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.
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.
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.
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.
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.
Lasing is reported for ridge-waveguide devices processed from a 40-stage InP-based quantum cascade laser structure grown on a 6-inch Ge-coated silicon substrate with a metamorphic buffer. The structure used in the proof-of-concept experiment had a typical design, including an Al0.78In0.22As/In0.73Ga0.27As strain-balanced composition, with high strain both in quantum wells and barriers relative to InP, and an all-InP waveguide with a total thickness of 8 µm. Devices of size 3 mm x 40 µm, with a high-reflection back facet coating, emitted at 4.35 µm and had a threshold current of approximately 2.2 A at 78 K. Lasing was observed up to 170 K. A preliminary surface morphology analysis suggests that laser performance for QCLs-on-Si devices can be significantly improved by reducing strain for the active region layers relative to InP bulk waveguide layers surrounding the laser core. Additional experimental data on material quality, including threading dislocation density, will be presented in the talk and compared for the same design grown on three different substrates to demonstrate how material quality impacts laser performance. The three substrates to be studied are the following: a native InP substrate, a GaAs substrate (~ 4 % lattice mismatch with InP), and a Si substrate (~ 8 % lattice-mismatch with InP).
Significant increase in continuous wave optical power from a single quantum cascade laser (QCL), beyond its current record of 5W, will likely require power scaling with active region lateral dimensions. Active region overheating presents a major technical problem for such broad area devices. Laser thermal resistance can be reduced and laser self-heating can be suppressed by significantly reducing active region thickness, i.e. by reducing number of active region stages and by reducing thickness of each stage in the cascade. The main challenge for quantum cascade lasers with a “thin” active region is to ensure that optical power emitted per active region unit area stays high despite the reduction in active region thickness, a condition critical for the power scaling. Experimental data demonstrating a multi-watt continuous wave operation for broad area QCLs, as well as various aspects of bandgap engineering, waveguide design, and thermal design pertinent to the broad area configuration, are discussed in this manuscript. The critical differences in broad-area laser design between mid-wave and long-wave QCLs is highlighted. Finally, semi-empirical model projections showing that the goal of reaching 20W from a single emitter is realistic is presented.
We report the experimental results of a 40-stage InP-based quantum cascade laser (QCL) structure grown on a 6-inch GaAs substrate with metamorphic buffer (M-buffer). The laser structure’s strain-balanced active region was composed of Al0.78In0.22As/In0.73Ga0.27As and an all-InP, 8 μm-thick waveguide. The wafer was processed into ridge-waveguide chips (3mm x 30 μm devices) with lateral current injection scheme. Devices with high reflection coating delivered power in excess of 200 mW of total peak power at 78K, with lasing observed up to 230K. Preliminary reliability testing at maximum power showed no sign of performance degradation after 200 minutes of runtime. Measured characteristic temperatures of T0 ≈ 460 K and T1 ≈ 210 K describes the temperature dependence for threshold current and slope efficiency, respectively, in the range from 78K to 230K. Partial high reflection coating was used on the front facet to extend the lasing range up to 303K.
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.
Conceived in ~1971 [1,2] and first experimentally demonstrated in 1994 [3], quantum cascade lasers have become the most importance sources of infrared laser radiation in the 3.5 μm to >12 μm spectral region. With needs already identified at even longer wavelengths, QCLs are being pursued vigorously as sources of terahertz laser radiation. The mid wave infrared (MWIR) and the long wave infrared (LWIR) regions are, however, significantly more important because of a number defense, homeland security and commercial applications critically require the capabilities of QCLs. These capabilities include size, weight and power considerations (SWaP), which make QCLs unique among all other potential sources of laser radiation in this region including optical parametric oscillators, optically pumped semiconductors and optically pumped solids. In this presentation, I will summarize some of the key advances and status of QCL technology as well as defense and civilian applications of the MWIR and LWIR quantum cascade lasers.
