Open Access
Add to BibSonomy
Abstract
A novel photonic-assisted method for radio frequency (RF) signal amplification with high-gain and narrowband based on a dual-loop optoelectronic oscillator (OEO) is proposed and experimentally demonstrated. In the proposed system, the low-power RF signal is injected into a dual-loop OEO which is below the threshold oscillation state. And the maximum gain is obtained when the frequency of the RF signal matches with the potential oscillation mode of the dual-loop OEO. The approach provides an average gain greater than 22 dB for the RF signal which matches with oscillation mode. After amplification, the signal-to-noise ratio (SNR) turns out to be 40 dB. Furthermore, the 3 dB bandwidth of the suggested system can be narrower than 1.2 kHz which can effectively remove the out-of-band noise and spurious effects. Meanwhile, the performance of sensitivity and phase noise are also investigated.
© 2022 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
The technology of ultra-stable and long-haul frequency transfer will enable significant advances in scientific researches and practical applications, including searches for dark matter, atomic clocks, and military systems [1–3]. These systems transmit a signal with constant frequency and require coherent frequency reference signals at each terminal to achieve phase coherence for performing interferometry [4]. Over the past few years, optical fiber has become an extremely promising medium for the frequency dissemination [5–7]. However, it is worth noting that the stability of the radio frequency (RF) signal is deteriorated since the optical fiber is influenced by mechanical perturbation and temperature variation [8,9]. To improve the stability of the RF signal, a remote frequency distribution system model based on the phase-locked loop (PLL) theory has been reported [10]. By transmitting 50 to 400 GHz RF signals over a 60 km spooled fiber to the remote ends, the experimental results show that the stability at 400 GHz is improved by an order of magnitude over an average time of 1000 s compared to that of 50 GHz, implying that the better stability can be achieved by propagating higher frequency RF signals. On the other hand, in order to extend the transmission distance of the RF signal, amplifiers are exercised at the terminals to compensate for the power loss of the signal. Nevertheless, the noise generated by the amplifiers is superimposed on the desired RF signal, which degrades the quality of signal [11]. Narrowband filters are usually introduced to eliminate the additive noise, but they are laborious to fabricate, especially at high frequencies [12]. Therefore, the manufacture of high-frequency amplifiers with narrowband is a critical problem.
Nowadays, many photonic-assisted techniques establish a perfect knowledge of the RF signal amplification, including dispersion element [13], premium tunable optical filters [14], and microwave channelization [15], which can overcome the limit of electronic bottleneck on the strength of prominent advantages of low loss and immunity to electro-magnetic interference [16,17]. Nonetheless, these techniques cannot amplify very low-power RF signal and suppress spurious signal simultaneously. Recently, various approaches based on multimode optoelectronic oscillator (OEO) has also been successively discovered to amplify low-power RF signal, such as injection locking [18], phase-shift-Bragg grating (PS-FBG) [19], chip-based Brillouin scattering effect [20], and direct modulation of DFB lasers [21]. The methods mentioned above selectively provides gain to RF signal that match the cavity modes based on traditional single-loop OEO, whereas the gain of the RF signal is insufficient in these systems. Moreover, the signal-to-noise ratio (SNR) and phase noise are not considered, which are crucial in the frequency transfer system.
In this paper, an effective method for low-power RF signal amplification based on dual-loop OEO was proposed and reduce the phase noise in the system. Differ from the conventional single-loop OEO, the dual-loop OEO, which can generate large mode spacing from short fiber and low phase noise from long fiber. Therefore, dual-loop OEO can achieve narrower bandwidth for better filtering out-of-band noise and spurious effects, which can further improve the phase noise. Furthermore, the frequency of the RF signal in the proposed method is kept single because in the actual fiber frequency transfer system it is constant [22,23]. An experiment is designed and carried out to analyze the performance of this scheme, and compare the performance with single-loop OEO ’s in terms of signal gain, SNR of the amplified signal and phase noise. And the results describe that the RF signal can be magnified greater than 22 dB on average, which is much better than that based on single-loop OEO. Notably, the 3 dB bandwidth of the system can be narrower than 1.2 kHz to filter out the noise effectively and amplify the desired RF signal more accurately, and the SNR of the amplified signal can attain 40 dB by using dual-loop OEO. Finally, by replacing with a higher center frequency filter to verify the theory, the experimental results are in agreement with our expectation.
