Amplifying Optical Fiber Non-Uniform Doping LP02 Mode Suppression
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Solution Overview
Problem
Double clad optical fibers that allow multi-mode light propagation suffer from the excitation of higher-order modes like LP02, which lowers the beam quality of output light, as these modes are amplified more than the fundamental LP01 mode, leading to difficulties in focusing and maintaining high beam quality, especially when higher-order modes are axially symmetric.
Innovation Solution
An amplification optical fiber design where the active element is doped at a higher concentration at positions where the intensity of LP02 mode becomes zero, relative to the center, and not uniformly across the core, suppressing the amplification of LP02 mode while enhancing the amplification of LP01 mode, thereby improving beam quality.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Power
If a double clad fiber allowing multi-mode light propagation is used as an amplification optical fiber, then high laser output can be obtained, but beam quality deteriorates due to excitation of higher-order modes such as LP02 mode
Solution Approach 1:
The patent applies local quality by creating a non-uniform doping concentration distribution within the core of the optical fiber. Specifically, the active element is doped at a higher concentration in a first region (central portion) and at a lower concentration in a second region (outer portion), allowing different regions to serve different functions: the central region preferentially amplifies LP01 mode while the outer region provides gain for higher-order modes but with reduced concentration to suppress their excessive amplification.
2Productivity
If active element is doped uniformly across the core, then amplification is provided throughout the core, but higher-order modes are amplified more than LP01 mode leading to poor beam quality
Solution Approach 1:
The patent implements local quality by spatially varying the doping concentration of the active element within the core. The first region (central portion) has higher doping concentration to provide strong amplification for LP01 mode, while the second region (outer portion) has lower doping concentration to reduce amplification of higher-order modes, thereby improving beam quality while maintaining overall amplification efficiency.
3Manufacturing precision
If a mode converter is used to excite only LP01 mode, then beam quality is maintained, but the system complexity increases and it is difficult to avoid exciting axially symmetric higher-order modes like LP02 mode
Solution Approach 1:
The patent extracts the mode selection function from a separate mode converter device and integrates it directly into the amplification optical fiber structure through spatially selective doping. By positioning the high-concentration doping region in the central portion of the core, the fiber itself preferentially amplifies LP01 mode while suppressing higher-order modes, eliminating the need for external mode converters and reducing system complexity.
Solution Approach 2:
The amplification optical fiber performs mode selection autonomously through its internal doping structure. The non-uniform doping concentration distribution creates a gain profile that naturally favors LP01 mode amplification over higher-order modes, allowing the fiber to self-regulate mode composition without requiring external control mechanisms or additional components.
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
The optical fiber amplifier outputs light with higher beam quality by preferentially amplifying LP01 mode over LP02 mode, even when both modes are present, resulting in improved focusing capabilities and reduced higher-order mode amplification.
Implementation Method 1
active element that stimulates to emit light of a predetermined wavelength
Implementation Method 2
amplifying light generated by a seed light source
Implementation Method 3
allows light having a predetermined wavelength to propagate
Implementation Method 4
double clad fiber that allows multi mode light to propagate through a core
Data Source
Figure 1~2
Figure 3(A)~3(C)
Figure 4(A)~4(C)
AI summary
There are provided an amplification optical fiber, and an optical fiber amplifier and a resonator using the same capable of outputting light of high beam quality even when a higher-order mode that is axially symmetric is excited in addition to LP01 mode. An amplification optical fiber 50 includes: a core 51; a clad 52 coating the core 51; and an outer clad 53 coating the clad 52, wherein the core 51 has a larger refractive index than the clad 52, the core 51 allows light having a predetermined wavelength to propagate in at least LP01 mode and LP02 mode, and in the core 51, active element that stimulates to emit light of the predetermined wavelength is doped at a higher concentration at a position where an intensity of the LP02 mode becomes zero than center of the core 51.