Experimental setup. BS1, pellicle beam splitter; BS2, cube beam splitter; MPC, multipass cell; RR, retroreflector; L, lens; PD1 and PD2, photodiodes; FC, fiber coupler; and OSA, optical spectrum analyzer. The dashed line corresponds to the He–Ne laser optical path.
Temperature reading of the multipass cell after the PID controller is turned on with set point at . The data are taken every . More than of data are taken, and the settling time is around . Inset, histogram of of data after the temperature is stabilized together with a Gaussian fit. The histogram is normalized to an area of 1. The rms value of the Gaussian fit is .
(a) The peak-to-peak variation of the He–Ne interferometer output when a ramp signal is applied to the piezo. (b) The error signal input to the PID controller when the He–Ne interferometer is locked. The sampling rate of the signal is .
(a) Interferograms in the time domain vs laser repetition rate when the multipass cell is filled with standard air or pumped out to a vacuum. (b) Interferograms in the frequency domain. The solid curve shows spectral modulation, and the dotted curve shows spectrum .
(Left panel) Nonlinear least-squares fitting of the measured refractive index of air. (Right panel) Difference between the fitted result and Edlén’s formula. The standard error of the data point is .
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