Logo PTB

Frequency Dissemination with Fibres

Working Group 4.34

Fiber-guided Optical Frequency Transmission

With the aid of optical clocks it is possible to obtain considerably smaller uncertainties and higher stabilities in the realization of the unit of time than with microwave standards. This fact will in future lead to a redefinition of the base unit "second". In order to fully utilize this potential, it is necessary to be able to compare also optical clocks with different optical frequencies and at different sites directly with each other. Important aids here are optical comb generators  (Kubina 2005).

The "transfer method" (Telle 2002) developed at the PTB enables a direct comparison to be made of  local optical standards by means of the frequency comb. By bypassing a microwave reference, a relative uncertainty in the range of 10-19 was attained thereby (Grosche 2008).

Since at the present and in future, the best standards are in general not operated in the immediate vicinity of one another, a phase-traceable frequency comparison over longer distances (within the borders of Europe, transatlantic) with an uncertainty in the range of < 10-15 is of great practical relevance for many questions in metrology, fundamental physics and astronomy. The phase-coherent transmission of the frequency information over longer distances thus becomes one of the central points of a redefinition of the unit of time.

Since conventional frequency comparisons using microwave standards and satellites do not attain the accuracy and stability required in order to use the outstanding stability of optical clocks, the only possible alternative for the carrying out of comparisons is the use of glass fibers in the telecommunication window at 1.5 µm, since here the smallest transmission losses are attained.

Different optical transmission variants as, e.g., transmission of a microwave reference which is modulated onto an optical carrier via amplitude or frequency modulation, or the direct transmission of the frequency comb spectrum, are investigated worldwide.

In contrast, we would like to use the optical frequency of a narrow-band cw transfer laser as carrier for the frequency information of the frequency standard. To do this, a cw laser is linked with the aid of a frequency comb generator phase-traceably to the optical frequency standard  (Grosche 2008). For this purpose, narrow-band fiber lasers (freewheeling linewidth < 10 kHz) with a wavelength  close to 1.5 µm are particularly suited so that a standard telecommunication fiber with relatively low attenuation losses can be used for transmission (Grosche 2007).