Study Of Coherent Population Trapping Atomic Clock

Passive coherent population trapping (CPT) atomic clocks can be widely used in microsatellites, submarine communication devices, telecommunication stations, missile guidance, etc., due to its small scale, low power dissipation and relatively good long-term frequency stability. This work focused on the research and improvement of components and performances of CPT atomic clocks. The aim is to get better frequency stability, smaller size and lower power dissipation, so as to improve the property and expand the application of CPT atomic clocks. The work involves the design of servo circuit, and the improvement of signal detecting circuit, frequency locking loop and physical package. Significant results are as follows.1. The digital servo of CPT atomic clocks is designed and realized by using a field programmable gates array (FPGA). The highly digitalized servo minimized the size and power dissipation of the atomic clock, decreased the total temperature drift of devices, enhanced the anti-jamming ability, and supplied a convenient environment to implement intelligent operations. Besides, the design provided a scheme easily to be transformed into application specific integrated circuit (ASIC) for miniature CPT atomic clock.2. The signal detection circuit, including the optical detection circuit, the filters, and the phase-sensitivity demodulation circuit is improved,. The improved optical detection circuit covered both wide dynamic range and high sensitivity. We designed a digital filter to replace the original analog one, which improved the short-term stability and decreased the power dissipation. The logic resource required by the digital filter is highly saved by advancing its structure, compared to the traditional digital filter. We also developed two kinds of phase-sensitivity demodulation schemes which need no filters. Though the short-term stability slightly decreased, the selection or design of main control chip is simplified, so the power dissipation is potential to be highly decreased. Therefore they are especially fit for the applications require extremely low power dissipation.3. The frequency locking loop is optimized by means of improving frequency locker, frequency scanning method and parameters fo the loop. We developed several variable-step frequency lockers to replace the old ones, which had the shortages of multiple stationary points, stead-state error, and lack of balance between correction speed and short-term stability. Algorithms are designed to recognize and handle circumstances that the clock is suffering vibration or shock. The stead-state error caused by the inconstant frequency drift speed of oscillator is also eliminated by changing the locker’s structure, so that the long-term stability is improved. We also analyzed the influence of integration time of the frequency locking loop, indentified best control speeds for small size and miniature CPT atomic clocks, relatively, and improved the short-term stability. The frequency retrace is also improved by changing the frequency scanning method, and the frequency accuracy is simultaneously enhanced.4. The structure of physical package is developed and the performance of the atomic clock is improved. We designed a special precise temperature-control system for vertical-cavity surface-emitting lasers (VCSEL). We also suppressed the temperature-frequency drift by studying the drift in different buffer gases ratios, modulating the ratio, and changing the operation temperature of the vapor cell. The work enables people to modulate the buffer gases ratio into best point according to the operation temperature the cell required, by the way of calculating the relation between inversion temperature and ratio. The work on C-field coil structure improved the uniformity and stability of magnetic field in the cell.