Wen-Hung Tseng

Enhancing long-term stability of the optoelectronic oscillator with a probe-injected fiber delay monitoring mechanism

Optoelectronic oscillators (OEOs), based on optical fiber loops to act as a high-Q cavity, are capable of generating stable radio-frequencies (RF). The long-term frequency stability of the OEO is then limited by the cavity variation that is mainly induced by temperature sensitivity of the optical fiber. In order to actively stabilize the OEO cavity, we employ the technique of RF transfer over optical fibers. We propose and experimentally demonstrate a dual-loop-OEO scheme to enhance the long-term stability with an injected probe signal to monitor the phase variation in the fiber loops. The experimental results show that the resulting spread-spectrum signal is useful in monitoring the fiber delay without observable interference. The relationships between the measured frequency and the monitored delay are theoretically and numerically discussed. We also estimate the long-term stability of the proposed OEO scheme with the cavity phase correction. The corrected result shows the long-term frequency stability of the proposed OEO is within 8.4×10(-8) at one day.

Sagnac Effect and Diurnal Correction on Two-Way Satellite Time Transfer

Precision of two-way satellite time and frequency transfer (TWSTFT) highly depends on the residual nonreciprocity delays; one of them is caused by the Sagnac effect. In this paper, we employ the satellite ephemeris data to compute the precise Sagnac effect and discuss the possible additive effect related to the Sagnac effect. We also describe a method to calculate the diurnal correction based on the time derivative of the Sagnac effect. Through the analysis of the link between the National Institute of Information and Communications Technology in Japan and the Telecommunication Laboratories in Taiwan, we demonstrate that the correction was useful to improve the TWSTFT data. The time deviation at an 8-h averaging time is reduced from 346 to 156 ps after the correction.

First international two-way satellite time and frequency transfer experiment employing dual pseudo-random noise codes

Two-way satellite time and frequency transfer (TWSTFT) is one of the main techniques used to compare atomic time scales over long distances. To both improve the precision of TWSTFT and decrease the satellite link fee, a new software-defined modem with dual pseudo-random noise (DPN) codes has been developed. In this paper, we demonstrate the first international DPN-based TWSTFT experiment over a period of 6 months. The results of DPN exhibit excellent performance, which is competitive with the Global Positioning System (GPS) precise point positioning (PPP) technique in the short-term and consistent with the conventional TWSTFT in the long-term. Time deviations of less than 75 ps are achieved for averaging times from 1 s to 1 d. Moreover, the DPN data has less diurnal variation than that of the conventional TWSTFT. Because the DPN-based system has advantages of higher precision and lower bandwidth cost, it is one of the most promising methods to improve international time-transfer links.

Stability improvement of an operational two-way satellite time and frequency transfer system

To keep national time accurately coherent with coordinated universal time, many national metrology institutes (NMIs) use two-way satellite time and frequency transfer (TWSTFT) to continuously measure the time difference with other NMIs over an international baseline. Some NMIs have ultra-stable clocks with stability better than 10−16. However, current operational TWSTFT can only provide frequency uncertainty of 10−15 and time uncertainty of 1 ns, which is inadequate. The uncertainty is dominated by the short-term stability and the diurnals, i.e. the measurement variation with a period of one day. The aim of this work is to improve the stability of operational TWSTFT systems without additional transmission, bandwidth or increase in signal power. A software-defined receiver (SDR) comprising a high-resolution correlator and successive interference cancellation associated with open-loop configuration as the TWSTFT receiver reduces the time deviation from 140 ps to 73 ps at averaging time of 1 h, and occasionally suppresses diurnals. To study the source of the diurnals, TWSTFT is performed using a 2 × 2 earth station (ES) array. Consequently, some ESs sensitive to temperature variation are identified, and the diurnals are significantly reduced by employing insensitive ESs. Hence, the operational TWSTFT using the proposed SDR with insensitive ESs achieves time deviation to 41 ps at 1 h, and 80 ps for averaging times from 1 h to 20 h.

