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.
Multispectral Imaging has recently made considerable improvements to the sensitivity, uniformity and
dynamic range of infrared FPAs based on capacitively read, bimorph microcantilever sensor technology. The
company is presently prototyping 160x120 imaging arrays with 50 μm pitch pixels and is actively pursuing the
development of next generation 25 μm pitch pixel arrays. Measured peak NETD values for recently fabricated 50μm pitch focal plane arrays are in the 40-50mK range, with individual pixels in the 10-15mK range. The modeled
and measured tradeoffs discussed in this paper lead to a possible 2-3 times further improvement in average NETD.
A number of factors influence the performance of these devices which includes the optimization of
sometimes competing design requirements. For example, the tuning and optimization of the infrared optical
resonant cavity structure while maximizing the change in sensor capacitance during IR irradiance. Similarly there
are tradeoffs between structural rigidity, which increases the structure resonant frequency improving noise
immunity, and thermal response times. These tradeoffs are discussed with reference to real world sensor
structures. Results from detailed thermo-electromechanical-optical modeling of the operation of the 25 μm pitch
pixels will be discussed in reference to the design and fabrication of 25 μm pitch test pixels. The most recent
infrared sensitivity and other performance measurements from the development of the company's first commercial
160 x 120 pixel imaging array product will also be presented.
This paper reports on the development of small pixel pitch infrared FPAs based on the capacitively read bimorph microcantilever sensor technology. The heat sensing bimorph microcantilever structures are fabricated directly onto the CMOS control and amplification electronics to produce a high performance, low cost imager that is compatible with standard silicon IC foundry processing and materials. Positional responsivities of greater than 0.3 μm/K have been modeled and measured for 50 μm pitch pixels, corresponding to a temperature coefficient of capacitance, &Dgr;C/C, (equivalent to TCR for microbolometers) above 30%/K. This responsivity, along with noise capacitances in the sub-attofarad range and nominal sensor capacitances of 15 fF, give modeled NEDT < 20 mK for these devices.
At smaller pixel pitches, the positional responsivity decreases rapidly with feature size resulting in increased system NEDTs. Modeling the performance of microcantilever based IR sensors with innovative sensor structures and pixel pitches down to 17 μm indicates NEDTs < 20 mK and thermal time constants in the 5 msec range, are feasible with this technology. Results from detailed thermo-electro-opto-mechanical modeling of the operation of the 25 μm pitch pixels are presented.