Temperature measurement and stabilization in a birefringent whispering gallery mode resonator
Spotlight summary: How precisely can temperature be measured? With patience and excellent eyesight, maybe you could read a mercury thermometer to a 10th of a degree, or with a good camera and a computer, maybe to a 100th of a degree. But at some point, uncertainties in the markings, the thermometer calibration, or even the physics of mercury expansion would limit the measurement precision. Systems like ultrastable lasers require control of temperature to better than a millionth—or even a billionth—of a degree Celsius. In this article, Strekalov and coworkers demonstrate a sensor that can measure temperatures to the nearest billionth of a degree (nano-Kelvin), and they do it by taking advantage of resonance.
One familiar way to demonstrate resonance is by rubbing your finger along the rim of a wine glass to generate a tone. The wine glass can only “sing” at frequencies where it has a resonance. The location of these frequencies depends on the radius of the rim and the speed of sound in the glass, and in turn, the speed of sound depends on temperature. So by listening to any change in tone, we could in principle determine a change in temperature. This is a very weak effect for an acoustic system, so we shouldn’t expect to see too many loud thermometers for sale (fortunately). But there’s an optical analog to the singing wine glass that is very sensitive to temperature changes.
The heart of the Cal Tech team's device is a circular glass disk, about 8 mm across and 2 mm thick. Feeding light from a laser into the disk using an optical fiber is like rubbing the rim of a wine glass to make it sing. At certain light frequencies, “Whispering Gallery Modes” (WGMs) are excited. They derive their name from circular buildings where a whisper near the wall on one side can be heard clearly near the exact opposite wall. When driven at resonance, the disk stores a large amount of optical energy, in the same way that a wine glass stores vibrational energy. As the temperature changes, the disk expands or contracts slightly, changing its radius and the speed of light in the glass. As a consequence, the frequency of the mode shifts slightly, which is easy to measure by sweeping the laser light frequency very quickly over some small range.
The disk actually has more than one resonance, and each resonance is affected by temperature a bit differently. The Strekalov group find that monitoring the difference between two particular resonances gives the most sensitive temperature measurement. They demonstrate their capability to measure the temperature to a billionth of a degree, and then they show how the system can be used to regulate the temperature of the glass disk. Whenever it gets cool, a circuit turns up the laser power to heat it. If it gets warm, the laser power is reduced. They show that over a few seconds or even a thousand seconds, the temperature stability achieved with their system far outperforms any other temperature control system in existence. They believe this will be especially useful for laser stabilization, where temperature changes can cause a shift in the frequency of laser light.
Using resonances to make sensitive measurements is not a new idea. Effects like this are used to image the brain, measure the length of DNA strands, and weigh single atoms. Perhaps someday even the radius of a wine glass will be used as a precise indicator of something other than future blood alcohol content.
-- Brad Deutsch
Technical Division: Optical Design and Instrumentation
ToC Category: Instrumentation, Measurement, and Metrology
|OCIS Codes:||(120.4800) Instrumentation, measurement, and metrology : Optical standards and testing|
|(120.6780) Instrumentation, measurement, and metrology : Temperature|
|(190.4870) Nonlinear optics : Photothermal effects|
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