Left and top center: Unipolar quantum-device communication system in which light is emitted by a quantum cascade laser (bottom right corner of left-hand image), then modulated with a high-speed Stark modulator (center of left-hand image and inset in bottom left corner) and finally detected by a quantum-well infrared photodetector (top left corner of left-hand image). Bottom center: Cross-section of modulator operation. Right: Eye diagram and histogram showing the quality of 31-m on-off-keying transmission at a data rate of 30 Gbits/s.
High-power quantum cascade lasers and detectors with large responsivity are now mature semiconductor devices operating in the mid-infrared range (4 µm < λ < 14 µm) at room temperature. These unipolar quantum optoelectronic devices are of paramount interest for implementing high-speed free-space communication systems.1 Yet fast modulation of light in this wavelength domain remains a critical bottleneck, hindering large deployment of telecom systems at wavelengths longer than 1.55 µm. In our work this year, we demonstrated a potential resolution to this bottleneck—a Stark-effect modulator operating at room temperature at 9 µm that, on a 31-m free-space link, reached a transfer speed of 30 Gbits/s.
One promising solution that has been proposed for extending the telecom wavelength range is based on frequency up- and down-conversion between telecom wavelengths and the chosen mid-infrared wavelength. In such a case, the signal is modulated and detected at 1.55 µm, while the transmission occurs in the mid-infrared. However, at the emitter side, this technique requires multiwatt near-infrared pump lasers to generate a mid-infrared beam of a few mW and is consequently bulky and inefficient.2
Another proposed option is to take advantage of the ultrashort photocarrier lifetime of graphene to perform all-optical modulation with a subwavelength-thick graphene–metal hybrid metasurface. Even if the response time with this type of 2D material is predicted in the picosecond range, state-of-the-art experiments are restricted to sub-GHz demonstrations so far.3 Furthermore, these two methods have not yet been able to reach the 8-to-12-µm wavelength range, which is the most relevant spectral domain for free-space applications due to the combination of low atmospheric attenuation, low scattering and background stealth.
Based on recent progress in the concept of unipolar quantum optoelectronics, we developed external Stark-effect modulators operating at 9-µm wavelength.4 These modulators combine a multi-GHz bandwidth with a large modulation depth, and are thus compatible with high-speed communications in the mid-infrared. By using unipolar devices, we have realized a system made of a single-mode quantum cascade laser, the aforementioned Stark-effect modulator and a fast quantum-well infrared photodetector, and demonstrated an unprecedented indoor free-space communication at 30 Gbits/s over a distance of 31 m.5
This breakthrough, in our view, provides a path toward a new paradigm in high-speed atmospheric data-transmission at mid-infrared wavelengths. It could also lead to transformative advantages in lidar, precision remote-sensing and ultrafast molecular spectroscopy, as many molecular fingerprints are in the mid-infrared domain.
Hamza Dely, Thomas Bonazzi, Etienne Rodriguez, Djamal Gacemi, Angela Vasanelli and Carlo Sirtori, Ecole Normale Supérieure, Université PSL, CNRS, Sorbonne Université, Université Paris Cité, Paris, France
Pierre Didier, Olivier Spitz and Frédéric Grillot, LTCI, Télécom Paris, Institut Polytechnique de Paris, Palaiseau, France
1. L. Flannigan et al. J. Opt. 24, 043003 (2022).
2. Q. Hao et al. Appl. Opt. 56, 2260 (2017).
3. A. Basiri et al. Light Sci. Appl. 11, 102 (2022).
4. H. Dely et al. Laser Photon. Rev. 16, 2100414 (2022).
5. P. Didier et al. CLEO: Science and Innovations, JTh6A.5, postdeadline paper (2022).