Information
-
Patent Grant
-
6570459
-
Patent Number
6,570,459
-
Date Filed
Monday, October 29, 200122 years ago
-
Date Issued
Tuesday, May 27, 200321 years ago
-
Inventors
-
Original Assignees
-
Examiners
-
CPC
-
US Classifications
Field of Search
-
International Classifications
-
Abstract
Physics package apparatus for a cell type atomic clock includes a cell structure having a central plate sandwiched between top and bottom plates. The central plate has a central interior aperture which together with the top and bottom plates forms an internal cavity for containment of an active vapor. The central plate includes a reservoir for holding a source of the active vapor, and a channel connecting the reservoir with the internal cavity. A heater is provided on the underside of the bottom plate for heating the vapor. The plates are batch processed on respective wafers which are subsequently joined together and cut into individual cell structures.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention in general relates to atomic frequency standards, and more particularly to components of a physics package for an atomic clock of the type which utilizes an optically pumped cell containing a vapor.
2. Description of Related Art
Atomic clocks are utilized in various systems which require extremely accurate and stable frequencies, such as in bistatic radars, GPS (global positioning system) and other navigation and positioning systems, as well as in communications, cellular phone systems and scientific experiments, by way of example.
In one type of atomic clock, a cell containing an active medium such as cesium (or rubidium) vapor is irradiated with both optical and microwave energy whereby light from an optical source pumps the atoms of the vapor from a ground state to a higher state from which they fall to a state which is at a hyperfine wavelength above the ground state. The microwave signal is tuned to a particular frequency so as to repopulate the ground state. In this manner a controlled amount of the light is propagated through the cell and is detected by means of a photodetector.
By examining the output of the photodetector, a control means provides various control signals to ensure that the wavelength of the propagated light and microwave frequency are precisely controlled.
There is a need, both in the military and civilian sectors, for an ultra small, completely portable, highly accurate and extremely low power atomic clock. The atomic clock must operate continuously for 24 hours per day to perform a useful function. For this reason, power levels approaching 100 milliwatts, or less, are desirable for military and many civilian uses.
The non-electronic portion of the atomic clock is often referred to as the physics package and, as will be described, includes power consuming elements such that the physics package promises to be the determiner of the size, low power capabilities and ultimate low cost of the final product.
It is a primary object of the present invention to provide physics package apparatus for an atomic clock, which is of small size, for example, 1 cm
3
, or less, and which meets low cost, ease of fabrication and low power usage requirements.
SUMMARY OF THE INVENTION
Physics package apparatus for a cell type atomic clock in accordance with the present invention includes a cell structure having a central plate sandwiched between top and bottom plates. The central plate has a central interior aperture which together with the top and bottom plates forms an internal cavity for containment of an active vapor. The central plate includes a reservoir for holding a source of the active vapor, and a channel connecting the reservoir with the internal cavity.
Apertures on either end of the central plate respectively accommodate a laser diode, which projects a laser beam through the vapor, and a detector for detecting the projected beam. End walls of the interior aperture may include curved portions for shaping and focusing the laser beam.
A microwave coupling arrangement, such as strip line conductors couples microwave energy into the vapor-containing cavity. For a complete physics package, insulation, a C-field coil and a surrounding metallic magnetic shield may be included.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1
is a simplified block diagram of a typical atomic clock.
FIG. 2
is an exploded view of an active vapor cell structure used in a physics package.
FIGS. 3A
,
3
B and
3
C illustrate components of the cell structure of
FIG. 2
, on respective wafers.
FIGS. 4A
,
4
B and
4
C illustrate certain steps in the fabrication of the cell structure of FIG.
2
.
FIG. 5
is a sectional view of a physics package in accordance with the present invention.
DESCRIPTION OF THE PREFERRED EMBODIMENT
In the drawings, which are not necessarily to scale, like or corresponding parts are denoted by like or corresponding reference numerals. In addition, the terms top and bottom are used herein for ease of explanation and not as structural or orientational limitations.
