Optical Fiber Sensor Interrogation Using Code and Wavelength Multiplexing
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Solution Overview
Problem
Current multiplexing techniques for interrogating optical fiber sensor networks, such as TDM, WDM, OFDR, and CDM, face limitations in speed, energy efficiency, and sensor density due to requirements for high-speed modulators, limited spatial resolution, and synchronization issues, which hinder fast and accurate measurement of densely spaced sensors.
Innovation Solution
A system and method using broadband light and modulators with predetermined codes to simultaneously interrogate multiple sensors, employing code-division multiplexing and wavelength-division multiplexing techniques, allowing for simultaneous illumination and distinction of sensor responses without the need for high-speed modulators, thereby enabling fast and accurate measurement of densely spaced sensors.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If time-division multiplexing (TDM) is used to interrogate optical fiber sensors, then each sensor can be addressed individually with distinct time slots, but the measurement time increases and the received optical power decreases due to sequential pulsing
Solution Approach 1:
The patent applies periodic action by using continuous wave illumination instead of sequential pulsing. The broadband light source continuously illuminates all sensors simultaneously, and the modulators apply periodic code sequences to encode and decode sensor responses in overlapping time slots, eliminating the sequential measurement bottleneck of TDM while maintaining sensor addressing accuracy through code-based discrimination.
Solution Approach 2:
The patent implements preliminary action by pre-modulating the broadband light signal with code sequences before the sensors reflect the light. This pre-modulation allows the system to prepare encoded illumination patterns that enable simultaneous interrogation of multiple sensors, rather than waiting for sequential responses as in traditional TDM approaches.
2Quantity of substance
If shorter pulse length is used in TDM to increase sensor density, then more sensors can be addressed, but the transmitted optical power decreases leading to lower signal-to-noise ratio
Solution Approach 1:
The patent applies continuity of useful action by using continuous wave illumination instead of pulsed illumination. The broadband light source continuously emits light through the sensor network, allowing all sensors to be illuminated simultaneously with sufficient optical power, while code-division multiplexing enables dense sensor packing without the SNR penalty associated with short pulses in traditional TDM.
Solution Approach 2:
The patent changes the temporal parameter from pulsed to continuous illumination, and uses code sequence parameters to differentiate sensors. This parameter transformation allows high sensor density to be achieved through code orthogonality rather than temporal separation, maintaining high optical power levels and signal-to-noise ratio while supporting dense sensor configurations.
3Quantity of substance
If wavelength-division multiplexing (WDM) is used to increase sensor density, then more sensors can be monitored, but the spectral slots must be non-overlapping which limits the number of sensors based on light source and interrogator spectral width
Solution Approach 1:
The patent transitions from the wavelength dimension (WDM) to the time-code dimension for sensor differentiation. By applying code-division multiplexing, sensors can be distinguished through temporal code sequences rather than requiring non-overlapping spectral slots. This dimensional shift allows overlapping spectral responses from densely packed sensors to be resolved through code orthogonality, dramatically increasing sensor density without constraining spectral resource allocation.
4Measurement precision
If optical frequency domain reflectometry (OFDR) is used to achieve good spatial resolution, then sensor locations can be precisely determined, but the method is limited to short fiber lengths of around 10 to 70 m
Solution Approach 1:
The patent applies segmentation by dividing the long fiber into multiple interrogation zones, each handled by a separate modulator with its own code sequence. This segmentation allows the system to maintain good spatial resolution within each zone while extending the total measurable fiber length beyond the limitations of conventional OFDR by processing multiple segments simultaneously through wavelength- and code-division multiplexing.
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 allows for simultaneous and accurate interrogation of multiple sensors with improved signal-to-noise ratio and reduced measurement time, enabling high-density optical fiber sensor networks with efficient energy use and minimal overlap between sensor responses.
Implementation Method 1
a light source for generating a broadband light signal, the broadband light signal spanning a wavelength range
Implementation Method 2
a first modulator for modulating the light signal using a first modulation according to a predetermined code comprising a plurality of identifiable chips
Implementation Method 3
a second modulator for modulating the received light signal with a second modulation according to a second predetermined code
Implementation Method 4
optical sensors are referred to as Fiber Bragg Gratings (FBGs)... As FBGs change their reflected wavelength when strain or temperature is applied
Implementation Method 5
a detection system for detecting the modulated received light signal, during a detection integration time, for simultaneously detecting the sensors
Data Source
AI summary
A system for interrogating sensors in a fiber optical sensor network includes groups of sensors. The sensors in one group operate at different wavelengths, and the sensors of other groups may have overlapping wavelengths. A light source generates a broadband light signal, input and output means for guides the broadband light to the fiber optical sensor network for illuminating the sensors and for coupling the light signal coming from the sensors of the fiber optical sensor network to the detection system, and a detection system detects the received light signal, during a detection integration time. The system is arranged for selecting pre-dominantly the received light coming from the different sensors of a selected group of sensors using a code-division multiplexing technique and simultaneously detecting sensors of the selected group of sensors using a wavelength-division multiplexing technique.