Stability of a high-finesse optical cavity at 493 nm in vacuum for cavity QED with Barium ions

Abstract

We explore the stability of a high-finesse optical cavity at 493 nm in vacuum for cavity QED with Barium ions. A high-finesse Fabry-Perot cavity is built using mirrors with high-reflectivity (HR) coatings that are implemented by stacking multiple thin films of low-loss dielectrics on substrates. Applications of such HR mirrors in the near ultraviolet (UV) range have been hampered by degradation of coatings in vacuum. Here, we explore the degradation of mirrors with HR coatings at 493 nm in vacuum. We study both vacuum-induced and laser-induced effects on oxide-coated cavity mirrors by probing changes in cavity loss using cavity lifetime measurements. We investigate the role of circulating power in the rate of increase in cavity loss and demonstrate methods of reversal of cavity degradation. While we observe no degradation without long exposure or with short exposures at lower circulating powers, we find evidence of degradation on long exposure to high circulating powers. We discuss potential causes and conclude that laser-induced deposition is the likely cause while ruling out thermally activated processes due to laser-induced heating.

Figures (8)

• Figure 1: (\ref{['figschematicwithatoms']}) Model for cavity QED with a single atom: $g$ is the coherent coupling of the atom to the cavity field. $\Gamma$ is the incoherent spontaneous emission rate of the atom into free space. The cavity decay rate $\kappa$ is the sum of the external coupling rate $\kappa_{\mathrm{ex}}$ and the internal loss of the cavity field $\kappa_{\mathrm{in}}$. (\ref{['figleveldiagram']}) A simplified level diagram of the ion. The ion can spontaneously decay to the lower $S$ or $D$ level from the excited $P$ level at a rate given by $\Gamma$ and the branching ratio $p$. (\ref{['table1']}) Table showing transition wavelengths ($\lambda$) of different ionic species, branching ratios ($p$) and their products ($p\lambda$).
• Figure 2: (\ref{['schematic']}) Schematic of the experimental setup. ECDL: External Cavity Diode Laser, APD 1 and 2: avalanche photodiodes detectors for transmitted and reflected signal respectively, BS: beamsplitter, AOM: acousto-optic modulator, EOM: electro-optic modulator, PZT: piezoelectric transducer for scanning the cavity length, DAQ: Data Acquisition system. The glowing blue lines indicate optical signal, while the orange line indicates electronic signals. (\ref{['cad']}) Schematic showing a cross sectional view of the cavity assembly. (\ref{['ringdown']}) A sample ringdown signal acquired by APD 1 showing exponential decay of cavity transmission after the input beam is turned off. The dashed vertical line indicates the time of extinction of the cavity input signal.
• Figure 3: Cavity loss measured in vacuum (blue open circles) over several months. The error bars are standard deviations of hundred measurements. Also shown in figure is the linear fit (red line) to the data.
• Figure 4: Schematic of the electronic control system for cavity relocking. Enclosed with a dashed line is the PI controller implemented on a PCB. DAC: digital-to-analog converter, P: P-gain, I: I-gain, SA: summing amplifier, SW: analog switch.
• Figure 5: Cavity loss measured as a function of time for two different runs, (a) and (b), detailed in the main text. Loss values inside the dashed boxes are measured at mean circulating powers denoted in the solid boxes.