Random distributed feedback fibre lasers are well known type of fiber lasers where the optical feedback is organized via amplified Rayleigh scattering on random in space sub-micron refractive index inhomogenities1. Random distributed feedback fiber lasers found their applications in telecommunications and distributed sensing systems, as well as attracted considerable amount of interest from researches2. It is well-known that the generation spectrum of random distributed feedback fiber laser is a wide spectrum of typical width of 1 nm. It can be specifically tailored to demonstrate multiwavelength, tunable operation etc. However, the main features of the generation spectrum should be defined by the nature of the feedback itself. Usually the smooth bell-shaped spectrum is attributed to the incoherent nature of the feedback3. It is well known however that the Rayleigh scattering is an elastic scattering and should be resulted in the coherent feedback, which in turn leads to narrow features in the generation spectrum. Recently, narrow modes have been observed in the generation of the random distributed feedback fiber laser by means of scanning Fabry-Perot interferometer4. The spectral width of modes was about tens of picometers and was limited by the spectral resolution of the scanning interferometer. Those modes were attributed to the stimulated Brillouin scattering.
The experimental investigation of formation of coherent structures from noise is essential for fundamental understanding of nonlinear systems. Here, we present switch-on dynamics in bidirectional mode-locked laser in spatio-temporal and frequency domains by using Dispersive Fourier Transform for both clockwise and counterclockwise directions. We have calculated cross-correlation of counter-propagating beams that reveals the dissimilarities between formation of the spectra of counter-propagating pulses. From cross-correlation we revealed periodic patterns which manifest complex exchange dynamics between the counter-propagating pulses at different stages of their mutual formation. These results will help to understand complex soliton dynamics and nonlinear systems in general.
We demonstrate a single pixel, compressed sensing (CS) approach for fibre Bragg interrogation. We reduce optical source and detector complexity by employing a broadband ASE source and a single-pixel detector respectively. Reconstruction is aided by convex optimisation, which provides the requisite sampling compression, yielding reconstruction accuracies approaching those of uncompressed measurements.
In present paper correlations between different parts of spectrum of a fiber laser with randomly distributed feedback (RDFL) were experimentally measured directly. Implemented statistical analysis demonstrate weak cross-correlations between different lines in generation spectrum. These correlations were vizualized by plotting 2-D probability density functions. Linear correlation coefficient (Pearson coefficient) was calculated for each pair of spectrum lines.
Random fibre lasers constitute the class of random lasers, where the feedback is provided by amplified Rayleigh scattering on sub-micron refractive index inhomogenities randomly distributed over the fibre length. It is known than the nature of Rayleigh scattering is elastic. However, as the laser generates a smooth spectra, the feedback mechanism in random fibre lasers has been assumed to be incoherent. In the present talk we will use a real-time spectral measurement technique based on a scanning Fabry-Perot interferometer to reveal fast dynamics of the random fibre laser spectrum. We observe long-living narrowband components in the generation spectrum, and make a statistical analysis of a large number single-scan spectra to reveal a preferential interspacing between narrow-components. Further, we will discuss the results of advanced real-time spectral measurements via heterodyne-based measurements. We will show that ultra-narrow spectral components (with spectral width down to 1 kHz) are generated. The existence of such narrowband spectral components, together with their observed correlations, establishes a long-missing parallel between the fields of random fibre lasers and conventional random lasers.
We present recent results on measurements of intensity spatio-temporal dynamics in passively mode-locked fibre laser. We experimentally uncover distinct, dynamic and stable spatio-temporal generation regimes of various stochasticity and periodicity properties in though-to-be unstable laser. We present a method to distinguish various types of generated coherent structures, including rogue and shock waves, within the radiation by means of introducing of intensity ACF evolution map. We also discuss how the spectral dynamics could be measured in fiber lasers generating irregular train of pulses of quasi-CW generation via combination of heterodyning and intensity spatio-temporal measurement concept.
In the present paper we numerically study instrumental impact on statistical properties of quasi-CW Raman fiber laser
using a simple model of multimode laser radiation. Effects, that have the most influence, are limited electrical bandwidth
of measurement equipment and noise. To check this influence, we developed a simple model of the multimode quasi-
CW generation with exponential statistics (i.e. uncorrelated modes). We found that the area near zero intensity in
probability density function (PDF) is strongly affected by both factors, for example both lead to formation of a negative
wing of intensity distribution. But far wing slope of PDF is not affected by noise and, for moderate mismatch between
optical and electrical bandwidth, is only slightly affected by bandwidth limitation. The generation spectrum often
becomes broader at higher power in experiments, so the spectral/electrical bandwidth mismatch factor increases over the
power that can lead to artificial dependence of the PDF slope over the power. It was also found that both effects
influence the ACF background level: noise impact decreases it, while limited bandwidth leads to its increase.
Multiwavelength lasing in the random distributed feedback fiber laser is demonstrated by employing an all fiber Lyot filter. Stable multiwavelength generation is obtained, with each line exhibiting sub-nanometer line-widths. A flat power distribution over multiple lines is also obtained, which indicates the contribution of nonlinear wave mixing towards power redistribution and equalization in the system. The multiwavelength generation is observed simultaneously in first and second Stokes waves.