A compact, sub-Hertz linewidth 729Â nm laser for a miniaturized 40Ca+ optical clock

We present a compact, sub-Hertz 729 nm laser for a miniaturized 40Ca+ single-ion optical clock. An external cavity diode laser is frequency-stabilized to a horizontally mounted, vibration-insensitive and high-fineness ultra-low-expansion (ULE) cavity with Pound-Drever-Hall (PDH) method. The laser linewidth is measured to be about 0.9 Hz from a heterodyne beat note with the other 729 nm laser. After removing the linear drift of about 0.1 Hz/s, the fractional frequency instability is less than 2×10−15 (1~100 s). This compact, ultra-stable laser system with a volume about 0.1 m3 excluding the electronics has been employed into a miniaturized 40Ca+ single-ion optical clock. The clock frequency instability has been measured to be 3.4×10−14/τ1/2 (1~10,000 s) with a 729 nm laser probe pulse time of 20 ms.