Burst Mode Laser Transmitter Power Control via Optical Sampling
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Solution Overview
Problem
Burst mode laser transmitters in GPONs face challenges due to high parasitic capacitance in photodiodes, which affects accurate tracking of light intensity and implementation of high-speed comparison circuits, leading to inefficient automatic power control.
Innovation Solution
The solution involves a burst mode laser transmitter apparatus with a current slicer, local oscillator, and automatic power control logic that compares photodiode current with reference currents using binary decisions and validation signals, updated by a timing determined by the transmit enable signal and local clock, to adjust bias and modulation currents effectively, even with high parasitic capacitance.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If photodiode is used to detect light intensity, then light intensity tracking is enabled, but high parasitic capacitance causes inaccurate tracking and slows down response speed
Solution Approach 1:
The patent extracts the photodiode from the direct feedback path by using a tap to sample a portion of the optical signal separately. This sampled signal is then processed through a transimpedance amplifier and comparator to generate error signals for APC, while the main optical path remains unaffected by the photodiode's parasitic capacitance, thus maintaining both accuracy and speed.
Solution Approach 2:
The patent introduces intermediary components including a transimpedance amplifier and comparator circuits that process the photodiode signal. These intermediaries buffer and condition the signal, isolating the photodiode's capacitive effects from the main control loop and enabling accurate light intensity measurement without compromising response speed.
2Productivity
If high-speed comparison circuit is implemented, then automatic power control speed is improved, but high parasitic capacitance of photodiode makes it challenging to implement
Solution Approach 1:
The patent extracts the comparison function from the direct photodiode output path and implements it after signal conditioning through the transimpedance amplifier. This separation allows the use of standard comparator circuits at optimized speeds without being constrained by the photodiode's capacitance, reducing implementation difficulty while maintaining high control speed.
Solution Approach 2:
The patent replaces direct electrical connection and comparison with an optical sampling approach followed by transimpedance conversion. This substitution transforms the high-speed electrical comparison problem into a more manageable signal processing sequence, reducing circuit complexity while achieving the desired control speed.
3Device complexity
If photodiode current is used directly for APC, then simple control is achieved, but inaccurate tracking due to parasitic capacitance occurs
Solution Approach 1:
The patent introduces a transimpedance amplifier as an intermediary between the photodiode and the APC control logic. This amplifier converts the photodiode current into a voltage signal and provides signal conditioning, improving tracking accuracy while adding minimal complexity to the overall control circuit.
Solution Approach 2:
The patent performs preliminary signal conditioning through the transimpedance amplifier before the signal enters the APC control loop. This preliminary action includes current-to-voltage conversion, gain adjustment, and noise filtering, which improve measurement accuracy before the signal is used for control decisions.
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 accurate and efficient automatic power control, ensuring the light intensity is maintained at target levels, improving the tracking of light intensity and reducing the impact of high parasitic capacitance on the photodiode's performance.
Implementation Method 1
a photodiode (MPD) 140 for receiving a part of the light signal from a back facet of the laser diode 120 and outputting a photodiode current IPD
Data Source
AI summary
An apparatus of automatic power control for burst mode laser transmitter and method are provided. In one implementation a method includes: generating an output current with a modulation pattern determined by a transmit data and a transmit enable signal, and a modulation level determined by a first control code and a second control code, wherein a light signal is generated in response to the output current; generating a first decision based on a comparison between a photodiode current and the first reference current, a second decision based on a comparison between the photodiode current and the second reference current, wherein the photodiode current is generated in accordance to the light signal; and generating the first control code and the second control code in response to the first decision and the second decision.