Fig. 3: Analysis of the phase evolution for a single longitudinal mode of the Fourier domain mode-locked (FDML) laser.

Consecutive sweeps (a) were overlaid (b) to gain access to the phase of a single longitudinal mode and its evolution from sweep to sweep. Oscillatory behaviour is observed (c) when plotting the phase evolution at a single time point relative to the sweep starts (here at 167,25 ns). d A fast Fourier transform (FFT) reveals an oscillation frequency of 44,6 kHz. This oscillatory behaviour can be explained by a phase slip between the continuous wave (CW) laser and the FDML laser since the optical frequency of the CW laser is not necessarily an integer multiple of the FDML sweep repetition rate. However, also a carrier-envelope phase slip (CEPS) of the FDML laser could be present and convoluted in this observed oscillation. The smooth nature of the observed phase oscillation indicates a constant phase slip that accumulates linearly over consecutive sweeps. This leads to the conclusion that the FDML laser output represents a comb-like structure.