Dynamics of Fourier Domain Mode Locked lasers

We describe and explain the dynamics in the output of a Fourier Domain Mode Locked laser (FDML) both experimentally and numerically using a delay equation model. The laser is formed using a semiconductor optical amplifier and a swept filter placed in a ring cavity. For FDML operation the filter is swept periodically with a period that matches the round trip time in the cavity [1] allowing an output that can be thought of as a train of highly chirped pulses with a fixed phase relationship between successive sweeps. In our experiment the cavity length was approximately 20 km and the filter width was approximately 75 pm. The intensity was analysed with a high frequency photodetector and a real-time oscilloscope with a 12 GHz acquisition bandwidth. We studied the laser output as a function of the detuning of the drive frequency of the filter away from the resonant frequency. There is a pronounced asymmetry in the output at the turning points of the filter sweep and our studies show that this asymmetry has a fundamental origin related to the field-matter interactions in the laser. We also show that the physics of the system can be understood in terms of a simpler system. By considering the FDML in a frame comoving with the filter, one can see that the high frequency operation should be identical to a static filter. By this reasoning, for a small positive detuning ε the “FDML + ε ” operation should be identical to that when the sweep rate is “Static + ε.” We show that this is borne out in the experiment. This could be very useful for understanding the system for very short cavity lengths beyond what is currently achievable for FDML swept sources.