The generation of a fourth-harmonic probe and its application in Nomarski interferometry at Shengguang-II

In this work, a design for the generation of a 4 omega (263-nm) probe converted from a 1 omega (1053-nm) laser is presented. The design is based on a beta-barium borate and potassium dihydrogen phosphate two-step frequency-conversion process. A suitable configuration for Nomarski interferometry based on the 4 omega probe is proposed, for measuring the electron density of laser-produced plasmas. The signal-to-noise ratio of the output 4 omega probe to 1 omega and 2 omega light after frequency quadrupling and harmonic separation is 10(3) with a 0.5 GW/cm(2 )1 omega input but decreases to similar to 10(2) at intensities below 0.1 GW/cm(2). Additional noise suppression by a factor of 10(4) is achieved using filters before the interferometer recording camera. The spatial resolution of the diagnostic can reach 5.2 mu m for a 10% modulation transfer function. An experiment validating the probe diagnostic system is conducted at the Shengguang-II laser facility. A clear interferogram of an aluminum plasma is obtained with 0.1 GW/cm(2) input, suggesting a maximal electron density of about 2.5 x 10(20) cm(-3 )as retrieved through an inverse-Abel transform. The design proposed in this paper is appropriate for a small laser device or a large laser facility that lacks a separate diagnostic beam, and it is an inexpensive solution as it requires small-aperture 1 omega input at a relatively low intensity. All the key parameters necessary to implement the design are provided in detail, making it straightforward to reproduce or transplant the system for specific uses.