Dielectric Materials for Thin-Film-Based Optical Communications Filters
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Dielectric Materials
for Thin-Film-Based Optical Communications Filters
Robert B. Sargent and Nada A. O’Brien Abstract Explosive growth in the fiber-optic telecommunications infrastructure during the past decade has driven noteworthy advances in thin-film-based optical interference filter technology. In particular, the widespread deployment of wavelength-division multiplexing (WDM), a means of increasing the communication capacity of an optical fiber by utilizing more than one wavelength of light, has motivated significant improvements in bandpass filter performance. A common requirement is for filters that combine or separate wavelengths spaced 100 GHz (0.8 nm) apart. The foundation for these recent breakthroughs has been laid by the development of optical coating technology over the past century. This article reviews the materials and processes used in the production of thin-film-based optical communications filters. Keywords: dielectric materials, photonic materials, optical communications, thin films.
Introduction In the 1970s, the infrastructure for longdistance communication began to shift from coaxial cable and free space microwave transmission systems to fiber-optic-based optical communications systems.1 Fiberoptic technology offered key advantages over previous technologies, including enormous bandwidth (information carrying capacity) and the potential for large distances between repeaters or amplifiers. Until about 10 years ago, requirements for capacity growth in the fiber-optic communications network were met by laying more fiber cable and through the use of time-division multiplexing (TDM). TDM increases the capacity of a communication channel by combining (i.e., interleaving in time) the pulses from a number of different digital streams into one digital stream. Such systems could transmit 2.5 Gbytes of information per second, enough capacity to support tens of thousands of phone calls on a single fiber.
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By the 1990s, the transmission of data on the long-distance network was fueling an increase in the already rising demand for long-haul bandwidth. Solutions were sought to further lower the cost of voice and data transmission. The invention and development of the optical amplifier, and notably the erbium-doper fiber amplifier (EDFA) around 1990,2 were important steps toward implementing lower-cost networks. Another was the introduction of wavelength-division multiplexing (WDM) in the mid-1990s.3 WDM is a technology that increases bandwidth by utilizing multiple communication channels on different wavelengths on each optical fiber in a cable. WDM is currently used in practical systems to increase the capacity of each fiber by 10–100 times over single-wavelength approaches. Thin-film interference filter technology is one of the important technologies employed in optical communications systems
that use EDFAs and WDM. We briefly review how thin-film optical filters work and then describe some typical requirements. Subsequent sections give a description of the most commonly used dielectric material
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