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.