Corresponding author: Metin Sabuncu (
Academic editor:
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.
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 Fig.
The visible VCSEL spectrum at various drive currents is given in the 680 nm
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) (
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:
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
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.
The circuit that allows for triangular wave modulation of the VCSEL drive current. The corresponding block diagram of the circuit scheme that drives the VCSEL is given below the drive circuit.