In this paper, we propose and demonstrate optical signal to noise ratio (OSNR) monitoring method for polarization-division-multiplexing (PDM) signal by using the uncorrelated signal power, which is generated by balanced subtraction in electrical domain. The proposed OSNR monitoring is insensitive to dispersion impairment by using low bandwidth receiver. The proposed OSNR monitoring method is tested from 5 dB to 27.5 dB in 100-Gb/s PDM-QPSK system experimentally.
© 2014 Optical Society of America
Polarization-division-multiplexing (PDM) technique is employed to increase optical spectrum efficiency (SE) in high speed optical fiber transmission system . During optical fiber transmission, amplified spontaneous emission (ASE) noise is generated and accumulated with the repeated amplification of optical signal, which degrades optical signal to noise ratio (OSNR) of the transmitted signal gradually. Since OSNR is a critical parameter that determines the performance of optical transmission system, OSNR monitoring becomes necessary for self-management of high-speed optical fiber transmission system . So far, there are several OSNR monitoring techniques being reported, which are based on different working principles. In , polarization-nulling technique is demonstrated on the basis of the polarization characteristics of optical signal and noise. However, it is not practical for the real optical fiber transmission system with polarization mode dispersion (PMD) and polarization dependent loss (PDL). In , electrical sampling technique derives OSNR monitoring parameter from 2-deminsion (2-D) phase portrait, since different level optical noise power can induce electrical waveform distortion in different degrees. Nevertheless, the OSNR monitoring accuracy is sensitive to chromatic dispersion (CD) and PMD, and the monitoring system requires expensive high bandwidth receiver and sampler. In , OSNR monitoring method relying on optical nonlinear effect does not require high bandwidth component. However, this method requires high pump power and highly nonlinear medium, so it is not a cost-effective solution. Compared with the OSNR monitoring methods in [3–5], the filtering effect of optical delay interferometer (ODI) is used for OSNR monitoring, which is a cost-effective method, and can be insensitive to CD and PMD . By tuning the phase shift of ODI, the center frequency of its passband and stopband can be placed at center of the monitoring signal separately, which can derive the signals that contain different ratios of signal to noise power. Thus, the power ratio of the output powers from the passband and stopband can vary as a function of signal OSNR, which can be used as a monitoring signal. Furthermore, ODI based OSNR monitoring method is demonstrated in PDM system . However, the OSNR monitoring accuracy would be affected by the fact that the phase shift of ODI is sensitive to environmental temperature change. In , the authors propose to sweep the phase shift of interferometer, so that the power ratio can be derived from the obtained transmittance curve of the ODI. Nevertheless, it needs certain testing time for single OSNR value measurement.
In , the authors propose uncorrelated signal based OSNR monitoring technique. By placing two optical filters symmetrically at two sidebands of the monitored signal separately, the optical spectrums of the two filtered sideband signals partially overlap at center frequencies. The overlapped frequency components are correlated, which is removed by balanced subtraction after photo detection. The remaining part is uncorrelated signal, whose high frequency power density is derived to estimate in-band OSNR. However, high frequency signal power related method is affected by CD and PMD, since both CD and PMD can induce high frequency RF tone power variation. Moreover, two-filter based setup increases the complexity of monitoring system. Thus, we propose to use single optical bandpass filter (OBPF) to generate uncorrelated signal, where the major correlated signal is removed by balanced electrical subtraction. The power ratio of the uncorrelated signal and original signal power is as a function of OSNR, which has been demonstrated in 10-Gb/s single polarization system by our simulation work . Compared with ODI based OSNR monitoring method, small amount of phase shift is not a critical issue for the two-branch electrical subtraction based method, which is an alternative solution of bandstop filtering. More importantly, by using low bandwidth receiver, the proposed method can be insensitive to CD. In this paper, we will experimentally demonstrate OSNR monitoring method by using the uncorrelated signal power in 100-Gb/s PDM-QPSK and 50-Gb/s QPSK systems. Moreover, OSNR monitoring performance is investigated in the presence of CD and PMD effects.
2. Operation principle and experimental setup
The experimental setup of our proposed method is shown in Fig. 1(a).The monitored signal is selected by a 0.4-nm optical tunable filter (OTF), and then, split into two branches by a 90:10 coupler. The 10% optical signal is directly detected and measured as reference signal power (Pref), while the 90% optical signal is split into two branches by another 3-dB coupler. One branch is connected with 0.2-nm OBPF with same center frequency as the monitored signal. The other branch is connected with optical attenuator and optical time delay line, which are used to keep time and power matched between two branches before the balanced receiver.
