Highly sensitive spectroscopy of residual absorption

Absorption spectroscopy of optical materials

When nonlinear optical and laser technology components are exposed to high (laser) light intensities, even very low levels of absorption may lead to local heating, eventually resulting in component malfunctions or even damage. It is essential to detect these minimal absorptions, which do occur even in highly pure materials, before the components are used. This requires extremely sensitive measuring processes. Fraunhofer IPM uses two such processes to verify the amount of absorbed energy.

Photoacoustic spectroscopy (PAS)

The principle of photoacoustic spectroscopy implies that high-frequent acoustic waves are generated within the material by using a pulsed laser (photoacoustic effect). A sensitive piezoelectric sensor detects the acoustic waves. The intensity of absorption is concluded from the amplitude of the sound wave.

© Fraunhofer IPM
Schematic setup of a photothermal common-path interferometer.
© Fraunhofer IPM
Absorption spectrum of a lithium niobate sample consisting of PCI and PAS data combined with transmission measurements (Cary, FTIR) and quality factor measurements from whispering gallery resonators (WGR).

Photothermal common-path interferometry (PCI)

Photothermal common-path interferometry (PCI) is a highly sensitive method, based on a pump-probe-technique: A strong continuous-wave (cw) pump beam is focused into the sample, heating it locally due to light absorption. The resulting local change of the refractive index causes refraction and diffraction effects on the crossing probe beam. This distortion is proportional to the absorption of the sample and can be measured by a lock-in technique.


Measuring over a broad spectral range

Continually pumped optical parametric oscillators (OPOs) developed by Fraunhofer IPM are used as tunable pump lasers for PCI. They enable the investigation of absorption properties over a broad spectral range (VIS – MIR).

We command many years of experience in highly sensitive absorption spectroscopy. A service lab offering a wide range of measurement facilities is at your disposal. Our staff will advise you in setting up appropriate measuring sites for your specific applications.

Absolute calibration

Tunable pump sources allow absorption structures within the material to be scanned. Comparing the measurement results with data from transmission measurements enables calibration of the system without the need for specially prepared samples. The result: reliable data.