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.
Highlights
Fourier domain mode-locked (FDML) lasers produce rapidly wavelength-swept light with bandwidths of more than 100 nm at tuning rates in the range of MHz [11, 14,15,16,17, 39]
We show that the intensity trace contains sufficient information to characterize the operation mode and the physical dynamics of the FDML laser, provided that sufficient analog bandwidth is available
It can be observed that the hole duration is not a constant quantity and has an asymmetric distribution around a dominating peak which is in Fig. 8a associated with the bar in the interval [40.625 ps 46.875 ps], containing 240,406 holes which corresponds to 25.24%
Summary
Fourier domain mode-locked (FDML) lasers produce rapidly wavelength-swept light with bandwidths of more than 100 nm at tuning rates in the range of MHz [11, 14,15,16,17, 39]. This is achieved by synchronizing the roundtrip time of the optical field in a ring laser setup with the sweep rate of a tunable Fabry–Pérot (FP) filter, acting as the wavelength tuning element. The high-frequency fluctuations can be classified as various types, such as irregular fluctuations, referred to as a modulational instability [28, 32] and the Eckhaus instability [20], periodic Turing-type formations [28, 32] and socalled holes [27, 33]
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