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Laboratory exercise TFE4165 Applied PhotonicsVersion: September 6, 2007

Dispersion and Simulation of Fiber Optic Link

Room: A478 (Koherentlab)Supervisor: ystein Mary, room A479, oystein.maroy@iet.ntnu.no

Motivation

The objective of this exercise is to study how chromatic dispersion affects theperformance of an optical high-speed network. We will use the simulation toolOptSim to calculate common performance parameters in a basic fiber optic link. Thus,we will also learn how to characterize a transmission system.

Before the Lab

Read through this text and get an overview of the exercise. A short introduction toOptSim will be given in the lab.

Introduction

In an optical communication system we use the intensity of the light to represent thedigital data bits 1 and 0. Normally, light on means 1 and light off is 0. The datarateor bit rateis the speed at which these bits are transmitted. Clearly, the temporal

width of the light pulses we use to represent the bits must not exceed the bit timeinterval.

If we look at the wavelength spectrum of an optical pulse coming from a laser source(Fig. 1), we see that the laser not only emits light at one wavelength but emits a smallcontinuous spectrum around a center wavelength !0. Unfortunately, the differentspectral components propagate through the fiber at different velocities due to awavelength dependent refractive index. Thus, the pulse will spread in time. This iswhat we call chromatic dispersion. After propagating a certain length, the pulses willexceed the bit time interval and we get intersymbol interference, as illustrated in Fig.2.

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Dispersion

The dispersion coefficientD(!) is a parameter describing the relation between theinitial spectral width "wof the pulse and the temporal pulse width "tat the fiber output

due to chromatic dispersion. Its unit is ps/(nm-km). This parameter tells us how manypicoseconds the pulse broadens per kilometer of fiber per nanometer of pulse spectral

Fig. 1: Wavelength spectrum of a laser pulse. !0is the center wavelength. The spectralbandwidth "wis often denoted the full pulse width at half maximum value.

Fig. 2: Two pulses before and after propagation through fiber. The pulses are spread in time and willeventually overlap (intersymbol interference).

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width [1]:

(1)

Lis the length of the fiber. An empirical formula used to estimate D(!) is [2]:

(2)

!0is the reference wavelength, i.e. the wavelength where the dispersion coefficient is0, which is ~1312 nm for a standard single-mode fiber. S0 is the dispersion slope,which tells us how much the dispersion changes with wavelength. These parametersare available from fiber specification sheets. OptSim uses Equation (2) to calculatechromatic dispersion.

At common communication wavelengths around 1550 nm, the dispersion coefficientis positive, which means that shorter-wavelength components travel faster than thelonger components. However, it is possible to produce fibers with customizedcoefficient values by altering the material composition and waveguide design.

The Fiber Optic Link

We will consider a basic fiber optic link with a 1550 nm laser source, a standardsingle-mode fiber and a receiver, as shown in Fig. 3. There are also two attenuatorscalled Normalizer and Attenuator for adjusting the launch power and the power

onto the receiver. Two monitors display the accumulated dispersion in the fiber andthe power at the receiver.

APRBSgenerator is a common tool for characterizing network performance. PRBS isshort for Pseudo-Random Bit Sequence and is a sequence of 2N-1 data bits with a

Fig. 3: Basic fiber optic link with test and monitoring components.

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random pattern of 1s and 0s. This sequence is transmitted repeatedly through thesystem and a bit-error ratio (BER) tester at the receiver counts the number oferroneously detected bits. The bit-error ratio is then the probability of detecting a 1when a 0 is transmitted, and vice versa.

OptSim does not count the number of errors, as one would do when measuring BERin a lab, but estimates the BER from the eye diagram in order to decrease thesimulation time. We will not delve into the details here but refer the interested studentto the OptSim manual [3]

In Fig. 3 the logical PRBS signal is transferred to an electrical component thatcontrols a direct modulated laser. Thus, the data signal is converted into an opticalsignal where no light means logical 0 and presence of light means 1. The BER testerat the receiver end is synchronized to the PRBS generator.

