Projector Laser Diode Pulse Driving to Suppress 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 projected onto rough surfaces or waveguides.
Innovation Solution
A light source system with a bias current less than 90% of the threshold current, utilizing a first converter to calculate current pulse amplitudes and a second converter to introduce pauses between pulses, breaking coherence and reducing interference effects.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Speed
If the bias current is set to equal the threshold current for lasing, then the laser diode response is fast and coherent laser pulses are emitted, but coherence-related interference effects such as speckles and fringes occur that deteriorate image quality
Solution Approach 1:
The patent applies periodic action by introducing pauses between current pulses, creating a periodic modulation pattern. The laser diode is driven with current pulses separated by pause periods where the driving current is below the threshold current. This periodic interruption breaks the coherence of consecutive laser pulses while maintaining fast response capability, as each pulse can still be generated quickly when the current exceeds the threshold during active periods.
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 regime of the laser diode, allowing it to operate in a pulsed mode with pauses below threshold, thereby breaking coherence while preserving the ability to generate bright laser pulses when needed.
2Object-affected harmful factors
If the bias current is reduced to break coherence, then interference effects are suppressed, but the amplitude of current pulses must be increased to compensate for lower bias current
Solution Approach 1:
The patent applies partial or excessive action by using current pulse amplitudes that exceed what would be needed if the bias current were at threshold levels. The current pulses are designed with amplitudes that are specifically calculated to compensate for the reduced bias current, ensuring that the sum of bias current and pulse amplitude achieves the desired laser output brightness. This excessive action during pulse periods compensates for the reduced baseline operation.
3Object-affected harmful factors
If pauses are introduced between current pulses, then coherence is broken and image quality improves, but the processing rate may be affected
Solution Approach 1:
The patent applies dynamics by making the pause duration between current pulses adaptable rather than fixed. The pause period is designed to be sufficient to break coherence but optimized to maintain high processing rates. The system dynamically balances the pause duration to achieve the minimum necessary time to interrupt coherence while maximizing the number of pulses that can be delivered within a given time frame, thus maintaining high pixel processing rates.
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 enhances image quality by suppressing coherence-related interference, allowing for high-quality projection even under varying ambient and interior conditions, while maintaining fast laser diode response and reducing energy consumption.
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
Implementation Method 2
the coherence of the emitted laser pulses causes undesired interference effects such as speckles... the pauses between two consecutive current pulses... break the coherence of consecutive laser pulses
Data Source
Figure 1
Figure 2
Figure 3~4
AI summary
The invention relates to a light source (4) for a projector (1) projecting pixels (Pi), which receives, for each pixel, a respective brightness value (BVi) indicating a target brightness (Bi), forms, for each brightness value, a respective current pulse (CPi), superimposes a constant bias current (Ib) and the current pulses for obtaining a driving current (Id) for driving a laser diode (14), wherein the laser diode transforms each current pulse into a respective laser pulse (LPi) 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 (Ai) 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 (P). The invention further relates to a light source system (4') and a projector (1).