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Abstract
We demonstrate a radio frequency (RF) phase-stable transmission over fiber based on microwave photonic phase shifter. In the proposed system, both assistant RF signals are applied to drive two arms of a dual-drive Mach-Zehnder modulator (DMZM), respectively. An optical bandpass filter is followed to filter out the first-order sideband of optical modulated signal. Due to the phase independence between two optical sidebands, the phase perturbation caused by fiber-length variations can be compensated automatically via controlling the direct-current bias voltage of the DMZM. We have performed RF transfer in a 155 km single-mode fiber with a frequency instability of 3.05 × 10−17 at 10,000 s averaging time.
© 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Ultrastable frequency standard remote transfer plays a significant role in numerous areas of human activity, such as scientific investigations, accurate navigation and military systems [1–4]. Currently, frequency standard remote distribution is usually accomplished by Global Positioning System (GPS) or Two-Way Satellite Frequency Transfer (TWSFT), but these conventional satellite links have met bottlenecks to deliver highly stable frequency source standards [5,6]. It has been shown that optical fiber link has the potential to disseminate highly stable frequency standards over very long distances, due to the advantages of large bandwidth, high reliability, low attenuation, and electro-magnetic interference immunity [2,7–9]. Although the optical fiber link is a promising alternative medium, fiber-based frequency transfer techniques still suffer from mechanical perturbation and temperature variation along the optical fiber link, which degrades the frequency stability.
Over the past few years, there have been many different fiber-based frequency distribution technologies greatly improved the frequency transfer field, including the transfer of optical frequency standard [10–12], optical comb [13,14], and radio-frequency (RF) [15–20]. The REFIMEVE project demonstrated optical frequency standard dissemination over a 1480 km optical fiber link with a relative frequency stability of $1{0}^{-16}$ at 1 s and $1{0}^{-19}$ at 10,000 s [11]. In 2016, an optical comb has been transferred over 1000 km optical fiber link with a fractional frequency stability of $2.75 \times {10^{ - 15}}$ at 1 s and $3.91 \times {10^{ - 21}}$ at 120,000 s [14]. Although optical frequency standard dissemination and optical comb transfer over fiber achieve incomparable advantages of transmission distance and frequency stability, the most practical applications of frequency transfer in science, commerce, and industry require RF signals to directly interface with their electronic systems [2]. Most of these RF phase stabilization actuators are based on piezoelectric fiber stretcher [15], temperature-controlled optical delay line [16], wavelength tunable laser [17], phase conjugation [18,19], or dual-parallel Mach-Zehnder modulator (DPMZM) [20]. However, these actuators above are complicated and have some performance limitation for their deployment in the future. For example, the piezoelectric fiber stretcher has a small compensation range. And, it will also induce an additional polarization mode dispersion and amplitude noise [21]. The temperature-controlled optical delay line has a relatively large compensation range but bulk and high power consumption. Adjusting laser wavelengths method causes channel crosstalk. The phase conjugation suffers from various levels of conversion loss.
In this paper, we report and demonstrate a novel technique for stable distribution of a RF standard signal over the conventional optical fiber link. Rather than the use of widely-reported piezoelectric fiber stretcher and temperature-controlled optical delay line, we utilize a microwave photonic phase shifter to compensate the phase fluctuations. The microwave photonic phase shifter is made up of a dual-drive Mach-Zehnder modulator (DMZM) and an optical bandpass filter (OBPF). In the proposed system, two assistant RF signals modulate either arm of the DMZM, respectively. After the following OBPF, the first-order optical sidebands of both auxiliary RF signals are filtered out, acting as a microwave photonic phase shifter, and then transferred to the remote site (RS). Due to the phase independence between two optical sidebands, the photocurrent detected by the remote photodetector (PD) has a continuously variable phase shift from 0 to 360 deg [22,23]. Therefore, the phase perturbation caused by fiber-length variations can be compensated automatically via controlling the direct-current (DC) bias voltage of the DMZM. The proposed system is not only relatively simple, but also overcomes periodic power fading due to chromatic dispersion [24]. Then the 100 MHz RF signal transfer over 155 km lab fiber experiment is demonstrated. The experimental result shows that the frequency instability of $3.05 \times {10^{ - 17}}$ at 10,000 s averaging time is achieved.
