Temporal coherence properties of laser modules used in headlamps determined by a Michelson interferometer

Abstract

In this work, an investigation of the temporal coherence properties of radiation which is emitted by laser modules integrated in headlamps is presented. The motivation for these measurements was difficulties concerning the field of classification for laser products which function as conventional headlamps. Based on an experimental setup including a Michelson interferometer, a goniophotometer and a spectrometer, coherence lengths of 92.5 and 147.0 μm are obtained for two different laser modules. The results show that the temporal coherence of the examined radiation is appreciably higher than the temporal coherence of conventionally produced white light. Therefore, at this point in time, laser modules used in headlamps cannot be considered as customary white light sources.

Mach-Zehnder-based measurement of light emitting diodes temporal coherence

“…On the other hand, the temporal coherence length of a light source measures the distance between two wavefronts emitted with a certain delay to each other from the same point that remain with constant relative phase between them. Recently, there has been an increasing interest about measuring the spatial and temporal coherence lengths of semiconductor-based light sources such as LEDs [11][12][13][14][15][16]18,19]. Usually, the temporal coherence length of light sources is measured by using a Michelson interferometer [14][15][16] or the double slit experiment [13,[16][17][18][19], where the interference pattern at the output of the system is related with the coherence degree of the source, γ.…”

“…Recently, there has been an increasing interest about measuring the spatial and temporal coherence lengths of semiconductor-based light sources such as LEDs [11][12][13][14][15][16]18,19]. Usually, the temporal coherence length of light sources is measured by using a Michelson interferometer [14][15][16] or the double slit experiment [13,[16][17][18][19], where the interference pattern at the output of the system is related with the coherence degree of the source, γ. In this work, we use for the first time a Mach-Zehnder interferometer, Fig.…”