Broad-area Semiconductor Lasers Research Articles

Spatial coherence research and passive speckle suppression for edge-emitting broad-area semiconductor lasers

The spatial coherence of filaments in edge-emitting broad-area semiconductor lasers (BALs) is studied based on speckle theory, and a passive speckle suppression method for BALs is demonstrated. A fly-eye lens is used to integrate the filaments and homogenize the laser beam. The speckle contrast is consistent with theoretical calculation, which proves the spatial incoherence of filaments. A light pipe is used to introduce an optical path difference between light with different incident angles, enabling more incoherent beamlets. These beamlets are then combined at different illuminating angles by a double-sided fly-eye lens and focusing lens. As a result, the subjective speckle contrast is reduced from 95.94% to 19.13% at 1.2 A while maintaining a high luminous efficiency of 77.8%. This work provides important design principles for laser display systems.

High-power narrow spectral width 975 nm semiconductor laser with high process compatibility achieved by high-order gratings.

We present broad-area semiconductor lasers with a 100th-order high-order distributed Bragg reflection (HO-DBR) grating structure, fabricated with the conventional UV lithography techniques. Based on the finite-difference time-domain (FDTD) algorithm, a maximum output power of 10.5 W, emitting around 975 nm with a spectral width of less than 0.5 nm FWHM has been achieved. This method provides insights for reducing the manufacturing costs of the high-power narrow spectral width DBR lasers.

High-energy picosecond pulses with a single spatial mode from a passively mode-locked, broad-area semiconductor laser

We present a mode-locked semiconductor laser oscillator that emits few picosecond pulses (5-8ps at a repetition rate of 379MHz and wavelength of 1064nm) with record peak power (112W) and pulse energy (0.5nJ) directly out of the oscillator (with no amplifier). To achieve this high power performance we employ a high-current broad-area, spatially multi-mode diode amplifier (0.3×5mm), placed in an external cavity that enforces oscillation in a single spatial mode. Consequently, the brightness of the beam is near-ideal (M2 = 1.3). Mode locking is achieved by dividing the large diode chip (edge emitter) into two sections with independent electrical control: one large section for gain and another small section for a saturable absorber. Precise tuning of the reverse voltage on the absorber section allows to tune the saturation level and recovery time of the absorber, providing a convenient knob to optimize the mode-locking performance for various cavity conditions.

Microwave photonics via cavity solitons in injected broad-area semiconductor lasers with saturable absorber

Microwave photonics via cavity solitons in injected broad-area semiconductor lasers with saturable absorber

Transverse Photonic Crystal Mode Engineering for Broad-Area Semiconductor Lasers With Narrow-Divergence Angles

Broad-area semiconductor lasers, coupled with asymmetric transverse photonic crystals (TPCs), are designed by the effective index method and the transfer matrix method. Due to the propagation constant of the fundamental mode lying in the forbidden band of the TPCs, but those of the high-order modes lying in the allowed band of the TPCs, the fundamental mode is mainly concentrated in the active ridge waveguide, while all the high-order modes extend into the lossy TPCs on both sides of the active ridge waveguide. Therefore, the fundamental mode possesses larger optical confinement factor than those of the high-order modes, and reaches the lasing threshold more easily, which is demonstrated by the single-lobe horizontal far-field pattern of the TPC broad-area laser under an injection current of 0.3 A near the threshold. The proposed TPC broad-area laser provides a new strategy of designing broad-area semiconductor lasers with narrow divergence angles.

Sensitive control of broad-area semiconductor lasers by cavity shape

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.

Lateral characteristics improvements of DBR laser diode with tapered Bragg grating

Broad area semiconductor laser (BAL) has poor lateral beam quality due to lateral mode competition, which limits its application as a high-power optical source. In this work, the distributed Bragg reflector laser diode with tapered grating (TDBR-LD) is studied. By changing the lateral width, the tapered grating increases the loss of high-order lateral modes, thus improving the lateral characteristics of the laser diode. The measuring results show that the TDBR-LD can achieve a single-lobe output under 0.9 A. In contrast to the straight distributed Bragg reflector laser diode (SDBR-LD), the lateral far field divergence of TDBR-LD is measured to be 5.23° at 1 A, representing a 17% decline. The linewidth of TDBR-LD is 0.4 nm at 0.2 A, which is reduced by nearly 43% in comparison with that of SDBR-LD. Meanwhile, both of the devices have a maximum output power value of approximate 470 mW.

Extreme events in a broad-area semiconductor laser with coherent injection

Spatiotemporal extreme events are interesting phenomena, both from a fundamental point of view, as manifestations of complexity in dynamical systems, and for their possible applications in different research fields. Here, we present some recent results for extreme events in spatially extended semiconductor laser systems (broad-area vertical cavity surface-emitting lasers) with coherent injection. We study the statistics of spatiotemporal intensity peaks occurring in the transverse $(x,y)$ section of the field perpendicular to the light propagation direction and identify regions in the parameter space where extreme events are more likely to occur. Searching for precursors of these phenomena, we concentrate, on the one hand, on the spatiotemporal dynamics of the field phase and, in particular, on the presence of optical vortices in the vicinity of an extreme event. On the other hand, we focus on the laser gain dynamics and the phase-space trajectories of the system close to the occurrence of an extreme event. Both these complementary approaches are successful and allow us to shed some light on potential prediction strategies.

