In the present communication, the characterization results of an in-house developed four-quadrants detection module based on HgCdTe APDs and a Si-CMOS ROIC pre-amplifier is discussed. The module has been designed to be employed as high data rate ground-segment detector for 1.55 μm long-distance free-space optical communication links in the framework of a project funded by the European Space Agency. The detector is characterized by a multiplication gain in excess of M = 150, a ROIC input referred noise of Ne = 45 electrons rms and a measured bandwidth of BW = 450 MHz. These characteristics enable the linear-mode detection of meso-photonic states ranging from tens of photons per pulse down to the single-photon level at high count rates exceeding 500 MHz per quadrant (and 2 GHz if the signal is dispatched over all four-quadrants). For the present module, the performance for PPM and OOK modulation formats was estimated and its potentiality for long-distance free-space optical communications employing these modulation formats was validated. In particular, for the PPM format, a detection probability of 0.9 and a false alarm probability of 10-2 , a minimum PPM slot width of 500 ps and a temporal jitter with a FWHM ~ 160 ps were estimated, for an incident photonic state with 10 photons/pulse. The potentiality of the detector for 625 Mbps OOK modulation format was also evaluated and compared with a quantum limited situation. In this case, a -3.9 dB penalty from the quantum limited BER was obtained. A new generation of detectors is currently in development, which is expected to further improve the performance.
HgCdTe Avalanche Photo Diodes (APDs) are developed at CEA/Leti to enable applications that require the detection of information contained in a low number of photons in each spatial and/or temporal bin, such as LiDAR and free space optical communications. The requirements for such detectors are strongly application dependent, which is why both the HgCdTe APD technology and the proximity electronics, used to extract the detected photocurrent, needs to be optimized for each application. The present communication reports results obtained from the development of detectors for high dynamic range LiDAR applications, made within the scope of the H2020 project HOLDON, and high data rate FSO, made in collaboration with Mynaric Lasercom AG. For FSO applications, we have measured 10 GHz bandwidth at unity gain for APDs with 10 μm diameter. At higher APD gain and diameter, the BW is presently limited by carrier transit and by resistance-capacitance product in small and large area APDs, respectively. For LiDAR we have developed APDs with an made of an array of diodes in parallel with a diameter up to 200 μm and large avalanche gain, M<100, that will be hybridized with a dedicated CMOS amplifier. This circuit was designed to enable photon shot noise limited linear detection over a dynamic range of 6 order of magnitude of signal for observation times ranging from ns up to μs. First characterizations made at unity APD gain shows that the HOLDON detector will meet most of the required performance parameters in terms of sensitivity and linear dynamic range.
We present CEA-LETI’s recent work on very small pitch HgCdTe focal-plane-arrays (FPA): materials, diode processing, readout circuit (ROIC) optimization and hybridization, done in the context of the common laboratory with SOFRADIR called DEFIR. We report on a 7.5μm pitch 640×512 FPA and a smaller 5μm pitch 64×152 FPA operating in middle wave infrared range (MWIR). The diode technology is n-on-p processed onto LPE grown HgCdTe. We will describe the two readout integrated circuits (ROIC) developed for 7.5μm and 5μm pitches and present the characterization of the IRFPAs hybridized to those ROICs. For these very small pitch detectors, we designed classic snapshot Direct-Injection (DI) Integrate-While-Read (IWR) ROICs that maximize the charge handling capacity by significantly increasing the dynamic range. For the 7.5μm ROIC, dedicated electronics has been embedded to measure the ROIC cross-talk. The 7.5μm pitch IRFPA operating at 110K displays nonlinearity under 0.5% across the maximum dynamic range, a full-well of 3.1 Me- with a 3.8V dynamic range, a ROIC noise of 210μV and SNR of 62 dB and NETD (Noise-Equivalent Temperature Difference) of 25 mK for an average current of 30 pA, and a responsivity of 1.3 pA/K.
We report high resolution (873 x 500), active-matrix, GaN-based LED microdisplays with a pixel pitch of 10 μm. They exhibit the highest resolution for the smallest pixel pitch ever reported for GaN microdisplays. High-density GaN μLED arrays were first patterned at 10-μm pitch on sapphire substrates. Arrays were then hybridized on CMOS active-matrix using the microtube technology. Blue and green monochrome prototypes have been realized. Full video, high-resolution images have been obtained. The performance of these GaN-based microdisplays make them suitable for a wide range of applications from augmented reality and head-up displays to pico- and compact projectors.
Dynamic X-ray imagers require large surface, fast and highly sensitive X-ray absorbers and dedicated readout
electronics. Monocrystalline photoconductors offer the sensitivity, speed, and MTF performances. Polycristalline
photoconductors offer the large surface at a moderate cost. The challenge for them is to maintain the first performances
at a compatible level with the medical applications requirements. This work has been focused on polycristalline CdTe
grown by Close Space Sublimation (CSS) technique. This technique offers the possibility to grow large layers with a
high material evaporation yield. This paper presents the results obtained with an image demonstrator using 350μm thick
CdTe_css layers coupled to a CMOS readout circuit with Indium bumping. The present demonstrator has 200 x 200
pixels, with a pixel pitch of 75μm ×75μm. A total image surface of 15mm × 15mm has then been obtained. The ASIC works in an integration mode, i.e. each pixel accumulates the charges coming from the CdTe layer on a capacitor, converting them to a voltage. Single images as well as video sequences have been obtained. X-ray performance at 16 frames per second rate is measured. In particular a readout noise of 0.5 X ray, an MTF of 50% at 4 lp/mm and a DQE of 20% at 4lp/mm and 600 nGy are obtained. Although present demonstrator surface is moderate, it demonstrates that high performance can be expected from this assembly concept and its interest for medical applications.
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