The phase shift between the carrier and the envelope of femtosecond pulse significantly affects nonlinear optical processes, such as the generation of attosecond pulses . The use of a heterodyne laser, whose frequency matched with the low-frequency range of the femtosecond laseremission spectrum, and its second harmonic, coincided with the high-frequencyone, made it possible to eliminate the offset of its frequency comb . Same result was achieved with the use of an acousto-optic modulator . In both cases, the phase shift between the envelope and the carrier has an arbitrary uncontrolled value.
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