A receiver needs a minimum optical input power in order to keep BER below a givenmaximum value [4]. By coupling the light from the laser directly into the receiver(except for a variable attenuator between the laser and the receiver), we can measureBER versus power. This is a back-to-back measurement, which is the reference towhich we compare all other measurements. The leftmost graph in Fig. 4 shows a

back-to-back measurement. Inserting fibers, amplifiers and other components into thetransmission path will introduce impairments like noise and dispersion, and we needto increase the power in order to maintain the same BER. The value of this power

increase is termed the power penalty and is often defined at BER=10-9

. The powerrequired to achieve BER=10-9is called thesensitivity. The second plot in Fig. 4 is the

Fig. 4: BER curves used to characterize an optical link. The leftmost line is aback-to-back measurement while the rightmost line shows the BER for the total

system

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BER of the complete system, and in this case there is a power penalty of 3 dB.

The last monitor in the system, Eye_Diagram, is an oscilloscope that displays asuperposition of many bits. This produces an eye like diagram (see Fig. 5) in whichthe eye opening is a measure of the degradation of the link. An open eye with sharplines means good performance, while noise and intersymbol interference appears asspreading of the rails.

Exercises

We will in the following exercises study what impact chromatic dispersion has on anoptical transmission system with a bit rate of 10 Gbps per channel. A widely deployed

bit rate in commercial systems today is 2.5 Gbps, with 10 Gbps as the next step up.We will use a PRBS sequence withN=9. S0of a standard fiber is ~ 0.090 ps/(nm2 km).

A: Back-to-back

First of all we need to characterize our system without any components between thetransmitter and the receiver, i.e. a back-to-back measurement. We will calculate BERversus received power and use this to evaluate performance of our transmissionsystem later on.

1. Open the back2back.moml file and look at the different network components. Byright-clicking the blocks, one can access and set the properties of the componentsor view the simulation results. We will not go into details of each component here,

Fig. 5: Eye diagram as calculated by OptSim. The top line is the signallevel of 1s, while the bottom line is the signal level of 0s.

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so most of the properties have already been set. There is a 1 m long fiber betweenthe transmitter and the receiver.

2. Run a simulation by pushing the F5 key and clicking OK in the dialog box thatopens. You can watch the status of the simulation in the lower left corner of theOptSim window.

3.

From the eye diagram (right-click Eye_Diagram and choose View Results) wecan see that there is a distinct difference between the zero level and the one level.The BER calculation (right-click BER-Tester, choose View Results anddouble-clik the name of the text file) shows error-free transmission, which is asexpected.

4.

Start the back-to-back simulation by pushing the F4 key and clicking OK in thedialog box. OptSim will now vary the attenuation of the attenuator in front of thereceiver and calculate the BER.

5. Use the values of Optical_Monitor and BER-Tester to plot BER as a functionof received power. What is the sensitivity of the receiver (i.e. the received powergiving BER = 10-9)?

B: Dispersion

1. Open the file called fiberdispersion.moml. We now have a 100 km long fiber asour transmission line.

2. Push F5 and run a simulation. Look at the eye diagram. We can clearly see theeffect of dispersion as the bits are starting to interfere with each other and close theeye. But still the BER is low.

3.

Push F4 and run the simulation. As before, use Optical_Monitor andBER_Tester to find BER as a function of received power. Plot BER in the samechart as the back-to-back graph in exercise A.

4.

What is the sensitivity of the system? What is the penalty?5.

What is the maximum transmission length, given that BER should be better than10

-9?

6. Change the bit rate to 2.5 Gbps (right-click PRBS_Generator, chooseProperties and under General change bitrate from 10e9 to 2.5e9.

7. Repeat step 3-5 and compare the results.

C: Dispersion compensation

1. Open the file called compensation.moml. A second fiber called DCF is added

to the link. This is a dispersion compensating fiber designed to have a large,negative dispersion coefficient. The length of the standard fiber is now 80 kmwhile the DC fiber is 20 km long.

2. Repeat steps 2 through 4 from exercise B. Compare the results. You can see howthe dispersion is accumulated and compensated by double-clickingDispersion_Monitor and then double-clicking the dispersion plot from the list.

3.

What is the maximum transmission length in this case?

Discussion

The bandwidth "wof the 10 Gbps data signal is approximately 0.07 nm. Find "tandcompare to the bit time interval. Are the simulation results OK?

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References

[1]

B. E. A. Saleh, M. C. Teich: Fundamentals of Photonics, John Wiley & Sons,

Inc., 1991.[2] J. M. Senior: Optical Fiber Communications Principles and Practice, PrenticeHall, second edition, 1992.

[3]

OptSim 4.0 Models Reference Volume II Block Mode, RSoft Design Group,2004.

[4]

G. P. Agrawal: Fiber-Optic Communication Systems, John Wiley & Sons, Inc.,second edition, 1997.