Physics package for cold atom primary frequency standard

This application claims the priority of US Provisional Application No. 61/087947 filed on August 11, 2009, and the entire disclosure of this application is incorporated herein by reference. This application relates to a US patent application entitled "COLD ATOM MICRO PRIMARY STANDARD" filed on the same day, the disclosure of which is incorporated herein by reference.

The primary frequency standard is an atomic clock that does not require calibration and operates autonomously for a long period of time with minimal time loss. One such atomic clock uses a laser-cooled alkali metal expansion cloud, such as cesium (Cs) or rubidium (Rb), in the non-electronic part of the atomic clock. The non-electronic part of an atomic clock is sometimes referred to as a physical package.

<p> Usually, these primary frequency standards and the corresponding physical packages are large and consume a lot of power. Although advances have been made to reduce the dimensions and power consumption of primary frequency standards and their physical packages, further reductions to achieve both military and civilian applications have been difficult.</p>

<p> An embodiment of the physical package provides a small chamber device that stores a cooling atom, which functions as a primary frequency standard as described below. More specifically, this small chamber device is a physical package used for atomic sensors (including accelerometers), especially for atomic clocks. The physical package is formed around a block with optical glass, glass-ceramic material, or some other suitable material. The outside of this block is shaped to include a plurality of surfaces located at predetermined angles to each other. The shape of this block has multiple angled bores, which are provided through the block, vacuum chamber cavities for alkali metals such as rubidium, light beams from light sources such as lasers. It functions as an optical path and a measurement port. Mirrors, such as those with optically clear windows, or metal or dielectric multilayers, are secured on the bore path to the outside of the block with a vacuum airtight seal. A fill tube made of a suitable material, such as a nickel-iron alloy, is secured to the outside of the block with a vacuum airtight seal at the end of the vacuum chamber cavity. Fill tubes are used for a variety of purposes, including introducing rubidium into the vacuum chamber of the physical package, evacuating the inside of the physical package to achieve the correct level of vacuum, and so on. After these are done, the fill tube is sealed to achieve a vacuum airtight seal and maintain a vacuum.</p><p> One embodiment of the physical package for an atomic clock has a block, the block comprising a plurality of faces located on the outside of the block at predetermined angles to each other, the block from one face of the block to the block. One or more measurement bores, which have a central bore that penetrates and extends to the opposite surface of the block, this central bore terminates with a fill tube and extends from one surface of the block through the block to the central bore. And it has a plurality of optical paths extending from one surface of the block through the block to the other surface of the block at a predetermined angle with respect to the angle of that surface, each of which is inside the block. It intersects at least part of the central bore and also intersects another one of the optical paths on one side of the block; the physical package has multiple optically clear windows, one of which. Is fixed to one surface of the block in place with a vacuum airtight seal, where one light path intersects the other light path and the remaining light path is the outer opening of the measurement bore. On top of, fixed using a vacuum airtight seal; the physical package has multiple mirrors, each of which is fixed to one side of the block, in another position, using a vacuum airtight seal. This other position is where one optical path intersects the other; the physical package has an inlet fill tube, which uses a vacuum airtight seal, one of the central bores. On one end, fixed to one side of the block and also has an outer fill tube, which uses a vacuum airtight seal, on the other end of the vacuum chamber cavity, on the other side of the block. It is fixed to the surface of.</p>

<figref num="1">It is a schematic perspective view (x-ray diagram) of one embodiment of the physical package of an atomic clock.</figref><figref num="2">It is an outside perspective view of one Embodiment of the physical package of an atomic clock.</figref><figref num="3">It is a schematic diagram of one embodiment of a physical package incorporated in an atomic clock.</figref><figref num="4">It is a flowchart which shows one Embodiment of the operation method of the physical package used to form an accurate frequency standard.</figref>

