4.6um QCL arrays with different emitter number are fabricated and their device results are reported; an 8-emitter laser array, epi-down mounted on copper heat sink, outputs ~12 watts peak optical power @ 20us pulse width 4% Duty Cycle, and ~8 Watts with 100us pulse width 20% duty cycle, at 9.6A current, room temperature. A similar laser array with 7 emitters was measured to have a Visibility (V) greater than 0.9 in an external cavity. A 3D thermal model has been established to simulate QCL arrays, including all the epi-layers and packaging elements; suggestions to improve the performances further are proposed.
We report on the experimental demonstration of a high-power (>100 mW) quantum cascade superluminescent (SL) emitter employing a spiral cavity with a passive loop back facet. By replacing the rounded, wet-etched, antireflection coated back facet of a spiral cavity with a passive loop facet, the lasing action was suppressed (i.e., threshold current density increased) due to a decrease in the back facet reflectivity. This type of facet allowed us to achieve a twofold increase in the peak SL power by employing a 16-mm-long spiral cavity.
In this paper we review our results on high power quantum cascade lasers in the mid- and long-wave infrared regions of the spectrum (4-12um). The specifications and characteristics of state-of-the-art QC lasers fabricated by MOCVD technology are illustrated, along with their key application requirements and potential issues for future improvements. Single emitter QC lasers in the Watt-class range are presented and analyzed. In addition, we explore the use of high power QCLs for THz generation at room temperature by non-linear mixing of high power mid-infrared beams in a monolithic intra-cavity design. The THz radiation so obtained is widely tunable by electrical injection. Experimentally, we demonstrate ridge waveguide single mode devices electrically tunable between 3.44 and 4.02 THz.
Quantum cascade (QC) lasers are compact and versatile light sources suitable for a broad range of absorption
spectroscopy based molecular sensing applications. However, for most of such sensing applications, single-mode
operation of QC lasers is a prerequisite. Conventional single-mode QC lasers, e.g., distributed feedback (DFB) [1] or
external cavity QC lasers [2], have much higher cost than multi-mode simple ridge QC lasers, mainly due to their
complicated and demanding device fabrication or time-consuming system integration and alignment processes. In order
to achieve more cost-effective single-mode QC lasers, we demonstrate a novel type of laser cavity design which consists
of an asymmetric Mach-Zehnder (AMZ) interferometer structure monolithically integrated in a conventional Fabry-Perot
(FP) cavity with simple ridge waveguide and as-cleaved facets. Strong wavelength selectivity is introduced by the
properly designed AMZ interferometer whose transmission spectrum comprises equidistantly spaced narrow peaks,
which in turn selects a specific FP mode associated with the entire laser cavity near the optical gain spectrum peak,
effectively facilitating single-mode operation of the laser. Continuously wavelength-tunable single-mode operation of
QC lasers is achieved in pulsed mode from 80 K to room temperature and in continuous-wave (CW) mode with high
side-mode suppression ratio (SMSR) up to ~35 dB. The observed spectral characteristics of the tested lasers are
described with satisfying accuracy by our model developed for such cavity structures. The fabrication process for such
AMZ interferometer type cavities is identical to that for simple ridge lasers, therefore providing a promising solution to
achieving more cost-effective single-mode QC lasers.
Quantum Cascade (QC) lasers are semiconductor devices operating in the mid-infrared and terahertz regions of the electromagnetic spectrum. Since their first demonstration in 1994, they have evolved rapidly into high power devices. However, they also have intrinsic challenges, such as beam steering at high power. Such phenomenon has been observed in QC lasers and attributed to the interaction between the two lowest transverse modes in the laser cavity.
In this project, we have used COMSOL Multiphysics simulations to first investigate how transverse mode propagation can be controlled with cavity spoilers. We have modeled this effect by creating short and lossy lateral constrictions from the top of the laser ridge to perturb the modes distributed more toward the sides of the laser ridge, while leaving the fundamental mode intact. After obtaining optimized dimensions for the constrictions, we have utilized focused ion beam (FIB) milling to etch two small trenches from the top of several laser ridges to create the simulated effect on our devices. We, then, filled them with platinum in an effort to completely suppress the propagation of higher order transverse modes in the cavity. The results obtained show minimal effect on threshold and a Gaussian far-field distribution at various current levels, indicating a complete suppression of the higher order transverse modes.
