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Amplitude noise and coherence degradation of femtosecond supercontinuum generation in all-normal-dispersion fibers

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Abstract

Supercontinuum (SC) generation via femtosecond (fs) pumping in all-normal-dispersion (ANDi) fiber is predicted to offer completely coherent broadening mechanisms, potentially allowing for substantially reduced noise levels in comparison to those obtained when operating in the anomalous dispersion regime. However, previous studies of SC noise typically treat only the quantum noise, typically in the form of one-photon-per-mode noise, and do not consider other technical noise contributions, such as the stability of the pump laser, which become important when the broadening mechanism itself is coherent. In this work, we discuss the influence of the amplitude and pulse length noise of the pump laser, both added separately and combined. We show that for a typical mode-locked laser, in which the peak power and pulse duration are anticorrelated, their combined impact on the SC noise is generally smaller than in isolation. This means that the supercontinuum noise is smaller than the noise of the mode-locked pump laser itself, a fact that was recently observed in experiments but not explained. Our detailed numerical analysis shows that the coherence of ANDi SC generation is considerably reduced on the spectral edges when realistic pump laser noise levels are taken into account.

© 2019 Optical Society of America

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Figures (4)

Fig. 1.
Fig. 1. (a) Measured (dashed blue) and modeled (solid black) dispersion profile, and fiber losses (solid red) of the NL-1050-NE-PM ANDi PCF. (b) Numerical SC spectra generated in 1 m of ANDi fiber with a 1054 nm pump with an average peak power and pulse duration of P0=100kW and T0=50fs, respectively. An amplitude noise of 0.5% was used, corresponding to a pulse duration noise of 0.4%.
Fig. 2.
Fig. 2. (a) Average spectral coherence |g12| of SC pulses generated with P0=100kW peak power pump pulses as a function of pump pulse duration T0 and propagation distance for an amplitude noise value of 0.3% (pulse duration noise 0.24%). The dotted line indicates the limit |g12|=0.9. (b) Limit |g12|=0.9 for a range of amplitude noise values from 0.1% to 1% (pulse duration noise 0.08%–0.8%).
Fig. 3.
Fig. 3. (a) RIN profiles for different amplitude noise values and mean spectral profile out of 1 m of ANDi fiber pumped with 100 kW peak power, 50 fs long pulses at 1054 nm. (b) Evolution of the RIN along the fiber length for an amplitude noise value of 0.5% (pulse duration noise 0.4%). Note: The color map has a dynamic range limited to a RIN equal to 0.5%, meaning RIN data is only visible for wavelengths 800–1430 nm.
Fig. 4.
Fig. 4. (a) RIN profile (blue line) and mean spectrum (red line) of an ensemble of 20 pulses after 10 cm of fiber with 100 kW peak power, 50 fs pulse duration at 1054 nm for an amplitude noise value of 0.5% (pulse duration noise of 0.4%). (b) RIN spectrum as a function of the input noise: OPM only (black line), amplitude noise plus OPM noise (red line), pulse duration noise plus OPM (pink line), and amplitude noise plus pulse duration noise and OPM (blue line).

Equations (6)

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Az=α(ω)2A+k2ik+1k!βkkATk+iγ(1+iτ0T)(A+R(T)|A(z,TT)|2dT),
δT0=0.8*(δAN1),
A(0,t)=P0δANsech(t/(T0(10.8(δAN1))))+F1{δQN},
|g12(ω)|=|A˜i*(ω)A˜j(ω)ij|A˜i(ω)|2|A˜j(ω)|2|,|g12|=0|g12(ω)||A˜i(ω)|2dω0|A˜i(ω)|2dω,
RIN(ω)=(|A˜(ω)|2|A˜(ω)|2)2/|A˜(ω)|2.
|A˜(ω)|2=P0T02|0+sech(x)(ei(ωω0+Δω)T0x+ei(ωω0Δω)T0x)dx|2,
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