Atmospheric pressure (AP) dielectric barrier discharges (DBDs) are increasingly used to treat thermally‐sensitive materials. Reliable measurements of the kinetic gas temperature, T, are therefore of capital importance. Spectroscopically determined rotational temperatures, Trot, are often tacitly assumed to be equal to T. Here, we measured T with fibre‐optic instrumentation that is a priori immune towards high voltages and high‐frequency electromagnetic fields generally encountered in plasmas. Finding Trot > T in AP glow discharge (APGD) DBDs in He and Ne, we believe that Trot ≈ T only during the short (≈ µs) current peaks that characterize APGD. Therefore, T represents the time‐averaged gas temperature; calorimetric measurements using a thermocouple buried in an electrode support this view.magnified image

Atmospheric pressure (AP) dielectric barrier discharges are frequently of interest for treating delicate substrates such as polymers or biological materials. In spite of its capital importance, thermometry in AP plasmas is subject to much uncertainty. We report temperature measurements in noble gases, nitrogen, and air using sensitive, accurate fibre-optic instrumentation that is a priori immune towards high voltages and high-frequency electromagnetic fields generally encountered in plasma environments.

We experimentally demonstrate tunable multiple-idler wavelength broadcasting of a signal to selective channels for wavelength division multiplexing (WDM). This is based on cascaded χ(2) nonlinear mixing process in a novel multiple-QPM 10-mm-long periodically poled LiNbO3 having an aperiodic domain in the center. The idlers' spacing is varied utilizing detuning of the pump wavelength within the SHG bandwidth. The temperature-assisted tuning of QPM pump wavelengths allows shifting the idlers together to different set of WDM channels. Our experimental results indicate that an overall idler wavelength shift of less than 10 nm realized by selecting pump wavelengths via temperature tuning, is sufficient to cover up to 40 WDM channels for multiple idlers broadcasting.

We demonstrate theoretically and experimentally, that the non-uniform spectra of second harmonic generation (SHG) from an unapodized step-chirped periodically poled nonlinear optical grating can be apodized utilizing tightly-focused Gaussian beams to suppress the ripple in its wideband response. In our example, by increasing focusing, a ripple-free response is progressively achieved over a 6-dB bandwidth of >5 nm, with a beam waist of 20 µm. With this tight focusing arrangement, a continuous tuning of 11-nm is also demonstrated by simply changing the focal point by 5.8 mm within the step-chirped grating based APPLN.

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