High-power Broad-area Laser Diodes Research Articles

Miniature long-range light beam transmitter resorting to a high-power broad area laser diode

A miniature long-range light beam transmitter, which taps into a high-power broad area laser diode (BALD), was realized to exhibit a uniform detectable width. An effective model was proposed to practically emulate the multimode characteristics of the beam generated by the BALD. The model, solely based on the emitting region and far-field divergence angle pertaining to the LD, is established through an incoherent superposition of multiple normalized Hermit–Gaussian modes. The feasibility of the proposed model was successfully verified in terms of the calculated and observed irradiance distributions of the light beams. A long-range light beam transmitter was then designed and constructed taking advantage of the BALD source in conjunction with a beam shaper. The manufactured transmitter was corroborated to provide an infrared beam with a constant detectable width of ~1m, over a distance ranging up to 400m, for a predefined threshold level.

Coherent beam combining of high power broad-area laser diode array with near diffraction limited beam quality and high power conversion efficiency

We explored a path of achieving high quality phase-locking of broad-area laser diode (BALD) array that operates at high electrical to optical power conversion efficiency (PCE). We found that (a) improving single transverse mode control for each individual BALD, (b) employing global Talbot optical coupling among diodes, and (c) enhancing strength of optical coupling among diodes are key factors in achieving high quality phase-locking of high power BALD array. Subsequently, we redesigned and improved a V-shaped external Talbot cavity and employed low reflectivity anti-reflection (AR) coated, low-"smile" BALD array to meet these three important requirements. We demonstrated near-diffraction limit far-field coherent pattern with 19% PCE and 95% visibility. The far-field angle (full-width at half-maximum (FWHM)) of center lobe was measured as 1.5 diffraction angular limited with visibility of 99% for 5A injection current and 1.6 diffraction angular limited with visibility of 95% for 14A injection current. Power scaling of diode array is discussed.

Suppression of Higher-Order Lateral Modes in Broad-Area Diode Lasers by Resonant Anti-Guiding

For a maximum fiber-coupled power, high power broad-area diode lasers must operate with a small lateral far-field divergence at high continuous wave (CW) powers. However, these structures are laterally multi-moded, with a low beam quality and wide emission angles. We present a new approach to suppress higher-order lateral modes based on an anti-guiding layer with a high refractive index inserted at the edges of the active stripe. Simulations of planar waveguides containing germanium layers reveal that strong mode-coupling effects occur. These are found to vary as a function of the thickness of the inserted layer. By embedding the germanium layer in the outer region of a ridge-waveguide, the mode coupling effects result in a reduction of the modal gain of higher-order lateral modes. The lateral far-field divergence of fabricated 90- μm stripe lasers emitting at 980 nm and containing such an anti-guiding layer is narrowed by 3° at a CW output power of 10 W.

Efficient High-Power Laser Diodes

High-power broad-area diode lasers are the most efficient light sources, with 90-μm stripe GaAs-based 940-980 nm single emitters delivering > 10 W optical output at a power conversion efficiency η <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">E</sub> (10 W) > 65%. A review of efforts to increase η <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">E</sub> is presented here and we show that for well-optimized structures, the residual losses are dominated by the <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">p</i> -side waveguide and nonideal internal quantum efficiency η <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">i</sub> . The challenge in measuring efficiency to sufficient precision is also discussed. We show that η <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">E</sub> can most directly be improved using low heat sink temperature T <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">HS</sub> with η <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">E</sub> (10 W) reaching > 70% at <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">T</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">HS</sub> = -50 °C. In contrast, increases in η <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">E</sub> at T <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">HS</sub> = 25 °C require improvements in both material quality and design, with growth studies targeting increased η <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">i</sub> and reduced threshold current and design studies seeking to mitigate the impact of the <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">p</i> -side waveguide. “Extreme, double asymmetric” (EDAS) designs are shown to substantially reduce <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">p</i> -side losses, at the penalty of increased threshold current. The benefit of EDAS designs is shown here using diode lasers with 30-μm stripes, (in development as high beam quality sources for material processing). Efficiency increases of ~ 10% relative to conventional designs are demonstrated at high powers.

High-Efficient and Reliable Broad-Area Laser Diodes With a Window Structure

We report on high-power 915-nm broad-area laser diodes with a window structure formed by a impurity-free vacancy disordering method. A maximum conversion efficiency of 68% and high output power of 19.8 W have been achieved at 20°C. A stable operation over 2000 h has also been confirmed under the condition of an output power of 15 W at 25°C. Far-field patterns suggest that the laser diode is suitable for coupling into an optical fiber with 105-μm core diameter and 0.15 numerical aperture.

