Four-port SNAP microresonator device.

It is well known from quantum mechanics that the transmission amplitude of a symmetric double-barrier structure can approach unity at the resonance condition. A similar phenomenon is observed in optics for light which propagates between two waveguides weakly coupled through a microresonator. Examples of microresonators used for this purpose include ring, photonic crystal, toroidal, and bottle microresonators. However, ring and photonic crystal photonic circuits, once fabricated, cannot be finely tuned to arrive at the mentioned resonant condition. In turn, it is challenging to predictably adjust coupling to toroidal and bottle microresonators by translating the input-output microfibers, since the modes of these resonators are difficult to separate spatially. Here we experimentally demonstrate a four-port micro-device based on a SNAP microresonator introduced at the surface of an optical fiber. The eigenmodes and corresponding eigenwavelengths of this resonator are clearly identified for both polarization states by the spectrograms measured along the length of the fiber. This allows us to choose the resonant wavelength and simultaneously determine the positions of the input-output microfiber tapers to arrive at the required resonance condition.