###### Abstract  
In this paper, a comparison study of a quantum cascade laser used for signal transmission by free-space optics is presented. The main goal is to define the capabilities of medium-wavelength infrared lasers operated in pulsed or continuous wave (cw) mode through testing and analyzing a laboratory setup of a data link operated at wavelengths of 4.5 \(\upmu\)m (pulsed, peak power 3 W) and 4.8 \(\upmu\)m (cw, average power -20 mW). In this spectral range, the link budget is also defined by radiation attenuation in the atmosphere (absorption, scattering, and turbulence interaction). The performed measurements define unique operational aspects of the quantum cascade lasers considering on-off keying modulation. The registered light pulse changes for different parameters of driving current signals determine some limitations in both rate and data range. Finally, we present eye diagrams of the signals obtained using two data links. free-space optics; quantum cascade lasers; wireless communications; quantum cascade lasers +
Footnote †: journal: _Photonics_  
at MWIR or LWIR wavelengths also minimizes the influence of photon scattering effects, defined as the relation between the size of the scattering particle and the radiation wavelength. For many years, there were no radiation sources that operated in these ranges characterized by compact construction, high modulation rate, room temperature operation, and low power consumption. In particular, the design of optoelectronic cascade structures (quantum cascade (QC) lasers and inter-band cascade (IC) lasers) has defined a new direction in the development of FSO systems that operate in a MWIR spectral range. Nowadays, the performance of state-of-the-art systems indicate that the technological readiness of these systems is related to laboratory devices. In Table 1, some FSO systems are listed. QC lasers are applied in most FSO systems. The aim of their construction is to achieve the highest data rates, which are usually obtained using cw lasers driven by DC biasing current and radio frequency (RF) modulation signal (T-bias electronic circuits). In this configuration, directed modulation at a few GHz can be obtained. The faster modulation speeds require specially designed devices, injection of RF signal, or cryogenic cooling. The main goal of this study is to define the capabilities of two MWIR QC lasers designed for pulsed or cw mode operation. According to their radiation wavelength (-4.5 \(\upmu\)m), comparable link budgets were performed considering different data sources of radiation attenuation, for example, the Kim model with visibility, the HITRAN molecular absorption database, and the PcModWin 6.0 software (Ontar Corporation, North Andover, USA) with environmental models. For a comparison, the same budget was calculated for the link operated at a wavelength of 1.5 \(\upmu\)m which corresponds to commercially available FSO systems. Finally, tests of newly designed FSO systems using a \"shelf-ready\" pulsed QC laser (-3 W peak power and -4.5 \(\upmu\)m wavelength) and a cw QC laser (-20 mW average power and -4.8 \(\upmu\)m wavelength) were conducted during operation at room temperature. The \"eye diagram\" analyses indicated that both QC lasers can be used in an FSO system providing a direct current modulation with a rate of 6 MHz (for pulsed laser) and 100 MHz (for cw laser). Additionally, the range calculations for these FSO systems indicated the need for a precise analysis of the spectral characteristics of both radiation source and atmospheric attenuation. It can significantly modify the data link range. For the designed links, which are spectrally separated by 300 nm, the high difference in QC lasers power has not so significant influence on the link distance. During the test of the cw laser, a significant relationship between amplitude and the frequency of light pulses was also identified. In practice, considering the speed and range of FSO links, a compromise should be established.