The combination of optical trapping and photonic crystal fiber enables distributed whispering-gallery-mode based measurements


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.

 combination of optical trapping and photonic crystal fiber enables distributed whispering-gallery-mode based measurements, Advances in Engineering

About the author

Richard Zeltner obtained both his Bachelor and Master degree in Integrated Life Science at the University of Erlangen-Nuremberg. In his Master thesis project he was working on refractometric sensing using crystalline whispering gallery mode resonators in the group of Professor Gerd Leuchs at the Max Planck Institute for the Science of Light (MPL) in Erlangen.

In 2014 he joined the division of Philip St.J. Russell at the MPL as a PhD candidate working on optical trapping of microparticles in hollow-core photonic crystal fibers. After graduating in 2018 he is currently working as a postdoctoral researcher in the Russell Division. His current research interests evolve around optical trapping, optomechanics, optofluidics and optical sensors.

About the author

Professor Philip Russell is a Director at the Max-Planck Institute for the Science of Light (MPL), a position he has held since January 2009 when MPL was founded. Since 2005 he has held the Krupp chair in experimental physics at the University of Erlangen-Nuremberg. He obtained his D.Phil. (1979) degree at the University of Oxford, spending three years as a Research Fellow at Oriel College, Oxford. At that time his interests were in the propagation of light in three-dimensional periodic structures, in particular the behaviour of photonic Bloch waves. From 1982 to 1983 he was a Humboldt Fellow at the Technical University Hamburg-Harburg (Germany), where he carried out a series of experiments exploring the propagation of photonic Bloch waves in periodic planar waveguides, observing such phenomena as negative refractive and diffraction and Bloch wave interference. From 1984 to 1986 he further developed his interests in photonic Bloch waves while working at the University of Nice (France) and the IBM TJ Watson Research Center in Yorktown Heights, New York. From 1986 to 1996 he was based mainly at the University of Southampton in the Optical Fibre Group, which later became part of the Optoelectronics Research Centre. During this period he worked on photosensitivity, second harmonic generation, fibre Bragg gratings and rocking filter formation in optical fibres. He also carried out theoretical studies on nonlinear holography and the interaction of light and sound in tapered optical fibre couplers and dual-mode optical fibres. From 1996 to 2005 he was professor in the Department of Physics at the University of Bath, where the main focus was on photonic crystal fibres—a new kind of optical fibre that he proposed in 1991.

His research interests currently focus on scientific applications of these fibres and related structures. He is a Fellow of the Royal Society and The Optical Society (OSA) and has won several awards for his research including the 2000 OSA Joseph Fraunhofer Award/Robert M. Burley Prize, the 2005 Thomas Young Prize of the Institute for Physics (UK), the 2005 Körber Prize for European Science, the 2013 EPS Prize for Research into the Science of Light, the 2014 Berthold Leibinger Zukunftspreis, the 2015 IEEE Photonics Award and the 2018 Rank Prize for Optoelectronics. He was OSA’s President in 2015, the International Year of Light. In June 2016 he received an honorary doctorate from the Universidad Internacional Menéndez Pelayo in Santander, Spain.

About the author

Riccardo Pennetta is currently a PhD candidate in the group of Philip Russell at the Max Planck Institute for the Science of Light in Erlangen, Germany. He received his Master degrees in Physics from University of Bari, Italy in 2014. His current research interests include optomechanicas, tapered fibres and photonic crystal fibres.

About the author

Shangran Xie received his B.E and Ph.D. degrees from Department of Electronics Engineering, Tsinghua University (P. R. China) in 2007 and 2013 respectively. From 2010 to 2011, he joined Fiber Optics Group in University of Ottawa in Canada as a joint Ph.D. student, working on long-distance distributed temperature, strain and birefringence sensing using Brillouin scattering in optical fibers. From 2013, he was appointed as a postdoctoral fellow in Russell Division, Max Planck Institute for the Science of Light in Germany, working on mid-IR supercontinuum generation, optical tweezers and optomechanics based on custom-designed specialty fibers, including hybrid waveguides, tapered waveguides and photonic crystal fibers.

He is currently the group leader for the optomechanics team in Russell Division. His research interests cover fiber-based optical tweezers and optomechanics, mid-IR supercontinuum generation and fiber sensing.


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), 1479

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