Optical Coherent Tomography Four Optical Couplers and Attenuator Balance System

OCT (Optical coherent tomography) is an imaging method used to generate a picture of the back of the eyes and retinas. By using an interferometer, part of the light is directed to the sample and another portion is sent to a reference arm with a well-known length [1]. Interferometers merge two or more sources of light to create interference patterns. We split the light into two beams that travel different optical paths and are combined to produce interference. There are two types of OCT: one is time domain (TD-OCT) and the other Fourier domain (FDOCT). TD-OCT is a system which depends on a mechanical cycle time of the moving reference mirror. In FD-OCT the reference mirror is fixed, and assists in sampling multiple points from the ocular structures simultaneously. Optical coherent tomography (OCT) generates sub-surface images of translucent or opaque materials at a resolution equivalent to a low power microscope. OCT build up clear 3D images of thick samples by rejecting background signals whilst collecting light directly reflected from a surface of interest. OCT systems use broadband sources and split light in a fiber coupler, first on a direct reference mirror, and as the sample is being measured. The noise types in OCT systems are receiver noise shot noise, intensity noise, and beat noise. In OCT systems there is balanced detection and unbalanced detection. In balanced detection, the total photo current variance or noise is σ2 i = σ2 re + σ2 sh + σ2 be. In contrast, OCT unbalanced systems contain Michelson interferometer. A balanced OCT system is an extended version of the unbalanced variant with two more beam splitters added for double detection. Balanced OCT systems can include additional attenuators at the detector arm to improve the detection efficiency. OCT balanced systems with four optical couplers and attenuator gives graphs of noise [V 2] as a function of typical values of IDC (detector photo current or mean photo current) for different attenuation values, and SNR as a function of mean photo current (IDC) for different attenuation values and other OCT system parameters. OCT SNR graphs for different K1, K2, K3 and K4 splitt