Free-space Optical Data Link Using Quantum Cascade Laser

The paper presents construction of a broadband optical system devoted to free space optical communication link. The main elements of the system are a quantum cascade laser and an HgCdTe heterostructural photodetector operating at the wavelength of 10"m. The described analyses showed that the system is characterized by lower sensitivity to adverse mete- orological conditions when compared with the systems operating in near infrared waveband. 1. INTRODUCTION Free-Space Optics (FSO) products are deployed in a line-of sight point-to-point conflguration. Free space optical systems ofier a ∞exible networking solution that delivers on the promise of broadband communications. Only FSO provides the essential combination of qualities required for modern networking. Since FSO transceivers can transmit and receive through windows, it is possible to mount FSO systems inside buildings, reducing the need to compete for roof space, simplifying wiring and cabling, and permitting the equipment to operate in a very favourable environment. The only essential for FSO is a line of sight between the two ends of the link. Free Space Optics is far more secure than RF technologies for several reasons: requires no RF spectrum licensing, FSO laser beams cannot be detected with RF meters or spectrum analyzers, the laser beams generated by FSO systems are narrow and invisible, making them harder to flnd and even harder to intercept and crack, FSO laser transmissions are optical and travel along a line of sight path that cannot be intercepted easily, data can be transmitted over an encrypted connection adding to the degree of security available in Free Space Optics network transmissions. Another advantage of FSO, when compared to RF, is signiflcant reduction in end-to-end delay. Most FSO products are plug-and- play units independent of the transmitted protocol and data rate. Quantum cascade lasers are very suitable for such applications because their emission wavelength can be chosen in the so- called atmospheric window regions, i.e., around 3{5"m and 8{14"m. In addition, the fast internal lifetimes of the devices should allow for reasonable modulation frequencies of up to 5{10GHz (1). In this paper, we analyze FSO-10"m system compared to shorter wavelengths, i.e., 0.8"m and 1.5"m. 2. ATMOSPHERIC INFLUENCES The atmosphere is a mixture of dry air and water vapour. Carrier-class Free Space Optics systems must be designed to accommodate heavy atmospheric attenuation, particularly by fog. Longer wavelengths are favoured in haze and light fog, but under the conditions of very low visibility this long-wavelength advantage does not apply. Additionally, the fact that 10-"m-based systems are allowed to transmit hundreds times more eye-safe power makes them very attractive for modern FSO links. The atmospheric transmission of optical signals, is given by the Beer's law equation