Danny J. Krebs

The Infrared Array Camera (IRAC) for the Spitzer Space Telescope

The Infrared Array Camera (IRAC) is one of three focal plane instruments on the Spitzer Space Telescope. IRAC is a four-channel camera that obtains simultaneous broadband images at 3.6, 4.5, 5.8, and 8.0 � m. Two nearly adjacent 5A2 ; 5A2 fields of view in the focal plane are viewed by the four channels in pairs (3.6 and 5.8 � m; 4.5 and 8 � m). All four detector arrays in the camera are 256 ; 256 pixels in size, with the two shorter wavelength channels using InSb and the two longer wavelength channels using Si:As IBC detectors. IRAC is a powerful survey instrument because of its high sensitivity, large field of view, and four-color imaging. This paper summarizes the in-flight scientific, technical, and operational performance of IRAC.

Compact, passively Q-switched Nd:YAG laser for the MESSENGER mission to Mercury

A compact, passively Q-switched Nd:YAG laser has been developed for the Mercury Laser Altimeter, an instrument on the Mercury Surface, Space Environment, Geochemistry, and Ranging mission to the planet Mercury. The laser achieves 5.4% efficiency with a near-diffraction-limited beam. It passed all space-flight environmental tests at subsystem, instrument, and satellite integration testing and successfully completes a postlaunch aliveness check en route to Mercury. The laser design draws on a heritage of previous laser altimetry missions, specifically the Ice Cloud and Elevation Satellite and the Mars Global Surveyor, but incorporates thermal management features unique to the requirements of an orbit of the planet Mercury.

Numerical model estimating the capabilities and limitations of the fast Fourier transform technique in absolute interferometry.

Anumerical model was developed to emulate the capabilities of systems performing noncontact absolute distance measurements. The model incorporates known methods to minimize signal processing and digital sampling errors and evaluates the accuracy limitations imposed by spectral peak isolation by using Harming, Blackman, and Gaussian windows in the fast Fourier transform technique. We applied this model to the specific case of measuring the relative lengths of a compound Michelson interferometer. By processing computer-simulated data through our model, we project the ultimate precision for ideal data, and data containing AM-FM noise. The precision is shown to be limited by nonlinearities in the laser scan.

InSb arrays for IRAC (infrared array camera) on SIRTF (Space Infrared Telescope Facility)

SIRTF requires detector arrays with extremely high sensitivity, limited only by the background irradiance. Especially critical is the near infrared spectral region around 3 micrometers , where the detector current due to the zodiacal background is a minimum. IRAC has two near infrared detector channels centered at 3.6 and 4.5 micrometers . We have developed InSb arrays for these channels that operate with dark currents of < 0.2 e/s and multiply-sampled noise of approximately 7 e at 200 s exposure. With these specifications the zodiacal background limited requirements has been easily met. In addition, the detector quantum efficiency of the InSb devices exceeds 90% over the IRAC wavelength range, they are radiation hard, and they exhibit excellent photometric accuracy and stability. Residual images have been minimized. The Raytheon 256 X 256 InSb arrays incorporate a specially developed (for SIRTF) multiplexer and high-grade InSb material.

The Lunar Orbiter Laser Altimeter (LOLA) laser transmitter

We present the final configuration of the space flight laser transmitter as delivered to the Lunar Orbiter Laser Altimeter (LOLA) instrument. The instrument was launched in 2009 and has been in operation for close to two years and accumulated over 1.3 billion laser shots in space.

Design considerations of the LOLA laser transmitter

We present the design of the Lunar Orbiter Laser Altimeter laser transmitter which consists of two oscillators on a single bench, each capable of providing one billion shots.

