Optical Pulse Emitter Space-Time Encoding for SNR Resolution Trade-off
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Solution Overview
Problem
Traditional optical time domain reflectometry (OTDR) techniques are limited by the energy of the optical pulse, leading to a trade-off between spatial resolution and signal-to-noise ratio (SNR), and encoding methods face challenges with increasing code word length due to complexity and measurement time constraints, restricting the maximum measurement distance.
Innovation Solution
A method and apparatus using a space-time encoding scheme with a control and processing unit generating binary sequences for an optical pulse emitter, launching multiple pulses into the DUT, and employing a decoding procedure with two successive steps to enhance coding gain and measurement distance, while maintaining spatial resolution and measurement time.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If the energy of the optical pulse is increased to improve the signal-to-noise ratio (SNR), then the measurement precision is improved, but the spatial resolution deteriorates because the pulse duration cannot be increased without worsening the spatial resolution
Solution Approach 1:
The optical pulse is segmented into multiple lower-energy sub-pulses arranged in a pseudorandom binary sequence. Each sub-pulse contributes to the overall signal, and through correlation processing, the signals are coherently integrated to achieve high SNR while each individual sub-pulse maintains short duration for good spatial resolution.
Solution Approach 2:
A periodic pseudorandom binary sequence of optical pulses is transmitted instead of a single continuous pulse. The periodic structure allows for correlation-based decoding that integrates signal energy over time, improving SNR without requiring each individual pulse to have high energy or long duration.
2Measurement precision
If encoding techniques with longer code words are used to improve SNR, then the measurement precision is improved, but the measurement time increases because the time of flight increases directly proportionally to the code word length
Solution Approach 1:
The patent transitions from single-wavelength OTDR to multi-wavelength Raman OTDR, utilizing the spectral dimension. By measuring both Stokes and anti-Stokes Raman scattering at different wavelengths, the system achieves differential temperature sensing that provides additional information dimensions, enabling improved precision without proportionally increasing time.
Solution Approach 2:
The patent changes the measurement parameter from single-wavelength backscatter to multi-wavelength Raman scattering. By exploiting the wavelength-dependent Raman effect and measuring intensity ratios at different wavelengths, the system achieves temperature measurement with improved precision while managing measurement time through efficient spectral utilization.
3Measurement precision
If encoding techniques with longer code words are used to improve SNR, then the measurement precision is improved, but the device complexity increases due to the quadratic increase in decoding operation complexity
Solution Approach 1:
The patent introduces wavelength as an additional dimension for signal differentiation. By separating Stokes and anti-Stokes signals spectrally, the system creates independent measurement channels that simplify the decoding process compared to relying solely on temporal coding, reducing the quadratic complexity burden.
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 method achieves increased coding gain and measurement distance, allowing for more accurate distributed monitoring of physical parameters along longer optical devices, such as optical fibers, by optimizing SNR and reducing decoding complexity.
Implementation Method 1
the energy of this pulse is partially backscattered during its path owing to physical phenomena that occur in the structure of the DUT, in particular owing to the inelastic scattering known as 'Raman scattering'
Implementation Method 2
the energy of this pulse is partially backscattered during its path owing to physical phenomena that occur in the structure of the DUT
Data Source
Figure 1~2
Figure 3~4
Figure 5
AI summary
A method for measuring a distributed physical value of an optical device under test (DUT), comprising the steps of: launching into the DUT (4) a probe signal (3c) that comprises a plurality of optical pulses at at least one test wavelength (λT), receiving at least one optical signal (4d) backscattered by the DUT (4), wherein the optical pulses are obtained with at least the following steps: generating a first time sequence (SC1) of first pulses that corresponds to a word of a first code (C1), the first time sequence (SC1) lasting not shorter than a time of flight and being formed by a number (N1) of time slots (D1) that is equal to the number of bits of the word of the first code (C1), each time slot corresponding to a respective first pulse of the sequence; generating a second time sequence (SC2) of second pulses that corresponds to a word of a second code (C2), the second time sequence (SC2) being periodic, with a period that is substantially equal to the duration of at least one of the time slots (D1); amplitude modulating the second time sequence (SC2) with the first time sequence (SC1).