Schematic diagram of digitally calibrated DCS. Comb1 and Comb2: optical frequency comb. CW1 and CW2: continuous wave laser. C1–C4: fiber circulator. FBG1–FBG4: fiber Bragg grating. FC1–FC6: fiber coupler. PC1–PC4: fiber polarization controller. BD1 and BD2: balance detector. PD: Photodetector. AD1–AD3: analog to digital converter. RAM: random-access memory. Black solid line: fiber. Blue solid line: cable.