Fig. 2: Schematics of TDS based on a balanced cross-correlator.
From: High-precision time-domain stereoscopic imaging with a femtosecond electro-optic comb
a Balanced imaging with two optical sampling cameras. PBS, polarization beamsplitter; QWP, quarter wave plate; DM, dichroic mirror; PPKTP, periodically poled KTiOPO4; I1, I2, electrical signal intensity for a pixel of camera 1 (camera 2). b Balanced signals as a function of repetition frequency (fr) tuning of an electro-optical (EO) comb. In (a), an EO comb emits ultrashort optical pulses that travel to the target, reflect off surfaces, and return to a balanced cross-correlator. Inside this correlator, the orthogonally polarized detection and reference pulses pass through the PPKTP crystal twice, generating bidirectional sum-frequency signals as they temporally overlap inside the crystal. The sum-frequency beams then project two correlated images of a target onto the respective cameras, which are synchronized to the fr tuning. By subtracting the intensities of two correlated pixels (each from a camera), one can derive the balanced signal (ΔI) at any spatial point. In (b), a target distance is calculated as D = c/(2nr·Δfr), where nr, c, and Δfr represent the reflective index of the air, the speed of light in a vacuum, and the frequency difference between two consecutive zero-crossing points (at fr1 and fr2).
