Abstract
In this dissertation, we study the precise time transfer techniques and extend their applications into the study of optoelectronic oscillators. In the microwave domain, two-way satellite time and frequency transfer (TWSTFT) is one of the main techniques used to compare atomic time scales over long distances. As more and more TWSTFT measurements have been performed, the large number of point-to-point two-way time transfer links has grown to be a complex network. For future improvement of the TWSTFT performance, it is important to reduce measurement noise of the TWSTFT results. One method is using TWSTFT network time transfer. We propose a feasible method to improve the short-term stability by combining the direct and indirect links in the network. Through the comparisons of time deviation (TDEV), the results of network time transfer exhibit clear improved short-term stabilities. For the links used to compare 2 hydrogen masers, the average gain of TDEV at averaging times of 1 hour is 22%. As TWSTFT short-term stability can be improved by network time transfer, the network may allow a larger number of simultaneously transmitting stations. In the other work, 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. 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 (ps, 10^-12 s) are achieved for averaging times from 1 s to 1 day (i.e., 86400 s). 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. Because there is no bandwidth limit in an optical fiber link, a new trend is to perform the time transfer through the optical fiber link. Hence, we present a two-way time transfer experiment through a 25 km optical fiber link. The fiber link, which is constructed to a common-path configuration, is used to replace the satellite link. The resulting data exhibits the time deviation of less than 7 ps at one-day averaging time. The frequency stability on the order of 1.9×10^-16 at 10^5 s has been demonstrated. In the final part of this dissertation, we point that the time transfer techniques are useful for the study of optoelectronic oscillators (OEOs). Based on optical fiber loops to act as a high-Q cavity, the OEOs 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. Finally, we study the impact of fiber delay fluctuation on reference injection-locked OEOs. We demonstrate that the phase shift of a reference injection-locked OEO varies as the change of its fiber delay over a long period of time. The 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. In future progression, to act as a local oscillator for an atomic clock, a tunable OEO can generate an oscillation frequency corresponding to the desired atomic transition without the use of a synthesizer.