Piezo-optic Phase Shifters for Integrated Astro-photonics

Recently, advances in integrated photonics have opened the door to new types of solid-state astronomical instruments that can manipulate light collected by a telescope using optical microchips. Leveraging precise optical phase control across multiple photonic channels, these 'astrophotonic' devices offer potential transformative capabilities for applications such as wavefront sensing and high-contrast imaging. The potential impact of astrophotonics to practical applications is highlighted by the integrated photonic beam combiner chip already used at VLTI-GRAVITY, offering benefits in terms of alignment stability, high interferometric contrast, and intrinsic filtering of modal content. At the same time, the chip's small size allows it to be placed in a cryostat to reduce thermal photon emission, which is desirable for mid-infrared astronomy. However, in this implementation, phase control for each path is implemented prior to the chip using mechanically-tuned fiber delay lines, adding additional size and complexity and limiting the achievable phase precision and tuning speed. In other ongoing work, concepts for mode-selective optical processing (i.e. coronagraph on a chip) are being experimentally investigated at JPL; these prototypes utilize on-chip thermal phase shifter elements, which are incompatible with cryostat operation and experience thermal crosstalk and hysteresis, and are relatively slow. New on-chip phase shifters, that do not dissipate excess heat and preserve linear operation with low added optical loss, would greatly benefit these and a variety of other astronomical instrument designs. The objective of the proposed work is to design and fabricate novel piezo-optic phase shifters in a scalable integrated photonic platform, with performance metrics in terms of speed, throughput, and stability targeted for future astrophotonics applications. These phase shifters operate by inducing mechanical strain in a piezoelectric thin film upon applying a voltage, which locally perturbs the refractive index of the material and modifies the phase velocity of light waves guided in buried optical waveguides. Because these kinds of phase shifters are capacitive, rather than resistive, they have extremely low DC power dissipation (sub-nanowatt) and can support highly linear operation from DC to MHz frequencies. As a result, they avoid many issues of conventional thermal phase shifters: They are compatible with cryostat operation, do not experience hysteresis, and allow for rapid closed-loop control necessary to achieve operations such as precise phase control or extremely high extinction ratio interferometry to meet future exo-Earth observation requirements. Our design, which has been validated through detailed multi-physics simulations, combines silicon nitride integrated photonic waveguides with novel piezoelectric transducers based on Scandium-doped Aluminum Nitride (ScAlN), a strong piezoelectric material compatible with standard microfabrication techniques. Fabrication and test will be carried out at JPL's Microdevices Laboratory, where we have demonstrated additive piezo-optic modulators in telecom-wavelength silicon waveguides, and an ultra-low-loss (1 dB/m at 780-1600 nm wavelengths) integrated silicon nitride component platform. Our design approach is directly compatible with existing architectures for astrophotonic integrated circuits, and may be extended to wider wavelength ranges with future engineering. Altogether, this work will result in a new class of phase shifter devices for flexible visible-NIR astrophotonics components, including nullers, beam combiners, and integrated photonic spectrometers, with application-specific tailorability.

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