Output light from an optical cavity includes, for each of a set of modes, an intensity function. Analyte can be positioned in the cavity, and a mode's intensity function can be encoded to include information about an optical characteristic of an analyte. For example, the intensity function can include a peak, and its central energy, maximum intensity, contrast, or intermediate intensity width (e.g. FWHM) can indicate the optical characteristic. For example, the information can be about both refractive index and absorption of an analyte.