FIG. 1
basically illustrates an atomic frequency standard, or atomic clock,
10
, of the type which includes a physics package
12
having a cell
14
filled with an active vapor
16
such as a vapor of cesium or rubidium.
An optical pumping means, such as a laser diode
20
is operable to transmit a light beam of a particular wavelength through the active vapor
16
, which is excited to a higher state. Absorption of the light in pumping the atoms of the vapor to the higher states is sensed by a photodetector
22
which provides an output signal proportional to the impinging light beam on the detector.
Adjacent to the cell
14
is a microwave cavity
26
, or the like, which couples a precisely controlled rf (radio frequency) signal to cell
14
. Assuming an active vapor
16
of cesium, the rf signal is tuned to the microwave atomic transition frequency of the cesium vapor
16
, of approximately 9.2 GHz, so that the ground state depleted by the laser diode
20
is repopulated at an enhanced rate.
The rf signal is provided by rf circuitry
28
and when the frequency of the rf signal is precisely at the desired hyperfine magnetic dipole transition frequency, the amount of light passing through cell
14
to detector
22
will be at a minimum. The output of detector
22
is provided, via feedback circuitry
30
, to a master control such as microprocessor
34
, which in turn controls the frequency provided by rf circuitry
28
. A separate output
36
of the rf circuitry
28
delivers the desired time standard, such as a 10 MHz clock signal.
A laser current regulator
40
, in response to signals from microprocessor
34
, controls the current to laser diode
20
, which in turn controls the wavelength emitted, to match the absorption of the vapor (852 nm for cesium). Typically the laser must also be controlled in temperature.
A laser current regulator
40
, in response to signals from microprocessor
34
, controls the current to laser diode
20
, which in turn controls the wavelength emitted, to match the absorption of the vapor (852 nm for cesium). Typically the laser must also be controlled in temperature to establish the desired wavelength. This is accomplished with the provision of laser heater
42
, under control of laser heater regulator
44
. A temperature sensor
46
monitors the laser temperature and provides a corresponding temperature output signal to the microprocessor
34
, via feedback circuitry
30
.
In order to generate the required vapor pressure in cell
14
, the vapor
16
is heated by a heater
48
. The precisely controlled cell temperature is accomplished with the provision of heater control
50
, in conjunction with temperature sensor
52
which monitors the cell temperature at the coldest point in the vapor envelope and provides this temperature indication, via feedback circuitry
30
, to microprocessor
34
.
The physics package
12
additionally includes a C-field coil
54
, under control of C-field regulator
56
, to generate a uniform background magnetic field, to minimize the effects of stray external magnetic fields. In addition, a magnetic field metallic shield
58
is generally provided to further isolate the cell
20
from external fields.
Further details of the components and operation of the atomic clock
10
are described in U.S. Pat. Nos. 5,192,921, 5,327,105, 5,606,291 and 5,852,386, all of which are hereby incorporated by reference.
The physics package, and its components, of the present invention meets the desired requirements of small size, for portability and low power consumption, for continual 24 hour use. In addition, the components can be batch fabricated, resulting in lower overall costs for the atomic clock. An embodiment of the invention is shown in
FIGS. 2
to
5
.
The exploded view of
FIG. 2
illustrates a cell structure
60
comprised of a central plate
62
which is sandwiched between top and bottom plates
63
and
64
. Central plate
62
includes a central interior aperture
70
extending completely through the plate and defining leg sections
72
and
73
, as well as end sections
74
and
75
.
An aperture
80
in end section
74
receives laser diode
81
, such as a vertical cavity surface emitting laser, and aperture
82
in end section
75
receives detector
83
. These apertures are an optional feature of the cell structure
60
in as much as one, or both, of the laser diode
81
and detector
83
may be positioned outside the end sections
74
and
75
, respectively.
A wall of interior aperture
70
may be curved, adjacent laser
81
, so as to define a lens portion
86
to collimate the laser beam projected through interior aperture
70
. Similarly, an opposite wall of interior aperture
70
, adjacent detector
83
may also be curved to define lens portion
87
, for focusing the projected laser beam onto the detector
83
.
