Single and coupled cavity mode sensing schemes using a diagnostic field.

Area of Science:

• Quantum optics
• Optical metrology

Background:

• Precise optical mode matching is crucial for squeezed-vacuum states in quantum experiments.
• Existing automatic spatial-mode matching schemes can reduce losses and improve stability.
• Mode matching sub-cavities is also essential for quantum-enhanced coupled-cavity systems like gravitational-wave detectors.

Purpose of the Study:

• To propose a new, simplified mode sensing scheme for both simple and coupled cavities.
• To enable routine use of high squeezing levels in gravitational-wave detectors.

Main Methods:

• A diagnostic field tuned to the HG20/LG10 mode frequency is employed.
• The scheme eliminates the need for moving parts or Gouy phase tuning.
• Error signals are derived, proportional to differences in waist position and Rayleigh ranges of sub-cavity eigenmodes.

Main Results:

• The proposed scheme generates separable error signals (90-degree demodulation phase).
• Reasonable error signals were demonstrated for a simplified Einstein Telescope optical design.
• The method is applicable to both simple and coupled cavities.

Conclusions:

• This new mode sensing scheme offers a robust and simplified approach to optical mode matching.
• It facilitates the practical implementation of advanced quantum technologies in sensitive instruments like gravitational-wave detectors.