We report the direct observation of the electromagnetic-field distribution of whispering?gallery modes in silica microcavities (spheres and toroids). It is revealed by their excitation efficiency with a tapered fiber coupler swept along the meridian. The originality of this method lies in the use of the coupler itself for the near field mapping, eliminating the need of additional tools used in previous work. This method is successfully applied to microspheres and microtoroid

Acousto-optic deflectors (AOD) are promising ultrafast scanners for non-linear microscopy. Their use has been limited until now by their small scanning range and by the spatial and temporal dispersions of the laser beam going through the deflectors. We show that the use of AOD of large aperture (13mm) compared to standard deflectors allows accessing much larger field of view while minimizing spatio-temporal distortions. An acousto-optic modulator (AOM) placed at distance of the AOD is used to compensate spatial and temporal dispersions. Fine tuning of the AOM-AOD setup using a frequency-resolved optical gating (GRENOUILLE) allows elimination of pulse front tilt whereas spatial chirp is minimized thanks to the large aperture AOD.

A method based on thermal bistability for ultra-low threshold microlaser optimization is demonstrated. When sweeping the pump laser frequency across a pump resonance, the dynamic thermal bistability slows down the power variation. The resulting lineshape modification enables a real-time monitoring of the laser characteristic. We demonstrate this method for a functionalized microsphere exhibiting a sub-microwatt laser threshold. This approach is confirmed by comparing the results with a step-by-step recording in quasi-static thermal conditions. c 2018 Optical Society of America OCIS codes: 140.3945, 140.3410, 140.3530, 140.6810, 130.3990 Optical microcavities have drawn a large interest in the last two decades and received numerous applications, including Cavity-QED and photonic devices, like light emitting diodes or microlasers. A special attention has been devoted to whispering gallery mode (WGM) microcavities induced by surface tension, like microspheres and microtoroids. They benefit from the sub-nanometer roughness of molten silica, which results in very high quality factors Q, enabling laser operation with a pump power in the sub-microwatt range [1]. These ultra-lowthreshold lasers have been obtained by silica functionalization with embedded rare earth ions. Different doping techniques, like fiber doping, sol-gel coating, ion implantation, co-deposition or co-sputtering have been successfully used with Nd 3+ , Er 3+ , and Yb 3+ or mixtures [1][2][3][4]. In this paper, we report on a new method for the fast measurement of the laser light-light characteristic based on thermal effect, and allowing real-time optimization. In monolithic microcavities, self-heating by the minute dissipated optical power induces a negative frequency shift, resulting in a bistable behavior when this shift exceeds the cold cavity linewidth [5][6][7]. For decreasing laser frequency, the heating pushes cavity resonance frequency in the same direction, and they remain close to resonance, as long as the thermal shift compensates the laser offset. The loss of the resonance results in a rapid drop of the coupling. In reverse direction, the thermal shift and the laser scan have opposite directions, and the line is narrowed. Due to the high-Q and the small heat capacity of the cavity, the bistability threshold is very low, typically in the microwatt range or less. At low scanning rate (typically up to 100 Hz) the heating affects the cavity as a whole, and only one cooling time is involved, while for increasing scanning rate a dynamical thermal effect appears which depends on the heat diffusion details. Our method exploits the low frequency regime, with a decreasing pump frequency so that the resonance shifts in the same direction as the laser. In our experiment, the microlaser is optically pumped by a laser injected in a WGM. In contrast to usual techniques, its frequency is not fixed, but is continuously swept across the resonance, at a nearly constant power. The thermal effect converts this frequency modulation into a smooth sweep of...