Experimental signatures of extreme optical fluctuations in lumped Raman fiber amplifiers

Abstract

Abstract:In this work, we experimentally investigate several temporal and spectral methods to highlight extreme fluctuations which can develop during the Raman amplification of an ultrashort pulse train. Forward and backward pumping schemes are compared to dual pass configurations.Keywords: Fiber optics; Raman amplifiers; extreme statistics 2 IntroductionThe study of extreme statistics in nonlinear fiber optics is a topic that has recently stimulated much attention. Following the pioneering work of Solli and coworkers [1] that have reported the observation of statistics analogous to hydrodynamic rogue waves in the formation of optical supercontinuum, most of the initial works have focused on the passive propagation of picosecond pulses in photonic crystal fibers [2]. Those different studies carried out in the anomalous dispersion regime have highlighted the specific role played by modulation instability, Akhmediev breathers [3,4] as well as Raman frequency shifted solitons that are also affected by third-order dispersion and resulting pulse-to-pulse interactions [5,6]. However, observation of statistics that strongly deviate from a Gaussian distribution is absolutely not restricted to supercontinuum generation and can also be observed in telecommunication applications in the context of transmissions [7][8][9], Raman fiber lasers [10,11] as well as discrete amplification [12,13]. More precisely, in this latest field, it has been shown that under certain conditions, statistical distribution of an amplified signal can be strongly reshaped during the amplification process.Examples of this deleterious degradation have been discussed for parametric amplifiers [12,14,15] and for Raman amplifiers [13,16,17]: in both cases, a quasi-instantaneous gain and a low walk-off between the signal and a partially coherent pump lead to a dramatic transfer of fluctuations from the pump to the signal in a co-propagating configuration. In a previous work,we have numerically and analytically described the evolution of a continuous signal in a copropagating amplifier in the presence of a low walk-off and pump depletion [17]. We have also Experimental SetupOur experimental setup is sketched in Fig. 1a. An actively mode-locked erbium-doped fiber laser delivers a train of 2.9 ps pulses at a repetition rate of 10 GHz and with an initial average power of 10 mW (corresponding to a peak power of ~300 mW) at a wavelength of 1550 nm. Let us note that our previous works [13,16] or [19] involved much lower repetition rates as the signal source was a MHz passively mode-locked fiber laser. The pump is delivered by a Raman continuous wave fiber laser centered at 1455 nm with an average power up to 2 W. An essential point is that this pump is a partially incoherent wave (~20 GHz of spectral width) that exhibits high contrast, fast intensity fluctuations [19]. Indeed, its intensity autocorrelation signal (plotted in Fig. 1b) presents a characteristic contrast of one half [13,20] with a temporal width of 25 ps. This temporal duration is a characteristi...

Extreme optical fluctuations in lumped Raman fibre amplifiers

“…We have also highlighted the impact on the phase and spectrum of the amplified signal and investigated the generation and properties of the cascaded Stokes wave. The numerical results presented here have confirmed several experimental observations detailed in ref [27] where a set of unconventional methods were used to qualitatively stress the highly damaging impairments of these pulse-to-pulse fluctuations. The very large range of intensity and phase fluctuations prevents the use of the highly nonlinear discrete Raman amplifier pumped in a co-propagating scheme for telecommunication applications.…”

“…From the RF measurements, no spectral broadening of the comb structure is visible, which confirms that the duration of the pulses after amplification does not radically differ from the input pulse stream. The optical spectrum (Figure 10b) confirms the dramatic drop of the optical signal to noise ratio (as usually measured in the telecommunication context [57]), in agreement with the experimental records detailed in [27]. The optical spectra of the cascaded Stokes wave do not exhibit clear 40 GHz components, which is consistent with the results of Figure 9(b) that have outlined the low correlation between structures delayed of 25 ps.…”

“…This level falls then drastically, which highlights the significant change of amplitude between two successive amplified pulses. Such a behaviour is in full agreement with the various experimental records based on the analysis of a 10 GHz pulse picosecond train and detailed in [27]. Let us stress once again here that due to the breakdown of the Gaussian assumption, the quantitative estimates that can be derived from the analytics developed for example in [54] should be questioned.…”

“…Using a series of temporal and spectral characterizations, we have experimentally confirmed the drastic consequences of the transfer of intensity fluctuations from the pump to a pulsed signal in a lumped Raman amplifier [27]. We have also shown that an adequate frequency shifted optical bandpass filter can isolate the most extreme structures [25].…”

“…More precisely, in this latest field, it has been shown that under certain conditions, statistical distribution of an amplified signal can be strongly reshaped during the amplification process. Examples of this deleterious degradation have been discussed for parametric amplifiers [19,23,24] and for Raman amplifiers [20,[25][26][27][28] : in both cases, a quasi-instantaneous gain and a low walk-off between the signal and a partially incoherent pump lead to a dramatic transfer of fluctuations from the pump to the signal in a co-propagating configuration.…”