Wavelength-Swept Light Source Linewidth Measurement via Noise Floor Analysis
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Solution Overview
Problem
Existing methods, such as the delayed self-heterodyne method, cannot measure the instantaneous linewidth of coherent light during wavelength sweeping, and methods that measure it require large interferometers or ultrahigh-speed processing, making it difficult to measure linewidth in applications like automatic driving where distances are long.
Innovation Solution
A spectrum measurement method and device that separates wavelength-swept light into measurement and reference sections with different optical path lengths, generates interfering light, and uses Fourier transforms to determine the coherence time, allowing for the measurement of linewidth based on noise floor variations without needing a long optical path length or high-speed processing.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If an interferometer that generates an optical path length difference greater than the coherence time is used to measure instantaneous linewidth, then the linewidth measurement becomes possible, but the size of the measurement device increases unavoidably
Solution Approach 1:
The patent changes the measurement parameter from direct optical path length comparison to noise floor analysis in the frequency domain. By performing Fourier transform on interference signals and analyzing noise floor characteristics, the system determines coherence time without requiring an interferometer with optical path length exceeding coherence time, thus reducing device size while maintaining measurement capability
Solution Approach 2:
The patent replaces the mechanical/optical interferometer system with a computational approach. Instead of using a physical interferometer to generate and measure interference patterns directly, the system uses a shorter-path interferometer combined with Fourier transform analysis of the interference signals to extract coherence information, substituting computational processing for extended optical paths
2Ease of manufacture
If the delayed self-heterodyne method is used to measure linewidth, then the measurement can be performed, but it cannot measure the linewidth during wavelength sweeping (instantaneous linewidth)
Solution Approach 1:
The patent utilizes the periodic wavelength sweeping action of the light source to enable instantaneous linewidth measurement. By performing multiple Fourier transforms on interference signals acquired during the periodic wavelength sweep and analyzing the noise floor characteristics at different optical path length differences, the system extracts coherence time information that reflects the instantaneous linewidth during the sweeping process
Solution Approach 2:
The patent introduces noise floor analysis as an intermediary measurement approach. Instead of directly measuring the interference pattern to determine linewidth, the system uses the noise floor characteristics of the Fourier transformed interference signals as an intermediate parameter to infer coherence time and instantaneous linewidth, enabling measurement during wavelength sweeping
3Productivity
If ultrahigh-speed processing is used to measure instantaneous linewidth, then the measurement speed improves, but the system complexity and cost increase
Solution Approach 1:
The patent applies partial action by using a moderate-speed acquisition system combined with Fourier transform analysis. Instead of requiring ultrahigh-speed processing for direct instantaneous linewidth measurement, the system acquires interference signals at manageable speeds, performs Fourier transforms to frequency domain, and extracts coherence time from noise floor characteristics, achieving instantaneous linewidth measurement without ultrahigh-speed processing requirements
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
Enables easy measurement of linewidth during wavelength sweeping, reducing the size of measurement devices and avoiding limitations imposed by the detection system's band, facilitating evaluation and calibration of coherent light sources, especially in automatic driving applications.
Implementation Method 1
having a wavelength-swept light source generate light to be measured that is wavelength-swept coherent light with a wavelength periodically changed
Implementation Method 2
separating the light to be measured into a measurement section and a reference section that have different optical path lengths, and then coupling the light to be measured in an interference section to generate interfering light
Implementation Method 3
detecting the interfering light at a light detector to generate an interference signal
Implementation Method 4
having an analyzer perform a Fourier transform of interference signals with an optical path length difference between the measurement section and the reference section changed
Data Source
AI summary
A wavelength-swept light source is configured to generate light to be measured that is wavelength-swept coherent light with a wavelength periodically changed. The light to be measured is separated into a measurement section and a reference section that have different optical path lengths, and is then coupled in an interference section to generate interfering light. An analyzer performs a Fourier transform of interference signals of the interfering light, and acquires an actual measured noise floor value for each of the optical path length differences based on a point spread function. An estimated coherence time is determined so that an actual measured amplitude value of the noise floor value and a calculated amplitude value coincide with each other. Linewidth of the light emitted from the coherent light source is measured based on the estimated coherence time.