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A phase fluctuation cancellation approach for anonymous microwave signal transmission over fiber link is proposed and demonstrated. Unlike most previous schemes that used for active systems, our proposal is suitable for passive systems by utilizing the optical signal feedback and electrical signal phase-locking. Experimental results show that phase drifts of 7.7-ps, 54-ps and 96-ps (RMS value) for 2.45-GHz signals could be reduced to 3.1-ps, 3.8-ps and 8.5-ps after 1-km, 10-km and 25-km SMF transmission over an eight-hour period, respectively. Overall system performance is limited by the coherent Rayleigh noise and could be further optimized.
© 2014 Optical Society of America
1. Introduction
Highly stable distribution of microwave signals over optical fiber links attracts growing interests and attentions due to its diverse applications, including remote clock synchronization [1,2], fundamental physics [3,4] and radio telescope arrays [5,6], etc. Recently, researchers reported that the remote radio frequency transfer using optical fiber technologies could enhance the signal stability by a few orders of magnitude comparing to satellite based techniques [7–10]. Many schemes have been proposed to compensate for the phase fluctuation introduced by environment perturbations such as physical vibration and temperature change along the transmission fiber [11–15]. All of them, to our best knowledge, are used for active systems, in which the local signals are generally generated at the local sites and then distributed to multiple remote sites [1–15]. On the other hand, with the development of multi-static radar systems and passive ranging/location in electronic warfare applications, multiple anonymous microwave signals received in the remote sites need to be transmitted to the local site over fiber links for centralized processing to obtain time or phase difference values among them [16–18]. Such system configuration is also known as passive system (Fig. 1). Similar to active systems, phase fluctuations introduced by fiber links in passive systems lead to time difference detection errors in the local site, therefore it’s also essential to effectively cancel phase fluctuations for such systems.
In this paper, we propose a new approach to stabilize the phase of anonymous microwave signals transmitted over fiber link from the remote site to the local site in passive systems. A fiber-Bragg-grating (FBG) at the remote side is used to automatically reflect the transmitted optical signal for reference, while an electrical phase-locked loop (PLL) based on phase difference detection and electronic phase shifters (EPSs) is utilized in the local site to compensate for the phase fluctuation. Optical Erbium-doped fiber amplifier (EDFA) and electrical low noise amplifiers (LNAs) are used to improve the signal-to-noise ratio (SNR) and adjust the power of microwave signal for optimized phase detection. Experimental results demonstrate that the phase fluctuation of 7.7-ps, 54-ps and 96-ps (RMS value) for 2.45-GHz microwave signals could be effectively reduced to 3.1-ps, 3.8-ps and 8.5-ps after 1-km, 10-km and 25-km SMF transmission over an eight-hour period, respectively. On the other hand, we note that the phase stability for 25-km link is mainly limited by coherent Rayleigh noise (CRN).
2. Operation principle
The schematic diagram of the phase fluctuation compensation system is shown in Fig. 2. Anonymous microwave signal with an initial phase of θref is amplitude modulated on the optical carrier with a centre 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 EPS1 by θ0-θc(t) and sent to the mixer, where θ0is the initial fixed phase shift corresponding to the reference voltage V0 applied to the EPS, θc(t) is the phase shift corresponding to proportion-integration (PI) of the output DC voltage of the mixer. (ii) The second branch modulates another optical carrier with a centre wavelength of λ2, which is launched back to the same fiber link (i.e. double-path) and then reflected to the local site (i.e. triple-path) by a fiber Bragg grating (FBG) with the Bragg wavelength of λ2. 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.
Fig. 2 Schematic diagram of the proposed phase fluctuation compensation system. EPS: electronic phase shifter; FBG: fiber-Bragg-grating; PLL: phase-locked loop.
Given that the angular frequency of the anonymous 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. θc(t) can be adjusted by the PI of the output DC voltage of the mixer that depends on the phase difference between the one-path and triple-path signals.We define the phase difference 2(θc(t) + θf(t)) between the one-path and triple-path signals as the phase error signal θe(t), then the following relationship can be obtained:
(1)where k is the proportion coefficient. As shown in Fig. 3, voltage uc(t) corresponds to the proportion-integration (PI) of the cosine value of the phase difference (i.e. 2(θc(t) + θf(t)). Only when we adjust θc(t) + θf(t) = π/4 and make cos(2(θc(t) + θf(t)) = 0, as well as θe(t) = π/2, uc(t) becomes a constant value and the PLL gets locked. 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. More detail explanations could be found in the description of the experimental setup in Section 3.
3. Experimental setup and results
Figure 4 illustrates the experimental setup of our approach. A 10-dBm and 2.45-GHz anonymous 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). The other branch issued to modulate an optical carrier (a tunable laser source set atλ1 = 1550nm) through a Mach-Zehnder modulator (MZM) biased at Vπ/2. The output power of the laser source is 10-dBm with a 3-dB linewidth of 100-KHz and an intensity noise of −145dBc/Hz. The modulated optical carrier is then passing through a FBG with Bragg wavelength ofλ2 = 1552nm 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. The output power of the EDFA is fixed at 4-dBm. The original microwave signal is recovered by PD-1 and then divided into three branches after passing through a 2400~2500MHz band pass filter (BPF) and amplified by a low noise amplifier (LNA).
