Open Access
Add to BibSonomyAbstract
Minimizing phase fluctuation along passive analog fiber link is proposed and experimentally demonstrated. By utilizing three different optical wavelengths, we could significantly reduce the effect of coherent Rayleigh noise (CRN). In addition, a phase-locked loop is employed for dynamic phase fluctuation compensation. The RMS phase jitter within two-hour period is reduced to ~1.7131 ps over 40-km fiber link.
© 2015 Optical Society of America
1. Introduction
Microwave signal transmission over optical fiber link, which is defined as microwave photonics [1, 2], attracts growing interests and attentions. Numerous applications about stable microwave phase delivery can benefit from this technology, including remote clock synchronization [3], fundamental physics research [4], radio telescope arrays [5] and so on. It could provide picosecond and femtosecond order synchronization accuracies instead of nanosecond order that is provided by IEEE1588 or GPS based synchronization technique [3, 6]. On the other hand, it can also preserve the stability delivered by high frequency source standards [3]. However, the optical fiber is sensitive to environment perturbations such as physical or mechanical vibration and temperature variations, resulting in phase fluctuations to the transmitted microwave signals [7, 8]. Various phase fluctuation cancellation schemes have been reported [9–13]. Generally, the reference clock signal is delivered from the local site to remote sites (i.e. active system), and a feedback controller is utilized in the local site for dynamic and real-time phase fluctuation compensation. However, in some other applications, e.g. radio-over-fiber (RoF) link [14] or passive location [15], microwave signals received in remote sites need to be transmitted to the local site over fiber links (i.e. passive system), while a feedback controller in the local site should be employed for the phase drift cancellation. In paper [15], we propose a phase fluctuation compensation approach used for passive applications with ~4-ps RMS phase jitter value over a 10-km fiber link. However, since coherent Rayleigh noise (CRN), which increases with the fiber length, seriously degrades the system performance for 25-km fiber transmission.
In this paper, we propose a new approach with CRN reduction to enhance the phase stability over fiber links for passive systems that transmit signals with single frequency. Three different optical wavelengths are applied to carry one-path, double-path and triple-path microwave signals, respectively. A phase-locked loop is utilized in the local site to minimize the phase difference between one-path signal and triple-path signal for dynamic phase drift cancellation. Compared to our previous CRN-limited scheme [15], the fiber length is extended from 10km to 40km, with ~2-ps root-mean-square (RMS) phase jitter over a two-hour period. Furthermore, obvious phase noise degradation in [15] is reduced to be negligible in the new scheme.
2. Operation principle
The schematic diagram of the phase fluctuation compensation system is shown in Fig. 1. The microwave signal received with an initial phase of θref is amplitude modulated on the optical carrier with a center wavelength of λ1 at the remote site. The modulated optical carrier is fed into the fiber link that introduces a phase fluctuation θf(t) along the one-way path and detected by a photo-detector (PD) at the local site. Then the detected one-path microwave signal is divided into three branches: (i) The first branch is phase-shifted via the electric phase shifter (EPS1) by θ0-θc(t) and sent to the mixer, where θ0 is the initial fixed phase shift corresponding to the reference voltage V0 applied to the EPS, θc(t) is the phase shift corresponding to Vc(t) (i.e. proportion-integration (PI) of the output DC voltage of the mixer). (ii) The second branch modulates another optical carrier with a center wavelength of λ2, which is launched back to the remote site through the same fiber link (i.e. double-path) and then transmit to the local site (i.e. triple-path) after λ2 is converted into λ3 through an optical frequency shifter (e.g. optical/electronic/optical (OEO) wavelength converter and acousto-optic frequency shifter (AOFS)). Subsequently the total fluctuation of this branch introduced by the fiber link is 3θf(t). The detected triple-path microwave signal is then phase-shifted via the EPS2 by θ0 + θc(t) and also sent to the mixer. (iii) The third branch is phase-shifted via the EPS3 by θ0 + θc(t) to be the final output signal. A key improvement of our proposal is that FBG employed in our previous scheme [15] is replaced by an optical wavelength converter, thus the CRN can be significantly reduced.
Fig. 1 Schematic diagram of the phase fluctuation compensation system. EPS: electronic phase shifter; PLL: phase-locked loop; E/O: electronic/optical conversion; O/E: optical/electronic conversion; PI: proportion-integration.
