Semiconductor Microcavity Lasers Research Articles

Optical frequency comb generation based on spectral broadening of a self-injection locked directly modulated microcavity laser.

A coherent optical frequency comb (OFC) with suppressed linewidth is demonstrated based on the spectral broadening of a directly modulated semiconductor microcavity laser with self-injection locking. Due to the high electro-optical response, the directly modulated microcavity laser provides a 10-GHz-spaced seeding OFC with 9 comb teeth in a 10 dB flatness window. Besides, an optical feedback fiber loop with two sub-loops is introduced to reduce the linewidth of the microlaser and suppress the undesired longitudinal modes. Self-injection locking is realized at the feedback power of 72 µW, and the laser linewidth is narrowed from 10 MHz to 14.5 kHz. Moreover, spectral broadening and pulse compression are used to broaden the optical spectrum and compress the pulse width, which results in an OFC with 36 comb lines in a 10 dB power deviation and a 1.8 ps optical pulse. Owing to the high coherence of the directly modulated OFC, all comb lines feature narrow linewidths in the range of 14.5∼18.2 kHz.

Low-threshold limaçon-shaped micro-cavity semiconductor lasers with an approximate ring-shaped top contact

Based on the whispering gallery mode (WGM) characteristics in the limaçon-shaped micro-cavity, we designed a micro-cavity laser with an approximate ring-shaped top electrical contact, which theoretically realized the annular current injection and low temperature rise features in the active gain region. Then we fabricated micro-cavity lasers with approximate ring-shaped top contact at different ring widths and compared them with conventional full disk-shaped top contact lasers, and observed a lower operating injection threshold current as a consequence of this relatively straightforward and feasible technique. The threshold current density of the approximate ring-shaped top contact laser is ∼0.41 kA/cm2, which is 16% lower than that of the conventional laser. Furthermore, we obtain a maximum light output peak power of 133 mW with directional emission at a far-field divergence angle of ∼38° at a full width of half maximum (FWHM).

Manipulation of lasing modes in a deformed octagonal microcavity laser

We have demonstrated a deformed octagonal microcavity semiconductor laser with manipulated lasing modes for bistable operation and direct modulation. There are two sets of degenerated four-bounced modes, S01 and S02, in the octagonal microcavity, and the degeneracy between them is broken by introducing a square hole into the center of the cavity. In the deformed octagonal microcavity laser, mode S01 dominates the lasing process during the current rising process. However, mode S02 also lases when the current decreases and interacts nonlinearly with S01, which is caused by the non-uniform distribution of the refractive index induced by the square-ring-shaped current injection. We observe a counterclockwise bistable hysteresis loop with continuous injection current ranging from 31 to 13 mA at 288 K. We also study the small-signal modulation response of the laser at high and low states with different injection currents. By utilizing the photon-photon resonance effect between modes S01 and S02, we effectively increased the 3-dB bandwidth of the laser from 12 GHz to 16.2 GHz.

Reservoir computing-based advance warning of extreme events

Physics-based computing exploits nonlinear or disorder-induced complexity, for example, to realize energy-efficient and high-throughput computing tasks. A particularly difficult but useful task is the prediction of extreme events that can occur in a wide range of complex systems. We prepare an experiment based on a microcavity semiconductor laser that produces statistically rare extreme events resulting from the interplay of deterministic nonlinear dynamics and spontaneous emission noise. We then evaluate the performance of three reservoir computing training approaches in predicting the occurrence of extreme events. We show that Dual Training Reservoir Computing (which in turn can be implemented with fast semiconductor laser dynamics) can provide meaningful early warnings up to 15 times the typical linear correlation time of the dynamics.

Self-pulsing and dual-mode lasing in a square microcavity semiconductor laser.

Self-pulsing and dual-mode lasing in a square microcavity semiconductor laser are studied experimentally. Self-sustained pulses originating from undamped relaxation oscillation induced by a two-mode interaction are obtained, as the injection current is slightly above the laser threshold. A repetition frequency of 4.4 GHz and a pulse width of 30-40 ps are obtained at a current of 8 mA. The laser switches to continuous-wave operation when the injection current is higher than a certain value, and dual-mode lasing with 30.7 GHz at 16 mA and 10.7 GHz at 27 mA are observed in the lasing spectra. Furthermore, the relative intensity noise spectra are presented to reveal the relationship between the lasing states and the dynamics induced by relaxation oscillation and mode beating.