In the last few years there has been significant progress made in the development of high power and high efficiency
quantum cascade lasers in the wavelength range of 4 to 5 microns, while QC lasers in the second atmospheric window
have been experiencing performance development at a slower pace. Now similar improvements in the QCL design and
growth used for the mid-wave IR (MWIR) can be applied to the long-wave IR (LWIR) with some important differences
and adaptations to the challenges presented by the operation at longer wavelengths. These include, among others, a
smaller optical confinement, larger losses and inter-miniband leakage, stronger sensitivity to background doping, and the
need for thicker waveguides. These factors generally result in the degradation of laser characteristics as the emission
wavelength increases. Here we present three new designs in the wavelength range of 8.9 to 10.6 μm and compare their
performance and design metrics along with two reference designs in the same spectral range. A selective strain design
emitting at 10.3 μm achieved threshold currents and slope efficiencies very close to the reference design emitting at 9.9
μm - thus providing longer wavelength emission with no performance deterioration. From the comparison of the designs
presented here, after taking into account the differences in performance metrics of devices designed to operate at longer
wavelengths, we can point out the contribution to the laser characteristics of the carrier leakage from the upper lasing
state to the upper miniband and to the continuum, and of the coupling strength between injector and upper lasing level.
We find that designs with similar metrics but larger splitting between ground injector and upper lasing level exhibit
superior performance than those with smaller coupling.
Quantum cascade lasers (QCLs) are unipolar devices composed of repeated stack of semiconductor multiple quantum
well heterostructures utilizing intersubband transitions and resonant tunneling. In this paper, 120 fs Mid-IR pulses are
used to investigate the nature of carrier transport through the quantum wells and barriers of a pulse biased, room
temperature operating ultrastrong coupling design QCL. Despite the low average power of Mid-IR pulses, we managed
to efficiently couple these pulses into the QCL waveguide so as to observe distinct phenomena by varying the pump and
probe's power. Biased just below the threshold, we observed a strong gain depletion dip at t=0 which is mainly caused
by the depletion of electrons from the upper lasing state mainly by stimulated emission. Ultrafast gain recovery within
the first 200 fs was observed. This is mainly attributed to phonon scattering and electrons resonantly tunneling through a
much thinner injector barrier, which overcomes the interface-roughness-induced detuning of resonant tunneling.
Electron transport through the injector region contributes to a slower gain recovery lifetime of 2-3 ps.
QC laser active regions with multiple transitions from strongly coupled upper laser states to lower laser states were
designed to achieve broad gain spectra as well as high performance. Two broad gain QC laser designs in the 7-9 μm
wavelength region were demonstrated based on the continuum-to-bound design. The first embodiment of one design at
~8 μm enables external cavity tuning over 190 cm-1 (7.5 - 8.8 μm) in pulsed mode operation at 0°C. We also
demonstrated a QC laser structure based on a continuum-to-continuum active region in the 4-5 μm wavelength region
with a gain spectrum of ~ 430 cm-1. External cavity tuning over 340 cm-1 (4.4 - 5.2 μm) was achieved with this design in
pulsed mode operation at 15°C. In spite of the broad gain spectrum, a low threshold current density (1.6 kA/cm2), large
slope efficiency (4.5 W/A), good temperature performance (T0=160 K), high peak power (up to 5 W) and high wall plug
efficiency (WPE, up to 20%) were achieved for ridge lasers with as cleaved facets, in pulsed mode operation at 295 K,
demonstrating that it is possible to make a wide gain spectrum compatible with high power and efficiency performance.
We discuss the design and performance of quantum cascade laser sources based on intra-cavity second harmonic
generation operating in at wavelengths shorter than 3.7μm. A passive heterostructure tailored for giant optical
nonlinearity is integrated on top of an active region and patterned for quasi-phasematching. We demonstrate operation of
λ≈3.6μm, λ≈3.0μm, and λ≈2.6m devices based on lattice-matched and strain-compensated InGaAs/AlInAs/InP
materials. Threshold current densities of typical devices with nonlinear sections are only 10-20% higher than that of the
reference lasers without the nonlinear section. Our best devices have threshold current density of 2.2kA/cm2 and provide
approximately 35μW of second-harmonic output at 2.95μm at room temperature. The second-harmonic conversion
efficiency is approximately 100μW/W2. Up to two orders of magnitude higher conversion efficiencies are expected in
fully-optimized devices.
