1.Xidian University, Xi’an 710071, China 2.National Time Service Centre, Chinese Academy of Sciences, Xi’an 710600, China 3.University of Chinese Academy of Sciences, Beijing 100039, China
Fund Project:Project supported by the National Key R&D Program of China (Grant No. 2016YFF0200200), the National Natural Science Foundation of China (Grant Nos. 91636101, 91836301, 11803041), the Strategic Priority Research Program of the Chinese Academy of Sciences (Grant No. XDB21000000), and the Key R& D Program of Guangdong Province, China (Grant No. 2018B030325001)
Received Date:06 August 2020
Accepted Date:31 October 2020
Available Online:19 March 2021
Published Online:05 April 2021
Abstract:To reduce the influence of fiber dispersion on accuracy of fiber-based time synchronization, a method of dispersion-error corrected dual-wavelength time synchronization is proposed in this paper. Specificlly, the method is to measure the dispersion coefficient of the fiber link, and then input it to each remote terminal, the time delay error caused by the fiber dispersion is eliminated through the delay phase controller. With the self-developed engineering prototypes, the experimental verifications are subsequently made both in laboratory and real field. Before the test, 16 devices of time synchronization are connected in series for calibration. The time synchronization system is able to keep delay difference within ± 15 ps after being calibrated. In the laboratory, the experimental setup is built by cascading 16 rolls of 50km-long fiber coils, and the total length of the fiber link is 800 km. The experimental results show that the dispersion coefficient of 800 km fiber link is 13.36 ps/(km·nm), and the delay error caused by dispersion is maintained within 10 ps after correction. The stability of the time transfer is 5.7 ps in standard deviation and the time deviation is 1.12 ps at an averaging time of 100000 s. In the real field test, a 1085-km-long field fiber link is utilized, along which 16 self-developed time-frequency transceiversare set at the cascaded fiber-optic stations. After being corrected with a dispersion coefficient of 16.67 ps/(km·nm) for 1085 km urban fiber link, the time transfer is demonstrated to have a dispersion-caused delay error of 60 ps. The experimental results show that the time standard deviation is 18 ps and the time transfer instability is 9.2 ps at an averaging time of 1 s and 5.4 ps at an averaging time of 40000 s. Finally, the time uncertainty of 800-km-long laboratory optical fiber link and 1085-km-long urban optical fiber link are evaluated, and the time uncertainty is 18.4 ps and 63.5 ps, respectively. This work paves the way for constructing the time synchronization fiber network in China. To further reduce the delay error caused by dispersion in a long-distance time transfer link, the more accurate thermal control of the lasers should be adopted to reduce the shifts of forward and backward wavelengths. Keywords:fiber link/ time synchronization/ synchronization network
表2800 km实验室光纤链路的详细参数 Table2.Detailed parameters of 800 km fiber link in the laboratory.
23.3.实验室光纤时间同步测试验证 -->
3.3.实验室光纤时间同步测试验证
为了验证双波长色散误差补偿方案的可行性, 首先利用自行研制的时间同步设备在实验室光纤链路上进行了时间同步的稳定度测试. 该实验室光纤链路由16盘标准的50 km光纤盘、16个远程端以及15个中继组成, 以满足实地光纤链路的测试模型. 该光纤测试链路的结构如图3所示, 将各个设备之间的光纤跳线改为50 km的标准光纤盘. 将1台本地端设备、15台中继设备、16台远程端设备以及测量设备放置在同一个地点, 便于对本地端的输入信号与各个远程端的输出信号进行精确比对测量. 当整个实验测试系统正常工作时, 将远程端16个输出的1PPS信号与本地端的1PPS信号输入到时差测量设备(SR620)中进行比对, 测量结果如图4(a)中黑色曲线所示, 其峰峰值约为50 ps, 通过计算得到其标准差值为5.7 ps, 其中蓝色曲线表示采用1 m光纤跳线时, 整个光纤时间同步链路的噪声极限. 将采集到的时差数据经过处理, 得到的时间同步传输稳定度如图4(b)中红色曲线所示, 分别为4.9 ps@1 s和1.12 ps@105 s. 整个光纤链路的时间信号传输稳定度极限如图4(b)中蓝色曲线所示, 分别为4.0 ps@1 s和0.083 ps@105 s. 从图4(b)中可以看出, 在1000 s以内, 该光纤链路的时间传输稳定度较为接近链路噪底. 在1000 s以上, 由于实验室环境温度变化的影响, 各个站点的设备时延和激光波长产生漂移, 从而导致长期稳定度出现了一定程度的恶化. 由于实验室空调温度变化的周期约为16 min, 因此导致系统的噪底在1000 s附近变差. 图 4 实验室光纤链路的时间同步测量结果 (a)光纤链路的时差测量结果(蓝色曲线: 系统噪底. 黑色曲线: 800 km实验室光纤链路); (b)光纤链路的时间同步稳定度测量结果(蓝色曲线: 系统噪底; 红色曲线: 800 km实验室光纤链路) Figure4. Time synchronization measurement results of the laboratory fiber link: (a) The measured time interval variation results of the laboratory fiber link (bule line: noise floor of the system; black line: 800 km fiber link in laboratory): (b) the measured time deviation results of the laboratory fiber link (bule line: noise floor of the system; black line: 800 km fiber link in laboratory).