In the drawings, the same reference numerals and indicators in the various figures indicate similar elements. FIG. 1 is a schematic X-ray diagram (perspective view) of an embodiment of the physical package 10 for an atomic clock. The physical package 10 contains a plurality of optical passages dug in the block 20; the first measurement bore 22 and the second measurement bore 24; and collectively referred to as the optical passages 30 in the block 20. The optical passages have first to fifth optical passages, respectively, designated by reference numerals 31 to 35. The physical package 10 has a plurality of mirrors collectively referred to as the mirror 40, the mirror 40 is fixed outside the block 20 at the intersection of certain optical passages 30, and the mirrors 40 are respectively from reference numeral 41. It has the first to fifth mirrors indicated by 45. The physical package 10 has a plurality of optically clear windows 50 that are collectively referred to by the window 50, and the window 50 is fixed to the outside of the block 20 at the intersection of some optical passages 30. It has a window 51 (the first window 51 is shown by a broken line, which indicates that the first window 51 is behind the physical package 10), and the window 50 is of the first measurement bore 22. It has a second window 52 fixed to the outer opening and a third window 53 fixed to the outer opening of the second measurement bore 24. Physical package 10 has a central bore 60 dug in block 20. The physical package 10 has a fill tube 70, which contains an inlet fill tube 71 and an outlet fill tube 72 fixed to the block 20 at each end of the central bore 60.

The plurality of light passages 30 are dug in the block 20 in a geometric arrangement that results in an angled bore so that only a single light source (not shown), such as a laser, needs to be used in the atomic clock. To do. This arrangement also allows multiple mirrors 40 to direct a light beam (not shown) from a single light source into the light passage 30 of block 20. The outside of the block 20 is shaped to accommodate the geometric arrangement of the sloping bores of the light passage 30. The fill tube 70 can be used to introduce the alkali metal (cesium, or other suitable alkali metal) required for the operation of the atomic clock into the system, and also to form a vacuum in block 20. It can be used to evacuate the interior. For example, the fill tube 70 can be used to place an alkali metal capsule or container prior to exhaust. After this is done, the fill tube is sealed to achieve a vacuum airtight seal and a vacuum is maintained using a variety of techniques. For example, this technique involves pinching and welding. The chamber is evacuated to create a sealed vacuum, after which the alkali metal is opened into the chamber by crushing the capsule under vacuum (or by any other suitable technique). In other words, the alkali metal is introduced into the chamber prior to exhaust, but the alkali metal atoms are not released until they are exhausted and sealed.

The fill tube 70 also functions as an electrode for forming plasma for discharge cleaning of the physical package 10 and also enhances evacuation (that is, evacuates the cavity) and baking the physical package 10 (that is, evacuates the cavity). That is, the block 20 is heated to promote exhaust). The embodiment of physical package 10 shown in FIG. 1 includes gettering of a material to limit the partial pressure of some gas (eg hydrogen).

In terms of functionality, the physical package 10 shown in FIG. 1 operates in an atomic clock as follows. Vertical Cavity Surface Emitting Laser, A beam of light from a single light source (not shown), such as a VCSEL) or other type of laser, is directed through the first window 51 into the physical package 10 and into the first light passage 31. Enter (check the order of the lights). The light beam then travels through the central bore 60 through the first light passage 31 to reach the fourth mirror 44. The fourth mirror 44 reflects the light beam, and the light beam passes through the central bore 60, follows the second light passage 34, and reaches the third mirror 43. After that, the third mirror 43 reflects the light beam, and the light beam passes through the central bore 60, travels through the third light passage 33, and hits the second mirror 42. Next, the second mirror 42 reflects the light beam, and the light beam passes through the central bore 60, travels through the fourth light passage 32, and reaches the first mirror 41. After that, the light beam is reflected by the first mirror 41 and passes through the first light passage 31. The light beam is reflected in the opposite direction by the fifth mirror 45, returns to the path, and exits the block 20 through the first window 51. The effect is that multiple mirrors 40 travel the light beam from a single light source through the light passage 30 of block 20 to form three oppositely reflecting beams that intersect each other at an angle of 90 °. The clock signal is read through a first measurement bore 22 and a second measurement bore 24 using a photodiode (not shown). The photodiode is located outside the second window 52 and the third window 53 and is attached to these windows. In an alternative embodiment of physical package 10, a large number of measurement ports may be used.