We present an overview of our recent results on the growth, fabrication, and characterization of high-power long-wave infrared quantum cascade lasers with multimode and single-mode waveguides. Powers of up to 1.2 W at wavelengths of = 6.1 µm are obtained with InGaAs/InAlAs buried heterostructure lasers grown lattice matched on InP substrates. For longer wavelengths, up to = 9 µm, powers of P = 800 mW are delivered from room-temperature-operated devices. Distributed-feedback waveguides have been fabricated with buried grating geometry, leading to single-mode emission of more than P = 150 mW output at = 7.74 µm when the device is operated at room temperature in continuous mode.
The ν1+ν3 combination band of uranium hexafluoride (UF6) is targeted to perform analytical enrichment measurements
using laser absorption spectroscopy. A high performance widely tunable EC-QCL sources emitting radiation at 7.74 μm
(1291 cm-1) is employed as an UF6-LAS optical source to measure the unresolved rotational-vibrational spectral
structure of several tens of wavenumbers (cm-1). A preliminary spectroscopic measurement based on a direct laser
absorption spectroscopy of methane (CH4) as an appropriate UF6 analyte simulant, was demonstrated.
We demonstrate active control of propagating surface waves on a mid-infrared extraordinary optical transmission
grating. The surface waves are excited at the interface between a GaAs substrate and a periodically patterned metal film
using a dual wavelength quantum cascade laser. The spectral properties of the laser and the transmission grating are
characterized by Fourier Transform Infrared spectroscopy. In addition, the far-field emission from excited surface
waves at the metal/GaAs interface is studied using a novel spatial and spectral imaging technique. By actively
controlling the optical properties of the grating, we demonstrate the ability to control the coupling of incident coherent
radiation to surface waves on the grating. With increased tunability of the grating, directional control of excited surface
waves should be achievable. These results suggest that the development of actively tunable plasmonic structures could
result in plasmonic routers and switches for interconnect or sensing applications.
MOCVD grown quantum cascade lasers (QCLs) have demonstrated about the same level performance as MBE
grown QCLs. With the regrowth capability to fabricate buried heterostructure (BH) waveguides, the QCL output
power has been dramatically increased and that opens the door to many mid-IR (and THz) applications. With the
stable and high growth rate to produce high performance and reliable BH lasers, commercialization of QCLs with
reasonable qualification and affordable price becomes possible. Furthermore with a good gain material and the
etching and regrowth capability, optoelectronic integration can be realized using MOCVD growth techniques. We
compare the MBE and MOCVD growth techniques and discuss important issues on growth rate stabilization and the
control of growth quality at the hetero-interface. We also go over a few growth and integration examples we are
working on that are preferentially done by MOCVD. Finally we describe a detailed QCL BH regrowth study and
discussed how that can be done right.
Adiabatic mode expansion laser diodes (AME-LDs) with small beam divergence fabricated by selective area growth
(SAG) are described. The AME-LDs have low thresholds of around 12 mA. The lateral and vertical far field angles
(FWHM) are reduced from 25° and 300 to 8° and 14°, respectively. The measured alignment tolerance to a cleaved single
mode fiber (SMF) is 3 pm.
We report preliminary results ofwidely tunable semiconductor laser materials using selective area growth techniques.
This material has very broad gain profiles ofmore than 250 nm and is promising to be used as widely tunable laser diodes for
future broadband wavelength division multiplexed (WDM) access networks.
The e-AO FB radiative transition is investigated by the 10K PL measurements for the unintentionally
doped MOCVD grown InGaAs. The intensity ratio of the e-AO FB transition to the BE exciton transition
is found to be greatly larger in bulk material than that in the quantum well samples. In quantum well
samples, this ratio is found to decrease rapidly as the well width decreases. A model is presented to
calculate the oscillator strength of the e-AO recombination and the 1 1H excitonic recombination in
quantum well, and this simulation can semi-quantitatively explain the experimental results.
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