Local-oscillator boosted tilt sensing using four-step on-axis phase-shifting digital holographic wavefront sensing

Abstract

Digital holographic wavefront sensors in the on-axis phase-shifting recording geometry (PSRG) can estimate the complex optical field while using a strong local oscillator (LO) to boost weak received signals above the focal-plane-array noise floor. Here, we experimentally demonstrate the measurement of tilt, while boosting the received signal with an LO, using a temporal four-step on-axis PSRG at 1550 nm. A spatial light modulator (SLM) applies a known tilt ramp, and the reconstructed phase along the pupil diameter is unwrapped and linearly fitted to recover the tilt. With a weak signal power of 8.3 nW, the 100 μrad tilt error RMS decreases from 360 μrad in the unboosted case of PLO = Psig to 46 μrad at the optimum LO power of 1.7 μW. This result is benchmarked against a high visibility reconstruction test where tilt error RMS is 7.1 μrad. These results show that LO boosting can improve low-order angular-mode sensing with on-axis PSRG, providing an experimental step toward weak-signal adaptive-optics wavefront sensing for free-space optical links