2. Principle and experimental setup
The schematic of the low-power RF signal amplification system is shown in Fig. 1. A continuous-wave (CW) light from a laser is intensity modulated by the Mach-Zehnder modulator (MZM). Next, a polarization controller (PC) is employed to control the power distribution of the modulated light in the two optical paths. Two different lengths of single-mode fibers (SMFs) with 50 m and 10 km are utilized to exploit the Vernier effect. Subsequently, utilizing polarization-beam splitter (PBS) and polarization-beam combiner (PBC) to split and combine the optical signals, while avoiding the interference and beating which are the main problems of coupling in the optical domain. The merged signal is then launched into an erbium-doped fiber amplifier (EDFA) to compensate the power loss. Afterwards, the amplified signal is connected to a photodetector (PD) for photoelectric conversion. In addition, the utilization of an electrical bandpass filter (EBF) at the output of the PD ensures that the selected mode will oscillate within the desired spectral range, while reducing the interference caused by out-of-band noise and improving the quality of the RF signal. Then the electrical signal is amplified by the electrical amplifier (EA) and divided into two parts by a 50 : 50 microwave power splitter (MPS). One part is sent to the electrical spectrum analyzer (ESA) for real-time analysis, and the other part is fed into the MZM via an electrical coupler (EC) to close the OEO. At last, the low-power RF signal produced from the signal generator is injected into the OEO cavity via the another part of EC. Signal amplification can be achieved when the frequency of the RF signal matches to the frequency of oscillation mode.
In this paper, the working principle of high-gain and narrowband amplification of low-power RF signal is mainly based on a dual-loop OEO. It is well known that the open loop gain of its feedback loop greater than unity is a condition for any oscillator to self-sustaining oscillate. Similarly, in a dual-loop OEO, it can remain self-sustaining oscillate as long as the combined open loop gain of the two loops is greater than one. In the meantime, in order to calculate the oscillation threshold of the dual-loop, the corresponding RF power is known as [24]
Fig. 1. Schematic diagram of the proposed RF signal amplification (LD: laser diode, MZM: Mach-Zehnder modulator, PC: polarization controller, PBS: polarization-beam splitter, PBC: polarization-beam combiner, SMF: single-mode fiber, EDFA: erbium-doped fiber amplifier, TA: tunable attenuator, PD: photodetector, EBF: electrical bandpass filter, EA: electrical amplifier, MPS: microwave power splitter, EC: electrical coupler, ESA: electrical spectrum analyzer, SG: signal generator).
3. Experimental results
An experiment is implemented on the basis of the configuration shown in Fig. 1. Initially, the power of the CW laser is set to 13 dBm and the gain of the EDFA is adjusted to allow the system to oscillate in a multimode state. Then the tunable attenuator (TA) is precisely controlled to ensure that the OEO remains below the oscillation threshold. In this case, the system has no mode oscillation, which makes the OEO extremely sensitive to externally injected signal. In the experiment, two different frequency RF signals with the same power of -60 dBm, i.e., 5.419 GHz and 5.420 GHz, are injected into the OEO when the oscillation mode is 5.419 GHz. As can be seen in Fig. 2, the power of the RF signal at 5.419 GHz is -37.24 dBm, yielding a gain of 22.76 dB. Nevertheless, the system gives a loss of 8.32 dB to the RF signal at 5.420 GHz. Therefore, the injected RF signal can be amplified when its frequency matches the oscillation mode, otherwise the loss is acquired.
Fig. 2. Electrical spectra of the measured RF signals: (a) injected signal frequency at 5.419 GHz, (b) injected signal frequency at 5.420 GHz.
Next, in order to demonstrate the advantage of the dual-loop OEO in amplifying the signal more effectively, different input powers of the RF signal with the same frequency of 5.419 GHz are injected into the three systems, which from -70 to -20 dBm with the step of 10 dBm. The output powers and the gains of the RF signal are observed in Fig. 3(a)-(b). It can be indicated that the average gain corresponding to the system based on dual-loop OEO is calculated to be greater than 22 dB. Instead, the corresponding gain of the system based on single-loop OEO with 50 m and 10 km are 9 and 6 dB, respectively. It can be concluded that the gain of the RF signal based on dual-loop OEO is much larger than that of the single-loop OEO.
