Figure: Octave spanning frequency comb generation in a microresonator. Panel (a) shows the experiment with a glass nano-fiber 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.
Max Planck In-stitute of Quantum Optics have developed octave-spanning frequency comb with a microresonator. 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.
We report the generation of an octave-spanning optical frequency comb in a continuous wave laser pumped microresonator. The generated comb spectrum covers the wavelength range from 990 to 2170 nm without relying on additional external broadening. Continuous tunability of the generated frequency comb over more than an entire free spectral range is demonstrated. Moreover, the linewidth of individual optical comb components and its relation to the pump laser phase noise is studied. The ability to derive octave-spanning spectra from microresonator comb generators represents a key step towards f-2f self-referencing of microresonator-based optical frequency combs.