Optical Parametric Generator with a periodically poled Lithium Niobate crystal (PPLN, left side of the picture) covering the mid infrared spectral range. The multiple colours visible on the right side are mainly produced by higher harmonic generation in the nonlinear crystal. The infrared radiation is made visible by a temperature-sensitive foil (central screen).
Tried and tested and efficient solid state laser materials are only available for a few wavelength ranges, and typically these lasers emit in the near infrared region. To verify harmful and hazardous substances or to detect atmospheric trace gases, however, UV sources for laser induced fluorescence (LIF) detection and sources in the mid infrared region for absorption measurements on rotational and vibrational transitions of molecules are particularly suitable. To be able to generate radiation with these wavelengths and with the necessary frequency stability, the FN department investigates different methods of nonlinear frequency conversion and develops these in a targeted manner for the respective applications. This includes frequency multiplication, the generation of sum frequency and difference frequency and the use of optical parametric oscillators and amplifiers.
A further field of application of nonlinear frequency conversion relevant to security is the interception of missiles equipped with heat seeking heads. In some scenarios, radiation sources are required that deliver high laser power or high pulse energy in the wavelength ranges used by these seeker heads. Suitable solid state laser sources do not exist for this purpose; therefore the FN department is investigating methods of nonlinear frequency conversion for these wavelength ranges.