We use a tunable diode laser operating near 1570 nm to investigate various effects of the heat transfer from fused-silica
microspheres, with and without thin-film coatings, to the surrounding gas in a vacuum chamber. The resonance
frequencies of microsphere whispering-gallery modes (WGMs), excited by a tapered-fiber coupler, shift with changing
temperature (about -1.6 GHz/K at 1570 nm). This shift, primarily due to the temperature dependence of the refractive
index of fused silica, enables the measurements whose results are reported here: determination of the thermal
accommodation coefficient of air on different surfaces, and measurement of the optical absorption coefficients of surface
water layers and of a thin film coating. Our method for determining thermal accommodation coefficients involves
deducing the thermal conductivity of the air as a function of pressure by measuring the relaxation rate of an externally
heated microsphere to room temperature. Then, in a separate experiment, by observing thermal optical bistability of the
WGM resonances caused by absorption of the probe laser, the contribution of water or film absorption to the total loss is
found.
A prototype evanescent-wave sensor using resonant whispering-gallery modes of a fused-silica microsphere has been developed and is being applied to the detection of trace amounts of carbon dioxide, carbon monoxide, ammonia, and acetylene in the 1530 - 1580 nm wavelength region. Its sensitivity is comparable to that of typical multipass-cell systems, but our sensor is much more compact because of resonant cavity enhancement. It represents a significant improvement over current total-internal-reflection or fiber evanescent-wave sensors, owing to the longer effective absorption path length (tens of meters in a sphere less than a millimeter in diameter). The present sensitivity is about a hundred parts per million, equivalent to the level detected by a household carbon monoxide sensor. This sensitivity is for direct absorption measurements; wavelength-modulation spectroscopy is being implemented and should provide two or three orders of magnitude improvement. By extending the same techniques, using fluorozirconate- glass microspheres and stronger transitions in the mid- infrared, the sensitivity is expected to improve to about a part per billion.
We report advances in compression tuning of fused-silica microsphere whispering-gallery resonances and a practical use of the improved compression tuner. The advances include extending the tuning range and enhancing the tuner’s response speed; these lead to the new application of using the tuner to lock to a laser, keeping a single whispering-gallery mode on resonance as the laser frequency is scanned. The resonance frequency of the mode to be locked is weakly modulated by axial compression of the microsphere, and phase-sensitive detection of the fiber-coupled optical throughput is used for locking. Using a laser wavelength of either 1570 nm or 830 nm, we demonstrate a locked tracking range exceeding 30 GHz for a microsphere of 120 GHz free spectral range. The improved tuner design that makes this application possible allows coarse tuning over 1 THz and piezoelectric tuning over 80 GHz. Compression modulation rates of up to 13 kHz have also been achieved with this tuner, producing a tuning speed of at least 16 GHz/ms.
The high Q of a microsphere whispering-gallery mode allows for sensitive resonant detection of atoms or molecules. The species being detected absorbs energy from the mode's evanescent field. It can be identified by knowing the resonant wavelength of the driving laser, and its concentration can be determined from the absorption signal on the light in reflection or transmission. High sensitivity results from the long effective absorption path length provided by the whispering-gallery mode's large Q. There are many possible implementations of and applications for such a sensor; several of each are described herein. In particular, for atmospheric trace-gas sensing, the microsphere has the potential to rival the performance of the multipass cell, but in a much more compact and rugged system. Our construction of a prototype system for detection of carbon monoxide, carbon dioxide, and ammonia is described.
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