Whispering gallery mode (WGM) res- onators have been suggested for use as reference cavities for laser stabilization. Because of their unique properties (small size, high stability, narrow line width), such application appears to hold great promise. It is expected to allow for the exceptionally high optical frequency stability.

The main practical difficulty associated with using WGM resonators for laser stabilization arises from the fundamental fact that the light propagates inside an optical material. This appears to be a disadvantage compared with low-expansion Fabry-Perot resonators that are essentially vacuum-filled. In WGM resonators, the material’s optical properties set the limit not only for the resonator’s Q-factor, but also for its stability.

The most important factors contributing to variation of the WGM resonance frequency are the thermal refractivity and thermal expansion. These variations can be suppressed if the resonator temperature is stabilized to the micro-Kelvin level. To achieve this, the temperature dependence of the resonator’s own anisotropy is used.

Temperature dependence of the resonator anisotropy leads to a temperature-dependent frequency difference between the TE and TM mode families. This frequency difference can be measured with a high precision, and the temperature variations are extracted. These variations are then suppressed by two digital control loops.

The first practical implementation of the dual-mode stabilization approach has been achieved, and active temperature stabilization of a WGM resonator at above room temperature has been demonstrated. Temperature stabilization at the level of 200 nK was achieved when integrated for one second, and below 10 nK when integrated for 10,000 seconds. This considerably surpasses state-of-the-art temperature sensors; especially temperature control techniques. This result is significant not only in the context of laser stabilization, but also as a demonstration of a novel temperature sensor with a broad spectrum of potential applications.

A WGM-based temperature sensor with nano-Kelvin sensitivity operating at room temperature was demonstrated. It was used to stabilize a WGM resonator at the level of a few nano-Kelvin, which will allow use as an ultra-stable laser lock reference. The demonstrated technique can be used in a variety of other applications requiring high temperature stability, as well as ultra-sensitive measurements of temperature variations. These include thermal stabilization of a quartz oscillator, mid- or far-IR sensitive bolometers, precise calorimetric measurements in chemistry, and study of optical and mechanical aging effects in various crystalline resonators.

This work was done by Dmitry V. Strekalov, Nan Yu, Robert J. Thompson, and Ivan S. Grudinin of Caltech for NASA’s Jet Propulsion Laboratory. NPO-48222

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Temperature Measurement and Stabilization in a Birefringent Whispering Gallery Resonator

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This article first appeared in the February, 2014 issue of NASA Tech Briefs Magazine.

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