Apparatuses and methods for sensing rotations are provided. One embodiment of the apparatus includes a cell containing alkali and active nuclear magnetic resonance (NMR) isotope(s) atoms, a magnet providing a first magnetic field, a light source, and optics which circularly polarize light to generate a pump beam for optically pumping the alkali atoms and, together with a second magnetic field orthogonal to the first magnetic field or a modulation of the light, causing the alkali and the NMR isotope atoms to precess about the first magnetic field. The apparatus further includes a partial reflector opposite the light source and configured to, in conjunction with a first linear polarizer, generate a reflected linearly-polarized probe beam from a portion of the pump beam, and one or more polarizing beam splitters configured to split light of the probe beam incident thereon into orthogonally polarized components that are detected and used to determine rotations.

BACKGROUND OF THE INVENTION

Field of the Invention

Embodiments disclosed herein relate generally to atomic sensing devices, and more specifically, to a chip-scale atomic gyroscope or magnetometer.

Description of the Related Art

Gyroscopes are devices that may be used to sense rotations of objects. Gyroscopes have applications in many areas including navigation, where, combined with accelerometer data, rotations sensed via gyroscopes may help provide the positions of airplanes, submarines, satellites, and the like, without having to rely on the Global Positioning System (GPS).

Traditionally, gyroscopes having high performance in terms of, e.g., bias stability and low angle random walk (ARW), have been large, expensive, and power-hungry. For example, some gyroscopes currently in use are lunchbox-sized devices that cost tens of thousands of dollars to manufacture. The size, expense, and power requirements of such devices are significant drawbacks that limit their utility in many applications.

SUMMARY OF INVENTION

In one embodiment, an apparatus for sensing rotations is provided. The apparatus generally includes a cell containing at least a vaporized source of alkali atoms and atoms of one or more active nuclear magnetic resonance (NMR) isotopes, a magnet that generates a first magnetic field, and a light source configured to emit light. The apparatus further includes optics configured to circularly polarize the light emitted from the light source to generate a pump beam for optically pumping the alkali atoms and, in conjunction with one of a second magnetic field orthogonal to the first magnetic field or a modulation of the emitted light, causing the alkali atoms and the one or more active NMR isotope atoms to precess about the first magnetic field. The apparatus also includes a partial reflector that is positioned opposite the light source and configured to, in conjunction with a first linear polarizer, generate a reflected linearly-polarized probe beam from a portion of the pump beam. In addition, the apparatus includes one or more polarizing beam splitters, each polarizing beam splitter configured to split light of the probe beam incident thereon into orthogonally polarized components, and one or more photodetectors, each photodetector being configured to detect one of the polarized components and generate a signal indicative of intensity of the detected polarized component.

In another embodiment, a method for sensing rotations is provided. The method generally includes applying a first magnetic field, emitting light from a light source, passing the light through optics to circularly polarize the light to generate a pump beam, and passing the pump beam through a cell to optically pump alkali atoms in the cell and, in conjunction with one of a second magnetic field orthogonal to the first magnetic field or a modulation of the emitted light, cause the alkali atoms and the one or more active NMR isotope atoms to precess about the first magnetic field. The method further includes passing the pump beam through a first linear polarizer, and attenuating and reflecting light of the pump beam passed through the first linear polarizer, where the attenuated and reflected light is further passed through the first linear polarizer to generate a probe beam. The method also includes passing the probe beam through the cell, and splitting the probe beam that has passed through the cell using one or more polarizing beam splitters, each polarizing beam splitter configured to split light of the probe beam incident thereon into orthogonally polarized components. In addition, the method includes detecting each of the polarized components via at least one respective photodetector, and determining the rotations based on the detected polarized components.

In yet another embodiment, an apparatus for sensing an external magnetic field is provided. The apparatus generally includes a cell containing at least a vaporized source of alkali atoms, a magnet that generates a first magnetic field, and a light source configured to emit light. The apparatus further includes optics configured to circularly polarize the light emitted from the light source to generate a pump beam for optically pumping the alkali atoms and, in conjunction with one of a second magnetic field orthogonal to the first magnetic field or a modulation of the emitted light, causing the alkali atoms to precess about the first magnetic field. The apparatus also includes a partial reflector that is positioned opposite the light source and configured to, in conjunction with a linear polarizer, generate a reflected linearly-polarized probe beam from a portion of the pump beam. In addition, the apparatus includes one or more polarizing beam splitters, each polarizing beam splitter configured to split light of the probe beam incident thereon into orthogonally polarized components, and one or more photodetectors, each photodetector configured to detect one of the polarized components and generate a signal indicative of intensity of the detected polarized component.