Fig. 4 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; FBG: fiber Bragg grating; OC: optical circulator; WDM: wavelength division multiplexer; PD: photo-detector; BPF: band pass filter; LNA: low noise amplifier; EPS: electronic phase shifter; LPF: low pass filter; PLL: phase-locked loop; 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 similar laser source with a centre wavelength of λ2 = 1552nm. 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 (i.e. double-path) and reflected by the FBG to the local site (i.e. triple-path). After EDFA, the triple-path optical signal is detected by PD-2. The detected three-path signal is amplified by another LNA after passing through the similar 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 to be the final output signal.
A highly accurate compensation circuit is designed to generateV0 ± Vc(t). 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, the control voltage V0 + Vc(t) applied to EPS3 becomes stable, subsequently the phase difference between the final output signal and the original signal becomes a constant value. EPS1-3 used in the experiment are with 5-MHz dynamic bandwidth to achieve fast locking of the PLL [19], and with −75dBc/Hz@2.45GHz phase noise at 10-Hz frequency offset to have negligible effects on the phase noise of the final output signal.
To evaluate the performance of our proposed phase fluctuation compensation scheme, we implement a comparative study by using three different lengths of fiber links (i.e. 1-km, 10-km and 25-km). Figure 5 shows the phase difference between input original signal and final output signal through (a) 1-km, (b) 10-km and (c) 25-km fiber link over 8 hours measured by the oscilloscope. As an example, for the 1-km fiber link case shown in Fig. 5(a), the root-mean-square (RMS) phase jitter is 7.7516ps without compensation. While after compensation, the RMS phase jitter is reduced to 3.1718ps. The phase jitter values without compensation for 10-km and 25-km are 54.1456ps and 96.1845ps, while the corresponding jitter values after compensation are 3.7643ps and 8.5469ps, respectively. The results in Figs. 5(a)-5(c) demonstrate that the phase fluctuation can be effectively compensated by the proposed approach. Frequency noise of the compensated signal that is higher than the uncompensated signal is also observed in Fig. 5, which is caused by the intensity noise of the control voltage (i.e. partially by the bias-control circuit).
Fig. 5 Measured phase difference between the input anonymous signal and the final output signal over (a) 1-km (b) 10-km and (c) 25-km fiber link (blue: uncompensated; red: compensated).
4. Affect of the CRN
As shown in Figs. 6(a)-6(c), when the fiber length is 1-km and 10-km, the phase difference after compensation exhibits very stable performance over time (i.e. eight hours), while it does exhibit a slight increasing trend versus time for the 25-km fiber link case. It indicates that the phase shift value of the EPS3 is not strictly equal to the phase fluctuation introduced by the fiber link. It should be caused by the inaccuracy of the control voltage, which is decided not only by the phase difference between the one-path signal and triple-path signal, but also the amplitudes of them. Therefore we measure the output electrical power stabilities of the one-path signal and triple-path signal for three link cases by using the “max_hold” and “min_hold” function of the spectrum analyzer. Since the peak-power jitter of one-path signal is very slight, we only show the triple-path results in Fig. 6.
Fig. 6 Max and min power value of the triple-path signal sent to mixer after (a) 1-km (b) 10-km and (c) 25-km fiber link transmission (blue: min value; red: max value).
The results in Figs. 6(a)-6(c) correspond to 1-km, 10-km and 25-km fiber link cases for 10-minute measurements, respectively. With the increase of the fiber length, the jitter of the peak power gets bigger. It is mainly caused by the stronger CRN due to forward and backward transmission of the optical carrier with the centre wavelength λ2. The peak power jitter of the triple-path signal through 25-km fiber link (i.e. 5.0488dB) is about 11 times and 19 times higher than that of the signals over 10-km (i.e. 0.4618dB) and 1-km fiber link (i.e. 0.2626dB), respectively. The higher power jitter over 25-km fiber link leads to higher inaccuracy on the control voltage of EPS3 and the appearance of the slight increasing trend shown in the red line of Fig. 6(c).
In addition, CRN also affects the phase noise of the measured final output signal, because the CRN induced intensity noise (i.e. power jitter) added on the control voltage can convert into phase noise of the final output signal through EPS3. The single-sideband phase noise between 10Hz and10MHz of the final output signal with/without compensation through three fiber links (i.e. 1-km, 10-km and 25-km) are measured by an RF spectrum analyzer and plotted in Figs. 7(a)-7(c), respectively. For 1-km and 10-km fiber link cases, there’s no phase noise degradation being observed, however, for 25-km fiber link case, an obvious degradation on the phase noise is measured. It is possible to further reduce CRN by various approaches, such as utilizing AGC-LNA [2].
Fig. 7 Phase noise of the measured final output signal through (a) 1-km (b) 10-km and (c) 25-km fiber link with/without compensation (blue: uncompensated; red: compensated).
5. Conclusion
In this paper, aiming for its applications in passive systems, we proposed and demonstrated a phase fluctuation cancellation approach for anonymous microwave signal transmission over fiber links. The approach utilizes the optical reference between one-path and triple-path signals for electrical phase locking. Results over eight-hour measurements indicate the effectiveness of our approach, while the CRN-limited performance for long fiber links (i.e. 25-km) could be further optimized.
Acknowledgments
The research is supported in part by the National Basic Research Program of China (2012CB315704), National Natural Science Foundation of China (No. 61325023, 61335005), Key Grant Project of Chinese Ministry of Education (No.313049), and the International Science and Technology Cooperation Program of China (2014DFA11170).
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