microangiography

OMAG is a functional extension of FD-OCT. The key difference between OMAG and FD-OCT is that, in OMAG, the spatial interferogram in the lateral direction (B-scan) is modulated with a Doppler frequency that can separate the moving and static scattering components within the sample. This Doppler modulation frequency can be introduced by mounting the reference mirror in the reference arm onto a linear piezo-translation stage, which moves the mirror at a constant velocity across the B-scan (i.e., x direction scan), or simply via inherent scattering fluid flow within the sample.

We achieved this by mounting the reference mirror in the reference arm onto a linear piezo-translation stage that moved the mirror at a constant velocity across the B-scan (i.e., the x direction scan). However, the latest version of OMAG utilizes the spatial modulation frequency provided by the inherent blood flow rather than reference arm modulation.