We present direct real-time experimental measurements and numerical modeling of temporal and statistical properties for the Ytterbium-doped fiber laser with spectral bandwidth of ~2 GHz. The obtained results demonstrate nearly exponential probability density function for intensity fluctuations. A significant decrease below the Gaussian probability has been experimentally observed for intensity fluctuations having value more than 2.5 of average intensity that may be treated as indication of some mode correlations.
Recently, temporal and statistical properties of quasi-CW fiber lasers have attracted a great attention. In particular, properties of Raman fiber laser (RFLs) have been studied both numerically and experimentally [1,2]. Experimental investigation is more challengeable, as the full generation optical bandwidth (typically hundreds of GHz for RFLs) is much bigger than real-time bandwidth of oscilloscopes (up to 60GHz for the newest models). So experimentally measured time dynamics is highly bandwidth averaged and do not provide precise information about overall statistical properties. To overpass this, one can use the spectral filtering technique to study temporal and statistical properties within optical bandwidth comparable with measurement bandwidth [3] or indirect measurements [4]. Ytterbium-doped fiber lasers (YDFL) are more suitable for experimental investigation, as their generation spectrum usually 10 times narrower. Moreover, recently ultra-narrow-band generation has been demonstrated in YDFL [5] which provides in principle possibility to measure time dynamics and statistics in real time using conventional oscilloscopes.In this work we experimentally study in real-time temporal and statistical properties of narrow-band YDFL radiation in full optical bandwidth. At the same time, full numerical modeling of time dynamics and statistical properties is performed for the first time of our knowledge. Experimental and numerical data are in good qualitative agreement.The YDFL based on 4 meter long cavity delivers up to 3.5W of output power within only 4GHz (7 pm) of optical bandwidth being still highly multimode (hundreds of different longitudinal modes are generated). Thus we can measure intensity dynamics in real-time in full optical bandwidth that have not been done previously for any quasi-CW fiber laser. Experimentally measured intensity evolution reveals high contrast intensity fluctuations being as high as several mean intensity value, Fig. 1a. The typical fluctuation scale is about 300 ps, Fig.1b. Experimentally measured intensity statistics, Fig.1c, reveals partial mode correlations as it is not completely exponential similar to statistics in quasi-CW RFLs [6].We also numerically model a laser using an NLSE-based model combined with material equations [7], and calculate, for the first time to our knowledge, the temporal and statistical properties of YDFL. Modelling demonstrates the same stochastic nature of the YDFL radiation as observed in experiment. Fig. 2 (a) Time dynamics of output radiation in experiment and simulation (pump power equals to 6.88 W); (b) Intensity autocorrelation at 6.88 W in experiment and simulation; (c) Intensity PDF at 6.88 W in experiment and simulation.
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