2. Principle
The diagram of the stable RF dissemination scheme based on microwave photonic phase shifter is illustrated in Fig. 1. In order to deliver the RF signal with angular frequency of $\omega _{0}$ to the RS, two assistant RF signals with angular frequency of $\omega _{1}$ and $\omega _{2}$ are utilized in the local site (LS), where ${\omega _1} - {\omega _2} = {\omega _0}$. Mathematically, we can assume that both assistant RF signals at the LS have the initial phase of zero. The continuous-wave (CW) light wave from a tunable laser with the angular frequency of $\omega _{c}$ is modulated by two auxiliary RF signals via the dual-drive Mach-Zehnder modulator (DMZM). It is worthwhile to note that each arm of the DMZM can be considered as a phase modulator. After we apply an optical bandpass filter (OBPF) to filter out the first-order optical sideband of both ancillary RF signals, a phase tunable optical single sideband (SSB) signal at the output of the microwave photonic phase shifter is generated, which can be expressed as
Fig. 1. Schematic of stable RF dissemination. LS, local site; RS, remote site; CWL, continuous-wave laser; DMZM, dual-drive Mach-Zehnder modulator; OBPF, optical bandpass filter; OC, optical coupler; EC, electrical coupler; CIR, circulator; PD, photodetector; Bi-EDFA, bi-directional Erbium Doped Fiber Amplifier; PDM, phase demodulator; PID, proportional-integral-derivative regulator module.
At the LS, two phase demodulators (PDMs) are used to obtain the phase errors between RF standard signal and phase-shifted RF signal, $\theta$; between RF standard signal and round-trip probe RF signal, $\theta + 2\varphi$. Then, the sum of phase errors, which is given as
is integrated in a proportional-integral-derivative (PID) regulator module to generate the DC bias voltage applied to DMZM. When the loop is locked, the steady state error is zero, i.e., Afterwards, it can be seen that the delivered RF signal ${V_2}\left ( t \right )$ is independent of the phase fluctuation induced by the optical fiber link. As a result, we can obtain a replication of the RF standard signal at the RS.3. Experiment setup and results
To verify the performance of the proposed scheme, a proof-of-concept experiment was carried out based on Fig. 2. Firstly, at the LS, an optical carrier with the power of 13.5 dBm is generated by a tunable laser source (Southern Photonics, TLS150D) and injected into a commercial DMZM (Fujitsu, FTM7937EZ). Secondly, both signal generators (Agilent, E8267D and E8257D) locked to an atomic clock (Quartzlock, A1000) provide two assistant RF signals with the frequency of 18.1 GHz and 18GHz. One branch of two assistant RF signals are mixed with each other to generate a 100 MHz RF standard reference signal, which is then used to compare with the phase-shifted RF signal, round-trip RF signal and the replicate RF signal by time interval counter (TIC) (Keysight, 53230A). The TIC has the advantage of high-speed comparison between the two input RF with up to 350 MHz. In addition, the other branch of two ancillary RF signals are applied to drive the up arm and down arm of the DMZM, respectively. Thirdly, in order to filter out the first-order optical sideband of two assistant RF signals, the center frequency of TLS is set at 193.3746 THz to match the OBPF (Max-ray, FBG-501). We measured the optical signal spectrum before and after OBPF by a high-resolution optical spectra analyzer (OSA) (Advantest, Q8384). As shown in Fig. 3(a), two $-1st$ optical sidebands are suppressed by more than 43 dB compared with two $+1st$ optical sidebands. Fourthly, the optical fiber link, which consists of a 75 km and 80 km standard G.652 single-mode optical fiber spools, is employed to simulate the real fiber link. Between the optical fiber spools, the bi-directional Erbium Doped Fiber Amplifier (Bi-EDFA) is used to compensate the optical fiber transmission loss. And all of the optical-to-microwave conversion are achieved by the photodetector (KangGuan, KG-APR-200M). Fifthly, the sum of phase differences between RF reference signal and phase-shifted RF signal, between RF reference signal and round-trip probe RF signal are sent to computer for digital PID process. At last, by carefully adjusting the parameters of PID, the programable direct current source controlled by PID can change the DC bias voltage applied to DMZM to compensate the phase drift, automatically.
