RFOG Bias Error Reduction via Spatial Mode Filtering
Find Innovative SolutionsGenerate Solutions
Solution Overview
Problem
Resonator fiber optic gyros (RFOGs) face significant bias stability issues due to the interference of high order spatial modes, which can compromise the accuracy of resonance frequency measurements and rotation rate determination, especially in hollow core fiber gyroscopes that may weakly support these modes.
Innovation Solution
The implementation of spatial mode filters within the resonator to increase round trip losses of high order spatial modes, ensuring that only the fundamental mode is supported, thereby reducing interference and enhancing bias stability through the use of single mode optical elements and polarizing filters.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If hollow core fiber is used in the resonator, then environmental sensitivity is reduced and nonlinear refraction is minimized, but high order spatial modes are weakly supported causing bias stability degradation
Solution Approach 1:
The patent extracts and removes the harmful high order spatial modes from the resonator by introducing mode filters (single mode fibers or waveguides) that selectively couple only the fundamental mode while rejecting higher order modes. This extraction principle directly addresses the problem of mode interference degrading measurement precision while preserving the benefits of hollow core fiber.
Solution Approach 2:
The patent introduces mode filters as intermediary elements between the hollow core fiber resonator and the detection system. These filters act as mediators that selectively transmit the fundamental mode while blocking high order modes, thereby resolving the contradiction between maintaining hollow core fiber advantages and eliminating mode-related measurement errors.
2Measurement precision
If mode filters are introduced to suppress high order modes, then measurement precision is improved, but device complexity increases
Solution Approach 1:
The patent merges the mode filtering function with existing resonator components by integrating single mode fibers or waveguides directly into the resonator loop. This combining approach allows mode suppression to be achieved without adding separate, complex filtering subsystems, thereby improving measurement precision while minimizing increases in device complexity.
3Reliability
If spatial averaging is used to suppress high order mode interference, then bias stability is improved, but suppression is compromised by imperfect spatial averaging
Solution Approach 1:
The patent replaces the mechanical/spatial averaging approach with an optical field-based solution using mode filters. Instead of relying on spatial integration to suppress mode interference, the invention uses the evanescent field coupling properties of single mode fibers or waveguides to selectively transmit only the fundamental mode, thereby achieving superior bias stability and measurement precision.
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 approach significantly reduces bias errors by suppressing unwanted high order spatial modes, leading to more accurate resonance frequency measurements and improved rotation rate determination in RFOGs.
Implementation Method 1
The mode filter comprises a single mode optical fiber or waveguide that is brought into close proximity to the resonator loop fiber, allowing evanescent field coupling between the mode filter and the resonator loop fiber. This coupling enables selective interaction with different spatial modes based on their field distributions.
Implementation Method 2
Due to modal dispersion, high order modes travel in different optical paths and have different round-trip phase delays with respect to the fundamental mode.
Implementation Method 3
When the gyro has nonzero rotation rate around an axis normal to the plane of the ring cavity (also called resonator), the effective round trip path lengths for lightwaves propagating in the two opposite directions are different. This so-called Sagnac effect causes the ring cavity to exhibit different resonance frequencies for the two opposite directions
Implementation Method 4
When the resonator output light is detected, light from the higher order mode may interfere with the fundamental mode, producing resonance asymmetry that varies with environmental changes.
Data Source
AI summary
Multiple resonator fiber optic gyroscope (RFOG) configurations comprising one or more mode filters inside the resonator are adopted to effectively suppress unwanted high order spatial modes which can be a significant source of gyro bias errors. The resonator comprises at least a loop fiber, either two or more in/out coupling elements, and connectors that link elements into a circulating loop. Directional elements may be used to separate output light from input light in some of the embodiments. In all embodiments, mode filters are placed in the resonator to guarantee that the light reaching the photodetector is filtered by at least one mode filter in the resonator at least once. The mode filters may contain both spatial mode filters (such as single mode fibers or waveguides) and polarization mode filters (such as polarizing elements) so that both spatial and polarization mode filtering can be implemented simultaneously.