We developed a new radio frequency dissemination system based on an optical fiber link. A 1.55 μm mode-locked fiber laser was used as optical transmitter in the system. To actively reduce the phase fluctuation induced by the fiber length variations with high resolution, we proposed a novel compensation technique. In our technique, we directly control the phase of optical pulses generated by the laser to compensate the fluctuation. The phase-controlling method is based on both pump power modulation and cavity length adjusting. We performed the transfer in a 22-km outdoor fiber link, with a transfer stability of 3.7 × 10−14 at 1 s and 6.6 × 10−18 at 16000 s. The integrated timing jitter in 24 hours was reduced from 14 ps to 35 fs.
©2012 Optical Society of America
High-stable radio frequency transfer over fiber links has attracted a growing research interest in the past few years. The primary aim is to share state-of-the-art atomic frequency standards in distantly located laboratories. Recently, some experiments have demonstrated that the transfer of microwave over fiber link can provide stabilities a few orders of magnitude higher than satellite-based techniques [1, 2]. In the research, using femtosecond mode-locked lasers (MLLs) as optical transmitter is one most attractive way, as the regularly spaced optical pulses from the laser can provide high-accuracy interferometry measurement  and can also be directly converted into microwave frequencies [2, 4]. Using MLLs in microwave transfers can be applied in radiotelescope arrays  and remote clock synchronization , which requires high correlation of the signals received in different locations. The technique also benefits the synchronizations of microwave in particle accelerators and x-ray free-electron lasers [7, 8].Typically, transfer distance ranging from a few hundred meters up to 20 km and sub-100 fs integrated timing jitter is required in these facilities.
Although using fiber link is a promising method for microwave distribution, the fiber length is affected by environment perturbations such as physical vibration and temperature changes, which result in phase fluctuation of the transferred microwave. The phase stability at the remote end is therefore degraded. One way to suppress the fluctuation is using reflection methods at the remote end to measure and compensate the fluctuation at the local end . In 2004 and 2005, Holman et al. proposed two compensation methods for a 6.9-km-long fiber-based transfer, in which variable optical attenuator and optical delay line were used to compensate the length fluctuation [10, 11]. Kim et al. reported two ~300-m fiber-based transfers. A fiber stretcher was utilized for the stabilizing the fiber length [12, 13]. Using both fiber stretcher and thermally controlled spool, Marra et al. compensated the fiber-induced fluctuation in long-distance transfers [14, 15]. An integrated timing jitter as small as 64 fs was successfully achieved by them.
Though the aforementioned research proves that it is very effective to use optical group-delay actuators for compensation, the compensation ranges of the optical devices are limited. Especially in transfers with long fiber links, the phase drift, which may reach tens or hundreds of picoseconds, could be beyond the adjusting range of the devices. To solve this problem, Hou et al. proposed a new compensation system . Instead of stabilizing the fiber to some reference, they physically controlled the cavity length of the MLL and generated a phase shift to cancel the fluctuation. However, one vital disadvantage of the idea is that accuracy of the compensation is low, because the accuracy of the phase shift provided in this way is strongly limited by the resolutions of the intra-cavity piezo actuator. As a result, the compensated fluctuation is still in picosecond scale (6.3 ps) and cannot reach the requirements of many applications.
In this paper, we propose a new compensation scheme. Instead of physically controlling the cavity length of a MLL, we utilized pump power modulation as well as a phase shifter to achieve the phase shift with much higher resolution. Results show that the phase fluctuation is reduced to sub-100-femtosecond scale.
2. Technique description
In , a commercial piezoelectric transducer (PZT) was used in an Er-doped fiber MLL, of which the repetition rate is 100 MHz. The extreme resolution of the PZT is 0.6 nm (PI, P840.2). Based on the theory of MLL , the extreme minimal phase shift provided by controlling the PZT can be calculated as:16], and it is very close to the response time of the physical cavity length adjusting. The result implies that the resolution phase shifting is larger than 100 fs and thus it is very difficult to achieve sub-100 fs integrated timing jitter in this way.
