Development of an Ultra Stable Fiber Optic Frequency Distribution System Using an Optical Delay Control Module

This thesis describes investigation and development of an ultra stable frequency distribution system, of which response speed for phase fluctuations is fast, to achieve the required ultra stable phase stability level of 1×10-16 in Allan standard deviation (ASD) for 1000 seconds averaging time for the frequency distribution. The thesis consists of six chapters. In Chapter 1 (Introduction), the objectives of the thesis are described. In Chapter 2, statistics for frequency stability measurement are reviewed to evaluate phase stability in frequency distribution. Specifically, a method to evaluate phase stability of a frequency distribution system in a time domain is explained to apply the theory for evaluating the development system. Histories of development of fiber optic links for frequency distribution are also reviewed. Chapter 3 reviews the fiber optic link systems applied in frequency distribution. The phase stabilized optical fiber and the two-way system are mentioned. Chapter 4 presents development of the ultra stable fiber optic frequency distribution system. In this chapter, the stability requirement for a frequency distribution system is studied. The components for a fiber optic link are evaluated. Also, the developed system configuration is described. The developed system is evaluated with laboratory measurements in Chapter 5 and the test results are also presented. Chapter 6 gives conclusion of this thesis with discussions of the measured results, including some recommendations for the direction of further improvement and development. This ultra stable frequency distribution system is the first equipment which an optical delay module (ODM) is applied to, and achieved the best stability in frequency distribution systems. This system responds to phase change caused by change of environmental conditions within one second and compensates it within several seconds. The new phase stability measurement system was also developed to measure high stability of this system. The system consists of a local unit and a remote unit connected by a fiber optic cable. The phase stabilized optical fiber (PSOF) cable is used for the reference frequency transmission. An optical signal, which is modulated by an injected reference signal, is transmitted to a remote unit through a fiber optic cable. An optical signal, which is modulated by the received reference signal in the remote unit, is sent back from the remote unit to the local unit through the same optical fiber cable. On the other hand, the delay at the remote end of the cable has exactly half the round trip delay. The phase difference between the transmitted and the returned signals at the transmitting end in the local unit is proportional to the round trip delay. Therefore, the phase at the remote end of the cable would be zero, when the phase difference between the transmitted and the returned signals from the remote unit, is adjusted by just a half of this round trip delay. A 1.55 μm laser diode for the signal transmission from the local unit to the remote unit and a 1.31 μm laser diode for the signal transmission from the remote unit to the local unit are installed in the developed system. The phase difference between the transmitted and the returned signals is compensated by an optical delay control module (ODM). This ODM induces no internal electronic noise in the fiber optic frequency distribution system. The system noise level of conventional frequency stability measurement systems is commonly the order of 10-16 for 1000 second integration time in ASD and this order is insufficient to measure the stability of this developed ultra stable frequency distribution system with stability level of 10-17. Thus, the frequency stability measurement system, of which system noise is the order of 10-17 for 1000 second integration time, has been designed this time. The vector voltmeter outputs a DC analog signal proportional to phase difference between a reference signal and a test signal from the direct analog output terminal. The DC analog outputs are AD converted by a multimeter with long integration time to achieve the lowest system noise in a frequency stability measurement system. This is the first direct DC analog output usage of a vector voltmeter to be implemented for use in a frequency stability measurement system. The system noise level which is used for the laboratory measurement this time is better than any dual mixer time difference systems. The stabilities of the developed ultra stable fiber optic frequency distribution system in ASD are 7.5×10-17 and 1.1×10-17 at 1000 second and 10,000 second averaging time respectively while the environmental temperature of the PSOF cable varies in the range of 10℃ and at the rate of 10℃/12 hours. This system has the best stability in frequency distribution systems developed so far. This system can be applied for the distribution of reference frequency from the ultra stable new frequency standard such as an ion storage frequency standard, to the differential VLBI system used for the precise measurements of the distance to galactic radio sources and to the connected radio interferometer in sub-millimeter range