Directly Modulated Laser Pre-Compensation for Dispersion

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Solution Overview

Problem

Directly modulated semiconductor lasers in optical communications systems face challenges due to non-linearities such as damped oscillatory transient responses, frequency chirp, and longitudinal mode spatial hole burning, which lead to dispersion-induced distortion and limit the development of low-cost, high-data-rate, long-reach systems.

Innovation Solution

An optical communications system with a current-driven directly modulated laser and a current controller that applies pre-compensation to the laser input current waveform to counteract the effects of laser non-linearities and fiber chromatic dispersion, using a memory-based look-up table to store profiles for different fiber span dispersions.

Engineering Contradictions & Design Principles

VSEngineering Contradiction Analysis

1Ease of manufacture

If directly modulated lasers are used to reduce cost, then manufacturing cost is reduced, but dispersion-induced distortion increases due to non-linearities

Engineering Contradiction:

Improvemanufacturing costVSAvoiddispersion-induced distortion

Core Design Contradiction:

Ease of manufactureVSObject-affected harmful factors

Solution Approach 1:

The patent applies pre-compensation to the laser drive current waveform before modulation, using a memory-based look-up table to pre-correct for chromatic dispersion and laser non-linearities. This preliminary action counteracts the distortion that would otherwise occur during transmission, enabling the use of cost-effective directly modulated lasers while maintaining signal quality.

Inventive Principle:

Principle #10Preliminary action

2Productivity

If current modulation is applied to achieve high data rates, then productivity increases, but non-linearities such as frequency chirp and spatial hole burning increase

Engineering Contradiction:

Improvedata rateVSAvoidlaser non-linearities

Core Design Contradiction:

ProductivityVSObject-generated harmful factors

Solution Approach 1:

The patent modifies the drive current waveform parameters through pre-compensation, adjusting the amplitude and temporal profile of the current signal to counteract non-linear effects. By changing the current waveform parameters in advance, the system achieves high data rates while reducing frequency chirp and spatial hole burning effects.

Inventive Principle:

Principle #35Parameter changes

3Device complexity

If simple directly modulated lasers are used instead of external modulators, then device complexity is reduced, but frequency chirp increases

Engineering Contradiction:

Improvemodulation system complexityVSAvoidfrequency chirp

Core Design Contradiction:

Device complexityVSObject-generated harmful factors

Solution Approach 1:

The patent replaces the need for complex external modulation systems with a simplified directly modulated laser approach, using electrical pre-compensation instead of optical modulation complexity. This substitution maintains low device complexity while addressing frequency chirp through electronic pre-correction of the drive current.

Inventive Principle:

Principle #28Mechanics substitution (Replace mechanical system)

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 effectively compensates for chromatic dispersion and laser non-linearities, reducing distortion and enabling low-cost, high-data-rate, long-reach optical communications systems by pre-processing the signal in the electrical domain before modulation.

Implementation Method 1

Directly modulated lasers are current-controlled, and the current modulation applied determines the power in the '1' and '0' levels, as well as the output optical frequency

Methodology Applied

Scientific EffectDirect modulation:

Implementation Method 2

The presence of non-linearity in the gain mechanism due to intra-band carrier relaxation effects, whereby the gain is not only dependent on the injected carrier density, but also on the photon density or power, gives rise to adiabatic frequency chirp

Methodology Applied

Scientific EffectFrequency chirp:

Implementation Method 3

Chromatic fibre dispersion remains a limiting factor in the development of low cost, high data rate and long reach optical communications systems

Methodology Applied

Scientific EffectChromatic dispersion: Dispersion (of waves)

Implementation Method 4

the current waveform applied to the laser is determined to compensate for the effects of the laser non-linearities and the fiber chromatic dispersion

Methodology Applied

Scientific EffectPre-compensation:

Data Source

PatentUS7805082B1Optical laser control for optical communications systems

Publication Date: 2010.09.28 CIENA CORP

AI summary

An optical communications system comprises a transmitter, a receiver and an optical communications link between the transmitter and receiver. The transmitter comprises a current-driven directly modulated laser for providing a modulated optical signal and a current controller for controlling the current waveform applied to the laser. The current waveform applied to the laser is determined to compensate for the effects of the laser non-linearities and the fiber chromatic dispersion. This system applies pre-compensation to the directly modulated laser input current waveform to provide both pre-compensation for chromatic dispersion and compensation for the non-linearities of the directly modulated laser. These are two of the main limiting factors in providing a low cost high data rate and long reach optical communications system.