GRIN Lens Mode Filter for Multimode Fiber Spatial Filtering
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Solution Overview
Problem
Large mode area fibers used in high power lasers and amplifiers become multimoded, leading to interference with fundamental signal modes and increased nonlinearity due to the propagation of higher order spatial modes, which existing methods like coiling and tapering fail to effectively address without causing additional issues like reduced mode area and fragility.
Innovation Solution
A mode filter utilizing the Fourier transform property of a graded index (GRIN) lens in combination with a pinhole element, either fiber-based or bulk optic, to selectively block higher order modes by shifting them away from the optical axis, allowing only the fundamental mode to propagate through a small core fiber.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Area of moving object
If the mode area of fibers is increased to reduce signal impairment and increase energy storage capacity, then the fiber becomes multimoded which causes higher order modes to interfere with the fundamental signal mode, altering beam center of mass and increasing minimum focused spot size
Solution Approach 1:
The invention segments the optical signal into different spatial modes using a GRIN lens that performs Fourier transformation, separating the fundamental mode from higher order modes in the spatial frequency domain. This allows selective filtering of higher order modes while preserving the fundamental mode, thus maintaining signal quality in large mode area fibers.
Solution Approach 2:
A GRIN lens acts as an intermediary element that transforms the spatial distribution of optical modes. The lens converts higher order modes into distinct spatial frequencies that can be selectively blocked by a pinhole aperture, enabling mode filtering without directly modifying the fiber structure.
2Reliability
If coiling and tapering LMA fibers are used to strip away higher order modes, then higher order modes are removed, but tight bending shifts the mode away from the fiber axis reducing mode area and increasing nonlinearity, and the tapered fiber section is fragile requiring special packaging
Solution Approach 1:
The invention replaces mechanical fiber manipulation methods (coiling, tapering) with an optical field-based filtering approach. A GRIN lens performs Fourier transformation of the optical field to separate modes spatially, and a pinhole aperture blocks higher order modes optically, avoiding mechanical stress and fragility associated with fiber deformation.
Solution Approach 2:
The GRIN lens serves as an intermediary that enables mode filtering through optical field transformation rather than mechanical fiber manipulation. This intermediary approach allows higher order mode removal without physically altering the fiber structure, maintaining fiber strength and eliminating packaging requirements.
3Reliability
If fiber taper lengths are used to remove higher order modes, then higher order modes are stripped, but the long taper length results in increased nonlinearity from propagation in small mode area
Solution Approach 1:
The invention segments the propagation process into two distinct stages: mode separation in the GRIN lens (Fourier domain) and selective mode transmission through the pinhole aperture. This eliminates the need for long taper sections where energy would be lost to nonlinear effects during extended propagation in small mode areas.
Solution Approach 2:
The invention substitutes the mechanical fiber tapering process with an optical field transformation approach using a GRIN lens. This replacement achieves mode filtering in a compact configuration without the extended propagation path that causes nonlinear energy loss in tapered fibers.
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
Effectively removes higher order modes with low fundamental mode loss, reducing nonlinearity and maintaining energy storage capacity, while being more robust and efficient than existing methods by periodically filtering unwanted modes along the fiber length.
Implementation Method 1
the use of the Fourier transform property of a graded index (GRIN) lens, in combination with a pinhole element, to block further propagation of higher order modes
Implementation Method 2
a pinhole element preferably comprises a small core fiber coupled to the output of the GRIN lens to collect only that portion of the optical signal propagating along the optical axis
Data Source
AI summary
A mode filter for eliminating the propagation of higher-order modes along a section of optical multimode fiber comprises a graded index (GRIN) lens, preferably of a quarter-pitch length, and a pinhole element in the form of a small core fiber. This configuration creates a Fourier spatial filter assembly that removes higher order modes propagating along an optical fiber while capturing the fundamental mode of the optical signal. A section of GRIN fiber is preferably used as the lens, with the small core fiber disposed at the output of the GRIN fiber lens to collect substantially only the on-axis fundamental mode of the optical signal. Since the higher order modes are shifted away from the origin by the GRIN fiber lens, only the fundamental mode signal is captured by the small core fiber.