Lamb dip spectrum in cavity ringdown spectroscopy at Balmer-α line of atomic hydrogen: toward sheath electric field measurement in plasmas

We reported in a previous work (S. Nishiyama et al., J. Phys. D: Appl. Phys. 50 (2017) 234003) that the sheath electric field was determined by the Stark spectrum of the Balmer-α line of atomic hydrogen which was measured by saturation spectroscopy. The goal of the present work is to apply the Stark spectroscopy at the Balmer-α line to plasmas with weaker optical absorption. The strategy is saturated cavity ringdown spectroscopy (CRDS). A hydrogen plasma was generated inside an optical cavity. A single-mode cw diode laser beam was truncated using an acousto-optic modulator when the cavity length was resonant with the laser wavelength. The temporal decay of the laser intensity transmitted through the cavity was deviated from an exponential curve. A saturation parameter of ~ 100 was evaluated from the ringdown curve by fitting it with the theory of saturated CRDS. The maximum saturation parameter was observed at the center of the 2p²P^o_3/2 − 3d²D_5/2 transition, indicating that saturated CRDS is applicable to the measurement of the Lamb-dip spectrum.