AC-Coupled Single-Ended Photodetectors for High Baud Rate Coherent Receivers
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Solution Overview
Problem
Conventional balanced photodetectors in coherent optical receivers face challenges in maintaining high common mode rejection ratio (CMMR) at high baud rates, such as 100 Gigabaud and beyond, due to bandwidth limitations of opto-electronic components.
Innovation Solution
The use of single-ended photodetectors AC-coupled to a digital signal processor, which generates electrical signals by removing the DC component and digitizing the AC component for processing, allowing for estimation of time-averaged power and phase recovery of quadrature modulated optical signals, thereby simplifying the receiver architecture and relaxing ADC resolution requirements.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Speed
If conventional balanced photodetectors are used in coherent optical receivers, then common mode rejection ratio (CMMR) can be maintained, but bandwidth limitations of opto-electronic components prevent operation at high baud rates (100 Gbd and higher)
Solution Approach 1:
The patent replaces the conventional balanced photodetector architecture with single-ended photodetectors combined with AC coupling and digital signal processing. This substitution moves the common mode rejection function from the analog opto-electronic domain to the digital signal processing domain, enabling operation at high baud rates while maintaining CMMR performance.
Solution Approach 2:
The patent changes the operating parameters by using AC coupling instead of DC coupling in single-ended photodetectors. This parameter change allows the system to reject common mode signals effectively at high frequencies by filtering out DC components digitally, thereby achieving both high baud rate operation and maintained CMMR.
2Device complexity
If single-ended photodetectors with DC coupled electrical outputs are used, then receiver architecture is simplified, but common mode rejection ratio deteriorates at high baud rates
Solution Approach 1:
The patent introduces AC coupling as an intermediary element between the single-ended photodetector and the subsequent signal processing stages. This AC coupling mechanism acts as a mediator that enables the simple single-ended architecture to achieve common mode rejection by blocking DC components while allowing AC signal transmission, thus resolving the contradiction between architectural simplicity and CMMR performance.
Solution Approach 2:
The patent substitutes the traditional DC-coupled balanced photodetector system with an AC-coupled single-ended photodetector system where common mode rejection is achieved through digital signal processing rather than analog balancing. This substitution maintains architectural simplicity while achieving the required CMMR at high baud rates.
3Measurement precision
If balanced photodetectors are used to maintain CMMR, then signal quality is preserved, but ADC resolution requirements increase due to DC component handling
Solution Approach 1:
The patent extracts and removes the DC component from the photodetector output through AC coupling before the signal reaches the ADC. By taking out the DC component early in the signal chain, the system eliminates the need for high ADC resolution to handle DC levels, while preserving signal quality through subsequent digital processing that maintains the AC signal integrity.
Solution Approach 2:
The patent performs preliminary action by AC coupling the photodetector output to remove DC components before analog-to-digital conversion. This preliminary processing step reduces the dynamic range requirements for the ADC, allowing lower resolution ADCs to achieve the same signal quality that would require high resolution ADCs in a DC-coupled balanced photodetector system.
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 enables coherent optical receivers to operate effectively at high baud rates by simplifying the receiver architecture and reducing the complexity of ADC requirements, while maintaining signal quality through digital signal processing.
Implementation Method 1
The at least one optical mixer is configured to receive an OLO signal from the OLO and a data-modulated optical signal and to generate first and second light mixtures by interfering parts of said data-modulated optical signal and said OLO signal with corresponding first and second relative phases
Implementation Method 2
Each single-ended photodetection circuit is connected to generate a single-ended electrical measure of a corresponding one of the light mixtures responsive to receipt thereof
Data Source
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Figure 2
Figure 3~4
AI summary
An optical coherent receiver for an fiber-optic communication system includes an optical mixer configured to mix data-modulated optical signal and an optical local oscillator signal, and to provide two mixed optical signals to two single-ended photo detector (SEPD) circuits. Each of the SEPD signal circuits is AC-coupled to a digital signal processor (DSP). The DSP is configured to generate estimates of the average powers of the data-modulated optical signals and the OLO signal. The DSP may be further configured to control the OLO to signal power ratio at the optical mixer input based on a ratio of the average power estimates. The DSP may be further configured to generate estimates of the in-phase and quadrature signal components from the AC-coupled SEPD signals using the DSP-generated average power estimates.