Temperature Sensors Based on WGM Optical Resonators

NASA’s Jet Propulsion Laboratory, Pasadena, California

A proposed technique for measuring temperature would exploit differences between the temperature dependences of the frequencies of two different electromagnetic modes of a whispering-gallery-mode (WGM) optical resonator. An apparatus based on this technique was originally intended to be part of a control system for stabilizing a laser frequency in the face of temperature fluctuations. When suitably calibrated, apparatuses based on this technique could also serve as precise temperature sensors for purposes other than stabilization of lasers.

The Lasers Would Be Mode-Locked to resonance frequencies f₀ and 2f₀ + δf, respectively, of the WGM resonator. The heterodyne frequency, δf, would vary with temperature and, therefore, would be measured for use as an indication of temperature.

A sensor according to the proposal would include (1) a transparent WGM dielectric resonator having at least two different sets of modes characterized by different thermo-optical constants and (2) optoelectronic instrumentation for measuring the difference between the temperature-dependent shifts of the resonance frequencies of the two sets of modes. The figure schematically depicts an example of such a sensor. Laser 1, operating at frequency f₀, would be locked to a mode in the first of the two sets of WGM modes to be exploited; the mode locking would be accomplished by established means that would include photodetector 1, an oscillator, polarizers, mixer 1, and electro-optical modulator EOM 1. Laser 2, operating at frequency 2f₀ + δf, would be locked to a mode in the second of the two sets of WGM modes to be exploited; in this case, the mode locking would be accomplished by established means that would include photodetector 2, the oscillator, mixer 1, and electro-optical modulator EOM 2.

Part of the modulated output of laser 1 would be fed through a frequency doubler to obtain a modulated beam at frequency 2f₀. In a beam splitter, the 2f₀ output from the frequency doubler would be combined with part of the modulated output of laser 2 at 2f₀ + δf. The interference between these combined beams would cause the output of photodetector 3 to include a component at the heterodyne frequency, δf, which would have the desired temperature dependence. Inasmuch as f₀ and δf could readily be chosen to place δf within a suitable radio-frequency range and means for measuring radio frequency precisely are readily available, it would be straightforward to measure δf. Then the temperature could be calculated by inversion of the known temperature dependence of δf. It has been estimated that for a typical CaF₂ WGM resonator having a resonance quality factor (Q) of 2 × 10¹⁰, the temperature measurement sensitivity would be characterized by a temperature increment of about 40 μK for a frequency increment of half the width of one of the resonance spectral peaks.

This work was done by Anatoliy Savchenkov, Nan Yu, Lute Maleki, Vladimir Iltchenko, Andrey Matsko, and Dmitry Strekalov of Caltech for NASA’s Jet Propulsion Laboratory.

This invention is owned by NASA, and a patent application has been filed. Inquiries concerning nonexclusive or exclusive license for its commercial development should be addressed to