Projector Laser Diode Pulse Biasing to Reduce Speckle
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Solution Overview
Problem
Conventional laser diode-based projectors suffer from coherence-related interference effects such as speckles and fringes, which degrade image quality, particularly when projecting onto rough surfaces or through waveguides.
Innovation Solution
A light source system with a laser diode driver that uses a bias current below 90% of the threshold current, incorporating a first converter to calculate current pulse amplitudes and a second converter to introduce pauses between pulses, breaking coherence and reducing interference.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Speed
If the bias current is set equal to the threshold current for lasing, then the response of the laser diode is fast and coherent laser pulses are emitted, but coherence-related interference effects such as speckles and fringes occur which deteriorate image quality
Solution Approach 1:
The patent applies periodic action by introducing pauses between current pulses, creating a periodic modulation pattern where the bias current is periodically reduced below the threshold current. This periodic interruption breaks the coherence of consecutive laser pulses while maintaining fast response capability, as the laser diode can quickly respond to each pulse cycle without sustained coherence buildup
Solution Approach 2:
The patent changes the bias current parameter from the conventional threshold current level to a reduced level (less than 90% of threshold current). This parameter change fundamentally alters the operating point of the laser diode, enabling fast response while preventing coherence-related interference by operating below the lasing threshold during pause periods
2Object-affected harmful factors
If the bias current is reduced to less than 90% of the threshold current, then coherence-related interference effects are suppressed, but the amplitude of current pulses must be increased to compensate
Solution Approach 1:
The patent applies preliminary action by pre-calculating and adjusting the current pulse amplitudes through a converter before the pulses are applied to the laser diode. This preliminary adjustment compensates for the reduced bias current effect, ensuring that each pulse achieves the required brightness level without excessive power consumption, as the compensation is optimized in advance
3Object-affected harmful factors
If pauses are introduced between consecutive current pulses, then the coherence of laser pulses is broken and interference effects are reduced, but the processing time per frame increases
Solution Approach 1:
The patent applies partial action by introducing pauses only between consecutive pulses within the same brightness value sequence, rather than after every single pulse regardless of conditions. The pause duration is optimized to be sufficient to break coherence but minimal enough to maintain frame rate performance, achieving interference suppression without excessive time loss
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
The solution effectively suppresses coherence-related interference, enhancing image quality and reducing energy consumption while maintaining fast response and adaptability to ambient conditions.
Implementation Method 1
the laser diode is configured to lase when the driving current exceeds a threshold current and to transform each current pulse into a respective laser pulse
Data Source
AI summary
The disclosed subject matter relates to a light source for a projector projecting pixels, which receives, for each pixel, a respective brightness value indicating a target brightness, forms, for each brightness value, a respective current pulse, thus forming current pulses, superimposes a constant bias current and the current pulses for obtaining a driving current for driving a laser diode, wherein the laser diode transforms each current pulse into a respective laser pulse having the target brightness of the respective pixel, wherein said bias current is less than 90% of a threshold current for lasing and the light source calculates amplitudes of the current pulses from the brightness values and forms the current pulses such that each two consecutive current pulses are separated by a pause. The disclosed subject matter further relates to a light source system and a projector.