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.
High peak-power, room-temperature operation is reported for ridge waveguide quantum cascade lasers (QCLs) monolithically integrated onto a silicon substrate. The 55-stage laser structure with an AlInAs/InGaAs core and InP cladding was grown by molecular beam epitaxy directly onto an 8-inch diameter germanium-coated silicon substrate template via a III–V alloy metamorphic buffer. Atomic force microscope imaging demonstrated a good quality surface for the full QCL structure grown on silicon. Fabricated 3mm by 26µm lasers operate at room temperature, deliver more than 3W of peak optical power, and show approximately 3% wall plug efficiency and 4.3 kA/cm2 threshold current density with emission wavelength centered at 11.5µm. The lasers had a high yield with only around 15% max power deviation and no signs of performance degradation were observed over a 10h burn in period at maximum power. Singled-lobed high quality output beam was measured for 3mm by 22 µm devices. Correlation between laser performance and defect density in the laser core for several QCL structures grown on lattice-mismatched substrates will also be discussed in this talk.
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.
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.
Brightness is often listed among the most important laser characteristic for practical applications. It is a function of both output optical power and mode quality. Multi-watt continuous wave (CW) operation has been demonstrated for broad-area Quantum Cascade Lasers (QCLs) emitting at ~4.6µm. Transition of the broad-area configuration to shorter wavelengths is however non-trivial as laser thermal behavior rapidly deteriorates with reduction in emission wavelength below 4.6µm. In this work we discuss the main design principles of high brightness, broad area QCLs emitting at ~4.0µm. Building off a power scaling approach to increasing broad area QCL CW power, a figure of merit is utilized to predict dominant lasing transverse modes for QCLs. A discussion follows on the role of laser core dimensions on mode selection within a waveguide, including design guidelines for maintaining single transverse mode behavior while altering broad area QCL design for increased power.
Quasicontinuous wave operation of midinfrared quantum cascade lasers are shown to have increased average output power with good beam quality. The ability to enhance average power by a significant fraction of CW power motivates the development of a model to estimate and project performance at varying duty cycles based on a previously developed continuous wave power projection model. The model takes into account pulse to pulse changes in temperature profile to project a transient steady-state temperature distribution. This temperature distribution is used to project both peak and average power in agreement with measurements. Preliminary model projections suggest that high average brightness may be achieved using a reduced number of stages and a greater scaling of core width than would be permissible for CW lasing.
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