Central plate
62
additionally includes a well, or reservoir
90
into which will be placed the source of the vapor, for example, cesium, which migrates, in gaseous form, into the interior aperture
70
, via channel
92
. When sealed with the top and bottom plates
63
and
64
, the interior aperture
70
forms an internal cavity
94
for the cesium vapor, as well as any buffer gas which normally may be utilized.
Bottom plate
64
includes, at either end, respective apertures
100
and
101
to accommodate the insertion of laser diode
81
and detector
83
into apertures
80
and
82
in central plate
62
, after which, the apertures
100
and
101
may be sealed.
In order to maintain the cesium in a gaseous state, at a precise temperature and pressure, the internal cavity
94
is heated. This is accomplished with the provision of serpentine heater
104
on the underside of bottom plate
64
, which also includes a temperature sensor
106
for obtaining a temperature indication of the cesium in reservoir
90
. This temperature sensor
106
provides an indication of the coldest spot in the vapor system, which determines the cesium vapor pressure within cavity
94
. Another heater,
108
, is affixed to a surface of the laser diode
81
to control its temperature and also to double as a temperature sensor.
Microwave signals may be coupled into the cesium cell by several different means.
FIG. 2
illustrates, by way of example, a microstrip coupling arrangement. More particularly, top plate
63
includes strip line electrodes
110
and an input electrode
112
, all of which are deposited on the surface thereof. As will be shown, a ground plane (not illustrated in
FIG. 2
) is also provided on the opposite side of cavity
94
. Microstrip coupling arrangements are described in more detail in the aforementioned U.S. Pat. Nos. 5,192,921 and 5,327,105.
The cesium cell structure
60
of
FIG. 2
lends itself to batch processing methods whereby many tens of such structures can be fabricated simultaneously. For example
FIG. 3A
shows a portion of a wafer
116
. By well-known photolithographic techniques a photo resist material is deposited over the surface of the wafer and thereafter masked with a pattern of central plates
62
.
The masked assembly is exposed to ultra-violet light, making the photo resist material soluble in the areas to be removed. These areas, apertures
70
,
80
,
82
, are then formed by an etching process. Further, the cesium reservoir
90
and channel
92
may also be etched all the way through the thickness of the plate
62
, whereby bottom plate
64
will serve as the bottom of reservoir
90
and channel
92
, when fully assembled.
As indicated in
FIG. 3B
, bottom plate
64
may be fabricated on wafer
118
by similar methods, after which, heaters
104
and sensors
106
(
FIG. 2
) may be deposited on the undersurface of the bottom plates
64
.
As indicated in
FIG. 3C
, top plates
63
may also be fabricated on a wafer,
120
, however, only strip line deposition is required.
Cesium is an element which reacts violently in air and water and is corrosive to many materials. All of the plates
62
,
63
and
64
are exposed to the cesium vapor and accordingly, the wafers
116
,
118
and
120
, from which they are made must be of a material which is inert to the cesium. Sodium borosilicate glasses are known to satisfy this condition.
In addition, when assembled, the plates form a sandwich which must be sealed. The sealing of the wafers may be accomplished by well-known techniques which utilize pressure, increased temperature and electric field technology to result in diffusion and drift-driven bonding between elements. Alternatively, the sealing may be realized with a wax material which is impervious to the cesium. This wax material should have a softening point of greater than around 85° C., have low vapor pressure of around 10
−6
Torr, or less, at 7° C., an application temperature of around 130° C., or lower, and must maintain the necessary sealing properties at the highest operating temperature of the cell. One example of such material is a commercially available wax known as Apiezon wax W, a product of Apiezon Products, a business unit of M & I Materials Ltd.
Another potential wafer material for one or more of the wafers is a single crystal, high resistivity semiconductor such as silicon, to which can be applied well-established fabrication techniques. This material has the added advantage in that integration of electronic components on a single substrate, along with the cesium cell may be possible.