A strain-balanced, AlInAs/InGaAs/InP quantum cascade laser structure, designed for light emission near 9μm, was grown by molecular beam epitaxy. Laser devices were processed in buried heterostructure geometry. Maximum pulsed and continuous wave room temperature optical power of 4.5 and 2W and wallplug efficiency of 16% and 10%, respectively, were demonstrated for a 3mm by 10μm laser mounted epi-side down on an AlN/SiC composite submount. Pulsed laser characteristics were shown to be self-consistently described by a simple model based on rate equations using measured 70% injection efficiency for the upper laser level.
QCLs represent an important advance in MWIR and LWIR laser technology. With the demonstration of
CW/RT QCLs, large number applications for QCLs have opened up, some of which represent replacement of
currently used laser sources such as OPOs and OPSELs, and others being new uses which were not
possible using earlier MWIR/LWIR laser sources, namely OPOs, OPSELs and CO2 lasers.
Pranalytica has made significant advances in CW/RT power and WPE of QCLs and through its invention of a
new QCL structure design, the non-resonant extraction, has demonstrated single emitter power of >4.7 W
and WPE of >17% in the 4.4μm-5.0μm region. Pranalytica has also been commercially supplying the highest
power MWIR QCLs with high WPEs. The NRE design concept now has been extended to the shorter
wavelengths (3.8μm-4.2μm) with multiwatt power outputs and to longer wavelengths (7μm-10μm) with >1 W
output powers. The high WPE of the QCLs permits RT operation of QCLs without using TECs in quasi-CW
mode where multiwatt average powers are obtained even in ambient T>70°C. The QCW uncooled operation
is particularly attractive for handheld, battery-operated applications where electrical power is limited.
This paper describes the advances in QCL technology and applications of the high power MWIR and LWIR
QCLs for defense applications, including protection of aircraft from MANPADS, standoff detection of IEDs, insitu
detection of CWAs and explosives, infrared IFF beacons and target designators. We see that the SWaP
advantages of QCLs are game changers.
KEYWORDS: Quantum cascade lasers, Missiles, High power lasers, Reliability, Heatsinks, Defense and security, Laser systems engineering, Mid-IR, Laser applications, Lasers
Quantum cascade lasers are finding rapid acceptance in many defense and security applications. Our new multispectral
laser platform providing watt-level outputs near 2.0 μm, 4.0 μm and 4.6 μm in continuous wave regime at room
temperature. Individual lasers are spectrally beam combined into a single output beam with excellent quality. Our
rugged, compact (11 × 10 × 6.5 inches), and highly reliable, air-cooled multispectral laser platform is already finding
acceptance at system level. Our uncooled devices produce > 2W at 4.6 μm and >1.5W at 4.0 μm at room temperature,
and maintain watt-level output at 67°C with real wallplug efficiencies >10%. Finally, all of our QCLs undergo 100-hour
pre-delivery burn-in and pass shock, vibration, and temperature testing according to MIL-STD-810G.
A strain-balanced, AlInAs/InGaAs/InP quantum cascade laser structure, designed for light emission at 4.0 μm using nonresonant
extraction design approach, was grown by molecular beam epitaxy. Laser devices were processed in buried
heterostructure geometry. An air-cooled laser system incorporating a 10 mm by 11.5 μm laser with antireflection coated
front facet and high reflection coated back facet delivered over 2 W of single-ended optical power in a collimated beam.
Maximum continuous wave room temperature wallplug efficiency of 5.0% was demonstrated for a high reflection coated
3.65 mm by 8.7 μm laser mounted on an aluminum nitride submount. Lasers processed from a 3.5μm structure with a
similar design delivered 50 mW in CW mode and 300 mW of average power in high duty cycle mode at 265K. The low
performance of the 3.5 μm structure is attributed to the fact that the bottom of indirect profile is located below the upper
laser level for this design.
Because of their compact size, reliability, tunability, and convenience of direct electrical pumping, quantum cascade lasers have found a number of important civilian and defense applications in the midwave infrared and long-wave-infrared spectral range. Most of these applications would benefit from higher laser optical power and higher wall-plug efficiency. We describe some of the most important features of high-efficiency quantum cascade laser design and realization of high-power quantum cascade laser systems. Specifically, optimization of the active region and waveguide, thermal management on the chip level, and impact of the laser facet coating on laser efficiency and scaling of optical power with cavity length are discussed. Also, we present experimental results demonstrating multiwatt operation with reliability of at least several thousands of hours on a system level.
