To control the laser parameters and meet the requirements of precise physical experiments, it is necessary to accurately obtain the spectral width of the frequency-modulated (FM) light. The FM light has complex comb distribution that the morphological characteristics are different from the typical continuous spectrum. In this paper, we define the spectral width of the FM pulse according to the nature of the phase modulation and develop a correlation traversal algorithm based on spectral morphology matching. Then, we establish the relationship between the measured spectrum of the medium-precision spectrometer and the simulated spectrum and complete the spectral width calibration. The correlation of the measured spectrum and the simulated spectrum in 2.5 GHz frequency modulation, within the spectral width range of 0.05 nm ~ 0.30 nm is calculated. The correlation traversal algorithm can obtain a strongly correlated theoretical spectrum with an R-value greater than 0.9 for any measured spectrum. Through linear fitting of all data, we obtain the relationship curve between the -3dB spectral width measured by the medium-precision spectrometer and the calibrated spectral width. And the determination coefficient of the relationship curve is 0.9989. The results show that the spectral width calibration method has the advantages of strong correlation and high linearity. The spectral width calibration method completes the spectral width calibration of FM light for the first time, which lays a foundation for precise control of spectral parameters in inertial confinement fusion precision physics experiments.
The Integration Test Bed (ITB) is a large-aperture single-beam Nd:glass laser system, built to demonstrate the key technology and performance of the laser drivers. The phase II designed output of the ITB at 1053nm is 18.2kJ with the peak power of 3.6TW. So it is important to keep a flat spatial intensity profile at the end of the system to avoid optical elements damage or small-scale self-focusing. Applying the Liquid Crystal Programmable Spatial Shaper (LCPSS) to compensate the beam non-uniformity related to amplification and transmission is an effective way at present. In this paper, we attempt to pre-compensate the beam nonuniformity by the LCPSS. Experiments were carried out to study the spatial fluence modulation and contrast improvement at the main laser output of the ITB laser facility. The results show that the peak-to-average fluence modulation in the near-field is typically 1.35:1; the contrast is about 0.08, at the designed energy and power, which meet the modulation less than 1.4:1 and the contrast under 0.1 design requirement.
The coherent amplification network (CAN) aims at developing a laser system based on the coherent combination of multiple laser beams, which are produced through a network of high beam quality optical fiber amplifiers. The scalability of the CAN laser facilitates the development of many novel applications, such as fiber-based acceleration, orbital debris removal and inertial confinement fusion energy. According to the requirements of CAN and the front end of high-power laser facilities, a millijoule polarized fiber laser system was studied in this paper. Using polarization maintaining Ytterbium-fiber laser system as the seed, and 10-μm core Yb-doped fiber amplifier as the first power amplifier and 40-μm core polarizing (PZ) photonic crystal fiber (PCF) as the second power amplifier, the all-fiber laser system outputs 1.06-mJ energy at 10 ns and diffraction limited mode quality. Using 85-μm rod-type PCF as the third power amplifiers, 2.5-mJ energy at 10-ns pulse width was obtained with better than 500:1 peak-to-foot pulse shaping ability and fundamental mode beam quality. The energy fluctuation of the system is 1.3% rms with 1-mJ output in one hour. When using phase-modulated pulse as the seed, the frequency modulation to amplitude modulation (FM-to-AM) conversion ratio of the system is better than 5%. This fiber laser system has the advantages of high beam quality, high beam shaping ability, good stability, small volume and free of maintenance, which can be used in many applications.
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