Free Space Optical Receiver Multi-Mode Signal Processing
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Solution Overview
Problem
In free space optical receivers, the increase in circuit size of the signal processing circuit leads to higher costs and complexity due to the need for efficient coupling of laser light with single-mode fibers, which is hindered by wave-front distortion and scintillation, especially with multi-mode fibers having large core diameters that excite numerous modes, requiring complex MIMO processes.
Innovation Solution
A free space optical receiver configuration that includes a light collecting unit, a multi-mode transmission medium, a multi-mode signal processing unit, a monitor unit, and a control unit to convert laser light into multi-mode light, monitor signal information, and synthesize output signals, thereby reducing the circuit size and improving coupling efficiency by selectively processing only strong signals and using mode conversion to concentrate optical energy into fewer modes.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If a multi-mode fiber with large core diameter is used to improve coupling efficiency and tolerate beam wander, then the coupling efficiency is improved, but the number of excited modes increases requiring complex MIMO signal processing
Solution Approach 1:
The patent segments the multi-mode fiber into multiple single-mode fibers, each carrying a specific mode group. This segmentation reduces the number of modes that need to be processed simultaneously, thereby simplifying the MIMO signal processing circuit while maintaining good coupling efficiency through the large core area of the multi-mode fiber.
Solution Approach 2:
The patent introduces mode selection based on propagation constants, adding a new dimension to the signal processing approach. By selecting specific mode groups with appropriate propagation constants and filtering out others, the system reduces the effective number of modes requiring complex MIMO processing, thus lowering circuit complexity while preserving coupling efficiency.
2Productivity
If single-mode fiber is used to achieve high bit rate transmission, then the bit rate is improved, but the coupling efficiency deteriorates due to beam spot variation and scintillation
Solution Approach 1:
The patent uses a multi-mode fiber with large core diameter that is segmented into multiple single-mode fibers, each handling specific mode groups. This approach maintains good coupling efficiency with the atmospheric channel by utilizing the large core area, while still achieving high bit rate transmission through the single-mode fiber segments that support high-speed modulation.
Solution Approach 2:
The patent changes the fiber parameter from single-mode to multi-mode with large core diameter to improve coupling efficiency and tolerate beam wander. By carefully selecting the core diameter and numerical aperture parameters, the system achieves both high coupling efficiency and high bit rate capability through mode-selective transmission.
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 suppresses the increase in circuit size, enhances coupling efficiency, and stabilizes the reception of high-capacity optical signals by reducing the number of input signals to the signal processing circuit and optimizing mode selection, leading to miniaturization and cost-effectiveness.
Implementation Method 1
laser light having propagated through a free space transmission path
Implementation Method 2
a multi-mode transmission medium for receiving input of the laser light and outputting multi-mode light
Implementation Method 3
a monitor unit for monitoring signal information based on the multi-mode light beam
Implementation Method 4
a photodetector for converting the multi-mode light beam into a plurality of received electrical signals
Data Source
AI summary
The circuit size of a signal processing circuit increases in a free space optical receiver, which increases in cost, because the signal processing becomes complex attempting to improve the coupling efficiency between received light and a fiber; therefore, a free space optical receiver according to an exemplary aspect of the present invention includes a light collecting means for collecting laser light having propagated through a free space transmission path; a multi-mode transmission medium for receiving input of the laser light and outputting multi-mode light; a multi-mode signal processing means for outputting a plurality of received electrical signals based on part of the multi-mode light; a monitor means for monitoring signal information based on the multi-mode light; a control means for controlling an operation of the multi-mode signal processing means based on the signal information; and a signal processing means for performing signal processing on the plurality of received electrical signals and outputting an output signal synthesized from the plurality of received electrical signals.