DETAILED DESCRIPTION

Embodiments of the present invention are directed to atomic sensing devices. The atomic sensing devices will be described herein primarily in relation to chip-scale atomic gyroscopes. It is to be understood, however, that embodiments of the atomic sensing devices may be used as magnetometers without departing from principles of the present invention.

A first embodiment provides a chip-scale atomic gyroscope having high performance, in terms of bias stability and ARW, as well as small size and low power usage. The chip-scale atomic gyroscope includes a vapor cell forming a closed gas cell containing one or more active NMR isotopes, alkali atoms, and buffer gas. The chip-scale atomic gyroscope gyroscope also includes a light source encircled by a polarization-selective photodetector assembly and a light reflective surface opposite the light source, forming a multiple light path through the vapor cell between the light source, the light reflective surface, and the polarization-selective photodetector assembly. The chip-scale atomic gyroscope further includes a quarter-wavelength optical phase retarder (“quarter-wave plate”) which serves two functions: (1) circularly polarizing light emitted by the light source to produce a pump beam for optically pumping alkali atoms, which in combination with orthogonal magnetic fields (or a modulating of the light from the light source) causes the alkali atoms and NMR isotope atoms to precess about the magnetic field; and (2) rotating the polarization of a returning, linearly polarized probe beam so that the signals arriving at the photodetector assembly produce roughly equal signals in each of the orthogonal polarization channels. Alternatively, the quarter-wavelength optical phase retarder may be positioned so that it circularly polarizes light emitted by the light source for optical pumping but does not intersect the probe beam. In addition, the chip-scale atomic gyroscope includes a polarization analyzer comprising four quadrants, each of which is a polarizing beam splitter that separates incident light into S and P components. These S and P component signals may optionally be purified using high-extinction ratio linear polarizers, and the orthogonal S and P polarizations for each quadrant are then measured to determine a rotation of the polarization of the probe beam due to interactions with precessing atoms in the vapor cell. The rotation of the polarization of the probe beam, as observed in the rotating frame of the gyroscope, may itself be used to determine a rotation of the gyroscope with respect to an inertial frame.

A second embodiment provides a chip-scale atomic gyroscope with similar components as the first embodiment, but with a polarizing beam splitter cube in lieu of the aforementioned polarization analyzer comprising four quadrants. The polarizing beam splitter cube splits incident light into S and P components that are measured with respective photodetectors to determine the rotation of the polarization of the probe beam due to interactions with precessing atoms in the vapor cell. The chip-scale atomic gyroscope according to the second embodiment may generally be larger and easier to assemble than the chip-scale atomic gyroscope according to the first embodiment, but the chip-scale atomic gyroscope according to the second embodiment cannot be used to determine rotations in different parts of the device and cannot be configured with its quarter-wavelength optical phase retarder in different positions, unlike the chip-scale atomic gyroscope according to the first embodiment.

The design of the chip-scale atomic gyroscopes described herein may be readily modified for use as magnetometers by removing external magnetic shields and using alternative gas mixtures in the vapor cell. For example, the magnetometer vapor cell may include alkali atoms and buffer gas, and the magnetometer may measure variations in the strength of the magnetic field based on the observed precession frequency of the alkali atoms in the vapor cell. Generally, the contents of the vapor cell may be chosen to minimize magnetic sensitivity in the chip-scale atomic gyroscopes and enhance magnetic sensitivity in the chip-scale atomic magnetometers.

To better understand the novelty of the atomic sensing devices of the present invention and the methods of use thereof, reference is hereafter made to the accompanying drawings.

FIG. 1illustrates a cross-sectional view of a chip-scale atomic device100, according to a first embodiment. As shown, the device100includes a light source105, a vapor cell110, a linear polarizer115, a partial reflector120, photodetectors1301-5, linear polarizers1351-4, a polarization analyzer140, a first magnet161generating a first magnetic field B0, a second magnet160generating a second magnetic field B1, a quarter-wavelength optical phase retarder (λ/4 plate)150, and a control circuit170. The light source105may be a semiconductor laser, such as a vertical cavity surface emitting laser (VCSEL) semiconductor laser diode, from which emitted light intrinsically diverges, or the light could be provided by an external laser source and coupled into the gyroscope either though an optical fiber or free-space optical transmission system.

