Sub-poissonian noise signatures in the carrier-envelope phase jitter of highly stabilized mode-locked lasers

Measurement and stabilization of the carrier-envelope phase (CEP) drift of femtosecond pulse trains has found widespread application in frequency metrology and high-field nonlinear optics. Stabilization of the CEP is typically accomplished in a feedback loop, acting on the pump power with a wide-bandwidth modulator. This stabilization strongly relies on the fact that a change of intracavity laser power results in a proportional change of the carrier envelope frequency f_CE. This proportionality may be turned around, with f_CE acting as a probe for the laser intracavity power. Doing so, we find that this novel method of intracavity power measurements is not limited by standard shot noise, with a sensitivity of the method down to -17 dB (i.e., a factor 50) below the shot noise floor of conventional photodetection in a photodiode or similar absorptive devices. This opens a perspective for highly sensitive interferometry applications.