A space mission called “Earth 2.0 (ET)” is being developed in China to address a few of fundamental questions in the exoplanet field: How frequently habitable Earth-like planets orbit solar type stars (Earth 2.0s)? How do terrestrial planets form and evolve? Where did floating planets come from? ET consists of six 30 cm diameter transit telescope systems with each field of view of 500 square degrees and one 35 cm diameter microlensing telescope with a field of view of 4 square degrees. The ET transit mode will monitor ~1.2M FGKM dwarfs in the original Kepler field and its neighboring fields continuously for four years while the microlensing mode monitors over 30M I< 20.6 stars in the Galactic bulge direction. ET will merge its photometry data with that from Kepler to increase the time baseline to 8 years. This enhances the transit signal-to-noise ratio, reduce false positives, and greatly increases the chance to discover Earth 2.0s. Simulations show that ET transit telescopes will be able to identify ~17 Earth 2.0s, about 4,900 Earth-sized terrestrial planets and about 29,000 new planets. In addition, ET will detect about 2,000 transit-timingvariation (TTV) planets and 700 of them will have mass and eccentricity measurements. The ET microlensing telescope will be able to identify over 1,000 microlensing planets. With simultaneous observations with the ground-based KMTNet telescopes, ET will be able to measure masses of over 300 microlensing planets and determine the mass distribution functions of free-floating planets and cold planets. ET will be operated at the Earth-Sun L2 orbit with a designed lifetime longer than 4 years.
Single-photon avalanche diodes (SPADs) are widely used for practical applications requiring single-photon detection. The readout circuit, or quenching electronics, plays an important role for the operations of SPADs. Sine wave gating (SWG) is one of the key techniques for synchronous single-photon detection that can easily operate SPADs with a gating frequency as high as GHz level. Here we present a monolithic readout circuit for 1.25 GHz SWG SPADs. The monolithic chip, including a low-noise amplifier and two low-pass filters inside, is designed for weak avalanche extraction in the SWG scheme and fabricated using the technology of low temperature co-fired ceramic with a size of 15 mm × 15 mm. We then apply the monolithic chip into an InGaAs/InP single-photon detector (SPD). After the characterization both on the monolithic chip and the InGaAs/InP SPD, the functionality of the monolithic readout circuit is effectively verified. Implementing the monolithic integration of readout circuit is a key step towards developing miniaturized InGaAs/InP SPDs.
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