Polarisation-Independent Coherent Receiver Using 3x3 Coupler
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Solution Overview
Problem
Current coherent optical receivers are polarization-sensitive, making them incompatible with cost-effective large-scale deployment in applications like access networks, as they require complex and costly solutions to maintain polarization independence, which increases manufacturing costs and energy consumption.
Innovation Solution
A coherent receiver design that splits the received signal and local oscillator into orthogonal polarization components and uses a 3×3 coupler to generate a resultant electrical signal with a frequency-shifted replica, allowing operation under intradyne conditions and suppressing the replica with a low-pass filter, thereby achieving polarization independence without precise frequency control.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Adaptability or versatility
If polarisation diversity is used to achieve polarisation independence, then the receiver can operate with random polarisation states, but the device complexity and manufacturing cost double due to duplicating the detection chain
Solution Approach 1:
The received optical signal is segmented into two orthogonal polarisation components using a polarising beam splitter. Each component is then processed through a separate 3×3 coupler and photodetector combination, but the processing chains are combined at the squaring and addition stage, achieving polarisation independence without full duplication of the entire receiver structure
Solution Approach 2:
The processing chains for the two orthogonal polarisation components are merged at the analogue processing stage by summing the squared outputs from both polarisation paths. This combining approach achieves polarisation independence while avoiding the need for completely separate detection chains, thereby reducing complexity compared to traditional polarisation diversity receivers
2Adaptability or versatility
If polarisation modulation or automatic polarisation alignment is used, then polarisation independence is achieved, but additional expensive components are required and performance degradation occurs
Solution Approach 1:
The receiver uses the inherent birefringence characteristics of the optical fibre and the fixed polarisation state of the local oscillator to automatically track and compensate for polarisation changes. The system self-adjusts by processing both orthogonal polarisation components through the 3×3 coupler and combining their outputs, eliminating the need for external polarisation control mechanisms or additional expensive components
3Reliability
If the received signal and local oscillator have mismatched polarisation states, then signal fading occurs, but maintaining fixed local oscillator polarisation simplifies the receiver design
Solution Approach 1:
The received signal is segmented into two orthogonal polarisation components, each of which can be independently mixed with the local oscillator. This segmentation ensures that at least one component will have adequate overlap with the fixed polarisation state of the local oscillator, preventing complete signal fading while avoiding the need for dynamic polarisation control
Solution Approach 2:
The system changes the processing approach by squaring the beat signals from both polarisation components before summing them. This parameter change in the signal processing domain compensates for polarisation mismatches and ensures stable signal detection regardless of the relative polarisation states between the received signal and local oscillator
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
The solution enables low-cost, polarization-independent operation of coherent receivers, suitable for wide-scale deployment in access networks, by utilizing a polarising beam splitter and existing analogue processing, reducing manufacturing costs and energy consumption while maintaining high sensitivity and selectivity.
Implementation Method 1
the beat between the received signal, which is an amplitude-modulated signal, and the signal from the local oscillator is carried out by means of a multiport optical coupler... three signals, which are each proportional to the optical beat between the received signal and the signal from the local oscillator and are phase shifted by a phase shift which is different for each of the three outputs
Implementation Method 2
The three signals are independently detected by respective photodetectors, which provide three analogue signals
Data Source
AI summary
In a coherent optical receiver, a received signal and an oscillator-generated signal, having frequency difference such that the receiver operates under intradyne conditions, are made to beat in a 3×3 optical coupler. A polarizing beam-splitter splits one of the signals into components with orthogonal polarization which are applied to inputs of the coupler, which receives the other of the received or oscillator-generated signal. After photoelectric conversion, the signals are fed to analog processing devices generating an electrical signal representing the received signal that is fed to a low pass filter before being demodulated. The frequency difference between the signals and the passband of the filter are such that a component of the electrical signal, oscillating at a frequency depending on the frequency difference and having amplitude and phase depending on the instant state of polarization of the received signal, is suppressed. A method is also provided.