Optical Modulator Impedance Matching for Low-Voltage Drive
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Solution Overview
Problem
Optical modulators face challenges in achieving low-voltage drive and stable modulation characteristics over a wide bandwidth due to impedance mismatch between the modulation electrode and external signal lines, leading to signal reflection and increased drive voltage, especially in high-frequency areas.
Innovation Solution
An optical modulator with an impedance adjustment part, configured as a lumped-constant circuit with a resistor and capacitor in parallel, and an impedance matching line, either multi-section or tapered, is used between the external signal line and the modulation electrode to adjust impedance, ensuring effective signal reflection over a wide bandwidth.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Power
If the distance between signal electrode and ground electrode is narrowed to increase electric field strength, then low drive voltage is achieved, but impedance of modulation electrode becomes lower than 50Ω causing impedance mismatch
Solution Approach 1:
An impedance matching circuit is introduced as an intermediary component between the modulation electrode and external signal line. This circuit includes a series connection of inductor and capacitor that transforms the low impedance of the modulation electrode to match the 50Ω impedance of external signal lines, thereby resolving the impedance mismatch caused by the narrow electrode distance required for low drive voltage operation
Solution Approach 2:
The impedance matching circuit changes the electrical parameter (impedance) of the modulation electrode by using frequency-dependent reactive components (inductor and capacitor). The series LC circuit transforms the low impedance at the modulation electrode to a higher impedance value that matches external signal lines, allowing the system to maintain both low drive voltage and proper impedance matching
2Speed
If electrode thickness is increased to match velocity of propagating light and modulation signal, then broadband characteristic is achieved, but impedance of RF line becomes lower and signal reflection increases
Solution Approach 1:
The impedance matching circuit acts as an intermediary that compensates for the low impedance caused by thick electrodes required for velocity matching. The series LC circuit transforms the impedance to match external signal lines, thereby reducing signal reflection while maintaining the velocity matching achieved through increased electrode thickness
Solution Approach 2:
The impedance matching circuit uses reactive components (inductor and capacitor) that provide frequency-dependent impedance transformation. This dynamic impedance transformation allows the system to maintain impedance matching across a broad frequency range, accommodating the broadband characteristic achieved through velocity-matched thick electrodes
3Reliability
If impedance matching line is formed on modulator device substrate, then impedance matching is improved, but in low-frequency area with insufficient line length, S11 characteristic is determined by connector and termination impedance causing reflection
Solution Approach 1:
The patent replaces the distributed impedance matching approach (long transmission line on substrate) with a lumped-element impedance matching circuit using discrete inductor and capacitor components. This substitution allows effective impedance matching in both low-frequency and high-frequency areas without requiring long line lengths, thereby reducing the frequency-dependent limitations of substrate-based matching lines
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 configuration improves the reflection characteristic of the modulation signal over a wide bandwidth, achieving low-voltage drive and stable modulation, while preventing signal attenuation and impedance mismatch issues.
Implementation Method 1
an impedance value of the modulation electrode in an active region in which an electric field formed by the modulation electrode is applied to the optical waveguide is set to be lower than an impedance value of the external signal line, and an impedance adjustment part having an impedance adjustment function with respect to mainly a modulation signal in a low-frequency area
Implementation Method 2
a modulation electrode for modulating light waves propagating through the optical waveguide
Data Source
AI summary
Provided is an optical modulator in which low-voltage drive and a stable modulation characteristic are secured over a wide bandwidth. An optical modulator includes: a substrate 10; an optical waveguide (not shown) formed in the substrate 10; a modulation electrode (a signal electrode 11 and a ground electrode 12) for modulating light waves propagating through the optical waveguide; and an external signal line (not shown, only a connection connector 4 is shown) which is provided outside the substrate and supplies a modulation signal to the modulation electrode, in which an impedance value of the modulation electrode in an active region S in which an electric field formed by the modulation electrode is applied to the optical waveguide is set to be lower than an impedance value of the external signal line, and an impedance adjustment part 21 having an impedance adjustment function with respect to mainly a modulation signal in a low-frequency area and configured of a lumped-constant circuit, and an impedance matching line L having an impedance adjustment function with respect to mainly a modulation signal in a high-frequency area are disposed between the external signal line and the active region of the modulation electrode.