[0001] This relates generally to packaging semiconductor chips, and more particularly to a package for a magnetometer or atomic clock.

BACKGROUND

[0002] Chip-scale atomic clocks (CSACs) and magnetometers both use vapor cells that enclose vapors of alkali metals--typically either rubidium (Rb) or cesium (Cs). A laser sends a signal at an optical wavelength through the vapor cell, exciting hyperfine transitions using a phenomenon called coherent population trapping (CPT). For example, a cesium-based CSAC may use a laser that is tuned to the D1 absorption line of cesium at 894 nm. The laser sweeps a frequency region around the absorption line and monitors, at a photodetector, the amount of the light that is absorbed in passing through the vapor cell. The region of maximum absorption is detected and used to stabilize a reference frequency that is provided by the CSAC or magnetometer. The intrinsic noise in the system can hamper attempts to increase sensitivity in the measurements.

[0003] CSACs and magnetometers use similar structures, with one exception. With no external magnetic field, the Zeeman levels of an electronic transition are degenerate. However, in the presence of an external magnetic field, the degeneracy is broken, and the Zeeman levels are split in energy by the gyrometric ratio and the quantum number of degeneracy mf=0, +/-1,… +/-n. Structurally, this means that when the vapor cell is used for a CSAC, magnetic shielding is provided around the package to eliminate the external field and allow for a bias to provide a fixed splitting of the Zeeman levels; however, a magnetometer uses the spacing of the split absorption lines to measure the intensity of the magnetic field. Accordingly, different packages need to be produced to satisfy the needs of these different uses. Improvements to the fabrication and stability of these devices are desirable.

SUMMARY

[0004] Described examples implement a package for a CSAC or magnetometer that reduces noise by using orthogonal signals. After the optical beam has traversed the vapor cell, a polarizing beam splitter splits the optical signal into two orthogonal signals. The two orthogonal signals, when processed by different photodetectors, provide differential signals that cancel noise when processed and increase sensitivity. Sets of coils, e.g. Helmholtz coils, can be arranged to surround sensitive portions of the package and coupled to contacts that can be used to energize the coils as electromagnets. With the addition of differential signals and the ability to selectively block a magnetic field, a single package can be used as a CSAC, a zero field magnetometer or a free induction decay magnetometer. With appropriate control circuity, the described package can cycle between the different uses.

[0005] In one aspect of a package for a chip scale atomic clock or magnetometer, the package includes: a vapor cell comprising an alkali metal vapor; first and second photodetectors; a laser operable at a frequency that excites an electron transition in the alkali metal vapor, the laser positioned to provide an optical signal directed through the vapor cell and towards the first photodetector; and a polarizing beam splitter, the polarizing beam splitter positioned between the vapor cell and the first photodetector to receive the optical signal and to split the optical signal into a first signal directed toward the first photodetector and a second signal directed toward the second photodetector, the first signal being orthogonal to the second signal.

[0006] In another aspect of a package for a chip scale atomic clock or magnetometer, the package includes: a vapor cell comprising an alkali metal vapor; a first photodetector; a laser operable at a frequency that excites an electron transition in the alkali metal vapor, the laser positioned to direct an optical signal through the vapor cell and towards the first photodetector; and first, second and third sets of conductive coils coupled to contact pads for a power source, the first, second and third sets of conductive coils arranged to magnetically isolate the vapor cell when powered.

[0007] In yet another aspect of a package for operation as both a chip scale atomic clock and a magnetometer, the package includes: a vapor cell comprising an alkali metal vapor; first and second photodetectors; a laser operable at a frequency that excites an electron transition in the alkali metal vapor, the laser positioned to provide an optical signal directed through the vapor cell and towards the first photodetector; a polarizing beam splitter, the polarizing beam splitter positioned between the vapor cell and the first photodetector to receive the optical signal and to split the optical signal into a first signal directed toward the first photodetector and a second signal directed toward the second photodetector, the first signal being orthogonal to the second signal; and first, second and third sets of conductive coils coupled to contact pads for a power source, the first, second and third sets of conductive coils arranged to magnetically isolate the vapor cell when powered.

BRIEF DESCRIPTION OF THE DRAWINGS

[0008] FIG. 1 depicts an example of a package for a CSAC/magnetometer according to an embodiment.

[0009] FIG. 2 depicts an example of a package for a CSAC/magnetometer according to an embodiment.

[0010] FIG.3 depicts an example of a conventional package for a CSAC or magnetometer.

[0011] FIG.4 depicts an example absorption line, e.g., of cesium at 852 nm.

