AC-Coupled MZI Driver Circuit for High Voltage Swing
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Solution Overview
Problem
Mach-Zehnder Interferometer (MZI) modulators in optical communications face a limitation in voltage swing due to the maximum allowable voltage in modern nanometer CMOS processes, restricting the optical modulation amplitude and link performance.
Innovation Solution
A driver circuit design that differentially drives both plates of the MZI modulator using two inverter-based CMOS drivers and AC coupling capacitors, allowing a voltage swing twice the supply voltage without increasing the supply voltage, thereby enhancing the phase change per unit length.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Strength
If the supply voltage is increased to increase voltage swing and optical modulation amplitude, then the optical signal strength improves, but the transistor voltage limits in modern nanometer CMOS processes are exceeded
Solution Approach 1:
The single voltage swing task is segmented into two complementary drivers working differentially. Each driver operates within safe voltage limits while their combined differential output achieves the desired total voltage swing, effectively dividing the voltage stress across multiple components
Solution Approach 2:
AC coupling capacitors are introduced as intermediary elements between the drivers and the modulator plates. These capacitors enable the differential voltage swing to be transmitted while blocking DC voltage levels, allowing each driver to operate within safe voltage ranges while achieving high total swing
2Ease of operation
If stacked drivers are used to achieve higher voltage swing, then the voltage compliance is improved, but reliability issues arise due to excessive voltage across transistors
Solution Approach 1:
Instead of stacking drivers in series where voltage adds up through multiple transistor channels, the invention uses parallel differential drivers with AC coupling. This inverts the conventional approach by having each driver push-pull against a common reference, achieving voltage swing without series voltage addition that stresses transistors
Solution Approach 2:
The voltage swing function is segmented across two independent driver circuits that operate differentially. Each driver handles half the total swing requirement, operating independently within safe voltage boundaries while their differential combination delivers the full required swing
Data Source
AI summary
A driver circuit for a Mach-Zehnder modulator is provided that includes a first driver having an input to receive one of an input data or input data complement, and an output to be coupled to a first application voltage node associated with a first arm of a Mach-Zehnder modulator. The driver circuit includes a second driver having an input to receive the other of the input data complement or input data, and an output to be coupled to a second application voltage node associated with the first arm of the Mach-Zehnder modulator. The first driver and the second driver differentially drive the first and second application voltage nodes associated with the first arm of the Mach-Zehnder modulator to result in a voltage swing associated with a voltage applied to the first arm that is twice the supply voltage.