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A novel low power chromatic dispersion monitoring technique employing SOA spectral shift
Optics Communications 230 (2004) 297–300 www.elsevier.com/locate/optcom
A novel low power chromatic dispersion monitoring technique employing SOA spectral shift Ying Shi *, Minghua Chen, Shizhong Xie Department of Electronic Engineering, Tsinghua University, Beijing 100084, PR China Received 14 June 2003; received in revised form 11 November 2003; accepted 12 November 2003
Abstract A novel approach for online chromatic dispersion monitoring by use of the spectral shift in the semiconductor optical amplifier (SOA) is proposed and experimentally demonstrated. Power on the long-wavelength side of the signal through the SOA is extracted for measuring the dispersion characteristics. Keeping a constant input power of 0 dBm, the experimental result shows that the ratio of the filtered power to the total power decreased by 10 dB when the introduced dispersion reaches 700 ps/nm. Ó 2003 Elsevier B.V. All rights reserved. PACS: 42.79.S Keywords: Chromatic dispersion; Monitoring; SOA; Spectral shift
1. Introduction In high bit-rate optical transmission systems, the system residual dispersion varies due to the changes of network or environment factors, which results in system performance degradation. So it is necessary to employ online residual dispersion monitoring and dynamic dispersion compensation in these systems. At present, online dispersion monitoring methods include vestigial sideband [1] or single sideband [2], subcarrier monitoring [3,4], clock level [5] and fiber spectral broadening [6]. The sideband, clock level and subcarrier methods involve
high speed O/E unit, and the fiber spectral broadening method needs high power optical amplifiers for the weak Kerr effect in fiber. In this paper, a novel dispersion monitoring scheme employing SOA frequency shift is demonstrated. It is shown that the dispersion of the input signal influences the gain saturation of the SOA, the output spectral shift, and the power of the long wavelength side of the output signal, which can be used to measure the dispersion. For the strong nonlinearity in SOA, the input power could be greatly reduced. 2. Principle
*
Corresponding author. Tel.: +86-010-62773197; fax: +86010-62788161. E-mail address: [email protected] (Y. Shi).
In our method, the dispersion is measured employing the frequency shift in SOA, where the gain
0030-4018/$ - see front matter Ó 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.optcom.2003.11.022
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Y. Shi et al. / Optics Communications 230 (2004) 297–300
saturation of the SOA and the frequency shift of the output signal vary with the width of the input pulse at different residual system dispersion. Assuming the case of a Gaussian pulse with residual dispersion d, the input electric field can be written as pffiffiffiffiffi P0 T2 p ffiffiffiffiffiffiffiffiffiffiffiffiffiffi ffi Ain ðT Þ ¼ exp 2s20 ð1 þ ikdÞ 1 þ ikd ð1Þ exp ð ix0 T Þ; where T ¼ t z=vg ; k ¼ k2 =ð2pcs20 Þ; s0 is the pulse width without dispersion; x0 is the carrier frequency; k is the optical wavelength of the carrier, and P0 is the peak power without dispersion. The differential gain of the SOA satisfies [7] dGðT Þ G0 GðT Þ Pin ðT Þ GðT Þ ¼ 1 ; e dT sc Esat
ð2Þ
where G0 is the unsaturated single-pass amplifier gain of the SOA; sc is the carrier lifetime; Esat is the saturation energy, and GðT Þ is the differential gain of the SOA. GðT Þ is got by solving Eq. (2), and thus the electric field of the output signal 1 ia Aout ðT Þ ¼Ain ðT Þ exp GðT Þ 2 pffiffiffiffiffi P0 T2 GðT Þ ¼ pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi exp þ 2 2s20 ð1 þ ikdÞ 1 þ ikd aGðT Þ exp i x0 T þ ; ð3Þ 2 where a is the chirp parameter of the SOA. The second exponent function determines the output phase. The instantaneous frequency shift of the output signal can be written as a dGðT Þ : ð4Þ 4p dT The waveforms of input pulses without dispersion and with dispersion of 300 ps/nm are shown in Fig. 1 and the frequency chirps of the corresponding output pulses are also depicted. At the leading edge of the pulse, GðT Þ decreases quickly for the gain saturation, which results in a negative value of dGðT Þ=dT , namely a negative chirp. At the trailing edge the GðT Þ recovers and leads to a positive chirp. From the figure, it can be seen that the gain saturates across the entire pulse, so the Dm ¼
Fig. 1. Contrast of the input pulse and the signal chirp of the SOA output with/without chromatic dispersion.
