Refractive Index Profile Measurement of Cylindrical Glass Bodies
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Solution Overview
Problem
Existing refractive index measurement techniques for cylindrical glass bodies with refractive index striae face accuracy and precision issues due to diffractive effects, making it difficult to obtain precise refractive index profiles, especially in optical fiber preforms with striae structures.
Innovation Solution
A method utilizing a transformation function to determine refractive indices based on the deflection angle of the zero-order optical beam, discarding higher and small angle diffracted beams, and using a near-infrared light source with low coherence to improve measurement precision, allowing for accurate refractive index profile measurement even with significant striae structures.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If conventional refractive index measurement techniques are used on cylindrical glass bodies with striae, then the measurement process can be performed, but the accuracy and precision of the refractive index profile measurement deteriorates due to diffractive effects
Solution Approach 1:
The patent segments the detected beam signal into different diffraction orders (zero-order, first-order, higher-order beams) and processes only the zero-order beam for refractive index calculation. By separating the useful signal (zero-order) from harmful signals (diffracted orders), the measurement precision is improved while eliminating the adverse effects of striae-induced diffraction.
Solution Approach 2:
The patent changes the parameter of beam selection by focusing exclusively on the zero-order beam trajectory rather than using total beam deflection. This parameter change transforms the measurement approach from one that is sensitive to all diffraction effects to one that is insensitive to striae-induced diffraction, thereby improving measurement accuracy.
2Loss of information
If the measurement system attempts to detect all beam portions including diffracted orders, then more information is captured, but the measurement accuracy deteriorates due to contamination from higher order diffracted beams
Solution Approach 1:
The patent extracts and utilizes only the zero-order beam information from the total beam signal, discarding the higher-order diffracted beams. This extraction of the essential signal component eliminates the contaminating information from diffraction, thereby improving measurement accuracy while retaining sufficient information for precise refractive index profiling.
Solution Approach 2:
The patent converts the harmful diffractive effects into a beneficial separation mechanism. The striae-induced diffraction, which was previously a source of measurement error, is now utilized to spatially separate the zero-order beam from higher-order beams, making it easier to isolate and measure the useful signal while discarding harmful components.
3Device complexity
If a coherent light source is used for measurement, then the measurement system is simpler, but the measurement precision deteriorates due to interference patterns from striae
Solution Approach 1:
The patent changes the coherence parameter of the light source from coherent to low-coherence (superluminescent diode). This parameter change eliminates interference patterns and diffraction-related measurement errors while maintaining sufficient brightness and spectral characteristics for accurate zero-order beam trajectory measurement, thereby improving precision without excessive system complexity.
Data Source
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AI summary
According to some embodiments a method of measuring the refractive index profile of a consolidated glass body having a cylindrical surface comprises the steps of: (a) forming an image of a slit behind the glass body; (b) optionally pre-scanning the cylindrical surface of the test glass body or a reference glass body and analyzing the data within a fixed window to determine the likely location of the zero-order, un-diffracted beam while ignoring other diffracted beams; (c) optionally adjusting the optical power to improve the intensity of the data within the fixed window in order to improve the analysis; (d) predicting the trajectory of the zero-order beam through the preform based on the sampling location xi (incidence position) of the light impinging on the cylindrical surface and the location at which the zero-order beam impinges on the detector; (e) measuring the cylindrical surface of a glass body while using the estimated location of the zero-order beam and adjusted optical power to set the center of a floating window and the beam power at each measurement point; (e) determining deflection angles of the exiting zero-order beam within the floating window at each sampling location; (e) calculating the refractive index profile of glass body by utilizing a transformation function which determines refractive index at each location based upon the measured deflection angle function of the beam.