Optical Local Oscillators
Optical frequency standards are based on atomic systems that exhibit narrow and reproducible spectral features. However, these spectral features are noisy for short measurement periods. The precision of future optical clocks is therefore critically dependent on the availability of optical local oscillators (lasers) that are sufficiently stable over periods long enough for this noise to be 'averaged down'.
The challenge is to control the frequency of a laser so that it remains constant to within a fraction of a cycle per second. However, even the best commercially available lasers fluctuate by more than a thousand cycles per second. To improve upon this, the laser is stabilised to an optical cavity. This consists of two highly reflective mirrors bonded to a spacer made from ultra-low-expansivity glass, and has the property that it resonates at a very specific optical frequency. The frequency of the laser is actively steered so that it always remains in resonance with the cavity. In this way the laser takes on the frequency characteristics of the cavity. To achieve state-of-the-art laser stability, careful attention must be paid to temperature control and vibration isolation of the cavity.
To test the performance of such a system, two nominally identical systems can be compared. In the arrangement shown below, two Nd:YAG lasers are stabilised to two independent cavities, and light from each combined on a photodetector. The cavity resonances are at different frequencies, so a beat signal is observed at the difference frequency. In this way we have observed a beat frequency linewidth of less than 0.5 Hz measured over 4 s, and a fractional frequency stability of 1 part in 1015 at 5 s.
We are also working on novel cavity designs that have a low sensitivity to vibration. This technology is being used to develop narrow linewidth laser sources for the strontium lattice clock and the strontium and ytterbium ion optical frequency standards.