We demonstrate phase-locked, high power, tree-array quantum cascade lasers based on ridge waveguides with near diffraction-limited beam quality from the single-emitter side at a wavelength of 8.6μm. Tree-arrays based on ridge waveguides are promising for power scaling of QCLs, and are simpler to fabricate than buried heterostructure waveguides. Understanding the fabrication sensitivity of ridge waveguide tree-array QCLs is important for assessing their viability for mass fabrication. An analysis of fabrication tolerance and guidelines for the design of efficient MMI couplers is presented.
The beam quality of ridge-waveguide quantum cascade laser arrays with broad-area emitters and Multi-Mode Interference (MMI) couplers is investigated both experimentally and numerically. Previous demonstrations of MMI QCL arrays had narrow ridge waveguides to ensure fundamental mode operation and phase locking between elements of the array. In the interest of scaling optical power with lateral waveguide dimensions, we demonstrate broad area tree-arrays with MMI couplers at a wavelength of 4.65μm and ridge widths between 13 μm and 17μm. The emitted beams from the stem’s side are characterized with M2 measurements. We show that the MMI coupled arrays generally have significantly improved beam quality compared to Fabry Perot resonators with the same dimensions. Optimized tree-array devices will be the cornerstone of the next generation high power infrared systems.
The low cross-plane thermal conductivity of Quantum Cascade Lasers (QCLs) is a significant limitation in their Continuous-Wave (CW) performance. Structural parameters such as individual layer thicknesses and interface density vary for QCLs with different target emission wavelengths, and these design parameters are expected to influence the cross-plane thermal conductivity. Though previous works have used theoretical models and experimental data to quantify thermal conductivity, the correlation between target wavelength and thermal conductivity has yet to be reported for QCLs. In this work, we observe a general trend across a group of QCLs emitting from 3.7 to 8.7 𝜇m: as the QCL design changes to reduce wavelength, the thermal conductivity decreases as well. Numerically, we measured an approximate 70% reduction in thermal conductivity, from 1.5 W/(m·K) for the 8.7 m device, to 0.9 W/(m·K) for the 3.7 𝜇m device. Analysis of these structures with the Diffuse Mismatch Model (DMM) for Thermal Boundary Resistance (TBR) shows that the largest contribution of this effect is the impact of superlattice interface density on the thermal conductivity. The observed changes in conductivity result in significant changes in projected CW optical power and should be considered in laser design.
Due to an unprecedented combination of high power, high efficiency, and small size, Quantum Cascade Lasers (QCLs) finding numerous applications in various mid-wave and long-wave infrared fields. The control of material composition, thickness, and doping level for each layer in the QCL superlattice offers a unique flexibility in optimizing laser characteristics to specific applications. Band gap engineering (laser core design) will be discussed in this talk in the context of spectroscopic applications, including heterogeneous laser core design that allows for either wavelength tuning in a broad spectral region around a single central wavelength or operation on multiple isolated spectral lines with significant spectral separation. The design and fabrication of QCLs with a low-cost top-metal Distributed Bragg Reflector for achieving narrow-spectrum emission will also be presented. Finally, our latest results on monolithic beam combining of multiple DBR QCLs using multi-mode interference and Y-junction couplers for increasing laser tuning range and/or increasing peak optical power will be presented. Employment of the high-power DBR QCL arrays in specific infrared applications will be discussed at the end of the talk.
Multi-watt continuous wave operation has been demonstrated for broad-area, Fabry-Perot Quantum Cascade Lasers (QCLs). In addition to high optical power, increase in operational range for infrared countermeasures requires low atmospheric propagation losses for emitted radiation. Single-line operation tailored to low atmospheric losses can be achieved for QCLs utilizing the distributed feedback grating etched into the laser waveguide along full cavity length. An alternative solution explored here is to utilize the grating as an outcoupler, so-called distributed Bragg reflector (DBR) configuration. Since output facet reflectivity of only several percent is needed for high-performance QCLs, the DBR section can be made very short, on the order of several hundred microns, leaving the rest of the (optimized) laser waveguide unchanged. Top-metal DBR configuration with grating etched into the top cladding layers of the QCL structure offers the advantage of a low fabrication cost. Therefore, broad-area DBR QCLs with a top-metal grating promise a significant improvement in spectral brightness and at the same time a low fabrication cost. The main design principles for these devices will be discussed in this talk along with preliminary experimental data.
Experimental and model results for high power broad area quantum cascade lasers are presented. Continuous wave power scaling from 1.62 W to 2.34 W has been experimentally demonstrated for 3.15 mm-long, high reflection-coated 5.6 μm quantum cascade lasers with 15 stage active region for active region width increased from 10 μm to 20 μm. A semi-empirical model for broad area devices operating in continuous wave 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 sub-linearity of pulsed power vs 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 x 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 300Å, pulsed roll-over current density of 6 kA/cm2 , and InGaAs waveguide layers; 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.
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 Å, 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.
