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Microwave Atomic Clock
The current position:Research < Microwave Atomic Clock
Traditional Rubidium Gas-cell Frequency Standard:
    As a good second-class frequency standard, rubidium frequency standard cannot be taken place in many areas. As it has lots of advantages, such as small volume, light weight, low price, long lifetime, good frequency stabilities, low drift, easy to produce, it becomes the first order among all the frequency standards. Rubidium frequency standard is generally composed of physical part and electric circuit part. The physical part gives out a quite accurate signal from rubidium atoms. By using voltage controlled crystal oscillator (VCXO) and electronic circuits, the frequency of VCXO will be controlled by the spectrum of rubidium. The structure of the physical part of traditional rubidium frequency standard is shown in Fig. 1.
Fig.1 the physical part of traditional rubidium frequency standard
    The lamp full of 87Rb gives out the light of two frequencies named ‘a’ line and ‘b’ line. When the light goes through 85Rb cell, ‘a’ line will be filtered, but ‘b’ line will be left. Then, as optical pumping light, ‘b’ line goes through the absorbing cell full of 87Rb and some buffer gas. One photo-detector is used to accept the transmitted light. After a little while, the intensity of the light will be steady. Now we use crystal oscillator and other electric circuits to give out a microwave which scans around the working frequency of 87Rb. The microwave influences the absorbing bulb. As the frequency of the microwave is nearer to the working frequency of 87Rb, the pumping light will be absorbed more, and so the light goes into the light probe will be weaker. So, as the microwave scans, through the light probe, we can see the microwave resonance line as Fig. 2.

Fig.2 Microwave resonance line
    This line contains the information of the high stability of rubidium atomic energy levels. Then with the electric circuit, we use the line to lock the output frequency, and at last we make a traditional rubidium frequency standard. The members of the group have been undertaking several national great research programs and the Rb atomic clocks which were successfully developed with the partner unions are applied on the Beidou navigation system.

Rubidium Beam Frequency Standard:
    Traditional rubidium atomic frequency standard is of the rubidium gas cell type. It is small, light, inexpensive for the absent of vacuum system, magnet poles or atomic beams. However, because of the poor accuracy and large frequency shifts, rubidium gas cell clock is used as secondary standard. Rubidium beam frequency standard(shown in Fig.3), a small type clock borrowing ideas from Cesium beam clock, uses a Ramsey cavity instead of gas cell. Optical pumping and fluorescence detection are also adopted. The clock is expected to reach higher stability and keep the Cs clock’s advantage of high accuracy and long term frequency stabilites meanwhile.

Fig.3
Basic principle of Rubidium beam clock
    Rubidium beam clock is working on the transition between the two hyperfine level of 87Rb |F=1, m=0>and |F=2, m=0>. The transition frequency is 6.834 682 614GHz. First, in order to tell whether the transition happens, the populations of the two levels need to be made different before the resonance. This is realized by pumping the atoms to a particular level, |F=2, m=0> for this case. Then the beam traverses the resonance cavity, where microwaves are fed in. reaction with microwaves of different frequency, atoms are transited to the other level, |F=1, m=0>. Finally, the emergent atoms are detected to find the quantity of atoms which has transited to the other level. According to atomic beam technology, the transition probability is the highest, only when the input microwave frequency equals the transition frequency. With sweep and modulation on the microwave frequency, the transition frequency is finally obtained.
Optical pumping
Fig.4 The levels of 85Rb and 87Rb
    As mentioned before, atoms need to be pumped to one level before resonance. We use the optical pumping method. The light from a 87Rb lamp going through a 85Rb filter cell, is used as the pumping light(b light in the figure), which pumps the atoms to |F=2, m=0> level.
Ramsey Cavity
Fig.5 Ramsey Cavity
    The Ramsey cavity adopts the theory of separated resonance field. The atoms traverse microwave region twice, with a long non-resonance distance between. With this method, the line width of final line is narrowed, or adopts the long microwave structure or the long helix structure.
Fig.6 Line shape(Full line for separated resonance field. Dashed line for single resonance field.)
    Detection The same light as the pumping part is used to detect the population on level (F=1, m=0). Fluorescence detection method is used.
Fig.7 the imitation of the magnetic filed in the cavity when resonance

Diode Laser-pumped Rubidium Atomic Clock:
Introduction
    The laser pumped atomic clock is one kind of atomic clocks. Atomic clocks have the same functions as the traditional clocks but more precise and steady. For example, the time deviation of the traditional clocks is 60 seconds per year, while the atomic clocks in our lab can reach the standard of 1 second deviation per 100 thousand years and the more precise atomic clocks can serve at 1 second deviation per tens of millions years. Atomic clocks play an significant role in the humans lives, though we don't have the notions of their existence mostly. The atomic clocks are fixed on the navigation satellites of GPS to obtain the time signal difference between the atomic clocks of an object on the ground and themselves. So it is available to calculate the distance between the satellites and the object. If the quantity of the satellites is enough, we can obtain sufficient data about the distance between the satellites and the object to locate the object precisely. Since the great significance of the atomic clocks, the governments and the military have payed great attention to them. What's more, the experiment physicists may be more excited about the most precise tools--atomic clocks to help them to testify the phenomenon of gravitational red shift and the variations of some basic constants. The communication system, exploration and many fields have great requirements of the atomic clocks.
Feature
    Because of the small type, low power consumption, reliable and reasonable of cost, Rubidium frequency standards are widely used in communication, electric power and satellite navigation. Traditional and applied Rubidium frequency standards adopt the Rubidium lamp as the pumped light resource. The spectrum is so wide that it contains 'a' line & 'b' line (87Rb) and 'A' line & 'B' line (85Rb). Only 'b' line will be left when the beam transmits the filter cell because of 'A' line and 'a' line overlap mostly. What's more, the remnant 'a' line and the luminance beam have a bad effect on the final result. So we adopt the narrow line width semiconductor laser as the pumped source.
Structure
    A narrow line width (1MHz) semiconductor laser whose frequency is locked by the absorption saturation method can be used as the pumped light resource. Then the laser is injected into the absorbing cell and other parts are same as the traditional Rubidium frequency standards.
Results at present
    We have accomplished the whole system and got the preliminary results. The relative frequency difference and frequency stability are shown in Fig.9. The short-term frequency stability is
3.7×10-12τ-1/2(1-100s).
Fig.9 Relative frequency difference measurement and short-term stability

Compact Cesium Beam Clock: