Over the last two years, our group has reported the first room-temperature continuous-wave (RTCW) fixed wavelength VCSELs operating above 3 microns, in both optically pumped and electrically pumped devices. Our optically pumped 3.3um devices employ one or two wafer-bonded GaAs/AlGaAs mirrors, in conjunction with a type I InGaAsSb/AlInGaAsSb quantum well active region. Our electrically pumped 3.3um devices employ a bottom waferbonded GaAs/AlGaAs mirror, top deposited ZnSe/ThF4 mirror, and type II interband cascade (ICL) active region. These fixed wavelength devices lay a foundation for tunable devices in the spectrally rich 3-5um region. Narrowly tunable devices can use thermal tuning, by variation of pump power (optically pumped devices), bias current (electrically pumped devices), or device temperature (both electrically and optically pumped devices). In this paper, we describe tunable CW optically pumped devices with >4nm of tuning near 3.3um using variation of pump power. CW electrically pumped devices show ~2nm tuning near 3.3um using variation of bias current. These results are a critical first step towards an inexpensive and high-speed methane sensing source. A first generation of MEMS-tunable optically pumped devices has achieved 70nm tuning range near 3.34um.
Here we present the design and results of distributed feedback (DFB) interband cascade lasers (ICL) with 1st order top surface grating. A partially corrugated sidewall waveguide was designed and implemented to suppress high order lateral modes. The DFB ICLs have 4mm long cavities and 4.5 μm wide base line ridge waveguide, and are mounted epi-up on SiC submounts. Continuous-wave (CW) operation of the DFB ICLs was demonstrated. The lasing wavelengths re around 3.3 μm at 25°C, and the measured side mode suppression ratio is 30 dB. The best device with low detuning of gain peak and DFB Bragg wavelength showed a maximum output power and a maximum wall plug efficiency of 46 mW and 2%, respectively, at 25 °C.
Here we present lifetime test results of 4 groups of quantum cascade lasers (QCL) under various aging conditions including an accelerated life test. The total accumulated life time exceeds 1.5 million device·hours, which is the largest QCL reliability study ever reported. The longest single device aging time was 46.5 thousand hours (without failure) in the room temperature test. Four failures were found in a group of 19 devices subjected to the accelerated life test with a heat-sink temperature of 60 °C and a continuous-wave current of 1 A. Visual inspection of the laser facets of failed devices revealed an astonishing phenomenon, which has never been reported before, which manifested as a dark belt of an unknown substance appearing on facets. Although initially assumed to be contamination from the environment, failure analysis revealed that the dark substance is a thermally induced oxide of InP in the buried heterostructure semiinsulating layer. When the oxidized material starts to cover the core and blocks the light emission, it begins to cause the failure of QCLs in the accelerated test. An activation energy of 1.2 eV is derived from the dependence of the failure rate on laser core temperature. With the activation energy, the mean time to failure of the quantum cascade lasers operating at a current density of 5 kA/cm2 and heat-sink temperature of 25°C is expected to be 809 thousand hours.
A quartz-enhanced photoacoustic spectroscopy (QEPAS) sensor system was developed for the sensitive detection
of hydrogen peroxide (H2O2) using its absorption transitions in the v6 fundamental band near 7.73 μm. The recent
availability of distributed-feedback quantum cascade lasers (DFB-QCLs) provides convenient access to a strong H2O2 absorption line located at 1295.55 cm-1. Sensor calibration was performed by means of a water bubbler that generated titrated average vapor concentrations. A minimum detection limit of 75 parts per billion (ppb) was achieved at a pressure
of 80 torr for a 1 sec data acquisition time. The long-term repeatability and stability of the sensor was investigated by
measuring time-varying H2O2 mixtures for ~2 hrs. An Allan deviation analysis was performed to investigate the long-term performance of the QEPAS sensor system, indicating the feasibility of a minimum detection limit of 12 ppb using the optimum data averaging time of 100 sec.
Using a range of grating pitches, we obtained distributed feedback (DFB) quantum cascade lasers (QCL) operating CW
at room temperature over the 9-11μm range. Single mode CW operation is demonstrated up to 10.8 μm, and up to 40 °C.
To the best of our knowledge, it is the longest wavelength reported for a DFB QCL operating CW above room
temperature. The wavelength coverage per wafer is larger than 110 cm-1. DFB QCLs from two wafers which have
different gain peaks have lasing wavelengths near optical absorption peaks of glucose.