Fig. 2. Experimental setup of frequency dissemination system. LS, local site; RS, remote site; TLS, tunable laser source; SG: signal generator; DMZM, dual-drive Mach-Zehnder modulator; OBPF, optical bandpass filter; OC, optical coupler; EC, electrical coupler; CIR, circulator; PD, photodetector; TIC: time interval counter; PDCS: programable direct current source.
Fig. 3. (a)Optical modulated signal spectrum before and after the OBPF. (b)Fractional frequency instability of the 155 km free running fiber link and the proposed compensated link.
It is worthwhile to mention that, systems for the LS and the RS were placed at our laboratory to check the performance of our compensation system. We used modified Allan deviation (ModADEV) to characterize the frequency instability in this paper. It is because ModADEV has the ability of identifying white and flicker phase noise, rather than standard Allan deviation [25]. In Fig. 3(b), the red and green lines stand for the frequency instability of the signal transfer with compensation, and free running, respectively. It can be seen that the 155 km phase compensated link can reduce the short-term instability compared with the free running link, from about $7.72 \times {10^{ - 11}}$ to $9.56 \times {10^{ - 14}}$ at 1 s. Moreover, the proposed system also reduces frequency instability near three orders of magnitude from $4.25 \times {10^{ - 14}}$ to $3.05 \times {10^{ - 17}}$ at 10,000 s averaging time. Figure 3(b) shows that the slope of curves change from $\tau ^{ - 3/2}$ to $\tau ^{ - 1}$ for averaging times of less than 100 s, indicating that the white phase noise has been suppressed and the residual flicker phase noise dominants the phase noise performance. In contrast, with auto-compensation for phase fluctuation, the ModADEV slope is close to $\tau ^{ - 1/2}$ for averaging times of more than 100 s, suggesting that the white frequency noise dominates.
Note that when the round-trip auxiliary signal using the same optical carrier transmitted in the same fiber link, the performance of RF signal will be degraded by the reflections from the connectors and Rayleigh backscattering originating on the optical fibers [26,27]. These phenomena has been well discussed in Refs. [2,28]. There are various methods to reduce the effects of reflections and Rayleigh backscattering. For the reflections, we can fusion-splice the fiber at all the previously-connectorized junctions [29]. For the Rayleigh backscattering, the transceiver at remote end can slightly shift the carrier frequency or return a harmonic of the modulation frequency, which allows optical filters to isolate the forward optical signal and the Rayleigh backscattering [2,10,30]. These solutions are able to eliminate these back reflections. Fortunately, our proposed technique is perfectly compatible with these solutions mentioned above. In addition, the results demonstrate that the employed microwave photonic phase shift technique can reduce frequency instability compared with the non-compensated case. Its performance and transfer distance can be further improved by utilizing TIC with higher resolution and more Bi-EDFAs.
4. Conclusion
In summary, we propose and demonstrate a stabilized RF signal delivery scheme based on microwave photonic phase shifter. Difference from the piezoelectric fiber stretcher, temperature-controlled optical delay line, wavelength tunable laser and hybrid RF modulation, the RF phase perturbation induced by the optical fiber link can be compensated by simply changing the DC bias voltage of the DMZM. Experimentally, a 100 MHz RF signal is transferred through a 155 km optical fiber link, which showed the frequency instability $9.56 \times {10^{ - 14}}$ at 1 s and $3.05 \times {10^{ - 17}}$ at 10,000 s averaging time. The experimental result shows that the proposed system has the potential to disseminate highly stable frequency standards over long distances. The performance and RF transfer distance can be further increased by applying digital phase discriminator with higher resolution and more Bi-EDFAs in optical transmission link.
Funding
National Natural Science Foundation of China (61427813, 61531003, 61690195, 61701040); BUPT Action Project for Promoting the Development of Scientific and Technological Innovation (2019XD-A18).
Disclosures
The authors declare that there are no conflicts of interest related to this article.
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