Optimum output coupling in optical oscillators using an antiresonant ring interferometer

We describe a simple and universal method for absolute optimization of output power from optical oscillators using interferometry. By incorporating an antiresonant ring interferometer in one arm of the oscillator cavity, simple adjustments to the interferometer provide continuously variable output coupling over a broad spectral range and under any operating conditions. We demonstrate the technique using a femtosecond optical parametric oscillator (OPO), where we show continuously adjustable output coupling from 1% to 60%. By operating the OPO under an optimized output coupling of approximately 30%, we obtain approximately 200 mW of extracted power, more than twice that with an approximately 4% conventional output coupler, across the full tuning range. We also show that the technique has no detrimental effect on the spatiotemporal characteristics of the output, with the extracted signal exhibiting a Gaussian beam profile and near-transform-limited pulse durations.