Open Access
Add to BibSonomy
Abstract
In this paper, we propose an active-compensation stable radio frequency (RF) transmission scheme based on a high-performance phase lock loop (PLL). In our PLL, a new structure for phase-detection is designed with only one standard RF signal to obtain a simple structure with no interference from other signals. In addition, different optical wavelengths carrying the same RF signal are utilized in the two directions to suppress Rayleigh scattering. The low phase noise homemade bi-directional erbium doped fiber amplifier (EDFA) module is used to reduce signal-to-noise ratio (SNR) deterioration. Hence, the transmission distance is greatly improved. The effects of polarization mode dispersion and phase noise produced by the EDFA on the transmission distance are discussed. Ultimately, a stable RF signal with 2.4 GHz transmitted over a 1007 km fiber link is obtained. The experimental results demonstrate that frequency instabilities of 1.2×10−13 at 1s and 5.1×10−16 at 20000s. Therefore, the system can be used for atomic clocks comparisons and provides frequency standard for time transfer systems over a long-haul fiber.
© 2018 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Stable radio frequency (RF) transfer via optical fiber links has become increasingly essential for a large number of applications, such as radio telescope array applications [1, 2], very long baseline interferometry (VLBI) [3] and deep space networks. In order to obtain a stable RF signal remotely, many schemes have been reported over the last decade [4–19]. The optical frequency comb transmission system achieves high transmission stability [15–18]. However, the structures of these schemes are complicated and stable operation over long periods is difficult to achieve [20]. Relatively, a class of active compensation schemes based on phase lock loop (PLL) that are simpler and more robust than their predecessors was developed. These schemes are a promising solution because of their sufficient compensation range and fast frequency response [13]. Wang B. et al., for example, proposed an active compensation scheme based on PLL to achieve a RF stable transfer over 80 km [11]. However, the transmission site is complex, and the transmission stability is affected by the relative fluctuations among the three frequency sources. In addition, the signal cannot be transmitted over long distances due to Rayleigh scattering in the same optical wavelength [21]. In another PLL-based scheme proposed by Akiyama et al. [12], the transmission distance was also limited by Rayleigh scattering, and the circuit structure used to prevent the nonlinear interference of the mixer is extremely complex.
In this letter, we propose a PLL-based active compensation scheme for stable RF transfer over a long-haul fiber link. In contrast to previous schemes [10–13]. Multiple RF signals can be transmitted stably in our proposed scheme. A new structure for phase-detection using frequency mixers is designed, and only one standard RF signal is used to obtain a simple structure with no interference generated from other signals. A high precision voltage control multi-frequency module (VCFM) and 10 low phase noise homemade bi-directional erbium doped fiber amplifier (EDFA) modules are utilized in the solution. The standard RF signal, phase detector, proportional-integral controller (P.I. controller), VCFM and optical fiber link constitute a high-performance PLL. These advantages allow the transmission distance to reach 1007 km. In our solution, the optical fiber-induced phase fluctuation is identified by the phase detector and compensated by the VCFM. The VCFM includes a high-precision oven-controlled crystal oscillator (OCXO) with low tuning sensitivity, which guarantees the fast frequency responses and super compensation accuracy [22]. To improve the distance of RF signal transmission, different wavelengths are utilized to carry the RF signal in the opposite transmission directions to prevent the Rayleigh scattering and the EDFA module is applied to ensure the signal-to-noise ratio (SNR). The main limitation of the transmission distance is the noise produced by EDFA and polarization mode dispersion (PMD) [23,24], which will be discussed late on the basis of the experimental results.
An experiment is designed and carried out to transfer a 2.4 GHz RF signal via a 1007 km optical fiber. The experimental results demonstrate that the normal allan deviation (ADEV) of our system is 5.3×10−15 at 1s after transmitted over 10 km, and 1.2×10−13 at 1s and 5.1 × 10−16 at 20000s after transmitted over 1007 km fiber link.
