Vincent Huet

Controling the coupling properties of active ultrahigh-Q WGM microcavities from undercoupling to selective amplification

Ultrahigh-quality (Q) factor microresonators have a lot of applications in the photonics domain ranging from low-threshold nonlinear optics to integrated optical sensors. Glass-based whispering gallery mode (WGM) microresonators are easy to produce by melting techniques, however they suffer from surface contamination which limits their long-term quality factor to a few 108. Here we show that an optical gain provided by erbium ions can compensate for residual losses. Moreover it is possible to control the coupling regime of an ultrahigh Q-factor three port microresonator from undercoupling to spectral selective amplification by changing the pumping rate. The optical characterization method is based on frequency-swept cavity-ring-down-spectroscopy. This method allows the transmission and dispersive properties of perfectly transparent microresonators and intrinsic finesses up to 4.0 × 107 to be measured. Finally we characterize a critically coupled fluoride glass WGM microresonator with a diameter of 220 μm and a loaded Q-factor of 5.3 × 109 is demonstrated.

Slow-light microcavities

Optical microcavities with high quality factors Q are of great interest in integrated micro- and nanophotonics. Their narrow resonance and long photon lifetime (τ<inf>cav</inf>) can be used in fundamental physics, sensing applications, microwave signal optical processing or for optical delay line and memory miniaturization [1]. Considering a Lorentzian resonance profile, the quality factor and the photon lifetime of a microcavity with a resonant angular frequency ωο = 2·πε/λ<inf>0</inf> reads: Q = ω<inf>0</inf> τ<inf>cab</inf> = n<inf>G</inf>(ω<inf>0</inf>)LF/λ<inf>0</inf>, (1) where n<inf>g</inf> is the group index of the material constituting the microcavity, F is the finesse and L is the cavity round-trip length. The usual method to obtain ultra-high quality factors consists in optimizing the manufacturing process to reach high finesse values. Following this approach record quality factors up to 10¹¹ have been measured in millimetric CaF2 whispering gallery mode (WGM) resonators [2]. Otherwise, Eq. (1) shows that for a given finesse, the use of resonators made with slow-light materials displaying high group indices can also increase the quality factor. This method has already been successfully implemented in bulky resonators including a highly dispersive slow-light medium [3]. We have recently demonstrated that this approach can be applied to solid-state optical micro-resonators [4]. The slow-light effect was introduced thanks to population oscillations in an Er³⁺ doped WGM fluoride glass microsphere (diameter 100 μm). Since the lifetime of the ⁴I<inf>13/2</inf> level is very long (T ≈ 10 ms) strong group index up to 10⁶ can be obtained. Using a cavity-ring-down method (see Fig. 1) in a pump probe configuration, we have measured a photon lifetime of 2.5 ms at 1530 nm corresponding to a quality factor of 3 × 10¹² [4].

Ultra-long photon lifetime in a slow-light microcavity

Ultrahigh-quality (Q) factor optical micro-resonators with very long photon storage time are important in the aim of integrating optical functions such as filters, tunable delay lines for microwave-photonics or highly sensitive sensor applications. To date, Q-factors up to 1011 have been measured only in millimetre-size crystalline whispering-gallery-mode (WGM) resonators. Semiconductor, Silica or glass WGM micro-resonator Q-factors are limited to 109 due to technological imperfections or residual absorption. We show that by introducing slow light effects in a monolithic WGM micro-resonator it is possible to enhance the photon lifetime by several orders of magnitude and circumvent fabrication limitations. We experimentally demonstrate Erbium-doped fluoride glass micro-resonators with a photon lifetime up to 2.5 ms at room temperature, corresponding to a Q-factor of 3 × 1012 at 1530 nm, by combining WGM resonance effect and population oscillations.

ERRATUM: Controling the coupling properties of active ultrahigh-Q WGM microcavities from undercoupling to selective amplification

ERRATUM: Controling the coupling properties of active ultrahigh-Q WGM microcavities from undercoupling to selective amplification

High gain selective amplification in whispering gallery mode resonators: analysis by cavity ring down method

We study both theoretically and experimentally the dispersive properties of single whispering gallery mode resonators. We present a simple experimental protocol which allows us to obtain in detail its coupling regime and thus their dispersive properties. We demonstrate a compact optical amplifier with a gain up to 20dB in an Erbium doped fluoride microsphere of 135μm in diameter coupled via a tapered fiber. The model is also applied to analyze the dynamic behavior of the modal coupling between two degenerate resonances of the same cavity. In particular, this can be used to describe the coupling of counterpropagating whispering gallery modes (WGM) by Rayleigh scattering. The theory is successfully compared to experiments carried out in silica microspheres