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Optics Express

Optics Express

  • Editor: C. Martijn de Sterke
  • Vol. 15, Iss. 14 — Jul. 9, 2007
  • pp: 8660–8666
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Simultaneous monitoring technique for ASE and MPI noises in distributed Raman amplified systems

H. Y. Choi, S. B. Jun, S. K. Shin, and Y. C. Chung  »View Author Affiliations


Optics Express, Vol. 15, Issue 14, pp. 8660-8666 (2007)
http://dx.doi.org/10.1364/OE.15.008660


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Abstract

We develop a new technique for simultaneously monitoring the amplified spontaneous emission (ASE) and multi-path interference (MPI) noises in distributed Raman amplified (DRA) systems. This technique utilizes the facts that the degree-of polarization (DOP) of the MPI noise is 1/9, while the ASE noise is unpolarized. The results show that the proposed technique can accurately monitor both of these noises regardless of the bit rates, modulation formats, and optical signal-to-noise ratio (OSNR) levels of the signals.

© 2007 Optical Society of America

1. Introduction

In distributed Raman amplified (DRA) systems, the system’s performance could be limited by the multi-path interference (MPI) noise as well as the amplified spontaneous emission (ASE) noise [1

1. J. Bromage, L. E. Nelson, C. H. Kim, P. J. Winzer, R.-J. Essiambre, and R. M. Jopson, “Relative impact of multiple-path interference and amplified spontaneous emission noise on optical receiver performance,” in Proc. of OFC 2002, TuR3 (2002).

]–[4

4. C. R. S. Fludger and R. J. Mears, “Electrical measurements of multipath interference in distributed Raman amplifiers,” J. Lightwave Technol. 19, 536–545 (2001). [CrossRef]

]. Thus, for the proper operation of DRA systems, it would be desirable to have the capability of monitoring both of these noises. Previously, we have demonstrated that the in-band ASE noise can be monitored accurately by using the polarization-nulling technique [5

5. J. H. Lee, D. K. Jung, C. H. Kim, and Y. C. Chung, “OSNR monitoring technique using polarization-nulling method,” IEEE Photon. Technol.Lett. 13, 88–90 (2001). [CrossRef]

]–[6

6. J. H. Lee, H. Y. Choi, S. K. Shin, and Y. C. Chung, “A review of the polarization-nulling technique for monitoring optical-signal-to-noise ratio in dynamic WDM networks,” J. Lightwave Technol. 24, 4162–4171 (2006). [CrossRef]

]. This technique utilizes the different polarization properties of the signal and ASE noise. However, the MPI noise, which is a replica of the optical signal, has been considered as a difficult parameter to monitor. Accordingly, there have been only a few reports on the measurement technique of MPI noise [3

3. S. A. E. Lewis, S. V. Chernikov, and J. R. Taylor, “Characterization of double Rayleigh scatter noise in Raman amplifiers,” IEEE Photon. Technol.Lett. 12, 528–530 (2000). [CrossRef]

]–[4

4. C. R. S. Fludger and R. J. Mears, “Electrical measurements of multipath interference in distributed Raman amplifiers,” J. Lightwave Technol. 19, 536–545 (2001). [CrossRef]

]. The time-domain extinction method measures the MPI noise in the sampling window (where there is no signal) obtained by using a high-extinction (>90 dB) acousto-optic switch after launching a short test pulse [3

3. S. A. E. Lewis, S. V. Chernikov, and J. R. Taylor, “Characterization of double Rayleigh scatter noise in Raman amplifiers,” IEEE Photon. Technol.Lett. 12, 528–530 (2000). [CrossRef]

]. However, this technique is not suitable for monitoring the MPI noise in a real DRA system as there can be multiple signals modulated in various formats at all times. The receiver noise analysis method estimates the MPI noise by subtracting the ASE noise density measured by using an optical spectrum analyzer (OSA) from the total noise density measured by using a homodyne receiver and an RF spectrum analyzer [4

4. C. R. S. Fludger and R. J. Mears, “Electrical measurements of multipath interference in distributed Raman amplifiers,” J. Lightwave Technol. 19, 536–545 (2001). [CrossRef]

]. However, this technique can be inaccurate in a dynamic networking environment since the in-band ASE noise cannot be measured accurately by using an OSA. In this paper, we show that the polarization-nulling technique can also be used to monitor the MPI noise [7

7. H. Y. Choi, S. B. Jun, S. K. Shin, and Y. C. Chung, “Monitoring technique for ASE and MPI noises in distributed Raman amplified systems,” in Proc. of OFC 2007, OMM2 (2007).

