Optical PAM4 Generation Using Dual EA Modulators for High Linearity
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Solution Overview
Problem
Existing PAM4 signal generation technologies, such as those based on single-ended electro-absorption modulated lasers (EML) and directly modulated lasers (DML), face challenges in achieving high linearity and quality due to limitations in device design, temperature sensitivity, and chirp effects, making it difficult to generate high-quality PAM4 signals.
Innovation Solution
A PAM4 signal generation apparatus utilizing a distributed feedback laser (DFB), two electro-absorption modulators, a semiconductor optical amplifier (SOA), and a polarization beam splitting rotator (PSR) to generate and multiplex NRZ optical signals, reducing linearity requirements and eliminating the need for expensive linear electrical devices, thereby enabling high-quality PAM4 signal generation in the optical domain.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Device complexity
If a single-ended EML is used to generate PAM4 signal, then the device structure is simple, but the linearity is poor and signal quality is low
Solution Approach 1:
The patent divides the modulation function into two separate EA modulators (first and second EA modulators) that independently modulate two different optical signals from the DFB laser. Each modulator handles one bit of the 2b1q coding scheme, avoiding the linearity requirements of a single high-complexity modulator while achieving PAM4 signal generation through optical domain combination.
Solution Approach 2:
The patent replaces the need for linear electrical amplification with optical domain processing. By using two EA modulators to directly modulate optical signals and combining them through optical coupling, the system eliminates the requirement for linear electrical devices that would otherwise be needed to maintain signal linearity in electrical-domain PAM4 generation.
2Manufacturing precision
If high-linearity devices are used to generate PAM4 signal, then signal quality is improved, but device design difficulty increases
Solution Approach 1:
The patent substitutes optical modulation for electrical amplification by using EA modulators that directly modulate optical signals. This approach achieves high signal quality through optical domain processing without requiring complex high-linearity electrical devices, thereby reducing overall design difficulty while maintaining signal integrity.
Solution Approach 2:
The patent segments the PAM4 modulation process into two independent NRZ modulation channels, each handled by a separate EA modulator. This segmentation allows each modulator to operate in its optimal non-linear region while the optical combination achieves the desired PAM4 linearity, simplifying individual device design requirements.
3Manufacturing precision
If amplitude-limiting devices are used in electrical domain, then linearity can be maintained, but device cost increases
Solution Approach 1:
The patent replaces electrical domain amplitude limiting with optical domain modulation. By using EA modulators that directly modulate optical signals according to 2b1q coding rules, the system achieves the necessary amplitude control without requiring expensive linear electrical amplifiers or amplitude-limiting devices, thereby reducing overall system cost.
Solution Approach 2:
The patent uses two copies of the same EA modulator structure to independently modulate two optical signals. This copying approach allows the use of simpler, lower-cost modulator designs that can be mass-produced, replacing the need for a single complex high-linearity device while achieving the same functional outcome through optical combination.
4Manufacturing precision
If DML is used for PAM4 generation, then linearity is better than EML, but temperature sensitivity and chirp effects degrade signal quality
Solution Approach 1:
The patent substitutes direct laser modulation (DML) with external optical modulation using EA modulators. This replacement eliminates the chirp effects and temperature sensitivity inherent in DML by using a stable DFB laser source and externally modulating the optical signal, thereby achieving high linearity without the harmful side effects of direct modulation.
Solution Approach 2:
The patent introduces EA modulators as intermediary devices between the stable DFB laser source and the final PAM4 signal output. These modulators serve as the active modulation element rather than directly modulating the laser, which eliminates chirp and temperature sensitivity while maintaining the linearity needed for high-quality PAM4 signal generation.
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
The apparatus achieves high-linearity PAM4 modulation in the optical domain, simplifying device design and reducing the need for costly linear electrical components, resulting in improved PAM4 signal quality and increased device yield.
Implementation Method 1
a distributed feedback laser (DFB)... generate a first optical signal and a second optical signal
Implementation Method 2
two electro-absorption modulators... obtain a first NRZ optical signal by means of modulation... obtain a second NRZ optical signal by means of modulation
Implementation Method 3
a semiconductor optical amplifier (SOA)... adjust an optical power of the second optical signal... to obtain a second optical signal that is after optical power amplification
Implementation Method 4
a polarization beam splitting rotator (PSR)... multiplex the first NRZ optical signal and the second NRZ optical signal
Data Source
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AI summary
Embodiments of the present invention disclose a PAM4 signal generation apparatus. The PAM4 signal generation apparatus includes a DFB, two EA modulators, an SOA, a PSR, a direct-current power source, two electrical-signal generators, and two amplitude-limiting amplifiers. The two electrical-signal generators and the two amplitude-limiting amplifiers are used to generate two NRZ electrical signals respectively, the DFB outputs two optical signals, the SOA amplifies an optical power of one of the optical signals, the two EA modulators use the NRZ electrical signals and the optical signals including "a large signal and a small signal" respectively to generate two NRZ optical signals respectively, and finally, the two NRZ optical signals are multiplexed by the PSR to generate a PAM4 electrical signal. According to this apparatus, a linearity requirement is greatly lowered. PAM4 modulation is performed in an optical domain, and this prevents a PAM4 signal from being generated on an electrical signal. In this way, the electrical signal can be modulated to the optical domain in a distortionless way. Therefore, a high-quality PAM4 signal can be generated in the embodiments of the present invention.