Various materials and methods can be used to construct the parts of the physical package 10. For example, suitable materials that make up block 20 include glass-ceramic materials such as MACOR® and optical glass such as BK-7 or Zerodur®. In general, the material used to make up the block should have the following properties: That is, it should be vacuum airtight, impermeable to hydrogen or helium, and not react with materials (eg rubidium) introduced into the central bore 60. Other properties of block 20 include having low permeability to inert gases (eg argon) and being suitable for frit bonding to connect the mirror 40 to the outer surface of block 20 and also block 20. Includes being able to bake at high temperatures (eg 200 ° C and above). The block 20 can be manufactured in various ways. In one embodiment of the physical package, where the physical package is formed from a glass-ceramic material, the solid material is cut to the desired dimensions and molded to fit the desired geometry of the light passage 30. The light passage 30 and the central bore 60 are then dug within the dimensioned and shaped block 20. The volume of the generated block 20 is about 1 cm³About 5 cm from³Can be in the range of. The diameter of the optical passage 30 of block 20 depends on the volume of block 20 and is 1 cm.³Dimensional dimensions can be tolerated. The diameter of the central bore 60 of the block 20 also depends on the volume of the block 20.

Following the formation of block 20, the formation of physical package 10 is completed by attaching other parts of physical package 10 to block 20. In general, a material that can provide a seal that maintains a vacuum in a physical package 10 without the need for active vacuuming of multiple mirrors 40, multiple optically clear windows 50, and fill tubes 70. And must be attached to block 20 using technology. About 10^-7from torr and 10^-8Vacuum pressure of torr is acceptable. In one embodiment of the physical package 10, the plurality of mirrors 40 are fixed to the outside of the block 20 at the intersection of several light passages 30 using various techniques for forming a vacuum airtight seal. Various types of mirrors can be used in physical packages, including high reflectance, optically smooth mirrors with single-layer or multilayer metal or dielectric multilayer films. The mirror 40 can be a planar mirror or, if desired, a curved mirror that slightly focuses the light beam. The dimensions of the mirror 40 will depend on the volume of the block 20. Multiple optically clear windows 50 are secured to the outer openings of the first and second measurement bores 22 using various known techniques such as frit sealing to form a vacuum airtight seal. To. Suitable materials that make up the optically clear window 50 include, for example, BK-7 glass with an antireflection coating. The dimensions of window 50 will depend on the volume of block 20. In an alternative embodiment of the physical package, the mirror 40 and / or the optically clear window 50 are placed inside the block 20 in a vacuum-tight manner. Fill tubes 71, 72 are secured to the central bore 60 of block 20 using a variety of techniques for forming vacuum airtight seals, such as frit sealing, or using Swagelok or O-rings. Suitable materials for inlet fill tube 71 and outlet fill tube 72 include, for example, nickel, iron, aluminum, and nickel-iron alloys such as INVAR . The inlet fill tube 71 and the outlet fill tube 72 can have diameters ranging from about 1 mm to about 5 mm.

FIG. 2 is an outside perspective view of an embodiment of the physical package 10 for an atomic clock. From FIG. 2 and the above description, the physical package 10 includes a block 20, a plurality of optical passages 30, an inlet fill tube 71 and an outlet fill tube 72. The block 20 is formed on the outside of the block so as to include a plurality of surfaces 22 at predetermined angles with each other. This shape is suitable for the geometric arrangement of the sloping bores for the light passage 30.

FIG. 3 is a schematic diagram of an embodiment of a physical package integrated into the sensor device 100. The sensor device 100 is an atomic sensor (eg, an accelerometer or an atomic clock) having a physical package 110. In the embodiment shown in FIG. 3, the sensor device 100 is an atomic clock. The physical package 110 has a vacuum chamber cavity 120 that holds an alkali metal atom 130 such as rubidium (eg Rb-87) or cesium under passive vacuum (with or without a gettering agent). The physical package 110 also has an arrangement of light passages 140 and mirrors 150 that direct the light beam 160 from a single laser light source 170 to the physical package 110, and also has at least one photodetection port 180 (the embodiment shown). Has two).

The atomic clock 100 also has a micro-optical bench 190 with a single laser light source 170, which is a semiconductor laser, the laser being, for example, a Vertical Cavity Surface Emitting Laser (VCSEL), distribution. It is a distributed feedback laser, or an edge emitting laser. Atomic clock 100 further has a microformed vapor cell 192 containing an alkali metal such as rubidium (Rb-87) or cesium, and a beam splitter for splitting the light beam 160 into the vapor cell 192 and the physical package 110. Has 194. The atomic clock 100 further has a plurality of magnetic field coils 200, such as a Helmholtz coil and an anti-Helmholtz coil, for generating a magnetic field.