We demonstrated the room temperature continuous wave (CW) operation of mid-infrared distributed feedback (DFB)
quantum cascade lasers (QCLs) made of strain balanced GaInAs/AlInAs material on InP substrates for sensing CO2
isotope and N2O gas for potential applications that need battery powered portable devices in a sensor network. For the
former device at 4.35 μm wavelength, we demonstrated a low threshold voltage of less than 8 V for battery operation
and a near circular far field pattern with small divergent angles of 33 by 28 degrees full width at half maximum
(FWHM) in vertical and horizontal directions, respectively, for easy collimation. For the latter device at 4.5 μm
wavelength, we demonstrated a low CW threshold power consumption of 0.7 W at 20 °C. A side mode suppression ratio
(SMSR) of 30 dB was achieved within the whole operating current and temperature ranges for both lasers.
We demonstrate room temperature lasing of quantum cascade lasers (QCLs) operating at 3.45 and 3.55 μm under pulsed
and continuous wave (CW) operations, respectively. To the best of our knowledge those are the shortest wavelengths
ever achieved at room temperature from QCLs with strain balanced GaInAs/AlInAs material on InP substrate. With the
back facet high reflection coated, a maximum output power of 60 mW was obtained at 10 °C under CW operation. A
tuning range of 124 cm-1 was obtained in a pulsed mode in external cavity configuration.
We report the life test results for 9 4.6-µm planar buried heterostructure quantum cascade lasers made of strain-balanced GaInAs/AlInAs/InP materials grown by metal organic vapor-phase epitaxy. No facet coating was deposited, and the devices were mounted on CuW submounts using AuSn solder. The aging condition is continuous wave operation at a heat-sink temperature of 22ºC with a constant current of 0.85A, corresponding to current densities of 4.7 and 2.7 kA/cm2, for lasers with widths of 4 and 7 µm, respectively. All lasers survived 5000 h aging, and most devices showed improved performance during the first 1000 h of aging.
We propose and theoretically investigate mid/far-infrared photodetectors based on frequency up-conversion in a
near-resonant cascade of interband and intersubband transitions in high optical nonlinearity asymmetric quantum
well structures. Such structures can yield high detectivity and responsivity in the bandwidth of the order of
30% of a central frequency in the mid-infrared range. Resonant up-conversion detectors can be designed for
both collinear and perpendicular pump and signal beams. They can be integrated with semiconductor pump
lasers to yield compact devices. We present specific device designs based on GaAs/AlGaAs and InGaAs/AlInAs
heterostructures and calculate their expected figures of merit.
We discuss peculiarities of the nonlinear optical processes utilizing intersubband nonlinearities in high band offset
heterostructures formed by three nearly lattice matched binaries, InAs, GaSb, AlSb, and their alloys. We show that these
materials offer unique benefits for nonlinear optics due to great flexibility in designing optical interaction schemes in a
wide frequency range and very large values of the nonlinear susceptibilities even involving short-wavelength transitions.
The resulting nonlinear conversion efficiency for the second-harmonic or sum-frequency generation is in the mW/W2
range even for very short coherence lengths of the order of several &mgr;m.
We propose a new design of quantum cascade lasers integrated with resonant intersubband nonlinearities, in which the laser is divided into two separately contacted and biased sections along the cavity length. One section operates as a laser active medium while another section serves as a nonlinear element. We show that such schemes turn out to be surprisingly flexible and efficient in implementing various resonant optical nonlinearities.
We discuss practical benefits of the nonlinear active quantum-cascade structures that support both laser action and, at the same time, nonlinear self-conversion of laser light into coherent radiation at different frequencies. We show that the proposed approach can greatly enhance the performance of quantum cascade lasers and provide new functionalities. Examples considered include extreme frequency up- or down-conversion, fast and wide-range electric tuning, and multi-frequency generation.
We overview coherent nonlinear optical phenomena that occur in active quantum-cascade structures which support both laser action and, at the same time, nonlinear self-conversion of laser light into coherent radiation at different frequencies. In other words, the laser field serves as an intracavity optical pump for the desired nonlinear optical process. In such systems, resonant absorption of the pump field is overcome by laser gain, and huge resonant nonlinearities of the intersubband transitions can be fully exploited. The proposed approach holds promise to extend the operating wavelength of QC lasers to the spectral regions where they become less efficient, constrained by material limitations, or operate only at cryogenic temperatures. It may also lead to the development of broadly tunable injection-pumped sources.
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