For the two-point spherical wave structure function, we propose a modified quadratic approximation, which can be used to investigate the second-order coherence properties (such as the beam spreading, average intensity, cross-spectral density function) of partially coherent beams through the turbulent atmosphere. We prove that the modified quadratic approximation, different from usual one, can be used to study the effects of inner and outer scales of turbulence, and is better than the usual one for a less coherent beam or for a very strong turbulence. A more accurately analytical expression for average intensity of Gaussian-Schell model beams is derived based on the modified quadratic approximation. These results are also illustrated by investigating the average intensity for Gaussian-Schell model beams in turbulence.
In the pulsed fiber amplifiers with repetition frequency of several tens kHz, amplified spontaneous emission (ASE) is easy to build up because of the low repetition frequency and weak pulse signal. The ASE rises the difficulty to amplify the weak pulse signal effectively. We have demonstrated an all-fiber preamplifier stage structure to amplify a 40 kHz, 10 ns bandwidth (FWHM) weak pulse signal (299 μW) with center wavelength of 1062 nm. Compared synchronous pulse pump with continuous wave(CW) pump, the results indicate that synchronous pulse pump shows the better capability of increasing the output power than CW pump. In the condition of the same pump power, the output power of synchronous pulse pump is twice as high as CW pump. In order to suppress ASE, a longer gain fiber is utilized to reabsorb the ASE in which the wavelength is shorter than 1062nm. We amplified weak pulse signal via 0.8 m and 2.1 m gain fiber in synchronous pulse pump experiments respectively, and more ASE in the output spectra are observed in the 0.8 m gain fiber system. Due to the weaker ASE and consequent capability of higher pump power, the 2.1 m gain fiber is capable to achieve higher output power than shorter fiber. The output power of 2.1 m gain fiber case is limited by pump power.
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