Measurement of air-refractive-index fluctuation from frequency change using a phase modulation homodyne interferometer and an external cavity laser diode

We present a method for air-refractive-index (n(air)) fluctuation measurement using a laser interferometer. The method is based on a combination of a phase modulation homodyne interferometer (PMHI), an external cavity laser diode (ECLD) and an ultralow thermal expansion material (ULTEM). The PMHI utilizes a Michelson interferometer which is constructed on the ULTEM plate under the condition of an air temperature fluctuation of less than 10 mK, so that the optical path change or the air-refractive-index fluctuation (Delta n(air)) caused by the thermal disturbance can be neglected. Meanwhile, the ECLD is controlled by adjusting its frequency to track some of the dark fringes of the interferometer, so that Delta n(air) can be derived from the ECLD frequency change. The uncertainty of the Delta n(air) measurement in the experiment is of 10(-8) order. However, it will be possible to decrease the uncertainty to 10(-9) or less if the signal-to-noise ratio (SNR) of the control system is improved.