Fig. 3. Output powers (a) and the gains (b) of three amplification systems for different input powers signal amplification.
To investigate the sensitivity and gain dynamic range of the reported system, experiments are conducted to measure the different output spectra corresponding to different input powers of the amplified signals at a constant frequency of 5.419 GHz. On the one hand, sensitivity is the minimum input power that can be amplified by the system. As depicted in Fig. 4(a), when the input signal power reaches -91 dBm, the output power of -69 dBm is equal to the noise floor. Furthermore, as indicated in Fig. 4(b), when the input power reaches 0 dBm, the gain of the injected signal is diminished to 0 dB due to the gain saturation effect. It can be concluded that the sensitivity of the system is approximately -91 dBm. On the other hand, the gain dynamic range is the range from the minimum input power that can be amplified to the 1 dB compression point input power. It is evident from Fig. 4(b) that when the input signal power is approximately -27 dBm, the 1 dB compression point is around -4 dBm, yielding a dynamic gain range of 64 dB.
Fig. 4. The sensitivity (a) and the output powers (b) of the dual-loop OEO with different input powers.
The 3 dB bandwidth is a critical parameter of the amplification system, which is defined as the bandwidth from the peak of gain to a drop of 3 dB. To measure the 3 dB bandwidth of the proposed system, the input power of the injected signal is set to -45 dBm and the frequency is tuned from the highest gain point to the 3 dB drop point. As can be noted from Fig. 5, the calculated 3 dB bandwidth is narrower than 1.2 kHz when the signal gain drops from 20.14 dB to 17.12 dB. The bandwidth is narrow enough so that the out-of-band noise and spurious effects can be removed more effectively.
Considering that the system performance is related to the SNR of the amplified signal, comparative experiments are carried out on these three systems. The output spectra corresponding to RF signal with the same input power of -45 dBm are evaluated and the results are presented in Fig. 6(a) to Fig. 6(c). Firstly, the measured SNR of the system based on 50 m single-loop OEO is estimated to be 30.29 dB. Then the SNR of the system based on 10 km single-loop OEO is 32.94 dB. Finally, in the case of dual-loop OEO, the SNR of the system exceeds 40 dB. Thus, compared to the single-loop OEO results, a striking improvement is achieved by using the dual-loop OEO with 50 m and 10 km.
Fig. 6. The output RF signal spectra of the systems based on (a) 10 km single-loop OEO, (b) 50 m single-loop OEO, and (c) dual-loop OEO.
Finally, in order to analyze the phase noise of the system, a proof-of-concept experimental system is built, as plotted in Fig. 7. The RF signal is transmitted to the remote ends via a long-haul fiber link. After long distance transmission, the deteriorated RF signal output of the PD is emitted into the proposed system for amplification. To reduce frequency drift and mode jumps, the amplification system is operated in a constant temperature chamber. The phase noise of the amplified signal is depicted in the blue curve in Fig. 8. It can be seen that the phase noise is -93.79 dBc/Hz at the 10 kHz frequency offset. As a comparison, phase noise measurements are also performed on OEO with a loop length of 50 m, 10 km, and on the original RF signal. The experimental results demonstrate a 9.1 dB improvement in phase noise for a dual-loop OEO compared to a 50 m single-loop OEO and 18.87 dB for a 10-km single-loop OEO at 10 kHz offset frequency. Similarly, at an offset frequency of 1 MHz, the phase noise of the dual-loop OEO improves by 17.6 dB compared to the 50 m single-loop OEO and by 43.77 dB against the 10 km single-loop OEO respectively. Theoretically, the noise of OEO are dominated by thermal, shot, relative intensity noise (RIN) and amplifier spontaneous emission(ASE) noise. Since all this noise is bandwidth dependent, the use of a dual-loop OEO in our method makes the bandwidth of signals much narrower than that of a single-loop OEO [25], which allows for better filtering of spurious effects and side modes noise, providing an improvement of the phase noise. Therefore, the advantage of dual-loop OEO over single-loop OEO is not only amplify the signals but also significantly reduce the phase noise.