Full Utilization of TWSTT Network Data for the Short-Term Stability and Uncertainty Improvement

The two-way satellite time and frequency transfer (TWSTFT) is a precise time transfer technique and has being used to generate the international atomic time since 1999. Nowadays, TWSTFT links have formed a worldwide network and the large amount of TWSTFT data become highly redundant. To fully utilize the data of TWSTFT network and improve the TWSTFT results, a feasible method of processing data is proposed in this paper. According to the stability of each link, we assign a weighting value to each related equation and then solve the matrix equation with the weighted least squares method. We perform data analysis with practical time transfer data, and the results show that it is an effective method to improve the short-term stabilities and uncertainties of TWSTFT results.

Improving TWSTFT short-term stability by network time transfer

Two Way Satellite Time and Frequency Transfer (TWSTFT) is one of the major techniques to compare the atomic time scales between timing laboratories. As more and more TWSTFT measurements are performed, large numbers of point-to-point two-way time transfer links have grown to be a complex network. For a future improvement of the TWSTFT performance, it is getting important to reduce measurement noise of the TWSTFT results. One of the methods is using the TWSTFT network time transfer. The Asia-Pacific network is an exceptional case of simultaneous TWSTFT measurements. Some indirect links through relay stations show better short-term stability than the direct link because the measurement noise may be neutralized in a simultaneous measurement network. In this paper, the authors propose a practicable method to improve the short-term stability by combing the direct and indirect links in the network. Through the comparisons of time deviation (TDEV), the results of network time transfer show clear improved short-term stabilities. For the links based on H-maser, the average gain of TDEV at 1h averaging times is 20%. As TWSTFT short-term stability can be improved by network time transfer, the network may allow larger number of simultaneously transmitting stations.

The comparison between TWSTFT and GPS time transfer result of PTB-TL LINK

In this paper we compare the PTB-TL time transfer link data of TWSTFT and GPS carrier phase solutions. We investigated in detail the double difference between the time transfer data of TWSTFT and GPS IGSC/TAIPPP. Our results show the PTB-TL link has a conspicuously diurnal effect. The periodicity is almost exact 1 day and the peak-to-peak amplitude is about 1∼1.5 ns. We also found a phase drift which is about 5 ns between TWSTFT and GPS data during a period of 130 days.

Analysis of the Asia-Pacific TWSTFT network

Two way satellite time and frequency transfer (TWSTFT) is an important method to compare the atomic time scales between time keeping laboratories. The aim of this paper is to analyze the Asia-Pacific TWSTFT network, in which the measurements are performed synchronously by using multi-channel modems. As a result of the synchronous measurements, the clock drift and the majority path instabilities could be neutralized in the calculation of the closure links. This paper discusses what kinds of noise could be cancelled and what kinds of noise would still be kept in the result of the closure links by taking all three stations at the same time into account. From the re-examination of TWSTFT equations, we deduced that the time difference between two laboratories could be obtained via relay stations without debasing the stability in the synchronous TWSTFT measurements. The results show that the noises of the closure links in the Asia-Pacific TWSTFT network are very low. The smallest standard deviation of triangle closure is only 121 ps for the data over four months. The closing errors are closed to zero after removing calibration values. The analysis of modified Allan deviation confirms that the stability of data via relay station is in competition with the direct TWSTFT link. This paper also shows that the stability of TWSTFT could be improved by the weighted mean of the redundant TWSTFT links.

Impact of fiber delay fluctuation on reference injection-locked optoelectronic oscillators

We demonstrate that the phase shift of a reference injection-locked optoelectronic oscillator (OEO) varies as the change of its fiber delay over a long period of time. The dynamic variation of the fiber delay is monitored using an injected probe signal and is compared with the phase shift. With actively stabilized fiber delays according to the monitored data, the long-term frequency stability of the reference injection-locked OEO is evaluated.

Long-term inconsistency between TWSTFT and GPS time transfer results in PTB-TL and NICT-TL time links

In this paper, we compared the TWSTFT and GPSPPP data for the PTB-TL, PTB-NICT, and NICT-TL time transfer links. We investigated the double difference between the time transfer data of TWSTFT and GPSPPP from 2005 to 2010. The results show two time transfer techniques had a conspicuously inconsistency and the patterns of double difference results were similar in PTB-TL and PTB-NICT links. The double difference results of all direct links exhibited the annual varying patterns, whose peak-to-peak phase difference in 1-day moving average are 4.5 ns both for the PTB-TL and PTB-NICT links, and 2 ns for NICT-TL link.