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When a HgCdTe IR detector is used for low flux astronomy mission, persistence is one of the main characteristic to calibrate. However, it is not yet a true figure of merit since no generic persistence characterization protocol exist. Indeed, a protocol is usually developed for each mission in order to calibrate this parameter according to the specific needs and specific operating point of the detectors (Integration time, temperature, flux dynamics ...). It is consequently not possible to compare persistence performance of two detectors if the characterization protocol differs and no definitive calibration scheme exist.
In the frame of ALFA and ASTEROID programs, we started characterizing persistence at CEA LETI, CEA IRFU and Lynred a few years ago. The objective was to improve the performance of our built in house detectors in terms of persistence. The goal of this paper is to present our approach to identify which protocol is best suited to compare persistence between two detectors manufactured with different technological steps. With a dedicated instrument, we compare systematically persistence protocols in a controlled environment and on a single detector. Electrical and optical stimulation are compared as well as several stimulation parameters (amplitude and stimulation duration, ie soak time).
With the best-suited protocol for detector comparison, we show technological improvements on persistence comparing detectors manufactured at CEA Leti and Lynred in the frame of ASTEROID and NIRLFSA, a program prior to ALFA.
CAGIRE is the near infrared camera of the Colibrí robotic telescope, designed for the follow-up of SVOM alerts, mainly Gamma Ray Bursts (GRBs), and the quick imaging of sky regions where transient sources are detected by the SVOM satellite. CAGIRE is based on the Astronomical Large Format Array (ALFA) 2k x 2k SWIR sensor from the French consortium CEA-LYNRED. In the context of CAGIRE the sensor is operated in “Up the Ramp” mode to observe the sky in a square field of view of 21.7 arcmin on a side, in the range of wavelengths from 1.1 to 1.8 μm. An observation with CAGIRE consists of a series of short (1-2 minutes) exposures during which the pixels are read out every 1.3 second, continuously accumulating charges proportionally to the received flux, building a ramp.
The main challenge is to quickly process and analyse these ramps, in order to identify and study the near infrared counterparts of the bursts, within 5 minutes of the reception of an alert. Our preprocessing, which is under development, aims at providing reliable flux maps for the astronomy pipeline. It is based on a sequence of operations. First, calibration maps are used to identify saturated pixels, and for each pixel, the usable (non saturated) range of the ramp. Then, the ramps are corrected for the electronic common mode noise, and differential ramps are constructed. Finally, the flux is calculated from the differential ramps, using a previously calibrated map of pixel non-linearities. We present here the sequence of operations performed by the preprocessing, which are based on previous calibrations of the sensor response. These operations lead to the production of a flux map corrected from cosmic-rays hits, a map depicting the quality of the fit, a map of saturated pixels and a map of pixels hit by cosmic-rays, before the acquisition of the next ramp. These maps will be used by the astronomy pipeline to quickly extract the scientific results of the observations, like the identification of uncatalogued or quickly variable sources that could be GRB afterglows.
In past years, LETI also developed infrared detectors for space astrophysics in the mid infrared range – the long wave detector of the ISOCAM camera onboard ISO – as well as in the far infrared range – the bolometer arrays of the Herschel/PACS photometer unit –, both instruments which were under the responsibility of the Astrophysics department of CEA (IRFU/SAp, Saclay, France).
Nowadays, the infrared detectors used in space and ground based astronomical instruments all come from vendors in the US. For programmatic reasons – increase the number of available vendors, decrease the cost, mitigate possible export regulations, …– as well as political ones – spend european money in Europe –, the European Space Agency (ESA) defined two roadmaps (one in the NIR-SWIR range, one in the MWIR-LWIR range) that will eventually allow for the procurement of infrared detectors for space astrophysics within Europe.
The French Space Agency (CNES) also started the same sort of roadmaps, as part of its contribution to the different space missions which involve delivery of instruments by French laboratories. It is important to note that some of the developments foreseen in these roadmaps also apply to Earth Observations.
One of the main goal of the ESA and CNES roadmaps is to reduce the level of dark current in MCT devices at all wavelengths. The objective is to use the detectors at the highest temperature where the noise induced by the dark current stays compatible with the photon noise, as the detector operating temperature has a very strong impact at system level. A consequence of reaching low levels of dark current is the need for very low noise readout circuits.
CEA and SOFRADIR are involved in a number of activities that have already started in this framework. CEA/LETI does the development of the photo-voltaic (PV) layers – MCT material growth, diode technologies–, as well as some electro-optical characterisation at wafer, diode and hybrid component levels, and CEA/IRFU/SAp does all the electro-optical characterisation involving very low flux measurements (mostly dark current measurements). Depending of the program, SOFRADIR can also participate in the development of the hybrid components, for instance the very low noise readout circuits (ROIC) can be developed either at SOFRADIR or at CEA/LETI.
Depending of the component specifications, the MCT epitaxy can be either liquid phase (LPE, which is the standard at SOFRADIR for production purposes) or molecular beam (MBE), the diode technology can be n/p (standard at LETI and SOFRADIR) or p/n (under development for several years now) [3], and the input stage of the ROIC can be Source Follower per Detector (SFD for very low flux low noise programs) or Capacitive Trans Impedance Amplifier (CTIA for intermediate flux programs) [4].
This paper will present the different developments and results obtained so far in the two NIR-SWIR and MWIR-LWIR spectral ranges, as well as the perspectives for the near future. CEA/LETI is also involved in the development of MCT Avalanche Photo Diodes (APD) that will be discussed in other papers [5,6].
In all those configurations, we might distinguish several categories of applications:
• low flux applications where the FPA is staring at space and the detection occurs with only a few number of photons.
• high flux applications where the FPA is usually staring at the earth. In this case, the black body emission of the earth and its atmosphere ensures usually a large number of photons to perform the detection.
Those two different categories are highly dimensioning for the detector as it usually determines the level of dark current and quantum efficiency (QE) requirements. Indeed, high detection performance usually requires a large number of integrated photons such that high QE is needed for low flux applications, in order to limit the integration time as much as possible. Moreover, dark current requirement is also directly linked to the expected incoming flux, in order to limit as much as possible the SNR degradation due to dark charges vs photocharges. Note that in most cases, this dark current is highly depending on operating temperature which dominates detector consumption. A classical way to mitigate dark current is to cool down the detector to very low temperatures.
This paper won't discuss the need for wavefront sensing where the number of detected photons is low because of a very narrow integration window. Rigorously, this kind of configuration is a low flux application but the need for speed distinguishes it from other low flux applications as it usually requires a different ROIC architecture and a photodiode optimized for high response speed.
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