Here, a new compensation technique which provides much higher accuracy is proposed. We first introduce a novel method for phase-controlling the MLL, which is crucial in the technique. Then, the detailed setup for the compensation loop is given.
2.1 Pump power modulation for high-accuracy phase shift
In 2006, Hudson et al. proved that by using an intra-cavity electro-optic modulator (EOM) to change the refractive index inside the cavity, the repetition rate of a MLL can be controlled, and remotely located MLLs can be synchronized with 10-fs accuracy . Here, instead of using intra-cavity EOM, we explore the possibility of using pump power modulation to control the repetition rate through refractive index adjusting. In our research, a 1.55-μm Er-doped MLL, in which a 25-cm Er-doped fiber (EDF), is used. The repetition rate frep = 144.5 MHz. We use pump power modulation to generate an accurate phase shift of the laser. The underlying theory is that if the pump light intensity is linearly adjusted, the susceptibility of the gain media in the EDF is affected by nonlinear effects and also changes linearly . This change will lead to a linear shift of the EDF’s refraction index. Besides, due to the nonlinear effect caused by the third-order susceptibility, refraction index of single-mode fiber in cavity also changed. When intra-cavity light intensity increases, very slight linear shift of the refraction index also increases . As a result, n of the cavity can be adjusted by modulating pump power in a linear way. The frequency/phase of pulses is therefore controlled, as frep 1/n in MLLs [4, 20].
To evaluate the feasibility of the theory and corresponding performance, we respectively measured the phase shifts provided by power modulation and cavity length adjusting in 1 ms. A voltage-controlled current supply (Thorlabs, ITC110), of which the input range is 0 V-5 V and the response time is less than 10 μs, was used to modulate the power of a 976-nm laser diode (LD) from ~50 mW to ~500 mW. For the cavity length adjusting, the same PZT in  was used. A travel range of 30 μm is achieved with 0 V-5 V input voltages by using a 150-V PZT driver. The phase shifts were measured with a signal analyzer (Agilent 9010A) by detecting the 40th harmonic of frep through a fast photodiode, and the data is given in Fig. 1 . We can see that for the cavity length controlling method, the generated phase shift goes up with the input voltage in an inconsistent way. This inaccuracy of the phase-controlling is due to the PZT’s resolution. Based on Eq. (1), the inaccuracy is in hundreds-of-femtoseconds scale. The resolution of phase-controlling is therefore strongly limited. However, by using power modulation, a very smooth phase-controlling is achieved. This result implies that accuracy of the phase-controlling is greatly enhanced. Moreover, the low sensitivity of the phase shifts to input voltage change makes the phase-controlling easy to operate. Considering the PZT’s resolution (0.3 nm) and the current supply’s minimal tuning voltage (15 μV), the corresponding extreme minimal phase shift provided can be calculated and is ~616 fs and ~2.5 fs. The greatly improved accuracy not only provides a high-resolution phase shift, but also ensures that a high harmonic of frep can be easily phase-synchronized with a microwave.
2.2. Phase-controlling MLL in long term
Based on analysis results, we built a phase-locked loop (PLL) to control the laser phase. Schematic of the phase-controlling method is shown in Fig. 2 . By tuning the pump power in the PLL, the 21st harmonic of frep is phase-locked to a 3.035-GHz microwave, of which phase is controlled with a resistance-capacitance (R-C) phase shifter. Therefore, by controlling the phase shifter, the laser phase can be shifted by an arbitrary value for an approximately fixed frep. The accuracy of the phase shift is enhanced by the high-frequency harmonic synchronization. Loop bandwidth of the PLL is set to approximately 1 kHz by using a low-pass filter. A fast photodiode and a microwave phase detector are used to detect the phase error between the harmonic and the microwave. Unwanted fluctuations and drifts in the phase error signal are removed by a proportional-integral-derivative regulator before it is used to modulate the pump power.
