1、1,Optical Communication Technology And Optical Networking,Yanhe Li Dept. of Electronic Engineering, Tsinghua University 2004. 9,Chapter 3,2,Components,Components 3.1 Couplers 3.2 Isolators and Circulators 3.3 Multiplexers and Filters 3.4 Optical Amplifiers 3.5 Transmitters 3.6 Detectors 3.7 Switches

2、 3.8 Wavelength Converters,3,3.1 Couplers,A direction coupler,A directional coupler is used to combine and split signals in an optical network. A 2 x 2 coupler consists of two input ports and output ports.,Construction: fuse two fibers together in the middle; use waveguides in integrated optics.,4,3

3、.1 Couplers,A coupler can be used as a power splitter if the coupling length l is adjusted such that half the power from each input appears at each output. Such coupler is called a 3-dB coupler.,Couplers are also used to tap off a small portion of the power from a light stream for monitoring purpose

4、s or other reasons.,Couplers are the building blocks for several other optical devices. We will explore the use of 3-dB directional couplers to construct optical switched.,Couplers are also the principle components used to construct Mach-Zehnder interferometers, which can be used as optical filters

5、and multiplexers/demultiplexers.,5,3.2 Isolators and Circulators,Couplers and most other passive optical devices are reciprocal devices, in that the devices work exactly the same way if their inputs and outputs are reversed. However, in many systems there is a need for a passive nonreciprocal device

6、. An isolator is an example of such a device. Its main function is to allow transmission in one direction through it but block all transmission in the other direction. Isolators are used in systems in front of optical amplifiers and lasers primarily to prevent reflections from entering these devices

7、, which would otherwise degrade their performance.,The two key parameters: insertion loss isolation,is around 1 dB.,is around 40 to 50 dB.,6,3.2 Isolators and Circulators,The state of polarization of light propagating in a single- mode fiber refers to the orientation of its electric field vector on

8、a plane that is orthogonal to its direction of propagation. At any time, the electronic field vector can be expressed as a linear combination of the two orthogonal linear polarizations supported by the fiber. We will call these two polarization modes the horizontal and vertical modes.,7,3.2 Isolator

9、s and Circulators,Principle of operation of an isolator that works only for a particular state of polarization of the input signal,8,3.2 Isolators and Circulators,The principle of operation of a circular is similar to that of an isolator, except that it has multiple ports, typically three pr four. I

10、n a three-port circular, an input signal on port 1 is sent out on port 2, an input signal on port 2 is sent out on port 3, and an input signal on port 3 is sent out on port 1. circulars are useful to construct optical add/drop elements.,9,3.2 Isolators and Circulators,Circulators: (a) three-port, an

11、d (b) four-port.,10,3.3 Multiplexers and Filters,Optical filters are essential components in transmission systems for at least two applications: to multiplex and demultiplex wavelengths in WD system these devices are called multiplexers/demultiplexers and to provide equalization of the gain and filt

12、ering of noise in optical amplifiers.,11,3.3 Multiplexers and Filters,Different applications for optical filters in optical networks. A simple filter. (b) A multiplexer or a demultiplexer. (c) A wavelength router.,12,3.3 Multiplexers and Filters,A variety of optical filtering technologies are availa

13、ble. Their key characteristics for use in systems are the following:,1. Low insertion losses.,2. The loss should be independent of the state of polarization of the input signals.,3. The passband of a filter should be insensitive to variations in ambient temperature.,4. To ensure reasonably broad pas

14、sbands at the end of the cascade, the individual filters should have very flat passbands.,5. At the same time, the passband skirts should be sharp to reduce the amount of energy passed through from adjacent channel.,13,3.3 Multiplexers and Filters,A static wavelength router.,14,3.3 Multiplexers and

15、Filters,Characterization of same important spectral-shape parameters of optical filters. 0 is the center wavelength of the filter, and denotes the wavelength of the light signal.,15,3.3.1 Gratings,Gratings have been widely used for decades in optics to separate light into its constituent wavelengths

16、. In WDM communication systems, gratings are used as demultiplexers to separate the individual wavelengths.,The term grating is used to describe almost any device whose operation involves interference among multiple optical signals originating from the same source but with different relative phase s

17、hifts.,16,3.3.1 Gratings,(a) A transmission grating and (b) a reflection grating.,17,3.3.1 Gratings,Multiple narrow slits are spaced equally apart on a plane, called the grating plane. The spacing between two adjacent slits is called the pitch of the grating. Light incident from a source on one side of the grating is transmitted through these slits. Since each slit is narrow, by the phenomenon know as diffraction, the light transmitted through each slit spreads out in all directions. Thus each slit acts as a secondary source of light.,18,3.3 Multiplexers and Filters,Principle of operation of