Requirements for passive optical communication components vary with the optical networks in which they are deployed. Optical network topologies include ultra-longhaul, long-haul, metro core, metro access, enterprise, and residential networks:

• Ultra-long-haul networks refer to point-to-point transport networks that send signals across several thousand kilometers without electrical signal regeneration, typically using either Raman amplification or solitons.
• Long-haul networks are the conventional long distance point-to-point transport networks that can send signals across 1000 km before the need for regeneration.
• Metro core networks refer to metropolitan area core ring and mesh networks that are typically hundreds of kilometers in length and either do not use amplification or use it sparingly.
• Metro access networks are the metropolitan area access ring networks, with stretches of a few to tens of kilometers; for distances this short, amplification is not needed.
• Enterprise networks refer to the intracampus or intrabuilding networks where distances are typically <1 km.
• Residential networks refer to the infrastructure needed to bring the fiber to the home; these types of networks are deployed scarcely today; however, when their build-out accelerates, there will be need for massive amounts of hardware [1].

The distances, use or non-use of amplification, and volume of hardware needed have direct consequences on the types of passive optical components that are needed in each type of network. In ultra-long-haul and long-haul networks, passive optical component performance is critical and cost is secondary. Although amplification is used, it is expensive and should be minimized. Therefore, the requirement for low-loss components is important; also, the long distances between regenerators require that dispersion be managed very precisely, since the effect accumulates over distance [1]. In metro core networks, cost and performance are important. As amplification is minimized and preferably avoided, there is a strict optical loss budget within which passive optical components need to stay [1].

In metro access, enterprise, and residential networks, cost is critical and performance is secondary. Since the distances are relatively short, the loss and dispersion requirements are relatively relaxed; however, the need for a large number of passive optical components makes cost the most important characteristic of optical components used in this area [1].

Optical networks of various topologies are increasingly exhibiting high speed, high capacity, scalability, configurability, and transparency, fueled by the progress in passive optical componentry. Through the exploitation of the unique properties of fiber, integrated, and free-space optics, a wide variety of optical devices are available today for the communication equipment manufacturers. Passive devices include the following:

• Fixed or thermooptically/electrooptically acoustooptically/mechanically tunable filters, based on arrayed waveguide gratings (AWGs), Bragg gratings, diffraction gratings, thin-film filters, microring resonators, photonic crystals, or liquid crystals
• Switches based on beam-steering, mode transformation, mode confinement, mode overlap, interferometry, holographic elements, liquid crystals, or total internal reflection (TIR; where the actuation is based on thermooptics), electrooptics, acoustooptics, electroabsorption, semiconductor amplification, or mechanical motion (moving fibers, microelectromechanical systems; MEMS)
• Fixed or variable optical attenuators (VOAs) based on intermediate switching, and using any of the switching principles
• Isolators and circulators based on bulk
• Faraday rotators and birefringent crystals or on integrated Faraday rotators/nonreciprocal phase shifters/nonreciprocal guided-mode-to-radiation-mode converters and half-wave plates
• Electrooptic, acoustooptic or electro-absorption modulators
• Wavelength converters using semiconductor optical amplifiers (SOAs) or detectors and modulators
• Chromatic dispersion (CD) compensators using dispersion-compensating fiber, allpass filters or chirped Bragg gratings
• Polarization-mode dispersion (PMD) compensators using polarization-maintaining fiber, birefringent crystal delays, or nonlinearly chirped Bragg gratings [1]

As for active devices (lasers, amplifiers, and detectors), they make use of heterostructures, quantum wells, rare-earth doping, dye doping, Raman amplification, and semiconductor amplification. These basic passive and active building block elements permit building higher functionality components such as reconfigurable optical add/drop multiplexers (OADMs), optical cross-connects (OXCs), optical performance monitors (OPMs), tunable gain flattening filters (TGFFs), interleavers, shared and dedicated protection switching modules, and modulated laser sources [1].