As a high precision time and frequency measurement equipment, Atomic clocks has been applied into many important fields such as aerospace, defense, national standard time. Because of its complexity and high accuracy, the volume of a system usually is relatively large, the miniaturization and transportable atomic clocks will inevitably break the bottleneck of the widely used of optical atomic clocks is , like CPT clock (microwave clock, currently the smallest CPT clock can make fingernail, low consumption, stability, 10-12 orders of magnitude) is widely used, its core competitiveness will be greatly improved.
In the 1980 s, the world began to carry out the miniaturization of atomic beam optical frequency target research. At present, Russia is engaged in 88 Sr research work, the German PTB, the university of western Australia, Australia, China's Beijing university Jingbiao Chen group are engaged in 40 Ca miniaturization research , by 2015, the fractional frquency unstability the Russian team was 1.56 * 10-13, Russia team do 9.2 * 10-14 seconds stability. At present, the university of western Australia group does the best work,and the Chen Jingbiao team also has already done 3 * 10-14 .
Above is a schematic diagram of passive optical clock, reference spectrum occurs when laser frequency source interactions with reference source (cavity, quantum reference system) , and the frequency of the laser frequency source can be lockedin the center of the atomic transition frequency of the quantum reference system.Followed is the relevant energy level diagram of the 40Ca.
Because the atoms in 3 P1 state level lifetime is 0.4 ms, the energy of a corresponding linewidth is 400 Hz, atoms in 1 P1 state level lifetime is 4.8 ns, corresponding to the level of 34.63 MHz. According to the definition of Allan variance, the narrower the detection of spectral linewidth is beneficial to reduce the uncertainty of the system, at the same time the atomic level lifetime is more than 3 p1 state, the interaction of atoms and photons time obviously direct detection of 657 nm radiation spectrum is impossible, so based on the above reasons for using 657 nm radiation signals as the clock transition, and adopt the method of Elecronic - Shevling to around 3 p1 state long life problem.
PDH method is a widely used for pressure narrow laser line width, and stability of laser frequency technology, by the Pound, Derver, John Hall three people is put forward, by producing the error of servo feedback, lock the frequency of the laser on the high precision of FP cavity.
Our team right now calcium miniaturization of optical atomic clock using the PDH method of laser frequency stability and first linewidth narrow, then calcium atom 657 nm spectral lines to detect laser lock in the center of the atomic transition frequency. Through the PDH method can ensure the short-term stability of clock, lock to ensure the long-term stability of the atom.