High-order harmonic noise-like pulsing of a passively mode-locked double-clad Er/Yb fibre ring laser

Abstract

In this paper, we study noise-like pulse generation in a km-long fibre ring laser including a doubleclad erbium-ytterbium fibre and passively mode-locked through nonlinear polarization evolution. Although single noise-like pulsing is only observed at moderate pump power, pulse energies as high as 120 nJ are reached in this regime. For higher pump power, the pulse splits into several noise-like pulses, which then rearrange into a stable and periodic pulse train. Harmonic mode locking from the 2nd to the 48th orders is readily obtained. At pump powers close to the damage threshold of the setup, much denser noise-like pulse trains are demonstrated, reaching harmonic orders beyond 1200 and repetition frequencies in excess of a quarter of a GHz. The mechanisms leading to noise-like pulse breaking and stable high-order harmonic mode locking are discussed.

High energy noise-like pulsing in a double-clad Er/Yb figure-of-eight fiber laser

“…This can be done in practice by applying twist to the low-birefringence fiber to moderate the effect of the residual birefringence [3][4][5]. The polarization-imbalanced NOLM has been successfully implemented for a number of applications, including passive mode-locking of F8L cavities [4,6,7].…”

“…First demonstrated by Horowitz et al [8], this particular regime of fiber lasers has been drawing attention due to its potential for generating very broad spectra covering hundreds of nanometers [9][10][11], for achieving high energy pulses [7,[12][13][14] and for applications requiring low temporal coherence [8]. NLPs are partially stationary pulses consisting of a bunch of extremely variable ultrashort sub-pulses conserving stable global properties like the total duration of the bunch, its average amplitude, energy and average spectrum.…”

“…Moreover, many aspects of their dynamics and the dependence of pulse properties on laser parameters remain unclear. For example, some authors have reported that changing the pump power affects the energy and the width of the bunch without modifying the average peak power of the pulse or the shape of the optical spectrum [16,19,20]; others observed that, beyond certain duration, a NLP tends to split into multiple pulses as pump power is increased [7]. In terms of pulse energies, high values up to 250 nJ have been reached without pulse breaking [7,[12][13][14].…”

“…Besides, NLPs have been observed in various spectral windows including EDF [6][7][8][9][10][11]13,16,20], YDF [21] and TDF [14,22]. Another remarkable aspect of NLPs is that they have demonstrated better efficiency than solitons in nonlinear processes such as cascaded stimulated Raman scattering [23], nonlinear frequency conversion [23,24] and supercontinuum (SC) generation [25,26], among others.…”

“…It consists in producing a train of periodic pulses at a repetition rate that is an integer multiple of the cavity fundamental frequency [38], which is particularly attractive for optical communications. HML is not the exclusivity of soliton lasers and several cases of HML noise-like pulsing regimes have been observed [7,39,40].…”