As shown, a central hole in the photodetectors1304-5, polarizers1351-2, and the polarization analyzer140permit light from the light source105, which is substantially encircled by the photodetectors1304-5forming a quadrant photodiode, to enter the vapor cell110via the λ/4 plate150. In alternative embodiments, the light source105may be situated elsewhere. For example, the photodetectors1304-5may lack a central hole in one embodiment and, in such a case, the light source105may be situated on top of the photodetectors1304-5. Optionally, additional optical components, such as a polarizer and/or lenses, may be incorporated between the light source105and the λ/4 plate150to improve polarization and/or divergence properties of the light.

In operation, linearly polarized light emitted from the light source105passes through the λ/4 plate150, which is an optical device oriented at a polarization angle of 45° and which circularly polarizes the light. The circularly polarized light106then enters the vapor cell110. The vapor cell110may be a closed gas cell containing an atomic gas and having transparent windows1111-2on opposing ends. In one embodiment, the vapor cell110may be an anodically bonded cell comprising silicon and Pyrex® windows1111-2. The interior of the vapor cell110may be coated with a chemical compound (e.g., alkali atoms, or other materials which reduce atomic decoherence) to limit interaction between the gas and the cell walls, although no coating may be used in some embodiments. The vapor cell110may include one or more active NMR isotopes, alkali atoms, and buffer gas. Here, the NMR isotopes may be, e.g., noble gases such as129Xe and131Xe, the alkali atoms may be, e.g.,133Cs atoms, and the buffer gas may be, e.g., N2. Other NMR isotopes, alkali atoms, and buffer gases may also be used, including well-known ratios of particular atoms, as the design of the chip-scale atomic device100is not limited to any choice of vapor atoms.

The circularly polarized light106is at a wavelength that is resonant with an optical transition in the alkali atoms and, combined with the magnetic field B0, spin polarizes the alkali atoms. Illustratively, the magnetic field B0is provided by a surrounding magnet161that is a solenoid controlled by the control circuit170to produce a null signal, and the magnet161may itself be surrounded by a magnetic shield (not shown). Although the magnet161is shown as a solenoid, any magnet assembly may be used that is capable of generating a substantially homogenous magnetic field to set a rotation axis along the light beam emitted by the light source105and centered on the vapor cell110. After the alkali atoms are spin polarized, spin exchange collisions transfer the spin polarization of the alkali atoms to the NMR isotopes, such that the NMR isotopes also become spin polarized. By applying a second magnetic field B1that is orthogonal to the magnetic field B0and generated by a second solenoid160controlled by the control circuit170to apply pulses/waveforms (or any other magnet assembly capable of producing a transverse magnetic field pulse/waveform that is perpendicular to light emitted by the light source105and centered on gas cell) or, alternatively, modulating the frequency of light from the light source105(not shown), similarly responsive to control circuit170, the spin-polarized atoms in the vapor cell110are driven to precess about the magnetic field B0at frequency ω=γB0, where γ is the gyromagnetic ratio of the NMR active isotope. The precession of magnetic moments about a magnetic field is generally referred to as Larmor precession. Second magnetic field B1is preferably of a short time scale, such that the precession frequency is ultimately responsive to magnetic field B0.

An observed frequency of Larmor precession in a rotating frame changes with a rotation of the gyroscope relative to the inertial frame. In particular, an atom of interest precesses at observed frequency
ω=γB0+Ω,  (1)
where Ω is the rate of rotation of the gyroscope relative to the inertial frame. In an NMR gyroscope, systematic errors in the magnetic field B0(e.g., variations in the field's strength) may be eliminated by using two types of NMR isotopes, producing the following equations, which permit removal of dependence on B0:
ω1=γ1B0+Ω,
ω2=γ2B0+Ω.  (2)
In magnetometer embodiments, variations in the strength of the magnetic field B0is itself determined based on the precession frequency ω and known values of the gyromagnetic ratio γ and the rate or rotation Ω (e.g., 0), and the vapor cell110may include active NMR isotopes (e.g., some alkali) and buffer gas. Complementary to the gyroscope application, two active NMR isotopes may optionally be interrogated in the magnetometer in order to eliminate dependency on Ω. In one magnetometer embodiment, a magnetic shield (not shown) that keeps the magnetic field B0substantially constant in the NMR gyroscope may be omitted, thereby increasing magnetic field sensitivity.