[0012] FIG.5 depicts the hyperfine-structure energy levels of Cs ¹³³ in a weak magnetic field. DETAILED DESCRIPTION OF EXAMPLE EMBODIMENTS

[0013] In the drawings, like references indicate similar elements. In this description, the term "couple" or "couples" means either an indirect or direct electrical connection, unless qualified as in "communicably coupled" which may include wireless connections. Thus, if a first device couples to a second device, that connection may be through a direct electrical connection, or through an indirect electrical connection via other devices and connections.

[0014] FIG.3 is a block diagram of a conventional package 300 for a CSAC or magnetometer. Package 300 includes a laser 302, a vapor cell 306 that encloses an alkali metal vapor and a photodetector 308. Laser 302 can be a vertical-cavity surface-emitting laser (VCSEL) that is arranged to send an optical beam 320 at a predetermined frequency through vapor cell 306 and to provide the optical beam 320 to photodetector 308. In one example, a vapor cell 306 contains Cs ¹³³ , while laser 302 is tuned to 894 nm. An optics package 304 can be placed between laser 302 and vapor cell 306 to provide appropriate processing of the optical beam 320. Optics package 304 can include a polarizer, a quarter-wave plate, a lens or any combination of these elements according to the specific design. For example, vapor cell 306 can be formed of three separate substrates 312A, 310, 312B bonded together. Substrate 310 can be silicon, glass, metal, ceramic or other material, through which a cavity 314 has been etched. Substrates 312A, 312B can be formed of glass that allows laser beam 320 to pass through largely un-attenuated.

[0015] At the time the three substrates 312A, 310, 312B are bonded to each other, an alkali metal or metal vapor, e.g., cesium or rubidium, is introduced into cavity 314. The metal vapor captured in vapor cell 306 will absorb some of optical beam 320 when laser 302 is operated at a specific, known frequency. The intensity of optical beam 320 is monitored at photodetector 308 while laser 302 sweeps across a frequency range that includes a selected absorption line of the alkali metal vapor. The frequency at which the maximum absorption occurs can be used to provide a clock reference signal that is highly stable.

[0016] In order to provide a more stable laser 302, a heater 318 can be coupled to provide thermal stability to laser 302. Various standoffs 316 are provided as shown and are used to separate components into different levels. For example, one set of standoffs 316A holds heater 318 away from ground plane 324 to slow thermal bleed-off. A second set of standoffs 316B places the optics package 304 at a distance for appropriate focus of laser 302 and standoffs 316C provide spacing between photodetector 308 and vapor cell 306. External packaging 322 is provided to protect the various dies. When package 300 is used for providing an atomic clock, the external packaging 322 may include a material that serves to shield the package 300 from magnetic fields, e.g. an iron alloy. When package 300 is fabricated for a magnetometer, magnetic shielding is not necessary. In this instance, the external packaging 322 may be plastic, ceramic or other suitable encapsulation material.

[0017] FIG.4 is a graph depicting an absorption line of light by an alkali metal, such as cesium vapor, at various wavelengths and depicts the drop in transmitted light at specific wavelengths, one of which for cesium occurs at a wavelength of 894 nm. An interesting phenomenon occurs to this absorption line in the presence of a magnetic field, such as the earth’s magnetic field, and is depicted in FIG. 5. Instead of providing a single absorption line, as shown in FIG. 4, the absorption line is split into three to seven equally spaced lines representing hyperfine-structure energy levels of the alkali metal, the Zeeman levels. The number of lines is related to the alkali metal, while the spacing between the lines is proportional to the strength of the magnetic field.

[0018] One issue that can arise in a package for an atomic clock or magnetometer is the variability of the laser over time. Even with the use of a heater to stabilize the transmissions from the laser, the frequency of light in a chip scale package can vary. This variation can introduce a“wiggle” or fluctuation in the frequency transmitted and thus will introduce a corresponding fluctuation in the signal received at the photodetector.

[0019] FIG. 1 shows a package 100 for a chip scale atomic clock or magnetometer according to an embodiment. Package 100 contains laser 102, vapor cell 106, two photodetectors 108A, 108B and a polarizing beam splitter 126. Laser 102 directs optical beam 120 through vapor cell 106 and towards polarizing beam splitter 126. Polarizing beam splitter 126 splits optical beam 120 into two orthogonal beams 121A, 121B that are directed towards photodetectors 108A and 108B respectively. In the embodiment shown in package 100, beam 121A continues along the path of optical beam 120 and is received at photodetector 108A, while beam 121B is directed to reflector 128, which reflects beam 121B to photodetector 108B. In one embodiment, reflector 128 can be a second polarizing beam splitter that reflects beam 121B towards photodetector 108B. In one embodiment, reflector 128 can be a mirror that reflects the beam 121B towards photodetector 108B. Other positions of one or both of photodetectors 108A, 108B can also be used. In one embodiment, shown in FIG. 2, reflector 128 is not used and photodetector 108B is positioned on spacer 116C such that as beam 121B exits polarizing beam splitter 126, beam 121B is directed to photodetector 108B. The use of orthogonal signals, both carrying information from vapor cell 106, operates to provide the same cancellation of noise as using differential electrical signals, decreasing the fluctuations caused by the variations in the laser signal, smoothing the received signal and providing a larger signal-to-noise ratio for the package.

