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In the evolving landscape of laser technology, multimode optical fiber lasers emerge as key players, promising to enhance the power of fiber lasers while maintaining beam quality. This innovation not only holds significance for applications in defense against low-cost drones but also extends its reach to areas like remote sensing. The primary challenge lies in overcoming nonlinear optical effects that can compromise the smooth output beam. Most techniques mitigating these effects are confined to single-mode fibers, but a recent breakthrough showcases the potential of multimode fibers in overcoming this limitation.
High-power fiber lasers have revolutionized various industries, ranging from manufacturing and defense to aerospace and healthcare. Their ability to deliver kilowatt (kW) power levels with exceptional beam quality and efficiency has made them a preferred choice for many applications. However, as power levels increase, challenges such as Stimulated Brillouin Scattering (SBS) and Thermal Mode Instability (TMI) arise, affecting the laser’s performance and reliability.
The Challenge: Nonlinear Optical Effects
The primary hurdle in achieving high-power delivery through optical fibers is the presence of nonlinear optical effects, notably stimulated Brillouin scattering (SBS). SBS is a process where light scatters due to acoustic phonons, limiting the maximum power transmitted through the fiber. Traditionally, suppressing SBS has involved strategies like enlarging the fiber core to reduce optical intensity. However, this approach introduces complications, such as the generation of random speckles, hindering the production of a clean beam suitable for certain applications.
Multimode Fiber: A Game-Changing Solution
In a paradigm shift, researchers have demonstrated that wavefront shaping of coherent input light to a highly multimode fiber can substantially increase the power threshold for SBS.
Researchers from the University of South Australia (UniSA), the University of Adelaide (UoA) and Yale University have demonstrated the potential use of multimode optical fibre to scale up power in fibre lasers by three-to-nine times but without deteriorating the beam quality so that it can focus on distant targets. The breakthrough is published in Nature Communications.
This breakthrough involves shaping the input wavefront of a narrowband signal to excite numerous fiber modes, effectively increasing the SBS threshold while focusing the transmitted light to a diffraction-limited spot. The key advantage is that this method does not compromise the narrow linewidth and high output-beam quality required for practical applications.
Experimental Validation of Multimode Excitation
The experimental setup involved varying the number of excited modes in a multimode fiber and measuring the corresponding SBS threshold. Using a continuous-wave fiber laser at 1064 nm, the researchers launched the signal into a multimode fiber with a 20-μm core diameter. Initially exciting only the fundamental mode (FM), the transmitted power exhibited a linear increase until reaching the SBS threshold. The key finding was that selectively exciting multiple modes in the multimode fiber significantly increased the SBS threshold while maintaining high output-beam quality.
Advantages Over Previous Approaches
Prior methods to suppress SBS predominantly focused on single-mode fibers, utilizing techniques like spectral broadening of input light or modifying fiber geometry. However, these approaches often sacrificed certain qualities crucial for practical applications, such as spectral and coherent beam combining. The multimode excitation technique presented in this research overcomes these limitations, offering an efficient, robust, and broadly applicable method for mitigating detrimental nonlinear effects in optical fibers.
Future Applications and Implications
This groundbreaking approach opens doors for further power scaling of high-power fiber systems, with potential applications in directed energy, remote sensing, and gravitational-wave detection. The ability to increase power thresholds while maintaining beam quality positions multimode optical fiber lasers as a versatile and powerful technology for various fields. As research progresses, the practical implications of this innovation are likely to unfold, ushering in a new era in laser technology with profound implications for defense and beyond.
