11-08 :: August 2011

nanotimes News in Brief

Ronald Holzwarth from Menlo Systems Ltd. and pro- mises 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 fre- quencies. By optimizing the geometry of the toroid microresonator, Dr. Pascal Del’Haye (MPQ) and Tobias Herr (EPFL), doctoral students at the refer- red 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 2170nm (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° Cel- sius (1,472° F) whereby the resonator is expanding and changing its index of refraction. Both effects lead to a shift of the comb lines towards lower frequen- cies, i.e. longer wavelengths. The broad range of

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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 pre- condition for the use of frequency combs in metro- logy.

Also optical telecommunications will profit from the new tool. Whereas in the conventional frequency comb the lines are extremely close and of very low intensity, the spectral lines of the monolithic fre- quency comb have a separation of about 850 giga- hertz 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 astro- physics. Due to the large variety of possible applica- tions many groups worldwide show interest in using the resonators for the miniaturization of photonic devices. A number of other geometries and materials are investigated, e.g. polished crystals, highly re- flective fiber cavities and silicon structures based on computer-chip technology.

P. Del’Haye, T. Herr, E. Gavartin, M.L. Gorodetsky, R. Holzwarth and T. J. Kippenberg: Octave Spanning Tuna- ble Frequency Comb from a Microresonator, In: Physical Review Letters, Vol. 107(2011), Issue 06, August 05, 2011, Article 063901 [4 pages], DOI:10.1103/PhysRev- Lett.107.063901:

http://dx.doi.org/10.1103/PhysRevLett.107.063901 http://www.mpq.mpg.de/