Self-stabilization mechanism in ultra-stable Fourier domain mode-locked (FDML) lasers

Abstract

Understanding the dynamics of Fourier domain mode-locked (FDML) lasers is crucial in order to determine the physical coherence limits and to find new superior ways for experimental realization. In addition, the rich interplay of linear and nonlinear effects in a laser ring system is of great theoretical interest. Here we investigate the dynamics of a highly dispersion compensated setup where over a bandwidth of more than 100 nm a highly coherent output with nearly shot noise limited intensity fluctuations was experimentally demonstrated, called the sweet spot. We show by numerical simulations that a finite amount of residual dispersion in the fiber delay cavity of FDML lasers can be compensated by the group delay dispersion in the swept bandpass filter, such that the intensity trace exhibits no dips or high frequency distortions which are the main source of noise in the laser. In the same way a small detuning from the ideal sweep filter frequency can be tolerated. Furthermore, we find that the filter's group delay dispersion improves the coherence properties of the laser and acts as a self-stabilizing element in the cavity. Our theoretical model is validated against experimental data, showing that all relevant physical effects for the sweet spot operating regime are included. arXiv:2002.07731v2 [physics.optics]

Coexistence of Intensity Pattern Types in Broadband Fourier Domain Mode Locked (FDML) Lasers

“…4 and 5, the simulation model reproduces the qualitative behavior of the experimental data in Section 2.3 extremely well and is therefore an excellent tool to study the FDML laser dynamics. Yet, differences in the exact shape exist in the case of the local fringes or the quasi-periodic pattern while holes can be reproduced almost exactly, see also [27]. This observation shows that the accuracy of the individual spectral shifts introduced by the SOA, the bandpass filter or the detuning are a bottleneck in modeling FDML lasers, which has not yet been fully addressed in literature in this detail, to the best of our knowledge.…”

“…At a wavelength of 1300 nm (231 THz) and a filter bandwidth with a FWHM of 165 pm (30 GHz), an offset of 0.02 nm (15 GHz) from the peak transmission already causes a power loss of 50 % when the reflected power is absorbed by an isolator. Such losses are compensated by the semiconductor optical amplifier (SOA) gain medium with a fast response time in the order of several tens of ps [27]. This interplay of frequency-shift, gain and loss over a long time scale, i.e.…”

“…Second, the simulation bandwidth is chosen to be 3.45 THz in order to show that no hidden intensity fluctuations exist which could not be captured with the limited measurement bandwidth of 33 GHz and 50 GHz, respectively. We added the effects of carrier heating (CH) and spectral hole burning (SHB) in the SOA gain medium to the model in [27]. A quasi-static approach is used, since these processes have time constants in the order of 100 fs which is much smaller than the temporal extension of the highfrequency fluctuations in FDML lasers.…”

“…The other parameters which are different from [27] are the dispersion coefficients of the optical fiber…”

“…The formation of different patterns can be associated with different positions of the instantaneous lineshape in the spectral transmission window of the tunable bandpass filter. The instantaneous lineshape is defined and computed as in [27], and this approach has also been used to reproduce measured lineshapes [19,39]. The lineshape was averaged over 25 roundtrips to reduce spectral fluctuations.…”