KEYWORDS: Signal to noise ratio, Picosecond phenomena, Laser applications, Beam path, Signal detection, Signal attenuation, Plasma, Electrons, Physics, Mirrors
With the development of ultrafast laser technology, the peak energy of ultrashort pulses continues to increase. In addition to the demand for energy enhancement, many frontier physical experiments also put forward more stringent requirements for signal-to-noise ratio (SNR) of lasers. When the petawatt-level laser interacts with the target for physical experiments, the pre-pulses interact with the target in advance, affecting the density scale length of the pre-plasma and changing the spectral distribution of the generated electrons. In order to meet the requirements of pre-pulse control schem, we developed a SNR active control module based on isolated pre-pulses, which generates an isolated pre-pulse with adjustable time delay and relative intensity. The time delay of the isolated pre-pulse can be continuously adjusted in the range of -1300 ps to 0 ps. At the same time, by controlling the number of attenuators in the pre-pulse optical path, the relative intensity of the pre-pulse can be adjusted in the range of 10-8 to 10-3. We placed the module in front of the main amplification chain of the ShenguangⅡ ninth picosecond petawatt laser, adjusted the time delay and relative intensity of the pre-pulse, and measured the SNR at the terminal. The results verified the feasibility of the SNR active control scheme based on isolated pre-pulse.
Accurate temporal characterization both in intensity and phase distribution is important in the diagnosis of the petawatt (PW) class. We present a single-shot picosecond frequency-resolved optical gating (ps-FROG) setup based on an autocorrelator with ps measurement range that is spectrally resolved through a fine grating. The modified ptychographic-based algorithm with a changing update coefficient was used for the reconstruction of the pulse distribution; it can better adapt to the reconstruction of pulse with a large time–bandwidth product. We calibrated and verified the homemade ps-FROG in a 100-μJ ps laser system and used it to characterize the pulse distribution generated by the PW laser system of the Shen Guang II facility. The system shows good performance and high accuracy in reconstructing the intensity and phase distributions of a ps pulse, which provides reference for accurately adjusting the grating pair to acquire the pulse width as a preset.
The spectrum is a crucial parameter to a petawatt laser which is adopting the chirped pulse amplification technique. In such complex systems with high gain and wide spectrum bandwidth, the shape of the spectrum is crucial to the final output pulse width. In daily operation, the width of the compressed pulse will have some abnormal fluctuation, and the shape of the spectrum before compressed is also changed at the same shot. It will mislead the power and intensity estimation in laser-matter interaction experiments. So far, no theory has been able to analyze the relationship between spectrum and pulse width completely. Because it is hard to describe the fluctuation of the compressed pulse width which the online measure spectral phase in the high power laser system is difficult. In this paper, we first found and analyzed the relation between spectral variation and pulse width in the petawatt laser. With the support of existing data, we establish an end-toend deep learning model to map the petawatt laser’s spectrum before the compressor to the compressed pulse width. The deep learning scheme which based on Bayesian Neural Network (BNN) can provide an estimate of uncertainty as a function of pulse width to improve the accuracy of the model. After 20000 iterations, the Mean Square Error (MSE) is reduced to 0.08 in the validation test. Under the experiment, the model realizes an effective predict of the compressed pulse width. With the help of deep learning, we can get more information on the spectrum rather than the center wavelength and spectrum width to predict the compressed pulse width. It should be emphasized that this method will help to avoid unstable pulse output caused by an abnormal spectrum and to improve the operating efficiency of the petawatt laser system.