[0020] In addition to the components described hereinabove, package 100 may also contain heater 118, optical package 104, ground plate 124, encapsulation structure 122 and standoffs 116A, 116B, 116C, which perform the same functions as their counterparts in FIG.3 and are not further described herein. Vapor package 106 includes a hermetic cavity 114 that is formed in substrate 110 and sealed between substrates 112A, 112B, which may be glass. Circuitry to control laser 102 and heater 118 and to process the data collected by photodetectors 108 is generally provided on a separate chip (not specifically shown), to which chip 100 can be mounted.

[0021] In the embodiment shown in FIG. 1, package 100 includes five sections, here labeled A-E. In one embodiment, level A has the largest horizontal dimensions and occupies a region approximately 12x12mm. Including offset 116A, which provides a thermal offset for heater 118 from ground plate 124, stage A has a height of approximately 1.2 mm. Offsets 116B form level B and are 3.3 mm high. Level C is the optics package 104, which occupies a space 8x8 mm and is 0.8 mm high; offsets 116B also occupy a space of 8x8 mm to provide support for optical package 104. Level D is occupied by vapor cell 106, which in this embodiment is 2.5 mm high and occupies a horizontal space of 10x10 mm. Level E forms the uppermost level of package 100 and includes photodetectors 108, polarizing beam splitter 126 and reflector 128; level E occupies a horizontal space of 7x7 mm and is approximately 1.5 mm tall. [0022] As described hereinabove, the components shown in FIG. 1 can be used for either an atomic clock or a magnetometer. When used for an atomic clock, magnetic shielding must be provided to prevent earth’s magnetic field, or any other magnetic field, from affecting the operation of the clock. FIG. 2 discloses a package 200 according to an embodiment that can obviate the need for separate packaging for the different applications of package 200. Package 200 has been simplified slightly from package 100, primarily by not showing the previous wiring connections, and discloses an alternative arrangement of photodetectors 108A, 108B. Also, package 200 contains three sets of coils 210, 212, 214 that surround vapor cell 106. As shown in package 200, coil 210A is arranged to the left of the vapor cell 106 and coil 210B is arranged to the right of the vapor cell 106. Similarly coil 212A is arranged above vapor cell 106 and coil 212B is arranged below vapor cell 106, while coil 214A is arranged in front of vapor cell 106 and coil 214B is arranged behind vapor cell 106, so that the vapor cell 106 is magnetically isolated in three dimensions. For example, each set of coils 210, 212, 214 may be solenoids or Helmholtz coils, and they are coupled to one or more contacts (not specifically shown) that can be used to power the coils as electromagnetics. When the package 200 is to be used as a magnetometer, the power source does not need to be connected to coils 210, 212, 214; however, when package 200 is to be used as an atomic clock, the coils 210, 212, 214 are coupled to a power source and energized as electromagnets that cancel any external magnetic fields.

[0023] Although not specifically shown, coils surrounding the vapor cell and operable as electromagnets can also be used in older packages, such as shown in FIG. 3, to allow the packages to operate as either a magnetometer or an atomic clock. In at least one embodiment, package 200 can be mounted with external circuitry that allows the package to be used alternately as an atomic clock and as a magnetometer. In this embodiment, package 200 can be operated as a magnetometer, i.e., without energizing the coils. After the magnetic field is measured, the coils can be energized to cancel the detected magnetic field and the package can be operated as an atomic clock. In one embodiment, the atomic clock can be used to discipline a lower stability clock that operates continuously. A cycle from magnetometer to atomic clock and back to magnetometer can be repeated as quickly as necessary or desired. If the magnetic field is changing very quickly over time, the cycle time can be adjusted to account for the changing of the magnetic field.

[0024] Improvements to the packaging of a CSAC or magnetometer have been shown. Orthogonal signals carrying the information gained from a laser passed through an alkali vapor cell have been provided by splitting an optical beam using a polarizing beam splitter. The orthogonal signals are processed by separate photodetectors and used to provide cancellation of noise in the signal. Magnetic shielding can be provided by sets of coils that are arranged to surround the vapor cell and are coupled to contacts for a power supply. When the coils are powered, magnetic shielding is provided, and the package can be used as a chip scale atomic clock. When the coils are unpowered, no magnetic shielding is provided, and the package can be used as a magnetometer. A package can also be operated to cycle between different uses.

[0025] Modifications are possible in the described embodiments, and other embodiments are possible, within the scope of the claims.