Cascaded Optical Modulators for High-Order Signal Constellations
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Solution Overview
Problem
Current optical transmitters face challenges in handling high-order modulation formats due to the limitations of single digital-to-analog converters (DACs) at high symbol rates, particularly with complementary metal-oxide-semiconductor (CMOS) technology, which restricts the capability to generate reliable optical data signals for large constellations.
Innovation Solution
The use of two or more serially connected optical modulators, each driven by a dedicated DAC, with digital signals generated using different subsets of bitstreams to ensure sufficient digital resolution and sampling rate, allowing for the generation of optical data signals with various modulation formats like PSK, ASK, PAM, and QAM, and supporting large constellations by managing average driving power differences between modulators.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Device complexity
If a single DAC is used to drive optical modulators for high-order modulation formats, then the device complexity is reduced, but the manufacturing precision and reliability deteriorate due to limitations in digital resolution and sampling rate at high symbol rates
Solution Approach 1:
The patent divides the single DAC function into multiple separate DACs, each driving a specific optical modulator in the cascade. This segmentation allows each DAC to operate at lower symbol rates with sufficient digital resolution, overcoming the limitations of a single DAC at high symbol rates while enabling high-order modulation formats.
2Adaptability or versatility
If multiple serially connected optical modulators are used, then the capability to generate large constellations is improved, but the device complexity increases due to additional modulators and drive circuits
Solution Approach 1:
The patent extends the modulation capability by adding another dimension - a cascade of multiple optical modulators operating in series. Each modulator adds its own modulation dimension, and their combined effect enables large constellation sizes and high-order modulation formats that cannot be achieved by a single modulator.
3Measurement precision
If bitstream subsets are allocated to different DACs, then the digital resolution requirement for each DAC is improved, but the system complexity increases due to bitstream distribution and coordination
Solution Approach 1:
The patent segments the input bitstream into multiple subsets, with each subset directed to a specific DAC-modulator pair. This segmentation reduces the digital resolution requirement for each individual DAC by distributing the total bitstream information across multiple parallel paths, making the system more manageable and less complex than requiring a single high-resolution DAC.
Data Source
AI summary
An optical data transmitter comprising two or more serially connected optical modulators, each driven using a respective DAC. The digital signals applied to the individual DACs are produced using different respective subsets of the set of bitstreams representing the digital waveform or data stream to be transmitted, with the bitstream subsets being selected, e.g., such that (i) each of the individual DACs is able to support the digital resolution and sampling rate needed for properly handling the subset of bitstreams applied thereto and (ii) differences between average driving powers applied to different optical modulators are relatively small. In different embodiments, the two or more serially connected optical modulators can be arranged for generating optical communication signals of different modulation formats, e.g., PSK, ASK, PAM, IM, and QAM. Some embodiments can advantageously be used for generating optical communication signals employing constellations of relatively large sizes, e.g., larger than 1000 symbols.