Fig. 7. Schematic diagram of the proposed RF signal amplification in the frequency transfer system. (LD: laser diode, MZM: Mach-Zehnder modulator, SMF: single-mode fiber, EDFA: erbium-doped fiber amplifier, PD: photodetector, ESA: electrical spectrum analyzer, SG: signal generator).
4. Conclusion
In conclusion, a unique high-gain narrowband amplifier based on a dual-loop OEO is propounded and verified, which sheds new light on amplifying low-power RF signal. In the proposed system, the dual-loop OEO is in a multimode oscillation state, with a short loop for large mode spacing and a long loop to reduce phase noise. The EDFA provides gain to compensate for the loss of the feedback loop to keep the OEO below the critical threshold. The discussed scheme provides an average gain greater than 22 dB for low-power RF signal with the sensitivity up to -91 dBm. It is worth noting that the 3 dB bandwidth of the demonstrated system is narrower than 1.2 kHz, which eliminates the noise effectively. In addition, the SNR of the amplified RF signal based on the dual-loop OEO is superior to that of the single-loop OEO, and phase noise improvement of 17.6 dB for dual-loop OEO compared to single-loop OEO of 50 m, and 43.77 dB over single-loop OEO of 10 km at 1MHz offset frequency. A key outcome of our work is to demonstrate that RF amplifier based on dual-loop OEO yields higher signal gain, narrower amplification bandwidth, and lower phase noise. Therefore, the proposed system has great potential to be applied in frequency transfer terminals to amplify low-power RF signal. Fortunately, our proposed technique is perfectly compatible with the stimulated Brillouin scattering. The oscillation frequency of the system can be easily adjusted over a wide range by tuning the frequency of the pump laser.
Funding
Open Fund of IPOC (BUPT) (IPOC2021A06); Science and Technology Program of Guangzhou (2019050001); Natural Science Foundation of Guangdong Province (2021A1515012652); National Natural Science Foundation of China (61774062, 61875057, 62175070).
Disclosures
The authors declare that there are no conflicts of interest related to this article.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
References
1. F. Riehle, “Optical clock networks,” Nat. Photonics 11(1), 25–31 (2017). [CrossRef]
2. Y. Guo, X. Wen, W. Lin, W. Wang, X. Wei, and Z. Yang, “Real-time multispeckle spectral-temporal measurement unveils the complexity of spatiotemporal solitons,” Nat. Commun. 12(1), 67 (2021). [CrossRef]
3. W. C. Swann, M. I. Bodine, I. Khader, J. D. Deschenes, E. Baumann, L. C. Sinclair, and N. R. Newbury, “Measurement of the impact of turbulence anisoplanatism on precision free-space optical time transfer,” Phys. Rev. A 99(2), 023855 (2019). [CrossRef]
4. L. C. Sinclair, H. Bergeron, W. C. Swann, E. Baumann, J. D. Deschenes, and N. R. Newbury, “Ultrastable long-haul fiber-optic radio frequency transfer based on dual-PLL,” IEEE Photonics J. 13(1), 1–8 (2021). [CrossRef]
5. B. P. Dix Matthews, S. W. Schediwy, D. R. Gozzard, E. Savalle, F. X. Esnault, T. Leveque, C. Gravestock, D. D Mello, S. Karpathakis, M. Tobar, and P. Wolf, “Point-to-point stabilized optical frequency transfer with active optics,” Nat. Commun. 12(1), 515 (2021). [CrossRef]
6. W. Xue, W. Zhao, H. Quan, Y. Xing, and S. Zhang, “Cascaded microwave frequency transfer over 300-km fiber link with instability at the 10−18 level,” Remote Sens. 13(11), 2182 (2021). [CrossRef]
7. R. Wu, J. Lin, T. Jiang, C. Liu, and S. Yu, “Stable radio frequency transfer over fiber based on microwave photonic phase shifter,” Opt. Express 27(26), 38109–38115 (2019). [CrossRef]
