Broad-area Laser Research Articles

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.

What Causes the Pulse Power Saturation of GaAs-Based Broad-Area Lasers?

With short current pulses, the output power of semiconductor lasers is limited by non-thermal internal loss mechanisms. Two-photon absorption (TPA) was recently proposed as dominating photon loss process. We investigate this claim by advanced numerical laser simulation that includes all relevant mechanisms self-consistently. Using common material parameters, we obtain good agreement with the measured performance up to 90-W output power. Carrier leakage and free-carrier absorption are identified as the main saturation mechanisms, followed by longitudinal spatial hole burning. Contrary to earlier studies, TPA and gain compression are found negligible.

High-power GaSb-based microstripe broad-area lasers

A simple and effective approach based on the microstripe broad-area (MSBA) structure was proposed, and high-efficiency and high-power mid-infrared GaSb-based quantum well lasers were demonstrated. It was shown that the MSBA structure can effectively suppress the lateral current leakage and improve the temperature behavior of GaSb lasers. Compared with the conventional broad-area structure, the energy conversion efficiency of MSBA lasers was more than threefold and threshold current density decreased above 50%. High characteristic temperature and high beam quality were realized.

Coherent addition of high power broad-area laser diodes with a compact VBG V-shaped external Talbot cavity

We introduced a compact V-shaped external Talbot cavity for phase locking of high power broad-area laser diodes. The length of compact cavity is ∼25 mm. Near diffraction-limit coherent addition of 10 broad-area laser diodes indicated that high quality phase locking was achieved. We measured the near-field emission mode of each individual broad-area laser diode with different feedback, such as a volume Bragg grating and a high reflection mirror. We found out that the best result of phase locking broad-area laser diodes was achieved by the compact V-shaped external Talbot cavity with volume Bragg grating feedback.

High-power high-brightness 980 nm lasers with >50% wall-plug efficiency based on asymmetric super large optical cavity.

High-power high-brightness super large optical cavity laser diodes with an optimized epitaxial structure are investigated at the wavelength of 980 nm range. The thicknesses of P- and N-waveguides are prudently chosen based on a systematic consideration about mode characteristics and vertical far-field divergences. Broad area laser diodes show a high internal quantum efficiency of 98% and a low internal optical loss of 0.58 cm-1. The ridge-waveguide laser with 7 μm ridge and 3 mm cavity yields 1.9 W single spatial mode output with far-field divergence angles of 6.8° in lateral and 11.5° in vertical at full width at half maximum under 2 A CW operating current. The corresponding M2 values are 1.77 and 1.47 for lateral and vertical, respectively, and the corresponding brightness is 76.8 MW‧cm-2‧sr-1. The far-field divergence angles with 95% power content are in the range of 24.7° to 26.1° across the whole measured range.

Suppressed power saturation due to optimized optical confinement in 9xx nm high-power diode lasers that use extreme double asymmetric vertical designs

Broad area lasers with novel extreme double asymmetric structure (EDAS) vertical designs featuring increased optical confinement in the quantum well, Γ, are shown to have improved temperature stability without compromising series resistance, internal efficiency or losses. Specifically, we present here vertical design considerations for the improved continuous wave (CW) performance of devices operating at 940 nm, based on systematically increasing Γ from 0.26% to 1.1%, and discuss the impact on power saturation mechanisms. The results indicate that key power saturation mechanisms at high temperatures originate in high threshold carrier densities, which arise in the quantum well at low Γ. The characteristic temperatures, T0 and T1, are determined under short pulse conditions and are used to clarify the thermal contribution to power limiting mechanisms. Although increased Γ reduces thermal power saturation, it is accompanied by increased optical absorption losses in the active region, which has a significant impact on the differential external quantum efficiency, . To quantify the impact of internal optical losses contributed by the quantum well, a resonator length-dependent simulation of is performed and compared to the experiment, which also allows the estimation of experimental values for the light absorption cross sections of electrons and holes inside the quantum well. Overall, the analysis enables vertical designs to be developed, for devices with maximized power conversion efficiency at high CW optical power and high temperatures, in a trade-off between absorption in the well and power saturation. The best balance to date is achieved in devices using EDAS designs with , which deliver efficiencies of 50% at 14 W optical output power at an elevated junction temperature of 105 °C.

