Capacitive Coupled Laser Diode Modulation Signal Stabilization
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Solution Overview
Problem
In optical transmitters using differential electric signals for modulation, capacitive coupling between the Laser Diode Driver (LDD) and Laser Diode (LD) leads to transient state fluctuations in optical signal output due to average potential fluctuations, making it difficult to stabilize the optical signal before transmission.
Innovation Solution
A control circuit equalizes the average potential fluctuations at the normal and reverse phase input terminals of the LD, ensuring they cancel each other as in-phase components of the differential signal, allowing normal optical signal transmission even before stabilization.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Productivity
If capacitive coupling is used between LDD and LD for differential electric signal modulation, then economic efficiency and high-speed access are improved, but transient state fluctuations occur in optical signal output due to average potential fluctuations
Solution Approach 1:
The patent applies equipotentiality by equalizing the time constants of average potential fluctuations at the normal phase and reverse phase input terminals of the LD. By making the time constants equal, the patent ensures that potential fluctuations at both terminals occur at the same rate, allowing them to cancel each other out as in-phase components of the differential signal. This resolves the technical contradiction by maintaining signal stability while preserving the benefits of capacitive coupling for high-speed operation.
2Power
If DC coupling is used between LDD and LD, then large modulation signal amplitude is achieved, but device complexity increases
Solution Approach 1:
The patent applies parameter changes by transitioning from DC coupling to capacitive coupling between the LDD and LD. This parameter change in the coupling method allows the system to achieve high-speed operation with differential electric signals while avoiding the complexity of DC coupling circuits. The patent further optimizes the capacitive coupling by equalizing time constants, thereby resolving the contradiction between signal amplitude and device complexity.
3Reliability
If average potential fluctuations are not equalized at normal and reverse phase input terminals, then waveform deformation occurs and optical signal transmission is impaired
Solution Approach 1:
The patent applies equipotentiality by ensuring that the time constants of average potential fluctuations at the normal phase and reverse phase input terminals are equalized. This is achieved by appropriately setting the capacitance values in the coupling circuits. When the time constants are equal, the potential fluctuations at both terminals occur simultaneously and cancel each other out, preventing waveform deformation and ensuring reliable optical signal transmission without requiring additional stabilization operations.
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 stable optical signal transmission by controlling the average potentials at the input terminals, reducing waveform deformation and ensuring consistent output before the potentials are fully stabilized.
Implementation Method 1
when the LDD and the LD are capacitively coupled, an average potential temporally largely fluctuates depending on a time constant at each terminal
Data Source
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AI summary
A modulation method according to the invention is applied to a modulation device in which a light source is driven by a differential signal while a modulation circuit and the light source are capacitively coupled. In the modulation method of the invention, when an average potential fluctuates at an input terminal of the light source, a potential fluctuation is externally provided using a control circuit such that a normal phase side and a reverse phase side become equal to each other in a time constant of the fluctuation in average potential, and transient states of the average potentials at the normal phase side and reverse phase side are equalized and cancel each other as in-phase components of a signal input to the light source, which allows an optical signal to be normally transmitted from the light source. The optical signal can normally be transmitted from the light source even before the average potentials at a normal phase input terminal and a reverse phase input terminal of the light source are stabilized.