Laser Diode Speckle Reduction via Temperature Modulation
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Solution Overview
Problem
Speckle noise in laser projectors used for active depth camera systems causes angle-dependent variations in the projected pattern, leading to poor feature matching and challenging depth estimation due to the coherent nature of the laser light source.
Innovation Solution
A method involving a processor-controlled coherent light projection source that emits light at different wavelengths during sub-intervals within an exposure interval by modulating the temperature of the laser diode, using a pseudo-random pulse-width-modulated signal to reduce speckle noise, allowing cameras to capture images with reduced speckle patterns similar to those from incoherent light sources.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Use of energy by moving object
If a coherent laser light source is used for projection, then energy efficiency and compact physical size are improved, but speckle noise appears in the projected pattern causing angle-dependent variations
Solution Approach 1:
The patent applies periodic action by modulating the laser diode's drive current in a pseudo-random binary sequence (PRBS) pattern, causing the laser to operate at different temperatures and emit different wavelengths during the exposure interval. This temporal modulation of the coherent light source creates varying speckle patterns that are averaged by the camera's integration process, effectively reducing speckle noise while maintaining the energy efficiency of the laser source.
2Volume of moving object
If a coherent laser light source is used for projection, then compact physical size is improved, but speckle noise appears in the projected pattern
Solution Approach 1:
The patent uses periodic modulation of the laser diode's operating conditions through pseudo-random binary sequencing, which changes the laser's temperature and emission wavelength over time. This temporal variation causes the speckle pattern to change during the camera's exposure, and the averaging effect reduces speckle visibility while maintaining the compact size advantage of using a single laser diode.
3Object-generated harmful factors
If multiple lasers with different wavelengths are used to reduce speckle noise, then speckle reduction is achieved, but device complexity increases
Solution Approach 1:
The patent changes the operating parameters of a single laser diode by modulating its drive current in a pseudo-random binary sequence, which causes temperature variations and corresponding wavelength shifts. This approach achieves multi-wavelength speckle reduction using only one laser source, avoiding the complexity of multiple lasers while still creating diverse speckle patterns through parameter modulation.
Solution Approach 2:
The laser diode's own temperature-dependent wavelength characteristics are exploited to generate multiple effective wavelengths. By allowing the laser's temperature to vary naturally during operation and modulating its current, the system uses the laser's inherent physical properties to create speckle diversity without requiring external wavelength-multiplying components.
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 significantly reduces speckle noise, improving the reliability of feature matching and depth estimation in active stereo depth camera systems while maintaining the energy efficiency and compactness of coherent light sources, without the need for multiple lasers or moving parts.
Implementation Method 1
a coherent light projection source... configured to emit light toward the fields of view of the cameras
Implementation Method 2
the projection source to have different temperatures during first and second sub-intervals of the at least two sub-intervals, wherein the projection source emits light having different wavelengths during the first and second sub-intervals in accordance with the different temperatures of the projection source
Implementation Method 3
The projection pattern is then imaged by an appropriate imaging sensor, such as a complementary metal oxide semiconductor (CMOS) image sensor or a charge coupled device (CCD) image sensor. In most applications, these imaging sensors are sensitive to the intensity of light
Data Source
AI summary
A method for applying power in a plurality of pulses to the projection source to control the projection source to emit coherent light during an exposure interval comprising at least two sub-intervals, the applied power causing the projection source to have different temperatures during first and second sub-intervals of the at least two sub-intervals; and emitting light from the projection source, wherein the projection source emits light having different wavelengths during the first and second sub-intervals in accordance with the different temperatures of the projection source.