Mid-infrared free-space optical communication has a large potential for high speed communication due to its immunity to electromagnetic interference. However, data security against eavesdroppers is among the obstacles for private free-space communication. Here, we show that two uni-directionally coupled quantum cascade lasers operating in the chaotic regime and the synchronization between them allow for the extraction of the information that has been camouflaged in the chaotic emission. This building block represents a key tool to implement a high degree of privacy directly on the physical layer. We realize a proof-of-concept communication at a wavelength of 5.7 μm with a message encryption at a bit rate of 0.5 Mbit/s. Our demonstration of private free-space communication between a transmitter and receiver opens strategies for physical encryption and decryption of a digital message.

This study deals with the communication capabilities of two kinds of semiconductor lasers emitting in one of the atmosphere transparency windows, around 4 $\mu$ m. One of these two lasers is a quantum cascade laser and the other one is an interband cascade laser. With the quantum cascade laser, a subsequent attenuation is added to the optical path in order to mimic the attenuation of free-space transmission of several kilometers. Direct electrical modulation is used to transmit the message and two-level formats, non-return-to-zero and return-to-zero, are used and compared in terms of maximum transmission data rate. The sensitivity to optical feedback is also analyzed, as well as the evolution of the error rate when reducing the optical power at the level of the detector. This work provides a novel insight into the development of future secure free-space optical communication links based on mid-infrared semiconductor lasers and sheds the light on improvements required to achieve multi-Gbits/s communication with off-the-shelf components.

Space-to-ground high-speed transmission is of utmost importance for the development of a worldwide broadband network. Mid-infrared wavelengths offer numerous advantages for building such a system, spanning from low atmospheric attenuation to eye-safe operation and resistance to inclement weather conditions. We demonstrate a full interband cascade system for high-speed transmission around a wavelength of 4.18 µm. The low-power consumption of both the laser and the detector in combination with a large modulation bandwidth and sufficient output power makes this technology ideal for a free-space optical communication application. Our proof-of-concept experiment employs a radio-frequency optimized Fabry–Perot interband cascade laser and an interband cascade infrared photodetector based on a type-II InAs/GaSb superlattice. The bandwidth of the system is evaluated to be around 1.5 GHz. It allows us to achieve data rates of 12 Gbit/s with an on–off keying scheme and 14 Gbit/s with a 4-level pulse amplitude modulation scheme. The quality of the transmission is enhanced by conventional pre- and post-processing in order to be compatible with standard error-code correction.

In environments where traditional fiber optic cables are impractical, free-space optical communications offer a promising solution for transmitting large amounts of data, especially in the mid-infrared wavelength range. Despite the advantages of minimal atmospheric interference and stable signals, the vulnerability of wireless optical communications to eavesdropping poses a significant challenge. This study addresses this challenge by demonstrating a method for privately transmitting optical data using photonic chaos from distributed feedback quantum cascade lasers operating at 9.3 µm. Signal processing techniques are applied to enhance the quality of the transmission over distances exceeding 30 m, accompanied by a comprehensive analysis of the photonic chaos complexity to ensure data confidentiality. These findings mark a significant advancement in developing private communications systems within the thermal atmospheric window, with a substantially reduced risk of interception by adversaries. The research not only contributes to secure communications but also has potential implications for enhancing security of data transmission in challenging environments, impacting various industries and applications.

