Light source for the coherence radar
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The coherence radar
allows the contactless measurement of three dimensional object surfaces with a lateral resolution of some microns.
Applications are mainly located in mechanical and electrical engineering, but can also be found in medicine and
cosmetics.
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Fig. 1 Scheme of the coherence radar
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In our working group we investigate within the third-party funds "Grundlagen breitbandiger Kurzpulslaser für die
Interferometrie" (FESMET) (funded by BMBF and
VDI) broadband radiation sources, in particular broadband emitting
lasers.
Large objects with weakly reflecting surfaces require a high illumination intensity which can only provide by laser
light sources. On the other hand a laser generally radiates light with a spectrally small bandwidth. The task is to
design a light source that emits radiation with a short coherence length (i.e. small temporal coherence) and
simultaneous a good beam quality (i.e. high spatial coherence). The coherence length is via the uncertainty relation
linked to the spectral width of the light source: the broader the spectrum the shorter is the coherence length. By
shortening the coherence length of the light the longitudinal resolution can be increased up to the surface
roughness of the object.
The light source for the coherence radar should emit in the visible spectral range with 200-300 mW average output
power in transversal fundamental mode (TEM00). For that purpose several designs with gain switched
Ti:sapphire laser or Cr:YAG laser has been
investigated. For smaller applications we designed broadband light sources based on anti-reflecting coated
diode lasers.
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Fig. 2 Broadband laser labor
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German
