Optical frequency measurement and the related developement of atomic clocks and optical frequency standards is an important and rapidly evolving area of research. Both the relation of the microwave and the optical region and the operation of an optical frequency standard as a clock requires a sort of a "clockwork" for a coherent, noise-free link of large frequency ratios.
For this purpose we built an optical frequency comb generator based on a Kerr-lens mode-locked femtosecond laser. This laser is capable of generating sub-10 fs pulses. As shown in fig. 1, the special feature of the laser is the frequency spectrum consisting of a large number of exactly equidistant modes, which, pictorially spoken, correspond to the mm markers on a ruler. The entire spectrum can be described by only two numbers: the mode spacing frep and an offset frequency fceo which characterizes the absolute position of the comb with respect to the frequency zero point. Finally, the absolute frequency of any coherent optical source can be determined by detecting its beat note frequency with the m-th comb mode. A good wave meter is sufficient to determine the "mode number m" . Thus, the coherent link means to express the external optical frequency by three radio frequencies (frep, fceo, and beat note) and an order number (m).
Fig. 1: The frequency spectrum of a periodic pulse train from a femtosecond laser is comb-like. The spectral width is proportional to the inverse pulse duration and the pulse repetition rate equals the mode separation.
The useable spectrum can substantially be broadened by self-phase modulation in a dispersion-modified microstructure fiber, without deteriorating the coherence of the comb. Depending on power, pulse length and fiber length the spectrum reaches 450 nm and 1200 nm. Fig. 2 shows the visible part of the spectrum.
We phase-coherently linked both PTB's calcium frequency standard and the ytterbium-ion frequency standard [2] with the primary cesium clock. For the ytterbium transition frequency we reached a total relative uncertainty of 1·10-14. The measurement technique itself, however, would allow even lower uncertainties.
Fig. 2: The visible part of the comb generator's spectrum after broadening in a microstructure fiber. The spectrum appears continuous, but in fact it consists of more than one million separate modes with exactly equal spacing (here 100 MHz). In the dark parts the light is about 100 times weaker than in the bright parts. This is a result of the complicated processes of broadening in the fiber, without being a limitation for frequency measurements.
Publications
[1] J. Stenger, T. Binnewies, G. Wilpers, F. Riehle, H. R. Telle, J. K. Ranka, R. S. Windeler, A. J. Stentz; Phys. Rev. A 63 (2001) 021802(R)
[2] J. Stenger, Ch. Tamm, N. Haverkamp, S. Weyers, and H. R. Telle; Optics Lett. 26 (2001) 1589
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