As shown, the precession frequency(ies) ω (ωi) may be measured by observing a rotation of a polarization of a linearly polarized probe beam106′ that passes through the vapor cell110and interacts with atoms therein. Illustratively, a divergent light beam106, which is emitted from the light source105and circularly polarized by the λ/4 plate150, passes through vapor cell110. The divergent light beam106then passes through a high-extinction ratio linear polarizer115oriented either aligned or orthogonal relative to the initial polarization of the light, as emitted from the light source105. The linear polarizer115filters the light beam106, letting through light which is substantially linearly polarized along one plane. Such light is then incident on a partial reflector120that reflects a small percentage of the light back through the linear polarizer115and into the vapor cell110as a linearly polarized probe beam106′. Only a small percentage of light is reflected, such that the probe beam106′ is much weaker than the pump beam106. In one embodiment, 0.1% of the light is reflected and the remainder is transmitted through the partial reflector120. The light which is transmitted through the partial reflector120may optionally be monitored via a photodetector1301for purposes of, e.g., diagnostic or laser frequency monitoring.

As shown, the probe beam106′ passes back through the vapor cell110. Due to the Faraday effect, the polarization of the probe beam106′ is rotated when the probe beam106′ interacts with precessing atoms in the vapor cell110. This rotation may in turn be observed using the polarization analyzer140and photodetectors1302-5. In particular, the linear polarizer115may be aligned at 0° or 90° with respect to polarization of the light emitted from the light source105. The probe beam106′ resulting from the double-pass of the linear polarizer115would then be equal parts circular left and circular right polarized. The differential absorption between the left and right circularly polarized light of the probe beam106′ is referred to as circular dichroism and appears as a rotation signal that can be measured using the polarization analyzer140and the photodetectors1302-5.

Before reaching the polarization analyzer, the probe beam106′ which has traversed the vapor cell110passes through the λ/4 plate150. The λ/4 plate150further rotates the polarization of the incident probe beam106′ by 45° in order to balance the signal observed in each polarization component on the outputs of the polarization analyzer140for photodetectors1302and1304, and photodetectors1303and1305, each pair of which acts as a balanced photodetector that receives distinct S and P components of the probe beam106′ after those components are split by the polarization analyzer140. That is, the polarization analyzer140, which includes four quadrants that are each configured to split incident light into S and P components as discussed in greater detail below, receives light which is rotated by 45° by the λ/4 plate150such that the maximum polarization is near balance of each photodetector pair. Note, the S and P designations used herein are somewhat arbitrary, and the S polarized light may be the P polarized light, and vice versa, in other configurations. In the balanced photodetector, the signal levels and therefore the signal-to-noise ratio is similar on each detector and noise common to both S and P components (e.g., due to laser intensity variations) may be substantively reduced by differential measurement to improve signal-to-noise ratio, as discussed in greater detail below. Note, the λ/4 plate150serves two functions: circularly polarizing light emitted by the light source105to produce the pump beam106, and rotating the returning linearly-polarized probe beam106′ such that S and P components of the rotated light may be used in balanced detection.

As shown, the polarization analyzer140receives the probe beam106′ whose polarization is rotated by the λ/4 plate150, and splits the beam106′ into orthogonal S and P components. One of the polarization components is transmitted through the polarization analyzer140, while the other is rejected out of the sides of the polarization analyzer140. Illustratively, the S component is transmitted, while the P component is rejected, although the opposite may also occur, as illustrated inFIG. 2. The key is that S and P components of the probe beam106′ are substantially split and thereafter independently detected. Optionally, high-extinction ratio linear polarizers1351-4may be placed between the polarization analyzer140and the photodetectors1302-5to filter light incident thereon and block light which is not either S or P polarized, as appropriate. For example, the polarizer1354may be a high-extinction ratio polarizer that blocks S polarized light, thereby filtering the P polarized light passing through polarizer1354for measurement by the photodetector1303.

