Programmable-Bias Optical Modulator Driver for High Swing Bandwidth
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Solution Overview
Problem
Existing optical communication systems face challenges in achieving high swing and bandwidth requirements for micro-ring modulators (MRMs) due to limitations in drive voltage, bias voltage, and power consumption in CMOS technologies, particularly with differential and stacked drivers.
Innovation Solution
A pseudo-differential AC-coupled driver combined with a DC-coupled latch is used to provide signal-dependent biasing, allowing for high swing and bandwidth while minimizing power consumption, using low-speed, high-voltage devices to meet the biasing requirements of MRMs.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Speed
If a differential or stacked driver is used to drive the micro-ring modulator, then the drive voltage and bandwidth requirements can be met, but the power consumption increases
Solution Approach 1:
The driver is segmented into two functional parts: a high-speed AC-coupled differential driver for bandwidth-critical signal transmission, and a low-speed DC-coupled latch for bias voltage generation. This segmentation allows each part to operate in its optimal performance range, with the AC driver handling high-frequency signals and the DC latch providing stable bias without contributing to high-frequency power consumption
Solution Approach 2:
An AC coupling capacitor is introduced as an intermediary element between the differential driver and the modulator. This capacitor blocks the DC bias voltage from the differential driver while allowing AC signal transmission, enabling the differential driver to operate at low power while still providing the necessary drive voltage swing through the coupled latch circuit
2Illumination intensity
If the drive voltage is increased to meet the high swing requirement, then the modulation amplitude improves, but the power consumption increases
Solution Approach 1:
The DC-coupled latch circuit preliminarily establishes the required bias voltage conditions before the AC differential driver applies the signal swing. By pre-setting the operating point with the latch, the differential driver can achieve the full voltage swing range without requiring continuous high power consumption, as the bias structure is already in place
Solution Approach 2:
The circuit changes the voltage parameters dynamically: the latch provides a DC bias voltage that sets the operating point, while the AC-coupled differential driver provides a time-varying voltage swing around this bias point. This parameter separation allows the modulation amplitude to be controlled by the AC swing while the DC bias level is independently controlled by the latch
3Speed
If a DC-coupled driver is used to provide bias voltage, then the bandwidth can be extended to DC, but the power consumption increases
Solution Approach 1:
The bandwidth-providing function is segmented and assigned specifically to the DC-coupled latch circuit, while the power-consuming differential driver is AC-coupled and operates only at signal frequencies. The latch extends bandwidth to DC by providing a stable bias reference without the continuous high power consumption of a fully DC-coupled differential driver
Data Source
Figure 1A~1B
Figure 2~3
Figure 4~5
AI summary
Embodiments herein relate to a driver circuit for an optical modulator such as a micro-ring modulator (MRM). The driver circuit includes first and second drivers which receive respective differential data signals, and which have their output nodes coupled to a cathode and anode, respectively of the MRM. One or both of the drivers can have their output nodes alternating-current (AC)-coupled to a bias circuit which biases the voltage of the output signal to shift its voltage swing. The bias circuit can include a latch which receives Vbc_high at its power supply terminal and Vbc_low at its ground terminal, where Vbc_high>Vbc_low>0 V.