On the importance of non-thermal far-field blooming in broad-area high-power laser diodes

High-power broad-area laser diodes often suffer from a widening of the slow-axis far-field with increasing current (lateral far-field blooming). This effect is commonly attributed to self-heating. Utilizing self-consistent electro-thermal-optical simulations, we analyze previous experimental investigations of 970 nm broad-area GaAs-based Fabry-Perot lasers and reproduce the blooming mechanism in good agreement with the measurements. The simulations reveal that a substantial part of the far field blooming is not caused by self-heating but by increasing carrier and gain non-uniformity in the quantum wells.

Inverse Thermal Lens Effects on the Far-Field Blooming of Broad Area Laser Diodes

High-power broad-area laser diodes often suffer from a widening of the lateral far-field with increasing current (slow-axis far-field blooming). This effect mainly originates in self-heating that generates a lateral thermal lens in the active region. Pedestal heat sinking was recently shown to mitigate the far-field blooming. In this letter, utilizing self-consistent electro-thermal optical simulations, we analyze such a far-field improvement and link it to the formation of an inverse thermal lens. At high injection current, the inverse thermal lens is shown to weaken high-order lateral modes and to even narrow the slow-axis far-field.

How does external feedback cause AlGaAs-based diode lasers to degrade?

The effect of external feedback on the degradation of 808 nm emitting AlGaAs-based high-power broad-area diode lasers is studied. For this purpose, early stages of gradual degradation are induced by accelerated aging at high power levels. While the quantum well that actually experiences the highest total optical load remains unaffected, severe impact by point defects is observed on the cladding layers and the waveguide. Extended defects such as dislocations, however, are not observed in such early stages of degradation, which are accompanied by gradual power loss of a few percent only.

Self-consistent analysis of thermal far-field blooming of broad-area laser diodes

High-power broad-area laser diodes often suffer from a widening of the lateral (slow axis) far-field with increasing current, called thermal blooming, which is mainly caused by the non-uniform self-heating of the laser and has been studied for several decades. This paper presents the first self-consistent electro-thermal-optical simulation and analysis of such thermal blooming. Using a real InGaAs/GaAs broad-area laser as an example, a 900A/cm2 higher current density is shown to lead to only 0.5K stronger lateral temperature drop inside the ridge waveguide but to a one degree wider slow axis far field. Small non-thermal blooming is also observed.

Coherent beam combining of high power broad-area laser diode array with a closed-V-shape external Talbot cavity

We have coherently combined a high-power broad-area laser diode array by using a feedback loop closed off-axis external Talbot cavity. The off-axis feedback from two gratings provides transverse-mode control of broad-area lasers. The Talbot configuration of the external cavity implements diffractive coupling among laser diodes. Feedback from two gratings increases external cavity quality factor and spectrum selection capability. As a result, spatial coherence was improved and spectral linewidth was narrowed down. The high visibility of the far-field profile indicates that high spatial coherence was achieved. We also observed symmetric far-field profiles indicating that laser array was phase locked to in-phase and out-of-phase super-modes, respectively. Transition between these super-modes was observed by tuning one grating's tilted angle.

Imaging Catastrophic Optical Mirror Damage in High-Power Diode Lasers

We image catastrophic optical mirror damage (COMD) in red- and infrared-emitting high-power broad-area diode lasers by combining highly COMD-selective thermography, near-field imaging, scanning electron microscopy, and cathodoluminescence. All techniques exhibit strong correlations in terms of COMD location and strength and allow for an unambiguous decision about COMD occurrence. In particular, temperatures en route to and during COMD are measured, and the concept of a critical facet temperature that induces thermal runaway is supported.

5.6-W Broad-Area Lasers With a Vertical Far-Field Angle of 31$^{\circ}$ Emitting at 670 nm

Highly efficient 670-nm high-power broad-area laser diodes with a single InGaP quantum-well embedded in AlGaInP waveguide layers and n-AlInP and p-AlGaAs cladding layers are presented. The developed vertical layer structure leads to a vertical far-field angle of 31deg. At 15degC, 100 mu-m-wide broad-area lasers reach an output power of 5.6 W limited by thermal rollover. The conversion efficiency was 41% at 1.5 W. A 7600-h reliable operation at 1.5 W and a mean time to failure of about 37550 h will be reported.

