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Abstract
An optical comb based stable frequency dissemination system is proposed and experimentally demonstrated over a multi-access optical fiber loop link. In the system, a new technique based on optical-microwave phase locking loop is designed for phase compensation. In the experiment, a mode-locked fiber laser at a repetition rate of 100 MHz is used to provide an optical source at local site, then it transmits along a 150 km fiber loop link. To testify the proposed system, two accessing nodes are measured in the loop link. The dissemination frequency instability is measured at 3.65 × 10−15/1 s and 7.8 × 10−18/1000 s at the intermediate node. The similar performance is shown at the other node. Hence, the system has the potential application in high-precision frequency transmission system via a long-haul multi-access loop link.
© 2018 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
A large number of applications require high stable frequency dissemination, which include fundamental physics measurement, antenna arrays, deep space network, dissemination of frequency standards, high precision navigation and long-distributed radio telescopes [1–4]. Due to the advantages of low cost, low attenuation, high reliability, and immunity to electromagnetic interference, optical fiber link plays an instrumental role in stable frequency transmission [5–7]. The stability of phase dissemination will be destroyed via a long distance fiber link, because fiber length is sensitive to the temperature changes and vibrations of environment. In order to solve the problem, various schemes have been reported to improve the stability of the signals transmitted via optical fiber link [8–14].
Recently, optical combs based frequency transfer techniques lead research in terms of improving transfer distance, enhancing stability, achieving different topologies, etc. Optical combs are crucial in providing a straightforward means for comparing atomic clocks with disparate transition frequencies [15]. In addition, transmission based on optical comb simultaneously combines low-noise optical waves and radio frequency (RF) signals. What’s more, multiple frequency signals can be obtained with multiple repetitions and deterioration of the generated signal is avoided owing to omission of frequency multiplier which is used in RF transfer by continuous wave (CW) lasers. The stable frequency transfer based on optical comb with compensation of piezoelectric-transducer and motorized stages achieves 6.5 × 10−19 level at 82500 s averaging time over a 2.3 km point-to-point link [4]. The transfer by optical comb compensated with fiber stretcher and thermally controlled spool reaches 4 × 10−17 after 1600 s over 86 km fiber link [7]. Many important applications require high-precision signal transmitted to multi-user in a fiber loop link, such as radio astronomy, metropolitan area network. Application scenario of frequency dissemination in metropolitan area loop link is shown in Fig. 1. The reference frequency generated from standard station is steadily transferred to each node. In the past few years, many ultra-stable multi-access frequency dissemination schemes have been demonstrated [16–19]. While, multi-access RF transmission in loop link is mainly based on Mach-Zehnder modulator and CW lasers [17]. Hence, optical comb based multi-access loop link system with a cover range over 100 km is required, but has not been proposed.
In this paper, we demonstrate an optical comb based stable RF transfer system over a multi-access fiber loop link. The phase compensation of the transmitted signal is accomplished by adopting an optical-microwave phase locking loop (OMPLL). The optical comb signal is directly injected into the loop link. Thus, the electro-optical modulation process is omitted and signal-to-noise ratio of the optical comb is maintained properly. The OMPLL consists of a loop-based optical-microwave phase detector (FLOM-PD) for suppressing the phase noise which results from direct optical-to-electronic conversion. The scheme is integrated with a wavelength division multiplexing (WDM) system for frequency dissemination by transmitting mode-locked laser (MLL) pulses referenced to a RF standard in a 150 km long fiber link. In the system, only one optical source is employed, which can not only simplify the system structure but also make it easier to insert more nodes. After compensation, phase stabilized signal can be obtained by simply mixing the transmission signals of two channels together. An experiment is carried out, the system without using mechanical or optical delay lines can offer a fractional instability of 3.65 × 10−15 and 2.67 × 10−15 for averaging time of 1 s at the intermediate node and the other node, respectively. Thus, the repetition rate and its harmonic signals from the disseminated optical comb can be reproduced stably at nodes along the loop link.
