Setup

A loop experiment attempts to simulate the transmission performance of a long system by reusing or recirculating optical data signal through a modest length amplifier chain ranging from tens to hundreds of kilometers. [12]

Fig. 5.10 shows the setup of our circulating loop experiment. The eight wavelengths of DFB lasers conform to the ITU channels from 1550.92nm to 1555.75nm with 0.8nm channel spacing.

Figure 5.10 The setup of the circulating loop experiment.

There are eight polarization controllers after the DFB lasers to control the polarization of going to the EO modulator. Then the eight channels are coupled into one path by an 8x1 coupler. After the coupler the signals go through the boost EDFA for compensate the loss of coupler. The EO modulator is used for modulating the continuous wave signals into the 10Gbit/s NRZ signals. The signals are coded the 2³¹− pseudo random binary 1 sequence data patterns. The pulse pattern generator provides the gigabit bit pattern that drives the EO modulator. The data output of the pulse pattern generator must be a high quality eye diagram, that means fast rise and fall time, low distortion, low jitter, and high Q factor. The Anritsu MP1763C has a rise and fall time less than 30ps, less than 10% distortion, less than 20ps peak to peak crossover jitter, and a Q factor larger than 40dB. After the transmitter the signals go through the variable optical attenuator (VOA) to make sure that the signal power before each AO switch and EDFA is the same. In other words, the signals come from the eighth EDFA must the same as which come from transmitter before each AO switch. By adjusting attenuators the loop gain is set to unity. This allows the data to recirculate without loss. The data generator provides synchronizing signals to the transmitter switch, loop switch, and error detector. The AO switches can control when the signals go in the loop and how many round-trips they circulate by the data generator. A 3dB coupler allows for data patterns to be loaded in and also lets them exit the loop after each round-trip. The switching in and out of the data trains needs to be synchronized both with the loop time and the bit error rate test set. The bit error rate after transmission of varying distances can be measured by using the data patterns exiting the loop after the desired number of round-trips, so that any transmission degradation with distance can be observed.

Our loop transmission part consists of six EDFAs (maximum output power=17dBm, fixed gain=22dB and noise figure=6.5dB) followed by 50km of LEAF fiber (D=4.1639ps/nm/km, and loss=0.2dB/km at 1553.33nm) and two DCFs in the appropriate position. Limits to the

optical effects (4.1) and the noise floor respectively.

The LEAF fiber has large core diameter to reduce the intensity of light and so the nonlinear effect by lifting the threshold of maximum power. Since the fiber loss in one span is less then the gain of EDFA, it needs VOA in each span to attenuate the optical power. For reducing the optical power into the fiber to minimize the nonlinearity the VOA is put just after each EDFA.

The data signals emerging from the loop on the output side of the coupler pass through the appropriate optical bandpass filter and then go into the bit error rate test set.

5.5 Experiment Result

The line width of optical pulse will be broader after modulation. This nature of adding information on carrier source is shown in Fig. 5.11. The line width of DFB laser is very narrow and suitable for WDM system. The optical spectrum can give us the information of OSNR. In the loop the optical power of each channel should be kept constant and the noise floor should be as low as possible. We can see the noise increases as signals propagating as shown in Fig. 5.12, 5.13, and thus the OSNR decreases with distance as shown in Fig. 5.14, 5.15. The channels are closed to the gain peak wavelength, thus the noise floor tilt in long wavelength. Fig. 5.14 shows the worst OSNR after 3000km is still larger than 22dB.

Figure 5.11 The spectrum of channel 1.

Figure 5.12 The optical modulated spectrum of loop 1 and 5.

Figure 5.13 The optical modulated spectrum of loop 8 and 10.

Figure 5.14 The OSNR of eight channels after propagating each loop.

We also measure the BER after one round-trip time. Fig. 5.16 shows the power penalty is about 1dB. We think the reason is about the noise figure of EDFAs. First, the original noise figure of EDFA is too large. Second, for the requirement of circulating loop experiment we close the mechanism of auto-shutdown of EDFA but it will introduces more amplified spontaneous noise because of the always on pump power. The mechanism of auto-shutdown is when there is not input power the EDFA will shutdown. The transmission distance causes the six EDFAs power up and shutdown asynchronous and thus signals can’t propagate correctly.

The best eye diagrams measured are shown in Fig. 5.17. They show the noise is too serious to degrade the quality of signals.

Figure 5.16 BER of back to back and 300km.

300km 600km

900km 1200km

1500km 1800km

2100km

Figure 5.17 The eye diagram of 300, 600, 900, 1200, 1500, 1800 and 2100km.

