M.Sc Thesis

M.Sc StudentSherman Alexander
SubjectElectro-optical systems with ultra-low phase noise and their
applications in RF electronic systems
DepartmentDepartment of Electrical and Computer Engineering
Supervisor PROF. Moshe Horowitz


Electronic warfare systems, radar, and modern communication systems are forced to utilize an ever increasing bandwidths and carrier frequencies over an ever increasing distances, simultaneously. All of those requirements place a great importance on the jitter performance of both the transmitter and the receiver systems used, which in most cases can't be met by conventional purely electronic timing sources.

In an effort to meet the ever increasing jitter performance levels a hybrid optoelectronic systems are often utilized. Those system utilizes optical components in order to overcome the limiting thermodynamic noise barrier of purely electronic sources. In this dissertation works we will discuss and demonstrate two such optoelectronic configurations that can be used to generate and sample low jitter and high bandwidth signals that are applicable to high precision synchronization and sensing applications.

We demonstrate a new method to improve the performance of photonic assisted analog to digital converters that are based on frequency down-conversion obtained by optical under-sampling. The under-sampling is performed by multiplying the radio frequency signal by ultra-low jitter broadband phase-locked optical comb. The comb wave intensity has a smooth periodic function in the time domain rather than a train of short pulses that is currently used in most photonic assisted analog to digital converters. Hence, the signal energy at the photo-detector output can be increased and the signal to noise ratio of the system can be improved without decreasing its bandwidth. We experimentally demonstrate a working system for electro-optical under-sampling with a 6-dB bandwidth of 38.5 GHz and a spur free dynamic range of 99 dB/Hz^(2/3) for a signal with a carrier frequency of 35.8 GHz. This result is in compare with 94 dB/Hz^(2/3) for a signal at 6.2 GHz that was obtained in the same system when a pulsed optical source was used. The comb spacing is equal to 4 GHz and its bandwidth was greater than 48 GHz. The temporal jitter of the comb measured by integrating the phase noise in a frequency region of 10 kHz to 10 MHz around comb frequencies of 16 and 20 GHz was only about 15 and 11 fs, respectively.

In addition, we demonstrate the generation of ultra-low repetition rate pulse train, in both an optical and electrical domain, with a repetition rate of 19.6 kHz by a passively mode-locking of an optoelectronic oscillator based on a long optical cavity implemented by an optical fiber. This configuration allows forming both optical and electrical pulses having both low jitter and a low pulse to pulse period. The pulse to pulse timing jitter equals 0.06 ppm of the repetition time of the pulses. No significant dependence of the pulse duration, waveform, and the timing jitter was observed when the cavity length was changed from 150 m to 10400 m. Such pulse trains can be used as a reference source for high performance radar systems that would allow high resolution tracking of low visibility targets at long ranges or as a timing and synchronization signal for time-critical distributed systems.