Phase Analog Coherent Demodulation for Optical Time Domain Reflectometry
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Solution Overview
Problem
Current phase sensitive optical time domain reflectometry systems face challenges in reducing the operating frequency required for quadrature demodulation, leading to high hardware and software design complexities and costs, as well as limited measurement distance and stability in vibration signal detection.
Innovation Solution
A phase analog coherent demodulation system that generates pulsed and continuous light with a frequency difference of 10 KHz to 100 MHz, using a difference frequency and pulse generator, optical fiber amplifier, and I/Q demodulation module to reduce the operating frequency and improve stability, enabling easier implementation and increased measurement distance.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If digital coherent detection technology is used to receive and analyze backward scattered light, then phase detection accuracy is improved, but hardware and software complexity increases and implementation becomes difficult
Solution Approach 1:
The patent replaces digital coherent detection (complex hardware/software system) with an analog coherent detection method using optical mixing. The mixing module performs optical mixing of the backward scattered light with a reference light to directly obtain the phase information through optical heterodyning, eliminating the need for complex digital signal processing hardware and software while maintaining phase detection accuracy.
Solution Approach 2:
The patent extracts only the essential function of phase detection from the complex digital coherent detection system. By using optical mixing to directly obtain phase information in the optical domain, the system removes unnecessary digital processing components while retaining the core capability of accurate phase measurement.
2Device complexity
If a single acousto-optic modulator is used to determine mixing frequency, then the system structure is simplified, but the operating frequency must be high (up to 100 MHz) requiring high-bandwidth mixers and high-speed data acquisition
Solution Approach 1:
The patent segments the frequency generation function into two independent acousto-optic modulators working in cascade. The first AOM generates an intermediate frequency and the second AOM generates the final mixing frequency. This segmentation allows each AOM to operate at lower frequencies individually, and the frequencies can be independently adjusted to achieve the desired mixing frequency without requiring a single high-frequency AOM.
Solution Approach 2:
The patent introduces dynamic frequency adjustment capability by using two independently controllable AOMs. The mixing frequency can be dynamically changed by adjusting the drive frequencies of the two AOMs, providing flexibility without requiring high fixed operating frequencies. This dynamic control allows the system to adapt to different measurement requirements while maintaining lower operating frequencies.
3Measurement precision
If high operating frequency is used for quadrature demodulation, then measurement precision is maintained, but cost and design difficulty increase
Solution Approach 1:
The patent changes the frequency parameters by using two AOMs with independently adjustable drive frequencies. Instead of using a single high-frequency AOM, the system uses two AOMs whose frequency parameters can be optimized and adjusted to achieve the desired mixing frequency. This parameter flexibility allows the use of lower-frequency, lower-cost components while maintaining measurement precision through proper frequency selection and optimization.
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 simplifies the hardware and software requirements for quadrature demodulation, reduces costs, and enhances the stability and measurement distance of vibration signal detection in phase sensitive optical time domain reflectometry.
Implementation Method 1
an optical fiber amplifier, configured to receive the pulsed light and amplifies an optical frequency of the pulsed light
Implementation Method 2
the I/Q demodulation module being further configured to mix the backward scattered light and the continuous light to generate a second frequency difference signal
Implementation Method 3
a balance detector, configured to process the continuous light and the backward scattered light mixed and generate the second frequency difference signal
Implementation Method 4
a 90° phase shifter, configured to receive the first frequency difference signal and inputs it into the first analog multiplier after phase shifting by 90°
Implementation Method 5
a first analog multiplier, configured to multiply the second frequency difference signal and the first frequency difference signal after 90° phase shifting
Implementation Method 6
a first low pass filter, configured to process the operation result and outputs the first low frequency signal
Data Source
AI summary
A phase analog coherent demodulation system includes a difference frequency and pulse generator configured to generate a pulsed light and a continuous light with different frequency from the pulsed light, a controller, and an I/Q demodulation module. The controller locks the frequency difference signal of the pulse light, and the pulsed light generates the backward scattered light of constant frequency in the measured medium, and the backward scattered light returns along the original path and mixing beat with the continuous light in the I/Q demodulation module, the difference frequency electrical signals of the two optical signals are obtained. The I/Q demodulation module orthogonally demodulates the difference frequency electrical signal by using the frequency difference signal of the pulsed light and the continuous light as the fundamental frequency signal of the orthogonal demodulation, to detect phase information of the vibration signal. The invention reduces the operating frequency required for quadrature demodulation in system phase detection.