Laser Diode Threshold Monitoring Using Photodiode Current Kinks
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Solution Overview
Problem
Conventional Vertical Cavity Surface Emitting Lasers (VCSELs) lack reliable methods to monitor output power degradation, leading to high early fail rates due to sensitivity issues with integrated photodiodes, which are prone to both laser and spontaneous light emissions, making it difficult to detect malfunctions or degradation effectively.
Innovation Solution
A laser device with a photodiode that measures photodiode currents at different driving currents to calculate a virtual laser threshold current, allowing for reliable detection of malfunctions or degradation without directly measuring output power, using circuitry to determine a ratio of gradients and identify kinks in the photodiode current curve.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Device complexity
If an integrated photodiode is used to monitor laser output power, then the device structure is simplified and cost is reduced, but the measurement precision deteriorates because the photodiode cannot reliably distinguish laser light from spontaneous emission
Solution Approach 1:
The patent applies preliminary action by measuring the photodiode current at multiple driving currents below the lasing threshold before the laser actually lases. This allows constructing the linear relationship between driving current and photodiode current in advance, establishing the baseline for later threshold detection without requiring high precision during the actual measurement phase
Solution Approach 2:
The patent changes the operating parameter by varying the driving current across multiple values (below threshold and at threshold) rather than using a single fixed current. This parameter variation enables the construction of current relationships and detection of threshold points through mathematical analysis, compensating for the photodiode's inability to distinguish light types
2Ease of operation
If conventional output power monitoring is used, then the measurement is straightforward, but the reliability deteriorates due to high early fail rates of 500 ppm that cannot be detected
Solution Approach 1:
The patent performs preliminary measurements at multiple driving currents below the lasing threshold to establish the linear relationship between driving current and photodiode current. This preliminary characterization enables early detection of laser diode degradation and threshold shifts, improving reliability before actual lasing operation begins
Solution Approach 2:
The patent implements feedback by continuously monitoring the photodiode current at multiple driving currents and comparing the measured values against the expected linear relationship. When deviations occur indicating threshold shifts or degradation, the system can alert operators or adjust operations, creating a closed-loop reliability monitoring system
3Measurement precision
If multiple measurement points are used to determine laser threshold current, then the measurement precision improves, but the computational expenditure increases
Solution Approach 1:
The patent changes parameters by measuring at multiple driving current values to capture the linear relationship between current and photodiode response. This multi-point measurement approach enables accurate threshold determination through linear regression or intersection calculation, achieving precision without requiring complex real-time analysis
Solution Approach 2:
The patent performs the computational analysis of multiple measurement points as a preliminary step to establish the linear relationship and threshold current. By pre-calculating these relationships during characterization, the system reduces the computational burden during actual operation while maintaining high measurement precision
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 method enables reliable monitoring of laser diode quality and detection of malfunctions or degradation using a single parameter, reducing computational expenditure and avoiding the need for external photodetectors, thus improving the reliability of VCSELs in optical sensing applications.
Implementation Method 1
a photodiode configured to receive the radiation emitted by the at least one laser diode, wherein the received radiation induces a photodiode current in the photodiode
Data Source
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AI summary
The present invention relates to a laser device, comprising at least one laser diode configured to emit radiation, with an output power of the radiation depending on a laser diode driving current, a photodiode configured to receive the radiation emitted by the at least one laser diode, wherein the received radiation induces a photodiode current in the photodiode in dependence on the output power of the received radiation, circuitry configured to measure the photodiode current for a laser diode driving current, determine a virtual laser threshold current (Ith2) of the at least one laser diode from the measured photodiode current as a measure of an actual laser threshold current of the at least one laser diode. Further, a method of monitoring the output power of a laser diode is described.