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A symmetrical atom interferometer with a thermal calcium atom beam has been developed using a narrow linewidth diode laser stabilized to the resonance of a high-finesse cavity. The linewidth of the diode laser was estimated to be less than 1 Hz relative to the cavity resonance in noise measurement over the range of 100 Hz to 1 MHz, and the phase instability of the interference fringes obtained from the Allan deviation was improved to 2 mrad at an integration time of 300 s. Using this atom interferometer, the ac Stark phase shift between the {sup 1}S{sub 0} and {sup 3}P{sub 1} states of a Ca atom was measured as a function of a laser power near the resonance of the {sup 1}S{sub 0}-{sup 1} P{sub 1} transition at a wavelength of 423 nm. The decay rate of the {sup 1}P{sub 1} state was determined to be {gamma}=1.91(33)x10{sup 8} s{sup -1}.

An external cavity diode laser stabilized to a high-finesse rigid cavity made of ultralow-expansion (ULE) glass maintained at a zero-crossing temperature of −3.3 °C showed a linear frequency drift of 25 mHz/s, which was the lowest ever reported with ULE glass. The linewidth of the beat note between two independent laser systems stabilized to independent ULE glass cavities was narrower than 1 Hz, and the Allan deviation of the beat note around 1 s of averaging time was close to the Brownian thermomechanical noise limit.