Described self-referencing cavity enhanced spectroscopy (SRCES) systems and methods are tailored to acquiring spectra in a middle regime, in which signals are lower than optimal for conventional absorption spectroscopy, and absorption is higher than optimal for cavity ring-down spectroscopy (CRDS). Longitudinal mode resonance spectral peaks are analyzed individually to extract intensity ratios (e.g. maximum to minimum) and/or curve-fitting parameters, obviating the need to measure or precisely control the input light intensity.