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Optical Synthesizer

Optical atomic clocks are being developed intensively worldwide. The necessary laser systems have to show an extremely high frequency stability and, at the same time, provide an optical frequency which has to fit to the relevant atomic clock. So far, the required frequency stability can only be attained with the frequency of the laser system locked to an ultra-stable optical cavity which is largely isolated from all external disturbances. This high technical effort must be repeated for every optical atomic clock. A much more flexible solution would be a type of optical synthesizer consisting of two lasers: a master laser, which is optimized to ultimate stability, and a connected slave laser which emits light at the required frequency. In this synthesizer it has to be ensured that the stability of the master laser is transferred to the slave laser. An experiment has now been performed at PTB showing that such a transfer of stability is possible.

The centre piece of the optical synthesizer is formed by a programmable frequency converter developed at the PTB on the basis of the direct digital synthesis. The device allows a phase-coherent frequency conversion with a freely adjustable factor of 48 bit resolution.

Two diode lasers whose optical frequencies differed by more than 30 THz were involved in this experiment. A complexly cavity-stabilized diode laser which was developed for an optical atomic clock with calcium atoms served as the master laser. Its frequency stability of 10–15 corresponds to a line width of only 1 Hz. The slave laser is used for the development of an optical atomic clock with strontium atoms.

In order to transfer the stability of the master laser to the slave laser, an optical frequency comb generator is used. The light from the two lasers is sent to the frequency comb generator via optical fibres and is transformed there into a radio frequency signal by means of the so-called transfer oscillator technique. The generated signal corresponds to a beat signal between the two laser frequencies and is independent of unavoidable technical fluctuations of the comb frequencies. This technique needs a special frequency converter which multiplies radio frequencies with freely selectable factors. At PTB, a freely programmable frequency converter based on direct digital synthesis has been developed specifically for this purpose. The frequency of the slave laser is controlled by electronic feedback in such a way that the frequency of the beat signal remains constant. Thereby, all the frequency fluctuations of the slave laser relative to the master laser are cancelled and the slave laser shows the same stability as the master laser.

As the light of both lasers is sent to the frequency comb generator via optical fibres, the lasers can also be located in different labs or buildings. In the future, this newly developed technology will be used to distribute the stability of a central ultra-stable master laser to various lasers of other experiments. The central laser will operate at an optical frequency which is optimized to achieve the highest possible frequency stability. The task of emitting light at the desired frequency is then taken up by the respective lasers on site.

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