Directly Modulated Laser Transmitter Dispersion Tolerance
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Solution Overview
Problem
Optical communication systems face significant challenges in transmitting signals over long distances without incurring excessive dispersion penalty due to accumulated group-velocity dispersion (GVD) in optical fibers, as conventional methods like dispersion-compensating modules and electrical pre-compensation increase costs and complexity.
Innovation Solution
A method involving a directly modulated laser transmitter followed by a passive optical spectrum reshaper, where the laser is modulated with a pre-distorted drive current derived from a conventional NRZ-formatted digital signal, generating optical signals that tolerate up to 4600 ps/nm accumulated GVD with less than 1 dB dispersion penalty, using a combination of adiabatic and artificial transient chirp.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If dispersion-compensating modules are inserted periodically along the optical transmission fiber, then the dispersion penalty is reduced, but the system cost and transmission loss increase significantly
Solution Approach 1:
The patent applies preliminary action by pre-distorting the electrical drive signal before it modulates the laser. The pre-distorted signal is designed to compensate for the expected dispersion effects, so that after propagation through dispersive fiber, the signal arrives at the receiver with minimal distortion. This eliminates the need for dispersion-compensating modules along the transmission path, reducing both system cost and transmission loss while maintaining signal quality.
2Length of stationary object
If electrical pre-compensation is used to transform signals, then the distance over which signals can be transmitted is improved, but additional optical modulators are required which increase cost and complexity
Solution Approach 1:
The patent replaces the conventional approach using additional optical modulators with an electrical signal processing solution. Instead of adding optical components to pre-compensate the signal, the invention pre-distorts the electrical drive signal that controls the laser. This substitution of electrical preprocessing for optical compensation reduces device complexity and cost while achieving the same goal of extended transmission distance through dispersion compensation.
3Reliability
If a directly modulated laser transmitter with external modulator is used, then arbitrary GVD can be pre-compensated, but electrical drive power consumption and system cost increase substantially
Solution Approach 1:
The patent extracts and removes the external optical modulator from the transmitter configuration, retaining only the directly modulated laser. By taking out the unnecessary external modulator component, the system achieves dispersion compensation through electrical pre-distortion alone, substantially reducing electrical drive power consumption and system cost while maintaining the capability to compensate for significant amounts of GVD.
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 dispersion tolerance, allowing for longer transmission distances with reduced system complexity and cost, as it does not require additional optical components and achieves similar performance to optically pre-compensated signals with significantly higher GVD.
Implementation Method 1
a directly modulated laser transmitter followed by a passive optical spectrum reshaper, wherein the laser is modulated with a prescribed, pre-distorted drive current
Implementation Method 2
an optical spectrum reshaper that converts frequency modulation of the optical signal to amplitude modulation
Implementation Method 3
generating optical signals that tolerate up to 4600 ps/nm accumulated GVD with less than 1 dB dispersion penalty, using a combination of adiabatic and artificial transient chirp
Data Source
AI summary
An optical transmitter is disclosed wherein a modulating signal, such as an NRZ signal, encoding data is combined with a time derivative of the modulating signal and coupled to a directly modulated laser in order to generate artificial transient chirp in the output of the laser effective to substantially compensate for dispersion experienced by the output of the laser traveling through a dispersive medium such as an optical fiber. In some embodiments, the time derivative is added to the modulating signal only at the falling edges of the modulating signal.