We have designed, built, and tested an uncooled THz imager based on optical readout photomechanical imaging technology, in which a MEMS-based sensor chip transduces the THz scene into a visible signal that is captured by a CCD imager. The performance of the 130x90 resolution, 100 μm pitch, 30 fps uncooled THz imager was measured using the λ = 119 μm (2.52 THz) emission line of a CO2-pumped methanol gas laser. Excellent linearity of the responsivity was observed over a wide range of laser power. The noise equivalent power (NEP), limited by shot noise from the optical readout, was 76 pW/Hz1/2. Switching to a high-capacity CCD imager to reduce shot noise and tailoring the photomechanical pixel structure for THz absorption will yield an NEP of less than 1 pW/Hz1/2. In addition, the uncooled THz imager successfully profiled the output beam of a λ=134 um (2.24 THz) quantum cascade laser (QCL) in real time, with performance far superior to a commercial pyroelectric array camera.
Hostile fire indication (HFI) systems require high-resolution sensor operation at extremely high speeds to capture hostile
fire events, including rocket-propelled grenades, anti-aircraft artillery, heavy machine guns, anti-tank guided missiles
and small arms. HFI must also be conducted in a waveband with large available signal and low background clutter, in
particular the mid-wavelength infrared (MWIR). The shortcoming of current HFI sensors in the MWIR is the bandwidth
of the sensor is not sufficient to achieve the required frame rate at the high sensor resolution. Furthermore, current HFI
sensors require cryogenic cooling that contributes to size, weight, and power (SWAP) in aircraft-mounted applications
where these factors are at a premium. Based on its uncooled photomechanical infrared imaging technology, Agiltron has
developed a low-SWAP, high-speed MWIR HFI sensor that breaks the bandwidth bottleneck typical of current infrared
sensors. This accomplishment is made possible by using a commercial-off-the-shelf, high-performance visible imager as
the readout integrated circuit and physically separating this visible imager from the MWIR-optimized photomechanical
sensor chip. With this approach, we have achieved high-resolution operation of our MWIR HFI sensor at 1000 fps,
which is unprecedented for an uncooled infrared sensor. We have field tested our MWIR HFI sensor for detecting all
hostile fire events mentioned above at several test ranges under a wide range of environmental conditions. The field
testing results will be presented.
Employing an optical readout architecture expands the capabilities offered by uncooled thermal imagers, such as
extremely fast frame rates, dual-band imaging, and multi-megapixel resolution. It also affords the ability to incorporate
multiple pixel designs on the same infrared sensor chip, which we have taken advantage of to fabricate an optical
readout photomechanical imager with 12 distinct pixel designs in the sensor chip layout. Using this methodology, we
were able to quickly sort the designs in terms of performance and suitability for manufacturing, and thus, in an expedient
and highly cost-effective manner, determine which pixel designs have merited future consideration for full-scale
prototyping. A fast frame rate MWIR photomechanical imager based on one of the best pixel designs was built and
tested for high-speed imaging of small arms fire.
In an optical-readout photomechanical imager, the infrared sensor array is physically separated from the ROIC. The
modularity of the optical readout architecture allows for extra design freedom that is not possible in bolometers, negating
fundamental trade-offs, such as NETD versus thermal time constant. For successful commercialization, the
photomechanical imager must meet application-specific performance and functional targets, and to this end, Agiltron has
advanced the photomechanical imaging platform over several technology generations. Improvements have been made to
both the optical readout system and the photomechanical sensor chip, which enabled reductions in size, weight, and
power (SWAP) and NETD over successive generations. The current-generation photomechanical imager has the size
equivalent to a digital camera and an /1-equivalent NETD and MDTD of less than 100 mK.
Agiltron has demonstrated an uncooled dual-band MWIR/LWIR imager based on our previously reported photomechanical sensor technology. The readout from two separate MWIR and LWIR photomechanical sensor arrays were fused together, permitting either dual-band, MWIR-only, or LWIR-only operation of the imager. Results of the dual-band imager in response to a wide-dynamic range infrared scene are described. With the dual-band MWIR/LWIR capability, the full dynamic range of the scene was successfully captured, yet good sensitivity was still maintained when imaging near-room temperature objects. Extensive comparative performance analysis was performed for the photomechanical thermal imager and HgCdTe and InSb photon imagers. A noise model for the optical readout photomechanical thermal imager is presented. It is shown that the photomechanical pixel can be engineered to nullify the contribution of shot noise to NETD without affecting the thermal properties of the pixel. In this regime, the performance of the uncooled photomechanical imager will approach that of cooled photon imagers.
Using low-pressure metalorganic chemical vapor deposition, we have grown GaInAs/InP QWIP structures on GaAs-coated Si substrate. First, the procedure to optimize the epitaxy of the InP buffer layer on Si substrate is given. Excellent crystallinity and a mirror-like surface morphology were obtained by using both a two-step growth process at the beginning of the InP buffer layer growth and several series of thermal cycle annealing throughout the InP buffer layer growth. Second, results of fabricated GaInAs/InP QWIPs on Si substrate are presented. At a temperature of 80 K, the peak response wavelength occurs at 7.4 micrometers . The responsivities of QWIPs on both Si and InP substrates with identical structures are equal up to biases of 1.5 V. At a bias of 3 V, the responsivity of the QWIPs on Si substrate is 1.0 A/W.
Quantum dots are recognized as very promising candidates for the fabrication of intersubband photodetectors in the infrared spectral range. At present, material quality is making rapid progress and some devices have been demonstrated. Examples of mid-infrared quantum dot intersubband photodetectors are presented along with device design and data analysis. Nonetheless, the performance of these devices remains less than comparable quantum well intersubband photodetectors due to difficulties in controlling the quantum dot size and distribution during epitaxy.
