Another variant of SDOCT uses a wavelength-tunable laser to rapidly sweep through a range of wavelengths, allowing the spectrum of the interferometer output to be recorded sequentially using a single detector. 2 is also commonly referred to as Frequency-Domain OCT (FDOCT). The particular implementation of SDOCT shown in Fig. The map of optical reflectivity versus depth is obtained from the interferometer output spectrum via a Fourier Transform. It can be shown that the measured spectrum of the interferometer output contains the same information as an axial scan of the reference arm. Instead of obtaining the depth information of the sample by scanning the reference arm length, the output light of the interferometer is analyzed with a spectrometer (hence the term Spectral-Domain). The key difference is that in an SDOCT system the reference arm length is fixed. Most of the components are identical to the setup of the Time-Domain technology. 2 shows the basic fibre-based SDOCT setup. Transverse scanning of the sample (to build up a two- or three-dimensional tomographic image) is achieved via rotation of a sample arm galvonometer mirror.įig. The act of translating (axially scanning) the reference arm reflector is equivalent to performing optical sectioning of the sample, allowing for the generation of map of optical reflectivity versus depth. Specifically, coherent interference is observed only when the optical pathlengths differ by less than the coherence length of the light source, a quantity that is inversely proportional to its optical bandwidth. The axial optical sectioning ability of the technique is due to the following reason: Because the light is emitted from a broadband source (large range of optical wavelengths), a strong interference signal is only detected when the light from the reference and sample arms has travelled the same optical distance. The interference signal between the reflected reference wave and the backscattered sample wave is then recorded. The reference arm is terminated by a mirror which can be scanned in the axial direction in the sample arm, the light is weakly focused into a sample. The Michelson interferometer splits the light from the broadband source into two paths, the reference and sample arms. 1 shows a schematic diagram of the basic fibre-based TDOCT setup. Spectral-Domain OCT is rapidly replacing the Time-Domain technology in most applications because it offers significant advantages in sensitivity and imaging speed. Time-Domain OCT technology is more intuitive to understand, and most early reasearch and commercial instrumentation was based on this technology. There are two main categories of OCT instrumentation: Time-Domain OCT (TDOCT) and Spectral-Domain OCT (SDOCT). The application of OCT imaging to other biomedical areas such as endoscopic imaging of gastro-intestinal and cardiovascular systems is currently an active field of research. The technique has already become established as a standard imaging modality for imaging of the eye, with numerous commercial instruments on the market. For this reason, OCT systems may combine high axial resolutions with large depths of field, so their primary applications have included in vivo imaging through thick sections of biological systems, particularly in the human body. Whilst the lateral resolution is determined by the spot size of the light beam, the depth (or axial) resolution depends primarily on the optical bandwidth of the light source. Three-dimensional images can then be created by scanning the light beam laterally across the sample surface. By using the time-delay information contained in the light waves which have been reflected from different depths inside a sample, an OCT system can reconstruct a depth-profile of the sample structure. Optical coherence tomography (OCT) is an imaging technique which works similar to ultrasound, simply using light waves instead of sound waves.
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