Introduction to WDM and Investigation of Wavy Characteristic

Introduction to WDM and Investigation of Wavy characteristic

Optical Communication Team #1
9931207 Dae-Hun Yang, 0041143 Woo-Sung Kim, 0040277 Jung-Youn Han

Abstract-At the opening of 1970's, many people gave attention to the light communication. Because if we use light to communicate, we would reduce the effect of channel because of low loss through light fibers. And technologies of the light communication have been improved. Now, the technology is greatly developed by introduction of Wavelength Division Multiplexing.

The light communication consists of receiver, channel, transmitter. There are many issues for each parts. However, we are going to talk about WDM because not only our professor is teaching about wave and field of electromagnetic, but also WDM is one of the most important and the latest technology in the light communication.

1. Introduction to Optical Communication

Optical communication is the method that uses light. Light has wavy character proved by Young's experiment[1]. For that reason, optical communication could transmit signal to the receiver. Basic concept of optical communication is explained below.

The innermost region of the fiber, or core, was used to transmit the light, while the glass coating, or cladding, prevented the light from leaking out of the core by reflecting the light within the boundaries of the core. This concept is explained by Snell’s Law which states that the angle at which light is reflected is dependent on the refractive indices of the two materials[2]— in this case, the core and the cladding. The lower refractive index of the cladding (with respect to the core) causes the light to be angled back into the core as illustrated in Fig 1-1.

2. What's the WDM

The fiber optic industry first deployed single wavelength transmission links. As requirements changed, the industry responded with wavelength-division multiplexing (WDM), which sends two distinct signals per fiber, doubling transmission capacity. Two important considerations in a WDM device are crosstalk[3] and channel separation. WDMs allow multiple independent data streams to be sent over one fiber. The most common WDM system uses two wavelengths, although four or more-wavelength systems are available.

Fig 2-1 illustrates two WDMs permitting two streams of data to be carried on a single fiber. The type of data does not matter.

WDM enables the utilization of a significant portion of the available fiber bandwidth by allowing many independent signals to be transmitted simultaneously on one fiber, with each signal located at a different wavelength. Routing and detection of these signals can be accomplished independently, with the wavelength determining the communication path by acting as the signature address of the origin, destination or routing. Components are therefore required that are wavelength selective, allowing for the transmission, recovery, or routing of specific wavelengths.

2-1 Advanced Devices of WDM

- CWDM(Coarse Wavelength-division Multiplexing)

The development of CWDM, an intermediate technology, CWDM allows a modest(several) number of channels, typically eight or less. Whereas DWDM systems use channel spacing as close to 0.4 nm, CWDM uses a spacing of 20 nm.

- DWDM(Dense Wavelength-division Multiplexing)

DWDM is a developed technology than CWDM. DWDM allows over 18 channels and uses very narrow channel spacing(0.4nm). It means DWDM allows the transmission of e-mail, video, multimedia, data, and voice—carried in Internet protocol (IP), asynchronous transfer mode (ATM), and synchronous optical network/synchronous digital hierarchy (SONET/SDH), respectively, over the optical layer.

- WWDM(Wide Wavelength Division Multiplexing)

WWDM was originally defined as a LAN WDM operating in the 1310nm window, while early CWDM was also described as a LAN technology, but operational in the 850nm window.

2-2 Basic Operation of WDM and Transmission line Character of Optic Fiber

In a simple WDM system shown in Fig 2-2, each laser must emit light at a different wavelength, with all the lasers’ light multiplexed together onto a single optical fiber. After being transmitted through a high-bandwidth optical fiber, the combined optical signals must be demultiplexed at the receiving end by distributing the total optical power to each output port and then requiring that each receiver selectively recover only one wavelength by using a tunable optical filter. Each laser is modulated at a given speed, and the total aggregate capacity being transmitted along the high-bandwidth fiber is the sum total of the bit rates of the individual lasers. An example of the system capacity enhancement is the situation in which ten 2.5 Gbps signals can be transmitted on one fiber, producing a system capacity of 25 Gbps. This wavelength-parallelism circumvents the problem of typical optoelectronic devices, which do not have bandwidths exceeding a few GHz unless they are exotic and expensive. The speed requirements for the individual optoelectronic components are, therefore, relaxed, even though a significant amount of total fiber bandwidth is still being utilized.

The concept of wavelength demultiplexing using an optical filter is illustrated in Fig 2-3. In the figure, four channels are input to an optical filter that has a nonideal transmission filtering function. The filter transmission peak is centered over the desired channel, in this case, l3, thereby transmitting that channel and blocking all other channels. Because of the nonideal filter transmission function, some optical energy of the neighboring channels leaks through the filter, causing interchannel, interwavelength cross-talk. This cross-talk has the effect of reducing the selected signal’s contrast ratio and can be minimized by increasing the spectral separation between channels.

If the transmitted signals have various frequency, the optical fiber should be changed transmission line characteristic impedance because optical fiber is a transmission line for the light. Transmission characteristic impedance value is governed by electrical components such that resistance, conductance, capacitance, inductance. WDM is to transmit several data types. For example, voice signal is on 4Hz~20kHz and video signal is on the several tens of mega Hz. So, if there is a lot of loss on the fiber, we can't send accurate data to the terminal. Because not-fixed frequency causes transmission line loss. And the loss will make a thermal noise. Thermal noise[4] can be considered AWGN[5] and loss may make distortion. For that reason, fiber which we are going to use don't have to include R and G factor. But all kind of materials have their own loss factor. On that account, we should think about noise effect a little.

