Optical coherence tomography (OCT) is used to capture a high-resolution cross-sectional image of scattering biological tissues and is based on fiber-optic interferometry. The core of an OCT system is a Michelson interferometer, wherein a first optical fiber is used as a reference arm and a second optical fiber is used as a sample arm. The sample arm includes the sample to be analyzed as well as a probe that includes optical components. An upstream light source provides the imaging light. A photodetector is arranged in the optical path downstream of the sample and reference arms.
Optical interference of light from the sample arm and the reference arm is detected by the photodetector only when the optical path difference between the two arms is within the coherence length of the light from the light source. Depth information from the sample is acquired by axially varying the optical path length of the reference arm and detecting the interference between light from the reference arm and scattered light from the sample arm that originates from within the sample. A three-dimensional image is obtained by transversely scanning in two dimensions the optical path in the sample arm. The axial resolution of the process is determined by the coherence length.
To obtain a suitably high-resolution 3D image, the probe typically needs to meet a number of specific requirements, which can include: single-mode operation at a wavelength that can penetrate to a required depth in the sample; a sufficiently small image spot size; a working distance that allows the light beam from the probe to be focused on and within the sample; a depth of focus sufficient to obtain good images from within the sample; a high signal-to-noise ratio (SNR); and a folded optical path that directs the light in the sample arm to the sample.
In addition, the probe needs to fit within a catheter, which is then snaked through blood vessels, intestinal tracks, esophageal tubes, and like body cavities and channels. Thus, the probe needs to be as small as possible while still providing robust optical performance. Furthermore, the probe operating parameters (spot size, working distance, etc.) will substantially differ depending on the type of sample to be measured and the type of measurement to be made.