KEYWORDS: High power lasers, Near field optics, Spatial filters, Laser development, Optical alignment, Near field, Laser systems engineering, Charge-coupled devices, Mirrors, Signal detection
Laser beam alignment is very important for high-power laser facility. Long laser path and large-aperture lens for alignment are generally used, while the proposed alignment system with a wedge by far-field sampling technique reduces both space and cost requirements. General alignment system for large-aperture laser beam is long in distance and large in volum because of taking near-field sampling technique. With the development of laser fusion facilities, the space for alignment system is limited. A new alignment system for large-aperture laser beam is designed to save space and reduce operating costs. The new alignment for large-aperture laser beam with a wedge is based on far-field sampling technique. The wedge is placed behind the spatial filter to reflect some laser beam as signal light for alignment. Therefore, laser beam diameter in alignment system is small, which can save space for the laser facility. Comparing to general alignment system for large-aperture laser beam, large-aperture lenses for near-field and far-field sampling, long distance laser path are unnecessary for proposed alignment system, which saves cost and space greatly. This alignment system for large-aperture laser beam has been demonstrated well on the Muliti-PW Facility which uses the 7th beam of the SG-Ⅱ Facility as pump source. The experimental results indicate that the average near-field alignment error is less than 1% of reference, and the average far-filed alignment error is less than 5% of spatial filter pinhole diameter, which meet the alignment system requirements for laser beam of Multi-PW Facility.
Ultrashort pulse is important to exploring laser acceleration in many areas, such as fast ignition, advanced radiography capability. Petawatt laser should not only improve output energy on a single beam, but also combine multi-beams coherently. Diagnostics of temporal and phase synchronization is developed for coherent beam combination on a 10ps laser pulse. When two pulses are guided into the diagnostics, one goes through a temporal delay unit and a lens with a focal length 500mm, then arrives at detector unit, the other goes through a phase delay unit and the same lens, and then arrives at detector unit, too. First, temporal synchronization is adjusted by temporal delay unit and monitored by a cross-correlation generator in the detector unit. Second, phase synchronization is adjusted by phase delay unit and monitored by a far field interferogram in the detector unit. In our design, temporal resolution is 6.7fs in temporal synchronization, and phase resolution is 0.007π in phase synchronization. Experiment has proved that this diagnostics is useful to realize synchronization between two ultrashort pulses both in temporal and in spatial.
KEYWORDS: Pulsed laser operation, Near field, Diagnostics, Picosecond phenomena, Scattering, Mirrors, High power lasers, Temporal resolution, High dynamic range imaging, Calibration
Pulse contrast is an important parameter for ultrafast pulses. It shall be 108 or higher in order to avoid effect from noise before main pulse. Diagnostics with cross-correlation can achieve high temporal resolution such as ~7fs. Cross-correlation has advantage in pulse contrast measurement than autocorrelation because it can distinguish noise before
or after main pulse. High dynamic range is also essential in pulse contrast measurement. Cross-correlation signal from a
single shot is converted into a signal series through fiber array, which can be analyzed by a set of a PMT and an
oscilloscope. Noise from nonlinear crystal and scatter needs decrease to improve dynamic range. And pulse power is also
discussed in pulse contrast experiments. Time delay τ is generated by travel stage in measurement for repetition pulses.
Then energy instability will generate error in this measurement. In measurement for single shot pulse, time delay τ is
generated by slant angle of beams. The scanning procession is completed with thousands parts of beam section within a single shot, and error will generated from no uniformity in near field. Performance test of pulse contrast measurement is introduced in subsequent sections. Temporal resolution is testified by self-calibration. Dynamic range is judged by a
parallel flat. At last pulse contrast of petawatt laser is diagnosed by a single shot cross-correlator with high confidence. The
ratio is 10-6 at 50ps before main pulse, and 10-4 at 10ps before main pulse.
KEYWORDS: Sensors, Photodetectors, Silicon, Superposition, Interference (communication), Data acquisition, Environmental sensing, Temperature metrology, Prisms, Signal to noise ratio
To accurately measure the linearity of photodetectors in near-infrared waveband, based on the beam superposition method, a new design idea which use the tow-beam path and correlation methods was proposed. Using the 1053nm laser, and the Si photodetector as the experimental subject, a linearity measurement system of highly accurate photodetectors was designed. This system has over seven orders of magnitude dynamic range. The joint uncertainty is superior to 0.08%. Meanwhile, the linear factor of four different conditions which include the different size of incident beam spots, incident angles, positions and the environment temperature have been measured and analyzed. The experiment shows that the linearity of Si photodetector is ideal when the size of beam spots are bigger, the incident angles are smaller and the environment temperature is lower, moreover, the linearity of margin area is unsatisfactory.
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