Simulations and experiments showing the origin of multiwavelength mode locking in femtosecond, Yb-fiber lasers

Abstract

A stable and self-starting femtosecond breathing-pulse Yb-fiber oscillator is reported, modelocked using the nonlinear polarisation evolution mechanism. A bifurcation between two distinct modes of operation is demonstrated experimentally, producing pulses with a single central wavelength in one state, or following adjustment of the intra-cavity waveplates, the emission of pulses with three distinct central wavelengths. The maximum bandwidth was 72 nm at the -10 dB level and the pulses were compressible externally to 70 fs with energies of 0.75 nJ. The multi-wavelength pulses reported here are significantly shorter than the pico-second pulses previously observed from similar modelocked multi-wavelength sources. Vector simulations based on the nonlinear Schrödinger equation show that the multi-wavelength behaviour is produced by overdriving the nonlinear polarisation evolution based saturable absorber at the peak of the pulse, leading to transmission of the two wings of the strongly chirped pulse. This new insight shows clearly that the three pulses output in the multi-wavelenght state are coherent. The agreement between simulation and experimental data shows nonlinear polarisation evolution based modelocked fiber lasers are a suitable platform for studying the nonlinear dynamics underlying the bifurcation of the output.

Simultaneous emission of Gaussian-like and parabolic-like pulse waveforms in an erbium-doped dual-wavelength fiber laser

“…The absence of resonant sidebands and the significant chirp of the output pulses indicate that the laser operates in the stretched-pulse regime. The dual-wavelength pulses reported here are significantly shorter than the picosecond pulses previously observed in other dual-wavelength mode-locked fiber lasers 20 – 27 . Figure 6(d) provides the long wavelength spectrum with a FWHM of about 30.2 nm (from 1602.4 nm to 1632.6 nm).…”

“…This view has also been confirmed in ref. 27 , which shows that the multi-wavelength behavior can be induced by variations in the ellipticity of the circular polarization state and an overdrive of the NPE by numerical simulation based on the generalized vector nonlinear Schrödinger equation. Meanwhile, the single circulating pulse mechanism eliminates the effect of group velocity mismatch between two independent pulses, and hence maintains a fixed temporal separation between the pulses corresponding to different wavelengths at the laser output.…”

Direct control of mode-locking states of a fiber laser

“…The technological importance of passively mode-locked fiber lasers is well recognized [1,2]. In addition, mode locking is of fundamental importance, since it is inherently nonlinear [3][4][5] and constitutes a non-equilibrium steady state [6]. The same nonlinearity that makes mode locking possible ultimately limits laser performance.…”

“…2(a)] when the intracavity energy exceeds the level a single pulse can hold. Further increases in nonlinearity lead to a bifurcation; most commonly, the pulse breaks up into multiple pulses or, less commonly, transitions into a noise-like state or into a multiwavelength state [5]. Even before such a bifurcation, the presence of a CW peak leads to an order-of-magnitude increase in intensity fluctuations of the laser [25].…”

Generation of burst pulses through multimodal interference in a passively mode-locked ytterbium fibre-ring laser

“…Multiwavelength operation in PMLFL was studied numerically and experimentally by Feehan et al [29], they proposed that, by the NPE in the SA's operation, various wavelengths can be selected to circulate into the cavity. In our case, results are more related to studies reported for other types of fibres, and wavelengths centred in 1550 and 1980 nm.…”