Measurement of air-refractive-index fluctuation from laser frequency shift with uncertainty of order 10-9

In the previous work (Ishige et al 2009 Meas. Sci. Technol. 20 084019), we presented a method of measuring the relative air-refractive-index fluctuation (Delta n(air)) from the laser frequency shift with the measurement uncertainty of order 10(-8) using a phase modulation homodyne interferometer (Basile et al 1991 Metrologia 28 455), which was supported by an ultralow thermal expansion material (ULTEM) and an external cavity laser diode (ECLD). In this paper, an improvement in the uncertainty of the Delta n(air) measurement is presented. The improvement method is based on a Fabry-Perot cavity constructed on the ULTEM, which has a thermal expansion coefficient of 2 x 10(-8) K-1 and an ECLD. The Pound-Drever-Hall method (Drever et al 1983 Appl. Phys. B 31 97) is also used to control the ECLD frequency to track the resonance of the cavity. Delta n(air) can be derived from the ECLD frequency shift. The estimated measurement uncertainty of Delta n(air) for a short time (similar to 150 s) in the experiment is of order 2.5 x 10(-9) or less.