Compact optoelectronics oscillators using WGM modes on fused silica and MgF2 mini-disks resonators
Résumé
This study deals with a design, fabrication and characterization of compact optoelectronic oscillators (OEO). Resonator behaves as a sphere because energy is trapped in whispering-gallery-modes in the equatorial region. For this purpose, Fused-silica and MgF2 are suitable, due to their mechanical characteristics and their low attenuation at 1.55 μm wavelength. In fact, 6-7 degrees Mohs hardness of these materials allows us to obtain a quite easy precision-processing. Our prototype owns a quality factor of approximately 3×108, which is certainly limited by the available technology. Resonator is coupled to an optical fiber including a taper-waveguide-based on a nm-position resolution. Microwave carrier is generated by locking optical phase modulation to a free-spectral-range (FSR) resonator, which occurs in the X-band. Moreover, this carrier is detected by a standard low-noise InGaAs p-i-n telecom photodiode. Oscillator prototype is assembled on a 0.12 m2 optical breadboard. In principle, this surface can be reduced to those of the oscillator main parts (resonator, laser, photodiode, amplifier and optical modulator). Oscillator phase noise measured by a dual-delay-line instrument, which has been developed in Besançon, corresponds to -90 dBrad2/Hz at 10 kHz off carrier. According to this result, oscillator suffers from severe noise-limitations due to several reasons: the thermal coefficient of the resonator, the low power that the resonator can accept, and the small volume of the energy-confinement region in the resonator (≈2×1014 m3) but our oscillator is packaged in a small volume, contrarily to classic OEO based on an optical fiber of a few km.