Broad Area Quantum Cascade Lasers Research Articles

Fundamental mode emission in tapered quantum cascade lasers for high brightness

Currently, the highest power quantum cascade lasers (QCLs) include (i) an efficient active region design, which minimizes the energy fraction transferred into heat, (ii) a relatively large number (40) of cascades, which increases the mode confinement factor, Γ, and (iii) narrow buried-heterostructure waveguides, which efficiently cool the active region through efficient lateral heat flow. This combination allows watt-level continuous wave (cw) emission powers at room temperature. Moreover, narrow waveguides favor the fundamental TM00 lateral mode, resulting in a nearly Gaussian beam. To the extent that power efficiency is already largely optimized, power scaling in narrow-stripe QCLs is only possible through increasing the laser length, which is limited by the needs for robust construction and transparent antireflection coatings. Broad-area (BA) QCLs with a small number of cascades can also deliver watt-level cw powers at room temperature, profiting from the enhanced vertical heat flow. Power can be scaled for these lasers also via the laser width and is nearly unlimited. The usual drawback of the BA lasers is poor beam quality. We discuss the use of tapered QCLs and the first demonstration of room temperature cw operation of the BA QCL in fundamental TM00 mode. A stable and reproducible transverse optical mode is achieved using a double-taper waveguide geometry. The laser emits at 4.6 μm and has a demonstrated room temperature cw power of 100 mW.

Figure-of-merit analysis of distributed sidewall loss method to suppress high-order modes in broad area quantum cascade lasers

We have modeled broad-area ridge-waveguide quantum cascade lasers (QCLs) using COMSOL and generated a rank-ordering of high-order transverse lateral modes as a function of cavity width on the basis of a figure-of-merit. In particular, we corroborate experimental results previously obtained using the distributed sidewall loss (DSL) method to suppress high-order transverse lateral modes and extract fundamental mode in broad-area QCLs. COMSOL modeling shows that waveguide losses preferentially incurred by the high-order modes, due to the direct sidewall contact with metal as prescribed by the DSL method, can be used to obtain single-lobed emission with enhanced brightness in broad-area QCLs.

Chirped coupled ridge waveguide quantum cascade laser arrays with stable single-lobe far-field patterns

Power scaling in a broad area quantum cascade laser (QCL) tends to deteriorate beam quality with the emission of a multiple-lobe far-field pattern. In this paper, we demonstrate a coupled ridge waveguide QCL array consisting of five elements with chirped geometry. In-phase mode operation is secured by managing supermode loss with properly designed geometries of ridges. A single-lobe lateral far-field with a near diffraction limited beam pattern was obtained in the whole current dynamic range. The devices were fabricated with the wet and dry etching method. The regrowth technique of the InP:Fe insulation layer and InP:Si waveguide layer was employed. Such a structure has the potential to optimize the beam quality of the recently reported high-power broad-area QCL with a reduced cascade number.

Tapered Quantum Cascade Laser Arrays Integrated with Talbot Cavities

Power scaling in broad area quantum cascade laser (QCL) usually leads to the deterioration of the beam quality with an emission of multiple lobes far-field pattern. In this letter, we demonstrate a tapered QCL array integrated with Talbot cavity at one side of the array. Fundamental supermode operation is achieved in the arrays with taper straight-end connected to the Talbot cavity. Lateral far-field of the fundamental supermode shows a near diffraction limited beam divergence of 2.7°. The output power of a five-element array is about three times as high as a single-ridge laser with an emission wavelength of around 4.8 μm. However, arrays with the taper-end connected to the Talbot cavity always show a high-order supermode operation whatever Talbot cavity length is.

Experimental investigation of broad area quantum cascade lasers under external feedback.

Mode creation and temporal response of broad-area quantum cascade lasers (BA-QCL) placed within an external feedback cavity are described in this publication. The critical feedback parameter becomes the mirror angle relative to the BA-QCL facet. With judicious angle choices, a plethora of curious modes can be created, each with their particular threshold and slope efficiency. These range from a nearly single far-field intensity peak to highly multimode emission similar to their diode counterparts. Dynamics are strongly dominated by transverse mode competition ranging for less than 20MHz to greater than 100MHz. When the mirror is parallel to the facet, higher frequency external cavity oscillations become undamped.

Distributed loss method to suppress high order modes in broad area quantum cascade lasers

We describe a method where the standard fabrication of broad area quantum cascade lasers is modified to provide a controlled amount of direct contact of device sidewalls with metal. We demonstrate that this provides sufficient levels of distributed losses to suppress the high order transverse modes in favor of the fundamental or near-fundamental transverse mode operation. We observe that the quantum cascade laser power and slope efficiency are degraded by a small amount, resulting in a large increase in brightness to accompany the power scaling.

Power scaling and experimentally fitted model for broad area quantum cascade lasers in continuous wave operation

Experimental and model results for 15-stage broad area quantum cascade lasers (QCLs) are presented. Continuous wave (CW) power scaling from 1.62 to 2.34 W has been experimentally demonstrated for 3.15-mm long, high reflection-coated QCLs for an active region width increased from 10 to 20 μm. A semiempirical model for broad area devices operating in CW mode is presented. The model uses measured pulsed transparency current, injection efficiency, waveguide losses, and differential gain as input parameters. It also takes into account active region self-heating and sublinearity of pulsed power versus current laser characteristic. The model predicts that an 11% improvement in maximum CW power and increased wall-plug efficiency can be achieved from 3.15 mm×25 μm devices with 21 stages of the same design, but half doping in the active region. For a 16-stage design with a reduced stage thickness of 300 A, pulsed rollover current density of 6 kA/cm2, and InGaAs waveguide layers, an optical power increase of 41% is projected. Finally, the model projects that power level can be increased to ∼4.5 W from 3.15 mm×31 μm devices with the baseline configuration with T0 increased from 140 K for the present design to 250 K.

