Free Fabry-Perot Cavity

Recently, Nature Communications published a landmark photonics research work. A research team from the CREOL College of Optics and Photonics at the University of Central Florida and other institutions innovatively proposed and experimentally realized a resonance-free Fabry-Pérot (FP) cavity.

By employing an intracavity holographic phase mask to manipulate the orbital angular momentum (OAM) of light beams, the study achieves unconstrained gradual order escalation of Laguerre–Gauss (LG) modes. This approach fundamentally breaks the discrete resonance characteristic of traditional optical resonators, creating a novel optical cavity that is nearly immune to cavity length perturbations. It opens up brand-new avenues for high-stability optical sensing, structured light lasers, and non-Hermitian photonics research.

1. Research Background and Existing ChallengesThe traditional Fabry-Pérot (FP) resonant cavity is a fundamental core device in laser technology, optical precision sensing, quantum optics and photon detection. However, it possesses inherent and unavoidable limitations:

1. It only generates resonant peaks at a few specific wavelengths, resulting in narrow operating bandwidth.

2. It is extremely sensitive to cavity length variations; vibration and temperature fluctuation both cause resonant wavelength drift, degrading sensing accuracy and laser stability.

Previous attempts to improve performance via dispersion engineering and intracavity optical field shaping have failed to fundamentally eliminate cavity-length dependence and discrete resonance, greatly restricting practical applications.2. Core Innovative Design and Working PrincipleThis research breaks away from the conventional route of modifying resonators through dispersion regulation. Instead, it innovatively starts with spatial mode manipulation of the optical field and fundamentally eliminates resonance from the physical mechanism.

The research team integrated a holographic phase mask (HPM) inside a compact planar FP cavity as the core element for mode conversion. It abandons the commonly used Hermite–Gauss (HG) modes and adopts Laguerre–Gauss (LG) modes carrying orbital angular momentum (OAM) as the propagation basis.

Higher-order LG modes generated during evolution feature inherent orthogonality, which breaks the coherent interference condition of multiple intracavity beams. The discrete resonant peaks of conventional cavities are completely suppressed, enabling the cavity to achieve an all-flat, resonance-free broadband spectral response.

Key differences between the two modes:

1. HG mode: A single component can only switch between two fixed modes, unable to achieve continuous order escalation, so resonance cannot be eliminated.

2. LG-OAM mode: Continuous escalation of all modal orders can be realized with only one phase mask, featuring simple structure and easy integration.

3. Experimental Results and Core HighlightsThe team fabricated the holographic phase mask using photothermorefractive glass and realized broadband mode conversion via volume Bragg gratings. The average conversion efficiency of LG modes exceeds 87.5% with low phase distortion, enabling orthogonal propagation of high-order modes.

Experimental comparison shows that traditional FP cavities exhibit discrete resonant peaks, and their spectra drift significantly with any change in cavity length. In contrast, the new resonance-free cavity completely eliminates resonant peaks, presenting a flat spectral response consistent with the input spectrum. It maintains stable performance even when the cavity length varies over a span of 350%, demonstrating excellent immunity to external perturbations. When Gaussian light is injected, up to the 19th-order LG modes can be excited without generating resonance at any order, which fully verifies the effectiveness of the resonance-free mechanism.