Fast and compact four-quadrant CEP detection with <i>f</i>-2<i>f</i> polarization interferometry

The carrier-envelope phase (<i>φ</i>_CEP) of few-cycle light pulses is of crucial importance for strong-field light-matter interaction on subcycle time scales. Standard <i>f</i>-2<i>f</i> interferometry rapidly records beat notes defined by the carrier-envelope offset frequency (<i>f</i>_CEO), but measuring <i>φ</i>_CEP at a high bandwidth is more involved. Here, we introduce <i>f</i>-2<i>f</i> polarization interferometry, which unambiguously measures <i>φ</i>_CEP over a range of 2π at the detection speed of the implemented photodiodes. It relies on evaluating the polarization state of the spectral overlap of a traditional <i>f</i>-2<i>f</i> interferometer with linear optical components and balanced photodiodes. In the first implementation, we detect <i>φ</i>_CEP modulations with a read-out rate of up to 60 kHz. Our scheme promises single-shot <i>φ</i>_CEP detection at intrinsic oscillator repetition rates in the MHz regime. It can be used in a wide range of ultrafast experiments, from <i>φ</i>_CEP stabilization and phase tagging to selective high-harmonic generation and precise lightwave control of electron motion.