dominant chirp is negative, the frequency of the output signal shifts toward the long wavelength side. When the dispersion increases, the input pulse turns flat, the gain saturation effect becomes weak, and thus the frequency chirp decreases as the dashed curve shows. As a result, the rising of the lower frequency part damps and the power of the long wavelength side decreases. Because the period of the pulses is much shorter than the carrier lifetime of the SOA (typically 200– 300 ps), the gain has not enough time to recover at the presence of a string of 1 s. Therefore, the gain saturation is reduced owing to the pattern effect. At the bitrate of 40 Gbit/s or beyond, the pattern effect will become considerable. However, since the monitoring result is the average power of the frequency shifted component, this method will not be impaired much by the pattern effect. So it is promising to be used in high bitrate systems.
3. Experimental setup The experimental setup is shown in Fig. 2. A 27 )1 pseudo random sequence (PRS) in 10 Gbit/s return-to-zero (RZ) code with 50% duty cycle having passed a chromatic dispersion emulator (CD Emulator) is sent into a SOA. The input average power of the SOA is 0 dBm and is kept unchanged. The output of the SOA is filtered by a
Y. Shi et al. / Optics Communications 230 (2004) 297–300
299
Fig. 2. Experimental setup: *CD Emulator – chromatic dispersion emulator (consists of SMF and DCF); PM – power meter.
0.16 nm fiber Bragg grating (FBG) filter followed by a power meter (PM), which are used to extract the long wavelength side of the frequency shifted signal and measure its power. The wavelength of the input pulses is 1553.37 nm, and the center of the FBG filter is 1553.62 nm, so as to get the low frequency part of the signal. The CD Emulator consists of some of a few segments of single mode fiber (SMF) and dispersion compensation fiber (DCF), and the total dispersion varies from )650 to 850 ps/nm when the length of SMF and DCF changes. To avoid the nonlinearity in the CD Emulator, the input power is set at )3 dBm. A part of the output of the SOA is tapped off and monitored to get the total power and the spectra of the frequency shifted signal.
4. Results and discussion
Fig. 3. Output spectra of the SOA: (a) without dispersion; (b) with dispersion of 850 ps/nm.
The spectra of the output show that the output frequency shifts strongly when dispersion is around zero, as shown in Fig. 3(a). Part of energy of the output signal shifts to the long wavelength side and the spectrum is distorted at the same time. Comparing Fig. 3(a) and (b), it can be seen that this frequency shift damps while dispersion rises. When dispersion increases to the value of 850 ps/ nm, the phenomenon of frequency shift almost disappears and the spectrum nearly remains the same as that before the SOA, as shown in Fig. 3(b). To obtain the frequency component shifted in the SOA, the 0.16 nm filter is set at the wavelength 0.25 nm red-shifted from the spectral peak. The
bandwidth of the optical filter influences the accuracy of the monitoring. It is unavoidable that some power of the baseband is included in the output power of the filter, and especially when the bandwidth of the filter is too broad, the baseband will exert crucial effects on the monitoring result. Theoretically, the monitoring sensitivity can be improved by using a filter with narrow bandwidth. But too narrow bandwidth leads to weak output power, which will result in that the electrical noise of the PM is comparable to the filtered power. The experimental results are shown in Fig. 4, also the eye diagram of the input signal of the
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Y. Shi et al. / Optics Communications 230 (2004) 297–300
of 100 ps/nm, which is due to the original chirp produced by the LiNbO3 modulator in the transmitter. When the value of the dispersion is 100 ps/ nm, the input pulses of the SOA is compressed, as the eye diagram shows. 5. Conclusion
Fig. 4. Experimental results.