]. This is because the degree-of-polarization (DOP) of the MPI noise is 1/9 and its state-of-polarization (SOP) is identical with that of the signal in DRA systems (assuming that the MPI noise is generated mostly by double Rayleigh backscattered signals) [8

8. M. O. van Deventer, “Polarization properties of Rayleigh backscattering in single mode fiber,” J. Lightwave Technol. 11, 1895–1899 (1993). [CrossRef]

]. Thus, if we nullify the signal by using the polarization-nulling technique and measure the in-band noise power, the measured value should have contributions from both the ASE and MPI noises. However, we can easily separate these contributions by using the deterministic DOP of the MPI noise (since the ASE noise is, in principle, unpolarized). For demonstration, we measure the MPI noises included in the 10-Gb/s and 40-Gb/s signals (modulated in NRZ and RZ formats) at various OSNR values in the ranges of 15 dB ∼ 30 dB. In addition, we generate the MPI noise in a real DRA system and measure it as a function of the on-off gain of the Raman amplifier. The results show that we can accurately measure both the ASE and MPI noises in DRA systems regardless of the data rates and modulation formats.

2. Principle of operation

Fig. 1. Configuration of the proposed monitoring technique.

Figure 1 shows the configuration of the proposed monitoring technique for both ASE and MPI noises. This configuration is identical with the setup used for the polarization-nulling method [6

6. J. H. Lee, H. Y. Choi, S. K. Shin, and Y. C. Chung, “A review of the polarization-nulling technique for monitoring optical-signal-to-noise ratio in dynamic WDM networks,” J. Lightwave Technol. 24, 4162–4171 (2006). [CrossRef]

]. It has been previously reported that, by using this configuration, the in-band ASE noise could be accurately measured even in the dynamic networking environment [6

6. J. H. Lee, H. Y. Choi, S. K. Shin, and Y. C. Chung, “A review of the polarization-nulling technique for monitoring optical-signal-to-noise ratio in dynamic WDM networks,” J. Lightwave Technol. 24, 4162–4171 (2006). [CrossRef]

]. In this paper, we show that the MPI noise could also be measured by using the same configuration.

Fig. 2. Operating principle (a) Signal spectrum in the presence of ASE and MPI noises, (b) Filtered signal at two different wavelengths, (c) Polarized noises.

Figure 2(a) shows the spectrum of the optical signal together with ASE and MPI noises incident on the monitoring module. The spectrum of the MPI noise has an identical shape with that of the signal (since the MPI noise represents the crosstalk component generated by double Rayleigh backscattering of the signal in DRA systems [2

2. S. H. Chang, S. K. Kim, M.-J. Chu, and J.-H. Lee, “Limitations in fibre Raman amplifiers imposed by Rayleigh scattering of signals,” Electron. Lett. 38, 865–867 (2002). [CrossRef]

]–[3

3. S. A. E. Lewis, S. V. Chernikov, and J. R. Taylor, “Characterization of double Rayleigh scatter noise in Raman amplifiers,” IEEE Photon. Technol.Lett. 12, 528–530 (2000). [CrossRef]

]), whereas the spectrum of the ASE noise is flat over the entire signal’s bandwidth. We first adjust the tunable bandpass filter in Fig. 1, which has a bandwidth much narrower than the signal’s spectrum, to the center and slope of the signal’s spectrum, and measure the signal powers at two different wavelengths (P1, P3) using a photodiode PD 1, as shown in Fig. 2(b). We then measure the noise powers (P2, P4) after nullifying the signal by adjusting the polarization controller orthogonal to the signal’s polarization using PD 2, as shown in Fig. 2(c). However, if there is non-negligible MPI noise, the measured noise powers (P2, P4) should have the contributions from both the ASE and MPI noises. Thus, P2 and P4 can be expressed as,