Influence of transverse carrier diffusion on two-dimensional optical rogue waves in broad-area semiconductor lasers with a saturable absorber

The statistics and dynamics of two-dimensional rogue waves in a broad-area semiconductor laser with an intracavity saturable absorber are numerically investigated under the effect of transverse carrier diffusion. We show that lateral diffusion of carriers alters the statistics of rogue waves by enhancing their formation in smaller ratios of carrier lifetimes in active and passive materials while suppressing them when the ratio is larger. The temporal dynamics of the emitted rogue waves is also studied and show that the finite nonzero transverse carrier diffusion coefficient gives them a longer duration. To further approach a realistic experimental situation, we also investigate the statistics and dynamics of rogue waves by simulating a circular disk-shape pump which replaces the flat pump profile typically used in numerical simulations of broad-area lasers. We show that a finite pump shape reduces the number of emitted rogue waves per unit area both below and above the laser threshold for every carrier lifetime ratio. The temporal width of the emitted rogue waves is also shown to decrease as a consequence of removing the nonphysical effects of the infinite flat pump on carrier dynamics.

Control of spatiotemporal rogue waves by harmonic pump modulation in a semiconductor laser with a saturable absorber

Through numerical simulations, statistical and dynamical properties of extreme events in a broad-area semiconductor laser with intracavity saturable absorber are investigated. By inclusion of a diffusion coefficient for the field, formation of rogue waves in a state of extended turbulence is studied and shown to be affected by harmonic perturbations. In particular, we propose a control technique based on periodic modulation of the pump parameter which can either drive the state of the system closer to or away from the chaotic attractors respectively enhancing or suppressing the generation of rogue waves. By statistical and dynamical analysis of the events in terms of intensity and optical gain, we show that when the system is under resonant modulation with frequency close to that of the dominant oscillations in the turbulent state (which is equal to the relaxation oscillation frequency typical of semiconductor lasers), more rogue waves are triggered with larger intensities and shorter lifetimes. On the other hand, off-resonant modulations restrain the formation of rogue waves where they appear in lower intensities and longer lifetimes. An example of special cases where the proposed scheme can completely forbid or allow the emission of rogue waves is also presented.

Spatial structure of lasing modes in wave-chaotic semiconductor microcavities

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.

Stabilized narrow-beam emission from broad-area semiconductor lasers

We provide a feasible and compact scheme to control and stabilize the spatiotemporal dynamics of BAS lasers. The proposal is based on the ability of non-Hermitian potentials with given local symmetries to manage the flow of light. A local PT-symmetric configuration allows to control, enhance and localize the generated light. We impose a pump modulation, with a central symmetry axis which induces in-phase gain and refractive index modulations due to the Henry factor. Both modulations are, in turn, spatially dephased by an appropriate index profile to yield to a local PT-symmetry within the modified BAS laser. Such local PT-symmetry potential induces an inward mode coupling, accumulating the light generated from the entire active layer at the central symmetry axis, which ensures spatial regularization and temporal stability. By an exhaustive exploration of the modulation parameters, we show a significant improvement of the intensity concentration, stability and brightness of the emitted beam. This approach produces a two-fold benefit: light localization into a narrow beam emission and the control over the spatiotemporal dynamics, improving the laser performance.

Numerical investigation of longitudinal spatial hole burning (LSHB) in high-power semiconductor lasers

Longitudinal spatial hole burning (LSHB) and its effect on output power of broad-area semiconductor lasers are examined using one-dimensional rate equations. It is shown that direct LSHB can be separated into two mechanisms – an increase in the effective threshold current and a decrease in the output efficiency. The LSHB dependence on laser diode parameters is analysed. In particular, a large optical confinement factor Γ is shown to suppress the LSHB-induced power decrease.

Photonic crystal spatial filtering in broad aperture diode laser

Broad aperture semiconductor lasers usually suffer from low spatial quality of the emitted beams. Due to the highly compact character of such lasers, the use of conventional intracavity spatial filters is problematic. We demonstrate that extremely compact photonic crystal spatial filters, incorporated into a laser resonator, can improve the beam spatial quality and, correspondingly, increase the brightness of the emitted radiation. We report the decrease in the M2 value from 47 down to 28 due to photonic crystal spatial intracavity filtering and the increase in the brightness by a factor of 1.5, giving a proof of principle of intracavity photonic crystal spatial filtering in broad area semiconductor lasers.

