Sending and receiving of messages from one place | U) another has always been a matter of great importance. So we should select the proper ! communication systems. Today optical fiber communication is widely used because of its advantages.
An important issue in the field of communication is the effective utilization of the capacity of a communication channel. Inorderto increase the capacity we use multiplexing techniques. WDM is a technique where modulated light from several sources of distinct wavelengths are required to the transmitted simultaneously over a single fiber. Wavelength selective optical multiplexes and demultiplexes are used at the start and end of the transmission route.
CWDM uses a special property of fiber optics. This property allows the combination of multiple signals into a single fiber. Each signal is assigned to a different wavelength of light. It provides an easy way to increase the Utilization of the high channel capacity of the optical fiber. It can be applicable in telecommunication areas.
TITLE PAGE NO:
MULTIPLEXING TECHNIQUES 3
Frequency division multipIexing(FDM)
Time division multiplexing (TDM) 4
Wavelength division multiplexing (WDM)
Comparison between FDM & WDM 5
Dense wavelength division multiplexing (DWDM)
Advantages of DWDM 9 j
Disad\outages of DWDM
Coarse wavelength division multiplexing^ WDM) 10
CWDM CIRCUIT COMPONENTS 12
AMPLIFICATION OF OPTICAL SIGNALS 14
Erbium doped fiber amplifiers Advantages of EDFAs
Optoelectronic amplifiers 15
ADVANTAGES OF CWDM 16
LIST 01 i i(aim
NAME OF FIGURE PAGE NO:
PROPAGATION OF FDM 6
PROPAGATION OF WDM 6
CHANNEL SPACING IN DWDM 8
PROPAGATION OF CWDM 11
In the modern communication systems, we use the optical fiber as the medium because it has many advantages. The security is the main advantage of optical fiber communication. The loss occurred in this communication is very less than that in ordinary copper wire transmission.
Inorder to transmit the information or messages^ we have to multiplex them. In optical communication system we commonly use the wavelength division multiplexing. Wavelength division multiplexing is a technique which uses a special property of liber optics. This property allows the combination of multiple signals onto a single strand of liber. Each signal is assigned to a different wavelength of light . Since one wavelength does not aiTect another wavelength, the signals do not interfere.
Coarse wavelength division multiplexing (CWDM) performs two functions. First, they filter the light, ensuring only the desired wavelengths are used. Second, they multiplex or demultiplex multiple wavelengths, which are used on a single fiber link. The difference lies in the wavelengths, which are used.
| Multiplexing is a technique of combining multiple
| channels over a transmission path. The multiplexer is a special combinational circuit .In combinational circuits, the outputs at any instant of time depend upon the inputs present at that instant of time. This means that there is no memory in these circuits. The multiplexer is also known as data selector. An important issue in the field of information- transmission is i the effective utilization of a communication channel under the requirements of I dynamically increasing traffic. In practice , the multiplexing of many low bandwidth or low 1 bit rate signals into high capacity traffic trunks do this. In digital communication this is done electronically by a hierarchical multiplexing. The single mode fiber has a capacity that often exceeds the information rate generated by conventional electronic multiplexing schemes. Some novel multiplexing schemes are therefore required to influence the capacity utilization of the single mode fiber. In electronic multiplexing , techniques such as frequency division multiplexing (FDM) and time division multiplexing (TDM) are well known. The : multiplexing procedures unique to optics are the wavelength division multiplexing(WDM) ; and optical frequency division multiplexing (OFDM).
1 Frequency division multiplexing (FDM)
2 Time division multiplexing (TDM)
3 Wavelength division multiplexing (WDM)
4 Dense wavelength division multiplexing (DWDM) Coarse wavelength division multiplexing (CWDM)
Frequency Division Multiplexing (FDM)
Frequency division multiplexing assigns different carrier frequencies . Cable television uses FDM. In this technique , the carrier bandwidth is divided into sub-channels of different frequency widths , each carrying a signal at the same time in parallel. FDM is used at radio frequencies . With FDM , information signals with the same bandwidth from multiple sources modulate different frequencies. FDM signals propagate at the same time , through the same medium and follow the same transmission path. In FDM , each frequency having its own [modulating circuit and its own modulation rate.
Time Division Multiplexing (TDM)
The multiple signals are carried over the same channel in alternating time slots. TDM is widely used in digital transmission. In TDM each node sends synchronized data either bit synchronized or block based so that collisions are automatically avoided. The method is impractical in a fiber optic set up because it requires all nodes to be bit-synchronized.
