Quantum fluctuations in the small Fabry-Perot interferometer

We consider the small, of the size of the order of the wavelength, interferometer with the main mode excited by a quantum field from a nano-LED or a laser. The input field is detuned from the interferometer mode with, on average, a few photons. We find the field and the photon number fluctuation spectra inside and outside the interferometer and identify the contributions of quantum and classical noise in the spectra. Structures of spectra are different for the field, the photon number fluctuations inside the interferometer; for the transmitted, and the reflected fields. We note asymmetries in spectra. Differences in the spectra are related to the colored (white) quantum noise inside (outside) the interferometer. We calculate the second-order time correlation functions; they oscillate and be negative under certain conditions. Results help the study, design, manufacture, and use small elements of quantum optical integrated circuits, such as delay lines and optical transistors.