Investigation in compact optoelectronic oscillator with mini-disk resonator

This study deals with a design, fabrication and characterization of compact optoelectronic oscillators (OEO). The resonator - a disk measuring a few millimeters in diameter with rounded edges - behaves as a sphere because the 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, the hardness of 6-7 degrees Mohs of these materials allows us to obtain a quite easy precision processing and surface polishing. Our prototype owns a quality factor of approximately 3×10⁸, which is certainly limited by the available technology. The resonator is coupled to an tapered optical fiber with a few nm position resolution system. The microwave carrier is generated by locking the optical phase modulation to a free spectral range resonator, which occurs in the 10 GHz region. Moreover, this carrier is detected by a standard low-noise InGaAs p-i-n telecom photodiode. The 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). The oscillator phase noise measured by a dual-delay-line instrument, which has been developed in Besancon, corresponds to -90 dBrad²/Hz at 10 kHz off carrier. According to this result, the 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 could be packaged in a small volume, contrarily to a classic OEO based on an optical fiber of a few km.