Free Space Optical Terminal Polarization Multiplexing
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Solution Overview
Problem
Free space optical communications systems face challenges in separating Quantum Key Distribution (QKD) photons from the main optical beam, particularly in aircraft and spacecraft where weight and space are limited, due to the need for different wavelengths which can result in inefficient propagation and require separate bulk optics.
Innovation Solution
The system employs circular polarization for both the main optical beam and QKD photons, allowing them to share similar wavelengths (differing by 10-30 nm) and using optics like quarter wave plates and wavelength filters to separate them effectively, enabling robust channel separation and shared bulk optics.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If different wavelengths are used for QKD photons and main optical beam, then channel separation is improved, but propagation efficiency deteriorates and separate bulk optics are required
Solution Approach 1:
The patent changes the separation parameter from wavelength difference to polarization state difference. By using circular polarization states (left-handed and right-handed) instead of different wavelengths, the system achieves channel separation while maintaining similar wavelengths for both QKD photons and main optical beam, thereby preserving propagation efficiency and allowing shared bulk optics
Solution Approach 2:
The patent enables bulk optics to serve multiple functions by using the same optical components for both QKD photons and main optical beam since they operate at similar wavelengths. The polarization-based separation allows a single set of bulk optics to handle both channels, eliminating the need for separate optical paths
2Reliability
If different wavelengths are used for QKD photons and main optical beam, then channel separation is improved, but device weight and space increase due to separate bulk optics
Solution Approach 1:
The patent merges the optical paths for QKD photons and main optical beam by using the same bulk optics for both channels. The polarization multiplexing technique allows both signals to coexist in the same optical path, reducing the overall system weight and space requirements compared to having separate optical paths for each wavelength channel
Solution Approach 2:
The bulk optics are designed to be universal, handling both QKD photons and main optical beam simultaneously. This multi-functionality is achieved through polarization-based channel separation, allowing a single set of optical components to serve dual purposes and reduce terminal weight
3Productivity
If circular polarization is used for both QKD photons and main optical beam, then shared bulk optics is enabled, but polarization separation complexity increases
Solution Approach 1:
The patent applies local quality by introducing polarization-selective components (such as polarizing beam splitters or wavelength filters with polarization sensitivity) at specific points in the optical path where separation is needed. The bulk optics remain general-purpose, while localized polarization control elements handle the channel separation, maintaining simplicity in the majority of the system
4Productivity
If similar wavelengths are used for QKD photons and main optical beam, then propagation efficiency is improved, but channel separation becomes difficult
Solution Approach 1:
The patent changes the separation parameter from wavelength to polarization state. By encoding QKD photons and main optical beam with orthogonal circular polarization states (left-handed and right-handed respectively), the system achieves reliable channel separation while maintaining similar wavelengths, thereby preserving propagation efficiency through the atmosphere
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 enhances the separation between QKD and classical channels, ensures effective propagation through varying atmospheres, and reduces the weight and space requirements of communication terminals, particularly beneficial for vehicles like drones and satellites.
Implementation Method 1
a first optics arrangement configured to: combine the first beam and the photons into a single, second, beam to be transmitted to a target; and transform the first linear polarization of the first beam into one of left and right circular polarization and transform the second linear polarization of the photons into the other of left and right circular polarization
Implementation Method 2
the first beam has a first wavelength and the photons have a second wavelength different to the first wavelength
Data Source
AI summary
A free space optical communications transmitter terminal including an optical source arranged to provide a first beam of light encoding information to be communicated; an optical source arranged to provide photons encoding bits of a key of a Quantum Key Distribution protocol; and an optics arrangement. The first beam and the photons are linearly polarised. The optics arrangement is configured to combine the first beam and the photons into a single, second, beam to be transmitted to a receiver terminal; and transform the linear polarisation of the first beam into one of left and right circular polarisation and transform the linear polarisation of the photons into the other of left and right circular polarisation. A receiver terminal receives the second beam, transforms the circular polarisations into orthogonal linear polarisations, and filters out the linear polarisation of the first beam to allow the photons to pass to a single photon detector.