Coherent photo-thermal noise cancellation in a dual-wavelength optical cavity for narrow-linewidth laser frequency stabilisation.

Optical resonators are used for the realisation of ultra-stable frequency lasers. The use of high reflectivity multi-band coatings allows the frequency locking of several lasers of different wavelengths to a single cavity. While the noise processes for single wavelength cavities are well known, the correlation caused by multi-stack coatings has as yet not been analysed experimentally. In our work, we stabilise the frequency of a 729 nm and a 1069 nm laser to one mirror pair and determine the residual-amplitude modulation (RAM) and photo-thermal noise (PTN). We find correlations in PTN between the two lasers and observe coherent cancellation of PTN for the 1069 nm coating. We show that the fractional frequency instability of the 729 nm laser is limited by RAM at 1 × 10-14. The instability of the 1069 nm laser is at 3 × 10-15 close to the thermal noise limit of 1.5 × 10-15.

Transportable clock laser system with an instability of 1.6 × 10-16.

We present a transportable ultra-stable clock laser system based on a Fabry-Perot cavity with crystalline Al0.92Ga0.08As/GaAs mirror coatings, fused silica (FS) mirror substrates, and a 20 cm-long ultra-low expansion (ULE) glass spacer with a predicted thermal noise floor of mod σy = 7 × 10-17 in modified Allan deviation at one second averaging time. The cavity has a cylindrical shape and is mounted at 10 points. Its measured sensitivity of the fractional frequency to acceleration for the three Cartesian directions are 2(1) × 10-12 /(ms-2), 3(3) × 10-12 /(ms-2), and 3(1) × 10-12 /(ms-2), which belong to the lowest acceleration sensitivities published for transportable systems. The laser system's instability reaches down to mod σy = 1.6 × 10-16.

Transportable interrogation laser system with an instability of mod σy = 3 × 10-16.

We present an interrogation laser system for a transportable strontium lattice clock operating at 698 nm, which is based on an ultra-low-expansion glass reference cavity. Transportability is achieved by implementing a rigid, compact, and vibration insensitive mounting of the 12 cm-long reference cavity, sustaining shocks of up to 50 g. The cavity is mounted at optimized support points that independently constrain all degrees of freedom. This mounting concept is especially beneficial for cavities with a ratio of length L over diameter DL/D > 1. Generally, large L helps to reduce thermal noise-induced laser frequency instability while small D leads to small cavity volume. The frequency instability was evaluated, reaching its thermal noise floor of mod σy ≈ 3 × 10-16 for averaging times between 0.5 s and 10 s. The laser system was successfully operated during several field studies.

Phase noise of frequency doublers in optical clock lasers.

Frequency doublers are widely used in high-resolution spectroscopy to shift the operation wavelength of a laser to a more easily accessible or otherwise preferable spectral region. We investigate the use of a periodically-poled lithium niobate (PPLN) waveguide frequency doubler in an optical clock. We focus on the phase evolution between the fundamental (1396 nm) and frequency-doubled (698 nm) light and its effect on clock performance. We find that the excess phase noise of the doubler under steady-state operation is at least two orders of magnitude lower than the noise of today's best interrogation lasers. Phase chirps related to changes of the optical power in the doubler unit and their influence on the accuracy of optical clocks are evaluated. We also observe substantial additional noise when characterizing the doubler unit with an optical frequency comb instead of using two identical waveguide doublers.