SOA. The frequency shift becomes weak and the ratio of the filtered power to the total output power of the SOA decreases as the dispersion of the CD Emulator increases from 100 to 850 ps/nm; similarly, the filtered power declines when the dispersion grows negatively. There are two tiny peaks at the points of )200 and +400 ps/nm, which are caused by the constructive interference between the adjacent pulses and will possibly bring about the problem of local extreme value in the monitoring. However, the peaks are insignificant and can be reduced by appropriate self-adjusting dispersion compensation algorithms. The left half of the curve is slightly higher than the right, since the negative dispersion causes the red part of the signal to transmit faster than the blue part and to accumulate in the rising edge of the pulse where the frequency red shift is strong, and thus the spectral shift is enhanced. When the absolute value of the dispersion increases beyond 700 ps/nm, the change of the filtered power will be too small for monitoring. Therefore, the dispersion monitoring range is limited within the range of 700 ps/nm. The highest point of the curve has a positive shift
In conclusion, a new dispersion monitoring technique employing frequency shift in SOA is demonstrated in this paper. There are not any high speed O/E units or microwave circuit being used in this method, or any extra modulation being added in the transmitter, and the required input power is greatly reduced because of the strong nonlinearity in SOA. So this technique is promising to be used in online chromatic dispersion monitoring in high speed optical communication systems.
Acknowledgements This work was supported by the National Natural Science Foundation under Grant No. 90104003.
References [1] Q. Yu, L.-S. Yan, Z. Pan, A.E. Willner, Technical Digest Series, OFC 2002, WE3, p. 197. [2] Akira Hirano, Shoichiro Kuwahara, Yutaka Miyamoto, Technical Digest Series, OFC 2002, WE2, p. 196. [3] C. Youn, Technical Digest Series, OFC 2003, WP2, p. 403. [4] Y. Wang, Z. Pan, A. Sahin, L. Yan, C. Yu, A.Willner, Technical Digest Series, OFC 2003, WP3, p. 404. [5] A.B. Sahin, L.-S. Yan, Qian Yu, M. Hauer, Z. Pan, A.E. Willner, ECOCÕ01, p. 446. [6] P.S. Westbrook, B.J. Eggleton, Gregory Raybon, IEEE Photon. Technol. Lett. 14 (2002) 346. [7] G.P. Agrawal, N. Anders Olsson, IEEE J. Quantum Electron. 25 (1989) 2297.
A novel low power chromatic dispersion monitoring technique employing SOA spectral shift Ying Shi *, Minghua Chen, Shizhong Xie Department of Electronic Engineering, Tsinghua University, Beijing 100084, PR China Received 14 June 2003; received in revised form 11 November 2003; accepted 12 November 2003
Abstract A novel approach for online chromatic dispersion monitoring by use of the spectral shift in the semiconductor optical amplifier (SOA) is proposed and experimentally demonstrated. Power on the long-wavelength side of the signal through the SOA is extracted for measuring the dispersion characteristics. Keeping a constant input power of 0 dBm, the experimental result shows that the ratio of the filtered power to the total power decreased by 10 dB when the introduced dispersion reaches 700 ps/nm. Ó 2003 Elsevier B.V. All rights reserved. PACS: 42.79.S Keywords: Chromatic dispersion; Monitoring; SOA; Spectral shift
1. Introduction In high bit-rate optical transmission systems, the system residual dispersion varies due to the changes of network or environment factors, which results in system performance degradation. So it is necessary to employ online residual dispersion monitoring and dynamic dispersion compensation in these systems. At present, online dispersion monitoring methods include vestigial sideband [1] or single sideband [2], subcarrier monitoring [3,4], clock level [5] and fiber spectral broadening [6]. The sideband, clock level and subcarrier methods involve
high speed O/E unit, and the fiber spectral broadening method needs high power optical amplifiers for the weak Kerr effect in fiber. In this paper, a novel dispersion monitoring scheme employing SOA frequency shift is demonstrated. It is shown that the dispersion of the input signal influences the gain saturation of the SOA, the output spectral shift, and the power of the long wavelength side of the output signal, which can be used to measure the dispersion. For the strong nonlinearity in SOA, the input power could be greatly reduced. 2. Principle
*
Corresponding author. Tel.: +86-010-62773197; fax: +86010-62788161. E-mail address: [email protected] (Y. Shi).
