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Selecting and tuning modes are useful in ultrasonic guided wave non-destructive testing (NDT) since certain modes at various center frequencies are sensitive to specific types of defects. Ideally one should be able to select both the mode and the center frequency of the launched waves. We demonstrated that an affordable laser diode array can selectively launch either the S0 or A0 ultrasonic wave mode at a chosen center frequency into a polymer plate. A fiber-coupled diode array (4 elements) illuminated a 2 mm thick acrylic plate. A predetermined time delay matching the selected mode and frequency was employed between the output of the elements. The generated ultrasound was detected by a 215 kHz piezo receiver. Our results imply that this array permits non-contacting guided wave ultrasonic NDT. The solution is small, affordable, and robust in comparison to conventional pulsed lasers. In addition, it does not require experienced operators. [ABSTRACT FROM AUTHOR]

Coherently coupled laser arrays can be described by the temporal coupled mode theory in which the array modal behavior can be classified according to the coupling matrix, M ¯ ¯ . Accounting for a nonuniform gain/loss distribution in a laser array makes M ¯ ¯ a non-Hermitian matrix, and experimentally we find phase-front tuning (beam steering) of the coherent supermode as a result of the non-Hermiticity. We report the experimental characterization of the supermodes in coherently coupled vertical cavity surface emitting laser diode arrays and demonstrate control of non-Hermiticity by spatially varying injection currents. Exceptional points are identified in these electrically injected microcavity diode arrays. [ABSTRACT FROM AUTHOR]

The thermal stress due to the thermal expansion mismatch could induce crystallographic defects such as buckling and cracking and degrade device performance. In this paper, the thermal stress distribution in a laser array structure selectively grown on V-groove-patterned Si substrates was investigated by two-dimension finite-element method. Surprisingly, unexpected results are observed that the top of the InGaAs active layer and the most region of the InP cap layer are in compression, which is far different from the thermal stress distribution in planar structures. Two mechanisms have been proposed and modeled to explain the difference—(i) the width of uncoalesced layers is smaller than that of the Si substrate, which causes thermal stress to change in epitaxial layers, and (ii) thermal stress in the InGaAs and InP layers is affected by the V-groove structure. The results show that whether or not the epitaxial layers are coalesced has significant effect on the thermal stress distribution. The effect of the height of the V-groove, the height and the width of the SiO2 mask on the thermal stress distribution was also studied. It is found that the height of V-groove and the height of SiO2 mask play a critical role in the stress distribution. These findings are useful for the optimal designs for the laser array and provide an important step towards the realization of photonic integration circuits on silicon. [ABSTRACT FROM AUTHOR]

Arrays of coupled semiconductor lasers are systems possessing complex dynamical behavior and are of major interest in photonics and laser science. Dynamical instabilities, arising from supermode competition and slow carrier dynamics, are known to prevent stable phase locking in a wide range of parameter space, requiring special methods to realize stable laser operation. Inspired by recent concepts of parity-time ( P T ) and non-Hermitian photonics, in this work, we consider non-Hermitian coupling engineering in laser arrays in a ring geometry and show, both analytically and numerically, that non-Hermitian coupling can help to mitigate the onset of dynamical laser instabilities. In particular, we consider in detail two kinds of nearest-neighbor non-Hermitian couplings: symmetric but complex mode coupling (type-I non-Hermitian coupling) and asymmetric mode coupling (type-II non-Hermitian coupling). Suppression of dynamical instabilities can be realized in both coupling schemes, resulting in stable phase-locking laser emission with the lasers emitting in phase (for type-I coupling) or with π/2 phase gradient (for type-II coupling), resulting in a vortex far-field beam. In type-II non-Hermitian coupling, chirality induced by asymmetric mode coupling enables laser phase locking even in the presence of moderate disorder in the resonance frequencies of the lasers. [ABSTRACT FROM AUTHOR]

We show a phase-locked array of quantum cascade lasers with an intracavity spatial filter based on the Talbot effect. All the laser arrays show in-phase operation from the threshold current to full power current with a near-diffraction-limited divergence angle. The maximum power is just about 5 times that of a single-ridge laser for an eleven-laser array device and 3 times for a seven-laser array device. The structure was analyzed by using the multi-slit Fraunhofer diffraction theory, showing very good agreement with the experimental results. Considering the great modal selection ability, simple fabricating process, and potential for achieving continuous wave operation, this phase-locked array may be a hopeful solution to obtain higher coherent power. [ABSTRACT FROM AUTHOR]

Single-mode, widely tunable laser diodes in the mid-infrared range are highly interesting for demanding spectroscopic applications involving multi-species discrimination. We report on an alternative approach using single frequency laser arrays. Single-mode laser arrays were fabricated using all-photonic-crystal electrically pumped distributed feedback cavities on GaSb. The fabricated lasers exhibit thresholds in the 3.2 kA/cm2 range in a continuous wave regime at room temperature. The maximum output power reaches 1 mW and single mode operation with a side-mode suppression ratio of 30 dB is demonstrated. These lasers were used to perform tunable diode laser absorption spectroscopy of several gases in standard gas cells. Continuous spectral coverage of a 40 nm band using 10 lasers seems an achievable goal using laser arrays with PhC lattice constant variations of 1 nm from laser to laser. ©2017 Author(s). All article content, except where otherwise noted, is licensed under a Creative Commons Attribution (CC BY) license [ABSTRACT FROM AUTHOR]

Terahertz radiation from tunneling ionization of gaseous atoms and molecules in the two-color laser fields with various polarizations has been investigated. We experimentally demonstrate that the efficiency of terahertz emission in the circularly polarized laser fields with the same helicity is 5 times higher than that with linearly polarized two-color femtosecond pulses in high laser intensity. By solving time-dependent Schrödinger equation, this enhancement is well explained based on the analysis of electron tunneling ionization and subsequent dynamics. [ABSTRACT FROM AUTHOR]

By employing vapor-phase transport method, ZnO microrods are fabricated and directly assembled on p-GaN substrate to form a heterostructural microlaser array, which avoids of the relatively complicated etching process comparing previous work. Under applied forward bias, whispering gallery mode ZnO ultraviolet lasing is obtained from the as-fabricated heterostructural microlaser array. The device's electroluminescence originates from three distinct electron-hole recombination processes in the heterojunction interface, and whispering gallery mode ultraviolet lasing is obtained when the applied voltage is beyond the lasing threshold. This work may present a significant step towards future fabrication of a facile technique for micro/nanolasers. [ABSTRACT FROM AUTHOR]

Coherently coupled vertical-cavity surface-emitting laser arrays offer unique advantages for nonmechanical beam steering applications. We have applied dynamic coupled mode theory to show that the observed temporal phase shift between vertical-cavity surface-emitting array elements is caused by the detuning of their resonant wavelengths. Hence, a complete theoretical connection between the differential current injection into array elements and the beam steering direction has been established. It is found to be a fundamentally unique beam-steering mechanism with distinct advantages in efficiency, compactness, speed, and phase-sensitivity to current. [ABSTRACT FROM AUTHOR]