5.6μm quantum cascade lasers based on Al0.78In0.22As/In0.69Ga0.31As active region composition with measured pulsed room temperature wall plug efficiency of 28.3% are reported. Injection efficiency for the upper laser level of 75% was measured by testing devices with variable cavity length. Threshold current density of 1.7kA/cm2 and slope efficiency of 4.9W/A were measured for uncoated 3.15mm x 9µm lasers. Threshold current density and slope efficiency dependence on temperature in the range from 288K to 348K can be described by characteristic temperatures T0~140K and T1~710K, respectively. Pulsed slope efficiency, threshold current density, and wallplug efficiency for a 2.1mm x 10.4µm 15-stage device with the same design and a high reflection-coated back facet were measured to be 1.45W/A, 3.1kA/cm2 , and 18%, respectively. Continuous wave values for the same parameters were measured to be 1.42W/A, 3.7kA/cm2 , and 12%. Continuous wave optical power levels exceeding 0.5W per millimeter of cavity length was demonstrated. When combined with the 40-stage device data, the inverse slope efficiency dependence on cavity length for 15-stage data allowed for separate evaluation of the losses originating from the active region and from the cladding layers of the laser structure. Specifically, the active region losses for the studied design were found to be 0.77cm-1, while cladding region losses – 0.33cm-1. The data demonstrate that active region losses in mid wave infrared quantum cascade lasers largely define total waveguide losses and that their reduction should be one of the main priorities in the quantum cascade laser design.
5.6 μm quantum cascade lasers based on Al 0.78 In 0.22 As/In 0.69 Ga 0.31 As active region composition with measured pulsed room temperature wall plug efficiency of 28.3% are reported. Injection efficiency for the upper laser level of 75% was measured for the new design by testing devices with variable cavity length. Threshold current density of 1.7kA/cm2 and slope efficiency of 4.9W/A were measured for uncoated 3.15mm × 9μm lasers. Threshold current density and slope efficiency dependence on temperature in the range from 288K to 348K for the new structure can be described by characteristic temperatures T0 ~ 140K and T1 ~710K, respectively. Experimental data for inverse slope efficiency dependence on cavity length for 15-stage quantum cascade lasers with the same design are also presented. When combined with the 40-stage device data, the new data allowed for separate evaluation of the losses originating from the active region and from the cladding layers of the laser structure. Specifically, the active region losses for the studied design were found to be 0.77 cm-1, while cladding region losses - 0.33 cm-1. The data demonstrate that active region losses in mid wave infrared quantum cascade lasers largely define total waveguide losses and that their reduction should be one of the main priorities in the quantum cascade laser design.
We demonstrate power scaling of an Nd:YAG picosecond master oscillator power amplifier system to over 200 W. The ‘z-slab’ amplifier design is a power scalable, edge-pumped zigzag slab amplifier architecture, and it is demonstrated here in two alternative multi-stage implementations at 1064 nm using a picosecond seed laser. In a simple design, an average power of 225 W was generated with up to 450 μJ pulse energy at 11 ps pulse duration. In a compact multi-pass design, 150 W was generated with M2 < 1.75.
The material in which a volume Bragg grating is made will always have some absorption at the grating's design wavelength. Thus, when exposed to a high power laser beam the grating will absorb some power, be heated such that a temperature gradient is formed and, consequently, become distorted. We developed an accurate model to calculate the reflection of a high power laser beam by a volume Bragg grating that experiences such distortion. We used the beam propagation method (BPM) to calculate the laser beam propagation in the grating numerically, and the BPM calculations are iterated to account for the counter propagation of the laser beam in the volume Bragg grating. We devised a new method to assure convergence in the iteration of the BPM calculations when the grating diffraction strength is very large. We also established a new formulation of the wave equation to include the grating period distortion in the BPM formulation. The surface distortion and temperature induced background index change are also included in the model. This model has been validated to be correct and very accurate. We applied it to calculate the reflection of a high power laser beam by a distorted volume Bragg grating which has large diffraction strength. Our calculation shows that a small amount of grating structure distortion could introduce significant changes of both the phase and intensity patterns of the reflected laser beam. Understanding such changes is critical to the application of volume Bragg grating to high power laser systems.
We evaluate the performance potential of a diode pumped Nd: YAG rod laser by finding the absorbed pump distribution using ASAP, pump induced thermal lensing, gain medium surface distortion and stresses using FEMLAB and depolarization losses using MATLAB. Beam propagation in the optically distorted Nd:YAG rod and the free space part of the cavity, and the output laser beam were determined with a computational scheme we developed which employs the beam propagation method combined with sparse matrix technology. We propose a special cavity design that can select the spatial eigen mode shape of the laser and simultaneously compensate for pump induced thermal lensing, gain medium surface distortion and birefringence. The converged solutions calculated this special cavity design give both high extraction efficiency and good output beam quality. Sensitivity of the output beam to mirror tilt, thermal induced mirror distortion, and errors in the cavity length or the optical distortions in the rod were also calculated.
We developed a computer model for simulating real solid state laser systems, by solving the paraxial wave Eq.s in the multi-physics modeling software FEMLAB. The reflection of the laser on the curved cavity mirror is calculated by an analytical method. This model was verified to be able to give very accurate results, by applying it to empty stable and unstable resonators, and a face pumped Yb:YAG disk stable resonator laser.
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