23.4.1085 km实地光纤时间同步测试 -->
3.4.1085 km实地光纤时间同步测试
为了验证自行研制的时间同步设备在实际应用中的可靠性, 利用千公里级实地光纤链路实现了ps量级的时间同步传递研究, 该链路的地理位置分布如图5所示. 以中科院国家授时中心临潼园区为光纤链路的始发站, 途径一长、涝峪、筒车湾、洋县、汉中、勉县、宁强7个站点, 最后整个光纤链路又返回到国家授时中心临潼园区, 其中勉县和宁强两个站点之间利用四根光纤进行了两次往返传输. 在国家授时中心临潼园区放置一台光纤时间同步本地端设备和一台远程端设备, 其余各个站点分别放置一台远程端设备和一台中继设备. 利用光时域反射计(OTDR)对实地光纤链路的长度和衰减情况进行了分段测试, 测试结果如表3所列. 经统计, 光纤链路全程为1085 km, 总衰减为287.5 dB. 实验测试前, 通过环回测试得到整段链路的平均色散系数为16.67 ps/(km·ns), 引入的色散误差为7180 ps, 修正后的色散误差优于60 ps. 图 5 实地光纤链路地理位置 Figure5. Geographical distribution of the field fiber link.
编号
站点
距离/km
衰减/dB
1
临潼
0
0
2
二长
41.69
12
3
捞浴
72.75
21
4
筒车湾
81.90
21
5
洋县
76.50
19
6
汉中
60.48
16
7
勉县
60.55
16
8
宁强
73.20
19
9
勉县
73.20
19
10
宁强
74.32
19
11
勉县
74.32
19
12
汉中
60.55
16
13
洋县
60.48
16
14
筒车湾
76.50
19
15
捞浴
81.90
21
16
一长
72.75
21
17
临潼
41.69
12
总计
1085
287.5
表31085 km实地光纤链路各个站点的详细参数 Table3.Detailed parameters of the individual sites in the 1085 km field fiber link.
以自行研制的工程样机在往返约1085 km的实地光纤链路上实现了光纤时间同步传输测试. 将位于国家授时中心的远程端1PPS时间信号与本地端1 PPS时间信号输入到时差测量设备(SR620)中进行比对, 测量结果如图6(a)所示, 其中黑色曲线表示光纤链路自由运转时的时差测量结果, 蓝色曲线表示的是光纤链路补偿后的时差测量结果. 从图6(a)中可以看出, 当光纤链路自由运转时, 时差峰峰值达到了160 ns, 时差变化的标准差为47 ns. 当对光纤链路补偿后, 时差峰峰值为170 ps, 时差变化标准差为18 ps. 从测量结果可以明显地看出, 在链路自由运转和锁定时, 链路中的时差测量结果呈现出24 h周期性变化, 这主要是由于昼夜环境温度变化导致光纤链路长度发生改变而引起的. 光纤链路补偿后的时间同步传输测量结果如图6(b)所示, 其时间传输稳定度为9.2 ps@1 s和5.4 ps@4 × 104 s. 由于外界环境的干扰以及系统控制带宽的限制, 使1085 km实地光纤链路的短期传输稳定度相对于实验室800 km光纤链路发生恶化. 同时, 由于昼夜环境温度变化以及各个站点之间温度变化的差异, 从而对光纤链路的长期传输稳定度产生影响. 图 6 1085 km实地光纤链路的时间同步测量结果 (a)光纤链路的时差测量结果(黑色曲线: 自由运转链路, 蓝色曲线: 补偿后的链路); (b)补偿后链路的时间同步稳定度测量结果 Figure6. Time synchronization measurement results of 1085 km field fiber link: (a) The measured time interval variation results of the laboratory fiber link (black line: free running fiber link; blue line: compensated fiber link); (b) the measured time deviation results of the field fiber link after compensated.