In an alternative embodiment, the λ/4-plate150may be configured such that it intersects the pump beam106but not the returning probe beam106′. For instance, the λ/4-plate150may be placed below the linear polarizers1352and1354. In such a case, light from the light source105passes through the λ/4-plate150to become the circularly polarized pump beam used for optical pumping in the vapor cell110. Upon exiting through the vapor cell110, however, the pump beam may pass through a linear polarizer similar to linear polarizer115but oriented with its principal axis at 45° with respect to the analysis axis of the polarization analyzer140. The probe beam that returns through the vapor cell110would then be linearly polarized at 45°, and the rotation of this linear polarization as the probe beam passes through the gas of the vapor cell110may be analyzed by balanced photodetectors to determine a rotation in the plane of polarization. That is, what is measured is birefringence, rather than the circular dichroism discussed above. Note, the returning probe beam106′ would not traverse the λ/4-plate150in this configuration. In addition, the birefringence measurement is optimized (i.e., gives the maximum signal) off-resonance, in contrast to the circular dichroism measurement which is optimized on-resonance.

As shown, a first balanced photodetector comprises photodetectors1302and1304and a second balanced photodetector comprises photodetectors1303and1305. Each balanced photodetector includes a pair of photodetectors which are configured to receive respective S and P components of probe beam. For example, a photon incident on the polarization analyzer140may be split into an S component transmitted through to the photodetector1305and a P component rejected out the side of the polarization analyzer140to photodetector1303. In one embodiment, the photodetectors1303and1305may be connected such that, initially, their photocurrents substantially cancel when observed at 45° relative to the polarization of the probe beam106′. This is due to the rotation of the probe beam160′ by the λ/4 plate150so that the maximum polarization is near balance of the balanced photodetector. As a result, the effective output of the balanced photodetector may be substantially zero, until one of the S and P components changes intensity due to a rotation of the probe beam106′ caused by interaction with atoms in the vapor cell110precessing at a different frequency which, as discussed, may occur where the gyroscope100is rotated relative to an inertial frame. In this way, rotation of the polarization of the probe beam106′ may be determined, which may, in turn, be used to determine rotation of the gyroscope100with respect to the inertial frame according to well-known theory. The control circuit170may include a processor (not shown) which performs calculations to make these determinations, and which outputs a signal indicative of determined rotations. In other embodiments, the signals from the photodetectors1302-5may be individually monitored by the control circuit170to, e.g., determine a magnetic field gradient across the vapor cell110.

FIG. 2illustrates the polarization analyzer140of the chip-scale atomic device100, according to an embodiment. The polarization analyzer140may be made from various materials, such as optical glass (e.g., BK7 glass), Pyrex®, quartz, without limitation. As shown, the polarization analyzer140includes a central hole203, through which light from the laser source105initially passes, and four quadrants1411-4, each of which is a polarizing beam splitter that splits incident light from a returning probe beam106′ (not shown) into S and P components. Any given photon that hits a quadrant of the polarization analyzer140is split, and rejected and transmitted in equal ratios to its polarization. More specifically, the statistical probability of detecting a rejected or transmitted photon depends on the polarization of the incident light.

Illustratively, a photon of probe beam106′ traveling along path201and incident on quadrant1411is split into an S component that is transmitted through the quadrant1411and a P component that is rejected out of the side of the quadrant1411. Similarly, a photon traveling along path202and incident on quadrant1412is split into a P polarization component that is transmitted through the polarization analyzer quadrant1412and an S polarization component that is rejected out of the side of the quadrant1412. As discussed, the S and P components split by the polarization analyzer140may then be optionally filtered by high-extinction ratio linear polarizers, after which the light is incident on photodetectors used to measure the intensities of those light components. Such intensities generally depend on the orientation of the polarization of the probe beam106′, which is affected by interactions with vapor cell atoms precessing at different frequencies due to different orientations of the gyroscope with respect to an inertial frame, and which may thus be used to determine the rotation of the gyroscope with respect to the inertial frame.