One thing should be concerned is that due to environmental influences , cavity length of the laser drifts, and when it happens, stabilizing frep with pump power modulation could be difficult. This is because that the adjusting range of frep provided in this way is limited by the maximum power of LD and the minimum power required for mode-locking. To solve the problem, we brought in cavity length adjusting. After frep is stabilized to the reference frequency by modulating pump power, the control signal for the power modulation is detected by an analysis system. If the signal changes, it means that the cavity length has drifted and the power has been tuned to stabilizing frep. The system then drives PZT so that the control signal returns to near its initial value. The drift is therefore compensated directly. The system filters out most alternating signals. Only smooth voltage signal with slow change is out for driving PZT. By limiting the control signal within a certain small range, the output power of the MLL remains approximately constant, while the phase is accurately controlled.
2.3 Compensation loop
The proposed compensation loop is illustrated in Fig. 2. The same MLL for Fig. 1 is used, which is 30-nm-wide and generates sub-150 fs optical pulses. By using the method proposed in last subsection, MLL is stabilized by phase-locking the 21st harmonic of the frep to the 3.035-GHz microwave, of which the phase is controlled and the frequency is directly derived from a coherent-population-trapping based Rb atomic clock. The pulse duration passes through a 90:10 power splitter, which is composed of a wave plate and a beam splitter. The 90% output, which is ~16 dBm, is used to propagate the pulses over 22 km of outdoor SMF to the remote site. The overall loss of the 22-km fiber link is ~6 dB. To avoid unwanted reflections and reduce optical loss in the fiber-based transfer, ends of fibers are all fusion-spliced. At the remote site, a portion is returned to the local site by a back reflector via the same fiber, while the other part is coupled out for the receiving. Both the sites are located at the same laboratory so that we can measure the stability of the travelled signal with the local references. The forward and backward transferred pulses are separated by an optical circulator. The powers of the received optics at the remote and local ends are −0.7 dBm and 1.7 dBm, respectively. EDFAs are used to amplify the received pulses at both ends. The amplification ensures that the microwaves generated by fast photodiodes can provide power and signal-to-noise ratios (SNRs) which are high enough for the operation of our microwave devices. The frep detected at the remote end also has a high SNR (>75 dB for 1 kHz measurement bandwidth). Therefore, we do not use common dispersion compensating devises so that further optic loss and nonlinearity are avoided. For transfer with longer fiber links, pre-chirp methods could be needed. The amplified values are tuned to make sure that the generated microwaves have best performances in phase noise . Using EDFAs after receiving rather than before or during transmitting could minimize the possibility of nonlinear effects arising from the amplification of short pulses, and no obvious nonlinearity was observed.
At the local end, the generated microwave corresponds the returned pulses is amplified by a radio frequency amplifier with automatic gain control to maintain stable high power. The amplified microwave is then fed into two frequency mixers to detect the phase differences between it and two local signals: the initial reference signal and the phase-shifted signal. The detections are carried out at the 21st harmonic of frep (3.035 GHz) to attain high detection sensitivity. Assuming the fiber-induced phase fluctuation observed at the remote site is φp, phase of the returned signal, φreturn, is given as:
3. Results and analysis
To test the performance of our compensation scheme, stability of the phase is measured. The measurement is performed by phase comparing the 21st harmonic of frep at the receiving end directly with the reference microwave signal by using a frequency mixer. To achieve stable measurement, thermal environment of the measurement setup for the long-term drift is controlled carefully. Most of the components in the measurement (including frequency mixer and photodiode) and the coaxial cables are in stable-temperature condition (< 0.1 K peak-peak), with the help of temperature-stabilized mounts and insulating material. The cables are in small-lengths (< 3 cm) to reduce thermal length drifts. All the components are placed in vibration-isolated condition and packed with aluminum box to stabilize thermal condition. To reduce the long-term influence of air flow and protect the thermal equilibrium, the tests were operated in a closed room with no human activity in it. We logged the output voltage of the frequency mixer by using an analog-digital converter (measurement bandwidth 7 Hz) and converted them into phase changes . The converting rate is 2 times per second. Figure 3(a) shows the timing drifts measured in an out-of-loop condition. When the compensation technique is not used, the integrated root-mean-square (rms) timing drift is ~14 ps in 24 hours, and the fluctuation range exceeds 50 ps. When using the technique, the rms drift is reduced to 35 fs and the suppressed fluctuation is mostly within 170 fs, while the noise floor of our measurement produces fluctuations within 33 fs (peak-peak).
