Intensity Noise of Normal-Pumped Picosecond Supercontinuum Generation

The noise properties of supercontinuum (SC) sources are of importance in many applications. This intensity noise of the SC arises from nonlinear amplification of the input-pulse shot noise and the spontaneous Raman scattering down the fiber [1]. Low noise femtosecond SC generation (SCG) has previously been demonstrated [2]. However, in commercial SC sources, the SC is generated by pumping with picosecond to nanosecond pulses and the SCG is thus initiated by modulation instability (MI). Therefore, the SC is characterized by low coherence and high shot-to-shot fluctuations, in particular at the spectral edges [3–5]. However, the influence of the noise properties when pumping in the normal dispersion regime has so far not been investigated for long pulse pumping. In this work we have measured and compared the relative intensity noise (RIN) properties of long-pulse MI initiated SCG in three different regimes: (i) all-normal SCG, (ii) normal-pumped SCG where higher-order Raman lines occur in the anomalous dispersion regime, and (iii) anomalous-pumped SCG. We show that the noise properties for the three regimes are similar: when increasing the input pump power, the intensity noise will continuously decrease for a given wavelength, and for a given input power the intensity noise will be lowest at the pump wavelength and increase when moving towards the spectral edge [6].

A high-power ytterbium laser, which delivers 10 ps pulses at 1064 nm at a repetition rate of 80 MHz was used to generate the SC in two kinds of photonic crystal fibres (PCFs); 10 m of LMA-15 (ZDW=1240 nm) and 10 m of SC-5.0-1040 (ZDW= 1040 nm). The dispersion profiles of the PCFs are shown in Fig. 1(a) and the power spectral density (PSD) as a function of power for normal-pumped SCG is shown in Fig. 1(b). The RIN was measured for different wavelengths (Fig. 1(c)) by passing the SC through bandpass filters (FWHM=10 nm) and detect the filtered SC by a low noise photodetector connected to an electrical spectrum analyzer [4,5]. In Fig. 1 (d-e) the spectra and the RIN of normal pumped SCG are compared to anomalous pumped SCG.