Given that the angular frequency of the microwave signal is ωRF, we could obtain the following phase relation at the local site:
where θone-path(t), θtriple-path(t) and θoutput(t) are the phases of one-path signal, triple-path signal and final output signal, respectively. The PI controller is applied to adjust the phase difference between one-path signal and triple-path signal (i.e. 2(θc(t) + θf(t))) to be π/2 automatically [15]. Subsequently the phase of the final output signal is constant as θref-θ0-π/4 and is independent of the phase contribution or fluctuation from the fiber link, i.e. the phase fluctuation accumulated along the fiber link is effectively compensated.3. Experimental setup
Figure 2 illustrates the experimental setup of our approach. A 10-dBm and 2.45-GHz microwave signal generated by a vector signal generator (VSG) is divided into two branches. One branch is sent to the four-channel oscilloscope (WaveMaster 8Zi-A with the phase difference measurement error of ± 0.37ps@2.45GHz). The other branch is used to modulate an optical carrier (a tunable laser source set at λ1 = 1550nm with a peak power stability ± 0.01dB/hour) through a Mach-Zehnder modulator (MZM) biased at Vπ/2. The output power of the laser source with a 3-dB linewidth of 100-KHz and an intensity noise of −145dBc/Hz is 10dBm. The modulated optical carrier is then passing through a wavelength division multiplexer (WDM) and sent to the local site through a standard single-mode fiber (SMF) link. At the local site, the optical beam firstly passes through an optical circulator (OC) and then amplified by an EDFA. After wavelength selection and amplified spontaneous emission (ASE) noise filtering through a WDM, the optical signal is converted into the microwave signal by PD-1 (i.e. one-path), which is divided into three branches after passing through a 2400~2500MHz band pass filter (BPF) and a low noise amplifier (LNA).
Fig. 2 Experimental setup of the proposed phase fluctuation compensation system. TLS: tunable laser source; MZM: Mach-Zehnder modulator; EDFA: Erbium-doped fiber amplifier; PC: polarization controller; OC: optical circulator; WDM: wavelength division multiplexer; PD: photo-detector; BPF: band pass filter; LNA: low noise amplifier; OSC: oscilloscope.
One branch of the one-path signal is phase shifted via the EPS1 and sent to the mixer. Another branch is applied to a MZM biased at Vπ/2 to modulate another laser source with a center wavelength of λ2 = 1551nm. A bias-control module is used for the MZM with ± 0.2dB optical output power stability. The modulated optical signal is then sent back to the same fiber link and detected by the PD-2 with a noise-equivalent power of 15.7pw/√Hz after passing through a WDM (i.e. double-path). The recovered double-path signal is employed to modulate the third laser source with a center wavelength of λ3 = 1552nm through a MZM biased at Vπ/2. The modulated optical carrier is sent to the same fiber link through the WDM and detected by PD-3 at the local site (i.e. triple-path). The detected triple-path signal is amplified by an LNA with a noise figure of 2.2-dB after the BPF, and then also sent to the mixer after phase-shifted via EPS2. The mixer works as a phase detector and its DC output voltage corresponds to the phase difference between the one-path signal and triple-path signal. The third branch is phase shifted via EPS3 driven by the output of the PLL (i.e. control voltage) to be the final output signal.
In addition to a sum and difference circuit that is utilized to generate V0 ± Vc(t), a highly accurate analog controller is designed to generate Vc(t) corresponding to the PI of the DC output of the mixer. Among them, V0 + Vc(t) is applied to EPS1 and EPS3, V0-Vc(t) is applied to EPS2. The corresponding phase shifts of EPS1, EPS2 and EPS3 are θ0 + θc(t), θ0-θc(t) and θ0 + θc(t), respectively. When the PLL gets locked (i.e. cos(2(θc(t) + θf(t))) equals to zero), the control voltage V0 + Vc(t) applied to EPS3 becomes stable, which means that the phase difference between the final output signal and the original signal becomes a constant value. Electric phase shifters (EPS1-3) with −75dBc/Hz@2.45GHz phase noise at 10-Hz frequency offset have negligible effects on the phase noise of the final output signal.
4. Results
Phase difference between the final output signal and the original signal over long time (i.e. several or dozens of hours) is usually measured to evaluate the long-term phase stabilities (LTPS). Figure 3 shows the LTPS through (a) 25-km and (b) 40-km fiber link over 2 hours measured by the oscilloscope. As an example, for the 25-km fiber link case shown in Fig. 3(a), the root-mean-square (RMS) phase jitter value is 30.6201ps without compensation. While after compensation, the RMS phase jitter value is reduced to 1.6696ps. Moreover, the phase jitter value without compensation for 40-km fiber link is 41.6276ps, while the corresponding RMS jitter value after compensation is 1.7131ps. LTPS is seriously degraded in 25-km case in [15], however, the results in Fig. 3 demonstrate that the phase fluctuation can be effectively compensated even the fiber length is extended from 25km to 40km, which indicates that LTPS is improved due to CRN reduction by our new scheme.
Fig. 3 Measured phase difference between the original signal and the final output signal over (a) 25-km and (b) 40-km fiber link (blue: uncompensated; red: compensated).
Phase noise of the final output signal is usually measured to evaluate the short-term phase stabilities (STPS), which would be distorted by CRN induced intensity noise [15]. The single-sideband phase noise between 10-Hz and100-KHz frequency offset of the original signal and final output signal with/without compensation through 25-km and 40-km fiber links are measured by a microwave spectrum analyzer (R&S®FSU67 with the phase noise floor of −140dBc/Hz@2.45GHz) and plotted in Fig. 4(a) and 4(b), respectively. The difference between the dotted black line and dashed blue line can be used to evaluate fiber link transmission induced phase noise degradation, which is mainly located from 1-KHz to 100-KHz frequency offset as shown in Fig. 4. On the other hand, the difference between the dashed blue line and solid red line can indicate CRN induced phase noise distortion, there is no difference being observed between these two lines due to successful CRN reduction. Subsequently, STPS of the final output signal with compensation is also enhanced in the new scheme.
