Speaker
Description
Squeezed light is a key technology for reducing quantum noise in gravitational-wave detectors. Frequency-dependent squeezing enables broadband noise suppression but is highly sensitive to mode mismatch between the squeezed field and the filter cavity. We experimentally demonstrate robust frequency-dependent squeezing under mode mismatch using a self-imaging filter cavity.
The self-imaging filter cavity is a multimode cavity that supports the simultaneous resonance of multiple spatial modes. Owing to this property, frequency-dependent phase rotation can be applied simultaneously to both the fundamental, mode-matched field and residual mismatched higher-order modes. Consequently, the matched and mismatched components acquire a common squeezing-angle rotation and become effectively indistinguishable at the filter cavity output. We realize frequency-dependent squeezing at 1560 nm using a self-imaging filter cavity, and use a conventional single-mode filter cavity for performance comparison. To introduce controlled mode mismatch, we displace the cavity input mode in the horizontal or vertical direction, and separately induce waist-size mismatch by shifting the mode-matching lens. Under these mismatch conditions, we experimentally observe that the self-imaging cavity preserves a clear squeezing benefit over a modest range of mode mismatch, whereas the single-mode cavity exhibits substantially stronger degradation. Our technique provides a viable approach to mitigating mode-mismatch effects in gravitational-wave detectors.