The most technologically mature optically pumped semiconductor lasers (OPSL) are based on InGaAs quantum wells (QW) for emission in the 900-1200 nm range. The low wavelength boundary is set by both the bandgap of InGaAs and the most common pump wavelength of 808 nm. To extend the wavelength coverage into 700 – 900 nm, a different QW system and a different pump wavelength are needed. In this work, we present the progress and result in the development of AlGaAs-based OPSL.
Optically pumped semiconductor lasers (OPSL) have been replacing legacy gas lasers and solid state lasers for over a decade, due to their superior properties such as compactness, high efficiency, low noise, wavelength scalability, and power scalability. It has wide applications in life sciences, medical therapeutics, light show, and other scientific researches. In this work, we present a gain model and couple it to the thermal management of high power OPSL.
Self-heating of Optically Pumped Semiconductor (OPS) chip has been identified as the major limiting factor of power scaling in OPS-based lasers in continuous wave (cw) mode. In this work, characterization of OPS lasers in short pulse (100 ns) and low duty cycle (1%) regime, where self-heating is negligible, as a function of the heat sink temperature is presented. This data, combined with a rigorous thermal model, allows us to predict OPS chip performance in new cooling configurations for power scaling.
Optically pumped semiconductor lasers (OPSL) offer the advantage of excellent beam quality, wavelength agility, and high power scaling capability. In this talk we will present our recent progress of high-power, 920nm OPSLs frequency doubled to 460nm for lightshow applications. Fundamental challenges and mitigations are revealed through electrical, optical, thermal, and mechanical modeling. Results also include beam quality enhancement in addressing the competition from diode lasers.
We demonstrate the first room temperature continuous wave THz sources based on intracavity difference frequency
generation from mid-infrared quantum cascade lasers. This accomplishment was enabled by integration of several key
technologies, resulting in a new high efficiency waveguide design and improved thermal dissipation. Room temperature
single mode emissions at 3.6 THz with an emitting power of 3 μW and a mid-IR-to-THz conversion efficiency of 0.44
mW/W<sup>2</sup> are obtained in continuous wave mode. THz peak power up to 1.4 mW in pulsed mode operation with a mid-IRto-
THz conversion efficiency of 0.8 mW/W2 at 3.5 THz is also demonstrated.
We present the recent development of high performance compact THz sources based on intracavity nonlinear
frequency mixing in mid-infrared quantum cascade lasers. Significant performance improvements of our THz
sources in the spectral purity, frequency coverage as well as THz power are achieved by systematic optimizing the
device's active region, waveguide, phase matching scheme, and chip bonding strategy. Room temperature
single-mode operation in a wide THz spectral range of 1-4.6 THz is demonstrated from our Čerenkov phase-matched
THz sources with dual-period DFB gratings. High THz power up to 215 μW at 3.5 THz is demonstrated via
epi-down mounting of our THz device. The THz power is later scaled up to mW level by increased the mid-IR
power and conversion efficiency. The rapid development renders this type of THz sources promising local oscillators
for many astronomical and medical applications.
We present the high performance THz sources based on intracavity difference-frequency generation from mid-infrared quantum cascade lasers. Room temperature single-mode operation in a wide THz spectral range of 1-4.6 THz is demonstrated from our Cerenkov phase-matched THz sources with dual-period DFB gratings. High THz power up to 215 μW at 3.5 THz is demonstrated via epi-down mounting of our THz device. The rapid development renders this type of THz sources promising local oscillators for many astronomical and medical applications.
This paper describes our development efforts at Northwestern University regarding dual-section sampled
grating distributed feedback (SGDFB) QCLs. These devices are the same size, but have much wider electrical tuning,
than a traditional DFB laser. In this paper, I will show how we have dramatically extended the monolithic tuning range
of high power quantum cascade lasers with high side mode suppression. This includes individual laser element tuning of
up to 50 cm<sup>-1</sup> and 24 dB average side mode suppression. These lasers are capable of room temperature continuous
operation with high power (<100 mW) output. Additionally, we have demonstrated a broad spectral coverage of over
350 cm<sup>-1</sup> on a single chip, which is equivalent to 87.5% of the gain bandwidth. The eventual goal is to realize an
extended array of such laser modules in order to continuously cover a similar or broader spectral range, similar to an
external cavity device without any external components.