P2=12PASE+1d2r1+r(P1PASE)P4=12PASE+1d2r1+r(P3PASE)
(1)

where PASE is the power of the ASE noise located within the bandwidth of the tunable optical filter, d is the DOP of the MPI noise, and r is the MPI noise level defined as the power ratio of the MPI noise to the optical signal (i.e., reciprocal of the signal-to-MPI noise ratio). In these equations, the first and second terms represent the contributions from the ASE and MPI noises, respectively. We assume that the MPI noise in DRA systems is mostly generated by double Rayleigh backscattering. In this case, the DOP of the MPI noise is determined to be 1/9 (since 5/9 of the MPI noise is co-polarized with the signal, while the rest is cross-polarized) [8

8. M. O. van Deventer, “Polarization properties of Rayleigh backscattering in single mode fiber,” J. Lightwave Technol. 11, 1895–1899 (1993). [CrossRef]

]. Thus, PASE and r can be expressed as

PASE=2(P1P4P2P3P1P32P2+2P4r=149P1P3P2P41
(2)

Fig. 3. Experimental setup.

Figure 3 shows the experimental setup used to evaluate the performance of the proposed technique. This setup was used to adjust the ASE and MPI noise powers independently. For the evaluation, we used 10-Gb/s and 40-Gb/s signals modulated in NRZ and RZ formats (pattern length: 231-1). To generate the MPI noise, we tapped 90 % of the modulated signal, transmitted through 1.5-km long single-mode fiber (SMF) (which was longer than the coherence length), and then sent to the MPI generator consisted of eight different noise paths (0 km ∼ 7 km). Although it may be desirable to use a large number of noise paths for the accurate simulation, we implemented the MPI generator by using only eight paths as in other literatures [10

10. C. R. S. Fludger, Y. Zhu, V. Handerek, and R. J. Mears, “Impact of MPI and modulation format on transmission systems employing distributed Raman amplification,” Electron. Lett. 37, 970–972 (2001). [CrossRef]

]–[11

11. M. Fujiwara, J. Kani, H. Suzuki, and K. Iwatsuki, “Impact of backreflection on upstream transmission in WDM single-fiber loopback access networks,” J. Lightwave Technol. 24, 740–746 (2006). [CrossRef]

]. We adjusted the polarization controllers to set the DOP of the MPI noise to be 1/9. The generated MPI noise was added to the modulated signal together with the ASE noise. The ratio between the MPI and ASE noise powers was determined by using the variable optical attenuators (VOA1 and VOA2). The 3-dB bandwidth of the tunable bandpass filter within the proposed monitoring module was 3 GHz. We measured the ASE noise power (PASE) and MPI noise level (r) by measuring the powers (P1, P2, P3, and P4) in Eq. (2), while varying r from -30 dB to -15 dB and OSNR (which was defined as the power ratio between the signal and ASE noise only) from 15 dB to 30 dB. Figure 4 shows the measured OSNR errors at various MPI noise levels. The OSNR error increased slightly with OSNR due to the difficulty of accurately measuring the small noise power (when the OSNR was very high). However, regardless of the data rates, modulation formats, and MPI noise levels, the proposed technique could measure the OSNR with accuracy better than 1 dB. On the other hand, the results in Fig. 5 show that the proposed technique could also measure the MPI noise level (r) accurately regardless of the data rates, modulation formats, and OSNRs. In particular, it is interesting to note that the proposed technique could accurately monitor the MPI noise level (r) even when the MPI noise was much smaller than the ASE noise.

Fig. 4. Measured OSNR errors at various MPI noise levels (a) 10-Gb/s NRZ signal, (b) 10-Gb/s RZ signal, (c) 40-Gb/s NRZ signal, (d) 40-Gb/s RZ signal. (oe-15-14-8660-i001 r = -15 dB, oe-15-14-8660-i002 r = -20 dB, ▿ r = -25 dB, oe-15-14-8660-i003 r = -30 dB)
Fig. 5. Measured MPI noise levels at various OSNR values (a) 10-Gb/s NRZ signal, (b) 10-Gb/s RZ signal, (c) 40-Gb/s NRZ signal, (d) 40-Gb/s RZ signal. (∎ OSNR = 15 dB, ▾ OSNR = 20 dB, ▴ OSNR = 25 dB, ● OSNR = 30 dB)

Fig. 6. MPI noise levels measured in a real DRA system. The solid curve and symbols represent the calculated values and the measured data, respectively.

