In the context of greenhouse gases monitoring such as carbon dioxide (CO2), a powerful emitter in the near infrared (1.5−2 μm) is needed to get a useful precision on concentration measurement (< 1 %) with reasonably space and time resolution. Energy pulses larger than several millijoules at a pulse repetition frequency (PRF) of several hundred of Hertz are usually required. Such requirements call for a solid-state laser configuration at least for a part of it as demanded pulse energy is well beyond current pulsed fiber laser potential performances. These DIAL emitters also call for a specific multiple wavelength emission around the chosen atmospheric gas absorption line, single mode operation, high spectral purity and stability, high pulse energy stability, good beam quality, linear pulse polarization and good overall wall plug efficiency, especially for space Integrated Path Differential Absorption (IPDA) lidar measurements. In this paper we report on the development and the demonstration of a two-wavelength single-frequency Ho:YLF oscillator that was developed in the framework of a ESA contract. This laser is especially suitable for atmospheric carbon dioxide (CO2) measurement using the R30 CO2 absorption line at 2050.967 nm. The oscillator consists in a fiber-coupled and freespace solid-state hybrid system and can be used in high-energy middle-rate or moderate-energy high-rate configurations depending the detection scheme of the lidar. The pulse energy and frequency stabilities are specially documented in twofrequency single-mode operations in the context of CO2 space borne IPDA measurements.
In the framework of CO2 monitoring in the Atmospheric Boundary Layer (ABL), a ground-based 2-μm Differential Absorption Lidar (DIAL) has been developed at the Laboratoire de Météorologie Dynamique (LMD) in Palaiseau. In order to derive flux information, this system has been set up with coherent detection, which allows to combine CO2 density measurements with wind velocity measurements. On the other hand, new advances in the field of Mercury Cadmium Tellure (MCT) Avalanche Photodiodes (APDs) open the way for high-precision measurements in direct detection ultimately from space. In this study, we first report on state of the art measurements obtained with the current coherent DIAL system before presenting expected results for a similar laser transmitter equipped with MCT APDs. For this latter part, we use a numerical model which relies on APDs performance data provided by the Laboratoire d'Électronique et de Technologie de l'Information (LETI).
In the absence of climate change policies, the fossil fuel emissions are projected to increase in the next decades. Depending on how the current carbon sinks change in the future, the atmospheric CO2 concentration is predicted to be between 700–1000 ppmv by 2100, and global mean surface temperature between 1.1–6.4°C, with related changes in sea-level, extreme events and ecosystem drifts. Keeping the atmospheric CO2 concentration at a level that prevents dangerous interference with the climate system poses an unprecedent but necessary challenge to humanity. Beyond this point, global climate change would be very difficult and costly to deal with. There are two main approaches that are currently analysed: (1) to reduce emissions; (2) to capture CO2 and store it, i.e. sequestration. For these two ways, some monitoring at different scales ultimately from space would be needed. Lidar remote sensing is a powerful technique that enables measurements at various space and time resolution.
HgCdTe APDs have opened a new horizon in photon starved applications due to their exceptional performance in terms of high linear gain, low excess noise and high quantum efficiency. Both focal plane arrays (FPAs) and large array single element using HgCdTe (MCT) APDs have been developed at CEA/Leti and Sofradir and high performance devices are at present available to detect without deterioration the spatial and/or temporal information in photon fluxes with a low number of photon in each spatio-temporal bin. The enhancement in performance that can be achieved with MCT has subsequently been demonstrated in a wide scope of applications such as astronomical observations, active imaging, deep space telecommunications, atmospheric LIDAR and mid-IR (MIR) time resolved photoluminescence measurements. Most of these applications can be used in space borne platforms.
HgCdTe avalanche photodiodes offers a new horizon for observing spatial or temporal signals containing only a few infrared (IR) photons, enabling new science, telecommunication and defence applications. A large number of HgCdTe APD based detectors have been developed at CEA LETI to address the increasing number of applications in which a faint photonic information needs to be extracted from the noise of the proximity electronics used to sample the signal. The performance of HgCdTe APDs is directly related to the multiplication process and the dark current generation in the APDs. The impact of these parameters is presented as a function of the Cd composition and geometry of the APDs. The obtained and expected performance of HgCdTe APD detectors is reported for applications ranging from very low flux observations with long observations times to high data rate telecommunications with up to single photon resolution.
The remarkable properties (internal gain larger than 100 and close to unity excess noise factor) of Short Wave Infrared
(SWIR) HgCdTe electron-initiated Avalanche Photodiodes (e-APDs) are put to good use to demanding applications, i.e.
spectroscopy and LIDAR. Knowing the requirements of both situations, we have designed specific models based on
highly sensitive single elements APDs and adapted proximity electronics.
On one hand, we use the e-APDs low noise equivalent power (NEP) at 180K (few fW/Hz1/2). We simultaneously
designed a specific Transimpedance Amplifier (TIA) which allows us to take advantage of the low APD NEP. The
combination of both elements along with a dedicated cryostat enables direct LIDAR detection at moderate bandwidth
(BW = 20 MHz) without the need for long time averaging, which is crucial in far field (≥ 5 km) analysis. One the other
hand, we have optimized a low-noise and low-frequency LN2 cooled prototype operating with an external commercial amplifier. It allows us to observe the photoluminescence of Ge nanostructures in the range 1.5-2.5 μm with a
significantly increased SNR along with a reduce pump laser power. The possibility to use these detectors in the photon
counting limit will be discussed in light of our recent results. In parallel, we present preliminary time response
measurements performed on SWIR APD suggesting that a higher GHz BW could be reached with this type of detector.
This is however subjected to optical optimization at the moment.
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