COherent mid-infrared RAdiometry and LIdar for atmospheric remote sensing – CORALI

The system developed within the framework of the project is based on optoelectronic devices: detectors, modulators, and lasers. These devices are unipolar semiconductor components. The term "unipolar" refers to the fact that these elements use only electrons. The operating wavelength of these devices, whether they detect light (in the case of detectors) or emit light (in the case of lasers), is defined according to the laws of quantum mechanics. The metamaterial architecture used in these components allows them to operate at room temperature. This characteristic is essential to avoid the need for cooling and to bypass the use of cryostats, which are very bulky and require a continuous flow of liquid nitrogen.

These devices can therefore be integrated with Peltier modules to maintain a stable operating temperature while keeping a very compact environment.

Another feature of our LIDAR is that it uses an external modulator to modulate the laser beam in both amplitude and phase without altering the laser current. The advantage of this approach is achieving a linear and highly controlled frequency modulation, which improves the performance of LIDAR measurements.

LPENS:

We demonstrated a first laboratory proof-of-concept of a 9 µm wavelength LiDAR for distance measurements over a few tens of centimeters, as well as velocity measurements. The measured object’s distance was limited by the available space on the optical table.

TRT:

TRT demonstrated a proof-of-concept frequency-modulated (FMCW) LiDAR system based on unipolar components. The measurement relies on coherent detection of a frequency-modulated signal reflected by a distant target. This coherent detection was implemented in two distinct ways: on an external detector (QCD/QWIP developed by LPENS) or directly within the laser cavity (self-mixing). Thanks to wideband, linear frequency modulation of a quantum cascade laser (up to 8 GHz in 65 µs with <1% nonlinearity) and efficient coherent detection, an absolute distance precision below 1% was achieved for outdoor targets up to 54 m away.

ONERA:

ONERA built an optical test bench serving as a prototype for a laser spectroradiometer with passive heterodyne detection (see Fig. 7 and Fig. 8). The bench is centered around two main components:

The local oscillator (LO), an infrared QCL emitting around 8.72 µm, driven by a low-noise current supply with temperature control.

The atmospheric transmission beam: in the lab, solar illumination is replaced by blackbody radiation, collimated through a 5 cm long, ½-inch diameter gas cell filled with 50 mbar N₂O (Air Liquide) to simulate atmospheric absorption. Gas filling is done via a dedicated pumping/filling setup. The beam intensity can be modulated using a mechanical chopper.

The bench can operate in several configurations with minor adjustments:

Linear scanning of the LO line combined with mechanical intensity modulation of the transmission beam: the main operating mode described here.

Linear scanning of the LO line with electro-optic QCL frequency modulation: the mechanical modulation is removed, and a function generator drives the QCL modulation input with a ramp and sinusoidal signal.

Fixed-frequency mode: if a broadband detector replaces analog processing, a high-bandwidth (>10 GHz) oscilloscope must be added

The work initiated in the CORALI project will continue along two main axes:

Demonstrator: The LIDAR will be optimized to be integrated into a compact optical device, making it portable and enabling field measurements.

Expansion into Other Areas (Quantum Optics): The expertise developed within the CORALI project will be further advanced to explore new research directions. All work on laser stabilization and the development of high-responsivity quantum detectors will pave the way for electric field quadrature measurements to detect non-classical light sources.

High-resolution spectroscopic measurements will also be conducted in collaboration with LPL, focusing on more fundamental goals such as testing fundamental constants or symmetries.

The mid-infrared spectral region (2-20µm) contains the characteristic vibrational transitions of important atmospheric gases (H2O, N2O, CO2, NF3, CH4, O3…). Moreover, in this range there are two atmospheric transparency windows at 3-5µm and 8-13µm and therefore a light beam can propagate over very long distances and investigate the atmosphere remotely. Furthermore, mid-infrared radiation has a lower sensitivity to atmospheric turbulence than at telecoms wavelengths. This is crucial to have accurate, precise and very sensitive measurements for environmental sensing, spectrometry and greenhouse gases (GHG) monitoring. The objective of our proposal is to provide an accurate and compact mid-infrared solar occultation heterodyne radiometry and LIDAR systems. They will employ quantum cascade laser as a source, while the room temperature ultrafast quantum well detectors recently developed at LPENS will be a key advantage in our systems.