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

Abstract

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.

Mode decomposition method for investigating the nonlinear dynamics of a multimode beam

“…It has been established that in the output radiation the fraction of the LG1,0 and LG−1,0 modes is from 20 to 30%, and the distribution itself is close to exponential (see Fig. 2 on the right), which explains the significant deviation of the M 2 parameter from unity [7].…”

“…SLM is a device, usually controlled by a computer, that imposes some form of spatial modulation (amplitude, phase, or amplitude-phase) on radiation. Existing works on mode decomposition included beam analysis at the output of fewmode fibers, but we were able not only to adapt this method for use in the case of fibers with a large number of modes and a parabolic refractive index profile [4], but to show its applicability in significantly different configurations [5][6][7].…”

Mode Decomposition Method for Investigating the Nonlinear Dynamics of a Multimode Beam

“…In particular, a bandpass filter was added, in order to separately analyze the pump and the Stokes beams. A full description of the setup can be found in [49]. In Fig.…”

“…5). It is clear that the first 5-7 principal quantum numbers (PQNs) are involved in the depletion process, which indicates the presence of significant interactions between the pump modes with different quantum numbers and the Stokes beam [49].…”

Метод Модовой Декомпозиции Для Исследования Нелинейной Динамики Многомодового Излучения

“…SLM -это прибор, обычно управляемый с помощью компьютера, который накладывает определенную форму пространственной модуляции (амплитудной, фазовой или амплитудно-фазовой) на излучение. Существующие работы по модовой декомпозиции включали в себя анализ пучка на выходе из маломодовых волокон, но нам удалось не только адаптировать этот метод для использования в случае волокон с большим числом мод и градиентным профилем показателя преломления [4], но показать его применимость в существенно различных конфигурациях [5][6][7].…”