5.2.1　Principles of OCT system

OCT is a macro-optical imaging modality using light-tissue interaction for generating images.The OCT system is composed of a light source，a beam splitter and a detector，also known as Michelson interferometer.OCT integrates semiconductor laser technique，optical technique，ultra-sensitive detection technique and computer image processing technique，and can create in vivo cross-sectional images of skin with a resolution of 1-15 μm.The beam splitter of OCT divides the light emitted by the near-infrared light source into two beams：one beam is directed to the measured substance，called the sample arm；the other beam is directed to the reference reflector，called the reference arm.The two beams of light reflected from the measured substance and the reference mirror are superimposed on the detector.Under certain conditions，interference can be observed：coherent waves superimpose and their electromagnetic field amplitudes add constructively（i.e.，they reinforce each other）or destructively（i.e.，they cancel out each other）or meet any condition in between.The associated light intensity can be measured as an electrical signal using a photo detector.This signal is a function of the difference in optical path length between both arms.For a low coherent light source（like a superluminescent diode or a pulsed laser source），interference is only possible if the optical paths are matched to be equal in length within the short coherence length of the source，which usually is in the order of micrometers.

OCT is a non-invasive and two-dimensional imaging technique through measuring reflected or backscattered light from tissue by correlating it with light.Compared with other imaging techniques，OCT has the following advantages：①a near-infrared light source that is harmless to human body；②interference technology to achieve tomographic imaging with extremely high resolution（micrometer）；③the optical fiber with a diameter of 100 μm transmits signals and can be connected to a variety of instruments，including catheters，endoscopes，laparoscopes and surgical probes，which allows imaging of organ systems in the body；④in situ imaging and real-time transmission image，without complicated mathematical calculations and image reconstruction，surgical guidance；⑤portability，an important factor for clinically viable devices.(https://www.daowen.com)

According to the working principle，there are two OCT systems，time domain OCT（TD-OCT）and frequency domain OCT（FD-OCT）（Figure 5-3）.In the case of TD-OCT，light from the light source is split into the reference beam and the sample beam.Back reflected light from both arms is combined again and recorded by the detector.To record one depth profile of the sample（A-scan），the reference arm needs to be scanned.This has to be repeated for each lateral scan position.FD-OCT is the second generation of OCT technology and provides a more efficient implementation of the principle of low-coherence interferometry.In contrast to TD-OCT，FD-OCT uses spectral information to generate Ascans without the need for mechanical scanning of the optical path length.Thus，the scanning imaging speed and the signal-to-noise ratio of FD-OCT are greatly improved.The advantages are no longitudinal scanning，great improvement of signal sensitivity，and large dynamic range.