PMD Measurement Using Depolarizer Carrier Averaging
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Solution Overview
Problem
Current methods for measuring polarization mode dispersion (PMD) in optical devices suffer from poor repeatability and accuracy, especially at lower PMD values, due to the inherent variations with temperature and strain, and are sensitive to polarization mode coupling, making it difficult to achieve precise measurements for high-bit-rate transmissions.
Innovation Solution
A device using a depolarizer with two birefringent elements oriented at acute angles to the polarization state of the optical test signal, creating multiple carrier components that average the PMD measurement, ensuring stable and repeatable results across a wide range of PMD values by analyzing the superposition of circularly, elliptically, and linearly polarized light at different wavelengths.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If traditional PMD measurement methods (JME, MMM, INTFER, FA-FT) are used, then PMD measurement is achieved, but measurement precision and repeatability deteriorate due to sensitivity to polarization mode coupling and temperature/strain variations
Solution Approach 1:
The patent segments the optical signal into multiple discrete wavelength components (e.g., five wavelengths spaced 100 GHz apart) within the spectral bandwidth. By measuring PMD at each discrete wavelength and averaging the results, the method reduces sensitivity to polarization mode coupling that varies with wavelength, thereby improving both measurement precision and repeatability compared to continuous spectral methods.
Solution Approach 2:
The patent changes the measurement parameter from continuous spectral analysis to discrete wavelength sampling. By selecting specific wavelength points across the spectral bandwidth and measuring PMD at each point, the method transforms the measurement approach to reduce sensitivity to environmental variations and polarization mode coupling, achieving better measurement reliability.
2Device complexity
If a single carrier frequency is used for PMD measurement, then the measurement setup is simple, but measurement precision deteriorates due to sensitivity to polarization mode coupling at that specific frequency
Solution Approach 1:
Instead of using a single carrier frequency, the patent segments the measurement into multiple discrete wavelength components. Each wavelength component is measured separately, and the results are averaged. This segmentation reduces the impact of polarization mode coupling at any single frequency while maintaining relatively simple measurement setup requirements.
Solution Approach 2:
The patent combines multiple PMD measurements taken at different discrete wavelengths into a single averaged result. By merging the measurements from multiple wavelength points, the method achieves improved precision that overcomes the limitations of single-frequency measurements while keeping the overall system complexity manageable.
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 device significantly improves the accuracy and repeatability of PMD measurements by generating four known carrier components, allowing for precise averaging and reducing dispersion, especially for low PMD values, thus enabling reliable transmission of high-bit-rate signals.
Implementation Method 1
a first birefringent element having an eigenaxis oriented at an acute angle to the polarization state of the optical test signal; and a second birefringent element having an eigenaxis oriented at an acute angle to the eigenaxis of the first birefringent element and the polarization state of the optical test signal
Data Source
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AI summary
A device for measuring polarization mode dispersion (PMD) in a device under test (DUT) includes a polarized light source for launching a test beam through the DUT, and a PMD analyzer, which uses one of a plurality of known techniques, e.g. fixed analyzer-Fourier transform (FA-FT) or interferometric, to calculate the PMD from the DUT. A passive depolarizer, made up of a plurality of birefringent elements is disposed between the light source and the PMD analyzer to generate a plurality of carrier frequencies that correspond to the delays imposed by the birefringent elements. The PMD content of the DUT is present around each of the carriers, and the plurality of PMD measurements can be averaged to obtain a more accurate and repeatable measure of the PMD.