US2956238A - Atomic resonance devices - Google Patents

Description

Oct. 11, 1960 F. H. REDER 2,956,238

ATOMIC RESONANCE DEVICES Filed June 10, 1958 3 32 [00 m PHASE SERVO DE MOTOR i 22 MAGNET 54 l 34 GRYS AL |8 036. 05c. 40

SYNTHESIZER rso 1 a 9|92.e3 e4 MC 48 FERRITE 26 MAGNET PUMP VALVE cs oven INVENTOR, FRIEDRICH H. REDER ATTORNEY.

United States Patent'Ob ATOMIC RESONANCE DEVICES Friedrich H. Reder, Long Branch, NJ., assignor to the United States of America as represented by the Secretary of the Army Filed June 10, 1958, Ser. No. 741,202 v, e

3 Claims. (Cl. 331-3) (Granted under Title 35, US. Code (1952), sec. 266) The invention described herein may be manufactured and used by or for the Government for governmental purposes, without the payment of any royalty thereon.

The present invention relates to atomic resonance devices, and more particularly to the devices of this type which utilize the changes of energy state of cesium atoms to produce resonance efiects having a high order of stability.

Among the objects of the invention are to reduce the amount of cesium wasted when the device is not in operation, to provide improved means for frequency modulating the output, and to decrease the physical height of I cavities.

the equipment and increase the signal to noise ratio there- The features of the invention which are believed to, be novel are set forth with particularity in the appended claims. The invention itself, however, both as to its organization and method of operation, together with further objects and advantages thereof, may best be understood by reference to the followingdescription taken in conjunction with the accompanying drawing,which is a schematic diagram of the invention applied to a cesium beam frequency standard, although it is applicable to other types of atomic frequency standards. A commercial example of this type of frequency standard is marketed under the trademark Atomichron.

Such a standard comprises an oven 10 where cesium is vaporized and passed through a manually operated, high-vacuum valve 12 into a beam tube 14. This valve is kept closed when the apparatus is not in operation to prevent loss of cesium, which is used up when the temperature is above 35 centigrade.

Along tube 14 are positioned two spaced cavity resonators 16 and 18, known as Ramsey cavities. Positioned above and below these cavities are permanent focusing magnets 20 and 22. Above magnet 22 is positioned a detector 24.

The atoms in the cavities 16 and 18 are excited by a wave applied thereto in like phase through a branch 26 opening into the main tube 14 midway between the cavities. The frequency of this wave should be as near as possible to the frequency of the cesium resonance, viz., 9192.63184 megacycles.

A vacuum pump 28, connected to the tube 14, immediately above the valve 12 keeps the gas at the required low pressure. Heretofore, this pump was positioned above the detector 24 so that all the gas with any impurities therein traversed the entire length of the tube. By connecting the pump immediately above the source, most of the impurities introduced into the beam tube are pumped out close to their source, with a resultant improvement in the signal to noise ratio. In addition, by positioning the pump at the side of the gas generating oven, the overall height of the equipment can be reduced about a foot.

In operation, the beam of cesium atoms enters into. the tube 14 and travels toward the cavities. The energy state of the atoms depends upon the orientation of their dipole moments. If it is assumed that the atoms of the beam 2,956,238 Patented ,Oct. 11, 1960.

focusin'gmagnet 20, because of the orientation of its magnetic field, the atoms of the higher energy state will be deflecteld toward' the wall of the tube, while thoseof lower energy state will pass on toward the cavities, where they are excited by the radio frequency field injected into the The atoms absorb energy from this field and as a result are raised to the higher energy state. A maximum number of atoms are thus transformed if the frequency of the radio frequency exciting field-is exactly equal "to the transition frequency of the atom. These atoms now travel past the second focusing magnet 22, which is so oriented that atoms in the lower energy state will be deflected away from the beam and those in the higher energy state will pass on to detector 24.

The number of atoms whichare converted to the higher energy state, and hence the number of such atoms which reach detector 24, depends upon the closeness of the injected signal'frequency to the frequency ofvcesium resonance. These atoms strike the ionizing target 30 and then are attracted by means of a mass spectrometer to the cathode '32 of an electron multiplier. The output of latter varies with the number and rapidity of impingement of cesiumatoms on the cathode. The detected output cantherefore be used to lock the frequency of the external oscillator which supplies this injected signal. The external oscillator is a five megacycle crystal oscillator 34 having a'control element therein to precisely adjust its frequency to a small extent. The output of this oscillator is applied to a conventional frequency synthesizer which converts the 5 'mc. output to a plurality of harmonies and-subharmonics which are combined in various ways to provide a number of frequencies to be utilize d at leads 40''48. t V

