Optical Waveguide Modulator MMI Tap Stray Light
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Solution Overview
Problem
Conventional optical waveguide modulators with in-line taps suffer from insufficient accuracy in tracking time-variant optical signals due to the tendency of three-port couplers to pick up stray light from the substrate, leading to tracking errors in monitoring photodetectors.
Innovation Solution
A planar optical waveguide circuit incorporating a 2×2 multi-mode interference (MMI) coupler as an in-line optical tap, which couples non-equal fractions of modulated light into output ports and includes an unused waveguide terminated to avoid picking up stray light, reducing tracking errors by acting as a four-port device.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Device complexity
If a conventional three-port optical coupler is used as an in-line tap, then the device complexity is reduced, but the tracking precision deteriorates due to picking up stray light from the substrate
Solution Approach 1:
The patent divides the optical coupling function into multiple independent waveguide paths within the MMI coupler structure. By segmenting the light propagation into distinct modes and paths, the design separates the desired signal transmission from stray light pickup, allowing precise control over which light components are coupled to the tap output.
Solution Approach 2:
The patent introduces an intermediary unused waveguide in the MMI coupler structure that acts as a mediator to cancel stray light. This unused waveguide serves as a reference path that interferes destructively with stray light components, thereby eliminating the harmful effect while maintaining the coupling function.
2Measurement precision
If a four-port MMI coupler is used as an in-line tap, then the tracking precision is improved by reducing stray light pickup, but the device complexity increases
Solution Approach 1:
The MMI coupler structure performs multiple functions simultaneously: it provides the optical coupling function for signal transmission, acts as an interferometer for stray light cancellation, and serves as a power splitter for the tap function. This multi-functionality reduces the need for separate components, thereby limiting the increase in overall device complexity.
Solution Approach 2:
The patent merges the coupling function, the stray light rejection function, and the power splitting function into a single integrated MMI coupler device. By combining these functions that would traditionally require separate components, the design achieves improved tracking precision while minimizing the increase in device complexity.
3Ease of manufacture
If the unused waveguide is left unterminated, then the ease of manufacture is improved, but harmful factors increase due to picking up stray light
Solution Approach 1:
The patent converts the potentially harmful effect of the unused waveguide (which could pick up and re-radiate stray light) into a beneficial effect by using it as a reference path for destructive interference. The same waveguide structure that could cause harm is instead utilized to cancel stray light, turning the problem into a solution.
Solution Approach 2:
The patent applies preliminary anti-action by designing the unused waveguide with a specific termination that prevents stray light pickup before it can cause harm. The termination is configured to create destructive interference with any stray light that might be picked up, thereby preemptively neutralizing the harmful effect.
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 use of a 2×2 MMI coupler significantly reduces tracking errors by minimizing the coupling of stray substrate light, enhancing the accuracy of monitoring photodetectors and maintaining the integrity of modulated light signals.
Implementation Method 1
The in-line optical tap comprises a multi-mode interference (MMI) coupler comprising first and second input ports and first and second output ports
Implementation Method 2
an unused waveguide coupled at one end thereof to the second input port of the MMI coupler... The second end of the unused waveguide is terminated so as to avoid picking up any stray portion of the input or modulated light propagating in the substrate
Data Source
AI summary
A planar optical waveguide circuit includes an optical modulator, such as that based on a Mach-Zehnder interferometer, that is followed by an in-line optical tap in the form of a 2×2 multi-mode interference coupler that is characterized a reduced tracking error as compared to Y-junction couplers.