WDM method is similar to FDM[6]. If the velocity of wave is constant, one over wavelength is equal to frequency. So, we can employ M-ary orthogonal signaling. Each type of signals that is carried orthogonal basis don't interfere each other. After that, through the consideration of noise effect and distortion, we are able to reach the goal that is to transmit correct data.

First, each signal passed by MUX has their own power. The unidirectional isolation is a measure of the part of the optical power at each wavelength exiting from the port at wavelengths different from the nominal wavelength relative to the power at the nominal wavelength. It is given by the following formula:

IU = aiox – aioc

The terms aiox and aioc are elements of the logarithmic transfer matrix[7], where i is the input port number, o is the output port number, c is the (channel) wavelength number associated with port o and x is the isolation wavelength number, where x is any wavelength number not equal to c. In each output port o there is one channel wavelength lc and k1 isolation wavelengths lx. This is illustrated in Fig 2-4. lcis used in this concept to denote channel wavelength and not fibre cut-off wavelength.

Fig 2-5 illustrates an example using the transfer matrix, if powers P1, P2, P3, …Pk were launched into a WDM DMUX device at wavelengths 1, 2, 3, …k respectively, then the signals emerging from port x would be: t1x1P1, t1x2P2, t1x3P3, … t1xkPk. So the isolation of port 2 to wavelength 3 would be a123a121. In the fiber the variance value is fixed because noise variance value is governed by channel's own material factor. So by the concept of power of signal passed MUX, SNR could be controlled. By using SNR factor, we can decide received signal error probability9. As you see below figure, the WDM is very row error performance method because in the fiber noise[8]variance is small.

Second, if there is a loss on the transmission line, transmitted signal could be changed their own shape. It is a distortion. If we know about the information of filter and power loss when signal is transmitted. The filter is made for the wanted purpose. So the only factor we should consider is power loss in the transmission line. It is the reduction in optical power between an input and output port of a WDM device in decibels (dB). It is defined as:

IL = -10log(Pout/ Pin)

where, Pin is the optical power launched into the input port and Pout the optical power received from the output port.

For a WDM device, it is an element aiow of the n*n*k element logarithmic transfer matrix. Here i is the input port number, o is the output port number and w is the wavelength number associated with port i or o, n is the total number of input + output ports and kis the total number of wavelengths of the logarithmic transfer matrix. For WWDM devices, it shall be specified as a maximum value and a minimum value at each operating wavelength range. For DWDM and CWDM devices, it shall be specified as a maximum value and a minimum value within the wavelength range as illustrated in Fig 2-6. Obtained power loss and known information about filter will help to equalize. Because each type of WDM method has different power loss, as shown in Fig 2-6, it should be applied to accurate value and equalizer.

Recent research interests in WDM networks include network control and management, faultmanagement, multicasting, physical-layer issues, IP over WDM, traffic grooming, and opticalpacket switching, just to name a few.

3. Conclusion

By investigation we’ve done, WDM is a good solution to improve potical communication. Especially, WDM is the most efficient method in the ultra-frequency and multi-communication field. If we try to improve these forms, without distribution optical infrastructure we can achieve the goal to transmit variety data types and high quality data. After that, this technology will be the best solution in communication field.

4. References

1)Book references

Physics for Scientist and Engineers-Fishbane, Gasiorowicz, Thornton. Chapter 36 [Wavy Character of Light]

Signal & System Openheim Chapter 8.[FDM, Frequency of Data types]

Cheng Chapter 9-[Transmission line]

Digital Communication-Proakis Chapter 4. Chapter 5[M-ary orthogonal signaling, Error performance]

Introduction to Digital Communication-Bernard Sklar Chapter 3[Equalizing]

fiber-optics communication third Gorvind P. AGRAWAL Ch.8, Ch.1

2)Internet references

5. Figures

1

Fig 1-1 Basic concept of Optical Communication

Fig2-1 WDM Application

Fig 2-2 Basic Operation

Fig 2-3 The concept of wavelength demultiplexing

Fig 2-4 Illustration of unidirectional (far-end) isolation of a WDM device

Fig 2-5Example of WDM demultiplexer device

Fig 2-6 Illustration of maximum and minimum insertion loss of a WDM device

1

[1] "Double Slit" experiment, demonstrating that light can be diffracted, and proving that light has wave properties.

[2],

[3]Crosstalk : 1. Undesired coupling from one circuit, part of a circuit, or channel to another. 2. Any phenomenon by which a signal transmitted on one circuit or channel of a transmission system creates and undesired effect in another circuit or channel.

[4]Thermal noise is the noise generated by thermal agitation of electrons in a conductor. The noise power, P , in watts, is given by P = 4kT Δf , where k is Boltzmann's constant in joules per kelvin, T is the conductor temperature in kelvins, and Δf is the bandwidth in hertz.

[5]Additive White Gaussian Noise. White noise has equal power per hertz over the specified frequency band

[6]FDM(FREQUENCY-DIVISION MULTIPLEXING)

[7]A general logarithmic transfer matrix(for G.671:ITU-T Standard) is shown below

9

where aio is the optical power reduction in decibels out of port o with unit power into port i, i.e.:

aio = 10 log (tio)

where tio is the transfer matrix coefficient. Similarly, for the off state, aoio=10 log(toio).

[8]Noise generated in an optical fiber system combination of mode-dependent optical losses and fluctuation in the distribution of optical energy among the guided modes or in the relative phases of the guided modes