We also measured the single-sideband phase noise between 1 Hz and 100 kHz at the remote end with a spectrum analyzer. The measured data is plotted in Fig. 3(b). We can see that when the phase noise suppression is activated, the ðber-induced phase noise is reduced by up to 30 dB. The reduced phase noise is −90 dBc/Hz at 1 Hz offset, which is very close to the noise measured when the 22-km SMF is replaced by a 0.5-m ðber while the other parts of the suppression system are still operating. This result implies that the suppression performance is still strongly limited by the measurement noise floor. To obtain signals with lower phase noises, reference microwave signals with higher purities and EDFAs with lower noises could be utilized. By integrating the phase noise between 1 Hz and 100 kHz, we calculate the short-term timing jitter to be less than 28.5 fs.
Using the measured data in Fig. 3 (a), the frequency stability at the remote site is calculated and shown in Fig. 4 . The fractional frequency instability (in terms of overlapping Allan deviation) is 3.7 × 10−14 at 1 s and 6.6 × 10−18 at 16000 s. The stability is more than 3 orders of magnitude higher than the stability measured in the previous research . We also measured the stability when the 22-km SMF is replaced by a 0.5-m ðber. By comparing the stability measured in two conditions, we can conclude that similar with the phase noise, the stability is also limited by the measurement noise floor. We believe the measurement noise floor is mainly imposed by the noise of the optical amplifiers and radio frequency amplifiers.
The focus of the research is to distribute the radio-frequency signal frep. To distribute frep and the carrier-envelope offset frequency (fceo)  together, it is possible to use intra-cavity prims  to compensate the pump-modulation involved fceo change  while maintain a stabilized frep. However, the transfer performance could be degraded as the system is very complex.
In summary, we demonstrated a novel phase fluctuation compensation technique for mode-locked laser based long-term transfer of microwave over fiber link. Instead of stabilizing the fiber link to some reference, we directly adjusted the phase of the laser to compensate the fiber-induced phase fluctuation. This technique is advantageous over optical group-delay actuators by its large compensation range. Unlike the previous research , in which the phase shift is generated by physically controlling the cavity length, we adjusted the refraction index of the cavity through both pump power modulation and cavity length controlling. Phase-locking a high-frequency harmonic of the repetition frequency to a reference signal can be easily achieved by using the technique, and by shifting the phase of the reference, phase of the laser is controlled .The resolution for the compensation is greatly enhanced.
To test the performance of technique, we transferred a microwave over a 22-km outdoor single-mode fiber. Results show that the integrated timing jitter is reduced to 35 fs in 24 hours, and the frequency stability reaches 3.7 × 10−14 at 1 s and 6.6 × 10−18 at 16000 s. Compared with the results in , the stability provided by us is greatly improved by more than 3 orders of magnitude and is high enough for most of the applications in which microwave transfer using mode-locked laser is needed. Compensating phase fluctuation with tens of picoseconds continuously in 24 hours was also achieved successfully. The results make it possible to use the proposed technique to transfer highly stable and accurate radio frequency from laboratories to other distantly located sites in long term. The technique is very easily implemented with very low cost since no optical group-delay actuators are used. The compensated system could potentially be used over much longer distances (>100 km) by adding bidirectional optical amplifiers with low noises.
The authors would like to thank Professor Zhigang Zhang and Professor Zhong Wang for helpful discussions and experimental assistants. This work was supported in part by the Nature Science Foundations of China (No. 11027404), 973 program (2010CB328201) and (2012CB315606).
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