Nonlinear dynamics of a semiconductor microcavity laser subject to frequency comb injection.

The nonlinear dynamical behaviors of a semiconductor microcavity laser with frequency comb injection have been experimentally and numerically investigated. The microcavity laser is harmonically locked to a unit fraction of the comb spacing due to the undamped relaxation oscillation at certain conditions, creating additional comb lines with reduced frequency spacing. The stability maps indicating various locking states are obtained based on rate equations, which demonstrates that the locking regions are closely related to the relaxation oscillation. Moreover, the microcavity laser with comb injection leads to spectral broadening of the original comb and the number of comb lines raises from 3 to 13. Owing to the large modulation bandwidth of the microcavity laser, the comb lines and the frequency spacing can be tailored over a wide range by varying the injection parameters.

Manipulation of lasing modes in a circular-side octagonal microcavity laser with a spatially distributed current injection.

We propose and demonstrate a circular-side octagonal microcavity (COM) semiconductor laser with a spatially distributed current injection for manipulating the lasing modes. There are two types of high-quality-factor whispering-gallery (WG) modes with distinct field patterns in a COM: the four-bounced quadrilateral modes and the eight-bounced octagonal modes. By designing two separated p-electrodes, the COM laser is divided into two regions that are pumped independently to select specific modes for lasing. The two types of WG modes lase simultaneously when the two regions are injected with equivalent currents. Degeneracy removal of the quadrilateral modes is observed in both simulation and experiment when the two regions are injected with inequivalent currents. The quadrilateral modes are suppressed when one of the two regions is un-injected or biased with a negative current, and single-octagonal-mode lasing is realized. The results show that the lasing modes can be efficiently manipulated with the spatially distributed current injection considering the distinct field patterns of different WG modes in the microcavities, which can promote the practical application of the microcavity lasers.

Optical feedback-induced dynamics and nonclassical photon statistics of semiconductor microcavity laser

We study a coherently driven semiconductor laser cavity containing a single quantum dot(QD) (as gain medium) with optical feedback under Markovian approximation. We have obtained coupled operator equations for the model Hamiltonian using standard input-output formalism of cavity-QED and have found that these equations do not have any finite steady state solutions. We have also used an exact numerical framework based on Matlab platform qotoolbox, to compute the temporal dynamics of the mean excitation number of laser cavity mode under high feedback coupling regime. We have further studied the photon correlations of both the cavity mode as well as external feedback mode to feedback identify the laser parameters and coupling strength that give the nonclassical sub-Poissonian photon statistics. This work is useful for coherent control of photon statistics and photon correlations in the semiconductor laser with optical feedback.

Genetic Algorithm Applied to the Optimized Design of Semiconductor Microcavity Lasers

Semiconductor microcavities have been used in important studies of several areas for technological or purely scientific purposes. However, the definition of the optimal parameters for the fabrication of microcavities is a difficult task. Moreover, some uncertainties related to the growth process can change the device features. These problems cannot be experimentally controlled, hindering the development of theoretical models. In this work we present a theoretical model to simulate the microcavities and also propose an evolutionary approach to optimize the device under uncertainty in order to ensure the growth with the desired features. Thus, based on the reflectance spectra of a AlxGa1-xAs semiconductor microcavity, the aluminum concentrations, x, and the number of layers that compose the heterostructure were optimized. This set of parameters may offer increased robustness in the growth process, while providing a considerable quality factor and the desired position of the cavity resonance, achieving the device’s operation limits. The device was optimized considering the cavity resonance between 700nm and 2000nm, where the results indicate that the proposed algorithm is able to find satisfactory solutions, minimizing the problems caused by inaccuracy in the growth process.

