RFOG Optical Phase Lock Loop Filtering for Bias Stability
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Solution Overview
Problem
Resonator fiber optic gyroscopes (RFOGs) using a multi-frequency laser source (MFLS) experience bias instability due to undesired error currents picked up by slave lasers in the optical phase lock loop, leading to frequency modulation and sidebands that cause errors over temperature.
Innovation Solution
Implementing notch filters and shielding in the optical phase lock loop to attenuate undesired frequency signals and reduce crosstalk, along with relocating common modulation to the Pound-Drever-Hall (PDH) loop to mitigate bias instability.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If slave lasers are used in the optical phase lock loop to provide CW and CCW signals, then the RFOG can measure rotation rate using counterpropagating laser light waves, but undesired error currents are picked up by the slave lasers causing frequency modulation and sidebands that lead to bias instability over temperature
Solution Approach 1:
The patent introduces an intermediary filtering stage between the mixer and slave laser drivers. The filter removes undesired mixer products and error currents before they reach the slave lasers, preventing frequency modulation and sideband generation while maintaining the phase-lock functionality required for rotation measurement
Solution Approach 2:
The patent extracts and removes the harmful mixer products and error currents from the optical phase lock loop signal path using filtering techniques. By taking out these undesired frequency components before they can modulate the slave laser frequencies, the system maintains measurement precision while eliminating the source of bias instability
2Measurement precision
If the optical phase lock loop uses mixer outputs to control slave laser frequencies, then phase locking can be achieved, but mixer products at undesired frequencies are introduced causing frequency modulation of the slave lasers
Solution Approach 1:
A filtering intermediary is placed in the feedback path between the mixer and slave laser drivers. This filter acts as a mediator that allows the necessary phase-locking signals to pass through while blocking and removing undesired mixer products, preventing them from modulating the slave laser frequencies
Solution Approach 2:
The patent converts the potentially harmful mixer products into removable noise by filtering them out. The filtering process transforms the harmful frequency modulation effect into a manageable signal processing task, where undesired components are identified and eliminated before affecting the laser frequencies
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
Significantly reduces bias errors over temperature, enhancing the accuracy and stability of RFOGs by minimizing undesired sidebands and mixer products.
Implementation Method 1
a Pound-Drever-Hall (PDH) stabilization loop that is configured to lock the master laser to the resonant frequency
Implementation Method 2
causes the first slave laser and the second slave laser to have an optical frequency modulation component at the frequency of the common modulation signal
Implementation Method 3
a first slave laser that is configured to transmit a light wave at a frequency that is phase locked to the frequency of the master laser in a first optical phase lock loop
Implementation Method 4
combining a light wave from a master laser with a light wave from a first slave laser to produce a first combined signal including a first beat note signal
Implementation Method 5
laser light waves—one in the clockwise (CW) direction and the other in the counterclockwise (CCW) direction—are frequency-tuned to propagate at resonance within an optical fiber ring resonator
Implementation Method 6
an optical fiber ring resonator
Implementation Method 7
In the presence of rotation rate, Ω, the resonance frequencies will be different in proportion to the rotation rate (due to the Sagnac Effect)
Data Source
AI summary
A method is provided that includes combining a light wave from a master laser with a light wave from a first slave laser to produce a first signal including a first beat note signal, detecting the first signal, providing the first signal to a first mixer in a feedback path of a first optical phase lock loop, receiving a signal from a first offset frequency source at the first mixer, applying a first notch filter in the feedback path of the first optical phase lock loop after the first mixer to remove mixer products from an output of the first mixer, locking a frequency of the light wave from the master laser to a resonant frequency of a fiber optic resonator, and phase locking the first slave laser to the frequency of the master laser in the first optical phase lock loop at a first offset frequency.