Frequency-agile, rapid scanning spectroscopy

Challenging applications in trace gas measurements require low uncertainty and high acquisition rates1–4. Many cavityenhanced spectroscopies exhibit significant sensitivity and potential5,6, but their scanning rates are limited by reliance on either mechanical or thermal frequency tuning7. Here, we present frequency-agile, rapid scanning spectroscopy (FARS) in which a high-bandwidth electro-optic modulator steps a selected laser sideband to successive optical cavity modes. This approach involves no mechanical motion and allows for a scanning rate of 8 kHz per cavity mode, a rate that is limited only by the cavity response time itself. Unlike rapidly frequency-swept techniques8–11, FARS does not reduce the measurement duty cycle, degrade the spectrum’s frequency axis or require an unusual cavity configuration. FARS allows for a sensitivity of ∼2 3 10212 cm21 Hz21/2 and a tuning range exceeding 70 GHz. This technique shows promise for fast and sensitive trace gas measurements and studies of chemical kinetics.