2. Method
Figure 1 shows a schematic of the proposed solution. The active-compensation frequency dissemination system consists of two parts, a local site (LS), and a remote site (RS). The LS and RS are connected by single-mode fiber spools. Laser diode 1 (LD1) and Laser diode 2 (LD2) are used to transmit the forward and the backward signal, their wavelengths are 1550.12 nm and 1550.92 nm, respectively. The two different wavelengths are used to prevent Rayleigh scattering during fiber transmission and in the Bi-EDFA module. At the LS, for brevity, the standard RF signal can be expressed as a simple cosine function without considering its exact amplitude
where ωr and φr are the angular frequency and initial phase, respectively. Another RF signal V0 can be expressed as V0 ∝ cos (ω0t + φ0). The V0 denotes a low-frequency signal relative to Vr, where ω0 and φ0 represent the angular frequency and initial phase, respectively, which can be eliminated in the following derivation. We multiply the two signals to obtain then V1 is filtered by bandpass filters (BPFs) to obtain
Fig. 1 Schematic diagram of the proposed active-compensation RF stable transmission system. P.I. control, proportional-integral controller; MZM, Mach-Zehnder modulator; VCFM, voltage control multi-frequency Module; PD, photodetector; BPF, bandpass filter; Bi-EDFA, bi-directional erbium-doped fiber amplifier module; WDM, wavelength-division multiplexer.
These two signals are phase-detected with the returned probe signal to generate an error signal. The probe signal is generated by the VCFM, which contains a tunable OCXO and a phase-locked dielectric resonant oscillator (PDRO) that can multiply the OCXO signal by a fixed factor and phase lock it. The VCFM output signal can be expressed as
and divided into two branches. One of these signals is mixed with the V2 signal to obtainThe other signal is modulated onto the optical carrier by a Mach-Zehnder modulator (MZM) and transmitted to the RS, and then detected by a photo-detector (PD). The signal filtered by the BPF can be written as
where ∆t is the propagation delay corresponding to the entire fiber link, which changes with physical factors such as temperature variation or mechanical vibration. The structure of the RS is relatively simple, a branch of V6 is modulated onto the optical carrier of LD2 and then transmitted back to the LS. The signal carrying the round-trip delay detected by PD2 can be expressed as V7 ∝ cos (ω4t + φ4 + 2ω4∆t). Then we mix the signal V7 and V3 to obtainMixing the signals V8 and V5 yields a DC error signal
Finally, the P.I. controller feedback-controls the phase of V4 in accordance with Ve. When the DC error is zero and the P.I. controller works in the steady-state condition, we obtain ωr = ω4 and φr + φξ = φ4 + ω4∆t, where φξ is a fixed phase due to the cable. In this way, V6 = Vr. We can also connect the OCXO and other output frequency PDRO. This PDRO signal has a linear relationship with V6 and it is also a stable signal that does not need to be transmitted back at RS.
3. Experimental setup
The schematic diagram of the experiment is shown in Fig. 2. At the LS, a 10 MHz rubidium oscillator (Quartzlock, A1000) passes through the phase-lock frequency multiplier to obtain a 100 MHz signal, one branch of which is phase-lock multiplied to be 2.4 GHz as the frequency standard by using the PDRO. Another branch with 7 dBm is used to multiply the reference signal. In addition, the harmonic effect of the 100 MHz in the mixer is shown in Fig. 3. The signal power observed after the 1.7 GHz frequency is lower than −70 dBm. Hence, the harmonic signal will not interfere with V1 after passing through the electric amplifier (EA). The P.I. controller that processes an error signal is a high-speed servo controller (Newport, LB1005) with an intuitive front panel to enable independent control of the P-I corner frequency, overall servo gain and low-frequency gain limit. Due to the accumulated phase noise caused by the 1007 km fiber link, we choose 10 Hz corner frequency for the experiment. The VCFM includes a tunable 100 MHz OCXO and two PDROs that phase-lock the 100 MHz signal to 2.4 GHz and 2.3 GHz respectively. Of these signals, that at 2.4 GHz is the probe signal, while that at 2.3 GHz is used to achieve stable transmission of multiple frequencies. The two signals are modulated on a 1550.12 nm single-mode optical carrier by a MZM that biased at the positive sloped quadrature point to avoid the generation of second harmonics. The fiber link contains 590 km G.652 single-mode fiber and 417 km G.655 single-mode fiber. Table 1 shows the length distribution and loss of the fiber, the values indicated are close to those in a real buried environment. Ten homemade low-noise Bi-EDFA modules are placed approximately every 90 km in our 1007 km optical fiber link and the module structure is shown in Fig. 2. The module includes two wavelength-division multiplexers (WDMs) to increase the isolation of different wavelength channels and prevent the reflection light signal from being amplified which is generated from fiber connection node. For the same wavelength signal light in the reflected light of the connection