Frequency detuning analysis in multi-chirp range-selective digital holography with temporal heterodyning

Range-selective digital holography (RSDH) is an advanced imaging technique that combines digital holography with the range-selective capabilities of frequency-modulated continuous-wave (FMCW) chirped lidar. Temporal heterodyne FMCW range-selective digital holography (TH FMCW RSDH) further extends this method by incorporating phase-shifting techniques to enable on-axis holographic imaging. In standard implementations, maximum hologram strength is achieved when the local oscillator (LO) frequency shift is set by the target distance, assuming that the hologram integration and chirp duration are aligned. However, in systems where hologram integration spans multiple chirps, this alignment is no longer guaranteed, and the optimal LO frequency shift may deviate from the nominal value. This work develops a theoretical framework to analyze the effects of LO detuning on range resolution and hologram strength in TH FMCW RSDH under multi-chirp integration. The model is experimentally validated using a time-of-flight (ToF) camera to perform phase-shifting and integration. Our results demonstrate that maximum hologram strength can occur at LO frequency shifts that differ from the nominal range-dependent value. These findings provide valuable insights for optimizing the performance of both TH and non-TH FMCW RSDH systems in scenarios where hologram integration spans multiple chirps.