In our method, the dispersion is measured employing the frequency shift in SOA, where the gain
0030-4018/$ - see front matter Ó 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.optcom.2003.11.022
298
Y. Shi et al. / Optics Communications 230 (2004) 297–300
saturation of the SOA and the frequency shift of the output signal vary with the width of the input pulse at different residual system dispersion. Assuming the case of a Gaussian pulse with residual dispersion d, the input electric field can be written as pffiffiffiffiffi P0 T2 p ffiffiffiffiffiffiffiffiffiffiffiffiffiffi ffi Ain ðT Þ ¼ exp 2s20 ð1 þ ikdÞ 1 þ ikd ð1Þ exp ð ix0 T Þ; where T ¼ t z=vg ; k ¼ k2 =ð2pcs20 Þ; s0 is the pulse width without dispersion; x0 is the carrier frequency; k is the optical wavelength of the carrier, and P0 is the peak power without dispersion. The differential gain of the SOA satisfies [7] dGðT Þ G0 GðT Þ Pin ðT Þ GðT Þ ¼ 1 ; e dT sc Esat
ð2Þ
where G0 is the unsaturated single-pass amplifier gain of the SOA; sc is the carrier lifetime; Esat is the saturation energy, and GðT Þ is the differential gain of the SOA. GðT Þ is got by solving Eq. (2), and thus the electric field of the output signal 1 ia Aout ðT Þ ¼Ain ðT Þ exp GðT Þ 2 pffiffiffiffiffi P0 T2 GðT Þ ¼ pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi exp þ 2 2s20 ð1 þ ikdÞ 1 þ ikd aGðT Þ exp i x0 T þ ; ð3Þ 2 where a is the chirp parameter of the SOA. The second exponent function determines the output phase. The instantaneous frequency shift of the output signal can be written as a dGðT Þ : ð4Þ 4p dT The waveforms of input pulses without dispersion and with dispersion of 300 ps/nm are shown in Fig. 1 and the frequency chirps of the corresponding output pulses are also depicted. At the leading edge of the pulse, GðT Þ decreases quickly for the gain saturation, which results in a negative value of dGðT Þ=dT , namely a negative chirp. At the trailing edge the GðT Þ recovers and leads to a positive chirp. From the figure, it can be seen that the gain saturates across the entire pulse, so the Dm ¼
Fig. 1. Contrast of the input pulse and the signal chirp of the SOA output with/without chromatic dispersion.
dominant chirp is negative, the frequency of the output signal shifts toward the long wavelength side. When the dispersion increases, the input pulse turns flat, the gain saturation effect becomes weak, and thus the frequency chirp decreases as the dashed curve shows. As a result, the rising of the lower frequency part damps and the power of the long wavelength side decreases. Because the period of the pulses is much shorter than the carrier lifetime of the SOA (typically 200– 300 ps), the gain has not enough time to recover at the presence of a string of 1 s. Therefore, the gain saturation is reduced owing to the pattern effect. At the bitrate of 40 Gbit/s or beyond, the pattern effect will become considerable. However, since the monitoring result is the average power of the frequency shifted component, this method will not be impaired much by the pattern effect. So it is promising to be used in high bitrate systems.
3. Experimental setup The experimental setup is shown in Fig. 2. A 27 )1 pseudo random sequence (PRS) in 10 Gbit/s return-to-zero (RZ) code with 50% duty cycle having passed a chromatic dispersion emulator (CD Emulator) is sent into a SOA. The input average power of the SOA is 0 dBm and is kept unchanged. The output of the SOA is filtered by a
Y. Shi et al. / Optics Communications 230 (2004) 297–300
299
Fig. 2. Experimental setup: *CD Emulator – chromatic dispersion emulator (consists of SMF and DCF); PM – power meter.
0.16 nm fiber Bragg grating (FBG) filter followed by a power meter (PM), which are used to extract the long wavelength side of the frequency shifted signal and measure its power. The wavelength of the input pulses is 1553.37 nm, and the center of the FBG filter is 1553.62 nm, so as to get the low frequency part of the signal. The CD Emulator consists of some of a few segments of single mode fiber (SMF) and dispersion compensation fiber (DCF), and the total dispersion varies from )650 to 850 ps/nm when the length of SMF and DCF changes. To avoid the nonlinearity in the CD Emulator, the input power is set at )3 dBm. A part of the output of the SOA is tapped off and monitored to get the total power and the spectra of the frequency shifted signal.