3.3 Wavelength Division Multiplexing (WDM)
In WDM , multiple signals are carried together as separate wavelengths of light in a multiplexed signal. WDM is used in optical Fiber networks. It uses the same principle of FDM but applying to wavelength of light in optical fiber. WDM resembles FDM in that idea is to send information signals that occupy the same band of frequencies through the same fiber at the same time without interfering with each other. This is accomplished by modulating injection laser diodes that are transmitting highly concentrated light waves at different wavelengths. Therefore WDM is coupling light at two or more discrete wavelengths into and out of an optical fiber. Each wavelength is capable of carrying large amounts of information in either analog or digital form and the information can already be time or frequency division multiplexed. Although the information used with lasers is almost always time division multiplexed digital signals, the wavelength separation used with WDM is analogous to analog radio channel frequencies.
Comparison between FDM & WDM
Although frequency and wavelength division
I multiplexing share similar principles, they are not the same. The most obvious difference is ; that optical frequencies are much higher than radio frequencies. Probably the most difference however, is in the way the two signals propagate through their respective transmission media. :,With FDM, information signals with the same bandwidth from multiple sources modulate ! different frequencies, with each frequency having its own modulation rate. FDM signals | propagate at the same time, through the same medium and follow the same transmission path.
The basic principle of WDM, is somewhat different.
Different wavelengths in a light pulse travel through an optical fiber at different speeds. In standard optical liber communication systems, as the light propagates down the cable, wavelength dispersion causes the light waves to spread out and distribute their energy over a longer period of time. Thus , in standard optical fiber systems, wavelength dispersion creates problems which impose limitations on the system's performance. With WDM, however wavelength dispersion is the essence of how the system operates. With WDM, information signals from multiple sources that occupy the same bandwidth modulate lasers operating at different wavelengths. Hence , the signals enter the fiber at the same time and travel through the same medium. However , they do not take the same path down the fiber. Since each wavelength lakes a different transmission path . they each arrive at the receiver at a slightly different time. Thus , data can be encoded sequentially onto the WDM channel using a single data modulator. The result is a series of rainbows of different colors simultaneously propagating down the cable. The figure below illustrates the basic principles of FDM and WDM signals propagating through their respective transmission .media.
Channel 4 bandwidth
Channel 3 bandwidth
Channel 2 bandwidth
Channel 1 bandwidth
Fig: 3.3a Propagation of FDM
Wavelength 2 out
Wavelength 1 out
Fig:3.3b Propagation of WDM
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3.4 Dense Wavelength Division Multiplexing (DWDM)
A more recent innovation in optical multiplexing is the DWDM, which, multiplexes and demultiplexes wavelengths in the same window. It is widely used in telecommunication applications to increase the capacity. In this technique the different wavelengths are separated by less than lnm. WDM is generally accomplished at approximate wavelengths of 1550nm with successive frequencies spaced in multiples of 100C.il lz. At 1550nm and 100GHz frequency spacing , the wavelength separation is approximately 0.8nm. Using a multiplexing technique called DWDM , the spacing between adjacent frequencies is considerably less. So WDM systems carrying multiple optical signals in the vicinity of 1550nm with less than 200GHz are considered DWDM. Obviously , the more wavelengths used in a WDM system . the closer they are to each other and the more dense the wavelength spectrum.
Figure (3.4) shows the channel separation in DWDM systems. Ideally the port should transmit all the light at the centre of the optical channel, and no light outside. As usual , reality is a compromise. Normally the peak transmission at the centre wavelength is not 100% with a typical loss of 3 to 5dB. Normally the channels are equally spaced , so the points where the curves intersect match the widths of the channels or equivalently the channel separation. In the figure , this is 100Hz for the tightly packed DWDM channels and 1000Hz for the loosely packed channels.
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Fig : (3.4) Channel spacing in DWDM
The points where the curves overlap is where crosstalk can occur . The amount of crosstalk also depends on the range of wavelengths emitted by the transmitter on each channel. The range of wavelengths emitted by the transmitter depends on the modulation scheme as well as on the light source itself.
3.5 Coarse Wavelength Division Multiplexing (CWDM)
Inorder to overcome the limitations of DWDM , we use CWDM. Here we can use four to eight wavelengths per fiber , sometimes more. It is commonly used in metropolitan and regional areas. The spacing between wavelengths in CWDM is about 10 to 20nm. In CWDM technique , multiple optical signal channels are carried across a single strand of fiber at different wavelengths of light. These channels are also called lambda circuits, Think of each wavelength as a different color of light in the
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The figure (3.5) shows a simple schematic of a CWDM system. Optical signals at eight different wavelengths from separate transmitters arc combined in a multiplexer at the left . The signals travel through the same fiber from the multiplexer to an add / drop multiplexer in the middle , which routes one wavelength, to a point at the bottom and picks up another signal at the same wavelength to show it is a different signal.
At the right side , the eight signals are split in a demultiplexers and routed to separate receivers ,one for each wavelength. The multiplexer i takes separate wavelengths and combines them , and the demultiplexer takes combined wavelengths and separates them. Key operating considerations differ between the two. Multiplexer should have low insertion loss and avoid scattering light back to any of the transmitter. Demultiplexer must reliably separate the optical channels , with low leakage of light from one optical channel into adjacent channels.