Complex coupling coefficient in laterally coupled microcavity laser diode arrays

The complex component of the coupling coefficient κ=κr+iκi, used to describe the coupling between adjacent semiconductor microcavity laser diodes, is studied. The complex component κi represents the gain or loss difference between the coherent in-phase and out-of-phase array supermodes obtained from two laterally coupled lasers. Steady-state analysis reveals that the threshold of the preferred coherent supermode is lower than that of an individual laser mode in proportion to κi. We show that the complex component κi can be experimentally extracted from a simple output power vs current measurement. Furthermore, the change in the lasing threshold at the onset of optical coupling perturbs the differential resistance of the coupled lasers. Therefore, an electrical signature of optical coupling can be detected in the diode array series resistance.

Lasing of scarred mode near above threshold in a semiconductor microcavity laser.

We study a lasing of mode groups in a fully chaotic rounded D-shape InGaAsP semiconductor microcavity laser when an electrode is smaller than a cavity (inward gap). Although there are numerous unstable periodic orbits supporting resonances, a mode group localized on period-5 unstable periodic orbit is more competitive than the others for our laser configuration of the inward gap. By means of theoretical and numerical analyses with ray and wave dynamics, we show that the analyses well agree with our experimental results.

Optically pumped semiconductor laser based on a type-II CdS/ZnSe heterostructure

An optically pumped semiconductor laser based on a type-II CdS/ZnSe nanoheterostructure containing 10 quantum wells (QWs) was studied. The structure was grown by metal-organic vapour phase epitaxy on a GaAs substrate. The lifetime of electron-hole pairs at a low pump level was measured by luminescence decay to be ∼0 ns. The peak power of the microcavity semiconductor laser at room temperature and longitudinal pumping by a repetitively pulsed N2 laser was 7.2 W at a wavelength of 514 nm. The relatively low laser slope efficiency (0.35 %) is explained by amplified spontaneous emission propagating along the structure. The peak power and efficiency of the laser in the case of transverse pumping increase to 70 W and 3.5 %, respectively.

Lowest threshold lasing modes localized on marginally unstable periodic orbits in a semiconductor microcavity laser.

The lowest threshold lasing mode in a rounded D-shape microcavity is theoretically analyzed and experimentally demonstrated. To identify the lowest threshold lasing mode, we investigate threshold conditions of different periodic orbits by considering the linear gain condition due to the effective pumping region and total loss consisting of internal and scattering losses in ray dynamics. We compare the ray dynamical result with resonance mode analysis, including gain and loss. We find that the resonance modes localized on the pentagonal marginally unstable periodic orbit have the lowest threshold in our fabrication configuration. Our findings are verified by obtaining the path lengths and far-field patterns of lasing modes.

Dual-Mode Microcavity Semiconductor Lasers

Dual-mode microcavity semiconductor lasers are investigated for potential applications in photonic microwave and optical frequency comb (OFC) generation. By regulating whispering-gallery modes (WGMs) in square microcavities, dual-mode lasing is achieved with frequency intervals ranging from tens to hundreds of gigahertz. In the square microcavities, the fundamental and first-order transverse WGMs have spatially separated field distributions, which allow stable dual-transverse-mode lasing with mode intervals inversely proportional to the cavity areas. Dual-transverse-mode square microcavity lasers are experimentally demonstrated with frequency intervals ranging from ∼35 to ∼150 GHz. Photonic microwaves and OFCs are successfully generated based on the dual-mode square microcavity lasers. To enhance the transverse-mode intervals, circular-side square microcavity lasers with a square hole in the center are proposed and demonstrated, and dual-transverse-mode lasing with a frequency interval up to ∼0.56 THz is obtained. Furthermore, a square-Fabry–Perot coupled-cavity microlaser is demonstrated to increase the output power and introduce an additional degree for manipulating the lasing modes. Dual-mode lasing is realized with a frequency interval of ∼0.5 THz, a single mode optical fiber coupled power of ∼3 mW, and a wavelength tuning range of ∼10 nm.

