Influence of mirror size on ringdown frequency in cavity-ringdown spectroscopy of slender premixed burner flame

We applied near-infrared cavity-ringdown absorption spectroscopy to a slender premixed burner flame with the intention of measuring the density of OH radicals. We found that the ringdown frequency was increased by the flame even when the laser wavelength was detuned from absorption lines. The additional ringdown frequency was dependent on the diameter of cavity mirrors, even though the expected spot size of the laser beam inside the cavity was much smaller than the mirror diameter.