The infrared unmodulated pyramid wavefront sensor: simulations, lab and on sky
Direct imaging of Earth-like exoplanets requires strong suppression of stellar light at small angular separations, achievable with extreme adaptive optics (XAO). The pyramid wavefront sensor (PWFS) is widely used for its sensitivity but suffers from nonlinearities and limited dynamic range. Modulation extends the range but reduces sensitivity and limits loop speed. We investigate an unmodulated infrared PWFS (IR-umPWFS), where the dynamic range scales with wavelength, removing the need for modulation and enabling faster operation. We adopt a 3-sided PWFS, which eliminates the roof defect of the classical 4-sided design. We assess its feasibility and performance through end-to-end simulations, lab tests, and on-sky observations on the PAPYRUS bench at the Observatoire de Haute-Provence (OHP), and compare it with a visible modulated PWFS (VIS-mPWFS). Simulations show reliable operation, reaching Strehl ratios up to ~60% at 1400 nm for seeing up to 4''. Infrared operation improves linearity, reduces optical-gain variations and modal cross-talk. Laboratory results validate the simulations and confirm the pyramid quality. On sky, stable closed-loop operation was achieved over three nights. Compared with the VIS-mPWFS at 500 Hz, the IR-umPWFS at 1000 Hz showed gains of ~1.63 in Strehl ratio and ~1.26 in encircled energy, and a ~2.03-fold reduction in residual wavefront error. The IR-umPWFS remained robust to tip-tilt offsets up to 3 lambda/D at 1400 nm. It is a strong candidate for future XAO systems: operating in the infrared extends the linear range sufficiently to remove the need for modulation, enabling higher loop speeds no longer limited by the modulation mirror, which contributed to the improved on-sky performance. The OHP turbulence (D/r0 ~ 17 at 1400 nm) is comparable to typical VLT conditions (D/r0 ~ 20), supporting the relevance of these results for VLT/RISTRETTO.