Stability improvement of ⁴⁰Ca⁺ optical clock by using a transportable ultra-stable cavity

The instability of the clock laser is one of the primary factors limiting the instability of the optical clocks. We present an ultra-stable clock laser based on a 30-cm-long transportable cavity with an instability of ∼3 × 10^-16 at 1 s-100 s. The cavity is fixed by invar poles in three orthogonal directions to restrict the displacement, meeting the requirements of transportability and low vibration sensitivity. By applying the ultra-stable laser to a transportable ⁴⁰Ca⁺ optical clock with a systematic uncertainty of 4.8 × 10^-18 and using the real-time feedback algorithm to compensate the linear shift of the clock laser, the short-term stability of the transportable ⁴⁰Ca⁺ optical clock has been greatly improved from 4.0×10^-15/τ/s to 1.16×10^-15/τ/s, measured at ∼100 s-1000 s of averaging time, enriching its applications in metrology, optical frequency comparison, and time keeping.