Optical Fiber Brillouin Measurement Using Second Harmonic Detection
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Solution Overview
Problem
The homodyne BOCDR method for measuring optical fiber characteristics is hindered by amplitude modulation (AM) noise, which degrades measurement accuracy and stability, particularly in the detection of Brillouin scattered light spectra.
Innovation Solution
The optical fiber characteristics measurement apparatus detects the second harmonic component of the signal yielded by homodyne detection of interference light, rather than the direct current (DC) component, using a bandpass filter to reduce AM noise and eliminate the need for normalization processing.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If the homodyne BOCDR method is used to measure the spectrum of Brillouin scattered light, then the measurement capability of optical fiber characteristics is achieved, but amplitude modulation (AM) noise degrades the measurement accuracy
Solution Approach 1:
The patent extracts only the second harmonic component from the homodyne detection signal through bandpass filtering, separating it from the AM noise-containing DC component and other frequency components. This extraction isolates the useful signal information while eliminating the harmful AM noise, directly resolving the contradiction between measurement capability and noise degradation.
Solution Approach 2:
The patent changes the detection parameter from the DC component (which contains AM noise) to the second harmonic component (which is free from AM noise). By shifting the detection frequency parameter, the system achieves accurate measurement of optical fiber characteristics without being affected by AM noise, thereby resolving the measurement accuracy degradation issue.
2Measurement precision
If the DC component is detected for BGS measurement, then the measurement process requires normalization processing to remove noise or bias, but this increases device complexity and reduces long-term stability
Solution Approach 1:
The patent extracts the second harmonic component from the detection signal, which inherently eliminates the need for normalization processing. By detecting this specific frequency component through bandpass filtering, the system obtains BGS measurements free from noise and bias without requiring additional normalization circuitry or processing steps, thus reducing device complexity.
Solution Approach 2:
The patent uses the second harmonic component as an alternative copy of the BGS information that does not require normalization. Instead of processing the DC component through complex normalization routines, the system directly measures the second harmonic component, which contains equivalent BGS information in a cleaner form, thereby simplifying the overall measurement system.
3Measurement precision
If normalization processing is applied to remove noise or bias from the signal, then short-term measurement accuracy is improved, but the normalized waveforms vary over a long period of time reducing stability
Solution Approach 1:
The patent changes the detection parameter from the DC component to the second harmonic component. This parameter change fundamentally eliminates the source of long-term instability associated with normalization processing. The second harmonic component naturally rejects AM noise and does not require normalization, providing both high measurement accuracy and excellent long-term stability simultaneously.
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 enhances measurement accuracy and stability by reducing the influence of AM noise, resulting in improved Brillouin scattered light spectrum analysis and simplified circuitry.
Implementation Method 1
a first optical splitter configured to split a modulated light subjected to frequency modulation into a pump light and a reference light
Implementation Method 2
a second optical splitter configured to cause the pump light to be incident on a fiber under test from one end and configured to extract Brillouin scattered light generated within the fiber under test
Implementation Method 3
an adjuster configured to change a frequency of at least one of the reference light and the pump light to a plurality of frequencies
Implementation Method 4
a calculator configured to measure characteristics of the fiber under test based on a second harmonic component among frequency components of a signal yielded by homodyne detection of interference light between the Brillouin scattered light and the reference light
Data Source
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AI summary
An optical fiber characteristics measurement apparatus (10) includes a first optical splitter (20) that splits modulated light into pump light (LP) and reference light (LR), a second optical splitter (24) that causes the pump light (LP) to be incident on a fiber under test (80) from one end and extracts Brillouin scattered light (LS) generated within the fiber under test (80), an adjuster (30) that changes frequency of at least one of the reference light (LR) and pump light (LP) to a plurality of frequencies, and a calculator (54) that measures characteristics of the fiber under test (80) based on a second harmonic component among frequency components of a signal yielded by homodyne detection of interference light between the Brillouin scattered light (LS) and the reference light (LR) at each frequency, the second harmonic component having a frequency that is two times the modulation frequency of the modulated light.