Twisted Polygonal Inner Cladding for Fiber Laser Skew Mode Suppression
Find Innovative SolutionsGenerate Solutions
Solution Overview
Problem
Fiber laser devices with a double cladding structure often experience a skew mode where pumping light is reflected at a certain angle within the inner cladding without entering the core, reducing the amount of light pumped into the core and affecting the rare earth element doping, which is not effectively suppressed by existing techniques such as twisting a polygonal cladding into a circular shape.
Innovation Solution
An amplification optical fiber with an inner cladding having a polygonal cross-sectional shape with rounded corners, where the angularity of the outer circumference is set to 0.15 or more, and the inner cladding is twisted to ensure that the pumping light is efficiently directed into the core, thereby suppressing the skew mode and reducing crystallization of the active element.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Ease of manufacture
If the inner cladding has a circular cross-sectional shape, then the fiber structure is simple and easy to manufacture, but pumping light is continuously reflected at a certain angle on the interface between the inner cladding and outer cladding, causing skew mode propagation without entering the core
Solution Approach 1:
The patent applies asymmetry by changing the inner cladding cross-section from a circular shape to a polygonal shape with rounded corners. This asymmetric geometry disrupts the continuous reflection path of skew rays at the inner cladding interface, forcing pumping light to enter the core instead of propagating as skew modes. The angularity parameter (0.15 or more) quantifies this deviation from circular symmetry to ensure effective skew mode suppression.
2Reliability
If the inner cladding cross section is made into a polygonal shape with sharp corners, then skew mode is suppressed, but the corners are rounded by heat during fiber drawing, resulting in a nearly circular shape that fails to suppress skew mode
Solution Approach 1:
The patent applies curvature in a controlled manner by introducing rounded corners at the vertices of the polygonal inner cladding cross-section. This rounded polygonal shape maintains the angularity parameter at 0.15 or more while being thermally stable during fiber drawing. The rounding radius is specifically controlled to preserve the polygonal character needed for skew mode suppression while avoiding the sharp corners that would be rounded away by heat.
Solution Approach 2:
The patent uses parameter changes by defining and controlling the angularity parameter (c) to be 0.15 or more. This dimensionless parameter characterizes the deviation from a circular shape and ensures sufficient polygonal character for skew mode suppression. By setting this specific parameter threshold, the patent provides a quantitative criterion that maintains effectiveness despite thermal rounding during manufacturing.
3Productivity
If the angularity of the inner cladding is increased to suppress skew mode, then more pumping light enters the core, but the manufacturing complexity increases
Solution Approach 1:
The patent applies asymmetry through a rounded polygonal cross-section that provides sufficient angularity (0.15 or more) to suppress skew modes and improve light pumping efficiency. This asymmetric shape is achieved through standard fiber drawing processes with controlled preform geometry, avoiding complex manufacturing steps while maintaining the angular characteristics needed for high pumping efficiency.
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 solution effectively suppresses the skew mode, ensuring that more pumping light is entered into the core, enhancing light amplification and reducing transmission loss by maintaining a higher concentration of active elements without crystallization, thus improving the performance of the amplification optical fiber.
Implementation Method 1
at least a part of pumping light entered to the inner cladding is reflected to the core side on the interface between the inner cladding and the outer cladding
Implementation Method 2
The light pumps the rare earth element doped in the core
Data Source
Figure 1
Figure 2
Figure 3~4
AI summary
An amplification optical fiber includes: a core (11) doped with an active element; an inner cladding (12) surrounding the core (11), the inner cladding (12) having a refractive index lower than a refractive index of the core (11); and an outer cladding (13) surrounding the inner cladding (12), the outer cladding (13) having a refractive index lower than a refractive index of the inner cladding (12), wherein: the inner cladding (12) is twisted about a center axis of the core (11); in a cross section vertical to a longitudinal direction, an outer circumference of the inner cladding (12) has a polygon with rounded corners; and an angularity c defined by Expression (1) and Expression (2) is 0.15 or more and 0.8 or less, A=cosπ/n c=1−d1/d21−A where a number of vertices of the polygon is defined as n, a diameter of a circumcircle (C2) of the outer circumference of the inner cladding (12) is defined as d2, and a diameter of an inscribed circle (C1) of the outer circumference of the inner cladding (12) is defined as d1.