InP Mach-Zehnder Bias Control for Wavelength-Dependent Loss
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Solution Overview
Problem
InP-based optical modulators experience significant wavelength-dependent insertion loss, affecting the accuracy and convergence time of auto bias control (ABC) due to the combination of high-frequency band compensation and wavelength-dependent optical absorption characteristics, which is not a significant issue in lithium niobate (LN) modulators.
Innovation Solution
An optical transmitter with a bias controller and monitor system that adjusts the gain of the monitor signal to compensate for wavelength-dependent insertion loss before auto bias control, using a digital potentiometer or adjusting the gain of the transimpedance amplifier and digital gain to correct the wavelength dependency at a preceding stage.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Adaptability or versatility
If high-frequency band compensation is applied to expand the operating band, then the transmission band is widened, but the insertion loss exhibits wavelength dependency that degrades ABC control accuracy
Solution Approach 1:
The patent applies preliminary action by pre-calculating and storing correction values for wavelength-dependent insertion loss in a lookup table before ABC control operation. The corrector retrieves the appropriate correction value based on the laser wavelength and applies it to the monitor signal gain, preparing the system in advance to compensate for wavelength variations without real-time computation delays.
Solution Approach 2:
The patent changes the gain parameter of the monitor signal based on the wavelength-dependent insertion loss characteristics. By adjusting the monitor signal gain according to the laser wavelength using the correction values, the system compensates for wavelength variations and maintains accurate ABC control across the expanded operating band.
2Stability of the object's composition
If band compensation is applied to flatten the wavelength-to-power characteristic, then the gain in lower frequency band is reduced, but loss variation occurs that affects ABC control
Solution Approach 1:
The patent implements feedback by using the monitor signal to detect the actual output light power of the optical modulator. The corrector adjusts the monitor signal gain based on wavelength-dependent correction values, creating a feedback mechanism that compensates for insertion loss variations and enables accurate ABC control despite band compensation effects.
3Volume of moving object
If InP-based material is used to reduce device size, then the modulator size is reduced, but wavelength-dependent optical absorption characteristics cause insertion loss variation
Solution Approach 1:
The patent applies local quality by implementing wavelength-specific correction values in a lookup table that account for the local optical absorption characteristics of the InP-based material at different wavelengths. Each wavelength range has its own correction factor, allowing precise compensation for the material's inherent wavelength-dependent absorption without changing the compact modulator design.
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 maintains convergence accuracy and reduces convergence time of the ABC control by compensating for wavelength-dependent insertion loss, ensuring stable operation across varying wavelengths.
Implementation Method 1
In an LN modulator or an optical modulator of a Mach-Zehnder type (MZM) using an InP-based material, a direct current (DC) bias that defines an operating point drifts due to change in temperature, change over the years, and the like.
Implementation Method 2
The band compensation is a process of flattening the gain of the entire band by reducing the gain in a lower frequency band, in order to flatten the wavelength-to-power characteristic in a higher frequency band.
Implementation Method 3
a monitor configured to monitor output light of the optical modulator to generate a monitor signal
Data Source
AI summary
An optical transmitter includes: an optical modulator of a Mach-Zehnder type using an InP-based material; a bias controller configured to control a DC bias applied to the optical modulator; a monitor configured to monitor output light of the optical modulator to generate a monitor signal; and a corrector configured to correct a gain of the monitor signal in a direction in which wavelength dependency of insertion loss of the optical modulator is compensated according to a wavelength at a preceding stage of bias control by the bias controller.