Photodiode Alignment Using Microscopic Focus and Magnetic Fixturing
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Solution Overview
Problem
The challenge lies in accurately positioning the active area of a photodiode within a hermetically sealed can relative to a lens in three axes (X, Y, Z) to achieve precise optical alignment, which is essential for applications like infrared missile guidance systems, where conventional methods fail to meet stringent tolerance ranges due to the inability to physically access the photodiode surface.
Innovation Solution
A fixturing system utilizing an oversized aperture and a microscope/video camera setup allows for the photodiode can to be moved in orthogonal directions using a magnetic plunger or vacuum fixturing tool, enabling precise alignment of the photodiode's active area at the focal point of the lens without physical contact, using micrometers and focus/defocus techniques.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If the photodiode is hermetically sealed inside the can, then the photodiode is protected from environmental damage, but the active area becomes inaccessible to physical manipulation for alignment
Solution Approach 1:
The patent introduces an intermediary optical system consisting of a lens and microscope that enables indirect observation and alignment of the sealed photodiode's active area. The lens focuses light onto the active area, and the microscope allows visual feedback on the position of the active area relative to the lens, serving as a mediator between the sealed photodiode and the alignment process
Solution Approach 2:
The patent replaces direct mechanical manipulation of the photodiode with an optical-based alignment system. Instead of physically touching or probing the sealed photodiode, the system uses optical focusing and visual feedback through a microscope to achieve precise alignment, substituting mechanical interaction with optical interaction
2Ease of manufacture
If conventional soldering methods are used to mount the photodiode, then the manufacturing process is simple, but the X, Y, and Z location of the active area cannot be controlled within tight tolerance ranges
Solution Approach 1:
The patent performs preliminary alignment of the photodiode can to the lens before final sealing and mounting. By using the optical feedback system to pre-position the active area at the focal point of the lens, the alignment is established before the photodiode is permanently fixed, allowing precise positioning to be achieved before the simplicity of conventional mounting is applied
Solution Approach 2:
The patent implements a feedback mechanism where the microscope provides visual information about the position of the active area relative to the lens focal point. This feedback allows the operator to adjust the photodiode can position and verify alignment, creating a closed-loop system that achieves precise positioning despite using simple mounting methods
3Productivity
If the photodiode active area is not precisely positioned at the focal point of the lens, then the mounting process is faster, but the optical alignment tolerance of plus or minus 0.003 inches cannot be achieved
Solution Approach 1:
The optical system acts as an intermediary that enables rapid visual verification of alignment without requiring slow, iterative mechanical adjustments. The microscope provides immediate feedback on whether the active area is positioned at the focal point, allowing quick verification and adjustment while maintaining tight tolerances
Solution Approach 2:
The patent replaces slow mechanical trial-and-error alignment methods with rapid optical observation and positioning. By using the lens-microscope system to visually determine alignment status, the process achieves both speed and precision, eliminating the need for time-consuming mechanical adjustments while maintaining the required optical alignment tolerance
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 method achieves precise alignment of the photodiode's active area within tight tolerance ranges (0.002 to 0.003 inches), ensuring maximum light collection and efficient optical communication systems by maximizing the signal-to-noise ratio, applicable beyond infrared seeker assemblies.
Implementation Method 1
temporarily affixing the bottom of the can to a magnetic plunger
Implementation Method 2
vacuum-operated fixturing tool
Implementation Method 3
an array of photodetectors is arranged to detect the reflections, with each of the photodetectors provided with an imaging or relay lens adjacent the photodetector so that the reflected light is focused onto the active area of the photodiode
Data Source
AI summary
A fixturing system and microscope/video camera setup enables an operator to manipulate a photodiode into position optically using known good targets for the X and Y location and using microscope focus/defocus/refocus for locating the active area of the avalanche photodiode exactly at the focal point of the lens.