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
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
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
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.
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
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>
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.)
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:
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.
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.
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
Fig.9 Relative frequency difference measurement and short-term stability
Compact Cesium Beam Clock: