High resolution spatially extended 1D laser scattering diagnostics using volume Bragg grating notch filters
Laser light scattering systems with volume Bragg grating (VBG) filters, which act as a spectral/angular filter, have often been used as a point measurement technique with spatial resolution as low as a few hundred um defined by the beam waist. In this work, we demonstrate how VBG filters can be leveraged for spatially resolved measurements with several um resolution perpendicular to the laser propagation axis over a few mm along the beam propagation axis. The rejection ring, as determined by the angular acceptance criteria of the filter, is derived analytically, and the use of the ring for 1D laser line rejection is explained. For the example cases presented, having a focused probe beam waist with a diameter of ~150 um, the rejection ring can provide up to several mm length along the beam propagation axis for a 1D measurement, which is also tunable. Additionally, methods to further extend the measurable region are proposed and demonstrated; using a collimation lens with a different focal length or using multiple VBG filters. The latter case can minimize the scattering signal loss, without the trade-off of the solid angle. Such use of multiple VBGs is to extend the measurable region along the beam axis, which differs from the commonly known application of multiple filters to improve the suppression of elastic interferences. 1D rotational Raman and Thomson scattering measurements are carried out on pulsed and DC discharges to verify this method. The system features compactness, simple implementation, high throughput, and flexibility to accommodate various experimental conditions.