专业英语(光电子)-03-1-Components.ppt

*,Optical Communication Technology,And Optical Networking,Yanhe Li,Dept.of Electronic Engineering,Tsinghua University,2004.9,Chapter 3,1,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,3.8 Wavelength Converters,2,3.1 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.,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 purposes 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 and multiplexers/demultiplexers.,4,3.2 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.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,which would otherwise degrade their performance.,The two key parameters:insertion loss,isolation,is around 1 dB.,is around 40 to 50 dB.,5,3.2 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 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.,6,3.2 Isolators and Circulators,Principle of operation of an isolator that works only for a,particular state of polarization of the input signal,7,3.2 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.In 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.,8,3.2 Isolators and Circulators,Circulators:(a)three-port,and(b)four-port.,9,3.3 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 filtering of noise in optical,amplifiers.,10,3.3 Multiplexers and Filters,Different applications for,optical filters in optical,networks.,A simple filter.,(b)A multiplexer or a,demultiplexer,.,(c)A wavelength router.,11,3.3 Multiplexers and Filters,A variety of optical filtering technologies are available.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 passbands 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.,12,3.3 Multiplexers and Filters,A static wavelength router.,13,3.3 Multiplexers and 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.,14,3.3.1 Gratings,Gratings have been widely used for decades in optics to separate,light into its constituent wavelengths.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 shifts.,15,3.3.1 Gratings,(a)A transmission grating and(b)a reflection grating.,16,3.3.1 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.,17,3.3 Multiplexers and Filters,Principle of operation of a,transmission grating.The,reflection grating works in,an analogous manner.The,path length difference,between rays diffracted at,angle,d,from adjacent slits,is,=a sin(,i,)sin(,d,).,AB -CD,18,3.3.1 Gratings,The path length difference between the rays traversing through,adjacent slits is the difference in lengths between the line segments,AB and CD,and is given approximately by asin(,i,)sin(,d,).,Thus constructive interference at a wavelength,occurs at the,imaging plane among the rays diffracted at the angle,d if the,following grating equation is satisfied:,asin(,i,)sin(,d,)=,m,.,for some integer,m,called the order of the grating.,19,