4. Summary

References and links

1.

J. Bromage, L. E. Nelson, C. H. Kim, P. J. Winzer, R.-J. Essiambre, and R. M. Jopson, “Relative impact of multiple-path interference and amplified spontaneous emission noise on optical receiver performance,” in Proc. of OFC 2002, TuR3 (2002).

2.

S. H. Chang, S. K. Kim, M.-J. Chu, and J.-H. Lee, “Limitations in fibre Raman amplifiers imposed by Rayleigh scattering of signals,” Electron. Lett. 38, 865–867 (2002). [CrossRef]

3.

S. A. E. Lewis, S. V. Chernikov, and J. R. Taylor, “Characterization of double Rayleigh scatter noise in Raman amplifiers,” IEEE Photon. Technol.Lett. 12, 528–530 (2000). [CrossRef]

4.

C. R. S. Fludger and R. J. Mears, “Electrical measurements of multipath interference in distributed Raman amplifiers,” J. Lightwave Technol. 19, 536–545 (2001). [CrossRef]

5.

J. H. Lee, D. K. Jung, C. H. Kim, and Y. C. Chung, “OSNR monitoring technique using polarization-nulling method,” IEEE Photon. Technol.Lett. 13, 88–90 (2001). [CrossRef]

6.

J. H. Lee, H. Y. Choi, S. K. Shin, and Y. C. Chung, “A review of the polarization-nulling technique for monitoring optical-signal-to-noise ratio in dynamic WDM networks,” J. Lightwave Technol. 24, 4162–4171 (2006). [CrossRef]

7.

H. Y. Choi, S. B. Jun, S. K. Shin, and Y. C. Chung, “Monitoring technique for ASE and MPI noises in distributed Raman amplified systems,” in Proc. of OFC 2007, OMM2 (2007).

8.

M. O. van Deventer, “Polarization properties of Rayleigh backscattering in single mode fiber,” J. Lightwave Technol. 11, 1895–1899 (1993). [CrossRef]

9.

A. Kobyakov, S. Gray, and M. Vasilyev, “Quantitative analysis of Rayleigh crosstalk in Raman amplifiers,” Electron. Lett. 39, 732–733 (2003). [CrossRef]

10.

C. R. S. Fludger, Y. Zhu, V. Handerek, and R. J. Mears, “Impact of MPI and modulation format on transmission systems employing distributed Raman amplification,” Electron. Lett. 37, 970–972 (2001). [CrossRef]

11.

M. Fujiwara, J. Kani, H. Suzuki, and K. Iwatsuki, “Impact of backreflection on upstream transmission in WDM single-fiber loopback access networks,” J. Lightwave Technol. 24, 740–746 (2006). [CrossRef]

12.

C. Xie and D. C. Kilper, “Influence of polarization scattering on polarization-assisted OSNR monitoring in dense WDM systems with NZ-DSF and Raman amplification,” in Proc. of OFC 2005, JWA40 (2005).

13.

C. Xie, D. C. Kilper, L. Moller, and R. Ryf, “Orthogonal polarization heterodyne OSNR monitoring technique insensitive to polarization effects,” J. Lightwave Technol. 25, 177–183 (2007). [CrossRef]

14.

B. Zhu, S. N. Knudsen, L. E. Nelson, D. W. Peckham, M. O. Pederson, and S. Stulz, “800 Gbits/s (80×10.664 Gbits/s) WDM transmission over 5200 km of fibre employing 100 km dispersion-managed spans,” Electron. Lett. 37, 1467–1469 (2001). [CrossRef]

15.

