Sound Measurement Apparatus Using Differential Detection
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Solution Overview
Problem
Current sound measurement devices using the acousto-optic effect face challenges in detecting minute optical phase modulation due to sound, as it is masked by noise from the average light intensity, limiting the minimum detectable phase modulation and signal-to-noise ratio.
Innovation Solution
A sound measurement device employing a differential detection method using two photodetectors and a differential signal generator to isolate the optical phase modulation due to sound, while adjusting the interferometer to maintain the phase of the interference fringe at mid-fringe, thereby canceling noise from the average light intensity and enhancing the signal-to-noise ratio.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If conventional sound measurement using optical interferometer is used, then sound measurement capability is achieved, but noise from average light intensity masks the minute optical phase modulation due to sound
Solution Approach 1:
The patent segments the light detection into two separate photodetectors: one detecting the interference light (containing phase modulation information) and another detecting the average light intensity. By separating these detection functions, the system can process the phase modulation signal independently from the noisy average intensity, thereby improving measurement precision while eliminating the masking effect of average light intensity noise
Solution Approach 2:
The patent extracts only the relevant phase modulation information from the optical signal by using a photodetector configured to detect interference patterns rather than total light intensity. This extraction method isolates the sound-induced phase modulation (φs) from the dominant average light intensity (IDC), allowing precise sound measurement without being affected by intensity fluctuations
2Measurement precision
If average light intensity IDC is reduced to improve SN ratio, then noise impact decreases, but the minimum detectable optical phase modulation amount is limited
Solution Approach 1:
The patent implements feedback control by continuously monitoring the average light intensity with one photodetector and using this information to adjust the interferometer operation. This feedback mechanism allows the system to maintain optimal light intensity levels while compensating for intensity fluctuations, thereby preserving both the signal-to-noise ratio and the minimum detectable phase modulation capability
3Measurement precision
If light source intensity stability is increased to improve measurement accuracy, then optical phase modulation detection improves, but system cost and complexity increase
Solution Approach 1:
The patent enables the system to self-correct for light intensity fluctuations by using one photodetector to monitor average intensity and another to detect phase modulation. The system automatically compensates for intensity variations through differential measurement, eliminating the need for expensive ultra-stable light sources and reducing both device complexity and cost while maintaining high measurement accuracy
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 allows for the accurate measurement of optical phase modulation due to sound without being affected by noise, significantly improving the signal-to-noise ratio and reducing the demand for light source intensity stability, thus enhancing measurement sensitivity and reducing costs.
Implementation Method 1
As one of sound measurement methods using light, there is a method using a refractive index change of a medium due to sound called an acousto-optic effect
Implementation Method 2
the two pieces of light are combined by the beam splitter. The phase difference between the two pieces of light is extracted as an electrical signal by detecting multiplexed light, that is, interference light by a photodetector
Implementation Method 3
The phase difference between the two pieces of light is extracted as an electrical signal by detecting multiplexed light, that is, interference light by a photodetector
Data Source
AI summary
Provided is a measurement technology of an optical phase modulation amount due to sound without being affected by noise included in average light intensity. The measurement technology includes: an interference light generator that obtains first light including light subjected to optical phase modulation by a sound measurement unit and second light different from the first light and including light subjected to optical phase modulation by the sound measurement unit from light emitted from a light source; a first photodetector that obtains a first electrical signal from the first light; a second photodetector that obtains a second electrical signal from the second light; a differential signal generator that obtains a differential signal that is a difference between the first electrical signal and the second electrical signal; and an optical phase modulation amount adjuster that adjusts an optical phase modulation amount φ0 due to an element other than sound by fixing an interferometer such that a phase of an interference fringe is in mid-fringe by using the differential signal as an error signal, in which the optical phase modulation amount φs due to sound is measured as a current Δi of the differential signal, and the first photodetector and the second photodetector are adjusted such that output voltages are saturated when light that causes a phase fluctuation exceeding a predetermined range around mid-fringe is input.