Fig. 2: Swept phase continuity measurements to determine the chirped phase characteristics of Fourier domain mode-locked laser.

The Fourier domain mode-locked (FDML) sweep can be idealized by a sinusoidal sweep in wavelength (a). The continuous wave (CW) laser is a monochromatic laser, represented here by a constant wavelength over time. Thus, around the intersection points, a beat note signal can be measured by a fast oscilloscope. The experimental data of this beat note signal is shown in (b). Due to the repetitive behaviour of the swept source FDML, this beat signal occurs periodically with the roundtrip frequency. c Zoom into one FDML sweep where the beat signal occurs over an optical bandwidth of 0.7 nm (±0.35 nm). This is limited by the electronic detection bandwidth of 63 GHz (see methods). d Zoom into the beat signal reveals the chirped signal with the zero frequency point at approximately 167.4 ns. e Zoom into the middle of the beat signal. The beat signals provide an insight into the swept phase continuity of the FDML laser.