node, the optical isolator in the EDFA can effectively avoid signal disturbances. The two EDFAs are used to amplify the amplitude of optical signals in both directions. The power of the optical signal entering every fiber spool is made appropriate to avoid SNR degradation due to fiber power attenuation, the stimulated Brillouin scattering and the non-linearity of light. Therefore the optical signal entering the next 90 km is amplified to approximately 2 dBm every time. Each section of the 90 km fiber contains a dispersion compensation fiber in order to reduce the asymmetry of the optical path in both directions caused by the dispersion. The entire fiber link is placed in a laboratory environment where the temperature changes over time. At the RS, the signal filtered by the WDM is amplified to 0 dBm through an EDFA and detected by PD2. The 2.3 GHz and 2.4 GHz signals are filtered at their respective center frequency. By using a mixer, signals with frequencies of 100 MHz, 2.3 GHz, and 2.4 GHz coherent to the frequency standard can be provided to users at the RS. Moreover, any frequency can be obtained by changing the PDRO in the VCFM. To increase the SNR of the returning light, the 2.4 GHz RF signal from the PD2 is amplified to 8 dBm by an EA, filtered and then modulated onto the 1550.92 nm optical carrier by the MZM2 which is also biased at the positive sloped quadrature point. To reduce the transmission noise as much as possible, the noise figure of all EAs in the system is lower than 3 dB. The BPFs are narrow band filters of 40 MHz, and the electrical isolators shown in the fig. 2 are installed to prevent the V1 signal reflected by the filter from affecting V2 and V3.
Fig. 2 Experimental setup of the proposed phase fluctuation cancellation scheme. ISO, electric isolator; PDRO, phase-locked dielectric resonant oscillator; OCXO, oven controlled crystal oscillator.
Table 1. Length distribution and loss of the fiber.
The reference 2.4 GHz signal and the regenerated signal are converted to a 20 MHz signal by the dual-mixing method [25]. Then we place the signals into a phase noise test set (Symmetricom, Inc., TSC5120A) to evaluate the system performance. The down converted signals are amplified to 17 dBm, according to the input power requirement of the TSC5120A.
4. Results and discussion
Figure 4(a) shows the ADEVs that express relative frequency stability. The ADEV of the free running system over a 1007 km fiber is 2.5×10−13 at 1s and 2.3 × 10−12 at 20000s. The ADEV over the 1007 km fiber link with compensation is 1.2×10−13 at 1s and 5.1 × 10−16 at 20000s. Comparison between the two ADEVs shows that the compensated system effectively addresses the phase noise caused by the 1007 km optical fiber link. The noise floor of the compensation system is measured by replacing the fiber link with a 1 m fiber, and the result reveals ADEV that reaches 4.1×10−15 at 1s and 3.7×10−17 at 10000s. In addition, the noise of the EA that placed in the measurement part will affect the long-term stability in the measurement result and cannot be eliminated.
To analyze the stability of short-distance transmission and the effect of PMD on transmission, we also transmit the signals over a 10 km fiber, the results are shown by using the green triangular curve in the Fig. 4(b). The short-term stability reaches 5.4 × 10−15 at 1s which is very close to the noise floor, and the tilt at around 100s is mainly due to the PMD [23]. By contrast, the polarization state of the light in the 1007 km fiber is changed more quickly by the influence of the environment. The front-section fiber is equivalent to a polarization scrambler for the next-section fiber. The variation of the polarization state tends to decrease the effect of the PMD over an average time (only a slight tilt occurs in approximately 400s) [26]. The noise of the EDFA mainly includes signal- amplified spontaneous emission (ASE) beat noise and ASE-ASE beat noise. Although, it is difficult to calculate accurately, the ultimate transmission distance can be deduced by estimating the SNR degradation that due to the noise of EDFA under the requirement of ADEV [24]. However, comparison of the atomic clock stability curve reveals that the stability of the transmission system is higher than that of the atomic clock itself, regardless of the distance. Thus the proposed system can be used for atomic clock comparisons and provides a frequency standard for time transfer system over a long-haul fiber. In fact, the stability of the clock source is related to the measurement result. The clock source with improved stability can excavate a more realistic non-interfering system transmission stability.
Figure 5 shows the time-domain phase drift between the reference signal and the regenerated signal. It can be clearly observed that the phase drift speed of the 1007 km is very high. The phase drift is mainly due to the temperature changes. The fluctuation of the compensated signal is caused by the asymmetry of the fiber. The peak to peak value of the timing drift is 416 ps when the RF transmitted freely and the value with compensation is 0.7 ps. The root mean square is 0.14 ps and 127.46 ps, respectively.
