Research Ideas and Outcomes : Research Idea
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Corresponding author: Metin Sabuncu (metin.sabuncu@deu.edu.tr)
Received: 17 Jun 2016 | Published: 24 Jun 2016
© 2016 Metin Sabuncu, İsmail Yilmazlar.
This is an open access article distributed under the terms of the Creative Commons Attribution License (CC BY 4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.
Citation: Sabuncu M, Yilmazlar İ (2016) Speckle Contrast Reduction with a Visible VCSEL Projector. Research Ideas and Outcomes 2: e9597. doi: 10.3897/rio.2.e9597
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Speckle contrast reduction in a laser projector that has a VCSEL emitting in the visible spectrum as its light source can be achieved electronically. Employing a drive circuit enables exciting independent longitudinal modes of the VCSEL.
By modulating the VCSEL current continuously with a triangular wave rather than with pulses, the speckle noise could be reduced.
Vertical cavity surface emitting lasers; visible VCSEL; speckle
VCSEL is an acronym that stands for vertical cavity surface emitting laser (
Like all other laser based projectors a VCSEL based projection system will also suffer from speckle noise. It is therefore important to devise methods that reduce speckle noise in VCSEL systems. Pulsed current modulation was used in an infrared VCSEL system to reduce speckle noise (
The VCSEL drive current modulator circuit scheme and the corresponding block diagram of the drive circuit is given in
The visible VCSEL spectrum at various drive currents is given in the 680 nm VCSEL datasheet. The visible VCSEL emits light between 678 and 684 nm depending on its drive current.
When the drive current is modulated with a continuous wave these 4 independent modes are excited. This will result in speckle reduction in the projected image (
Theoretically the speckle is quantified by the speckle contrast ratio (C) (
\(C=\sigma/I\)
where σ represents the standard deviation of the picture and I stands for the mean intensity of the picture.
By dividing the initial speckle ratio Ci by the final speckle ratio Cf, the speckle reduction factor (R) is calculated:
\(R=Ci/Cf\)
The speckle reduction factor will depend on the number of the different excited modes and their relative intensities.
When the VCSEL drive current is driven with a modulated current between 4 and 16 mA, 4 modes will be excited. The maximum and minimum value of the drive current are adjusted by the circuit elements.
The maximum speckle reduction factor for triangular modulation will be \(R ≈ 2\). If however the VCSEL drive current were to be modulated with a square wave (pulsed) then only two modes would be excited resulting in a maximum speckle reduction factor \(R ≈ 1.41\). Speckle reduction will be important when using VCSELs in projection systems (
We proposed a method of speckle contrast reduction in a laser projector that has a VCSEL emitting in the visible spectrum as its light source. Effective speckle reduction can be achieved electronically by modulating the VCSEL current continuously in a triangular fashion rather than pulsed.
Department of Electrical and Electronics Engineering, Dokuz Eylul University