Probing microcavity resonance spectra with intracavity emitters

We measure the fluorescence spectrum of broadband emitters in an open optical microcavity with radius of curvature $R=17.7\left(3\right)\mu m$ and finesse $F\approx 1000$. This geometry enables a combined measurement of emission spectra versus cavity length, which has several benefits over measurements at fixed wavelength or fixed cavity length alone. We demonstrate the role of the optical penetration depths on the cavity modes and provide practical working equations for its analysis. Furthermore, we show the ability to measure the coupling of cavity modes within a small scan range of the cavity length. By measuring these cavity emission spectra as a function of cavity length, we thus obtain a rich and complete picture of the optical microcavity.