The asymmetry of the fiber link is an important factor that affecting the transmission stability of an RF dissemination system. In our system, different fiber lengths in each EDFA render the round trip optical path asymmetrical, which could lead to the asymmetry of the frequency deviation (the derivative of the round trip phase fluctuation) and reduce the system stability. Measurement reveal that the asymmetry of our fiber link length is within 20 m, which is a small-to-negligible value relative to 1007 km. However, in the actual buried communication fibers, such an asymmetry could increase due to a number of factors, such as the Sagnac effect. At that time, the asymmetry of the fiber link is a factor that must be considered [27].
5. Conclusion
In summary, we have developed an active-compensation RF dissemination system based on PLL via a very long optical fiber link to enable the distribution of multiple stable RF signals. A new structure for phase-detection is designed to obtain improved performance on the circuit. Low phase noise Bi-EDFA modules are used to suppress the SNR deterioration over a long-distance fiber link, and two different wavelengths are transmitted in the link to avoid the Rayleigh scattering. A stable frequency of 2.4 GHz is experimentally transmitted over a 1007 km fiber link. The experimental results demonstrate that the proposed scheme can effectively compensate the phase noise caused by the optical fiber link, and the ADEVs of 1.2×10−13 and 5.1×10−16 are obtained at 1s and 20000s, respectively.
Funding
National Basic Research Program of China (2014CB340102); National Natural Science Foundation of China (NSFC) (61531003, 61690195, 61701040, 61427813); Fund of State Key Laboratory of Information Photonics and Optical Communications (Beijing University of Posts and Telecommunications [BUPT]); Youth Research and Innovation Program of BUPT (2017RC13); Open Funds of IPOC (IPOC2017ZT14).
References
1. W. Turner, T. Cornwell, A. McPherson, and P. Diamond, “Ska phase 1 system (level 1) requirements specification,” SKA-TEL-SKO-0000008 (2014).
2. B. Wang, X. Zhu, C. Gao, Y. Bai, J. W. Dong, and L. J. Wang, “Square kilometre array telescope--Precision reference frequency synchronisation via 1f-2f dissemination,” Sci. Reports 5, 13851 (2015). [CrossRef]
3. M. Tarenghi, “The atacama large millimeter/submillimeter array: overview & status,” in Science with the Atacama Large Millimeter Array, (Springer, 2008), pp. 1–7.
4. C. X. Liu, T. W. Jiang, M. S. Chen, S. Yu, R. H. Wu, J. M. Shang, J. T. Duan, and W. Y. Gu, “Gvd-insensitive stable radio frequency phase dissemination for arbitrary-access loop link,” Opt. Express 24, 23376–23382 (2016). [CrossRef] [PubMed]
5. Y. He, B. J. Orr, K. G. Baldwin, M. J. Wouters, A. N. Luiten, G. Aben, and R. B. Warrington, “Stable radio-frequency transfer over optical fiber by phase-conjugate frequency mixing,” Opt. Express 21, 18754–18764 (2013). [CrossRef] [PubMed]
6. S. W. Schediwy, D. R. Gozzard, S. Stobie, J. Malan, and K. Grainge, “Stabilized microwave-frequency transfer using optical phase sensing and actuation,” Opt. Lett 42, 1648–1651 (2017). [CrossRef] [PubMed]
7. Y. J. Cui, T. W. Jiang, S. Yu, R. H. Wu, and W. Y. Gu, “Passive-compensation-based stable rf phase dissemination for multiaccess trunk fiber link with anti-gvd and anti-backscattering function,” IEEE Photonics J. 9, 1–8 (2017). [CrossRef]
8. L. Q. Yu, R. Wang, L. Lu, Y. Zhu, J. L. Zheng, C. X. Wu, B. F. Zhang, and P. Z. Wang, “Wdm-based radio frequency dissemination in a tree-topology fiber optic network,” Opt. Express 23, 19783–19792 (2015). [CrossRef] [PubMed]
9. D. Hou, P. Li, C. Liu, J. Y. Zhao, and Z. G. Zhang, “Long-term stable frequency transfer over an urban fiber link using microwave phase stabilization,” Opt. Express 19, 506–511 (2011). [CrossRef] [PubMed]
10. Ł. Śliwczyński, P. Krehlik, A. Czubla, Ł. Buczek, and M. Lipiński, “Dissemination of time and rf frequency via a stabilized fibre optic link over a distance of 420 km,” Metrologia 50, 133 (2013). [CrossRef]
11. B. Wang, C. Gao, W. L. Chen, J. Miao, X. Zhu, Y. Bai, J. W. Zhang, Y. Y. Feng, T. C. Li, and L. J. Wang, “Precise and continuous time and frequency synchronisation at the 5 × 10-19 accuracy level,” Sci. Reports 2, 556 (2012). [CrossRef]
12. T. Akiyama, H. Matsuzawa, E. Haraguchi, T. Ando, and Y. Hirano, “Phase stabilized rf reference signal dissemination over optical fiber employing instantaneous frequency control by vco,” in Microwave Symposium Digest (MTT), 2012 IEEE MTT-S International, (IEEE, 2012), pp. 1–3.