Electrothermal Analysis of CW High-Power Broad-Area Laser Diodes: A Comparison Between 2-D and 3-D Modeling

We present a self-consistent electrothermal analysis for continuous wave (CW) high-power broad-area laser diodes using the finite-element method (FEM). A comparison between conventional 2D (lateral and longitudinal) and 3D models indicates that a 2D model not only overestimates the junction temperature but can also be inappropriate for evaluating the temperature profile in the active layer of large-aperture devices. Current spreading and facet heating make the active layer temperature distribution inherently 3D. Thus, a 2D model deviates even further from the 3D result as the injection current increases. We report here for the first time a full steady-state 3D electrothermal model including facet heating, and point out the importance of such an analysis for investigating the lateral mode behavior in high-power broad-area laser diodes.

3-W Broad Area Lasers and 12-W Bars With Conversion Efficiencies up to 40% at 650 nm

Highly efficient 650-nm high-power broad-area laser diodes and laser bars with single quantum well InGaP quantum wells embedded in AlGaInP waveguide layers, and n-AlInP and p-AlGaAs cladding layers are presented. Broad-area lasers that are 100-mum wide reach output powers of 3 W and conversion efficiencies of about 40% at 15degC. For 5-mm wide laser bars (filling factor of 20%), maximum output powers of 12 W in continuous-wave (CW) operation and 55 W in quasi-CW mode were obtained. Reliable operation of 5000 h at 800 mW for single emitters and at 5 W for laser bars will be reported.

Thermal properties and degradation behavior of red-emitting high-power diode lasers

The thermal properties and the degradation behavior of high-power broad-area diode lasers emitting at 650nm are analyzed. Imaging thermography is applied to assess the bulk temperature while the facet temperature is measured by micro-Raman spectroscopy. Although no visible facet alteration is observed, power degradation is found to be accompanied by increased temperatures at the facets. The immediate vicinity of them also turns out to be the starting point for the creation of defect networks within the quantum well seen in cathodoluminescence images. The observed behavior is compared to that known for near-infrared emitting devices.

Improvement of spatial and temporal coherence of a broad area laser diode using an external-cavity design with double grating feedback

We demonstrate a novel technique for narrow bandwidth and highly improved lateral mode operation of a high-power broad area laser diode. The system uses simultaneous feedback from the first diffracted order and the zeroth reflected order of a diffraction grating. The two feedback paths lead to simultaneously improvement of the spectral and spatial properties of the laser diode. The laser system operates in the well-known asymmetric double-lobed far field pattern with the larger lobe being extracted as the output. The bandwidth of the output beam is measured to 0.07 nm, which corresponds to an improvement of a factor of 17 compared to the bandwidth of the freely running laser. The output from the system contains 54% of the energy reaching the grating, or 75% of the power reflected into the zeroth order. The improvements in both the spatial and temporal coherence opens the possibility of using this laser system in applications such as frequency doubling and pumping of optical parametric oscillators.

Simultaneous injection locking of couples of high-power broad-area lasers driven by a common current source.

We experimentally demonstrate the simultaneous injection locking of pairs of high-power broad-area laser diodes in a 19-laser array driven by a common current source. Each pair is injection locked by use of a single-mode low-power semiconductor laser. The frequency and phase locking are verified bythe optical spectrum and the interference pattern between the injection-locked lasers. The influence of frequency detuning on the (simultaneous) injection behavior has been experimentally clarified. We validate a necessary condition for the injection locking of a broad-area laser array.

High power broad-area diode laser at 794 nm injected by an external cavity laser

The output of a single-mode laser diode mounted in an external cavity has been injected in a 500 mW broad-area diode laser. A maximum of 120 mW single-mode emission at 794 nm has been obtained for an injected power of 2.6 mW. Almost 90% of the total output could be produced in a diffraction limited single spatial Gaussian lobe. The spectral properties of the master laser have been transferred to the slave to within 1.5 kHz. We present a study of the injection efficiency under variation of various physical and geometrical parameters while the master laser is held at a fixed frequency to within 10 MHz. In particular, in the course of variation of slave temperature and injection angle we observe strong resonance behaviour.

Narrow bandwidth operation of high-power broad-area diode laser using cascaded phase-conjugate injection locking

A broad-area laser is injection-locked by another broad-area laser that is also injection-locked by a single-mode diode laser. Two double-phase conjugate mirrors of photorefractive BaTaO3 are used to couple the master laser beams to the first slave laser, and the first slave laser output to the second slave laser. One of the double-phase conjugate mirrors is built up with the beams from two broad-area lasers. Two slave lasers are oscillating in single longitudinal mode at 808.5 nm and the spectral width is the same as that of the master laser. Final single-mode output power from the second slave broad-area laser is 840 mW, which is limited by the power of the injection beam. This work verifies the possibility of the multi-stage cascaded injection locking of high-power diode lasers with phase-conjugate injection.

High-power broad-area diode lasers for laser cooling

High-power broad-area diode lasers for laser cooling