Fig. 4 Phase noise of the original signal and measured final output signal through (a) 25-km and (b) 40-km fiber link with/without compensation (black: original; blue: uncompensated; red: compensated).
5. Conclusion
In this paper, aiming for phase stability enhancement in passive systems, we proposed and demonstrated a phase fluctuation cancellation approach with three-optical-wavelengths. It utilizes the optical reference between one-path and triple-path signals for electrical phase locking, and the FBG in [15] is replaced by an OEO optical wavelength converter for CRN reduction and more accurate phase locking. Results of 2.45-GHz microwave signals over 25-km and 40-km fiber link transmission indicate the effectiveness of our approach, with obvious LTPS and STPS enhancement. By utilizing optical and electrical components with proper bandwidths, such scheme could be applied to other anonymous signals with the frequency range from several MHz to tens of GHz.
Acknowledgments
The research is supported in part by the National Basic Research Program of China (2012CB315704), the International Science and Technology Cooperation Program of China (2014DFA11170), the National Natural Science Foundation of China (No. 61275068) and the key project of Sichuan Province of China (2011GZ0239).
References and links
1. J. Capmany and D. Novak, “Microwave photonics combines two worlds,” Nat. Photonics 1(6), 319–330 (2007). [CrossRef]
2. X. Zou, W. Li, W. Pan, L. Yan, and J. Yao, “Photonic-assisted microwave channelizer with improved channel characteristics based on spectrum-controlled stimulated Brillouin scattering,” IEEE Trans. Microw. Theory Tech. 61(9), 3470–3478 (2013). [CrossRef]
3. D. Hou, P. Li, C. Liu, J. Zhao, and Z. Zhang, “Long-term stable frequency transfer over an urban fiber link using microwave phase stabilization,” Opt. Express 19(2), 506–511 (2011). [CrossRef] [PubMed]
4. K. Sato, T. Hara, S. Kuji, K. Asari, M. Nishio, and N. Kawano, “Development of an ultrastable fiber optic frequency distribution system using an optical delay control module [for frequency standard and VLBI],” IEEE Trans. Instrum. Meas. 49(1), 19–24 (2000). [CrossRef]
5. M. Tarenghi, “The Atacama large millimeer/submillimeter array: overview&status,” Astrophys. Space Sci. 313(1-3), 1–7 (2008). [CrossRef]
6. J. Levine, “A review of time and frequency transfer methods,” Metrologia 45(6), 162–174 (2008). [CrossRef]
7. L. Zhang, L. Chang, Y. Dong, W. Xie, H. He, and W. Hu, “Phase drift cancellation of remote radio frequency transfer using an optoelectronic delay-locked loop,” Opt. Lett. 36(6), 873–875 (2011). [CrossRef] [PubMed]
8. J. Kim, J. A. Cox, and F. X. Kartner, “Drift-free femtosecond timing synchronization of remote optical and microwave sources,” Nat. Photonics 2(12), 733–736 (2008). [CrossRef]
9. J. F. Cliche and B. Shillue, “Precision timing control for radio-astronomy: maintaining femtosecond synchronization in the Atacama Large Millimeter Array,” IEEE Control Syst. 26(1), 19–26 (2006). [CrossRef]
10. G. Marra, H. S. Margolis, S. N. Lea, and P. Gill, “High-stability microwave frequency transfer by propagation of an optical frequency comb over 50 km of optical fiber,” Opt. Lett. 35(7), 1025–1027 (2010). [CrossRef] [PubMed]
11. M. T. L. Hsu, Y. He, D. A. Shaddock, R. B. Warrington, and M. B. Gray, “All-digital radio-frequency signal distribution via optical fibers,” IEEE Photon. Technol. Lett. 24(12), 1015–1017 (2012). [CrossRef]
12. C. Daussy, O. Lopez, A. Amy-Klein, A. Goncharov, M. Guinet, C. Chardonnet, F. Narbonneau, M. Lours, D. Chambon, S. Bize, A. Clairon, G. Santarelli, M. E. Tobar, and A. N. Luiten, “Long-distance frequency dissemination with a resolution of 10(-17),” Phys. Rev. Lett. 94(20), 203904 (2005). [CrossRef] [PubMed]
13. G. Marra, H. S. Margolis, and D. J. Richardson, “Dissemination of an optical frequency comb over fiber with 3 × 10(-18) fractional accuracy,” Opt. Express 20(2), 1775–1782 (2012). [CrossRef] [PubMed]
14. W. Li, W. T. Wang, W. H. Sun, W. Y. Wang, and N. H. Zhu, “Stable radio-frequency phase distribution over optical fiber by phase-drift auto-cancellation,” Opt. Lett. 39(15), 4294–4296 (2014). [CrossRef] [PubMed]
15. Z. Li, L. Yan, Y. Peng, W. Pan, B. Luo, and L. Shao, “Phase fluctuation cancellation of anonymous microwave signal transmission in passive systems,” Opt. Express 22(16), 19686–19691 (2014). [CrossRef] [PubMed]
