Balanced Light Detector AC Coupling Noise Reduction
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Solution Overview
Problem
Conventional light detection circuits face limitations in amplifying AC signals due to supply voltage constraints, leading to 'railing' and loss of DC signal information, with inadequate methods for removing DC signals causing reduction in operational bandwidth and introduction of noise in balanced detection.
Innovation Solution
A system and method employing AC coupling with a common offset circuit that preserves photodiode bandwidth, allows optimal transimpedance amplifier performance, retains DC signal measurement capability, and eliminates noise from the balanced detection signal by applying a common offset to the outputs of two light detectors, ensuring the noise introduced is common to both.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If conventional photodiode receiver circuits amplify incoming signals with high gain (1k, 1M or more), then the AC signal detection capability is improved, but the supply voltage constraints cause the amplifier to rail and lose DC signal information
Solution Approach 1:
The circuit is divided into two separate detection paths (balanced detection configuration) where each photodiode processes signals independently. This segmentation allows the AC and DC components to be handled separately, with the differential amplifier extracting AC signals while preserving DC information in both channels.
Solution Approach 2:
The invention changes the operating parameters by using a differential amplifier configuration that accepts high DC bias currents without railinging. By measuring the difference between two photodiode outputs, the circuit can tolerate large DC components while maintaining sensitivity to AC signals, effectively changing how the amplifier handles signal levels.
2Measurement precision
If AC coupling is used to remove the DC signal in conventional circuits, then the AC signal detection is improved, but the operational bandwidth of the photodiode and transimpedance amplifier is reduced
Solution Approach 1:
Instead of using AC coupling to extract the AC signal, the invention extracts only the necessary information (the difference between the two photodiode outputs) while leaving the full bandwidth signal path intact. The differential amplifier directly processes the full-bandwidth signals from both photodiodes, removing the need for bandwidth-limiting AC coupling capacitors.
3Measurement precision
If conventional methods attempt to remove the DC signal by AC coupling, then the AC signal is preserved, but path dependent noise is inserted into the balanced detector
Solution Approach 1:
The invention merges the two photodiode outputs into a single differential signal that cancels common-mode noise. By combining the signals from both photodiodes through the differential amplifier, path-dependent noise that appears equally on both channels is rejected, while the desired differential signal is preserved and enhanced.
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 enables sensitive measurements like beam positioning and optical homodyne detection while maintaining high bandwidth and stability, effectively removing classical noise from the difference signal, thus enhancing the signal-to-noise ratio and allowing for the generation of random bits indicative of quantum noise.
Implementation Method 1
a first light detector having an first output, the first light detector configured to provide a first electrical signal from the first output, the first electrical signal representing light intensity of light received by the first light detector
Data Source
AI summary
A system and method with AC coupling that reserves photodiode bandwidth in a biased configuration, allows optimal transimpedance amplifier performance, retains DC signal measurement capability, and does not introduce noise into the balanced detection signal.