Dynamically tunable broadband output coupling of optical oscillators based on non-cyclic geometric phase mirror

We present a uniquely versatile and efficient mirror system capable of real-time fine-tuning in reflection and transmission properties across a broad wavelength range and at high optical power. Leveraging the principles of the non-cyclic geometric phase acquired by the clockwise and counter-clockwise beams of the Sagnac interferometer satisfying anti-resonant condition on propagation through the quarter wave-plate, half wave-plate, and the quarter wave-plate combination having fast axes oriented at 45⁰ (fixed), θ (variable), and -45⁰ (fixed) with respect to vertical, respectively, our mirror system offers dynamic transmission control across 0 - 100% without the need for realignment. Notably, the geometric phase-based mirror (GP- mirror) preserves the polarization state of the reflected beam, making it ideal for polarization-sensitive applications. Wavelength insensitivity of the geometric phase enables seamless operation of the mirror across a wide wavelength range. As a proof-of-principle, we use the GP-mirror as the output coupler of a continuous-wave, green-pumped, doubly-resonant optical parametric oscillator (DRO) based on a 30-mm-long MgO:sPPLT crystal and obtain stable operation at high power over a wide wavelength tuning range. For a pump power of 5 W, the DRO provides an output power of 2.45 W at an extraction efficiency as high as 49% when operated at optimum output coupling. The DRO shows a maximum pump depletion of 89% and delivers an optimum output power across a tuning range ≥90 nm. The demonstrated concept offers a promising approach for advancing the capabilities and control of coherent sources tunable across different spectral regions and in all time-scales from continuous-wave to ultrafast femtosecond domain.