Gain narrowing, amplified spontaneous emission

The onset of stimulated emission can be detected by a collapse of the broad emission spectrum to a narrow line. This line narrowing due to optical gain in a waveguide without resonator is commonly referred to as amplified spontaneous emission (ASE) [49], or travelling wave lasing, hi the case of additional optical resonators like gratings or microcavities, different modes with much narrower linewidths can be resolved. 

A serious problem in all laser-pumped dye lasers is the spontaneous background, emitted from the pumped volume of the oscillator and the amplifier cells. This spontaneous emission is amplified when passing through the gain medium. It represents a perturbing, spectrally-broad background of the narrow laser emission. This Amplified Spontaneous Emission... 

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. 

When spontaneous emission is amplified by stimulated emission, the spectrum is narrowed because the highest amplification occurs at the wavelength where the gain is the highest. Fig. Vll-8 shows a series of emission spectra from the... 

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