A clock laser with high frequency stability and highly precise transfer

A 729 nm diode laser is used for the development of an extremely narrow line 729 nm clock laser for a quadrupole transition of a single ⁴⁰Ca⁺ ion. The laser linewidth is decreased to 2 Hz, evaluated by a heterodyne beatnote between two narrow linewidth lasers that are individually stabilized to two independent ULE reference cavity systems. The ageing drift of the ULE cavity is observed over two years. It shows a decreasing with an exponential function. The long-term frequency drift is successfully minimized to 0.03 Hz/s by keeping the cavity at the zero expansion coefficient temperature. The Allan deviation is less than 5×10^-15 at the averaging time of 1 s~10 s. To observe the clock transition, the clock laser light is transmitted to Ca⁺ ion vacuum chamber by a polarization-maintaining single-mode optical fiber of 40 m length. Fiber optical phase noise has been canceled. In order to compare with other optical frequency standards or rf frequency standards, the clock laser frequency is locked to the ²S_1/2-²D_5/2 electric quadrupole transition of the single trapped ⁴⁰Ca⁺ ion. Stable locking has been observed over averaging times of 100 s. The Allan deviation approaches to 5×10^-15 over averaging times of 1000 s. We are developing an ultrastable laser based on a much higher finesses and vibration insensitive cavity [8]. The ultimate aim is building a clock laser with a subherz linewidth and Allan deviation of <; 1×10^-15 at 1 s.