D. F. Grosz, A. Agarwal, A. P. Kung, S. Banerjee, D. N. Maywar, and T. H. Wood, “Performance of a ULH single wide-band all-Raman DWDM transmission system over dispersion-managed spans,” IEEE Photon. Technol. Lett. 16, 1197–1199 (2004). [CrossRef]

OCIS Codes
(060.0060) Fiber optics and optical communications : Fiber optics and optical communications
(060.2360) Fiber optics and optical communications : Fiber optics links and subsystems

ToC Category:
Fiber Optics and Optical Communications

History
Original Manuscript: May 22, 2007
Revised Manuscript: June 15, 2007
Manuscript Accepted: June 15, 2007
Published: June 26, 2007

Citation
H. Y. Choi, S. B. Jun, S. K. Shin, and Y. C. Chung, "Simultaneous monitoring technique for ASE and MPI noises in distributed Raman Amplified Systems," Opt. Express 15, 8660-8666 (2007)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-15-14-8660


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References

  1. J. Bromage, L. E. Nelson, C. H. Kim, P. J. Winzer, R.-J. Essiambre, and R. M. Jopson, "Relative impact of multiple-path interference and amplified spontaneous emission noise on optical receiver performance," in Proc. of OFC 2002, TuR3 (2002).
  2. S. H. Chang, S. K. Kim, M.-J. Chu, and J.-H. Lee, "Limitations in fibre Raman amplifiers imposed by Rayleigh scattering of signals," Electron. Lett. 38, 865-867 (2002). [CrossRef]
  3. S. A. E. Lewis, S. V. Chernikov, and J. R. Taylor, "Characterization of double Rayleigh scatter noise in Raman amplifiers," IEEE Photon. Technol. Lett. 12, 528-530 (2000). [CrossRef]
  4. C. R. S. Fludger and R. J. Mears, "Electrical measurements of multipath interference in distributed Raman amplifiers," J. Lightwave Technol. 19, 536-545 (2001). [CrossRef]
  5. J. H. Lee, D. K. Jung, C. H. Kim, and Y. C. Chung, "OSNR monitoring technique using polarization-nulling method," IEEE Photon. Technol. Lett. 13, 88-90 (2001). [CrossRef]
  6. J. H. Lee, H. Y. Choi, S. K. Shin, and Y. C. Chung, "A review of the polarization-nulling technique for monitoring optical-signal-to-noise ratio in dynamic WDM networks," J. Lightwave Technol. 24, 4162-4171 (2006). [CrossRef]
  7. H. Y. Choi, S. B. Jun, S. K. Shin, and Y. C. Chung, "Monitoring technique for ASE and MPI noises in distributed Raman amplified systems," in Proc. of OFC 2007, OMM2 (2007).
  8. M. O. van Deventer, "Polarization properties of Rayleigh backscattering in single mode fiber," J. Lightwave Technol. 11, 1895-1899 (1993). [CrossRef]
  9. A. Kobyakov, S. Gray, and M. Vasilyev, "Quantitative analysis of Rayleigh crosstalk in Raman amplifiers," Electron. Lett. 39, 732-733 (2003). [CrossRef]
  10. C. R. S. Fludger, Y. Zhu, V. Handerek, and R. J. Mears, "Impact of MPI and modulation format on transmission systems employing distributed Raman amplification," Electron. Lett. 37, 970-972 (2001). [CrossRef]
  11. M. Fujiwara, J. Kani, H. Suzuki, and K. Iwatsuki, "Impact of backreflection on upstream transmission in WDM single-fiber loopback access networks," J. Lightwave Technol. 24, 740-746 (2006). [CrossRef]
  12. C. Xie and D. C. Kilper, "Influence of polarization scattering on polarization-assisted OSNR monitoring in dense WDM systems with NZ-DSF and Raman amplification," in Proc. of OFC 2005, JWA40 (2005).
  13. C. Xie, D. C. Kilper, L. Moller, and R. Ryf, "Orthogonal polarization heterodyne OSNR monitoring technique insensitive to polarization effects," J. Lightwave Technol. 25, 177-183 (2007). [CrossRef]
  14. B. Zhu, S. N. Knudsen, L. E. Nelson, D. W. Peckham, M. O. Pederson, and S. Stulz, "800 Gbits/s (80x10.664 Gbits/s) WDM transmission over 5200 km of fibre employing 100 km dispersion-managed spans," Electron. Lett. 37, 1467-1469 (2001). [CrossRef]
  15. D. F. Grosz, A. Agarwal, A. P. Kung, S. Banerjee, D. N. Maywar, and T. H. Wood, "Performance of a ULH single wide-band all-Raman DWDM transmission system over dispersion-managed spans," IEEE Photon. Technol. Lett. 16, 1197-1199 (2004). [CrossRef]

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