As shown, the polarization analyzer140includes four quadrants1411-4, each of which may be associated with a respective balanced photodetector. In other embodiments, the polarization analyzer140may be divided into more than four polarizing beam splitters, or fewer. Differences in photocurrent signals from different quadrants1411-4may indicate a magnetic field gradient across the vapor cell110, which is one of the systematic problems in NMR gyroscopes and which causes atoms across the vapor cell to precess at different frequencies. That is, a magnetic field gradient may be determined based on differences in the photodetector signals in different quadrants resulting from atoms precessing at different frequencies due to the magnetic field gradient. In one embodiment, complementary quadrants on opposite sides of the polarization analyzer140are monitored to detect magnetic field gradients across the cell. Such magnetic field gradients may then be eliminated by changing the magnetic field by, e.g., applying compensating fields or algorithmically via electronics or firmware. As a result, the impact of magnetic field gradients may be ameliorated for improved ARW and bias stability.

FIG. 3illustrates a cross-sectional view of a chip-scale atomic device300, according to a second embodiment. The chip-scale atomic device300may generally be larger, but easier to assemble, than the chip-scale atomic device100. As shown, the chip-scale atomic device300includes a light source305, a vapor cell310, a linear polarizer315, an partial reflector320, photodetectors3301-3, linear polarizers3351-2that are aligned orthogonal to each other, a polarizing beam splitter cube340, magnets360-361, and a quarter-wavelength optical phase retarder (λ/4 plate)350at 45°. The light source305, vapor cell310, partial reflector320, magnets360-361, and λ/4 plate350are similar to the light source105, vapor cell110, partial reflector120, magnets160-161, and λ/4 plate150, respectively, and descriptions thereof will not be repeated for conciseness.

In operation, light emitted from the light source305diverges and passes through a hole in the center of the photodetector3301, as well as the linear polarizer3351that is aligned for transmission with the polarizing beam splitter cube340. The light then enters the vapor cell310via the λ/4 plate350. Note, although the light source305is depicted as being located below the photodetector3301, the light source305may be situated elsewhere in alternative embodiments, such as on top of the photodetector1301if the photodetector3301did not have a hole. Optionally, additional optical components, such as a polarizer and/or lenses, may be incorporated to improve polarization and/or divergence properties of the light emitted from the light source305.

The λ/4 plate350circularly polarizes light passing through it, so that light entering the vapor cell310comprises a circularly polarized pump beam306resonant with an optical transition in alkali atoms of the vapor cell310. The physics discussed above with respect to the device chip-scale atomic100is also applicable to the chip-scale atomic device300. In particular, the pump beam306, combined with the magnetic field B0provided by the magnet361, spin polarizes alkali atoms in the vapor cell310, and spin exchange collisions transfer the spin polarization to NMR isotopes in the vapor cell310. By applying a second magnetic field B1orthogonal to the magnetic field B0, or alternatively modulating the frequency of light from the light source305, the spin-polarized atoms in the vapor cell310can be driven to precess about the magnetic field B0. The frequency of such precession in a rotating frame changes with a rotation of the device300relative to the inertial frame. The rotation of the device300relative to the inertial frame may thus be determined from the precession frequency, which can itself be measured by observing a rotation of a polarization of a linearly polarized probe beam306′ as the probe beam306′ passes through the vapor cell310and interacts with precessing atoms therein.

As shown, the probe beam306′ is light from circularly polarized pump beam306reflected by a partial reflector320that is configured to reflect a small percentage of the pump beam306, which has passed through the linear polarizer315, back through the linear polarizer315and into the vapor cell310. The photodetector3303is an optional photodiode that may be included to measure light transmitted by the partial reflector320for diagnostic or laser frequency monitoring purposes. The linear polarizer115is aligned relative to the light emitted from the light source305at 0° or 90°, and the double pass through the linear polarizer315produces the probe beam306′ with 0° or 90° polarization, i.e., equal parts left and right circular polarization. As the probe beam306′ passes through the vapor cell310, the polarization of the probe beam306′ becomes elliptical by circular dichroism of the vapor, which involves the differential absorption of the left and right circularly polarized light. After the probe beam306′ passes through the vapor cell310, the probe beam further passes through the λ/4 plate350, which splits the two circular components of the probe beam306′ into linear components of light.

The polarizing beam splitter cube340then splits the light into S and P polarization components. Illustratively, the P component is reflected by the polarizing beam splitter cube340, and further passes through the polarizer3352which is a high-extinction ratio polarizer that blocks S polarized light. The remaining light is substantially P polarized and detected by photodetector3302. Likewise, the S component of light is filtered by the polarizer3351which blocks P polarized light, and the remaining substantially S polarized light is detected by photodetector3301. Similar to the discussion above, the rotation of the polarization of the probe beam306′ may be determined, by a processor (not shown) of the control circuit370, based on the light detected by the photodetectors3301-2using well-known theory, and the rotation of the polarization of the probe beam306′ may itself be used to determine rotation of the gyroscope300with respect to the inertial frame according.

