Multimode Fiber With Double Bottleneck Core For Nonlinearity Suppression
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Solution Overview
Problem
High-power fiber lasers using multimode (MM) fibers face limitations in power scaling due to optical nonlinearities, particularly Stimulated Raman Scattering, which are exacerbated by the excitation of high-order modes (HOMs) and challenges in manufacturing fibers with low numerical aperture and high core diameter, leading to reduced beam quality and sensitivity to bending loads.
Innovation Solution
A MM fiber with a double bottleneck-shaped core and a controllably depressed refractive index profile, designed to minimize HOM excitation and amplification, features a Gaussian-to-ring profile transformation to maintain a high threshold for nonlinearities and ensure predominantly fundamental mode operation, with a dopant profile that provides gain only in the regions of the fundamental mode intensity peaks.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If the core diameter is increased to decrease power density and raise nonlinearity threshold, then the threshold for nonlinearities is improved, but the number of high-order modes that can be excited increases, deteriorating beam quality
Solution Approach 1:
The refractive index profile is designed with a depressed central region and elevated peripheral regions, creating local variations in optical properties. This allows the core to support a larger effective area for fundamental mode propagation while maintaining conditions that suppress high-order mode excitation through strategic index modulation zones.
Solution Approach 2:
The patent employs a specific refractive index profile with a depressed center region (lower index) surrounded by peripheral regions (higher index), rather than a uniform or linearly graded profile. This parameter configuration transforms the mode distribution, enabling the fundamental mode to expand while creating potential well barriers that confine and suppress high-order mode excitation.
2Ease of operation
If the numerical aperture is decreased to reduce HOM excitation, then beam quality is improved, but the power handling capacity and nonlinearity threshold are reduced
Solution Approach 1:
The refractive index profile creates localized regions of different index values - a depressed central region bounded by peripheral regions with higher index. This local differentiation allows the fiber to maintain a low effective numerical aperture for mode control while providing an expanded mode field area for power handling, resolving the contradiction between beam quality and power capacity.
Solution Approach 2:
The patent extends the solution into the radial dimension by creating a multi-region refractive index structure rather than a simple step or linear gradient. This dimensional complexity allows independent optimization of different spatial zones - the central depressed region controls mode confinement while the peripheral elevated regions expand the effective area, achieving both low NA and high power handling.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
This configuration effectively suppresses the amplification of HOMs, enhances the power handling capacity by increasing the mode field diameter, and maintains a high threshold for nonlinearities, allowing for high-power output predominantly in the fundamental mode, thus addressing the limitations of existing MM fiber lasers.
Implementation Method 1
guiding the excited fundamental mode without coupling thereof with high order modes
Implementation Method 2
controllably depressed region of refractive index which is centered along a core axis
Implementation Method 3
double bottleneck-shaped axial cross-section... Gaussian-to-ring profile transformation
Implementation Method 4
dopant profile configured to gainguide substantially a fundamental mode LPoi without coupling thereof with central symmetrical modes
Implementation Method 5
dopant profile configured to provide gain only in the regions of the fundamental mode intensity peaks
Data Source
Figure 1~5
Figure 6~8E
AI summary
A monolithic fiber is configured with a double bottleneck-shaped multimode (MM) core capable of supporting substantially only a fundamental mode at a given wavelength and having opposite end regions, frustoconically shaped transformer regions, which run inwards from the respective end regions, and a central uniformly dimensioned region, which bridges the transformer regions. The MM core has a refractive step-index profile which is configured with a centrally positioned dip having a variable width along the length of the fiber. The width of the dip is relatively small at the end regions of the MM core so as to support only the fundamental mode with a Gaussian profile. As the dip becomes larger along the input transformer region, it gradually shapes the Gaussian profile into the ring profile of the fundamental mode, which is guided along the central region of the MM core. The dip gradually becomes smaller along the output transition region so as to shape the ring profile back into the substantially Gaussian profile of the fundamental mode radiated from the output end region of the MM core. The refractive index profile has a ring area doped with one or more rare-earth elements and configured to amplify substantially only the fundamental mode.