Coherent Receiver Calibration for Common Mode Rejection
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Solution Overview
Problem
Coherent optical receivers are susceptible to common mode interference due to mismatch in photodiode responsivity, leading to degradation in signal quality, especially in applications where laser frequency accuracy is unpredictable and difficult to control.
Innovation Solution
A calibration algorithm is implemented using a switched input stimulus and a repurposed transmit laser as the local oscillator, allowing for in-field adjustment of photodiode bias to maximize common mode rejection ratio (CMRR) and minimize interference, leveraging existing transmit path components for calibration without additional circuitry.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If a double balanced photodiode configuration is used for coherent detection, then common mode rejection is improved, but photodiode responsivity mismatch still degrades performance
Solution Approach 1:
The patent applies preliminary action by performing calibration before actual signal reception. A calibration algorithm is executed that adjusts photodiode bias voltages to equalize responsivity values, preventing performance degradation before it occurs. This pre-adjustment ensures optimal common mode rejection is maintained throughout operation.
Solution Approach 2:
The patent implements feedback through a calibration algorithm that continuously monitors and adjusts photodiode bias settings. The system measures the actual responsivity of each photodiode and automatically adjusts bias voltages to equalize them, creating a closed-loop control system that maintains optimal performance despite environmental variations or aging effects.
2Adaptability or versatility
If laser frequency accuracy is not controlled, then system adaptability is improved, but common mode interference increases
Solution Approach 1:
The patent converts the harmful effect of laser frequency variations into a beneficial calibration opportunity. By using the actual operating laser (with its inherent frequency characteristics) as the local oscillator during calibration, the system adapts to and compensates for frequency-specific interference patterns, turning an uncontrolled variable into a calibration parameter.
Solution Approach 2:
The system performs self-calibration using its own internal components. The calibration algorithm uses the transmit laser itself as the local oscillator, eliminating the need for external reference equipment. This self-service approach allows the system to automatically equalize photodiode responsivity and compensate for its own frequency characteristics without external intervention.
3Measurement precision
If additional circuitry is added for calibration, then measurement precision is improved, but device complexity increases
Solution Approach 1:
The patent applies universality by making existing circuitry perform multiple functions. The transmit laser serves dual purposes: as the signal source during normal operation and as the local oscillator during calibration. Similarly, the photodiodes and signal processing circuits handle both calibration and operational signals, eliminating the need for dedicated calibration hardware.
Solution Approach 2:
The system uses its own operational components for calibration without requiring external equipment. The calibration algorithm repurposes the transmit path components (laser, modulator, photodiodes) to perform self-diagnosis and self-adjustment, maintaining measurement precision while avoiding additional hardware complexity.
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 CMRR of coherent optical receivers, reducing interference and improving signal-to-interference plus noise ratio (SINR) by equalizing photodiode responsivity and canceling phase noise, thus enhancing the performance and reliability of optical communication systems across varying environmental conditions.
Implementation Method 1
a first differentially configured pair of photodiodes, the photodiodes currents output being operably coupled to an analog to digital converter (ADC)
Implementation Method 2
adapting at least one bias setting of a photodiode associated with the first receiver in response to at least one first stimulus detected at the first receiver analog-to-digital converter (ADC) to an adaptive algorithm circuitry
Data Source
AI summary
A method for calibrating an optical transceiver. The method can include configuring optical switches to enable routing at least one output signal of modulator circuitry operably coupled to a first receive path of a coherent optical transceiver. The method can include configuring the input to at least one modulator to generate at least one first stimulus signal. The method can include configuring a path from the first receiver analog-to-digital converter to an adaptive algorithm circuitry. The method can include adapting at least one bias setting of a photodiode associated with the first receiver in response to at least one first stimulus detected at the first receiver analog-to-digital converter to an adaptive algorithm circuitry. The method can include determining an optimum value of a photodiode associated with the first receiver.