Whispering-gallery mode (WGM) resonators are used in a variety of applications such as quantum optics, nonlinear optics and sensing. As their optical resonances are sensitive to changes in the resonator environment, they are capable of detecting variations in physical quantities. Passive resonators are usually excited by prism coupling or tapered waveguide techniques that require close proximity between the coupling device and the resonator. In contrast, active WGM resonators that contain e.g. a fluorescent dye allow for remote excitation and collection of the emitted spectrum such that they can be employed in hardly accessible environments. Further, when operating an active WGM resonator above the lasing threshold the sensing performance can benefit from the decrease in line width and the increase in amplitude of the resonances.
While active and lasing WGM resonators were manipulated using optical tweezers and other optical trap configurations previously, the limited manipulation range of a few mm at best prevented researchers to harness their sensing capabilities for distributed measurements. However, it has been previously demonstrated that a microparticle can be optically manipulated over distances exceeding several meters inside hollow-core photonic crystal fiber (HC-PCF). Moreover, the trapped microparticle is protected from external interferences, such that the combination of optical trapping and HC-PCF represents a promising solution for the dilemma described above.
Researchers at the Max Planck Institute for the Science of Light: Dr. Richard Zeltner, Riccardo Pennetta, Dr. Shangran Xie and Professor Philip St.J. Russell developed a whispering-gallery mode sensor based on a lasing dye-doped microparticle optically trapped inside a liquid-filled HC-PCF. The microlaser was manipulated by a continuous wave trapping laser and pumped by a pulsed excitation laser. As both lasers were coupled into the fiber core modes, it was possible to propel and excite the laser along the whole length of the fiber. A fraction of the emitted lasing light couples into the core modes and could be analyzed at the fiber end face. By monitoring changes in the lasing wavelengths temperature variations along the fiber could be measured remotely. The authors reported that the developed sensor was capable of measuring temperature variations over centimeter length-scales with mm-spatial resolution. The work is published in the research journal Optics Letters.
The study is the first to demonstrate remote and distributed temperature measurements using an optically trapped WGM resonator. While the measurement range and duration in the current experiment was limited due to high optical loss of the HC-PCF and photobleaching of the gain medium, these obstacles could be easily overcome by performing the experiment in an air-filled or evacuated HC-PCF and by employing a more robust gain medium. Furthermore, the device’s sensing performance could be improved by using particles with high Q-factors and a detection system having a higher spectral resolution. Considering the fact that similar experiments may be conducted with other types of waveguides, the developed sensing method will extend the potential applications of whispering-gallery mode systems, especially in remote areas.
Zeltner, R., Pennetta, R., Xie, S., & Russell, P. (2018). Flying particle microlaser and temperature sensor in hollow-core photonic crystal fiber. Optics Letters, 43(7), 1479Go To Optics Letters