In conventional optical coherence tomography (OCT), the requirement of scanning limits image-acquisition time. Thus, Kin Pui Chan and his fellow researchers at Japan Science and Technology Corp. (Kawaguchi, Japan) and Yamagata University (Yonezawa, Japan), are working to develop a real-time OCT system with no moving parts for scanning. Although their efforts still require some improvements before being of practical use, Chan and colleagues have demonstrated non-scanning OCT imaging using a time-of-flight cross-correlator based on off-axis interferometry in conjunction with an angular dispersion method. "In a conventional OCT system, the reflectance profile is measured by scanning the reference mirror in the depth direction, and a cross-sectional image is produced by scanning the light beam across the sample while recording the reflectance profile at each transverse position," Chan explains. "Our objective was to achieve the same imagery without scanning."
In normal OCT systems, the interference fringe must be recorded in order to measure the correlation peak. But Chan's new system eliminates the need to record the fringe because the angular dispersion imaging method enables them to demodulate the cross-correlation function. "In an off-axis interferometer, the signal wave and the reference wave are incident upon an angular dispersion device, such as a grating, from two opposite sides," says Chan. The dispersion angle Θ results in a first-order diffraction at an angle β as given by
sin Θ + sin β = λ / d
where d is the spacing of the grating.
"By choosing an incident angle of Θ = sin -1 (λ0/d)," says Chan, "we can calculate that the lightwave at the center of wavelength λ0 will be diffracted at an angle of β = 0." The diffraction angle of the other wavelength components is β = λλ0 / λ0 sin Θ. Chan says the group sets the diffraction angle for λ=λ0 normal to the grating to downshift the spatial frequency of the superposition of the signal and reference waves to approximately zero.
Basically, the university team's non-scanning OCT system consists of an off-axis Mach-Zehnder interferometer with an 840-nm broadband (20 nm) source. The team used a pair of lenses to collimate the beam to about 5 mm in diameter, then split it into signal beam and reference beam. The signal beam was focused onto the sample through a cylindrical lens ( f = 15 mm). A reference beam sent from the opposite side at the same 30° angle interferes with the signal light on the surface of the grating.
Non-scanning OCT system imaged stack of 30-mm-thick plastic films attached to a glass slide. Depth resolution is about one-half the light source's coherence length.
Chan's first experiment evaluated a sample consisting of three roughly 30-µm-thick plastic films attached to a glass slide. The team used a CCD camera to record a 2-D image that resolved each of the plastic films. The result showed that the depth resolution was about one-half the light source's coherence length.
"Our 2-D image shows the advantage of our method," says Chan. "With our system, the incident wave is focused with a cylindrical lens, forming a narrow line strip on the sample surface. The CCD camera records the image where the horizontal coordinate corresponds to the depth direction of the sample, and the vertical coordinate corresponds to the one along the strip line."
Chan's team also found that a larger incident angle resulted in a wider detection range. In contrast, larger incident angles in conventional off-axis interferometers just yield higher spatial frequencies.
"Chan's method appears to provide a non-mechanical means of scanning the optical path length in an OCT imaging system. If it performs well and is practical to implement, his method could indeed impact our future products," says Joseph Schmidt, chief technology officer at Light Lamp Imaging LLC (Westford, MA).
Several practical issues such as the effect of amplitude modulation remain to be solved or improved, Chan admits. "But our work offers a new approach to non-scanning real-time OCT imaging."