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 $1\times10^{-14}$. The instability of the $1069\,$nm laser is at $3\times10^{-15}$ close to the thermal noise limit of $1.5\times10^{-15}$.Comment: 17 pages, 5 figure

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 × 10−1

A high-performance optical lattice clock based on bosonic atoms

Optical lattice clocks with uncertainty and instability in the $10^{-17}$-range and below have so far been demonstrated exclusively using fermions. Here, we demonstrate a bosonic optical lattice clock with $3\times 10^{-18}$ instability and $2.0\times 10^{-17}$ accuracy, both values improving on previous work by a factor 30. This was enabled by probing the clock transition with an ultra-long interrogation time of 4 s, using the long coherence time provided by a cryogenic silicon resonator, by careful stabilization of relevant operating parameters, and by operating at low atom density. This work demonstrates that bosonic clocks, in combination with highly coherent interrogation lasers, are suitable for high-accuracy applications with particular requirements, such as high reliability, transportability, operation in space, or suitability for particular fundamental physics topics. As an example, we determine the $^{88}\textrm{Sr} - ^{87}$Sr isotope shift with 12 mHz uncertainty

Excess noise and photo-induced effects in highly reflective crystalline mirror coatings

Thermodynamically induced length fluctuations of high-reflectivity mirror coatings put a fundamental limit on sensitivity and stability of precision optical interferometers like gravitational wave detectors and ultra-stable lasers. The main contribution - Brownian thermal noise - is related to the mechanical loss of the coating material. Owing to their low mechanical losses, Al\textsubscript{0.92}Ga\textsubscript{0.08}As/GaAs crystalline mirror coatings are expected to reduce this limit. At room temperature they have demonstrated lower Brownian thermal noise than with conventional amorphous coatings. %However, no detailed study on the noise constituents from these coatings in optical interferometers has been conducted. We present a detailed study on the spatial and temporal noise properties of such coatings by using them in two independent cryogenic silicon optical Fabry-Perot resonators operated at 4 K, 16 K and 124 K. We confirm the expected low Brownian thermal noise, but also discover two new noise sources that exceed the Brownian noise: birefringent noise that can be canceled via polarization averaging and global excess noise (10 dB above Brownian noise). These new noise contributions are a barrier to improving ultra-stable lasers and the related performance of atomic clocks, and potentially limit the sensitivity of third-generation gravitational wave detectors. Hence, they must be considered carefully in precision interferometry experiments using similar coatings based on semiconductor materials

Transportable ultra-stable laser system with an instability down to 10⁻¹⁶

In this work, a transportable ultra-stable laser system based on a Fabry-Pérot cavity with crystalline aluminium gallium arsenide (Al₀․₉₂Ga₀․₀₈As) / gallium arsenide (GaAs) mirror coatings, fused silica glass mirror substrates and a 20 cm-long ultra low expansion glass spacer was designed and built to serve as a clock laser for a ⁸⁷Strontium (Sr) lattice clock. The laser system uses an external-cavity diode laser, which is stabilized to a resonance frequency of the Fabry-Pérot cavity using the Pound-Drever-Hall method. This reduces the laser's fractional frequency instability down to the cavity's fractional length instability. Due to the high absorbance of Al₀․₉₂Ga₀․₀₈As/GaAs mirror coatings for visible light, the laser is operated at a wavelength of 1397 nm, which is twice the transition wavelength of a ⁸⁷Sr lattice clock. The laser system therefore includes frequency doubling and light distribution for operation of a ⁸⁷Sr lattice clock. The fundamental limit of the cavity's fractional length instability and thus the laser's fractional frequency instability is determined by the thermal noise floor resulting from Brownian, thermoelastic and thermorefractive noise of the cavity components. The calculated thermal noise floor limit given as modified Allan deviation of the fractional frequency instability mod σᵧ is below 1 · 10⁻¹⁶. Besides the thermal noise, technical noise caused by seismic noise, residual amplitude modulation, laser power, pressure, optical path length and temperature fluctuations affects the laser's fractional frequency instability. The single contributions of the technical noise were investigated and their impact on the laser's fractional frequency instability were suppressed below the thermal noise floor for averaging times around one second using passive or active stabilization. The laser system achieves an instability as low as mod σᵧ = 1.6 · 10⁻¹⁶, which is already a factor 1.3 lower than the theoretically possible instability of mod σᵧ = 2 · 10⁻¹⁶ for the same resonator with tantalum pentoxide (Ta2O5) / fused silica (SiO2) mirrors. This is the lowest fractional frequency instability among published transportable laser systems. Depending on the averaging time of interest, the fractional frequency instability has been reduced by a factor of up to seven compared to Physikalisch-Technische Bundesanstalt (PTB)'s current transportable laser system, which had the lowest fractional frequency instability until now. This reduced instability allows a reduction of the Dick effect limit by roughly a factor of four for interrogation times below 0.5 s, which would reduce the clock's instability limit significantly

Towards a Robust and Stand-Alone Ultra-Stable Laser System Based on a 124 K Si Resonator with an Instability of 4×10−174\times 10^{-17}

Ultra-stable laser systems are needed for precision measurements, e.g. with optical clocks, where the performance directly depends on the lasers' fractional frequency instability. This instability is fundamentally limited by the thermal noise in the systems' ultra-stable resonators. To reach the thermal noise floor, the technical noise of the laser system must be reduced below the thermal noise limit. This includes, noise resulting from laser power fluctuations, from residual amplitude modulation in the PDH servo or from seismic noise acting on the cavity

Supplementary document for Coherent photo-thermal noise cancellation in a dual-wavelength optical cavity for narrow-linewidth laser frequency stabilisation - 6788518.pdf

Equations used for fitting to the data in Figure 2 and