Spatiotemporal mode-locking in multimode fiber lasers (1705.05050v2)

Abstract: A laser is based on the electromagnetic modes of its resonator, which provides the feedback required for oscillation. Enormous progress has been made in controlling the interactions of longitudinal modes in lasers with a single transverse mode. For example, the field of ultrafast science has been built on lasers that lock many longitudinal modes together to form ultrashort light pulses. However, coherent superposition of many longitudinal and transverse modes in a laser has received little attention. The multitude of disparate frequency spacings, strong dispersions, and complex nonlinear interactions among modes greatly favor decoherence over the emergence of order. Here we report the locking of multiple transverse and longitudinal modes in fiber lasers to generate ultrafast spatiotemporal pulses. We construct multimode fiber cavities using graded-index multimode fiber (GRIN MMF). This causes spatial and longitudinal mode dispersions to be comparable. These dispersions are counteracted by strong intracavity spatial and spectral filtering. Under these conditions, we achieve spatiotemporal, or multimode (MM), mode-locking. A variety of other multimode nonlinear dynamical processes can also be observed. Multimode fiber lasers thus open new directions in studies of three-dimensional nonlinear wave propagation. Lasers that generate controllable spatiotemporal fields, with orders-of-magnitude increases in peak power over existing designs, should be possible. These should increase laser utility in many established applications and facilitate new ones.

• The paper demonstrates how integrating spatial and spectral filtering with nonlinear polarization rotation stabilizes both transverse and longitudinal modes in multimode fiber lasers.
• It employs coupled nonlinear Schrödinger equations for numerical modeling to predict and verify the formation of diverse ultrashort spatiotemporal pulse shapes.
• Experimental results confirm stable pulse energies (5-40 nJ) and highlight the potential for high-power multimode lasers in telecommunications and advanced laser applications.

Overview of Spatiotemporal Mode-Locking in Multimode Fiber Lasers

The research paper "Spatiotemporal Mode-Locking in Multimode Fiber Lasers," authored by Logan G. Wright, Demetrios N. Christodoulides, and Frank W. Wise, presents a detailed examination of multimode fiber lasers and their innovative capability to stabilize and lock both spatial and temporal modes. This paper advances the understanding of laser dynamics, exploring the coalescence of longitudinal and transverse modes through spatiotemporal mode-locking (STML) in fiber lasers.

The authors take a comprehensive approach to address the challenge of coherent superposition of modes in laser resonators. By integrating modal and chromatic dispersions with robust spatial and spectral filtering, they achieve the locking of multiple transverse and longitudinal modes to generate ultrashort light pulses featuring diverse spatiotemporal profiles. This technique primes multimode fiber lasers as a fertile domain for exploring nonlinear wave propagation while expanding the potential applications of laser technology.

Key Technical Insights

A primary focus of the paper is the investigation of three-dimensional electromagnetic field patterns within the laser resonator. The paper reveals a self-organized state where ultrashort pulses inherit various spatiotemporal shapes under multimode operation. Key factors enabling STML include spatial and spectral filtering and the strategic management of nonlinear polarization rotation acting as saturable absorbers. Through this, the research successfully demonstrates the coherence and stability of these pulses under a well-controlled experimental setup using Yb-doped gain fibers and passive graded-index multimode fibers.

The numerical modeling of this system, using a set of coupled nonlinear Schrödinger equations (NLSEs), enriches the understanding of pulse dynamics and helps to predict the formation of varied spatiotemporal pulse shapes. The authors provide a framework for simulating complex multimode laser behavior, evidencing stable states of spatiotemporally-mode-locked pulses as predicted by the simulations.

Experimental and Numerical Results

Experimental results corroborate the numerical simulations, revealing a rich variety of stable spatiotemporal pulses with simulations showing consistency in both temporal and spatial evolution of these pulses. The experiments also demonstrate the significance of filtering in managing the coupling between different types of laser modes, thereby achieving the desired spatiotemporal steady-state conditions. The paper reports pulse energies ranging from 5 to 40 nJ, showing that such complexity can be controlled at routinely available pump power levels.

Implications and Future Directions

The implications of this research are profound both in the theoretical exploration of wave propagation in multimode settings and in practical applications. The efficiency of large-mode-area multimode fibers in reaching high-power operations suggests potential utility in telecommunications through spatial division multiplexing and offers a platform for high-power laser development.

Moreover, the capability of these systems to support complex light field manipulations expands the horizon for frequency comb technology and coherent light field applications. The paper opens avenues for investigating related phenomena within microresonator systems, potentially leading to novel applications in optical communications and laser design.

Overall, this paper marks a significant step forward in understanding and harnessing multimode laser systems. Future research directions may encompass refining these lasers for higher power applications while ensuring environmental stability and exploring potential for phase control in 3D laser dynamics. As STML gains theoretical and practical traction, it promises to fuel advancements across numerous sectors relying on laser technology.