Random lasers (RLs) rely on light amplification in a gain material with feedback from multiple scattering in disordered media. They are unconventional light sources characterized by multiple narrow ...peaks emission with high potential in imaging and sensing applications. At variance with ordinary lasers, optical interaction between single RLs arranged in networks of multiply interconnected resonators remains unexplored. The typical RL architecture where gain material and scatterers are spatially mixed, is unsuitable for the realization of singular devices on the same platform. Here, we use RLs in which gain is restricted to the line between two scattering regions to establish and probe mutual coupling between many RLs. We prove the interaction in the network by detecting the spectral rearrangement of the compound emission as compared to individual RLs. The engineering of coupled RLs sets the basis for building structures with potential to function as optical neural networks.
We propose a pulse shaping and shortening technique for pulses generated from gain switched single mode semiconductor lasers, based on a Mach Zehnder interferometer with variable delay. The spectral ...and temporal characteristics of the pulses obtained with the proposed technique are investigated with numerical simulations. Experiments are performed with a Distributed Feedback laser and a Vertical Cavity Surface Emitting Laser, emitting at 1.5 µm, obtaining pulse duration reduction of 25-30%. The main asset of the proposed technique is that it can be applied to different devices and pulses, taking advantage of the flexibility of the gain switching technique.
We experimentally and theoretically investigate the photonic generation of microwave signals using a long-wavelength single-transverse-mode vertical-cavity surface-emitting laser (VCSEL) subject to ...two-frequency orthogonal optical injection. We study if a significant reduction of the linewidth is achieved in the double injection locking regime. In this regime, the VCSEL is subject to optical injection by two master lasers in such a way that stable locking is also observed if only light from one of the master lasers is injected. Our model includes the effect of the injected light reflected at the VCSEL's mirror generalizing previous modeling of reflection-mode optical injection-locked VCSELs. Our model also describes the high coherence associated to stable injection locking states and takes into account phase fluctuations in both master lasers. We observe no significant reduction of the linewidth in the double injection locking regime because the linewidth is mainly determined by the phase fluctuations of the two master lasers.
We report the experimental results obtained with a novel architecture for random lasing, in which the active material, free of scatterers, is placed between two large scattering regions. Lasing ...emission is investigated as a function of the illuminated area of the scattering regions, obtaining typical "resonant" and "non-resonant" random lasing spectra, depending on the device geometry. We propose a theoretical approach for the understanding of the observed phenomena, modelling the scattering elements with arbitrary spectral profiles in amplitude and phase and considering strong coupling between lasing modes. Good agreement between experiments and simulation results is obtained.
We present a novel implementation of the "phase reconstruction using optical ultra fast differentiation" (PROUD) technique and apply it to characterize the time resolved chirp of a gain switched ...semiconductor laser. The optical temporal differentiator is a fiber based polarization interferometer. The method provides a fast and simple recovery of the instantaneous frequency from two temporal intensity measurements, obtained by changing the spectral response of the interferometer. Pulses with different shapes and durations of hundreds of picoseconds are fully characterized in amplitude and phase. The technique is validated by comparing the measured pulse spectra with the reconstructed spectra obtained from the intensity and the recovered phase.
Two major elements are required in a laser device: light confinement and light amplification. Light confinement is obtained in optical cavities by employing a pair of mirrors or by periodic spatial ...modulation of the refractive index as in photonic crystals and Bragg gratings. In random lasers, randomly placed nanoparticles embedded in the active material provide distributed optical feedback for lasing action. Recently, we demonstrated a novel architecture in which scattering nanoparticles and active element are spatially separated and random lasing is observed. Here we show that this approach can be extended to scattering media with macroscopic size, namely, a pair of sand grains, which act as feedback elements and output couplers, resulting in lasing emission. We demonstrate that the number of lasing modes depends on the surface roughness of the sand grains in use which affect the coherent feedback and thus the emission spectrum. Our findings offer a new perspective of material science and photonic structures, facilitating a novel and simple approach for the realization of new photonics devices based on natural scattering materials.
We propose and demonstrate a coherent random laser in which the randomly distributed scattering centres are placed outside the active region. This architecture is implemented by enclosing a dye ...solution between two agglomerations of randomly positioned titanium dioxide nanoparticles. The same spectral signature, consisting of sharp spikes with random spectral positions, is detected emerging from both ensembles of titanium dioxide nanoparticles. We interpret this newly observed behaviour as due to the optical feedback given by back-scattered light from the scattering agglomerations, which also act as output couplers. A simple model is presented to simulate the observed behaviour, considering the amplitude and phase round trip conditions that must be satisfied to sustain lasing action. Numerical simulations reproduce the experimental reports, validating our simple model. The presented results suggest a new theoretical and experimental approach for studying the complex behavior of coherent random lasers and stimulate the realization of new devices based on the proposed architecture, with different active and scattering materials.
Electrically pumped random lasers with distributed feedback can be obtained by introducing random defects into the device active layer, modifying the epitaxial growth process and losing the ease of ...fabrication potentially offered by disordered structures. We recently demonstrated an alternative and more practical approach in which random lasing emission is obtained from a modified Fabry-Perot laser diode after pulsed laser ablation of its output mirror. Here, we improve our fabrication technique by sweeping the ablating laser beam along the output mirror at different speeds and with different pulse energies, obtaining control over the total energy delivered at each point. We optimize the ablation parameters by evaluating the device performances in terms of lasing threshold and output power and we present the device emission characteristics. The proposed technique is tunable, fast and reliable, allowing the fabrication of devices with different properties by proper selection of the ablation parameters.
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