|M.Sc Student||Matan Kahanov|
|Subject||Atomic Clocks Based on Coherent Population Trapping|
|Department||Department of Electrical Engineering||Supervisor||Professor Emeritus Eisenstein Gad|
|Full Thesis text|
Small scale atomic clocks have been a major research and development topic in recent years. These clocks, called chip scale atomic clocks, are a few cubic centimeter in volume and should be battery operated. The vast majority of these clocks are based on a quantum phenomenon called coherent population trapping (CPT) induced by a directly modulated VCSEL.
CPT is a quantum coherent phenomenon taking place in the so called L- system. When two coherent electromagnetic field impinge on a quantum L system, the population is trapped in the ground states and therefore the medium becomes transparent. The spectral width of the CPT resonance is very narrow and therefore it can serve as a reference to an atomic clock. The small size is possible since no microwave cavity is needed.
The contrast of CPT resonances is however rather low. A complex locking schemes is needed therefore in order to have a stable clock. A novel locking scheme that was recently developed and explored, the Double Field Frequency Modulation Spectroscopy (DFFMS), enables to enhance the contrast and there- fore to optimize the clock performance. In the DFFMS, a low rate signal is superimposed on the amplitude modulated bias current to the laser. A careful analysis and optimization of the DFFMS signal transversing a narrow resonance enables one to achieve an optimal signal to noise ratio (SNR). The optimum SNR
leads to an Allan Deviation (ADEV) of sy (t) = 3 X 10-11 at an integration time of 1 sec which is comparable to commercial standard clocks.
A diferent way to implement an atomic clock based on CPT is to generate self sustained oscillations at the resonance frequency using the optoelectronic oscillator method. Optoelectronic oscillators are hybrid oscillators that convert optical energy into a stable microwave oscillating field. Generally, these oscil- lators use an optical delay line in order to have high spectral purity. When the gain is above unity and the phase is an integer multiply of 2p self oscillation is created. Using the fact that the medium is transparent at the vicinity of the CPT resonance, the gain there is higher and therefore the oscillator should oscillate at the CPT frequency. Experimental results show a vast improvement in the spectral purity of an oscillator with the CPT resonance comparing to an oscilla- tor lack of the resonance. Since the CPT resonance is stable for long times this work may be a framework for realizing self oscillating atomic clock.