1D Confocal Broad Area Semiconductor Lasers (Confocal BALs) for Fundamental Transverse Mode Selection (TMS#0)

Previously in this journal we have reported on fundamental transverse mode selection (TMS#0) of broad area semiconductor lasers (BALs) with integrated twice-retracted 4f set-up and film-waveguide lens as the Fourier-transform element. Now we choose and report on a simpler approach for BAL-TMS#0, i.e., the use of a stable confocal longitudinal BAL resonator of length L with a transverse constriction. The absolute value of the radius R of curvature of both mirror-facets convex in one dimension (1D) is R = L = 2f with focal length f. The round trip length 2L = 4f again makes up for a Fourier-optical 4f set-up and the constriction resulting in a resonator-internal beam waist stands for a Fourier-optical low-pass spatial frequency filter. Good TMS#0 is achieved, as long as the constriction is tight enough, but filamentation is not completely suppressed.

Beam-combining scheme of high-power broad-area semiconductor lasers with Lyot-filtered reinjection: modeling, simulations, and experiments

A brightness- and power-scalable polarization beam-combining scheme for high-power, broad-area semiconductor laser diodes is investigated numerically and experimentally. To achieve the beam combining, we employ Lyot-filtered optical reinjection from an external cavity, which forces lasing of the individual diodes on interleaved frequency combs with overlapping envelopes and enables a high optical coupling efficiency. Unlike conventional spectral beam-combining schemes with diffraction gratings, the optical coupling efficiency is insensitive to thermal drifts of laser wavelengths. This scheme can be used for efficient coupling of a large number of laser diodes and paves the way toward using broad-area laser diode arrays for cost-efficient material processing, which requires high-brilliance emission and optical powers in the kW regime.

Efficient coupling of dynamic electro-optical and heat-transport models for high-power broad-area semiconductor lasers

In this work, we discuss the modeling of edge-emitting high-power broad-area semiconductor lasers. We demonstrate an efficient iterative coupling of a slow heat transport (HT) model defined on multiple vertical–lateral laser cross-sections with a fast dynamic electro-optical (EO) model determined on the longitudinal–lateral domain that is a projection of the device to the active region of the laser. Whereas the HT-solver calculates temperature and thermally-induced refractive index changes, the EO-solver exploits these distributions and provides time-averaged field intensities, quasi-Fermi potentials, and carrier densities. All these time-averaged distributions are used repetitively by the HT-solver for the generation of the heat sources entering the HT problem solved in the next iteration step.

Beam shaping of a broad-area laser diode using 3D integrated optics.

Typical high power broad-area semiconductor lasers exhibit a highly astigmatic beam profile. However, many applications require a homogenous and circular symmetric beam. Thus coupling into circular multimode optical fibers is often employed. The strip-like astigmatic output of the diode laser underfills the circular multimode fiber, thus a decrease in beam quality occurs after fiber coupling due to mode mixing inside the optical fiber. This Letter presents a 3D integrated optics approach to shape the output of a broad-area laser diode. Ultrafast laser inscription is utilized to create a pair of photonic lanterns connected back to back inside a glass chip that captures and shapes the output of a commercial 976nm wavelength broad-area laser diode with 95μm emitter width. Compared to coupling to a 105μm diameter, 0.15 numerical aperture step-index multimode fiber, the photonic chip-based approach results in a 13× higher beam quality and 7× greater brightness.

Stabilization of broad-area semiconductor laser sources by simultaneous index and pump modulations.

We show that the emission of broad-area semiconductor amplifiers and lasers can be efficiently stabilized by introducing two-dimensional periodic modulations simultaneously on both the refractive index and the pump (gain-loss) profiles in the transverse and longitudinal directions. The interplay between such index and gain-loss modulations efficiently suppresses the pattern-forming instabilities, leading to highly stable and bright narrow output beams from such sources. We also determine the stabilization performance of the device as a function of the pump current and linewidth enhancement factor.

Fundamental Transverse Mode Selection (TMS#0) of Broad Area Semiconductor Lasers with Integrated Twice-Retracted 4f Set-Up and Film-Waveguide Lens

Previously we focused on fundamental transverse mode selection (TMS#0) of broad area semiconductor lasers (BALs) with two-arm folded integrated resonators for Fourier-optical spatial frequency filtering. The resonator had a round-trip length of 4f, where f is the focal length of the Fourier-transform element (FTE), that is, a cylindrical mirror in-between the orthogonal resonator branches. This 4f set-up can be called “retracted once” due to the reflective filter after 2f; that is, the 2f path was used forwards and backwards. Now the branches are retracted once more resulting in a compact 1f long linear resonator (called “retracted twice”) with a round-trip length of 2f. One facet accommodates the filter, while the other houses the FTE, now incorporating a film-waveguide lens. The BAL facet with the filter represents both the Fourier-transform plane (after 2f, i.e., one round-trip) as well as the image plane (after 4f, two round-trips). Thus filtering is performed even after 4f, not just after 2f. Experimental results reveal good fundamental TMS for pump currents up to 20% above threshold and a one-dimensional beam quality parameter M1D2 = 1.47. The BALs are made from AlGaInAsSb, but the concept can equally well be employed for BALs of any material system.