Octave-Spanning Optical Ruler Is Precisely Tunable

Max Planck Institute of Quantum Optics (MPQ) scientists succeeded in generating optical frequency combs using chip-based quartz glass toroids with diameters on the micrometer scale. Taking it a step further, they have now created microresonators that can produce light over a range of more than an octave and are at the same time precisely tunable. This achievement brings a variety of applications into reach, such as optical telecommunications, miniaturization of photonic devices, or the precise calibration of spectrographs in astrophysics.

Octave-spanning frequency comb generation in a microresonator. Panel (a) shows the experiment with a glass nanofiber and a silicon chip with optical resonators. A scanning electron microscope 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. (Image: MPQ)

A frequency comb is a light source containing a large spectrum of colors, but the frequencies are not continuously distributed. Instead, up to one million spectral lines are spaced at 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. Developed by professor Theodor W. Hänsch of MPQ, the frequency comb is based on a mode-locking process in short-pulse lasers.

Using a glass nanowire, the scientists coupled light from a diode laser and stored it in the microresonator. The resulting extremely high photon densities produce new light fields that interact with the original light fields, creating new frequencies. By optimizing the geometry of the toroid microresonator, the scientists 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 900 to 2170 nm (near-IR), for the first time.

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 ºC, whereby the resonatorexpands and changes its index of refraction. Both effects lead to a shift of the comb lines toward lower frequencies (longer wavelengths). The broad range of frequencies, as well as the tunability, is an important precondition 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.

According to the MPQ scientists, this new tool will also be important to optical telecommunications. In the conventional frequency combs, the lines are extremely close and of very low intensity, whereas the spectral lines of the monolithic frequency comb have a separation of about 850 GHz and powers of the order of one milliwatt. This spacing and power level correspond to the typical requirements for the “carriers” of the data channels in fiber-based optical communications. Tunability and broad range make the device also suitable for precise calibration of spectrographs for astrophysics. A number of other geometries and materials are being investigated, such as polished crystals, highly reflective fiber cavities, and silicon structures based on computer-chip technology.

For more information, visit: www.mpq.mpg.de