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Linear Approximation in Frequency Domain

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Linear systems are characterized by two main properties: superposition and homogeneity. Superposition allows the response to multiple inputs to be the sum of the responses to each individual input. Homogeneity ensures that scaling an input by a scalar results in the response being scaled by the same scalar.
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Maximum likelihood sequence estimation for optical complex direct modulation.

Di Che, Feng Yuan, William Shieh

|April 26, 2017

Summary

This summary is machine-generated.

This study introduces complex direct modulation (CDM) for semiconductor lasers, enabling simultaneous intensity and angle modulation. This innovation achieves a record 100-Gb/s PAM-4 transmission over 1600-km fiber using low-cost lasers.

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Area of Science:

Photonics and Optical Communications
Semiconductor Laser Technology
Digital Signal Processing

Background:

Semiconductor lasers are key optical transmitters, typically used for 1-D modulation (intensity or angle) due to direct detection limitations.
Direct modulation (DM) offers intensity modulation via current-power mapping and angle modulation through frequency chirp.
Previous methods suppressed one modulation aspect, limiting laser diode capabilities.

Purpose of the Study:

To enable simultaneous intensity and angle modulation (2-D complex DM, CDM) using a single laser diode.
To enhance the accuracy of the frequency chirp model for CDM.
To demonstrate high-speed, long-haul optical transmission using cost-effective DM lasers.

Main Methods:

Digital coherent detection to enable simultaneous intensity and angle modulation.
Maximum Likelihood Sequence Estimation (MLSE) for joint demodulation of intensity and differential phase.
A novel statistical method to improve the accuracy of the frequency chirp model for MLSE transition probability.
Closed-form discrete signal approximation of frequency chirp.

Main Results:

Demonstrated the first single-channel 100-Gb/s Pulse Amplitude Modulation with 4 levels (PAM-4) transmission.
Achieved transmission over 1600-km of optical fiber.
Utilized only 10G-class direct modulation (DM) semiconductor lasers.

Conclusions:

Complex direct modulation (CDM) with enhanced chirp modeling significantly boosts single laser diode capabilities.
High-speed, long-haul optical communication is feasible with low-cost DM lasers.
This approach overcomes limitations of traditional direct detection in optical transmitters.