A major event in the evolution of OCT was the use of light wavelengths instead of time delay to determine the spatial location of reflected light. Through the use of Fourier transformation, this took the technology from the original method of TD-OCT to the development of SD-OCT. The original OCT method, known as TD-OCT, encoded the location of each reflection in the time information relating the position of a moving reference mirror to the location of the ,45 SD-OCT, instead, acquires all information in a single axial scan through the tissue simultaneously by evaluating the frequency spectrum of the interference between the reflected light and a stationary reference mirror. This method enables much faster acquisition times, resulting in a large increase in the amount of data that can be obtained during a given scan duration using SD-OCT. A comprehensive review of SD OCT by Joel Schuman is available(10)
The primary application of fiber Bragg gratings is in optical communications systems. They are specifically used as notch filters . They are also used in optical multiplexers and demultiplexers with an optical circulator , or optical add-drop multiplexer (OADM). Figure 5 shows 4 channels, depicted as 4 colours, impinging onto a FBG via an optical circulator. The FBG is set to reflect one of the channels, here channel 4. The signal is reflected back to the circulator where it is directed down and dropped out of the system. Since the channel has been dropped, another signal on that channel can be added at the same point in the network.