Continuous-filtering Vernier spectroscopy at 3.3 μm using a femtosecond optical parametric oscillator

Using a cavity-enhanced continuous-filtering Vernier spectrometer based on a femtosecond optical parametric oscillator we measure broadband spectra of atmospheric water and CH
₄
around 3.3 μm reaching 4 ppb detection limit for CH
₄
in 15 ms.