All Optical Fiber Dual WGM Resonators

Whispering gallery mode (WGM) resonator has many advantages such as high quality factor (Q), small mode volume, long photon life time and low power consumption, hence is widely used in optical filters, modulators, micro-lasers, frequency comb generators, nonlinear optics and high-sensitivity sensor devices. However, most of the WGM resonators are discrete optical devices, which cannot be coupled directly with incoming light radiations, and an external coupling scheme by use of tapered fiber, side-polished fiber, waveguide or prism has to be used. This creates a difficulty when applying WGM resonator in the optical fiber system. Here we propose and demonstrate an all optical fiber dual WGM resonator device, which is naturally integrated with optical fiber and being easily coupled by the light traveling in the fiber core. The device consists of an embedded cylinder inside a cylindrical air-cavity within the fiber core, fabricated by use of femtosecond laser micromachining and fast hydrofluoric acid etching technique. When the optical transmission path in the fiber core is tangent to the inner wall of the cylindrical air-cavity, the light is coupled into the cavity via the evanescent field and excites WGM. When light entering the cylindrical air-cavity is tangential to the surface of the embedded cylinder, it is coupled into the cylinder cavity, circulates along the cylinder surface, and excites WGM resonance. Thus a dual WGM operation is generated. The device is compact in size, flexible in construction, simple in fabrication and convenient in operation, and exhibits promising potential applications in photonics.