Chapter 6 Conclusion

We have accomplished the experimental setup of a circulating loop. By controlling the transmitter switch and loop switch we can allow the signals to circulate in the loop for designate times to simulate the long-haul transmission system. We use the WDM technology to increase the capacity in our experiment. The different wavelengths suffer from different dispersion. We use some DCFs to compensate the accumulated chromatic dispersion and the dispersion slope and yield good results. The SNR after 3000km can still large than 22dB and we believed that by fine tuning the gain and loss of each span and minimizing the amplified spontaneous noise we can get better SNR and thus get longer transmission length.

Bibliography

[1] C. R. Giles, E. Desurvire, J. R. Talman, J. R. Simpson, and P. C. Becker, ”2 Gb/s signal amplification at λ =1.53nm in an erbium-doped single-mode fiber amplifier,” J. Lightwave Technol., vol. 7, no. 4, Apr. 1989.

[2] N. Edagawa, S. Yamamoto, Y. Namihira, K. Mochizuki, and H. Wakabayashi, “First field demonstration of optical submarine cable system using LD-pumped Er-doped optical fiber amplifier,” Electron lett., vol. 25, no. 19, Sept. 1989.

[3] P. Trischitta, M. Colas, M. Green, G. Wuzniak, and J. Arena,”The TAT-12/13 cable network,” IEEE Commun. Mag., vol. 34, pp. 24-28, Feb. 1996.

[4] G. P. Agrawal, “Fiber-Optic Communication Systems,” 3^rd ed., John Wiley & Sons, New York, 2002.

[5]P. C. Becker and N. A. Olsson, “Erbium-Doped Fiber Amplifiers Fundamentals and Technology,” New Work: Academic, 1999.

[6]L. F. Mollenauer and K. Smith,” Demonstration of soliton transmission over more than 4000km in fiber with periodically compensated by Raman gain,” Opt, Lett. 13, no. 8, p. 675, 1998.

[7] P. M. Morse and H. Feshbach, “Methods of Theoretical Physics,” McGraw-Hill, New York, 1953, Chap. 9.

[8]G. P. Agrawal, “Nonlinear Fiber Optics,” 2^nd ed. New Work: Academic, 1995.

[9] Harry J. R. Dutton,” Understanding Optical Communications,” New York: Academic, 1988.

[10] A. Yariv and P. Yeh, “Optical Waves in Crystals,” John Wiley & Sons, New York, 1984.

[11] “Introduction to Acoustco-Optics,” Brimose.

[12] N. S. Bergano and C. R. Davidson,” Circulating Loop Transmission Experiments for the Study of Long-Haul Transmission Systems Using Erbium-Doped Fiber Amplifiers,” J.

Lightwave Tech., 13, 879, 1995.

[13] M. I. Hayee, A. E. Willner, “NRZ Versus RZ in 10-40-Gb/s Dispersion Managed WDM Transmission systems,” IEEE Photon. Tech. Lett., Vol. 11, No. 8, Aug. 1999.

[14]P. Kaiser and D. B. Kech,”Fiber types and their status,” in Optical Fiber Telecommunications II, New York: Academic, 1988, ch. 2

[15] N. S. Bergano and C. R. Davidson,” Wavelength Division Multiplexing in Long-Haul Transmission Systems,” J, Lightwave Tech, 14, 1299, 1996.

[16]P. F. Wysocki, J. R. Simpson and D. Lee, IEEE Photon. Tech. Lett., 6, 1098, 1994.

[17] P. Blondel, J. F. Marcerou, J. Auge, H. Fevrier, P. Bousselet, and A.

Dursin,“Erbium-doped fiber amplifier spectral behavior in transoceanic links,” in Optical Amplifiers and Their Applications, vol. 13, 1991 OSA Technical Digest Series(Optical Society of America, Washing ton, D.C.,1991),pp.82-85.

[18] J. P. Blondel, A. Pitel, and J. F. Marcerou,“ Gain-filtering stability in ultralong distance links,” in Conference on Optical Fiber communication, Vol. 4, 1993 OSA Technical Digest Series (Optical Society of America, Washington, D.C., 1993),pp. 38-39.

[19] B. M. Desthieux, M. Suyama, and T. Chikama, “Self-filtering characteristic of concatenated erbium-doped fiber amplifiers,” in Optical Amplifiers and Their Applications, Vol. 14, 1993 OSA Technical Digest Series(Optical Society of America, Washington, D.C., 1993),pp. 100-103.