Precision cavity enhanced velocity modulation spectroscopy

► Optical cavity was integrated with velocity modulation using 2f demodulation. ► Transitions from the v = 1 ← 0 band of the A2Πu-X2Σg+ system of N2+ were observed. ► Lamb dips were observed with linewidths of ∼40 MHz. ► Optical frequency comb was used for absolute calibration accuracy of ∼1 MHz.