Fiber Optic Sensor Demodulation for Small High-Frequency Signals
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Solution Overview
Problem
Existing fiber optic sensors face challenges in detecting small, high-frequency signals such as sonic and ultrasound due to environmental perturbations and require expensive, low-noise laser sources, and heterodyne demodulation lacks sensitivity for these signals.
Innovation Solution
A sensor system using a coiled optical fiber with fiber Bragg gratings and phase modulation to generate cosine and sine terms, combined with filters and amplifiers to extract the signal of interest, allowing for high sensitivity and omnidirectional detection of small, higher-frequency signals.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Ease of manufacture
If intensity demodulation is used with a single-frequency laser, then cost is reduced and speed is increased, but sensitivity for detecting small dynamic signals deteriorates and reliability worsens due to environmental perturbations moving the operation point
Solution Approach 1:
The system segments the detected signal into multiple frequency components through heterodyne demodulation, separating the small dynamic signal from environmental perturbations. The photodetector output is processed to extract specific frequency components that correspond to the dynamic signal of interest, while rejecting other frequency components including those from environmental disturbances.
Solution Approach 2:
A carrier frequency is introduced as an intermediary to enable heterodyne detection. This carrier frequency serves as a reference that allows the system to detect small dynamic signals through frequency mixing, improving sensitivity without requiring expensive low-noise laser sources or complex feedback control systems.
2Productivity
If heterodyne demodulation is used to extract phase of fringes, then dynamic range is increased and linearity is improved, but sensitivity for detecting small dynamic signals deteriorates
Solution Approach 1:
The system changes the detection parameter from direct phase measurement to frequency component analysis. By analyzing the amplitude of specific frequency components in the heterodyne demodulated signal, the system achieves both large dynamic range and high sensitivity for small dynamic signals simultaneously.
3Measurement precision
If wavelength-tunable laser sources with feedback control systems are used to maintain operation point, then measurement precision is improved, but device complexity increases and cost increases
Solution Approach 1:
The system uses frequency-based feedback through heterodyne demodulation rather than intensity-based feedback. The carrier frequency serves as a stable reference that automatically maintains the operation point without requiring complex wavelength-tunable laser sources or intensity-based feedback control systems.
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
The system provides high sensitivity, reliability, and accuracy in detecting small, high-frequency signals, reduces cost, and is less sensitive to environmental drifts and laser polarization variations, enabling multiplexing capabilities.
Implementation Method 1
The beam of laser light is phase modulated with a sinusoidal wave whose frequency is at least a factor-of-two higher than the highest frequency of a signal of interest
Implementation Method 2
Fiber optic sensors utilizing various optical interferometers or fiber Bragg gratings (FBGs) that sense temperature, strain, pressure and refractive index are generally known
Implementation Method 3
power of the laser light reflected from the sensor is measured by a photodetector (PD)
Data Source
AI summary
A system, apparatus, and method for demodulation of a fiber optic sensor is provided. An aspect of the system provides an optical fiber, a laser, a phase modulator configured to be coupled to the optical fiber, and a sensor. The laser emits a laser beam into the optical fiber. The phase modulator receives the laser beam from the laser and directs the laser beam to the sensor. The sensor includes a coiled portion of the optical fiber, uncoiled segments adjacent the coiled portion, and at least two fiber Bragg gratings configured to be coupled to opposite uncoiled segments adjacent the coiled portion of the optical fiber. The sensor system may further include a photodetector configured to receive a reflected portion of the laser beam from the sensor. The reflected portion is divided into at least two paths where at least two sub-outputs are generated for demodulation and sensing.