FIG. 4illustrates a cutaway view of a chip-scale atomic gyroscope package400, according to an embodiment. Although the chip-scale atomic gyroscope package400is depicted as including the chip-scale atomic device100, the chip-scale atomic gyroscope package400may be configured to include the chip-scale atomic device300as well.

As shown, the package400includes vacuum packaging405, which may be fabricated from ceramic, metals, or other non-magnetic materials, in which an NMR gyroscope (or magnetometer) is mounted on a thermal isolation platform4102and suspended in the vacuum. Here, the thermal isolation platform4102, may be fabricated from, e.g., a thin glass, a polymer such as polyimide, or plastic. In operation, vapor cell110of the NMR gyroscope (or magnetometer) may be heated by a heater (not shown) to vaporize atoms in the vapor cell110. For example, the vapor cell110may be heated to 100° C. in one embodiment. The suspension of the NMR gyroscope (or magnetometer) in conjunction with the vacuum in vacuum packaging405, which reduces heat convection and conduction, may lower power consumption required to heat the vapor cell110. In one embodiment, the suspension in the package400may be accomplished according to techniques disclosed in U.S. Pat. No. 7,215,213 entitled “Apparatus and System for Suspending a Chip-Scale Device and Related Methods,” which is hereby incorporated by reference in its entirety. In other embodiments, different suspension techniques, or no suspension, may be used.

In one embodiment, the package400may have length, width, and height that are each approximately 7 mm (i.e.,FIG. 4depicts the package400cut substantially in half, with the package400actually being a 7 mm cube). Other embodiments may have different dimensions. In one embodiment, the NMR gyroscope (or magnetometer) suspended in the vacuum packaging405may be the chip-scale atomic device100discussed above with respect toFIG. 1. As discussed, the chip-scale atomic device100includes a vapor cell110which, when used as a gyroscope, contains one or more active NMR isotopes, alkali atoms, and buffer gas. The chip-scale atomic gyroscope further includes a light source (not shown) encircled by a polarization-selective photodetector assembly and a light reflective surface120opposite the light source, forming a multiple light path through the vapor cell110between the light source, the light reflective surface120, and the polarization-selective photodetector assembly. Diverging light emitted from the light source is initially incident on λ/4 plate150which circularly polarizes the light. Combined with a magnetic field B0provided by, e.g., a surrounding solenoid161(not shown) or permanent magnet assembly, the circularly polarized light optically pumps the alkali atoms to become spin polarized. Through spin-exchange collisions with the alkali atoms, the NMR isotope atoms also become spin polarized. By applying an additional magnetic field B1orthogonal to B0and generated by a second solenoid160(not shown), or modulating the frequency of the light from the light source, the spin-polarized atoms are driven to precess about B0.

As discussed, the pump beam, upon reaching the opposing side of the vapor cell110, is attenuated, linearly polarized, and reflected back through the vapor cell110as a probe beam which interacts with precessing atoms in the vapor cell110. The returning linearly polarized probe beam passes through the λ/4 plate150and is rotated in order to balance the signal observed in each polarization component on the outputs of the polarization analyzer140. The rotated probe beam is then incident upon the polarization analyzer140, which may comprise four quadrants, each of which is a polarizing beam splitter that separates incident light into S and P components, which may optionally be purified using high-extinction ratio linear polarizers. The orthogonal S and P light components for each quadrant may then be measured using a balanced photodetector to determine a rotation of the polarization of the probe beam due to interactions with precessing atoms in the vapor cell. The rotation of the polarization of the probe beam, as observed in the rotating frame of the gyroscope, may itself be used to determine a rotation of the gyroscope with respect to an inertial frame.

Illustratively, the shape of components of the chip-scale atomic device100, including the vapor cell110and the polarization analyzer140, are substantially rectangular. This permits the components to be glued together such that relative vibration between components is reduced. As a result, the tight integration between components may provide a low-mass rigid structure for rapid time-to-act and minimal vibration sensitivity.

