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.
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.
Broad area type-I GaSb based diode lasers have recently exceeded 100 mW continuous wave room temperature powers
in 3.1-3.2 μm spectral region. Certain applications such as single frequency sources for spectroscopy and efficient
coupling to single mode fiber require single lateral mode laser operation. We characterize and compare two types of
lasers with similar structures and various ridge widths emitting at 3.1 and 3.2 μm. We obtain 35 and 25 mW of
continuous wave single lateral mode power from 8 and 13 μm wide ridge lasers emitting at 3.1 and 3.2 μm respectively.
This constitutes a threefold improvement compared to the previous result. Both devices had ridges etched to the depth
leaving approximately 300 nm of the top p-cladding in the areas outside the ridges. For 3.2 μm emitting lasers the
dielectric thickness was 220 nm while it was 510 nm for 3.1 μm emitting lasers. Gain spectra were measured by Hakki-
Paoli technique for various ridge widths. From gain spectra we extract differential gain and internal loss. We find that the
internal loss in thin dielectric, 3.2 μm emitting laser is about 14 cm-1 while it is 7 cm-1 in thick dielectric, 3.1 μm emitting
laser for the ridge widths of 13 and 8 μm exhibiting single lateral mode operation respectively. Internal losses measured
on broad area, 100 μm wide lasers processed from the same materials are similar and around 6-7 cm-1. We discuss
reasons for the internal loss increase with the aid of simulation of optical mode field and loss in our waveguide
structures.
Recent progress and state of GaSb based type-I lasers emitting in spectral range from 2 to 3.5 μm is reviewed. For lasers
emitting near 2 μm an optimization of waveguide core width and asymmetry allowed reduction of far field divergence
angle down to 40-50 degrees which is important for improving coupling efficiency to optical fiber. As emission
wavelength increases laser characteristics degrade due to insufficient hole confinement, increased Auger recombination
and deteriorated transport through the waveguide layer. While Auger recombination is thought to be an ultimate limiting
factor to the performance of these narrow bandgap interband lasers we demonstrate that continuous improvements in
laser characteristics are still possible by increasing hole confinement and optimizing transport properties of the
waveguide layer. We achieved 190, 170 and 50 mW of maximum CW power at 3.1, 3.2 and 3.32 μm wavelengths
respectively. These are the highest CW powers reported to date in this spectral range and constitute 2.5-fold
improvement compared to previously reported devices.
The optical spectral gain characteristics and overall radiative efficiency of MOCVD grown InGaAs quantum dot lasers
have been evaluated. Single-pass, multi-segmented amplified spontaneous emission measurements are used to obtain the
gain, absorption, and spontaneous emission spectra in real units. Integration of the calibrated spontaneous emission
spectra then allows for determining the overall radiative efficiency, which gives important insights into the role which
nonradiative recombination plays in the active region under study. We use single pass, multi-segmented edge-emitting in
which electrically isolated segments allow to vary the length of a pumped region. In this study we used 8 section devices
(the size of a segment is 50x300 μm) with only the first 5 segments used for varying the pump length. The remaining
unpumped segments and scribed back facet minimize round trip feedback. Measured gain spectra for different pump
currents allow for extraction of the peak gain vs. current density, which is fitted to a logarithmic dependence and directly
compared to conventional cavity length analysis, (CLA). The extracted spontaneous emission spectrum is calibrated and
integrated over all frequencies and modes to obtain total spontaneous radiation current density and radiative efficiency,
ηr. We find ηr values of approximately 17% at RT for 5 stack QD active regions. By contrast, high performance InGaAs
QW lasers exhibit ηr ~50% at RT.
The integration of thin film edge emitting lasers onto silicon enables the realization of planar photonic structures for
interconnection and for miniaturized optical systems that can be integrated in their entirety at the chip scale. These thin
film emitters are compound semiconductor lasers that are optimized for operation without the growth substrate.
Removal of the laser growth substrate, coupled with bonding to the silicon host substrate, enable the integration of high
quality edge emitting lasers with silicon. This paper explores the challenges, approaches, fabrication processes, and
progress in the integration of thin film edge emitting lasers integrated onto silicon.
Intersubband Quantum-Box (IQB) lasers; that is, devices consisting of 2-D arrays of ministacks (i.e., 2-4 stages) intersubband QB emitters are proposed, as an alternative to 30-stage quantum-cascade (QC) devices, as sources for efficient room-temperature (RT) emission in the mid-infrared (4-6 µm) wavelength range. Preliminary results include: 1) the design of devices for operation with 50% wallplug efficiency at RT; 2) realization of a novel type of QC device: the deep-well (DW) QC laser, that has demonstrated at =4.7µm low temperature sensitivity of the threshold current, a clear indication of suppressed carrier leakage; 3) the formation of 2-D arrays at nanopoles by employing nanopatterning and dry etching; 4) the formation of 40nm-diameter, one-stage IQB structures on 100nm centers by preferential regrowth via metal-organic vapor phase epitaxy (MOVPE).
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