Time amplified spontaneous emission

An alternative way to preform a channeled plasma consists in exploiting the nanosecond precursor that usually precedes a short femtosecond pulse in the output of a multi-terawatt laser system. In fact, the amplified spontaneous emission (ASE) pedestal has typically an intensity 106-1010 times lower than the main pulse, which, however, can be sufficient to ionize a gas-jet or a solid target. This drawback can be turned into a benefit assuming that this long precursor can prepare the plasma channel for the short pulse propagation. 

The harmonics are particularly attractive in terms of the radiation peak power, which can be 10 times higher than for synchrotron undulators. Thus, it should open the way for nonlinear laser spectroscopy in the short-wavelength (10 nm) regime. A competing technology requiring much larger installations is Self Amplified Spontaneous Emission used for Free-Electron-Laser action (SASE-FEL) [9.262]. 

The first observation of random laser emission in polycrystalline ZnO was made by Cao et al. [161,162]. Figure 3.34 shows the evolution of the emission spectra as the pump intensity was increased [162]. At low excitation intensities, the spectrum consisted of a single broad spontaneous emission peak. As the pump power increased, the emission peak became narrower due to the preferential amplification at frequencies dose to the maximum of the gain spectrum. When the exdtation intensity exceeded a threshold, very narrow peaks emerged in the emission spectra. The linewidth of these peaks was less than 0.3 nm, which was more than 30 times smaller than the linewidth of the amplified spontaneous emission peak below the threshold. When the pump intensity increased further, more sharp peaks appeared. 

We only had to add one more term - spontaneous emission - to the equation. Well, we did that, and then I had a hard time solving the equation, but a mathematician, Dr. [G.] Takahashi solved it. So Takahashi, Shimoda and I then wrote a paper on the maser amplifier, what the noise had to be, the theory of the noise fluctuations, and so on. Again, just trading ideas back and forth. 

The main contribution to the residual laser linewidth comes from iphase fluctuations. Each photon which is spontaneously emitted into the laser mode can be amplified by induced emission and this amplified contribution is superposed on the oscillating wave. This does not essentially change the total amplitude of the wave because, due to gain saturation, these additional photons decrease the gain for the other photons such that the average photon number n remains constant. However, the phases of these spontaneously initiated photon avalanches show a random distribution, and so does the phase of the total wave. There is no such stabilizing mechanism for the total phase as there is for the amplitude. In the course of time a "phase diffusion" occurs which can be described in a thermodynamic model by a diffusion coefficient D [6.37,38]. 

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