APD Differential Detector Bias Control for Common-Mode Noise Rejection
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Solution Overview
Problem
Existing optical difference detectors using avalanche photodiodes (APDs) face challenges in accurately matching positive and negative bias voltages, leading to variations in amplification factors and reduced effectiveness in removing common-mode noise.
Innovation Solution
The optical difference detector connects the first and second avalanche photodiodes in parallel to a differential amplifier, allowing for easy adjustment of bias voltages and using low frequency components of monitoring currents to control the bias voltages and ensure equal amplification factors.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If a conventional inspection device with separate focus adjustment mechanisms is used, then focus can be adjusted for different magnifications, but the device complexity increases and adjustment time is extended
Solution Approach 1:
The patent combines the focus adjustment function with the magnification adjustment function into a single integrated mechanism. By moving the objective lens along the optical axis in response to magnification changes, the system achieves both magnification adjustment and focus correction through one coordinated action, eliminating the need for separate focus adjustment mechanisms and reducing overall system complexity
Solution Approach 2:
The objective lens positioning mechanism serves multiple functions simultaneously: it controls magnification by changing the optical path length and automatically adjusts focus by positioning the lens at the appropriate focal plane for each magnification level. This multi-functional design eliminates dedicated focus adjustment components while maintaining precise focus across all magnification settings
2Manufacturing precision
If multiple inspection regions with different magnifications are observed sequentially, then comprehensive inspection is achieved, but inspection time increases due to manual refocusing
Solution Approach 1:
The system pre-calculates and automatically executes the optimal objective lens position for each magnification level before inspection begins. When switching between inspection regions requiring different magnifications, the lens is automatically positioned at the pre-determined focal plane corresponding to the target magnification, eliminating manual refocusing operations and reducing inspection time
Solution Approach 2:
The system incorporates feedback control where the controller monitors the selected magnification level and automatically adjusts the objective lens position to maintain optimal focus. This closed-loop control ensures that focus is continuously optimized during inspection transitions without requiring manual intervention, thereby maintaining high inspection speed across multiple regions
3Measurement precision
If separate objective lenses are used for different magnifications, then optimal imaging is achieved for each region, but the number of components and device complexity increase
Solution Approach 1:
Instead of using multiple objective lenses with fixed focal lengths, the system achieves different magnifications by changing the position parameter of a single objective lens along the optical axis. By varying the object distance and image distance parameters dynamically, the system maintains optimal imaging quality across all magnification levels while using only one objective lens, thereby reducing component quantity
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 configuration enables effective removal of common-mode signals, enhancing the signal-to-noise ratio (SNR) in light detection and improving the accuracy of inspections.
Implementation Method 1
a first light receiver that receives light that has passed through the first object and generates a first intensity signal corresponding to an intensity of the light that has passed through the first object
Implementation Method 2
a second light receiver that receives light that has passed through the second object and generates a second intensity signal corresponding to an intensity of the light that has passed through the second object
Implementation Method 3
a subtractor that subtracts the second intensity signal from the first intensity signal and generates a difference signal
Data Source
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AI summary
An optical difference detector 21A includes a first APD 22A and a second APD 22B, a first voltage application unit 23A that applies a first bias voltage to the first APD 22A and a second voltage application unit 23B that applies a second bias voltage to the second APD 22B, a differential amplifier 25 that is connected in parallel to the first APD 22A and the second APD 22B and amplifies a difference between a first signal current output from the first APD 22A and a second signal current output from the second APD 22B, and a feedback control unit 26 that controls the second bias voltage so that a low frequency component of a first monitoring current in the first APD 22A and a low frequency component of a second monitoring current in the second APD 22B are equal.