A narrow linewidth and frequency-stable probe laser source for the 88Sr+ single ion optical frequency standard

In this paper, we describe in detail a narrow linewidth and frequency-stable laser source used to probe the 5s (2) S (1/2)-4d (2) D (5/2) clock transition of the Sr-88(+) optical frequency standard. The performance of the laser system is investigated with studies of its frequency drift rates and with high resolution spectra of the Sr-88(+) clock transition. The observed short-term drift rates are typically in the range of 10 to 23 mHz/s, and the current long-term drift rate is 13.9(3) mHz/s. The laser stability, after subtraction of linear drifts, reaches 5x10(-16) at an averaging time of 3000 s. This high level of stability is attributed for the most part to stabilization of the reference cavity at the temperature where the coefficient of linear thermal expansion crosses zero. An upper bound for the laser linewidth is given by the observation of a Fourier-transform limited resonance of 4.3 Hz (Delta nu/nu=1x10(-14)) on the Sr-88(+) clock transition. The effective averaging time during the linewidth measurements was about 100 s.