Baohua Wang

A precise measurement of quality factor for Planar Microtoroid Resonators

The quality factor(Q) parameter is the most important parameter for optical micro cavities. In this paper, we present the Planar Microtoroid Resonators fabricated in our laboratory. Then, measuring methods common used are introduced. After that, a precise measurement of quality factor is demonstrated in which the photon lifetime was directly measured by cavity ringdown. This is done by repeatedly scanning the laser into resonance with a mode that was critically coupled to the taper. As the laser scanned into resonance, the power transferred into the cavity increased until the maximal power of the resonant mode was attained. At this moment, an external modulator is used to cut-off the pumping laser. As the resonant power outpouring, a cavity ringdown spectrum is observed and recorded. The cavity lifetime can be obtained through analyzing the spectrum. Finally, the quality factor is estimated from the cavity lifetime which is consistent with the widely used measurement of the frequency line shape.

The analysis of microtoroid cavity coupling system based on fullwave

The parameters of coupling system composed by microtoroid cavity and fiber-taper such as coupling distance, the size of the microtoroid cavities and the wavelength of the input light directly affect its coupling property. It has been fabricated in our laboratory and Rsoft Fullwave based on FDTD is used to simulate different size of micro-cavities, coupling distance from 0.1µm to 0.5µm, and the wavelength of input light around 0.85µm, 1.31µm and 1.55µm corresponding to individual optical-windows of optical fiber. It shows that the relationship among parameters mentioned can get an analysis through evanescent wave theory.

Water vapor detecting using high-Q microcavity

The target of this paper is to design a novel water-vapor sensor used in humidity transducer system based on high-quality factor (Q) spherical microcavities, which at present obtained the highest Q value induced by surface tension. Sensitive mechanics and its high sensitivity are discussed according to optical loss sensitization to coupling system composed of a microsphere cavity and a tapered fiber. After that, fabrication methods of master parts of the sensor in our lab are introduced. Additionally, a sensing structure composed of a gas-chamber and a vacuum-chamber is also designed. According to water absorption band, telecom band around 1550nm is used to pump the microsphere optical mode. In a vacuum environment, a transmitted spectrum is obtained through output end, revealing some information about the core system including the size of the microcavity and the optical loss of the regime. However, when the core system is placed in a water vapor environment, the transmitted spectrum will change due to extra optical loss induced by water molecules absorption, behaving as spectral shift, free spectrum range (FSR) or line width broadening. Contrasting and analyzing two different spectrums in the two situations, the gas concentration can be deduced. Indispensable experiments were also carried out, showing that, the two spectrums are different from resonance hump, formant strength and formant line width. Even an ultra low water vapor concentration induced a measurable output signal, ensuring a high detecting sensitivity. Certainly, the analyted vapor should not only be water vapor. This kind of sensitive mechanics is versatile. Changing the pumping light corresponding to the analyted absorption band, we are able to detect a series of vapor.