Optical Transmitter Phase Noise Reduction via Feedback Segmentation
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Solution Overview
Problem
Conventional techniques for narrowing the linewidth of light emitted by a wavelength tunable laser diode (t-LD) in optical coherent systems, particularly digital coherent systems, face challenges in reducing phase noise, which degrades the relative intensity noise (RIN) and spectral linewidth performance due to the interdependence of phase noise compensation across different sections of the LD.
Innovation Solution
The optical transmitter employs a chirped sampled grating distributed feedback (CSG-DFB) section and a chirped sampled grating distributed Bragg reflector (CSG-DBR) section, with an optical detector feedback loop that adjusts the bias current to the SG-DFB section, effectively reducing phase noise and narrowing the emission spectrum linewidth.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If conventional phase noise compensation techniques are applied to one section of the t-LD, then the phase noise in that section is reduced, but the phase noises in other sections are enhanced, degrading the RIN and spectral linewidth performance
Solution Approach 1:
The t-LD is divided into multiple independent sections (gain section, phase shifting section, Bragg reflector sections) that can be individually controlled. The patent applies separate feedback loops to each section, allowing independent phase noise compensation without interfering with other sections, thus resolving the contradiction between reducing phase noise in one section and preventing enhancement in others.
Solution Approach 2:
The patent implements feedback control mechanisms where the output of optical detectors is superposed on bias currents supplied to each section of the t-LD. This feedback loop continuously monitors and compensates for phase noise in each section independently, achieving overall phase noise reduction while maintaining stable RIN and spectral linewidth performance.
2Adaptability or versatility
If the optical transmitter uses a wavelength tunable laser diode with multiple sections for wavelength tuning, then the emission wavelength can be tuned, but each section causes phase noises that degrade the spectral linewidth
Solution Approach 1:
The wavelength tuning function is segmented across multiple specialized sections (phase shifting section, Bragg reflector sections) rather than using a single monolithic structure. Each section is optimized for its specific function and can be independently controlled through separate feedback loops, allowing wavelength tuning while minimizing the cumulative phase noise impact on spectral linewidth.
Solution Approach 2:
The patent dynamically adjusts bias currents and operating parameters of each section based on real-time feedback from optical detectors. By changing the operating parameters of individual sections independently, the system achieves wavelength tuning while maintaining optimal phase noise performance and spectral linewidth characteristics.
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 enables the optical transmitter to achieve a spectral linewidth of less than 300 kHz, improving the RIN and spectral performance by independently controlling the phase noise across different sections of the t-LD, thereby meeting the requirements of digital coherent systems.
Implementation Method 1
The optical detector receives an output of the optical component
Implementation Method 2
The optical component, which has a wavelength dependent transmittance, receives the light emitted from the t-LD
Implementation Method 3
The combination of the SG-DFB section and the CSG-DBR section forms an optical cavity that sets an emission wavelength of the t-LD at the target wavelength
Data Source
AI summary
An optical transmitter that narrows the linewidth of its output light is disclosed. The optical transmitter includes a wavelength tunable laser diode (LD) known as a CSG-DR LD integrated with a semiconductor optical amplifier (SOA) driven in a constant magnitude mode. The wavelength of the light output from the LD is determined by transmission through an etalon filter. The optical transmitter feeds the output of the etalon filter back to an injection current supplied to the LD to reduce phase noise inherently contained in the output light.