Broad area quantum cascade lasers operating in pulsed mode above 100 °C λ ∼4.7 μm**Project supported by the National Basic Research Program of China (No. 2013CB632801), the National Key Research and Development Program (No. 2016YFB0402303), the National Natural Science Foundation of China (Nos. 61435014, 61627822, 61574136, 61306058, 61404131), the Key Projects of Chinese Academy of Sciences (No. ZDRW-XH-20164), and the

We demonstrate a broad area (400 μm) high power quantum cascade laser (QCL). A total peak power of 62 W operating at room temperature is achieved at λ ∼4.7 μm. The temperature dependence of the peak power characteristic is given in the experiment, and also the temperature of the active zone is simulated by a finite-element-method (FEM). We find that the interface roughness of the active core has a great effect on the temperature of the active zone and can be enormously improved using the solid source molecular beam epitaxy (MBE) growth system.

Beam shaping in high-power broad-area quantum cascade lasers using optical feedback

Broad-area quantum cascade lasers with high output powers are highly desirable sources for various applications including infrared countermeasures. However, such structures suffer from strongly deteriorated beam quality due to multimode behavior, diffraction of light and self-focusing. Quantum cascade lasers presenting high performances in terms of power and heat-load dissipation are reported and their response to a nonlinear control based on optical feedback is studied. Applying optical feedback enables to efficiently tailor its near-field beam profile. The different cavity modes are sequentially excited by shifting the feedback mirror angle. Further control of the near-field profile is demonstrated using spatial filtering. The impact of an inhomogeneous gain as well as the influence of the cavity width are investigated. Compared to existing technologies, that are complex and costly, beam shaping with optical feedback is a more flexible solution to obtain high-quality mid-infrared sources.

Extracting fundamental transverse mode operation in broad area quantum cascade lasers

Power scaling in broad area quantum cascade lasers results in the operation of high order transverse modes with a far-field profile consisting of two lobes propagating at large angles relative to the optical axis. We report a method of suppressing the high order transverse modes that can extract the fundamental mode and provide emission along the optical axis. By generating a lateral constriction in the waveguide in the form of short trenches defined by the focused ion beam milling technique, we report broad area devices in which most of the power is contained in a near diffraction-limited beam that provides high brightness.

Above room temperature continuous wave operation of a broad-area quantum-cascade laser

We describe the design and implementation of a broad-area (w ≈ 30 μm) quantum-cascade laser operating in a continuous wave mode up to heat-sink temperatures beyond +100 °C. The room-temperature emission wavelength is 4.6 μm. The temperature gradient in the active region of such a wide laser stripe is essentially perpendicular to the epitaxial layers and the resulting steady-state active region temperature offset scales approximately with the square of the number of cascades. With only 10 cascades in the active region, the threshold electrical power density in the current quantum-cascade laser in the continuous-wave mode is as low as Vth × Ith = 3.8 V × 0.9 kA/cm2 = 3.4 kW/cm2 at room temperature for 2 mm-long two-side high-reflectivity coated laser stripe. A 4 mm-long one-side high-reflectivity coated laser stripe delivers in continuous-wave mode above 0.6 W at +20 °C and above 1.3 W at −27 °C (cooled with a single-stage Peltier element). A 2 mm-long two-side high-reflectivity coated laser stripe demonstrates continuous-wave lasing up to at least +102 °C (375 K). The thermal conductance, Gth, ranges between 235 W/K cm2 and 140 W/K cm2 for temperatures between −33 °C and +102 °C. This demonstration opens the route for continuous-wave power scaling of quantum-cascade lasers via broad-area laser ridges.

Gain-guided broad area quantum cascade lasers emitting 23.5 W peak power at room temperature.

We report gain-guided broad area quantum cascade lasers at 4.55 μm. The devices were processed in a buried heterostructure configuration with a current injector section much narrower than the active region. They demonstrate 23.5 W peak power at a temperature of 20°C and duty cycle of 1%, while their far field consists of a single symmetric lobe centered on the optical axis. These experimental results are supported well by 2D numerical simulations of electric currents and optical fields in a device cross-section.

High-power, low-lateral divergence broad area quantum cascade lasers with a tilted front facet

We introduce a simple technique to improve the beam quality of broad area quantum cascade lasers. Moderately tilted front facets of the laser provide suppression of higher order lateral waveguide modes. A device with a width of 60 μm and a front facet angle of 17° shows a nearly diffraction limited beam profile. In addition, the peak output power and the slope efficiency of the device are increased since most of the light inside the cavity is emitted through the tilted front facet by an asymmetric light intensity distribution along the cavity.

Angled cavity broad area quantum cascade lasers

Angled cavity broad area quantum cascade lasers (QCLs) are investigated with surface grating-based distributed feedback (DFB) mechanisms. It is found that an angled cavity incorporating a one dimensional DFB with grating lines parallel to the laser facet offers the simplest solution for single mode and diffraction limited emission in the facet normal direction. A room temperature single mode QCL with the highest output power for wavelengths longer than 10 μm is demonstrated. This structure could be applied to a wide range of laser structures for power scaling along with spectral and spatial beam control.

High power broad area quantum cascade lasers

Broad area quantum cascade lasers (QCLs) are studied with ridge widths up to 400 μm, in room temperature pulsed mode operation at an emission wavelength around 4.45 μm. The peak output power scales linearly with the ridge width. A maximum total peak output power of 120 W is obtained from a single 400-μm-wide device with a cavity length of 3 mm. A stable far field emission characteristic is observed with dual lobes at ±38° for all tested devices, which suggests that these broad area QCLs are highly resistant to filamentation.