Polychromatic pyramid wavefront sensor with MKID technology for high contrast imaging

TL;DR

This work introduces a polychromatic pyramid wavefront sensor enabled by Microwave Kinetic Inductance Detectors (MKIDs), exploiting per-photon energy and arrival time to sense wavefronts across 800–1800 nm. It defines a real-time optical-gain tracking framework that uses wavelength-dependent measurements to estimate and compensate gains within a polychromatic reconstruction, minimizing noise propagation. End-to-end Keck II AO simulations demonstrate that polychromatic sensing can increase limiting magnitude by 1–2 magnitudes and boost contrast by factors of 1.5–4 relative to single-band PWFSs, with significant gains when more than two bands are used. The paper also addresses practical deployment issues, including spectral band selection, atmospheric dispersion correction, pupil registration, modulation, and daytime calibrations, outlining a concrete pathway toward on-sky implementation and bench validation of MKID-based PWFS for next-generation high-contrast AO systems.

Abstract

The high sensitivity of the pyramid wavefront sensor has made it the preferred sensor in high contrast adaptive optics systems. Future higher contrast systems, like the Extremely Large Telescope's Planetary Camera System, will require higher performance wavefront sensing. A further performance improvement could be achieved with a polychromatic pyramid wavefront sensor by using additional information over a broader wavelength range. The development of such systems is becoming more feasible with the emergence of new detector technologies such as Microwave Kinetic Inductance Detector arrays. These are arrays of superconductor detectors that give a position, arrival time and measure of the energy for each incident photon. This paper introduces the polychromatic pyramid wavefront sensor concept by defining the technologies and techniques employed and their requirements. A method is developed to track the optical gains, taking advantage of the additional wavelength information, and used to compensate for optical gains within an optimised reconstructor to minimise noise propagation. An overview of expected performance improvement, using end-to-end simulations, is provided using the Keck II adaptive optics system as a reference design. The polychromatic pyramid wavefront sensor was shown to increase the limiting magnitude by 1 to 2 magnitudes, and the contrast by factors of 1.5 to 4, versus single band pyramid wavefront sensors, by sensing over a wavelength range approximately five to ten times broader (800-1800 nm) compared to Z band (152 nm wide) and H band (300 nm wide). Practical design and implementation issues have also been considered.

Figures (20)

• Figure 1: An example of a MKID output data stream showing the characteristic resonator phase change against time for detection of six different photons over 3 milliseconds at wavelengths of 450 nm and 900 nm.
• Figure 2: Illustration of different intensities read by a PWFS detector. From left to right: non-energy sensitive detector detecting a narrow waveband, MKID detecting multiple wavebands, MKID detecting multiple wavebands with an exaggerated dispersion and a 'flattened pyramid' case for the central pupil images.
• Figure 3: The PWFS response at different wavelengths to KL mode 30 with a wavefront rms of 100 nm. In each case, the modulation is 3 $\lambda/D$. The choice of wavelengths in this paper is discussed in section \ref{['sec:ao_system_keck']}.
• Figure 4: PWFS response to varying amplitude of an incoming wavefront aberration (KL mode 30). For each wavelength, the PWFS is operating with a modulation radius of 3 $\lambda/D$.
• Figure 5: Optical gains computed with a fitting error PSF for $r_0 = 8$ cm at different wavelengths ($\lambda$ = 1.03 $\mu$m for Y-band) using the convolutional model. The parameters used for this simulation are given in Table \ref{['tab:params']}.