Abstract
Tunable diode laser absorption spectroscopy using microresonator whispering-gallery modes (WGMs) is demonstrated. WGMs are excited around the circumference of a cylindrical cavity 125 mum in diameter using an adiabatically tapered fiber. The microresonator is very conveniently tuned by stretching, enabling the locking of an individual WGM to the laser. As the laser is scanned in frequency over an atmospheric trace-gas absorption line, changes in the fiber throughput are recorded. The experimental results of cavity-enhanced detection using such a microresonator are centimeter effective absorption pathlengths in a volume of only a few hundred microns cubed. The measured effective absorption pathlengths are in good agreement with theory.
Highlights
Dielectric microresonators supporting whispering-gallery modes (WGMs) are becoming increasingly useful for numerous applications in optics
This feature, combined with small mode volume and efficient coupling using prisms, angle-polished fibers, or tapered fibers, makes high intracavity power enhancement easy to achieve. Various applications such as filtering, lasing, modulation, nonlinear optics, sensing, and spectroscopy are enabled by these properties of WGMs2
The microcavity can be modeled as a four-mirror ring cavity, as depicted in Fig. 5.1, in which one of the mirrors is partially transmitting while all the others are assumed to be perfect reflectors
Summary
Dielectric microresonators supporting whispering-gallery modes (WGMs) are becoming increasingly useful for numerous applications in optics These resonators, often made of fused silica because of its extremely low absorption and scattering losses, can be spherical, cylindrical, disk-shaped, or toroidal; they are typically a few hundred mm in diameter. The high Q of a WGM means that light makes many round trips in the resonator before being lost from the mode by outcoupling, scattering, or absorption This feature, combined with small mode volume and efficient coupling using prisms, angle-polished fibers, or tapered fibers, makes high intracavity power enhancement easy to achieve. Various applications such as filtering, lasing, modulation, nonlinear optics, sensing, and spectroscopy are enabled by these properties of WGMs2
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