On enhanced tuning capabilities of external cavity lasers using acousto-optic modulators

For an enhanced tuning of an external-cavity Littrow diode laser at 670 nm a new approach based on the frequency controlling of intra-cavity acousto-optic modulators (AOM) has been developed and tested. Using a method called “mode shaking” a nearly continuously laser tuning over a frequency range of 186 GHz (0.279 nm) within a time interval of 20 ms has been achieved. A further method called “far laser frequency jump” allows frequency changes of up to 1080 GHz (1.6 nm) between two spectral regions of the diode laser emission range within about 50 μs.     

CARS Diagnostic and Modeling of a Dielectric Barrier Discharge

Dielectric barrier discharges (DBD) with planar- and knife-shaped electrodes are operated in N₂O₂NO mixtures under a pressure of 20 and 98 kPa. They are excited by means of consecutive unipolar or bipolar high-voltage pulse packages of 10 kV at a pulse repetition rate of 1 and 2 kHz. The rotational and vibrational excitation of N ₂ molecules and the reduction of nitric oxide (NO) in the discharge have been investigated using coherent anti-Stokes Raman scattering (CARS) technique. Rotational (gas) temperatures near the room temperature and vibrational temperatures of about 800 K at atmospheric pressure and 1400 K at a pressure of 20 kPa are observed. Therefore, chemical reactions of NO with vibrationally excited N ₂ are probably insignificant. One-dimensional kinetic models are developed that balance 35 chemical reactions between 10 species and deliver equations for the population density of excited vibrational levels of N ₂ together with a solution of the Boltzmann equation for the electrons. A good agreement between measured vibrational temperatures of N ₂ , the concentration of NO, and calculated data is achieved. Modeling of the plasma discharge verifies that a DBD operated with a N₂NO mixture reduces the NO content, the simultaneous presence of O ₂ , already 1%, is enough to prevent the NO reduction.     

Narrow-band BoxCARS applied to CO2 laser discharges

Summary CARS (Coherent Anti-Stokes Raman Scattering) has developed into a powerful tool for studying molecular systems. One of its possibilities is to derive vibrational and rotational temperatures as well as concentrations of molecules from measurements of the energy level population differences. A very good spatial resolution of CARS technique is one of its important advantages. This feature has been utilized for making spatially resolved measurements of the vibrational and rotational temperatures of N₂ in a d.c.-excited transverse-flow CO₂ laser discharge. Apart from that also spectra of CO₂, CO and O₂ in the discharge have been taken, which allowed us to evaluate the spatial distributions of those components in the discharge. Additionally first investigations of a microwave-excited CO₂ laser module have been performed for comparison. Paper presented at the ?XI European CARS Workshop?, Florence, Italy, 23–25 March, 1992.