Differential absorption emulation for systematic-error characterization in a range-resolved coherent lidar

Differential absorption lidar (DIAL) is a key remote-sensing technique for atmospheric trace-gas measurements. Power-dependent instrumental errors are difficult to separate and quantify under atmospheric speckle and long averaging times. Here we report an in-fiber differential-absorption optical depth (DAOD) emulation method for systematic-error characterization in DIALs based on a polarization-maintaining recirculating fiber delay-line with distributed polarization-mode coupling. The method imposes a common, range-resolved emulated DAOD with a controlled return-power gradient, enabling rapid, repeatable analysis of power-dependent errors. We demonstrate the approach in DIAL and observe signal-level-dependent DAOD deviations of up to ±0.01, with a precision down to 5×10⁻⁴ at 3 Hz.