Frequency comb generation in whispering gallery mode silica microsphere resonators

Optical frequency combs (OFC) using different kinds of whispering-gallery-mode (WGM) microresonators have already shown different applications and especially their applications in fiber optical communication systems as replacements of laser-arrays. For this application the free spectral range (FSR) of 200 GHz or less is desirable. Besides the fabrication material for microspheres, the resonator radius can be modified to change the FSR. In this paper use of silica microspheres for OFC represents an inexpensive alternative over the other microcombs: microring, microdisk, and microtoroid. We experimentally present a microsphere fabrication process from a different kind of silica (SiO₂) fibers by use of the hydrogen-oxygen melting technique. We experimentally review the OFC generation process the main microresonator parameters as FSR, Q-factor and evaluate the resulting WGM resonator generated OFC comb light source for further applications. An OFC was excited inside a 166 μm silica microsphere WGM resonators using a 1548 nm laser light. The obtained broadband OFC spanned from 1400-1700 nm with FSR of (397 ± 10) GHz.