We present our latest results on the development of high power, high efficiency room temperature quantum
cascade lasers. Strain-balanced, InP-based quantum cascade structures, designed for light emission at 4.6 μm using a
new non-resonant extraction design approach, were grown by molecular beam epitaxy and processed as buried
heterostructure lasers. Maximum single-ended continuous-wave optical power of 3 W was obtained at 293 K for
devices with stripe dimensions of 5 mm by 11.6 μm mounted on diamond submounts. Corresponding maximum
wallplug efficiency and threshold current density were measured to be 12.7% and 0.86 kA/cm2. 7 mm-long, 8.5 μm-wide
devices mounted on aluminum nitride submounts with optimized reflectivity coatings on the output facet emitted
2.9 W under the same conditions and 1.2 W in uncooled pulsed operation. Leveraging this research, we developed
fully packaged, air-cooled, table-top turn-key laser systems delivering in excess of 2 W of collimated continuous-wave
radiation. The high performance and level of device integration make these quantum cascade lasers the primary choice
for various defense and security applications, including directional infrared countermeasures, mid-wave infrared
illuminators and free space optical communications.
Strain-balanced, InP-based quantum cascade laser structures, designed for light emission at 4.6 μm
using a new non-resonant extraction design approach, were grown by molecular beam epitaxy.
Removal of the restrictive two-phonon resonance condition, currently used in most structure designs,
allows simultaneous optimization of several structure parameters influencing laser performance.
Following the growth, the structure was processed to yield buried heterostructure lasers. Maximum
single-ended continuous-wave optical power of 3 W was obtained at 293 K for devices with stripe
dimensions of 5 mm by 11.6 μ;m. Corresponding maximum wallplug efficiency and threshold current
density were measured to be 12.7% and 0.86 kA/cm2. Fully packaged, air-cooled lasers with the same
active region/waveguide design and increased laser core doping delivered approximately 2.2 W in
collimated beam. The high performance and level of device integration make these quantum cascade
lasers the primary choice for various defense applications, including directional infrared
countermeasures, infrared beacons/target designators and free space optical communications.
Early detection of explosive substances is the first and most difficult step in defeating explosive
devices. Many currently available methods suffer from fundamental failure modes limiting their realworld
suitability. Infrared spectroscopy is ideal for reliable identification of explosives since it probes
the chemical composition of molecules. Quantum cascade lasers rapidly became the light source of
choice of IR spectroscopy due to their wavelength agility, relatively high output power, and small size
and weight. Our compact, rapid, and rugged multi-explosives sensor based on external grating cavity
QCLs simultaneously detects TNT, TATP, and acetone while being immune to ammonium nitrate
interference. The instrument features low false alarm rate, and low probability of false negatives.
Receiver operation characteristics curves are presented.
KEYWORDS: Quantum cascade lasers, Laser systems engineering, Defense and security, Packaging, Fiber optic illuminators, Reliability, High power lasers, Quantum efficiency, Collimation, Defense systems
Leveraging Pranalytica's fundamental research into high power and high wallplug efficiency QCL devices and high
performance/reliability QCL packaging technologies, we developed several models of turn-key QCL systems for
security-related applications. Our tabletop high power system produces, at room temperature, more than 2W of
nominally collimated continuous wave radiation at 4.6 μm. Our flashlight-size portable illuminator at 4.6 μm produces
over 100 mW average power portable illuminator at 9.6 μm produces more than 20 mW, both with runtime of ~10 hrs.
These systems are opening the window of QCL acceptance into
real-world security and defense applications. At chip
level, we have demonstrated 3W of CW power at room temperature from a single, high reflectivity coated chip.
We have analyzed light absorption in a quantum cascade laser structure under forward and reverse bias. Strong
absorption modulation at the laser frequency is predicted and observed experimentally for the voltage variation
within the high differential resistance voltage range. We propose to use this mechanism for monolithically
integrated intracavity modulation of quantum cascade lasers, promising suppressed thermal chirp and fast modulation
capability. In addition, the described method allows for extraction of the intersubband absorption from
the total waveguide losses.
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