Optical Transmitter PAM Signal Level Control
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Solution Overview
Problem
Optical transmitters using pulse-amplitude modulation (PAM) face challenges due to non-linearity in the transfer characteristic of light-generating devices, leading to uneven optical signal levels and compromised data transmission quality.
Innovation Solution
An optical transmitter system that includes a light-generating device and a driver with a level controller to adjust electrical levels based on the transfer characteristic, ensuring optical levels have preset ratios and even differences, thereby compensating for non-linearity and improving signal quality.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Device complexity
If a light-generating device is driven by an electrical signal with evenly distributed levels to generate a PAM optical signal, then the device structure remains simple, but the optical levels become uneven due to non-linearity in the transfer characteristic
Solution Approach 1:
The driver pre-calculates and applies compensation to the electrical signal levels before they reach the light-generating device. By determining the transfer characteristic in advance and adjusting the electrical levels accordingly, the system compensates for non-linearity before optical generation occurs, ensuring uniform optical levels without complex real-time correction mechanisms.
2Reliability
If the electrical levels are adjusted to compensate for non-linearity and achieve even optical levels, then the optical signal quality improves, but the driver complexity increases
Solution Approach 1:
The driver performs self-characterization by measuring its own transfer characteristic between the electrical input signal and optical output signal. This self-measured data is then used to automatically determine appropriate electrical levels that compensate for non-linearity, eliminating the need for external calibration equipment or complex lookup tables, thereby improving reliability while keeping the driver structure relatively simple.
Solution Approach 2:
The system incorporates feedback mechanisms where the driver measures the actual transfer characteristic and uses this information to adjust electrical levels dynamically. This closed-loop approach ensures accurate compensation for non-linearity, maintaining high data transmission quality while adapting to variations in device performance over time or environmental conditions.
Data Source
AI summary
A method of controlling an optical transmitter comprising a determining step, a dividing step and a determining step. In the determining step, a full range of an optical signal output from a light-generating device is determined by setting an upper and a lower limit thereof by keeping power of a continuous wave (CW) light constant and varying an electrical driving signal, the optical signal having 2n optical levels where n is an integer. In the dividing step, the full range of the optical signal is divided into sub-ranges each between the neighbor optical levels including the upper limit and the lower limit of the full range, the sub-ranges having preset ratios. In the determining step, electrical levels of the electrical driving signal are determined based on a non-linear transfer characteristic of the light-generating device between the electrical driving signal supplied thereto and the optical signal output therefrom.