Fig. 1: Setup for beat signal measurements.

The core of the investigation is the superposition of the temperature stabilized Fourier domain mode-locked (FDML) laser (a) and one continuous wave (CW) laser (b) as shown in red. An auxiliary CW laser (c) is superimposed with the first one to simultaneously generate a second beat signal to monitor the short-term linewidth of the CW lasers. One optical spectrum analyser (OSA 1) is used to monitor the spectrum of the FDML laser (d). The transient beat signals between FDML laser and CW laser (e) are measured on a photodiode PD 1 and recorded with a fast real-time oscilloscope (shaded rectangle). The time trace of the second beat signal was measured on the second channel of the oscilloscope with a photodiode PD 2. A live Fourier transform was performed on the signal showing the radio frequency spectrum (f) to estimate an upper limit of the convoluted linewidth of both CW lasers. Only measurements for which this second monitoring signal shows a narrow linewidth were used for the analysis of the FDML laser, because only then we can assume that the CW laser had a sufficiently narrow linewidth to see fluctuations on the FDML laser’s linewidth. For easy tuning of the CW lasers to generate a certain desired beat frequency, their optical spectra (g) were detected with OSA 2. The optical spectrum of both CW lasers corresponds to the shown ~10.3 GHz beat note frequency. SOA = semiconductor optical amplifier, ISO = optical isolator, PC = polarization controller, FP = tunable Fabry-Pérot filter, CFBG = chirped fibre Bragg grating, FC = fibre coupler, PD = photodiode, OSA = optical spectrum analyser.