Exploration of VCSEL ultra-low biasing scheme for pulse generation

The generation of optical pulses at ultra low bias level, thus low energy cost, is explored in a commercial microcavity semiconductor laser in view of testing the principle of energy efficient information encoding in potential integrated schemes. Sequences of regular, highly nonlinear pulses with acceptable amplitude stability are obtained from a commercial device as potential sources of bits where the information is added by post-treatment (pulse removal). A discussion on the energy expenditure per bit is offered, together with the optimal frequency for pulse generation, which is found to lie slightly below the above-threshold value declared by the manufacturer.

Unidirectional emission of high-Q scarred modes in a rounded D-shape microcavity.

We propose a deformed microcavity laser, where a high-Q mode group emits unidirectionally. The cavity comprises three circular arcs and one linear section. To minimize diffraction effects from the boundary, three circular arcs and one linear section are tangentially connected. By adjusting the sizes and the positions of the two sub-circular arcs, unidirectionality is maximized. In an experiment with an InP based InGaAsP semiconductor microcavity laser, a lasing mode group localized on aperiod-7 unstable periodic orbit emits unidirectionally. In our resonance calculation, a high-Q factor is confirmed.

Alternation of Defects and Phase Turbulence Induces Extreme Events in an Extended Microcavity Laser.

Out-of-equilibrium systems exhibit complex spatiotemporal behaviors when they present a secondary bifurcation to an oscillatory instability. Here, we investigate the complex dynamics shown by a pulsing regime in an extended, one-dimensional semiconductor microcavity laser whose cavity is composed by integrated gain and saturable absorber media. This system is known to give rise experimentally and theoretically to extreme events characterized by rare and high amplitude optical pulses following the onset of spatiotemporal chaos. Based on a theoretical model, we reveal a dynamical behavior characterized by the chaotic alternation of phase and amplitude turbulence. The highest amplitude pulses, i.e., the extreme events, are observed in the phase turbulence zones. This chaotic alternation behavior between different turbulent regimes is at contrast to what is usually observed in a generic amplitude equation model such as the Ginzburg–Landau model. Hence, these regimes provide some insight into the poorly known properties of the complex spatiotemporal dynamics exhibited by secondary instabilities of an Andronov–Hopf bifurcation.

Pulse train interaction and control in a microcavity laser with delayed optical feedback.

We report experimental and theoretical results on the pulse train dynamics in an excitable semiconductor microcavity laser with an integrated saturable absorber and delayed optical feedback. We show how short optical control pulses can trigger, erase, or retime regenerative pulse trains in the external cavity. Both repulsive and attractive interactions between pulses are observed, and are explained in terms of the internal dynamics of the carriers. A bifurcation analysis of a model consisting of a system of nonlinear delay differential equations shows that arbitrary sequences of coexisting pulse trains are very long transients towards weakly stable periodic solutions with equidistant pulses in the external cavity.

Spatiotemporal Chaos Induces Extreme Events in an Extended Microcavity Laser.

Extreme events such as rogue waves in optics and fluids are often associated with the merging dynamics of coherent structures. We present experimental and numerical results on the physics of extreme event appearance in a spatially extended semiconductor microcavity laser with an intracavity saturable absorber. This system can display deterministic irregular dynamics only, thanks to spatial coupling through diffraction of light. We have identified parameter regions where extreme events are encountered and established the origin of this dynamics in the emergence of deterministic spatiotemporal chaos, through the correspondence between the proportion of extreme events and the dimension of the strange attractor.

Synchronous characterization of semiconductor microcavity laser beam.

We report on a high-resolution double-channel imaging method used to synchronously map the intensity- and optical-frequency-distribution of a laser beam in the plane orthogonal to the propagation direction. The synchronous measurement allows us to show that the laser frequency is an inhomogeneous distribution below threshold, but that it becomes homogeneous across the fundamental Gaussian mode above threshold. The beam's tails deviations from the Gaussian shape, however, are accompanied by sizeable fluctuations in the laser wavelength, possibly deriving from manufacturing details and from the influence of spontaneous emission in the very low intensity wings. In addition to the synchronous spatial characterization, a temporal analysis at any given point in the beam cross section is carried out. Using this method, the beam homogeneity and spatial shape, energy density, energy center, and the defects-related spectrum can also be extracted from these high-resolution pictures.