Quantum Cascade Lasers (QCLs), operating in continuous wave (cw) at room temperature
(rt) in 3-3.5μm spectral range, which overlaps the spectral fingerprint region of many hydrocarbons,
is essential in spectroscopic trace gas detection, environment monitoring, and pollution control. A
3μm QCL, operating in cw at rt is demonstrated. This initial result makes it possible, for the most
popular material system (AlInAs/GaInAs on InP) used in QCLs in mid-infrared and long-infrared, to
cover the entire spectral range of mid-infrared atmospheric window (3-5μm).
In<sub>0.79</sub>Ga<sub>0.21</sub>As/In<sub>0.11</sub>Al<sub>0.89</sub>As strain balanced superlattice, which has a large conduction band
offset, was grown. The strain was balanced with composite barriers (In0.11Al0.89As /In0.4Al0.6As) in
the injector region, to eliminate the need of extremely high compressively strained GaInAs, whose
pseudomorphic growth is very difficult.
We demonstrate room temperature terahertz (THz) quantum cascade laser (QCL) sources with a broad spectral coverage based on intracavity difference-frequency generation. Dual mid-infrared (mid-IR) active cores based on the single-phonon resonance scheme are designed with a THz nonlinearity specially optimized for the high operating fields that correspond to the highest mid-infrared output powers. Integrated dual-period distributed feedback (DFB) gratings with different grating periods are used to purify and tune the mid-IR and THz spectra. Two different phase matching schemes are used for THz generation. The first is the collinear modal phase matching scheme, wherein the wafer is grown on a n+ InP substrate. Room temperature single mode operation THz emission with frequency tuning range from 3.3 to 4.6 THz and THz power up to 65 mW at 4.0 THz are realized. The mid-IR to THz power conversion efficiency is 23 uW/W2. The second is the Čerenkov phase-matching scheme, wherein the wafer is grown on a semi-insulating InP substrate, and device’s facet is polished into 20-30 degrees for THz extraction. Room temperature single mode emissions from 1.0 to 4.6 THz with a side-mode suppression ratio and output power up to 40 dB and 32 µW are obtained, respectively. The mid-IR to THz power conversion efficiency is 50 uW/W2.
We demonstrate room temperature, continuous wave operation of quantum cascade ring lasers around 5 μm with single
mode operation up to 0.51 W output power. Single mode operation persists up to 0.4 W. Light is coupled out of the ring
cavity through the substrate with a second order distributed feedback grating. The substrate emission scheme allows for
epilayer-down bonding, which leads to room temperature continuous wave operation. The far field analysis indicates that
the device operates in a high order mode.
We demonstrate room temperature, high power, single mode and diffraction limited operation of a two dimensional
photonic crystal distributed feedback (PCDFB) quantum cascade laser emitting at 4.36 μm. Total peak power up to 34 W
is observed from a 3 mm long laser with 400 μm cavity width at room temperature. Far-field profiles have M<sup>2</sup> figure of
merit as low as 2.5. This device represents a significant step towards realization of spatially and spectrally pure broad
area high power quantum cascade lasers.
For many practical applications that need bright sources of mid-infrared radiation, single mode operation and good beam
quality are also required. Quantum cascade lasers are prominent candidates as compact sources of mid-infrared radiation
capable of delivering very high power both CW and under pulsed operation. While 1D photonic crystal distributed
feedback structures can be used to get single mode operation from quantum cascade lasers with narrow ridge widths,
novel 2D photonic crystal cavity designs can be used to improve spectral and spatial purity of broad area quantum
cascade lasers. In this paper, we demonstrate high power, spatially and spectrally pure operation at room temperature
from narrow ridge and broad area quantum cascade lasers with buried 1D and 2D photonic crystal structures. Single
mode continuous wave emission at λ = 4.8 μm up to 700 mW in epi-up configuration at room temperature was observed
from a 11 μm wide 5 mm long distributed feedback quantum cascade laser with buried 1D gratings. High peak powers
up to 34 W was obtained from a 3mm long 400 μm wide 2D photonic crystal distributed feedback laser at room
temperature under pulsed operation. The far field profile had a single peak normal to the laser facet and the M<sup>2</sup> figure of
merit was as low as 2.5. Emission spectrum had a dominating single mode at λ = 4.36 μm.