Further, the atomic clock 100 shown in FIG. 3 has a control electronic device 210. The arrangement of the light passage 140 and the mirror 150 guides the light beam 160 from a single laser light source 170 through the physical package 110, forming three back-reflecting optical beams that intersect each other at 90 ° within the vacuum chamber cavity 120. To do. The magnetic field generated by the optical beam and magnetic field coil 200 also slows, cools, and traps alkali metal atoms 130 (eg, Rb-87 atoms) from background steam, and also Rb-87 atoms. Used in combination to trap in the MOT (about 10 million atoms present at about 20 μK at the center of the intersection of the optical beams). The folded back-reflected beam path efficiently uses a single light source 170. Mirrors 150 (eg, dielectric mirrors) and diffractive optics are used to manipulate the optical beam and control the polarization of the optical beam, respectively, while minimizing scattered light and dimensions. The alkali metal-containing steam cell 192 is used to stabilize the frequency of the light beam 160 from a single laser light source 170 to a given atomic transition of the alkali metal.

Also, an embodiment of the atomic clock 100 has a Local Oscillator (LO) (not shown), an antenna (not shown), and a photodetector (not shown). One photodetector is used for each of the photodetection ports 180 in FIG. LO is used to generate a microwave signal corresponding to a given atomic transition of an alkali metal. The antenna is used to transmit the microwave signal from the LO to the alkali metal atom 130 in the physical package. Photodetectors are used to detect the emission of alkali metal atoms 130 (eg Rb-87 atoms).

FIG. 4 is a flowchart showing an embodiment of a method of operating a physical package used to form an accurate frequency standard. Method 400 has a step of storing atoms in a physical package (block 410). Method 400 also has a step of evacuating the physical package to near vacuum (block 420). The embodiment of the degree of vacuum is about 1 × 10.^-8The pressure is less than torr. In some embodiments of manipulating the physical package, the step of storing the atoms in the physical package (block 410) and the step of evacuating the physical package to near vacuum (block 420) are performed only once.

Method 400 further comprises the step of forming a magneto-optical trap (block 430) using a magnetic field and a light beam from a light source, where the light enters the physical package through one optically clear window and is multiple. It is back-reflected through the optical path of. The embodiment of Method 400, which manipulates the physical package used to form an accurate frequency standard, further extinguishes the magnetic and magneto-optical traps and allows the atom to transition from a high energy state to a low energy state. It has a step (block 440) that gives a small bias magnetic field for it. Time domain Ramsey or Rabi spectroscopy is performed using microwave signals generated by local oscillators and coupled to atoms by antennas to detect frequency division of atoms (block 450). Method 400 further comprises the step of measuring the emission of atoms with a light detector (block 460), determining the proportion of atoms at high ground state energy levels and determining the frequency of the microwave signal generated by the local oscillator. , Stabilize for frequencies that maximize the number of atoms in the high energy state (block 470). The LO frequency corresponds to the energy difference between the two ground hyperfine levels. In some embodiments of Method 400, some blocks are repeated to maintain the clock signal and lock the LO to atomic resonance. For example, blocks 430 through 470 can be made to loop between operations on physical packages.

The physical package design allows the use of only one light / laser beam (instead of six separate beams or three sets of back-reflecting beams or several combinations) in an atomic clock. The position of the mirror and the tilted bore allow a single light / laser beam to form an optical path by the mirror around the physical package so that it forms three back-reflecting beams that intersect each other at 90 °. .. The clock signal is read using a photodiode, which is mounted outside the optically clear window.

The above physical package design enables the manufacture of atomic clocks with many distinct advantages over existing atomic clocks. Thus advantages include reduced dimensions and power consumption, the ability to maintain an ultra-high vacuum without active evacuation, compatibility with high-vacuum manufacturing, and more.

Although embodiments of the present invention have been illustrated and described above, many modifications can be made without departing from the scope and gist of the present invention. The features described for one embodiment can be combined with or replaced with features of another embodiment. Therefore, the scope of the present invention is not limited to the disclosure of preferred embodiments. The present invention is determined by the appended claims.