Results of detector characterization are presented for quantum well infrared photodetectors (QWIPs) fabricated from a variety of III-V material systems lattice-matched to InP substrate. Extremely large responsivities of 33.2 A/W were obtained from GaInAs/InP QWIPs operating at (lambda) equals 9 micrometers which represents to the authors' knowledge the largest value of responsivity for any QWIP in this wavelength range. Devices made from AlGaInAs/InP and GaInAs/AlInAs have also been realized that extend the wavelength range of sensitivity from 3 micrometers out to 20 micrometers while remaining lattice-matched to InP. Lattice-matched multispectral detectors are demonstrated for sensitivity at both 4 micrometers and 8.5 micrometers . Localized epitaxy of GaInAs/InP superlattice structures lattice-matched to InP was performed on Si substrate for the purpose of monolithic integration of III-V QWIPs with Si-based readout integrated circuitry.
We have studied the dependence of the well doping density in n-type GaInAs/InP quantum well IR photodetectors (QWIPs) grown by low-pressure metalorganic chemical vapor deposition. Three identical GaInAs/InP QWIP structures were grown with well sheet carrier densities of 1 by 1011 cm-2, 3 by 1011 cm-2, and 10 by 1011 cm-2; all three samples had very sharp spectral response at (lambda) equals 9.0 micrometers . We find that there is a large sensitivity of responsivity, dark current, noise current, and detectivity with the well doping density. Measurements revealed that the lowest-doped samples had an extremely low responsivity relative to the doping concentration while the highest-doped sample had an excessively high dark current relative to doping. The middle-doped sample yielded the optimal results. This QWIP had a responsivity of 33.2 A/W and operated with a detectivity of 3.5 by 1010 cmHz1/2W-1 at a bias of 0.75 V and temperature of 80 K. This responsivity is the highest value reported for any QWIP in the (lambda) equals 8-9 micrometers range. Analysis is also presented explaining the dependence of the measured QWIP parameters to well doping density.
Multi-quantum well structures of GaxIn1-xAsyP1-y were grown by metalorganic chemical vapor deposition for the fabrication of quantum well IR photodetectors. The thickness and composition of the wells was determined by high-resolution x-ray diffraction and photoluminescence experiments. The intersubband absorption spectrum of the Ga0.47In0.53As/InP, Ga0.38In0.62As0.80P0.20 (1.55 micrometers )/InP, and Ga0.27In0.73As0.57P0.43 (1.3 micrometers )/InP quantum wells are found to have cutoff wavelengths of 9.3 micrometers , 10.7 micrometers , and 14.2 micrometers respectively. These wavelengths are consistent with a conduction band offset to bandgap ratio of approximately 0.32. Facet coupled illumination responsivity and detectivity are reported for each composition.
We report the first self-assembled InGaAs/InGaP quantum dot intersubband infrared photoconductive detectors (QDIPs) grown on semi-insulating GaAs substrate by low pressure metal organic chemical vapor deposition (MOCVD). The InGaAs quantum dots were constructed on an InGaP matrix as self assembling in Stranski-Krastanow growth mode in optimum growth conditions. The detector structure was prepared for single layer and multi-stacked quantum dots for active region. Normal incident photoconductive response was observed at a peak wavelength of 5.5 micrometer with a high responsivity of 130 mA/W, and a detectivity of 4.74 X 107 cm Hz1/2/W at 77 K for multi-stack QDIP. Low temperature photoresponse of the single quantum dot photodetector was characterized. Peak response was obtained between 16 K and 60 K. The detailed dark current noise measurements were carried on single and multistack quantum dot infrared detectors. High photoconductive gain as 7.6 X 103 biased at 0.5 V results in increasing the intersubband carrier relaxation time as two order of magnitude compared quantum well infrared photodetectors.
In this work, we present our recent achievements for the reliability of the Al-free lasers at high temperatures and high powers. Laser operations up to 30,000 hours were achieved without any degradation in the lasers characteristics from 7 randomly selected InGaAsP/GaAs diodes for (lambda) equals 808 nm. The test were performed for lasers without mirror-coating for optical power of 0.5 to 1 W CW at 50 approximately 60 degree(s)C. To the best of our knowledge, this is the first direct demonstration of the extremely high reliability of Al-free diodes operations at high powers and temperatures for periods of time much longer than practical need (approximately 3 years). The characteristics during the tests are discussed in detail.
In this paper, we studied the effects of the active region structure (one, two and three quantum wells with same total thickness) for high-power InGaAsP-GaAs separate confinement heterostructure lasers emitting at 0.8 micrometers wavelength. Experimental results for the lasers grown by low pressure metalorganic chemical vapor deposition show excellent agreement with the theoretical model. Total output power of 47 W from an uncoated 1 cm-wide laser bar was achieved in quasi-continuous wave operation.
GaInAsP lattice matched to GaAs in the entire bandgap range has been grown by low-pressure metalorganic chemical vapor deposition. Small mismatch and strong interference fringes in the x-ray spectrum, sharp photoluminescence (PL) peak, and high electron mobility indicate good control of the quaternary compositions, smooth epilayer interfaces, and coherent growth of the epilayers. Temperature coefficient of bandgap is measured from the temperature dependence of the PL peak to be 4.09 XT 10-4eV/K at 300K. Anomalous temperature dependence of PL at low temperature, similar to that reported for GaInP, is reported for GaInAsP/GaAs for the first time. It questions the attribution of the uncommon behavior to the crystal defects related to the long-range ordered structure.
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