Miniaturized calcium beam optical frequency standard using fully-sealed vacuum tube with 10^-15 instability

We implement a miniaturized calcium beam optical frequency standard using specially-designed fully-sealed vacuum tube, and realize the comparison with another calcium beam optical clock whose vacuum tube is sealed by flanges. The electron shelving detection method is adopted to improve the signal-to-noise ratio of the clock transition spectroscopy, and the readout laser is locked by modulation-free frequency locking technology based on Doppler effect. Injection locking is carried out to boost the power of the 657 nm master clock transition laser, thus ensuring the comparison. The fractional instability of the miniaturized calcium beam optical frequency standard using fully-sealed vacuum tube is 1.8×10^-15 after 1600 s of averaging. Total volume of the system except for electronics is about 0.3 m³. To our knowledge, it's the first time to realize the optical frequency standard using fully-sealed vacuum tube. This work will promote the miniaturization and transportability of the optical clock based on atomic beam.