Electromagnetic coupling through apertures and shielding effectiveness of a metallic enclosure housing electro-optic pockels cell in a high power laser system

摘要

This paper presents mathematical analysis and measurement results of electromagnetic couplings through a pair of intentional apertures in a metallic enclosure and its effect on the electromagnetic shielding effectiveness. The enclosure houses an electro-optical device (Pockles cell) and its high voltage drive circuitry. Two different aperture shapes i.e. rectangular and circular of equal areas are compared over frequency range of 200 MHz to 2 GHz. Pockels cells are commonly used in applications involving fast switching of polarization to block or transmit laser light. Pockels cell assemblies are housed in metallic enclosures to reduce noise emissions from high voltage pulser circuits driving the cell and to minimize the field couplings from external sources which may result into spurious switching of the cell. However, apertures are required in the enclosure to enable the laser beam to be incident on the Pockels cell and for onward transmission of the modulated beam. The apertures result into reduced shielding effectiveness of the metallic enclosure and increase in noise couplings. There is a need to understand the effect of apertures and to properly select their shapes and sizes such that the electromagnetic noise couplings are kept to a minimum acceptable level. Apart from the functional requirements of a reliable and noise free system operation, this issue is also important to meet the growing and stringent demands of electromagnetic compatibility. The analysis and measurements presented in this work pertain to the noise couplings from an external source into the enclosure through two differently shaped apertures i.e. rectangular and circular. For the comparison purpose, the planar areas of the apertures for the two cases are kept equal. The analysis and measurement techniques described in this work will be of interest to designers of electrical and electro-optical components for laser systems.