The optical spectrums of 100-Gb/s PDM-QPSK signal before the two branches of the balanced receiver are shown in Fig. 1(b). The optical spectrum of upper branch is overlapped with the center frequencies of lower branch. The overlapped area can be tuned by varying the bandwidth of OBPF. The overlapped frequency components of these two signals are correlated, which are cancelled out by balanced subtraction after photo detection. Therefore, the output of balanced receiver is uncorrelated signal, which is measured as PUN that can be generally expressed by Eq. (1). Moreover, the detected reference signal power (Pref) is expressed by Eq. (2).Eq. (3), where NEB and Br are noise equivalent bandwidth and resolution bandwidth separately.
In , we numerically investigate the impacts of filters’ bandwidth and shape on monitoring dynamic range by this method. In this paper, the bandwidth of OTF and OBPF are fixed at 0.4 nm and 0.2 nm individually for experimental demonstration. More importantly, since we propose to use low bandwidth receiver and measurement equipment to achieve low-cost OSNR monitoring scheme, we place two 465-MHz low-pass filters (LPF) after DC-coupled high-speed balanced receiver and single DC-coupled high-speed photo detector to emulate low bandwidth receivers, as shown in Fig. 1(a). Additionally, LPF can reject the high frequency RF components that are affected by dispersion effect, so it can reduce the signal power variation caused by dispersion. For power measurement, we use a 20-GHz sampling oscilloscope that servers as analog-digital-converter (ADC) to derive the amplitude of electrical signals, which can be converted into signal power. Since LPF placed before the sampling oscilloscope, the bandwidth of ADC can be low as the receiver bandwidth. Thus, it is possible to use low bandwidth receiver and ADC to fabricate a cost-effective OSNR monitoring module.
The whole experimental system is shown in Fig. 2.50-Gb/s single polarization QPSK signal is modulated by I/Q modulator, while pol-mux is used to push the 50-Gb/s signal to 100-Gb/s PDM signal. First order PMD emulator is used to introduce differential group delay (DGD) into the system. Additionally, different values of CD in the system are emulated by using different lengths of single mode fiber (SMF). OSNR value in the system is varied by output power of the noise source, which is coupled with optical signal by a 3-dB coupler. The bandwidth of noise source is 0.8 nm. Before monitoring module, an erbium doped fiber amplifier (EDFA) is used to compensate power loss during fiber transmission.
3. Experimental result and discussion
3.1 OSNR monitoring performance by using proposed uncorrelated signal power with low bandwidth receiver
For demonstration of the uncorrelated signal generation, the correlated and un-correlated signal of 10-Gb/s NRZ-OOK signal is shown in Fig. 3(a).In the un-correlated signal, major part of correlated signal is removed, where the uncorrelated noise and signal are at comparable power level. The generation of uncorrelated signal is also applicable in phase modulation format and PDM signal. As the experimental result in Fig. 3(b) shows, our proposed power ratio varies as a function of OSNR value in both 50-Gb/s QPSK and 100-Gb/s PDM-QPSK systems. The demonstrated OSNR monitoring range is from 5 dB to 27.5 dB. The OSNR monitoring performances in the two tested systems are same, due to same optical spectrum.
When OSNR of the monitored signal is high (larger than 20 dB), signal power is dominant in PUN, which is less sensitive to noise power variation, so the monitoring dynamic range decreases. The monitoring dynamic range can be increased by tuning the bandwidth of OTF and OBPF . By increasing the bandwidth of OTF and OBPF, the coefficient α in Eq. (1) is reduced, while the coefficient β and γ are increased in Eq. (1) and 2. The proportion of noise power in the uncorrelated signal power is increased, so that the measured uncorrelated signal power is more sensitive to noise power variation, which contributes to the increase of monitoring dynamic range at high OSNR. On the contrary, by using narrower bandwidth of OTF and OBPF, signal power takes up more proportion in the uncorrelated signal power. In high OSNR case, signal power becomes dominant in the uncorrelated signal power earlier, so that the increase of power ratio will enter saturation region faster as OSNR increases, as shown in Fig. 3. Thus, the filters with narrow bandwidth would lead to poor monitoring dynamic range. Although larger bandwidth of OTF and OBPF can enlarge the monitoring dynamic range, larger bandwidth of OTF may increase the monitoring error induced by power leakage from neighbor channels in WDM system. Thus, there is a tradeoff between monitoring dynamic range and monitoring accuracy.
3.2 The proposed OSNR monitoring performance in the presence of CD and PMD
Since the proposed method is based on electrical signal power, both CD and PMD induced high frequency RF power variation will reduce monitoring accuracy. Thus, by using low bandwidth receiver, OSNR monitoring error induced by CD and PMD can be minimized. As the experimental result shows in Fig. 4, in the demonstrated OSNR monitoring range (from 5 dB to 27.5 dB) of 100-Gb/s PDM-QPSK signal, the OSNR monitoring performance is robust to both 1st order PMD and CD effects. As DGD increases from 0 ps to 50 ps, OSNR monitoring error is within 1-dB. For CD impact on OSNR monitoring performance, when it increases up to 1360 ps/nm, OSNR monitoring error can be kept below 1 dB.
