The carrier-envelope phase (CEP) or carrier-envelope offset (CEO) phase is an important feature of an ultrashort laser pulse and gains significance with decreasing pulse duration, in a regime where the pulse consists of a few wavelengths. Physical effects depending on the fall into the category of highly nonlinear optics.
CEP in the time domain
The CEP ϕ 0 is the phase between the carrier wave and the position of the intensity envelope of the pulse (cf. figure in the time domain). In a train of multiple pulses it is usually varying due to the difference between phase and group velocity. The time, after which the phase increases resp. decreases by 2 π is called T C E O >. Ideally, it is an integer multiple of the duration T r e p between two pulses and the pulses are picked at the corresponding rate to obtain a constant phase over all picked pulses. Besides this linear evolution, fluctuations which are common in conventional femtosecond laser systems usually cause a nonlinear shot-to-shot fluctuation of the CEP. This is why measuring and controlling it is very important for many applications.
CEP in the frequency domain and measurement
In the frequency domain, a pulse train is represented by a frequency comb. Here, the carrier-envelope frequency f C E O = 1 T C E O = d ϕ 0 d t = is exactly the offset frequency of the pulse train, cf. figure. This makes it possible to perform a multi-shot measurement of the CEP, for example by using an f-2f interferometer. Here, the pulses to be measured are broadened to a bandwidth of at least one octave. A long-wavelength part of the pulse is frequency doubled and the beat note between it and the short-wavelength part of the fundamental pulse is measured. This is better known as the offset phase.
With a phase-locked loop, a property of the laser oscillator such as the optical path length can be adjusted correspondingly to the obtained offset frequency and thus the phase can be stabilized.