We present recent performance highlights of midinfrared quantum cascade lasers (QCLs) based on an InP material system. At a representative wavelength around 4.7 µm, a number of breakthroughs have been achieved with concentrated effort. These breakthroughs include watt-level continuous wave operation at room temperature, greater than 50% peak wall plug efficiency at low temperatures, 100-W-level pulsed mode operation at room temperature, and 10-W-level pulsed mode operation of photonic crystal distributed feedback quantum cascade lasers at room temperature. Since the QCL technology is wavelength adaptive in nature, these demonstrations promise significant room for improvement across a wide range of mid-IR wavelengths.
Some of the recent advances in high power quantum cascade laser development will be reviewed in this paper. Research
areas explored include short wavelength (λ<4 μm) lasers, high performance strain-balanced heterostructures, and high
power long wavelength (7< λ< 16 μm) lasers. Near λ=4.5 μm, highlights include demonstration of 18% continuous
wave wallplug efficiency at room temperature, 53% pulsed wallplug efficiency at 40 K, and 120 W of peak power output
from a single device at room temperature. Near λ~10 μm, up to 0.6 W of continuous output power at room temperature
has also been demonstrated, with pulsed efficiencies up to 9%.
We demonstrate very high wall plug efficiency (WPE) of mid-infrared quantum cascade lasers (QCLs) in low
temperature pulsed mode operation (53%), room temperature pulsed mode operation (23%), and room temperature
continuous wave operation (18%). All of these values are the highest to date for any QCLs. The optimization of WPE
takes the route of understanding the limiting factors of each sub-efficiency, exploring new designs to overcome the
limiting factor, and constantly improving the material quality.
We demonstrate optimization of continuous wave (cw) operation of 4.6 μm quantum cascade lasers (QCLs). A 19.7 μm
by 5 mm, double channel processed device exhibits 33% cw WPE at 80 K. Room temperature cw WPE as high as 12.5%
is obtained from a 10.6 μm by 4.8 mm device, epilayer-down bonded on a diamond submount. With the semi-insulating
regrowth in a buried ridge geometry, 15% WPE is obtained with 2.8 W total output power in cw mode at room
temperature. This accomplishment is achieved by systematically decreasing the parasitic voltage drop, reducing the
waveguide loss and improving the thermal management.
We demonstrate electrically pumped, room temperature, single mode operation of photonic crystal distributed feedback
(PCDFB) quantum cascade lasers emitting at λ ~ 4.75 μm. Ridge waveguides of 50 μm and 100 μm width were
fabricated with both PCDFB and Fabry-Perot feedback mechanisms. The Fabry-Perot device has a broad emitting
spectrum and a broad far-field character. The PCDFB devices have primarily a single spectral mode and a diffraction
limited far field characteristic with a full angular width at half-maximum of 4.8 degrees and 2.4 degrees for the 50 μm
and 100 μm ridge widths, respectively.
Over the past several years, our group has endeavored to develop high power quantum cascade
lasers for a variety of remote and high sensitivity infrared applications. The systematic
optimization of laser performance has allowed for demonstration of high power, continuous-wave
quantum cascade lasers operating above room temperature. In the past year alone, the efficiency
and power of our short wavelength lasers (λ~4.8 μm) has doubled. In continuous wave at room
temperature, we have now separately demonstrated ~10% wallplug efficiency and ~700 mW of
output power. Up to now, we have been able to show that room temperature continuous wave
operation with >100 mW output power in the 3.8< λ<11.5 μm wavelength range is possible.
Laser-based free-space communications have been developed to serve specific roles in "last mile" high-speed data
networks due to their high security, low cost, portability, and high bandwidth. Conventional free-space systems based
on near infrared optical devices suffer from reliability problems due to atmospheric scattering losses and scintillation
effects, such as those encountered with storms, dust, and fog. Mid-infrared wavelengths are less affected by
atmospheric effects and can significantly enhance link uptime and range. This paper will discuss some of the recent
advances in high-power, high temperature, high reliability mid-infrared Quantum Cascade Lasers and their potential
application in highly reliable free space communication links.