13. X. C. Wang, Z. W. Y. Liu, S. W. Wang, D. N. Sun, Y. Dong, and W. S. Hu, “Photonic radio-frequency dissemination via optical fiber with high-phase stability,” Opt. Lett 40, 2618–2621 (2015). [CrossRef] [PubMed]
14. D. R. Gozzard, S. W. Schediwy, B. Courtney-Barrer, R. Whitaker, and K. Grainge, “Simple stabilized radio-frequency transfer with optical phase actuation,” IEEE Photonic Tech L 30, 258–261 (2017). [CrossRef]
15. 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, 1775–1782 (2012). [CrossRef] [PubMed]
16. K. Jung, J. Shin, J. Kang, S. Hunziker, C.-K. Min, and J. Kim, “Frequency comb-based microwave transfer over fiber with 7× 10-19 instability using fiber-loop optical-microwave phase detectors,” Opt. Lett 39, 1577–1580 (2014). [CrossRef] [PubMed]
17. B. Ning, S. Y. Zhang, D. Hou, J. T. Wu, Z. B. Li, and J. Y. Zhao, “High-precision distribution of highly stable optical pulse trains with 8.8 × 10-19 instability,” Sci. Reports 4, 5109 (2014). [CrossRef]
18. X. Chen, J. L. Lu, Y. F. Cui, J. Zhang, X. Lu, X. S. Tian, C. Ci, B. Liu, H. Wu, T. S. Tang, K. B. Shi, and Z. G. Zhang, “Simultaneously precise frequency transfer and time synchronization using feed-forward compensation technique via 120 km fiber link,” Sci. Reports 5, 18343 (2015). [CrossRef]
19. R. Wilcox, J. Byrd, L. Doolittle, G. Huang, and J. Staples, “Stable transmission of radio frequency signals on fiber links using interferometric delay sensing,” Opt. Lett 34, 3050–3052 (2009). [CrossRef] [PubMed]
20. M. Kumagai, M. Fujieda, S. Nagano, and M. Hosokawa, “Stable radio frequency transfer in 114 km urban optical fiber link,” Opt. Lett 34, 2949–2951 (2009). [CrossRef] [PubMed]
21. M. Nakazawa, “Rayleigh backscattering theory for single-mode optical fibers,” JOSA 73, 1175–1180 (1983). [CrossRef]
22. F. M. Gardner, Phaselock techniques (John Wiley & Sons, 2005). [CrossRef]
23. O. Lopez, A. Amy-Klein, M. Lours, C. Chardonnet, and G. Santarelli, “High-resolution microwave frequency dissemination on an 86-km urban optical link,” Appl. Phys. B 98, 723–727 (2010). [CrossRef]
24. M. Amemiya, M. Imae, Y. Fujii, T. Suzuyama, F.-L. Hong, and M. Takamoto, “Precise frequency comparison system using bidirectional optical amplifiers,” IEEE T Instrum Meas 59, 631–640 (2010). [CrossRef]
25. F. Nakagawa, M. Imae, Y. Hanado, and M. Aida, “Development of multichannel dual-mixer time difference system to generate utc (nict),” IEEE T Instrum Meas 54, 829–832 (2005). [CrossRef]
26. P. Ciprut, B. Gisin, N. Gisin, R. Passy, P. Von Der Weld, F. Prieto, and C. W. Zimmer, “Second-order polarization mode dispersion: Impact on analog and digital transmissions,” J. Light. Technol. 16, 757–771 (1998). [CrossRef]
27. J. Geršl, P. Delva, and P. Wolf, “Relativistic corrections for time and frequency transfer in optical fibres,” Metrologia 52, 552 (2015). [CrossRef]