4. Results and discussion
Fig. 3. Output spectra of the SOA: (a) without dispersion; (b) with dispersion of 850 ps/nm.
The spectra of the output show that the output frequency shifts strongly when dispersion is around zero, as shown in Fig. 3(a). Part of energy of the output signal shifts to the long wavelength side and the spectrum is distorted at the same time. Comparing Fig. 3(a) and (b), it can be seen that this frequency shift damps while dispersion rises. When dispersion increases to the value of 850 ps/ nm, the phenomenon of frequency shift almost disappears and the spectrum nearly remains the same as that before the SOA, as shown in Fig. 3(b). To obtain the frequency component shifted in the SOA, the 0.16 nm filter is set at the wavelength 0.25 nm red-shifted from the spectral peak. The
bandwidth of the optical filter influences the accuracy of the monitoring. It is unavoidable that some power of the baseband is included in the output power of the filter, and especially when the bandwidth of the filter is too broad, the baseband will exert crucial effects on the monitoring result. Theoretically, the monitoring sensitivity can be improved by using a filter with narrow bandwidth. But too narrow bandwidth leads to weak output power, which will result in that the electrical noise of the PM is comparable to the filtered power. The experimental results are shown in Fig. 4, also the eye diagram of the input signal of the
300
Y. Shi et al. / Optics Communications 230 (2004) 297–300
of 100 ps/nm, which is due to the original chirp produced by the LiNbO3 modulator in the transmitter. When the value of the dispersion is 100 ps/ nm, the input pulses of the SOA is compressed, as the eye diagram shows. 5. Conclusion
Fig. 4. Experimental results.
SOA. The frequency shift becomes weak and the ratio of the filtered power to the total output power of the SOA decreases as the dispersion of the CD Emulator increases from 100 to 850 ps/nm; similarly, the filtered power declines when the dispersion grows negatively. There are two tiny peaks at the points of )200 and +400 ps/nm, which are caused by the constructive interference between the adjacent pulses and will possibly bring about the problem of local extreme value in the monitoring. However, the peaks are insignificant and can be reduced by appropriate self-adjusting dispersion compensation algorithms. The left half of the curve is slightly higher than the right, since the negative dispersion causes the red part of the signal to transmit faster than the blue part and to accumulate in the rising edge of the pulse where the frequency red shift is strong, and thus the spectral shift is enhanced. When the absolute value of the dispersion increases beyond 700 ps/nm, the change of the filtered power will be too small for monitoring. Therefore, the dispersion monitoring range is limited within the range of 700 ps/nm. The highest point of the curve has a positive shift
In conclusion, a new dispersion monitoring technique employing frequency shift in SOA is demonstrated in this paper. There are not any high speed O/E units or microwave circuit being used in this method, or any extra modulation being added in the transmitter, and the required input power is greatly reduced because of the strong nonlinearity in SOA. So this technique is promising to be used in online chromatic dispersion monitoring in high speed optical communication systems.
Acknowledgements This work was supported by the National Natural Science Foundation under Grant No. 90104003.
References [1] Q. Yu, L.-S. Yan, Z. Pan, A.E. Willner, Technical Digest Series, OFC 2002, WE3, p. 197. [2] Akira Hirano, Shoichiro Kuwahara, Yutaka Miyamoto, Technical Digest Series, OFC 2002, WE2, p. 196. [3] C. Youn, Technical Digest Series, OFC 2003, WP2, p. 403. [4] Y. Wang, Z. Pan, A. Sahin, L. Yan, C. Yu, A.Willner, Technical Digest Series, OFC 2003, WP3, p. 404. [5] A.B. Sahin, L.-S. Yan, Qian Yu, M. Hauer, Z. Pan, A.E. Willner, ECOCÕ01, p. 446. [6] P.S. Westbrook, B.J. Eggleton, Gregory Raybon, IEEE Photon. Technol. Lett. 14 (2002) 346. [7] G.P. Agrawal, N. Anders Olsson, IEEE J. Quantum Electron. 25 (1989) 2297.