FIG. 5illustrates a method500for sensing rotations, according to an embodiment. As shown, the method500begins at step510, where a first magnetic field (e.g., magnetic field B0) is applied and a light source (e.g., light source105or305) emits light. At step520, the light emitted by the light source intrinsically diverges and is passed through optics (e.g., λ/4 plate150or λ/4 plate350) to circularly polarize the light and generate a pump beam (e.g., pump beam106or306). As discussed, the optics that circularly polarizes the light may be placed in alternative locations in the geometry of the chip-scale atomic device100to measure either circular dichroism or birefringence.

At step530, the pump beam is passed through a vapor cell (e.g., vapor cell110or vapor cell310). The pump beam acts to optically pump alkali atoms in the vapor cell, which become spin polarized and cause one or more active NMR isotope atoms in the vapor cell to become spin polarized as well through spin-exchange collisions. The spin polarized atoms are then made to precess about the first magnetic field through application of a second magnetic field (e.g., magnetic field B1) orthogonal to the first magnetic field or modulation of the emitted light.

At step540, the pump beam is further passed through a linear polarizer, attenuated and reflected, and passed back again through the linear polarizer, to generate a probe beam (e.g., probe beam106′ or306′). The linear polarizer may be aligned at either 0° or 90° relative to the polarization of light emitted from the light source, or at 45°, depending on the placement of the optics that circularly polarizes the light. The probe beam that is generated is then linearly polarized at 0° or 90°, or at 45°, and weaker than the pump beam.

At step550, the probe beam passes through the vapor cell. As discussed, the probe beam is linearly polarized, and a polarization of the probe beam may be rotated due to the Faraday effect when the probe beam interacts with precessing atoms in the vapor cell. This rotated polarization may, in turn, be observed and used to determine the rotation of the device itself relative to an inertial frame.

At step560, one or more polarizing beam splitters split light of the probe beam that has passed through the vapor cell. In one embodiment, the polarizing beam splitters may be configured to form a polarization analyzer, such as the polarization analyzer140. As discussed, the polarization analyzer140comprises four polarizing beam splitters that are each configured to split light of the probe beam incident thereon into orthogonally polarized components. In such a case, the probe beam may first pass through the λ/4 plate, which rotates the polarization of the probe beam to balance a signal observed in photodetectors on outputs of the polarization analyzer140. In an alternative embodiment, the one or more polarizing beam splitters may include a single polarizing beam splitter cube, such as the polarizing beam splitter cube340, which is configured to split light of the probe beam incident thereon into orthogonally polarized components.

At step570, each of the polarized components output by the one or more polarizing beam splitters is detected using at least one respective photodetector (e.g., photodetectors1302-5or3301-2). Optional high-extinction linear polarizers (e.g., polarizers1352-5or3351-2) may be added to filter the polarized components before those components are detected by the photodetectors. Further, an optional photodetector (e.g., photodetector1301or3303) may be placed to detect light transmitted through the partial detector, discussed above, for diagnostic or laser frequency monitoring.

At step580, a control circuit or component therein (e.g., a processor of control circuit170or370) determines rotation of the device based on the detected polarized components and well-known theory. Then at step590, the control circuit or component outputs a signal indicative of the determined rotations. The output signal may be used for navigational purposes, displayed on a display screen, among other things.

The design of the chip-scale atomic gyroscopes described herein and the associated method for measuring rotations may be readily modified for use as magnetometers for measuring strength and/or rotation of an external magnetic field by removing magnetic shields, and the magnetometers may also use alternative gas mixtures in the vapor cells. For example, a magnetometer vapor cell may include alkali atoms and buffer gas, and the magnetometer may measure variations in the strength of the magnetic field which affects the precession frequency of the alkali atoms in the vapor cell.

Advantageously, embodiments disclosed herein provide high-performance gyroscopes which are compact, low-power, and relatively inexpensive to manufacture. Alternative magnetometer embodiments provide similar performance, power, size, and cost advantages. Such chip-scale gyroscopes and magnetometers may have a number of applications, such as navigation, and may be used in situations unsuitable for larger, power-hungry gyroscopes and magnetometers, such as personal use by individuals who carry the devices.

While the forgoing is directed to embodiments of the present invention, other and further embodiments of the invention may be devised without departing from the basic scope thereof.