Fig. 4: All-Mode phase evolution experimental data.

By analysing 83 consecutive sweeps, it is possible to analyse the phase evolution over all modes and many sweeps. We overlaid successive sweeps (a), generated single-mode phase evolutions (b) and calculated the fast Fourier transform (FFT)s (c) for each timepoint similar to the analysis in Fig. 3. Here, four exemplary modes are indicated by coloured circles. The phase evolution for all modes and all times is presented in (d) as a plot of the FFT frequency maximum over the time points. This plot reveals that the individual modes have a distinct and smooth phase relation between each other and over consecutive sweeps. Thus, we deduce that the Fourier domain mode-locked laser (FDML) laser output represents a well-defined wavelength sweep with a continuously chirped phase light field both along a sweep (i.e. from mode-to-mode) and over multiple sweeps (i.e. from sweep-to-sweep). e We repeated this measurement several times and found that the phase always has a connected and locally smooth shape apart from a step-like resolution artefact which is sometimes visible and is likely caused by the limited frequency resolution due to the finite record length of the measurement (see Supplementary Note 4 for discussion). We also found that the averaged frequency value drastically changes from measurement to measurement even if all parameters were the same. That is due to continuous wave (CW) laser mode hops in between measurements. Slight changes within the frequency evolution could be traced back to changes in the FDML light field. Larger changes between different measurements could be traced back to small alterations of the CW ring laser wavelength.