Hervé Tavernier

Determination of coupling regime of high-Q resonators and optical gain of highly selective amplifiers

We present a simple method to determine simultaneously the main characteristics of passive or active high-Q optical resonators. The method is based on cavity ringdown spectroscopy, where the probe wavelength is rapidly swept across the resonance. It has already been shown that this technique allows the loaded cavity lifetime of passive resonators to be obtained. We show that we can also infer the coupling regime for passive resonators and the resonant gain for active resonators. The method is tested on Er3+ doped fiber resonators and also applied to determine the intrinsic and external Q-factors of an MgF2 whispering gallery mode resonator.

Dynamic instabilities of microwaves generated with optoelectronic oscillators

We introduce a time-domain model to study the dynamics of optoelectronic oscillators. We show that, due to the interaction between nonlinearity and time delay, the envelope amplitude of ultrapure microwaves generated by optoelectronic oscillators can turn unstable when the gain is increased beyond a given critical value. Our analytical predictions are confirmed by numerical simulations and experiments.

Magnesium Fluoride Whispering Gallery Mode Disk-Resonators for Microwave Photonics Applications

We have manufactured a high-Q whispering gallery mode resonator with magnesium fluoride for microwave photonics applications in the gigahertz frequency range. This crystal is scarcely used for resonator fabrication despite its numerous advantages, that are mainly high mechanical hardness, low sensitivity to water vapor pollution, and low sensitivity to photo- and thermo-refractive fluctuations at optimal temperatures. Using a customized machining procedure, we have successfully fabricated and characterized a 5-mm resonator with a surface rugosity of the order of 1 nm (from 3 to 12 atoms). Cavity ring-down measurements enabled us to determine that the resonator has a quality factor Q = 3.4 × 108 at 1550 nm.

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.

Dynamical instabilities in opto-electronic ultra-pure microwave generators

An interesting alternative to reduce phase noise is the generation of ultre-pure microwave frequencies with opto-electronic oscillators (OEOs). The originality of OEOs is that energy is stored in a very long delay line instead of a high-finesse resonator. Beyond the scope of continuous-wave radars, OEOs can generate ultra-pure microwaves for many other applications, ranging from satellite telecommunication systems to ultra-low jitter clocks and laser pulses.