Garching, Germany--Scientists at the Max Planck Institute of Quantum Optics (MPQ) have been hard at work improving the optical frequency comb, which they invented more than a decade ago. Their newest achievement: taking a microtoroidal frequency comb generator that they recently developed and coaxing it to produce precisely tunable light over a frequency range of more than an octave (an octave is a doubling of the frequency).1
Precise frequency increments
The frequency comb, invented by Theodor Hänsch, is a source of light with a spectrum that looks, in fact, like a comb. Its many thousands (in some cases, up to a million) spectral lines are all separated from their nearest neighbors by the same frequency. 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. This quality is invaluable for metrology (for example, precise timekeeping) and optical communications.
The frequency comb developed by Hänsch is based on a mode-locking process in short-pulse lasers and is being marketed by an MPQ spinoff, Menlo Systems (Munich, Germany). A couple of years ago, Tobias Kippenberg and his team at MPQ were able to generate optical frequency combs using chip-based quartz microtoroids with diameters of less than 100 micrometers. Using a glass nanowire, the scientists couple light from a diode laser into this monolithic structure, where it is stored for a length of time, leading to extremely high photon densities that produce nonlinear effects such as four-wave mixing. The newly produced frequencies can in turn interact with the original light fields, producing new frequencies; from this cascade emerges a broad, discrete spectrum of frequencies.
By optimizing the geometry of the toroid microresonator, Pascal Del'Haye of MPQ and Tobias Herr of the Ecole Polytechnique Fédérale de Lausanne (Lausanne, Switzerland) were able to compensate the effects of dispersion, making the photon round-trip time inside the resonator the same for all light frequencies. As a result, the microresonators now produce light over a 900 to 2170 nm range in the near IR.
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 C, causing the resonator to expand and change its index of refraction. Both effects lead to a shift of the comb lines towards lower frequencies. 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.
Optical telecommunications should also benefit from the new tool. While in a conventional frequency comb the lines are extremely close and of low intensity, the spectral lines of the monolithic frequency comb have a separation of about 850 GHz and powers of the order of 1 mW. This spacing and power level corresponds to the typical requirements for the carriers of the data channels in fiber-based optical communications.
1. P. Del'Haye et al., Physical Review Letters 107, 063901, 1 August 2011.