Light from the light source is split into the reference beam and the central 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.
Figure reprinted from 
Superluminescent Diodes (SLDs) are the light source of choice in Spectral Domain Optical Coherence Tomography (SD-OCT) imaging. The broad spectrum and short coherence length of the emitted light improves the depth resolution in OCT images. LD-PD Inc offers SLDs designed for OCT applications with center wavelengths of 810, 830, 850, 880, 930, 970, 1064, or 1310,1550nm nm. Each SLD is shipped with an individualized product data sheet, which includes information on the spectrum and operating parameters of the device. Raw test data for each SLD is also available upon request; please contact Tech Support with inquiries. For more details about each SLD, please see the Specs links.
These devices are built into 14-pin butterfly packages, each with an integrated thermoelectric cooler (TEC) and thermistor to ensure output stability. The output is coupled into either an SM or PM fiber terminated with a 2.0 mm narrow key FC/APC connector. Please note that optical feedback can diminish the output power or damage the SLD. We do not recommend using these SLDs with components that are prone to back reflections, such as FC/PC connectors.