The concept of extreme pulses is adapted to almost any kind of population. Those extreme events can be studied in a number of diversified fields, such as hydrodynamics,¹ electronic circuits,² climatology,³ and epileptic crises.⁴ More generally, extreme events, which also include freak waves or rogue waves, describe random isolated events with amplitudes well above those of neighboring ones, and which occur more often than expected from the distribution of lower amplitude events.⁵ Research is carried out in various fields of science to report on their generation and to try to find a policy by which extreme events could possibly abide. In optics, extreme events have been studied from several viewpoints,⁶ in varied configurations such as semiconductor lasers,⁷ fiber lasers,⁸ photonic crystals,⁹ and microwave antennas.¹⁰ On the other hand, theoretical work followed experimental demonstrations of optical extreme events,^11,12 providing different insights into the formation of these giant pulses. As opposed to strict rogue waves, extreme events can pop up through the variation of a controlled parameter, as it will be shown later on in this article. These extreme events can, for instance, take place in telecommunication data streams, where the emergence of high intensity peaks inside an optical fiber sets the limit of a high-speed optical communication link. Despite a plethora of studies dealing with extreme events driven by various mechanisms, an intensive and comprehensive investigation of the emergence of such extreme pulses in the mid-infrared field is still missing. Hence, it remains of prime importance to study them beforehand and understand their origins, in order to be able to better control them in future practical applications.¹³ Experimentally, means of identifying extreme events vary with the type of dynamics studied. Several criteria are thus generally applied to the data, and the strictest one, leading to the identification of the lowest number of extreme events, should be used. To detect extreme events from experimental data, the probability density function (PDF) of an appropriate attribute of the data needs to be analyzed, and crest or wave heights are commonly used.¹⁴ Once extracted from the data, heights are compared to a threshold value, above which events can be considered extreme. This threshold is generally defined as twice the significant height $H_s$ of the dataset, which represents the average of the higher third of heights.¹⁵ Another threshold widely used relies on the standard deviation ($\sigma$) and the mean value ($\mu$) of the data, and an event is considered extreme if it exceeds $\mu +8\times \sigma$.¹⁶ However, a criterion with a lower threshold ($\mu +6\times \sigma$ or even $\mu +4\times \sigma$) is also commonly used without loss of generality.^17,18 This threshold alone is however not sufficient to identify extreme events, and it is necessary to verify that the PDFs represent long-tailed distributions. In this study, we use both thresholds because, on the one hand, the $\mu +8\times \sigma$ threshold is more restrictive, so we can ensure that we observe extreme events. On the other hand, the threshold related to $H_s$ allows us to take into account more events so that we can display relevant statistics with enough data. The mechanism leading to the birth of such extreme events can be explained by an external crisis¹⁹ triggered by an external parameter, as we will explain hereafter. The optical source generates a chaotic output, and the related bifurcation diagram is bounded because the system is stable. After the crisis, trajectories can explore a much larger region in the phase space, and the bifurcation diagram expands. This process creates pulses whose maxima are well above those usually found in the chaotic output. However, the large pulses must seldom occur, because as long as the extreme events are very rare, the average value of the intensity of the peaks and its variance remain similar to the values before the crisis. If the large events become the norm, the threshold for extreme events will subsequently increase and all the events will be considered as regular. This means that the bifurcation diagram becomes stable again, and the output displays chaos without extreme events. Given the fact that chaos and positive Lyapunov exponents have already been encountered in quantum cascade lasers (QCLs) under external optical feedback (OF),²⁰ it is not surprising to observe that these lasers can also be prone to extreme pulses. Despite that, it is clear that the formation of such events in QCLs can be further explained through strong numerical modeling, taking into account the peculiar characteristics of QCLs, in particular, the intersubband carriers dynamics and the transport properties.

This study investigates chaotic and spiking dynamics of mid-infrared quantum cascade lasers operating under external optical feedback and emitting at 5.5 $\mu m$ and 9 $\mu m$ . In order to deepen the understanding, the route to chaos is experimentally studied in the case of continuous-wave and current modulation operation. The non-linear dynamics are analyzed with bifurcation diagrams. While for quasi-continuous wave operation, chaos is found to be more complex, pure continuous wave pumping always leads to the generation of a regular spiking induced low-frequency fluctuations dynamics. In the latter, results show that by combining external optical feedback with periodic forcing and further induced current modulation allows a better control of the chaotic dropouts. This work provides a novel insight into the development of future secure free-space communications based on quantum cascade lasers or unpredictable optical countermeasure systems operating within the two transparency atmospheric windows hence between 3 $\mu m$ - 5 $\mu m$ and 8.5 $\mu m$ - 11 $\mu m$