Random lasers represent a relatively undemanding technology for generating laser radiation that displays unique characteristics of interest in sensing and imaging. Furthermore, they combine the classical laser’s nonlinear response with a naturally occurring multimode character and easy fabrication, explaining why they have been recently proposed as ideal elements for complex networks. The typical configuration of a random laser consists of a disordered distribution of scattering centers spatially mixed into the gain medium. When optically pumped, these devices exhibit spectral fluctuations from pulse to pulse or constant spectra, depending on the pumping conditions and sample properties. Here, we show clear experimental evidence of the transition from fluctuating (uncorrelated) to persistent random laser spectra, in devices in which the gain material is spatially separated from the scattering centers. We interpret these two regimes of operation in terms of the number of cavity round trips fitting in the pulse duration. Only if the cavity round-trip time is much smaller than the pulse duration are modes allowed to interact, compete for gain, and build a persisting spectrum. Surprisingly this persistence is achieved if the pumping pulse is long enough for radiation in the cavity to perform some 10 round trips. Coupled-mode theory simulations support the hypothesis. These results suggest an easy yet robust way to control mode stability in random lasers and open the pathway for miniaturized systems, as, for example, signal processing in complex random laser networks.

中文翻译：

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控制随机激光的光谱余辉


随机激光器代表了一种相对要求不高的产生激光辐射的技术，它在传感和成像方面表现出令人感兴趣的独特特征。此外，它们将经典激光器的非线性响应与自然发生的多模特性和易于制造相结合，解释了为什么它们最近被提议作为复杂网络的理想元件。随机激光器的典型配置由空间混合到增益介质中的散射中心的无序分布组成。当进行光泵浦时，这些装置表现出从脉冲到脉冲的光谱波动或恒定光谱，具体取决于泵浦条件和样品特性。在这里，我们展示了在增益材料与散射中心在空间上分离的设备中从波动（不相关）到持续随机激光光谱转变的清晰实验证据。我们根据脉冲持续时间中腔往返次数来解释这两种操作方式。只有当腔往返时间远小于脉冲持续时间时，模式才允许相互作用、竞争增益并建立持久频谱。令人惊讶的是，如果泵浦脉冲足够长以使腔内的辐射进行大约 10 次往返，则可以实现这种持久性。耦合模式理论模拟支持了这一假设。这些结果提出了一种简单而可靠的方法来控制随机激光器的模式稳定性，并为小型化系统开辟道路，例如复杂随机激光网络中的信号处理。