Laser scanning technology is one of the most integral parts of today's scientific research, manufacturing, defense, and biomedicine. In many applications, high-speed scanning capability is essential for scanning a large area in a short time and multi-dimensional sensing of moving objects and dynamical processes with fine temporal resolution. Unfortunately, conventional laser scanners are often too slow, resulting in limited precision and utility. Here we present a new type of laser scanner that offers ∼1,000 times higher scan rates than conventional state-of-the-art scanners. This method employs spatial dispersion of temporally stretched broadband optical pulses onto the target, enabling inertia-free laser scans at unprecedented scan rates of nearly 100 MHz at 800 nm. To show our scanner's broad utility, we use it to demonstrate unique and previously difficult-to-achieve capabilities in imaging, surface vibrometry, and flow cytometry at a record 2D raster scan rate of more than 100 kHz with 27,000 resolvable points.
The laser is an out-of-equilibrium nonlinear wave system where the interplay of the cavity geometry and nonlinear wave interactions, mediated by the gain medium, determines the self-organized oscillation frequencies and the associated spatial field patterns. In the steady state, a constant energy flux flows through the laser from the pump to the far field, with the ratio of the total output power to the input power determining the power-efficiency. While nonlinear wave interactions have been modelled and well understood since the early days of laser theory, their impact on the power-efficiency of a laser system is poorly understood. Here, we show that spatial hole burning interactions generally decrease the power efficiency. We then demonstrate how spatial hole burning interactions can be controlled by a spatially tailored pump profile, thereby boosting the power-efficiency, in some cases by orders of magnitude.Power-efficiency of lasers is a key property to be maximized to reduce energy requirements for on-chip applications, facilitate thermal management and pack lasers into smaller volumes. Earlier work on solid-state lasers addressed the quantum design of the gain medium and the optimization of carrier transport, demonstrating strong improvements in power-efficiency [1,2]. Far less systematic work has addressed the fundamental factors limiting the power efficiency of lasers and effective methods to overcome these, taking into account specific resonator properties. In particular, the impact of spatial hole burning interactions is poorly understood [3]. A case in point are the findings of Ref. [4], where the output power of microcavity quantum cascade lasers was found to increase exponentially with boundary deformation, resulting in a power enhancement over two orders of magnitude with respect to identical lasers of circular cross-section. After more than a decade of theoretical development, the fundamental factors behind this dramatic increase in power efficiency remain unknown. The goal of this paper is to provide a theoretical analysis of the mechanisms at work that enable such dramatic improvements in laser power efficiency.Typically, the pump power in a microlaser is deposited uniformly across the entire cavity area [ Fig. 1(a)] and lasing naturally occurs in cavity modes with the longest cavity lifetimes. Here we show the existence of a maximally power-efficient "optimally out-coupled mode" that may never turn on in a uniformly pumped cavity due to spatial-hole burning interactions. We further show that a non-uniform pump distribution designed to selectively pump the optimally out-coupled mode can lead to strong power enhancements.Spatially non-uniform pump distributions can be realized by fabricating patterned contacts [5], by spatially non-uniform doping in the case of current injection lasers [6,7], or by using spatial light modulators or special lenses for optically pumped lasers [8,9] (see Fig. 1(b)). We discuss the optimal spatial profile of the pump for a given resonator to maximize the power effic...
We report high-throughput optical coherence tomography (OCT) that offers 1,000 times higher axial scan rate than conventional OCT in the 800 nm spectral range. This is made possible by employing photonic time-stretch for chirping a pulse train and transforming it into a passive swept source. We demonstrate a record high axial scan rate of 90.9 MHz. To show the utility of our method, we also demonstrate real-time observation of laser ablation dynamics. Our high-throughput OCT is expected to be useful for industrial applications where the speed of conventional OCT falls short.
Studying time-dependent behavior in lasers is analytically difficult due to the saturating nonlinearity inherent in the Maxwell-Bloch equations and numerically demanding because of the computational resources needed to discretize both time and space in conventional FDTD approaches. We describe here an efficient spectral method to overcome these shortcomings in complex lasers of arbitrary shape, gain medium distribution, and pumping profile. We apply this approach to a quasi-degenerate two-mode laser in different dynamical regimes and compare the results in the long-time limit to the Steady State Ab Initio Laser Theory (SALT), which is also built on a spectral method but makes a more specific ansatz about the long-time dynamical evolution of the semiclassical laser equations. Analyzing a parameter regime outside the known domain of validity of the stationary inversion approximation, we find that for only a narrow regime of pump powers the inversion is not stationary, and that this, as pump power is further increased, triggers a synchronization transition upon which the inversion becomes stationary again. We provide a detailed analysis of mode synchronization (aka cooperative frequency locking), revealing interesting dynamical features of such a laser system in the vicinity of the synchronization threshold.Lasers are very rich dynamical systems which exhibit various time-dependent phenomena characteristic of nonlinear systems such as phase and mode locking, selfpulsing and breathing, and generally, spatio-temporal pattern formation and dynamical chaos. Almost all these effects can be understood and quantitatively studied using the semiclassical laser theory in the form of Maxwell-Bloch (MB) equations [1-3], a set of coupled non-linear equations for the space-and time-dependent electric field amplitude E(r, t), and the polarization and inversion of the gain medium P (r, t) and D(r, t). Early work made abundant use of spectral methods, where the field amplitudes entering the MB equations are expanded in a complete basis of spatial modes, reducing MB equations to a set of coupled non-linear ordinary differential equations for time-dependent amplitudes. These early theoretical investigations made a number of simplifying assumptions on the spatial aspects of the problem. The lasing modes were assumed to be simple (uniform, trigonometric, or gaussian) and unmodified from their passive cavity modes, and the openness (optical leakage) was taken into account phenomenologically. While these assumptions are sufficiently general to reproduce qualitatively almost all features of laser dynamics in macroscopic cavities, new laser systems have emerged in the past two decades that raised questions not easily addressable by these spectral approaches.Most novel laser systems are motivated by their deployment as compact and tunable light-sources for on-chip applications [4]. Typically, these lasers feature complex sub-wavelength patterning of the cavity volume to employ light-confinement mechanisms that are based on optical interferen...
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