2. Principle
The schematic diagram of the frequency dissemination is depicted in Fig. 2. For receiving frequency without phase variations at remote site (RS), the compensation system based on OMPLL is set up at the local site (LS). The OMPLL is comprised of a FLOM-PD, a proportional-integral controller and an optical comb. An arbitrary node is picked as RS in the link. At LS, a rubidium atomic clock is used as the reference of a RF source. A MLL based on a nonlinear polarization rotation mechanism is employed as the optical source at LS. 80% of the laser output power is coupled to a WDM. The comb bandwidth is reduced to a standard optical communication channel. It overcomes the major comb transfer limitations of the fiber dispersion and the high pulse intensity-induced nonlinearity and becomes less sensitive to dispersion of the comb transmission. Furthermore, a reduced spectral bandwidth also results in a longer pulse to a few ps and a significantly reduced peak power of the pulses. The transmitted pulse is ∼4 ps. λ1 and λ2 are used for the clockwise and anticlockwise transferred signals. The LS and RS are at the same location for dissemination evaluation measurement. To avoid the pulse broadening due to the fiber dispersion, the comb is filtered to 0.8 nm by a WDM channel bandwidth. At the LS, the backward pulse train is received at channel λ1, which makes the system free of reflection perturbation for detection. The 100 MHz RF signal is extracted from the round-trip transmitted pulse train and the source laser, respectively, by low noise photodiodes. The phase error is then detected by a frequency mixer. When direct photo-detection is used to lock MLL with feedback signal excess phase noise is added in the optical-to-electronic conversion process due to nonlinearity, saturation, temperature drift and amplitude-to-phase conversion in photodiodes [20]. Therefore, the synchronization between the compensated RF signal and the MLL is made through a FLOM-PD. The FLOM-PD is used for suppressing the amplitude of phase noise resulting from direct optical-to-electronic conversion by using photodiodes and has been demonstrated as an efficient RF-optical synchronizer [4]. Direct photo-detections are used for recovery of the RF signals. Because, there is no need for the lock of optical frequency comb here. These problems in direct photo-detection can be generally ignored with proper optical power into the photo detector (PD), when PD is working at linear area.
Fig. 2 Schematic of frequency dissemination. MLL: mode locked laser; FM: frequency multiplier; FLOM-PD: fiber loop optic microwave phase detector; PD: photo detector; FD: frequency divider; OBPF: optical band-pass filter; EBFP: electrical band-pass filter; SMF: single-mode fiber; WDM: wavelength division multiplexing; LS: local site; RS: remote site; PIC: proportional-integral controller; OMPLL: optical-microwave phase locking loop.
The active phase locking with passive phase-conjugate noise cancellation of the proposed scheme can be described as follows. Our goal is to transmit a optical comb signal to an arbitrary remote station via single-mode fiber (SMF) with stabilized phase. In the LS, the optical pulses form the MLL are divided into two parts by WDM, one for the loopback transmitting and the other transferred to the RS.
The multiple-frequency signal from standard RF signal can be expressed as cosine function:
where ω0 and ϕ0 are its angular frequency and initial phase, respectively. The amplitude is normalized for conciseness. The signal from the comb can be denoted as where ω1 and ϕ1 are its angular frequency and initial phase, respectively.The optical comb is transmitted back to the LS along the entire optical fiber loop link as a probe signal. The system is based on the well-known assumption that the signal experiences the same phase fluctuation in the forward and backward links [21]. The difference of phase shifts between forward and backward signals, which is caused by the polarization state drifts and polarization mode dispersion (PMD), is ignored in the upper SMF. At the LS, the probe signal is detected by a low noise PD and goes through an electrical band-pass filter (EBPF) with the center frequency of ω1 to get a cleared round-trip probe signal which can be denoted as
where Δϕ is the phase fluctuation corresponding to the entire fiber loop link.The phase fluctuation varies with thermal and mechanical fluctuations. Another branch of the standard signal is mixed with the cleared round-trip probe signal E2. The output of mixer1 is filtered by BPF with the center frequency of ω1 to generate a beat signal, which can be written as
At the RS, a 2 × 2 fiber coupler is placed at an arbitrarily chosen node. It is used to couple out the clockwise and the anticlockwise transferred optical signals in the fiber link. The forward-traveling signal and the backward signal can be denoted as
Here, ϕa and ϕb are the phase fluctuations of the clockwise and anticlockwise transferred signals corresponding to the angular frequency ω1 respectively. They have a relationship of Δϕ = ϕa + ϕb. Then the two signals are mixed and the term of sum frequency is filtered. The output signal can be written as
When the mode-locked laser is locked with the signal E3 by a proportional-integral controller (PIC), then E1 = E3. The relation between ω0 and ω1 can be expressed as
The relation between ϕ0 and ϕ1 can be expressed as
From the view of Eqs. (8) and (9), the Eqs. (2) and (7) can be written as
It’s clear that the signal E6 received at an arbitrary remote station has the same frequency and phase with the standard RF signal generated at the central station. Therefore, the vibration of the phase is effectively reduced.