The reactivity of cesium with pure silicon is unknown. If excessive, all surfaces exposed to the cesium must be protected. This may be accomplished by passivating the silicon surfaces with borosilicate sodium oxides. In addition, the seal between plates may be accomplished by electrostatic and pressure sealing at moderate temperatures over a period of time, followed by a stabilizing hydrogen treatment. A hybrid of silicon and glass may also be used.
If silicon is used for the central plate
62
, then windows transparent to the laser light must be formed in the end sections
74
and
75
, in the vicinity of lens portions
86
and
87
. Such windows may be formed by
If silicon is used for the central plate
62
, then windows transparent to the laser light must be formed in the end sections
74
and
75
, in the vicinity of lens portions
86
and
87
. Such windows may be formed by converting these regions to silicon oxide or silicon dioxide, with subsequent passivation.
Due to the highly reactive cesium, the assembly, or partial assembly, of the cell structure
60
should take place in a manufacturing chamber under vacuum conditions. Such vacuum manufacturing chamber is denoted by numeral
122
in FIG.
4
A. Within chamber
122
is a thermal plate
124
operable to be heated as well as cooled. Depending from plate
124
is a plurality of needles
125
in an array that matches the positions of the cesium reservoirs
90
. A source
128
of cesium within the chamber
122
is opened resulting in an emission of cesium vapor
129
.
Plate
124
is initially cooled, causing the vapor
129
to condense on the needles
125
. For ease of assembly, prior to its introduction into the chamber
122
, the wafer
116
, containing the central plates
62
may be brought into registration with wafer
118
containing the bottom plates
64
, and the two wafers sealed, thus defining the cesium reservoirs
90
.
As illustrated in
FIG. 4B
, the joined wafers
116
and
118
are positioned below the needles
125
, which are spaced in two dimensions to correspond to the spacing of the reservoirs
90
. The plate
124
is then heated causing the condensed cesium to liquefy and drop into the respective reservoirs
90
.
Thereafter, and as illustrated in
FIG. 4C
, subsequent to an application of a sealant, applied within the chamber plurality of simultaneously fabricated cesium cell structures
60
. The depositions of strip line electrodes
110
and
112
may take place prior to operations within the chamber
122
, or may be deposited subsequent to removal of the joined wafers, as long as the deposition process temperature is compatible with the sealant utilized to join the wafers.
Once the cell structure
60
has been formed, other components may be added to make a complete physics package. One such example of a physics package
140
is illustrated in the cross-sectional view of FIG.
5
. As previously brought out, the microwave excitation arrangement comprises strip line electrode
110
(as well as
112
shown in
FIG. 2
) deposited on the top surface of top plate
63
. A metallic layer
142
serves as the ground plane for the microwave excitation arrangement and is separated from the deposited heater
104
and sensor
106
by an insulating layer
144
.
A solenoidal magnetic C-field coil
146
surrounds insulating layer
148
, and C-field coil
146
is surrounded by another insulating layer
150
. A mu-metal, or other high permeability magnetic shield
152
is provided, and forms the outside of the physics package
140
.
By utilizing the manufacturing techniques described herein, the individual plates
62
,
63
and
64
may have an outside area of around 2 cm
2
, or less, and when joined, will form a cell structure
60
of around 0.8 cm
3
or less. The thickness of the components of
FIG. 5
are greatly exaggerated for clarity, however they will add less than around 0.2 cm
3
, making a total volume of around 1 cm
3
, or less, for the entire physics package.
It will be readily seen one of ordinary skill in the art that the present invention fulfills all of the objects set forth herein. After reading the foregoing specification, one of ordinary skill in the art will be able to effect various changes, substitutions of equivalents and various other aspects of the present invention as broadly disclosed herein. It is therefore intended that the protection granted hereon be limited only by the definition contained in the appended claims and equivalents. Having thus shown and described what is at present considered to be the preferred embodiment of the present invention, it should be noted that the same has been made by way of illustration and not limitation. Accordingly, all modifications, alterations and changes coming within the spirit and scope of the present invention are herein meant to be included.
