Random numbers are widely used for information security, cryptography, stochastic modeling, and quantum simulations. Key technical challenges for physical random number generation are speed and scalability. We demonstrate a method for ultrafast generation of hundreds of random bit streams in parallel with a single laser diode. Spatiotemporal interference of many lasing modes in a specially designed cavity is introduced as a scheme for greatly accelerated random bit generation. Spontaneous emission, caused by quantum fluctuations, produces stochastic noise that makes the bit streams unpredictable. We achieve a total bit rate of 250 terabits per second with off-line postprocessing, which is more than two orders of magnitude higher than the current postprocessing record. Our approach is robust, compact, and energy-efficient, with potential applications in secure communication and high-performance computation.

Spatiotemporal instabilities are widespread phenomena resulting from complexity and nonlinearity. In broad-area edge-emitting semiconductor lasers, the nonlinear interactions of multiple spatial modes with the active medium can result in filamentation and spatiotemporal chaos. These instabilities degrade the laser performance and are extremely challenging to control. We demonstrate a powerful approach to suppress spatiotemporal instabilities using wave-chaotic or disordered cavities. The interference of many propagating waves with random phases in such cavities disrupts the formation of self-organized structures such as filaments, resulting in stable lasing dynamics. Our method provides a general and robust scheme to prevent the formation and growth of nonlinear instabilities for a large variety of high-power lasers.

We design and fabricate an on-chip laser source that produces a directional beam with low spatial coherence. The lasing modes are based on the axial orbit in a stable cavity and have good directionality. To reduce the spatial coherence of emission, the number of transverse lasing modes is maximized by fine-tuning the cavity geometry. Decoherence is reached in a few nanoseconds. Such rapid decoherence will facilitate applications in ultrafast speckle-free full-field imaging.The high spatial coherence of conventional lasers can introduce coherent artifacts due to uncontrolled diffraction, reflection and optical aberration. A common example is the speckle formed by the interference of coherent waves with random phase differences [1,2]. Speckle noise is detrimental to full-field imaging applications such as displays [3], microscopy, optical coherence tomography, and holography [4]. It also poses as a problem for laserbased applications like material processing, photolithography [5], and optical trapping of particles [6].Various approaches to mitigate speckle artifacts have been developed. A traditional method is to average over many independent speckle patterns generated by a moving diffuser [7,8], colloidal solution [9], or fast scanning micromirrors [10]. However, the generation of a series of uncorrelated speckle patterns is time-consuming and limited by the mechanical speed. A more efficient approach is to design a multimode laser that generates spatially incoherent emission, thus directly suppressing speckle formation [11]. Low spatial coherence necessitates lasing in numerous distinct spatial modes with independent oscillation phases. For example, a degenerate cavity [12][13][14] allows a large number of transverse modes to lase, but the setup is bulky and hard to align. Complex lasers with compact size such as random lasers [15][16][17][18] have low spatial coherence and high photon degeneracy, but are mostly on optically pumped. For speckle-free imaging applications, wave-chaotic semiconductor microlasers [19] have the advantages of electrical pumping and high internal quantum efficiency. However, disordered or wave-chaotic cavity lasers typically have no preferential emission direction, and the poor collection efficiency greatly reduces their external quantum efficiency. Our goal is creating an electrically pumped multimode semiconductor microlaser without disordered or wave-chaotic cavity to combine low spatial coherence and directional emission.Moreover, the speed of speckle suppression is crucial for imaging applications. For instance, time-resolved optical imaging to observe fast dynamics requires specklefree image acquisition with a short integration time, so * hui.cao@yale.edu the oscillation phases of different spatial lasing modes must completely decorrelate during the integration time to attain decoherence. The finite linewidth ∆ν of individual lasing modes leads to their decoherence on a time scale of 1/∆ν. The frequency difference between different lasing modes can lead to even faster decoherence....

We present experimental and numerical studies of broad-area semiconductor lasers with chaotic ray dynamics. The emission intensity distributions at the cavity boundaries are measured and compared to ray tracing simulations and numerical calculations of the passive cavity modes. We study two different cavity geometries, a D-cavity and a stadium, both of which feature fully chaotic ray dynamics. While the far-field distributions exhibit fairly homogeneous emission in all directions, the emission intensity distributions at the cavity boundary are highly inhomogeneous, reflecting the non-uniform intensity distributions inside the cavities. The excellent agreement between experiments and simulations demonstrates that the intensity distributions of wave-chaotic semiconductor lasers are primarily determined by the cavity geometry. This is in contrast to conventional Fabry–Perot broad-area lasers for which the intensity distributions are to a large degree determined by the nonlinear interaction of the lasing modes with the semiconductor gain medium.

The ray dynamics of optical cavities exhibits bifurcation points: special geometries at which ray trajectories switch abruptly between stable and unstable. A prominent example is the Fabry–Perot cavity with two planar mirrors, which is widely employed for broad-area semiconductor lasers. Such cavities support lasing in a relatively small number of transverse modes, and the laser is highly susceptible to filamentation and irregular pulsations. Here, we demonstrate experimentally that a slight deviation from this bifurcation point (planar cavity) dramatically changes the laser performance. In a near-planar cavity with two concave mirrors, the number of transverse lasing modes increases drastically. While the spatial coherence of the laser emission is reduced, the divergence angle of the output beam remains relatively narrow. Moreover, the spatiotemporal lasing dynamics becomes significantly more stable compared to that in a Fabry–Perot cavity. Our near-planar broad-area semiconductor laser has higher brightness, better directionality, and hence allows shorter integration times than an incandescent lamp while featuring sufficiently low speckle contrast at the same time, making it a vastly superior light source for speckle-free imaging. Furthermore, our method of controlling spatiotemporal dynamics with extreme sensitivity near a bifurcation point may be applied to other types of high-power lasers and nonlinear dynamic systems.