Fourier domain mode locking (FDML) is a recently developed technique for lasers to generate ultra-rapid wavelength sweeps, equivalent to a train of extremely chirped pulses. FDML lasers are the light sources of choice for fastest megahertz optical coherence tomography (MHz-OCT). Measuring the coherence properties of FDML lasers is of particular importance for the image quality in OCT but it is also crucial to develop a better understanding of this unconventional mode locking mechanism. Usually, experiments to analyze the phase stability of FDML lasers use interferometers to generate interference of a single laser by delaying a part of the output to generate a beat signal. Here, for the first time, we present real time beat signal measurements between two independent FDML lasers over the entire sweep range of ~5 THz width for more than 80 roundtrips (~200 μs), evaluate their phase stability and explain the consequence for our understanding of the FDML mechanism. Beat signal measurements allow direct access to the phase difference between the FDML lasers and therefore the difference in timing of the circulating sweeps as well as their instantaneous frequency.     «

Fourier domain mode locking (FDML) is a recently developed technique for lasers to generate ultra-rapid wavelength sweeps, equivalent to a train of extremely chirped pulses. FDML lasers are the light sources of choice for fastest megahertz optical coherence tomography (MHz-OCT). Measuring the coherence properties of FDML lasers is of particular importance for the image quality in OCT but it is also crucial to develop a better understanding of this unconventional mode locking mechanism. Usually,...     »