Measurement of 2D density profiles using a second-harmonic, dispersion interferometer

• Published in: Review of scientific instruments 2023-02, Vol.94 (2)
• Main Authors: Chavez, C. T., Egly, A., Sepulveda, I., Wessel, F. J.
• Format: Article
• Language: English
• Subjects: Instruments & Instrumentation; Physics
• ISSN: 0034-6748

A second-harmonic, dispersion interferometer is used to image large-area (≃5 cm 2 ) plasma-jet and gas-jet density profiles. Achromatic telescopes magnify the diameters of the primary-laser beam (1064 nm) and its second-harmonic (532 nm) before probing the sample and de-magnify the beam diameters after the sample, where the primary beam transfers its phase change to a second, second-harmonic beam, allowing the sample’s dispersive-phase change to be measured between two, orthogonally polarized second harmonic beams. The telescopes produce an azimuthally symmetric, dispersive-phase shift in the sample + background phase-change image and in the background phase-change image, which is removed by digital subtraction. The interferometer’s performance was verified using standard-optical components as dispersive elements (BK7 lenses and wedge plates), resolving a minimum, phase-change sensitivity of Δ ϕ min ≳ 15 mrad and spatial resolution of Δ x res ≃ 100  μm. The phase change produced by unknown-density objects (a pulsed-plasma-jet and a pulsed-gas-jet) was measured, and their data were used to recover the original, 2D density profiles using an inverse Abel transform: peak-number density, N gas ≃ 6 × 10 20  cm −3 and N e ≃ 5 × 10 16  cm −3 ; line-integrated density, ∫N gas dl ∼ 2 × 10 19 cm −2 and ∫N e dl ∼ 1 × 10 16 cm −2 . The techniques and methods developed here are scalable to even larger probe-beam diameters and frame-capture rates, leading to a diagnostic capability that is well-suited for applications involving the real-time measurement of density.