Intensity pattern types in broadband Fourier domain mode-locked (FDML) lasers operating beyond the ultra-stable regime

Abstract

We report on the formation of various intensity pattern types in detuned Fourier domain mode-locked (FDML) lasers and identify the corresponding operating conditions. Such patterns are a result of the complex laser dynamics and serve as an ideal tool for the study of the underlying physical processes as well as for model verification. By numerical simulation we deduce that the formation of patterns is related to the spectral position of the instantaneous laser lineshape with respect to the transmission window of the swept bandpass filter. The spectral properties of the lineshape are determined by a long-term accumulation of phase-offsets, resulting in rapid high-amplitude intensity fluctuations in the time domain due to the narrow intra-cavity bandpass filter and the fast response time of the semiconductor optical amplifier gain medium. Furthermore, we present the distribution of the duration of dips in the intensity trace by running the laser in the regime in which dominantly dips form, and give insight into their evolution over a large number of roundtrips.

Influence of the linewidth enhancement factor on the signal pattern of Fourier domain mode-locked lasers

“…Ideally, FDML lasers should offer a distortion free output signal pattern. However, due to the internal dynamics of the semiconductor optical amplifier (SOA), irregular dips and fluctuations occur in the output signal pattern of FDML lasers [5][6][7]. The complete physical mechanism behind the formation of these power fluctuations is rather complicated due to the complexity of the overall system dynamics as imposed by the nonlinear electrical response of the SOA, the chromatic dispersion and self-phase modulation within the long optical fiber, and the time-varying frequency response of the tunable Fabry-Perot filter for the cyclic filtering of each spectral component within the gain-bandwidth of the SOA.…”

“…As the number of roundtrips in an FDML laser cavity increases, the frequency of power-dips may gradually increase and eventually push the system out of stability. Consequently, the signal to noise ratio is likely to keep on decreasing as the number of roundtrips increases, unless certain stability measures are taken [5,7,8]. The FDML laser dynamics are mainly determined by the dynamics of the SOA; therefore, the power-dips should originate due to the nonlinear SOA active region dynamics [6].…”

“…We intend to compute the effect of the LWEF on the quantity, strength, and duration of the dips. Measuring the duration of the dips may not always be very straightforward as some of the dips are not isolated from each other but rather occur intricately and interferingly [5,7], thereby forming a complex pattern of fluctuations that may last for a much longer duration within a single roundtrip. Therefore, rather than focusing only on the duration of the dips, a concurrent investigation of the correlation of the LWEF with the number and strength of th e dips is more useful.…”

“…Notably, as the number of roundtrips in the FDML laser cavity increases, dips of longer duration with complex patterns (Fig. 1) may arise [5,7].…”

“…Figures 1 and 2 illustrate the occurrence of power-dips based on the simulation parameters given in Table 1 [5,7]. The identification of the dips and their counting along the signal envelope is based on the computation of the standard deviation in optical power within a sliding frame of M grid-points (Eq.…”