Birefringent Optical Sensor Interrogation via Polarization Control
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Solution Overview
Problem
Birefringent optical sensors, such as birefringent Fibre Bragg Gratings (Bi-FBGs), face challenges in high precision monitoring and interrogating polarization-dependent wavelengths due to polarization dependent frequency shift (PDFS) issues, which affect accuracy and speed in sensing applications.
Innovation Solution
A measurement system utilizing a tunable laser source and a polarization control device, including modules to sweep the laser at a predetermined frequency range and detect polarization-dependent spectral features, such as wavelength peaks, to calculate measurements like temperature, strain, or pressure, with a polarization control device that uses passive or active polarization scrambling and Mach-Zehnder Interferometer (MZI) to synchronize orthogonal responses.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Ease of manufacture
If birefringent optical sensors are interrogated with polarized frequency swept sources, then polarization dependent frequency shift (PDFS) problems occur, but standard FBG interrogation methods can be used
Solution Approach 1:
The patent segments the polarization state detection into multiple discrete measurements by sweeping the laser frequency across the FBG reflection spectrum multiple times with different fixed polarization states. This allows the system to separately measure the reflection spectrum for each polarization state and then reconstruct the full polarization-dependent response, avoiding the need for continuous polarization tracking while maintaining measurement precision.
Solution Approach 2:
The patent performs preliminary action by pre-sweeping the laser frequency to characterize the polarization-dependent frequency shift (PDFS) of the FBG sensor before actual measurements. This preliminary characterization allows the system to store reference data about how the FBG responds to different polarization states, which is then used to correct or interpret subsequent measurements, thereby improving measurement precision without adding complexity to the manufacturing.
2Measurement precision
If high precision monitoring of polarization-dependent wavelengths is implemented, then measurement accuracy improves, but system complexity increases
Solution Approach 1:
The patent implements periodic action by performing repeated frequency sweeps of the laser across the FBG spectrum at different fixed polarization states. Instead of using complex continuous polarization tracking, the system periodically switches between a small number of fixed polarization states and performs spectral measurements at each state. This periodic sampling approach achieves high measurement accuracy by capturing the full polarization-dependent response while keeping the system complexity low through the use of simple, discrete polarization states and standard swept-source equipment.
3Adaptability or versatility
If birefringent FBGs are used for simultaneous strain and temperature measurements, then sensing capability is enhanced, but polarization dependent responses increase
Solution Approach 1:
The patent implements feedback by using the measured reflection spectra at different polarization states to calculate the orthogonal polarization axes of the birefringent FBG. The system continuously monitors the polarization-dependent responses and uses this information to determine the principal stress axes and birefringence characteristics. This feedback mechanism allows the system to compensate for polarization effects and accurately separate strain and temperature measurements, thereby maintaining high sensing capability while managing polarization dependent responses.
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
Enables high-speed and high-resolution monitoring of birefringent optical sensors by accurately detecting orthogonal spectral features, reducing complexity and improving measurement precision, allowing for strain-independent temperature measurements and temperature-independent pressure measurements without additional sensors.
Implementation Method 1
birefringent optical sensors, such as birefringent Fibre Bragg Gratings (Bi-FBGs), face challenges in high precision monitoring and interrogating polarization-dependent wavelengths due to polarization dependent frequency shift (PDFS) issues
Implementation Method 2
the polarization control device comprises a passive polarization birefringent fibre adapted to scramble a polarization state of the tunable laser
Implementation Method 3
an output of the polarization control device is connected to a Mach-Zehnder Interferometer (MZI) device, wherein measurement peaks from the MZI device are synchronous with polarization change in the system such that two Bi-FBG responses corresponding to two orthogonal polarizations are generated
Data Source
AI summary
The invention provides a measurement system to interrogate at least one birefringent optical sensor and a method to interrogate birefringent optical sensors at high speed and high resolution. The system and method detects, at least, a first and a second spectral feature that are polarization dependent, wherein the detected first and second spectral features correspond to different responses of at least one birefringent optical sensor.