7 .One of these frequencies is 9192.62184 mc., the exact frequency of cesium resonance, which is applied to branch 26 of'tube 14. To'provide locking-information this frequency'is frequency modulated at a low rate by means of a ferrite modulator within branch 26. This comprises apiece of ferrite 50, subjected to the varying magnetic field of a'permanent magnet 52 having a coil to which is applied a sinusoidal modulating current from a cycle modulating oscillator 54." Permanent magnet 52 provides a predetermined steady magnetic bias on the ferrite element-5m The l00cycle signal modulates the phase of the RF. exciting wave, and this produces a concomitant frequency modulation thereof. Since any departure of the frequency of the RF. exciting wave from the frequency of cesium resonance effects the output of detector 24, the amplitude of said output is a measure of said departure, and the direction of phase displacement of said output, relative to the phase of the modulating signal,

is a measure of the direction of departure. The 100 cycle detector output signal is amplified by a tuned amplifier 56 and applied, together with the signal from modulating oscillator 54 to a phase comparing detector 58 where the phase of the two signals are compared. If the exciting frequency departs from the frequency of cesium resonance, the phase detector will yield an output having an' amplitude which is a function of the extent of the departure of the frequency of the exciting wave from the frequency of cesium resonance, and a direction which is a function of the direction of said departure. This operates .a servo motor 60 which rotates the frequency controlling elements of oscillator 34 until the output of the phase detector is zero or minimum, at which time said departure is minimized.

While there has been described what is at present considered to be a preferred embodiment of the invention, it will be obvious to those skilled in the art that various changes and modifications may be made thereinwithout departing from the invention, and it is, therefore, aimed in the appended claims to cover all such changes modifications as fall within the true spirit and scope of the invention.

What is claimed is:

1. A cesium beam frequency standard comprising an oven for vaporizing cesium, an atomic beam tube into which a beam of cesium atoms is introduced from said oven, a high vacuum valve between said oven and said beam tube for controlling the flow of cesium into said tube; said beam tube having positioned therealong, in the order named, a first focusing magnet, a pair of spaced Ramsey cavity resonators tuned to the frequency of cesi: um resonance, a second focusing magnet, and a detector; a vacuum pump connected to said beam tube at a point near said oven. and positioned beside said oven; a branch connected to said beam tube substantially midway between said cavities for injecting into said cavities radio frequency energy of like phase and of a frequency in the vicinity of cesium resonance to cause a change of energy state of atoms in said cavity; a source of said radio frequency energy connected to said branch, a magnetic ferrite modulator in said branch, a source of low frequency current coupled to said modulator for frequency modulating said radio frequency energy and thereby periodically varying the amplitude and phase of the output of said detector, and means responsive to said output and said low frequency current for reducing any departure of the frequency of said radio frequency energy source from the frequency of cesium resonance.

, 2. An atomic frequency standard comprising a source of atoms, an atomic beam tube into which a beam of said atoms is introduced from said source; a first focusing magnet, a pair of spaced cavity resonator means tuned the fre quency of cesium resonance, a. second focusing magnet, and a detector arranged in said tube in the order named, a vacuum pump connected to said beam tube at a point near said source, whereby impurities in said beam are removed close to said source, a branch connected to said beam tube substantially midway between said cavities for injecting into said cavities radio frequency energy of like phase and of a frequency in the vicinity of the transition frequency of said atoms to cause a change of energy state of atoms in said cavity, a source of said radio frequency energy connected to said branch, a modulator in said branch, a source of low frequency current coupled to said modulator for frequency modulating said radio frequency energy and thereby periodically varying the amplitude and phase of the output of said detector, and

means responsive to said output and said low frequency current for reducing any departure of the frequency of said radio frequency energy' source from said transition frequency.

3. A cesium beam frequency standard comprising an elongated atomic beam tube, said beam tube having positioned therealong, in the order named from one end to the other, a first focusing magnet, first andsecond spaced Ramsey cavity resonators, tuned to therfrequencyv of cesi; um resonance, a second focusing magnet, and a detector; an oven for vaporizing cesium at said' oneend of the beam tube whereby abearn. of cesium atoms is introduced into said tube, a high vacuum valve at said one end connecting the output-of said oven to said beam tube for controlling the flow of cesium into said tube, a vacuum pump connected to saidbeam tube at a point immediately above said valve and positioned beside said oven; a branch connected to said-beam tube substantially midway between said cavities for injecting into said cavities radio frequency energy of like phase and of a frequency in the vicinity ofces-ium resonance to cause a change of energy state of atoms; in-t said cavity; a source of said radio frequency energy connected to said branch, a magnetic ferrite modulator in said branch, a, source of low frequency current coupled to said modulator for frequency modulating said radio frequency energy and thereby periodically varying the amplitude and phase of the output of said detector, and means responsive to said output and said low frequency current for reducing any departure of thefrequency of' said radio frequency energy source from the frequency of cesium resonance. 1 7

References Cited in the file of this patent- UNITED' STATES PATENTS Dicke Nov. 13, 1956 2,831,974 Wright et al. Apr. 22, 1958 2,836,722 Dicke et a1 May 27, 1958 2,894,209 Chodorow et al July 7, 1959 OTHER REFERENCES

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