3. Experiment setup and results
To verify the performance of the proposed scheme, an experiment is carried out based on Fig. 2. The experimental setup of the system is illustrated in Fig. 3. The system is experimentally examined via a 150 km fiber in the laboratory. The repetition rate and average power of MLL are 100 MHz and 22 mW, respectively. Two channels are carried out from WDM for the transmission system. The OBPFs select channels 33 and 34 on the 100 GHz International Telecommunication Union (ITU) grid. Central wavelengths of 33 and 34 ITU channels are 1550.92 nm and 1550.12 nm, respectively. It should be noted that we employ two signal generators as frequency multipliers. In the experiment, a 10 MHz rubidium oscillator (Quartzlock, A1000) passes through the synchronization of signal generator (Stanford Research Systems, SG386) to obtain 20 MHz RF signal. ω0 is set to be 20 MHz at the LS. 900 MHz signal generated by another signal generator is synchronized from E2 as shown in Fig. 3. A high-speed servo controller (Newport, LB1005) as the PIC is used for locking the signal to the MLL. The lock between the feedback signal and MLL is achieved through PZT. During this experiment, clockwise light does not return to the central station. Thus, unidirectional erbium doped fiber amplifiers (EDFAs) meet the demand of optical amplification. Five low phase noise EDFAs are used to amplify the amplitude of optical signals. Table 1 shows the EDFA powers, gains and output powers of EDFA1 to EDFA5 at the intermediate node. The EDFA powers are the internal pump power of EDFAs. The optical powers detected by PD1 to PD3 are 0.34 mW, 0.34 mW, and 0.36 mW, respectively. The same amplifiers are employed at the other node. At the RS, the signals filtered by the BPFs are detected by PD1 and PD2. By using a mixer, signal with frequency of 20 MHz coherent to the frequency standard can be provided to users.
Fig. 3 Schematic of frequency dissemination at intermediate node. MLL: mode locked laser; FLOM-PD: fiber loop optic microwave phase detector; PZT: piezoelectric transducer; PD: photo detector; FD: frequency divider; OBPF: optical band-pass filter; SMF: single-mode fiber; PD: photo detector; EBFP: electrical band-pass filter; EA: electrical amplifier; LS: local site; RS: remote site; PIC: proportional-integral controller; DAQ: data acquisition card.
Table 1. EDFA powers, gains and output powers of the EDFA1 to EDFA5.
A data acquisition card (NI USB-6251) logs the output voltage of the phase comparison module at the sampling rate of 100 Hz. The recorded output voltage from the phase comparison detector is used for calculating the fractional frequency instability. Following the route in [9] for shortening the data acquisition time, we applied the so-called Λ-type data averaging process for calculating the Allan deviation. Figure 4(a) shows the stability of the proposed system at intermediate node. The uncompensated instability and the system noise floor are also plotted. The noise floor is measured by replacing a long-haul fiber link with a 6 m long SMF connector. The instability of the free-running system is measured as 5.66 × 10−11 at 1 s. With the compensation enabled, the frequency instability is reduced to 3.65 × 10−15 and 7.8 × 10−18 at 1 s and 1000 s averaging time, respectively.
Fig. 4 Fractional frequency instability of the 150 km free running fiber link and the proposed compensated link with (a) a clockwise distance of 75 km (anticlockwise distance of 75 km) and (b) a clockwise distance of 50 km (anticlockwise distance of 100 km) node.
In order to verify that the experimental scheme is applicable to any node of the optical loop node, we set another accessing node between 100 km and 50 km standard SMF spools. The anticlockwise distance is 100 km in the loop link. The same photoelectric devices are used when testing the dissemination performance. Figure 4(b) shows the fractional frequency instability for the link with and without compensation at the other node. It presents compensation performance of the stable transmission. The instability of the free running system is 1.49 × 10−11 at 1 s. With the compensation enabled, the instability drops to 2.67 × 10−15 at 1 s and reaches 7.07 × 10−18 at 1000 s. The results verify that multi-access can be inserted in the optical fiber loop link. From the principle of the proposed system, the two nodes can get the stable signal with almost the same performance. While, in the experiment, there is a micro-difference of frequency instabilities at the two nodes caused by systematic error without temperature of optical and electrical devices. Challenges may be faced for us to obtain the long-term stability of the system, such as loss of lock of optical frequency comb. For reducing the impact of the polarization state drifts and PMD in the long-term stability, polarization scrambler may be used at the output of transmitters. Further studies will be devoted in evaluating thoroughly the instability at longer averaging time and greater distance in this paper.
4. Conclusion
In summary, we propose and demonstrate a multi-access stable phase dissemination scheme based on OMPLL via 150 km loop fiber link. The OMPLL is designed to achieve low noise optical-microwave phase synchronization. To avoid Rayleigh scattering and the interference between clockwise and counterclockwise signals, two different channels of optical wave are adopted. A stable frequency signal of 20 MHz is experimentally obtained over the loop fiber link. The fractional instability of the frequency transfer through a long-distance fiber loop link is demonstrated to be as low as 7.8 × 10−18 at 1000 s around middle node. The results prove that the proposed scheme can ensure not only particularly practical but also high fidelity frequency transfer over long fiber link. The frequency transfer distance can be further increased by applying low phase noise bi-directional EDFAs in loop link.
Funding
National Natural Science Foundation of China (NSFC) (61531003, 61690195, 61701040, and 61427813); Fund of State Key Laboratory of Information Photonics and Optical Communications of BUPT; Youth Research and Innovation Program of BUPT (2017RC13); Open Funds of IPOC (IPOC2017ZT14)
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