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Two important considerations in a CWDM device are crosstalk and channel separation. Both are of concern mainly in the receiving or demultiplexing end of the system. Crosstalk or directivity refers to how well the demultiplexed channels are separated. Each channel should appear only at its intended port and not at any other port. The crosstalk specification expresses how well a coupler maintains this port to port separations. A crosstalk of 20dB means that 1% of the signal appears at the unintended port.
Channel separation describes how well a coupler can distinguish wavelengths. In most couplers . the wavelengths must be widely separated. The limited number of channels for CWDM form the spectral requirements of a multi channel CWDM device. LED sources have wide spectral widths , which require each channel to be separated widely from the others. Constructing a CWDM device with many channels requires a narrow band source. Lasers , which have sufficiently narrow spectral widths allow many channels in the same optical window. CWDM allows the potential information carrying capacity of an optical fiber to be increased significantly.
A fiber cable guides light from end to end. A signal is injected in one end by an LED or by semiconductor lasers. Lasers for silica-based fiber optic cables produce light in a range called " window". These windows occupy the near infrared range.
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4 CWDM CIRCUIT COMPONENTS
The circuit components used with CWDM are similar to those used with conventional radio wave and metallic wire transmission systems.They Are given below.
1. Wavelength division MUX & DEMUX
2. Wavelength division add / drop MUX / DEMUX
3. Wavelength division routers
4. Wavelength division couplers
i 1. Wavelength division MUX & DEMUX
Multiplexers or combiners mix or combine optical signals with different wavelength in a way that allows them to all pass through a single optical fiber without interfering with one another. Demultiplexers or splitters separate signals with different wavelengths in a manner similar to the way filters separate electrical signals of
! different frequencies. Wavelength demultiplexes have as many outputs as there are
! wavelengths, with each output going to a different destination. Multiplexes and ; demultiplexes are at the terminal ends of optical fiber communication systems.
2 Wavelength division add / drop MUX / DEMUX
Add / drop multiplexers and Demultiplexers are similar to regular multiplexers and demultiplexers except they are located at intermediate points in the system. Add / drop multiplexers and Demultiplexers are devices that separate a wavelength from a fiber cable and reroute it on a different fiber going in a different direction. Once a wavelength has been removed, it can be replaced with a new signal at the same wavelength. In essence, add / drop multiplexers and demultiplexers are used to reconfigure optical fiber cables.
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Wavelength division routers
CWDM routers direct signals of a particular wavelength to a
specific destination while not separating all the wavelengths present on the cable. Thus, a router can be used to direct or redirect a particular wavelength in a direction from that followed by the other wavelengths on the fiber.
Wavelength division couplers
CWDM couplers enable more efficient utilization of the
transmission capabilities of optical fibers by permitting different wavelengths to be combined and separated. There are three basic types of WDM couplers: diffraction grating , prism and diehroic filter. With diffraction gratings or prisms, specific wavelengths are separated from the other optic signal by reflecting them at different angles. Once a wavelength has been separated, it can be coupled into a different fiber. A diehroic fiber is a mirror with a surface that has been coated with a material that permits light of only one wavelength to pass through while reflecting all other wavelengths. Therefore, the diehroic filter can allow two wavelengths to be coupled in different optical fibers.
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5.2 Optoelectronic amplifiers
Prior to the development of EDFAs , optoelectronic amplifiers were used to boost optical signals.This process is often called "3R" regeneration , referring to reamplify , regenerate and retime. Weak incoming light is converted to a voltage signal. amplified , and then converted back to light. This is impractical in high speed networks.
6 ADVANTAGES OF CWDM
1. Enhanced capacity
2. Optical components used in this are simpler
3. More reliable
4. Less cost
5. It requires only one data modulator
6. Insensitive to optical reflection
7. High temperature operation
8. Wide operating temperature ranges
9. High coupling efficiency
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The development of CWDM
technology , responded to the growing fiber network demand. With a capacity greater than WDM and smaller than DWDM , CWDM allows a modest number of channels, typically eight or less, to be stacked in the 1550nm region of the liber called the C-BAND. To dramatically reduce the cost , CWDMs use uncooled lasers with a relaxed tolerance of 3nm. CWDM transmission may occur at one of eight wavelengths : typically 1470nm. 1490nm ,1510nm ,1530nm ,1550nm ,1570nm. 1590nm, 1610nm.
and DWDM technologies arc the
foundation of ultrahigh aggregate bandwidth data transmission in metro applications. As the required capacity of metropolitan and network systems increases uncooled CWDM systems are becoming more popular.
4. Electronic communication systems by WAYNE TOMASI