###### Abstract
They have a high quantum efficiency (QE) due to the high absorption coefficient for the photon energy corresponding to the fundamental energy gap. The QE is the best one to compare other technologies. However, MCT detectors have large dark currents. There are also some production problems due to a weak mercury-telluride bond. This causes poor material uniformity, surface instability, a low yield, and high-cost processing. A quantum well infrared photoconductor (QWIP) is an alternative detector characterized by better thermal stability and higher uniformity. What is more, it provides lower dark currents, higher detectivities, and easier fabrication. The short carrier lifetime of a QWIP ensures achieving a faster response (>100 GHz bandwidth). However, the QE is limited by no optical transition for the normal incidence of light. To minimize this effect, a frontside scattering filter (reflecting gratings) is applied but some light intensity is still guided by the residual substrate. QWIP detectors should work at a low temperature, increasing the QE and decreasing the dark current. The IR response speed and low power consumption are interesting for the construction of commercial OWC receivers. The SLS technology provides a responsivity comparable to MCT, but it is in an early stage of development. The applied developed techniques of these structures ensure precise control of the layer thicknesses and interfaces. Due to a low dark current, normal incidence \begin{table}
\begin{tabular}{c c c c} \hline \hline  & **MCT** & **QWIP** & **SLS** \\ \hline Technological status & advanced & commercial & research \\ Quantum Efficiency & high & low & medium \\ Spectral range & wide & narrow & wide \\ Detectivity [cmHz\({}^{1/2}\)/W] & & \(1\times 10^{11}\) (78 K) [10] & \(7.1\times 10^{11}\) (150 K) [12] \\ (Temperature) & & \(2\times 10^{8}\) (210 K) [11] & \(4.6\times 10^{9}\) (300 K) [13] \\ Response time & from tens/hundreds of ps up to a few ns [8] & a few ps [14] & a few ns [15] \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Essential properties of some infrared detectors. absorption, room temperature operation, and band-gap, tunable type-II superlattice (T2 SL) detectors are desired in an OWC system. However, a fairly narrow effective band gap between the minibands makes this technology less effective for shorter wavelength applications. Despite this, SLSs can be a future-proof alternative to MCT as the third generation of infrared detectors. A detailed analysis of the infrared detector technologies, which are currently used in OWC systems and also are being developed, is presented by [PERSON] et al. [7] and [PERSON] et al. [8]. Additionally, quantum dot infrared photodetectors (QDIIPs), quantum cascade detectors (QCDs), and interband cascade infrared photodetectors (ICIPs) are described also. Regarding Table 2, the parameters of three defined groups of QC lasers and photodetectors used to transmit optical signals are listed. Group 'A' presents some lasers tested for transmission of analog or digital signals. They have a special lasing structure (with RF signal injection) driven by the bias-T circuit. These features provide for the lowest driving currents and the highest bandwidth. The lasers listed in group 'B' are ultra high-power ones with phased arrays (QCL-PA), photonic crystal distributed feedback (QCL-PCDF), and master-oscillator power-amplifier (QCL-MOPA) configurations. Their power level is a few tens of Watts, driving the current up to 25 A. Their preliminary test showed tens of kHz modulation frequencies. Group 'C' describes the results of the author's work to design the data link up to 2 km with the transfer of 10 Mb/s for real operation scenarios. Regarding that setup, QC lasers operated at a wavelength of 9.35 \(\upmu\)m with a peak power of 300 mW, and a maximal modulation speed of 4 MHz. Additionally, some high-power QC lasers are available commercially. Pranalytica, for example, offers a QC laser operated at the wavelength of 4.6 \(\upmu\)m with an average power above 4 W [25].