PC detector passivation for high performance

Probably the most important factor in producing HgCdTe detectors of high performance is surface passivation. A good passivant for PC HgCdTe detectors accumulates the surface thereby reflecting minority carriers from surface imperfections and increasing minority carrier lifetime. A variety of passivants are known and used to various degrees, including sulfides, fluorides, oxides, and others. One of the problems with known passivants is that they tend to accumulate the surface too strongly, thus producing non-optimal results. We have developed a new passivation process which passivates the surface without strong accumulation. This results in detector resistance that is about 50%higher than that achieved with traditional KOH passivation, responsivities that are from 100 to 200% higher, and detectivities that are from 50to 100% higher for long wavelength devices. Materials and processing details used to achieve these results, as well as experimental data will be presented. Keyword: passivation, infrared, HgCdTe, detector

HgCdTe detector technology and performance for the Composite Infrared Spectrometer (CIRS)/Cassini mission

The composite infrared spectrometer (CIRS) instrument, an important component of the Cassini mission, consists of 3 focal plane arrays for sensing IR radiation of the Saturnian planetary system. Goddard Space Flight Center has fabricated, tested, and delivered high performance, 10- element HgCdTe photoconductive (PC) arrays for use on CIRS FP3, the focal plane responsible for detection of radiation in the 9.1 to 16.7 micrometers spectral band. The delivered flight array has peak responsivity 100 percent above CIRS specification, detectivity 30 percent or more above specification, and a cutoff wavelength of 17.3 micrometers at the operating temperature of 80 K. In order to achieve high performance at low frequency while maintaining limited power dissipation, we adopted a split-geometry detector structure. This design also ensured the buttability of the PC arrays to photovoltaic arrays supplied by CE-Saclay-France for detection of radiation in the 7.1 to 9.1 micrometers range. The detector structure is also noteworthy for its use of 0.05 micrometers Alumina powder-loaded epoxy to minimize reflection at the epoxy/HgCdTe interface, thus spoiling undesired optical resonance. This was done in order to meet the CIRS spectral uniformity requirement, which would have been difficult at these long wavelengths without this feature.

High-performance HgCdTe infrared detectors for the GOES long-wave sounder

GOES long wave sounder (LWS) detector requirements have always pushed the state-of-the-art for longwave detectors operating in the vicinity of 102 K. Performance and yield of acceptable detectors have always been problems and continue to be important issues affecting the performance of instruments of both present and future design. GSFC has been examining new device and operational concepts aimed at producing significant improvements in performance and yield. Our approach has been directed towards mitigating the deleterious effects of operating small geometry HgCdTe PC devices under heavy bias, that is, under minority carrier sweepout, as is typical in conventional LWS detector operation. Specifically, theory indicates that detectors of the new design operating under optimal bias conditions have significantly higher responsivity, lower power dissipation,and lower 1/f noise knees than conventional LWS detectors. In this paper we will describe the new LWS detectors fabricated at GSFC, present detector data, and review the theory of operation of these devices.

Split-geometry detectors, our eyes in space

Infrared detectors have projected our ability to explore our planet and our solar system far beyond the spatial, temporal, and spectral limitations of our natural vision. As such, they are our eyes in space, constantly searching the heavens, and sending back information about the origin, constitution, and dynamics of planetary atmospheres, and other processes of interest. Their ability to do this effectively depends on their sensitivity. Today, long wave PC (photoconductive) HgCdTe detectors are the detectors of choice for applications requiring high sensitivity at long wavelengths and elevated temperature. However, planetary exploration and space surveillance of the earth’s climatic condition are presently still limited by the sensitivity of available detectors. This paper will describe detectors developed at Goddard to provide enhanced performance for applications such as the CIRS/Cassini mission to Saturn and Titan, and the GOES weather satellite. Specifically, this paper will show theoretically and experimentally how detectors of split-geometry design can be exploited to increase detector resistance, responsivity, and detectivity, while decreasing 1/f noise and power dissipation. Photomicrographs of split-geometry detectors will be shown, and data demonstrating theoretical split-geometry design advantages will be presented for flight arrays built for the CIRS/Cassini mission, and for advanced detectors for GOES.