Mr. Kent A. Marlett Profile

Kent A. Marlett

Senior Engineer at Bechtel Nevada

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Proceedings Article | 17 March 2005 Paper

Identification and removal of high-frequency temporal noise in a Nd:YAG macro-pulse laser assisted with a diagnostic streak camera

Kent Marlett, Ke-Xun Sun

Proceedings Volume 5580, (2005) https://doi.org/10.1117/12.567582

KEYWORDS: Streak cameras, Calibration, Diagnostics, Nd:YAG lasers, CCD cameras, National Ignition Facility, Pulsed laser operation, Electrons, Charge-coupled devices, Photons

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This paper discusses the use of a reference streak camera (RSC) to diagnose laser performance and guide modifications to remove high frequency noise from Bechtel Nevada’s long-pulse laser’s output. The upgraded laser used now exhibits less than 0.1% high frequency noise in cumulative spectra, exceeding NIF calibration specifications. ICF experiments require full characterization of streak cameras over a wide range of sweep speeds (10 ns to 480 ns). This paradigm of metrology poses stringent spectral requirements on the laser source for streak camera calibration. Recently, Bechtel Nevada worked with a laser vendor to develop a high performance, multi-wavelength Nd:YAG laser to meet NIF calibration requirements. For a typical NIF streak camera with a 4096x4096 pixel CCD, flat field calibration at 30 ns requires a smooth laser spectrum over 33 MHz to 68 GHz. Streak cameras are the appropriate instrumentation for measuring laser amplitude noise at very high frequencies since the upper end spectral content is beyond the frequency response of typical optoelectronic detectors for a single shot pulse. The SC was used to measure a similar laser at its second harmonic wavelength (532 nm) establishing baseline spectra for testing signal analysis algorithms. The RSC was then used to measure the custom calibration laser. In both spatial-temporal measurements and cumulative spectra, 6~8 GHz oscillations were identified. The oscillation was diagnosed as inter-surface reflections between amplifiers. In addition, RSC spectral data changes were found due to temperature instabilities in the seeding laser. Upgrades were made on the findings and high frequency noises were removed from the laser output.

Proceedings Article | 26 October 2004 Paper

Time-frequency analysis using peak-value sampling of spatial-temporal image data from streak camera

Ke-Xun Sun, Kent Marlett

Proceedings Volume 5559, (2004) https://doi.org/10.1117/12.560070

KEYWORDS: Streak cameras, Charge-coupled devices, Electrons, Image processing, CCD cameras, Sensors, Pulsed laser operation, Imaging systems, Photons, Time-frequency analysis

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A streak camera is a recording instrument in which spatial image is swept in time, creating a spatial-temporal image on a charge-coupled device (CCD). Traditional analysis for captured image data has been using uniform grid as sampling points, in which a block of CCD pixel readouts are summed to give one reading. Equivalently simple area moving averages are applied concurrently while sampling, and high frequency content is reduced. To solve this problem, we use peak-value sampling procedure, based on the view from photoelectron statistics. After background correction, maximum values in spatial dimensions are selected to obtain time series data. A DSP filter is then applied and optimized for this time series. A Welch algorithm fast Fourier transform is applied to obtain power spectra. Segmented cumulative spectra is then calculated for global statistics and related to time domain fluctuations. Self similarity at different sweeping time-scales is used to recognize CCD pattern noise. Sinusoidal pattern noise is automatically corrected by peak-value sampling. Computational results show that time-frequency analysis using peak-value sampling algorithm and similar variants is far more effective in discovering high frequency oscillatory noise than traditional uniform binned sampling. We have applied this algorithm to analyze data produced by a 4096x4096 CCD streak camera illuminated with a macro pulse laser. High frequency oscillations in 6~10 GHz region were found in laser spectra. Spatial-temporal oscillations of this range are difficult to diagnose with conventional optoelectronic detectors on a per-shot basis. This work has led to improvement of laser design.

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