Optical Dissemination Of Stabilized Microwave Clock Reference

Remote distribution of microwave local oscillator signal is one of the core technologies used in many applications, including very long baseline interferometry and clock comparison systems. Clock signal can take many forms, such as microwave frequency local oscillator or a highly stable optical frequency. Clock distribution using optical fiber has gained growing research interest due to its high stability, ease of use and low cost. Due to mechanical stress and temperature variations on the fiber, phase noise of the remote signal has deteriorated and requires feedback correction. The thesis introduces the basic concepts of clock distribution using optical fiber and its basic methods. Some of the important aspects of system design are discussed. A new scheme using intermediate frequency voltage controlled oscillator to steer phase more accurately is proposed, thus increasing transmission frequency without sacrificing system complexity and phase correction range.Two basic methods used in remote clock distribution by optical fiber is intensity modulated optical carrier and direct transfer of high stability optical frequency. The fundamental difference between these two branches are of the phase (or transmission delay) detection technique. Apart from that, phase correction systems could also be different, ranging from acousto-optic modulator, fiber stretcher to voltage controlled oscillator. In this thesis, the common techniques used in long-haul frequency transmission is analyzed, with their merits and shortcomings compared.The basic theories behind the design of local oscillator remote distribution systems are discussed. From the linear model of the system, fundamental design problems such as system properties, stability, as well as loop filter design is discussed. Some of the noise encountered in the system is discussed. The impact of additive white noise on phase noise is discussed, with the result applied on optical receivers. The impact on phase noise from common electrical and optical components and modules are measured.Usually the higher the transmission frequency, the more sensitive the system is to the variations in transmission delay, thus higher transmission stability. However, due to limitations in radio frequency voltage controlled oscillator and loop bandwidth, it is hard to have accurate as well as wide range control of the phase of high frequency signal, consequently few schemes were reported to be able to transmit local oscillator of up to 10 GHz over a long distance and have infinite phase compensation range. In view of this problem, a new method based on intermediate frequency phase correction is proposed, in which the phase correction signal is generated using intermediate frequency voltage controlled oscillator and carried to radio frequency using single sideband modulator. An experimental system is designed and implemented to have accurate phase control of a 10.03 GHz signal, and basing on which a long distance local oscillator distribution system. The instability at 10,000 s is lower than 1×10-16, phase noise at 1 Hz frequency offset is measured to be -70 dBc/Hz, peak to peak phase difference between local and remote end is less than ±0.03π. The system have high stability, very few radio frequency components. It is easy to use and very few cycle slips occur, making it suitable for long distance distribution of radio frequency local oscillator distribution.