Claims
- 1. Physics package apparatus for a cell type atomic clock, comprising:A) a cell structure having a central plate sandwiched between top and bottom plates; B) said central plate having a central interior aperture which together with said top and bottom plates forms an internal cavity for containment of an active vapor; C) said central plate including a reservoir for a source of said active vapor and a channel connecting said reservoir with said internal cavity.
- 2. Apparatus according to claim 1 wherein:A) said central plate includes first and second end sections; and B) said first end section includes a first aperture for placement of a laser diode operable to project a laser beam through said vapor in said cavity.
- 3. Apparatus according to claim 2 wherein:A) said second end section includes a second aperture for placement of a detector operable to detect said laser beam projected through said vapor in said cavity.
- 4. Apparatus according to claim 2 wherein:A) said central interior aperture includes a curved wall portion adjacent said first aperture, forming a lens for shaping and focusing said laser beam.
- 5. Apparatus according to claim 3 wherein:A) said central interior aperture includes a curved wall portion adjacent said second aperture, forming a lens for shaping and focusing said laser beam projected through said vapor, onto said detector.
- 6. Apparatus according to claim 2 wherein:A) said reservoir is in one of said end sections.
- 7. Apparatus according to claim 1 which includes:A) a heater disposed on the underside of said bottom plate and operable to heat said vapor in said internal cavity.
- 8. Apparatus according to claim 1 which includes:A) a temperature sensor disposed on the underside of said bottom plate below said reservoir and operable to obtain an indication of the temperature of the contents of said reservoir.
- 9. Apparatus according to claim 1 which includes:A) a microwave coupling arrangement operable to couple microwave energy into said vapor-containing internal cavity.
- 10. Apparatus according to claim 9 wherein:A) said microwave coupling arrangement includes i) at least two conducting strip line electrodes deposited on the top surface of said top plate and ii) a ground plane disposed below said bottom plate.
- 11. Apparatus according to claim 1 wherein:A) said central, top and bottom plates are of borosilicate glass.
- 12. Apparatus according to claim 1 wherein:A) at least one of said central, top and bottom plates is of a single crystal semiconductor having a coating thereon, said coating being impervious to said vapor.
- 13. Apparatus according to claim 12 wherein:A) said semiconductor is silicon.
- 14. Apparatus according to claim 1 which additionally includes:A) a laser diode positioned to project a laser beam through said vapor in said cavity; B) a detector positioned to detect said laser beam projected through said cavity; C) a heater disposed on the underside of said bottom plate and operable to heat said vapor in said internal cavity; D) a microwave coupling arrangement operable to couple microwave energy into said vapor-containing internal cavity; E) a C-field winding surrounding said microwave coupling arrangement to generate a uniform background magnetic field, to minimize the effects of any stray external magnetic fields; F) an insulating layer disposed between said microwave coupling arrangement and said C-field winding; and G) a magnetic field metallic shield surrounding said insulating layer to further isolate said cell structure from external fields.
- 15. Apparatus according to claim 1 wherein:A) a plurality of said central plates are formed on a first wafer; B) a plurality of said top plates are formed on a second wafer; C) a plurality of said bottom plates are formed on a third wafer; and wherein D) said plates are joined together, with said first wafer sandwiched between said second and third wafers, and the joined wafers subsequently cut into individual said cell structures.
- 16. Apparatus according to claim 15 wherein:A) at least said first and second wafers are joined in a vacuum chamber.
- 17. Apparatus according to claim 16 wherein:A) at least said first and second wafers are sealed with a wax material which has a softening point of greater than around 85° C., has a low vapor pressure of around ≦10−6 Torr at 70° C., and has an application temperature of around ≦130° C.
Foreign Referenced Citations (1)
Number |
Date |
Country |
0 550 240 |
Jul 1993 |
EP |