Applications of Organic and Inorganic Optical Thin Films in Telecommunications
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eptance was rapid due to the fact that, from a network point of view the wavelengths were completely logically separate and might as well have been on separate fibers. No change in network architecture was required for initial implementation. Commercial systems are now shipping with 80 wavelengths (not necessarily all lit) and systems have been announced with 160-170 channels, 50 GHz channel spacing and 10 Gb/s for each channel. Frequencies
1
1
M U X
2 3
D M U X
(1+2+3+4…N)
4
2 3 4
Figure 1. Dense Wavelength Division Multiplexing (DWDM) However, it was not long before people began to envision more powerful uses for DWDM beyond its initial role as a “dumb” fiber multiplier. In many different forms and in many different companies and laboratories, the concept of an “all optical” network was born. A schematic diagram of some of the elements in such an all optical network is shown in Figure 2. Argument still rages as to whether the all optical network should be “transparent” or “opaque” or “digital” or “analog”. However, the common vision is a network where high bandwidth optical data streams flow unobstructed and are only broken down into their substituent electronic digital parts where absolutely necessary.
CO FTTH Module Distribution Node
OADM OADM Metropolitan Ring
OXC
CO
OADM
FTTC Module
CO
OADM WDM
Figure 2. Emerging Optical Networks
However, optical networks such as that shown schematically in Figure 2 require many more optical components, and different types of components, than the initial point to point systems. 80 channel systems with dual band amplifiers are very complex. One calculation has shown that an 80 channel system with 3 amplifier stages requires 281 passive components, as compared to 13 for a 4 channel point to point system. Furthermore, new types of components and subsystems are required to manage and protect such complex systems. For example, Optical Add/Drop Multiplexers are necessary to make the provisioning of such massive bandwidth efficient. Optical crossconnects are required to provide restoration and protection as well as provisioning. Materials For Integrated Optical PLCs Large channel count, high bandwidth, spatially extensive, optical networks are extremely expensive and much of the cost is in the optical components used in the system. For the vision of the all optical network to become a reality, the cost per function of optical components must come down dramatically. One approach to reducing the cost of optical components is through integration. Shown in Figure 3 is the fabrication process for forming an optical waveguide on a Silicon substrate.
Core Cladding Silicon Metal Mask Core Cladding Silicon Waveguide Cladding Silicon Cladding Waveguide Cladding Silicon
Figure 3. Fabrication of Planar Lightwave Circuit for Optical Integration
First, a lower cladding layer of index ncl is deposited, followed by a core layer of index nco, with nco typically 10GHz – Polarization and Integration Limitations
• Silica and Glass – Very Low Loss – Inefficient for Active Functions
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