Fiber Optic Distribution Networks for Military Applications

A number of military applications require ad-hoc wireless communication and networking systems that employ low phase noise reference signals for upand downconversion of communication signals and further processing of data signals. High performance fiber optic link s are important for distribution of signals while phase noise degradation induced by AM-PM conversion in fiberoptic links impacts phase coherency of local oscillator (LO) signals in distributed systems. This paper focuses on issues associated with directly and externally modulated fiber optic links and their performance limitations in terms of gain, noise figure, nonlinearity, and dynamic range. The performance-cost aspects of both types of links are compared and it is pointed out that directly modulated links meet performance-cost requirements in most applications. Analysis of phase noise degradation of frequency reference is presented for directly modulated fiber optic distribution networks. SRS induced fiber nonlinearity is also discussed. Since the response of a Fabry-Perot laser diode can be altered by adding an external feedback, resulting in a resonance peak, results of a monolithically integrated FP laser are discussed. Finally, opto-electronic mixing of IF and LO signals are also demonstrated for the mode-locked case. 1.0. INTRODUCTION Recently a lot of efforts has been directed toward increase in global interconnectivity through electronic media and many market studies has identified a significant increase in demand for personal communication services in the 21 century. One of the most important proponents of this market is in the inter-office communication, where the high-speed fiberoptic local area networks (LAN) are combined with wireless communication to provide interconnectivity among many users. Conceptual system architecture of a hybrid wireless and fiber optic LAN with applications to both civilian and military systems is shown in Fig. 1. In this figure, personal computers (PC) are networked together through this hybrid wireless and fiberoptic networks. The digital data from the PC is up-converted by a millimeter wave (MMW) stable carrier and is then radiated by a low cost omni-directional antenna. The modulated received RF signal is then down-converted to the coded digital signals using the same stabilized MMW local oscillator (LO). The coded digital signal is then networked to other users through a high-speed fiberoptic network. The fiber-optic LAN is envisioned to operate either in analog format or digital data rates well above Gb/s. An important component of a reliable inter-office communication for the portable PC is the use of the low power consuming frequency translation circuits, which upand down-converts the information without any degradation in the its spectral purity. Frequency stability of the local oscillators used in the MMW wireless communication, and the clock recovery circuits used in the decision circuits, are critical in accurate data retrieval. Paper presented at the RTO SET Lecture Series on "Optics Microwave Interactions", held in Jouy en Josas, France, 2-3 September 2002; Duisburg, Germany, 5-6 September 2002; Budapest, Hungary, 9-10 September 2002, and published in RTO-EN-028.