In Fig. 5, the experimental result shows the OSNR monitoring performance between the systems that uses low bandwidth receiver (465 MHz) and high bandwidth receiver (42 GHz) in the presence of 1st order PMD and CD. By using high bandwidth receiver, OSNR monitoring performance is seriously affected by both PMD and CD, especially for high OSNR cases. It is due to that dispersion induced power variation is greater than noise power in the uncorrelated signal. In low bandwidth receiver scheme, OSNR monitoring errors are minimized below 1 dB for all cases at the demonstrated CD range. Since the low bandwidth receiver successfully removes CD induced high frequency power variation, this scheme can be insensitive to CD value which is much larger than demonstrated one.
3.3 Time mismatch between balanced receiver
Low bandwidth scheme cannot only reduce dispersion induced monitoring errors, it can also increase the time mismatch tolerance between two branches of the balanced receiver. As the experimental result shows in Fig. 6, 1-dB OSNR monitoring error tolerance to time mismatch is less than 4 ps in high bandwidth scheme. The time mismatch induced OSNR monitoring error is larger in higher OSNR case (20 dB). After using low bandwidth receiver, the 1-dB OSNR monitoring error tolerance can be increased to 40 ps in high OSNR case.
In this paper, we experimentally demonstrate OSNR monitoring method by using uncorrelated signal power in both single and duo polarization systems. By using low bandwidth receiver, both CD and PMD induced monitoring error can be minimized, while the tolerance to time mismatch between balanced receiver can be increased. More importantly, the bandwidth receiver scheme reduces the cost of monitoring module.
The authors would like to thank the support of AcRF Tier 2 Grant MOE2013-T2-2-145 from MOE Singapore and R-2012-N-009 from National University of Singapore (Suzhou) Research Institute.
References and links
1. G. Charlet, J. Renaudier, H. Mardoyan, P. Tran, O. B. Pardo, F. Verluise, M. Achouche, A. Boutin, F. Blache, J.-Y. Dupuy, and S. Bigo, “Transmission of 16.4-Tbit/s capacity over 2550 km using PDM QPSK modulation format and coherent receiver,” J. Lightwave Technol. 27(3), 153–157 (2009). [CrossRef]
2. Z. Pan, C. Yu, and A. E. Willner, “Optical performance monitoring for the next generation optical communication networks,” Opt. Fiber Technol. 16(1), 20–45 (2010). [CrossRef]
3. 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(1), 88–90 (2001). [CrossRef]
4. F. N. Khan, A. P. T. Lau, Z. Li, C. Lu, and P. K. A. Wai, “OSNR monitoring for RZ-DQPSK systems using half-symbol delay-tap sampling technique,” IEEE Photon. Technol. Lett. 22(11), 823–825 (2010). [CrossRef]
5. Z. Chen, L. Yan, A. Yi, W. Pan, and B. Luo, “Simultaneous OSNR monitoring for two polarization tributaries of a PDM signal using a polarization-diversity nonlinear loop mirror based on FWM,” J. Lightwave Technol. 30(14), 2376–2381 (2012). [CrossRef]
6. X. Liu, Y. H. Kao, S. Chandrasekhar, I. Kang, S. Cabot, and L. L. Buhl, “OSNR monitoring method for OOK and DPSK based on optical delay interferometer,” IEEE Photon. Technol. Lett. 19(15), 1172–1174 (2007). [CrossRef]
7. M. R. Chitgarha, S. Khaleghi, W. Daab, M. Ziyadi, A. Mohajerin-Ariaei, D. Rogawski, M. Tur, J. D. Touch, V. Vusirikala, W. Zhao, and A. E. Willner, “Demonstration of WDM OSNR performance monitoring and operating guidelines for Pol-Muxed 200-Gbit/s 16-QAM and 100-Gbit/s QPSK data channels,” OFC, OTh3B.6, (2013). [CrossRef]
8. E. Flood, W. H. Guo, D. Reid, M. Lynch, A. L. Bradley, L. P. Barry, and J. F. Donegan, “In-band OSNR monitoring using a pair of Michelson fiber interferometers,” Opt. Express 18(4), 3618–3625 (2010). [CrossRef] [PubMed]
9. W. Chen, R. S. Tucker, X. Yi, W. Shieh, and J. S. Evans, “Optical signal to noise ratio monitoring using uncorrelated beat noise,” IEEE Photon. Technol. Lett. 17(11), 2484–2486 (2005). [CrossRef]
10. Y. Yu, J. Yang, and C. Yu, “Low cost and CD insensitive optical signal to noise ratio monitoring method using beat noise,” in Proc. IEEE International Conference on Communication Systems (ICCS), 41–44, 2012. [CrossRef]