Nonlinear polarization dynamics of Kerr beam self-cleaning in a GRIN multimode optical fiber

We experimentally study polarization dynamics of Kerr beam self-cleaning in a graded-index multimode optical fiber. We show that spatial beam cleaning is accompanied by nonlinear polarization rotation, and a substantial increase of the degree of linear polarization.Comment: 5 pages, 6 figure

Mode decomposition of Kerr self-cleaned beams by phase only SLM

Graded-index multimode optical bers have recently attracted a renewed attention, thanks to the discovery of new nonlinear eects, such as Kerr beam self-cleaning. In essence, Kerr self-cleaning involves a ow of the propagating beam energy into the fundamental mode of the ber, accompanied by a redistribution of the remaining energy among high-order modes. Increasing the fundamental mode energy leads to a signicant improvement of the output beam quality. A standard method to determine beam quality is to measure the M2 parameter. However, since self-cleaning involves the nonlinear redistribution of energy among a large number of ber modes, measuring a single beam quality parameter is not sucient to characterize the eect. A properly informative approach requires performing the mode decomposition of the output beam. Mode decomposition permits to evaluate the energy distribution among all of the excited ber modes, which enables investigations of nonlinear mode coupling processes at a qualitatively new level. In this work, we demonstrate an eciency mode decomposition method based on holography, which is suitable for analyzing the self-cleaning eect. In a theoretical study, we describe the solution of the mode decomposition problem for the modes of the gradedindex multimode ber. In an experimental investigation, we demonstrate the decomposition of both low-power (speckled) and self-cleaned beams, involving more than 80 modes

Multicolour nonlinearly bound chirped dissipative solitons

The dissipative soliton regime is one of the most advanced ways to generate high-energy femtosecond pulses in mode-locked lasers. On the other hand, the stimulated Raman scattering in a fibre laser may convert the excess energy out of the coherent dissipative soliton to a noisy Raman pulse, thus limiting its energy. Here we demonstrate that intracavity feedback provided by re-injection of a Raman pulse into the laser cavity leads to formation of a coherent Raman dissipative soliton. Together, a dissipative soliton and a Raman dissipative soliton (of the first and second orders) form a two (three)-colour stable complex with higher total energy and broader spectrum than those of the dissipative soliton alone. Numerous applications can benefit from this approach, including frequency comb spectroscopy, transmission lines, seeding femtosecond parametric amplifiers, enhancement cavities and multiphoton fluorescence microscopy

All-fiber highly chirped dissipative soliton generation in the telecom range

A high-energy (0.93 nJ) all-fiber erbium femtosecond oscillator operating in the telecom spectral range is proposed and realized. The laser cavity, built of commercially available fibers and components, combines polarization maintaining (PM) and non-PM parts providing stable generation of highly chirped (chirp parameter 40) pulses compressed in an output piece of standard PM fiber to 165 fs. The results of the numerical simulation agree well with the experiment. The analyzed intracavity pulse dynamics enables the classification of the generated pulses as dissipative solitons

Experimental Method of Temperature and Strain Discrimination in Polymer Composite Material by Embedded Fiber-Optic Sensors Based on Femtosecond-Inscribed FBGs

Experimental method of temperature and strain discrimination with fiber Bragg gratings (FBGs) sensors embedded in carbon fiber-reinforced plastic is proposed. The method is based on two-fiber technique, when two FBGs inscribed in different fibers with different sensitivities to strain and/or temperature are placed close to each other and act as a single sensing element. The nonlinear polynomial approximation of Bragg wavelength shift as a function of temperature and strain is presented for this method. The FBGs were inscribed with femtosecond laser by point-by-point inscription technique through polymer cladding of the fiber. The comparison of linear and nonlinear approximation accuracies for array of embedded sensors is performed. It is shown that the use of nonlinear approximation gives 1.5–2 times better accuracy. The obtained accuracies of temperature and strain measurements are 2.6–3.8°C and 50–83 με in temperature and strain range of 30–120°C and 0–400 με, respectively

Ultra-Broadband NPE-Based Femtosecond Fiber Laser

A dissipative soliton mode-locked Yb-doped fiber laser is investigated experimentally and numerically from the point of view of generating ultra-broadband ultrashort pulses. An energy up to 2.2 nJ and a spectral bandwidth over 60 nm (at the −10 dB level) were obtained experimentally without dispersion compensation in the cavity. Almost a 100-fold compression coefficient has been achieved, so the resulting pulse duration was 149 fs. The numerical simulation has shown that a further scaling up to 3.5 nJ and a 100 nm spectral bandwidth is possible by reducing the low power transmission coefficient of the NPE-based SAM and increasing the amplification. At the same time, the tolerance of the SAM to a low power radiation is responsible for the transition to a multi-pulse operation regime

Mode decomposition of output beams in LD-pumped graded-index fiber Raman lasers

All-ber Raman lasers have demonstrated their potential for the ecient conversion of highly multimode pump beams into a high-quality Stokes beams. However, the modal content of these beams has not been investigated yet. Here we apply, for the rst time, a mode decomposition technique for revealing intermodal interactions in dierent operational regimes of CW multimode Raman lasers. Our approach allows for analyzing the output laser radiation in terms of the amplitude and phase distributions of a huge number of excited modes for both the pump and the Stokes beams, which enables a new insight into nonlinear mode coupling processes. The measured contribution of the rst three modes of the residual pump beam after overcoming the SRS threshold decreased on average by 25%, whereas the signal beam mainly consists of fundamental mode (40%) and the modes of the rst group (20%)

Mode dynamics during transition into Kerr self-cleaning regime for laser beams propagated in a multimode GRIN fiber

For a long time, interest in multimode (MM) optical fibers has remained limited, owing to the poor quality of laser beams emerging at their output. A typical indicator of beam quality is inversely proportional to the number of excited modes. However, recent intensive research of nonlinear propagation effects, involving high-power laser beams in MM graded-index (GRIN) fibers, has led to the discovery of previously unexpected phenomena, such as the Kerr-activated self-cleaning of a beam [1] . The manifestation of self-cleaning is that most of the beam energy flows into the fundamental mode of the fiber. This process is accompanied by a redistribution of energy towards higher-order modes [2] . Increasing the fundamental mode energy leads to improving the output beam quality. The standard method to determine the beam quality is to measure the M2-parameter (m-squared). This parameter measures the divergence of the beam with respect to the Gaussian beam. However, since self-cleaning involves a nonlinear redistribution of energy carried by a large number of fiber modes, this approach is not entirely correct, and a full mode decomposition (MD) of the output beam appears to be a much more informative method

Mode-resolved analysis of pump and Stokes beams in LD-pumped GRIN fiber Raman lasers

All-fiber Raman lasers have demonstrated their potential for efficient conversion of highly multimode pump beams into high-quality Stokes beams. However, the modal content of these beams has not yet been investigated. In this work, based on a mode decomposition technique, we are able to reveal the details of intermodal interactions in the different operation regimes of continuous wave multimode graded-index fiber Raman lasers. We observed that, above the laser threshold, the residual pump beam is strongly depleted in its transverse modes with principal quantum number below 10. However, the generated Stokes signal beam mainly consists of the fundamental mode, but higher-order modes are also present, albeit with exponentially decreasing population