tailieunhanh - Advanced Topics in Mass Transfer P14

Công nghệ cơ khí hay kỹ thuật cơ khí là ngành ứng dụng các nguyên lý vật lý để tạo ra các loại máy móc và thiết bị hoặc các vật dụng hữu ích. Cơ khí áp dụng các nguyên lý nhiệt động lực học, định luật bảo toàn khối lượng và năng lượng để phân tích các hệ vật lý tĩnh và động, phục vụ cho công tác thiết kế trong các lĩnh vực như ô tô, máy bay và các phương tiện giao thông khác, các hệ thống gia nhiệt và làm lạnh, đồ dùng gia đình, máy. | Use of a-SiC H Photodiodes in Optical Communications Applications 379 long been an established practice. This multiplexing technology uses multiple wavelengths to carry information over a single fiber. For WDM two key-elements are indispensable a multiplexer MUX and a demultiplexer DEMUX . The multiplexer takes optical wavelengths from multiple fibers and makes them converge into one beam. At the receiving end the system must be able to separate out the components of the light so that they can be detected. Demultiplexers perform this function by separating the received beam into its wavelength components and coupling them to individual fibers Fig. 1 . Fig. 1. Multiplexing and Demultiplexing operations. The standard MUX DEMUX devices are well known for infrared telecom systems M. Bas 2002 . These devices can be either passive or active in design. Passive designs are based on interference filters and plane diffraction gratings while active designs combine passive devices with tunable filters. Most of these devices require additional collimating and focusing optics that need alignment and lead to complicated designs. Figure 2 shows the sketch of the basic concept of a DEMUX device for optical communication where the different wavelengths are separated using a prism. Fig. 2. Demultiplexer device based on an optical component. The primary challenges of these devices are to minimize cross-talk and maximize channel separation. Cross-talk is a measure of how well the channels are separated while channel separation refers to the ability to distinguish each wavelength. Between multiplexing and demultiplexing points in a DWDM system there is an area in which multiple wavelengths exist. It is often desirable to be able to remove or insert one or more wavelengths at some point along this span. An optical add drop multiplexer OADM performs this function. 380 Advances in Photodiodes Rather than combining or separating all wavelengths the OADM can remove some while passing others