8. C. Clivati, R. Aiello, G. Bianco, C. Bortolotti, P. De Natale, V. Di Sarno, P. Maddaloni, G. Maccaferri, A. Mura, M. Negusini, F. Levi, F. Perini, R. Ricci, M. Roma, L. Santamaria Amato, M. Siciliani de Cumis, M. Stagni, A. Tuozzi, and D. Calonico, “Common-clock very long baseline interferometry using a coherent optical fiber link,” Optica 7(8), 1031–1037 (2020). [CrossRef]
9. H. Zhang, L. He, X. Li, Y. Xiao, P. Qu, and L. Sun, “Wideband microwave frequency distribution for multi-access along a single fiber link,” J. Lightwave Technol. 38(7), 1688–1692 (2020). [CrossRef]
10. Y. Dong, Z. Liu, X. Wang, N. Deng, W. Xie, and W. Hu, “Distribution of millimeter waves over a fiber link with high frequency stability,” Chin. Opt. Lett. 14(12), 120006 (2016). [CrossRef]
11. T. Hao, J. Tang, D. Domenech, W. Li, N. Zhu, J. Capmany, and M. Li, “Toward monolithic integration of OEOs: From systems to chips,” J. Lightwave Technol. 36(19), 4565–4582 (2018). [CrossRef]
12. M. Li, T. Hao, W. Li, and Y. Dai, “Tutorial on optoelectronic oscillators,” APL Photonics 6(6), 061101 (2021). [CrossRef]
13. M. S. Rasras, D. M. Gill, S. S. Patel, K. Y. Tu, Y. Chen, A. E. White, A. T. Pomerene, D. N. Carothers, M. J. Grove, and D. K. Sparacin, “Demonstration of a fourth-order pole-zero optical filter integrated using CMOS processes,” J. Lightwave Technol. 25(1), 87–92 (2007). [CrossRef]
14. X. Xie, Y. Dai, K. Xu, J. Niu, R. Wang, L. Yan, Y. Ji, and J. Lin, “Digital joint compensation of IMD3 and XMD in broadband channelized rf photonic link,” Opt. Express 20(23), 25636–25643 (2012). [CrossRef]
15. L. V. Nguyen, “Microwave photonic technique for frequency measurement of simultaneous signals,” IEEE Photonics Technol. Lett. 21(10), 642–644 (2009). [CrossRef]
16. J. Yao, “Microwave photonics,” J. Lightwave Technol. 27(3), 314–335 (2009). [CrossRef]
17. J. Capmany and D. Novak, “Microwave photonics combines two worlds,” Nat. Photonics 1(6), 319–330 (2007). [CrossRef]
18. P. S. Devgan, M. W. Pruessner, V. J. Urick, and K. J. Williams, “Detecting low-power RF signals using a multimode optoelectronic oscillator and integrated optical filter,” IEEE Photonics Technol. Lett. 22(3), 152–154 (2010). [CrossRef]
19. Y. Shao, X. Han, M. Li, and M. Zhao, “RF signal detection by a tunable optoelectronic oscillator based on a PS-FBG,” Opt. Lett. 43(6), 1199–1202 (2018). [CrossRef]
20. Z. Zhu, M. Merklein, D. Y. Choi, K. Vu, P. Ma, S. J. Madden, and B. J. Eggleton, “Highly sensitive, broadband microwave frequency identification using a chip-based brillouin optoelectronic oscillator,” Opt. Express 27(9), 12855–12868 (2019). [CrossRef]
21. X. Zhang, T. Pu, Y. Zhang, H. Zhou, J. Li, H. Zhu, X. Chen, and J. Zheng, “Ultra simple low-power RF signal detection based on an optoelectronic feedback DFB laser,” Opt. Express 29(19), 29641–29650 (2021). [CrossRef]
22. D. Wang, T. Jiang, C. Liu, S. Zhou, and S. Yu, “Stable radio frequency dissemination via a 1007 km fiber link based on a high-performance phase lock loop,” Opt. Express 26(19), 24479–24486 (2018). [CrossRef]
23. B. Wang, C. Gao, W. Chen, J. Miao, X. Zhu, Y. Bai, J. Zhang, Y. Feng, T. Li, and L. Wang, “Precise and continuous time and frequency synchronisation at the 5× 10-19 accuracy level,” Sci. Rep. 2(1), 556 (2012). [CrossRef]
24. X. S. Yao and L. Maleki, “Multiloop optoelectronic oscillator,” IEEE J. Quantum Electron. 36(1), 79–84 (2000). [CrossRef]
25. T. Bánky, B. Horváth, and T. Berceli, “Optimum configuration of multiloop optoelectronic oscillators,” J. Opt. Soc. Am. B 23(7), 1371–1380 (2006). [CrossRef]
