Directly Modulated Laser Linearization via Pre-Distorted Drive Current
Find Innovative SolutionsGenerate Solutions
Solution Overview
Problem
Directly modulated lasers exhibit intrinsic high-frequency dynamic non-linearity, leading to skew, level thickness, and increased jitter in optical eye diagrams, particularly with modulation formats like PAM-4, which cannot be mitigated by linear equalization.
Innovation Solution
A method to drive directly modulated lasers by generating a modulating current waveform that approximates an ideal current, correcting non-linearities by including constant, first derivative, and second derivative terms, allowing for practical implementation using analog or digital circuits to produce a linear optical output waveform.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Speed
If direct modulation of laser is used for optical data communications, then transmission speed is improved, but non-linearity in optical output waveform increases
Solution Approach 1:
The patent applies preliminary action by pre-distorting the drive current waveform before it reaches the laser. The drive current is shaped in advance to compensate for the known non-linear response of the laser, so that when the laser modulates the light, the output waveform becomes linear. This is achieved by adding derivative terms (first and second derivatives of the data signal) to the drive current, which pre-corrects the non-linearities before they occur in the optical output.
2Productivity
If high frequency modulation is used, then data rate is improved, but dynamic non-linearity of laser increases
Solution Approach 1:
The patent applies parameter changes by modifying the drive current waveform parameters to compensate for frequency-dependent non-linearities. By incorporating derivative terms with specific time constants (τ1 and τ2) into the drive current equation, the system dynamically adjusts the current waveform shape based on the modulation frequency, thereby maintaining linear optical output across a wide bandwidth and high data rates.
3Device complexity
If approximation method is used for drive current calculation, then implementation complexity is reduced, but correction precision may be affected
Solution Approach 1:
The patent applies partial action by implementing only the essential components of the full non-linearity compensation - specifically the first and second derivative terms with dominant time constants. Rather than calculating all possible non-linear correction terms, the patent selectively includes the most significant terms that provide the majority of the correction benefit, achieving good linearity with reduced computational and implementation complexity.
Data Source
AI summary
Techniques for driving a directly modulated laser whilst correcting non-linearities an optical output waveform, based on generating a modulating current waveform that approximates an ideal modulating current that produces a linear optical output waveform. The techniques enable useful, practical approximations to the ideal modulating current to be determined.