More than a decade ago, the frequency comb technique was developed at the Max Planck In-stitute of Quantum Optics by Professor Theodor W. Hänsch. The new tool has stimulated fun-damental research as well as laser development and its applications because it gave rise to a major increase in the accuracy of measuring optical frequencies.
Octave spanning frequency comb generation in a microresonator. Panel (a) shows the ex-periment with a glass nano-fiber and a silicon chip with optical resonators. A scanning electron mi-croscope picture of a resonator is shown in panel (b). Panel (c) shows the optical spectrum of the frequency comb generated in such a microresonator seeded by a single frequency laser.
Already a couple of years ago, a team of scientists around Dr. Tobias Kippenberg, formerly Leader of the Max Planck Research Group “Laboratory of Photonics and Quantum Measurements” at MPQ, who has since then become Associate Professor at the Ecole Polytechnique Fédérale de Lausanne (EPFL), succeeded for the first time in generating optical frequency combs using chip-based quartz glass toroids with diameters on the micrometer scale.
Now the scientists made a big step further: their new microresonators produce light over a range of more than an octave and are at the same time precisely tunable (PRL 107, 063901, 1 August 2011). This achievement brings a variety of applications into reach, such as optical telecommunications or the precise calibration of spectrographs in astrophysics.
A frequency comb is a light source containing – similar to a rainbow – a large spectrum of colours. However, the frequencies are not continuously distributed. Instead, up to a million spectral lines are spaced in exactly the same distance. The superposition of this “comb” with another laser beam results in a pattern from which the unknown laser frequency can be determined with very high accuracy. The frequency comb developed by Prof. Hänsch is based on a mode-locking process in short-pulse lasers. This set-up consists of many optical components, even though it is made today relatively compact and commercially available. Indeed, Menlo System a spin-off company established by MPQ which is meanwhile marketing the frequency comb technology worldwide.
A couple of years ago, the group “Laboratory of Photonics and Quantum Measurements”, which was associated with the Laser Spectroscopy Division of Professor Hänsch, has succeeded in generating a frequency comb by means of a tiny microstructure, a toroidal glass resonator with a diameter of less than 100 micrometres. This was done in cooperation with Dr. Ronald Holzwarth from Menlo Sys-tems Ltd. and promises to radically reduce the size of frequency comb generators.
Using a “nanowire” made of glass the scientists couple light from a diode laser into this monolithic structure, where it is stored for a rather long time. This leads to extremely high light intensities inside the resonator, i.e. photon densities, which again produce nonlinear effects such as ‘four-wave mixing’ induced by the Kerr effect: two light quanta of equal energy are converted to two photons of which one light quantum has a higher energy, the other a lower energy than the original ones. The newly produced light fields can in turn interact with the original light fields, thereby producing new frequencies. From this cascade emerges a broad, discrete spectrum of frequencies. By optimizing the geometry of the toroid microresonator, Dr. Pascal Del’Haye (MPQ) and Tobias Herr (EPFL), doc-toral students at the referred experiment, have managed to compensate the effects of dispersion, such that the photon round-trip time inside the resonator remains the same for all light frequencies. Now the microresonators produce light over the range of more than an octave, from von 900 bis 2170 nm (near IR), for the first time. (As on the keyboard of a piano, the range of an octave corresponds to a doubling of the frequency.)
By raising the intensity of the light coupled into the resonator the frequencies of the comb can be shifted simultaneously. The higher intensities increase the temperature of the glass structure by up to 800 degree Celsius whereby the resonator is expanding and changing its index of refraction. Both effects lead to a shift of the comb lines towards lower frequencies, i.e. longer wavelengths. The broad range of frequencies as well as the tunability is an important pre-condition for self-referencing, where the lower range of the spectrum is doubled and compared to the upper part. Self-referencing is an important precondition for the use of frequency combs in metrology.
Also optical telecommunications will profit from the new tool. Whereas in the conventional fre-quency comb the lines are extremely close and of very low intensity, the spectral lines of the mono-lithic frequency comb have a separation of about 850 gigahertz and powers of the order of one milliwatt. This spacing and power level corresponds to the typical requirements for the “carriers” of the data channels in fibre-based optical communications. Tunability and broad range make the device also suitable for very precise calibration of spectrographs for astrophysics. Due to the large variety of possible applications many groups worldwide show interest in using the resonators for the miniaturi-zation of photonic devices. A number of other geometries and materials are investigated, e.g. pol-ished crystals, highly reflective fiber cavities and silicon structures based on computer-chip technol-ogy. [Olivia Meyer-Streng]Prof. Tobias J. Kippenberg (PhD)
Solar wind impacts on giant 'space hurricanes' may affect satellite safety
19.09.2017 | Embry-Riddle Aeronautical University
Integrated lasers on different surfaces
19.09.2017 | The Agency for Science, Technology and Research (A*STAR)
Using ultrafast flashes of laser and x-ray radiation, scientists at the Max Planck Institute of Quantum Optics (Garching, Germany) took snapshots of the briefest electron motion inside a solid material to date. The electron motion lasted only 750 billionths of the billionth of a second before it fainted, setting a new record of human capability to capture ultrafast processes inside solids!
When x-rays shine onto solid materials or large molecules, an electron is pushed away from its original place near the nucleus of the atom, leaving a hole...
For the first time, physicists have successfully imaged spiral magnetic ordering in a multiferroic material. These materials are considered highly promising candidates for future data storage media. The researchers were able to prove their findings using unique quantum sensors that were developed at Basel University and that can analyze electromagnetic fields on the nanometer scale. The results – obtained by scientists from the University of Basel’s Department of Physics, the Swiss Nanoscience Institute, the University of Montpellier and several laboratories from University Paris-Saclay – were recently published in the journal Nature.
Multiferroics are materials that simultaneously react to electric and magnetic fields. These two properties are rarely found together, and their combined...
MBM ScienceBridge GmbH successfully negotiated a license agreement between University Medical Center Göttingen (UMG) and the biotech company Tissue Systems Holding GmbH about commercial use of a multi-well tissue plate for automated and reliable tissue engineering & drug testing.
MBM ScienceBridge GmbH successfully negotiated a license agreement between University Medical Center Göttingen (UMG) and the biotech company Tissue Systems...
Pathogenic bacteria are becoming resistant to common antibiotics to an ever increasing degree. One of the most difficult germs is Pseudomonas aeruginosa, a...
Scientists from the MPI for Chemical Energy Conversion report in the first issue of the new journal JOULE.
Cell Press has just released the first issue of Joule, a new journal dedicated to sustainable energy research. In this issue James Birrell, Olaf Rüdiger,...
12.09.2017 | Event News
06.09.2017 | Event News
06.09.2017 | Event News
19.09.2017 | Earth Sciences
19.09.2017 | Materials Sciences
19.09.2017 | Physics and Astronomy