The present invention relates generally to atomic standards and a method of providing a frequency and/or time based on an atomic standard, and more particularly to atomic frequency and/or time standards and methods employing a beam tube.
The following is a list of prior publications generally related to the field of the present invention:
Current commercially atomic frequency primary standards typically employ a Cesium beam tube to control the frequency of a crystal-controlled oscillator. Such commercially-available units have a traditional Cesium beam tube configuration. The construction and operation of traditional Cesium beam tube atomic frequency standards is described in reference 1.
The output of detector (6) is used to control oscillator (7) by means of FLL circuit (8). The detection signal on output of detector (6) varies in accordance with the quantity of Cs atoms that have undergone a transition in the cavity (4), and functions as an error signal for controlling the frequency of oscillator (7).
The Ramsey microwave cavity (3) is composed of two symmetrical arms and an in-between free zone. The width of detected line and the resultant stability of the frequency standard is proportional to the flying time of the atoms in the free zone; the longer the free zone the better the stability.
An Optical Cs beam tube atomic standard employing optical pumping for state preparation and optical pumping for detection has been the subject of investigation for many years now. Such optically pumped Cs beam tube standards employ optical state selection and state detection which replace the “A” magnet the “B” magnet, as well as a Ramsey microwave cavity and oscillator similar to the ones used in the classical Cs beam tube standard described above. In the 1st interaction region, optical pumping creates a hyperfine population difference. The resonance phenomenon is detected through fluorescent light variations as a function of the microwave frequency in a second interaction region. This fluorescent light variation, which is a direct measure of the number of Cs atoms which have undergone a transition, is detected and used to control the frequency of the microwave oscillations. A number of different pumping schemes are possible which may prepare cesium atoms Such schemes are well known and described in the literature (see references 1-3).
As far as it is known, such optical beam tube standards are either of the laboratory-type or are in development, and are not commercially available. See references 1-3.
The Cs atoms then enter Ramsey cavity (12). Again the RF magnetic field in cavity (12) causes some of the Cs atoms passing through cavity 16 to undergo a transition as a result of resonance phenomenon. After leaving cavity (12), Cs atoms enter the 2nd interaction region (14), where the resonance phenomenon in cavity (12) is detected through fluorescent light variations as a function of the microwave frequency. This is accomplished by optically pumping Cs atoms back to the hyperfine state, which generates photons and the fluorescence described above.
Again, the output of the (photo) detector (17) is used to control oscillator (15) by means of FLL circuit (16). The detection signal on output of detector (17) varies in accordance with the quantity of Cs atoms that have undergone a transition in the cavity (12), and functions as an error signal for controlling the frequency of oscillator (15).
Coherent Population Trapping (CPT) is a phenomenon which is being used since few years to make atomic clocks which are based on vapor cells without using the traditional microwave cavity. CPT state occurs when an alkali atom (for example) is subject to two optical fields with wavelengths that correspond approximately to the D1 or to the D2 transitions of the said alkali atom and where the frequency difference between the two optical wavelengths matches the hyperfine 0-0 transition. Under these conditions the atoms are trapped in a superposition state of the two hyperfine levels and cease to absorb or emits radiation, i.e., they become transparent to the said optical radiation and in addition one observe a dark line in their fluorescence. For more details see references 3-7. To our knowledge CPT has not been used in atomic standards based on atomic beam.
The invention concerns an atomic beam tube that uses single or double atomic beam combines with single or double optical beams and where the Coherent Population Trapping (CPT) phenomenon is employed. Two main configurations are proposed, which are set forth in claims 1 and 2, respectively and will be disclosed in details in the following. These are described in the following two sections.
In the drawings:
1. Counter Propagating Optical Beams, Single One-Way Atomic Beam
In this configuration a round optical beam configuration is employed, such as shown in
Finally a photo detector (38) detects the fluorescence emitted by the atoms crossing the said 2nd interaction region. The photo detector output is used by a Frequency-Lock-Loop (FLL) circuit (28) to lock an Oscillator (24) to the CPT dark line which is related to the said hyperfine transition frequency. The said Oscillator (24) also generates an output for the user.
Not shown in the figure are the “C-field” coil that generates a constant homogeneous magnetic field perpendicular to the atomic beam and a set of magnetic shields to shield the atoms from the environmental stray field. The principles and advantages of this configuration are as follows:
2. One-Way Optical Beams, Two-Ways Counter-Propagating Atomic Beams
In this configuration a round optical beam configuration is employed, such shown in
Two atomic sources (37, 38) generate two overlapping counter-propagating atomic beams inside a vacuum envelope (not shown in the figure). A laser (39) emits optical beam at a wavelength which corresponds to the D1 or the D2 transition of the used alkali atom (for Cs D1 wavelength is 894.35 nm and D2 wavelength is 852.1 nm). The optical beam is modulated by a modulator (40) at a frequency around half the hyperfine transition frequency of the used alkali atom. The said modulation generates two sidebands separated by the hyperfine 0-0 transition frequency (for Cs the hyperfine frequency is around 9.2 GHz). The modulation frequency is generated by an Oscillator (41). The optical beam is then split into two symmetrical left and right beams by a beam splitter (42). Each beam is transmitted through a circular polarizer (43-44) (a quarter-lambda plate). The said left beam is circulated right wise and the right beam is circulated left wise by means of 2 mirrors (45-46) in such a way that it crosses the atomic beam at two interaction regions (47) and (48) separated by (for example) 20 cm. In the present configuration the two interaction regions are symmetrical with respect the two atomic sources. The said mirrors and the said splitter are mounted on tilters and translators to enable adjustments of optical beam lengths and angles in order to achieve a given set of the following conditions:
In primary frequency standard Cs beam tubes the end to end phase shift is generally evaluated by running alternatively the atomic beam from left to right or from right to left. In this way if there is a phase change between the first and the second Ramsey interaction zone the sign of the shift change and average to zero. Similarly if there is a residual first order Doppler shift due to progressive wave inside the microwave cavity, by changing the speed of the atoms the shift change sign and average to zero.
The idea is to have two counter-propagating atomic beams. This will cause a very high order cancellation of the end-to-end phase shift as well as of the first order Doppler shift.
This idea would probably not be implemented in a magnetically selected beam, because of the need of the hot wire detector. Instead it can surely be implemented in an optically pumped device where the pumping and detection zone are made equal and symmetric. A negative effect in this case is the doubling of the scattered light; a possible source of light shift.
In a device which utilizes CPT beam instead, no change in the optical system is required. A major engineering effort should probably be done in order to guarantee a good retracing of the beams and an efficient way to stop the two beams without interfering with the nozzle.
From the point of view of the signal there will be an increased background florescence in the detection zone because of the transverse pumping process happening in each zone, but there will be also two statistically independent signals. Since the main noise contribution will probably came form straight scattering of the laser this will not change.
From the loop point of view the two signal should be considered of the same importance, and the microcontroller should lock the VCO to their average frequency. In this way we zero the end to end and the first order Doppler shift.
A device where the end to end phase shift is compensated by two counter-propagating atomic beams, can be almost considered a Primary frequency standard, because almost all other shift can be evaluated with programmable experiments.
Number | Date | Country | |
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61129039 | Jun 2008 | US |