High-temperature operation of electrical injection type-II (GaIn)As/Ga(AsSb)/(GaIn)As \u201cW\u201d-quantum well lasers emitting at 1.3 \xb5m

Electrical injection lasers emitting in the 1.3 μm wavelength regime based on (GaIn)As/Ga(AsSb)/(GaIn)As type-II double “W”-quantum well heterostructures grown on GaAs substrate are demonstrated. The structure is designed by applying a fully microscopic theory and fabricated using metal organic vapor phase epitaxy. Temperature-dependent electroluminescence measurements as well as broad-area edge-emitting laser studies are carried out in order to characterize the resulting devices. Laser emission based on the fundamental type-II transition is demonstrated for a 975 μm long laser bar in the temperature range between 10 °C and 100 °C. The device exhibits a differential efficiency of 41 % and a threshold current density of 1.0 kA/cm2 at room temperature. Temperature-dependent laser studies reveal characteristic temperatures of T0 = (132 ± 3) K over the whole temperature range and T1 = (159 ± 13) K between 10 °C and 70 °C and T1 = (40 ± 1) K between 80 °C and 100 °C.

Wavelength‐Stabilized High‐Pulse‐Power Laser Diodes for Automotive LiDAR

For automotive light detection and ranging (LiDAR) systems diode lasers emitting short optical pulses with a good beam quality and a low wavelength shift over a wide operating temperature range is needed. In this paper, theoretical and experimental results of broad area laser diodes specially designed for this application are presented. Optical pulse powers of tens of watts and pulse lengths in the 10 ns‐range for wavelengths near 905 nm and environmental temperatures between 15 and 85 °C are achieved. Due to the integrated Bragg grating the wavelength shift with temperature is as low as 65 pm K−1. The contact mesa with a width of 50 μm leads to a lateral M2 value of about 9. Streak camera measurements of the temporal evolution of the lateral near field intensity reveal that the highest intensity is emitted near the mesa edges. Simulation results of power current characteristics indicate that the power saturation experimentally observed can be attributed to nonlinear effects such as two‐photon‐absorption and gain compression.

宽发射面激光二极管光束整形系统的光学设计

宽发射面激光二极管光束整形系统的光学设计

Delayed feedback control of self-mobile cavity solitons in a wide-aperture laser with a saturable absorber.

We investigate the spatiotemporal dynamics of cavity solitons in a broad area vertical-cavity surface-emitting laser with saturable absorption subjected to time-delayed optical feedback. Using a combination of analytical, numerical, and path continuation methods, we analyze the bifurcation structure of stationary and moving cavity solitons and identify two different types of traveling localized solutions, corresponding to slow and fast motion. We show that the delay impacts both stationary and moving solutions either causing drifting and wiggling dynamics of initially stationary cavity solitons or leading to stabilization of intrinsically moving solutions. Finally, we demonstrate that the fast cavity solitons can be associated with a lateral mode-locking regime in a broad-area laser with a single longitudinal mode.

Efficient coupling of the inhomogeneous current spreading model to the dynamic electro-optical solver for broad-area edge-emitting semiconductor devices

We extend a 2 (space) + 1 (time)-dimensional traveling wave model for broad-area edge-emitting semiconductor lasers by a model for inhomogeneous current spreading from the contact to the active zone of the laser. To speedup the performance of the device simulations, we suggest and discuss several approximations of the inhomogeneous current density in the active zone.

Power scaling in quantum cascade lasers using broad-area stripes with reduced cascade number

For many applications, optical emission power of many watts in the midinfrared part of the optical spectrum is required. Quantum cascade lasers for these applications typically contain 30 to 40 cascades and have laser ridge widths of about 10 μm. These lasers cannot, however, be readily scaled up in power. This paper demonstrates a path for further power scaling that is based on broad stripes with fewer than 15 cascades. Although the confinement factor Γ for this design approach is lower than for designs with 30 to 40 cascades, the ability to efficiently extract heat from the top and bottom of the lasers more than offsets the Γ disadvantage. We expect this approach to deliver tens of watts of power with high beam quality leading to high brilliance. The paper discusses the design approach, thermal conduction considerations, and confinement factor. Room temperature cw operation of broad-area lasers based on this approach is demonstrated and described.

Two-dimensional photonic crystal Bragg lasers with triangular lattice for monolithic coherent beam combining

We demonstrate an electrically pumped, single-mode, large area, edge-emitting InGaAsP/InP two-dimensional photonic crystal (PC) Bragg laser with triangular lattice. The laser operates in the single transverse and longitudinal modes with a single lobe, near-diffraction-limited far field. We compare the performance of the triangular-lattice PC Bragg laser with the rectangular-lattice PC Bragg laser fabricated from the same wafer and find that their performances are comparable. Then, we combine two single triangular-lattice PC Bragg lasers that tilt to opposite directions by taking advantage of the symmetry of the single emitter cavity mode. The measurement results show that the combined PC Bragg lasers provide the near-diffraction-limited output beam, and the single wavelength operation is also maintained in the coherently combined broad-area PC Bragg lasers.

Resonant excitation of transverse patterns in broad-area lasers by periodic temporal pump modulation

We demonstrate numerically that the temporal periodic modulation of the pump can excite regular transverse spatiotemporal intensity patterns in broad-area lasers. The Floquet stability analysis is used to characterize dynamics of modulated systems. Pattern formation occurs when the modulation frequency is approximately equal to the relaxation frequency or twice the value of this frequency. Instability of homogeneous oscillations leads to the formation of regular optical patterns, such as stripes and hexagons. The characteristic sizes of the observed patterns are in good agreement with predictions of the Floquet analysis.

Thin-film filter wavelength-stabilized, grating combined, high-brightness kW-class direct diode laser

We report on kW-class dense wavelength beam combining of a laser diode module consisting of ten broad-area laser diode bars by using a novel multi-laser cavity approach based on a thin-film filter (TFF) as a dispersive optical element. The wavelength-stabilized output of the TFF cavity is beam combined upon a -1st order transmission grating. Hereby, a cylindrical telescope is used for linear dispersion-matching between the TFF and the combiner grating. On the basis of simulations of the resulting beam quality deterioration, we are able to optimize the cavity and the combiner setup for optimal beam quality preservation. We demonstrate a highly efficient direct diode laser with 1.1-kW output power and a symmetrical beam parameter product of about 6mm × mrad (95 % power content) in both beam axis.

DFB lasers with apodized surface gratings for wavelength stabilization and high efficiency

We present results of GaAs-based distributed-feedback (DFB) broad-area (BA) lasers with integrated surface-etched gratings. Lasers with longitudinally varying (apodized) Bragg gratings are compared to lasers with uniform gratings. DFB-BA lasers with apodized gratings show improved conversion efficiency and output power compared to those with uniform gratings. DFB-BA lasers with narrow contact stripe width W = 30 μm and resonator length L = 6 mm show a peak conversion efficiency of 60% and maintain efficiency >50% to an output power of >6 W. The spectral FWHM linewidth is below 0.5 nm. Furthermore DFB lasers with implemented apodized gratings are less sensitive to fabrication tolerances.

Impact of longitudinal refractive index change on the near-field width of high-power broad-area diode lasers

Typical for broad-area laser (BAL) diodes operating in a continuous-wave mode is a narrowing of the near-field (NF) width at the output facet for high injection currents (output powers). This phenomenon increases the facet load of BALs, resulting in a reduction in the level of catastrophic optical mirror damage. In this letter, we demonstrate theoretically that thermally induced changes in the refractive index in both lateral and longitudinal directions not only cause the NF narrowing at the front facet but also a broadening of the NF at the back facet. In contrast, a sole lateral self-heating induced variation in the refractive index (commonly referred to as thermal lensing) does not result in a NF narrowing. Our theoretical findings are confirmed by measurements of the current-dependent profiles of the NF at the back and front facets of a BAL with a stripe width of 120 μm emitting at 960 nm. Furthermore, our quasi three-dimensional thermo-electro-optic simulations indicate that a longitudinally homogeneous device temperature can reduce the front-facet load while keeping the beam quality unchanged compared with the experimental results.

Beam Quality Deterioration in Dense Wavelength Beam-Combined Broad-Area Diode Lasers

We study various effects of beam quality deterioration in dense wavelength beam-combined direct diode laser systems based on broad-area laser diode bars of different filling factors. We mainly concentrate our research on external cavity architectures, where a transmission grating is used for simultaneous spectral stabilization and beam combining the emitters of a single laser diode bar. In this context, the residual spectral linewidth of the stabilized diode emitters, diode bar smile and spectral emitter crosstalk are the most relevant parameters which influence the resulting beam quality. On the basis of our results, we can deduce a lower limit for the beam quality of dense wavelength beam-combined broad-area diode lasers.

High-power broad-area buried-mesa lasers

Broad area lasers emitting near 940 nm are fabricated using a process based on two-step epitaxy. The n-side of the layer structure and the active layer are grown during the first epitaxial step, the p-side during the second. Between the first and the second step a shallow etching is used to remove the active layer from the two sides and at the two facets. This simple approach allows the creation of buried mesa lasers with non-absorbing mirrors, resulting in a reduced lateral current leakage, lower threshold current and higher efficiency, plus an increased robustness with respect to catastrophic optical damage.

Beam control of high-power broad-area photonic crystal lasers using ladderlike groove structure

The high-power broad-area (BA) photonic bandgap crystal (PBC) diode laser is promising as a high-brightness laser source, however, it suffers from poor lateral beam quality owing to the intrinsic drawback of BA lasers. In this paper, a ladderlike groove structure (LLGS) was proposed to improve both the lateral beam quality and emission power of BA PBC lasers. An approximately 15.4% improvement in output power and 25.2% decrease in the lateral beam parameter product (BPP) were realized and the underlying mechanism was discussed. On the basis of the one-dimensional PBC epitaxial structure, a stable vertical far field was demonstrated.