Monitor Photodiode Leakage Current Control in Laser Readers
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Solution Overview
Problem
Recent changes in laser manufacturing to reduce costs have resulted in lasers where the cathode of the chip and the monitor photodiode are both grounded, requiring a negative power supply to reverse bias the monitor photodiode, which is not available in existing electro-optical readers, leading to unpredictable photodiode leakage current that corrupts the main photodiode current.
Innovation Solution
A controller and signal processor arrangement that forward biases the monitor photodiode with a positive voltage power supply, maintaining a constant leakage current, and removes this current from the total current to isolate the main photodiode current, allowing low-cost lasers from various sources to be used without corrupting the main current.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If the monitor photodiode is reverse biased to operate in photoconductive mode, then the laser output power monitoring precision is improved, but the device complexity increases due to requiring a negative power supply
Solution Approach 1:
The patent inverts the conventional approach by using forward bias instead of reverse bias for the monitor photodiode. This allows the photodiode to operate in photovoltaic mode without requiring a negative power supply, thereby reducing device complexity while still enabling laser output power monitoring through signal processing that removes leakage current effects.
Solution Approach 2:
The patent adopts a simpler, more cost-effective power supply configuration by eliminating the need for a negative power supply. This approach uses readily available positive voltage sources and ground connections, making the system easier to manufacture and maintain while achieving the same functional goal through alternative means.
2Ease of manufacture
If both the laser chip cathode and monitor photodiode cathode are grounded to reduce manufacturing cost, then the ease of manufacture is improved, but the measurement precision deteriorates due to unpredictable photodiode leakage current
Solution Approach 1:
The patent extracts and removes the harmful photodiode leakage current from the monitoring signal through signal processing. By separating the leakage current component from the total current signal, the system maintains the simple grounded configuration while eliminating its adverse effects on measurement precision.
Solution Approach 2:
The patent employs feedback mechanisms through signal processing circuitry that continuously monitors and compensates for leakage current effects. The system uses the relationship between the monitor photodiode current and main photodiode current to accurately determine laser output power despite the presence of leakage current.
3Speed
If the forward bias voltage to the monitor photodiode is increased, then the photodiode response speed is improved, but the leakage current increases which corrupts the main photodiode current
Solution Approach 1:
The patent optimizes the forward bias voltage parameter to a specific range (0.1V to 0.6V) that balances response speed and leakage current. By carefully selecting this voltage parameter, the system achieves adequate response speed while keeping leakage current at manageable levels that can be compensated through signal processing.
Solution Approach 2:
The patent uses feedback signal processing to continuously monitor and compensate for leakage current effects. The signal processor analyzes the relationship between the monitor photodiode current and main photodiode current to accurately extract laser output power information despite the presence of leakage current.
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 solution enables the use of low-cost lasers from multiple sources in electro-optical readers by maintaining a constant photodiode leakage current, preventing corruption of the main photodiode current and ensuring accurate laser power monitoring.
Implementation Method 1
an internal light detector, e.g., a semiconductor monitor photodiode, is typically mounted inside the laser adjacent a semiconductor laser chip, for monitoring the output power of the laser beam
Implementation Method 2
A positive voltage power supply is connected to an anode of the monitor photodiode, for forward biasing the monitor photodiode at a substantially constant positive voltage below a threshold voltage of the monitor photodiode
Data Source
AI summary
In an electro-optical reader, an aiming or a scanning laser has an internal laser chip and a monitor photodiode. A controller deenergizes the chip in a deenergized state, and energizes the chip to emit a laser beam with an output power in an energized state. A positive voltage power supply forward biases and maintains the photodiode at a substantially constant positive voltage below a threshold voltage, to support a substantially constant photodiode leakage current in both states. A signal processor supports a total current comprised of a main photodiode current indicative of the output power of the laser beam in the energized state and of the leakage current which flows opposite to the main current, and removes the leakage current from the total current to output only the main current in the energized state.