Light-pulse atom interferometric device
An atomic interferometric device useful, e.g., for measuring acceleration or rotation is provided. The device comprises at least one vapor cell containing a Raman-active chemical species, an optical system, and at least one detector. The optical system is conformed to implement a Raman pulse interferometer in which Raman transitions are stimulated in a warm vapor of the Raman-active chemical species. The detector is conformed to detect changes in the populations of different internal states of atoms that have been irradiated by the optical system.
1. Apparatus comprising an atomic interferometer of the kind in which an optical system stimulates Raman or single-photon transitions in an atomic population of a chemical species that has a metastable optically excited state and in which changes in the populations of different internal states of atoms of the chemical species are detected, wherein the atomic interferometer comprises:
a vapor cell;
a vapor source situated within the vapor cell and conformed to provide a warm atomic vapor of the chemical species;
at least one Raman laser controllable so as to irradiate the warm atomic vapor with optical pulses that stimulate Raman or single-photon transitions in atoms of the warm atomic vapor, thereby populating a metastable optically excited state; and
at least one detection system conformed to detect changes in the population of the metastable optically excited state.
2. Apparatus of claim 1 , wherein at least two Raman lasers are controllable so as to irradiate the warm atomic vapor with coincident optical pulses that stimulate Raman transitions in the atoms of the warm atomic vapor.
3. The apparatus of claim 2 , further comprising a controller adapted to tune at least one of the Raman lasers so that the Raman transitions are stimulated in a selected velocity class within the atomic population.
4. The apparatus of claim 3 , wherein:
the detection system comprises one or more probe lasers, each arranged to transmit an optical beam through the atomic vapor, and at least one photodetector arranged to detect changes in the interaction of one or more of the probe beams with the atomic vapor; and
the controller is adapted to tune at least one of the probe beams so that the changes in the population of the metastable optically excited state are detected in a selected velocity class within the atomic population.
5. The apparatus of claim 4 , wherein the controller is adapted to tune one or more of the Raman lasers and one or more of the probe lasers to multiple wavelengths so that the Raman transitions are concurrently stimulated in two or more selected velocity classes within the atomic population and the changes in the population of the metastable optically excited state are detected in the two or more selected velocity classes.
6. The apparatus of claim 1 , wherein:
the Raman laser or lasers are tuned or tunable so as to stimulate the Raman or single-photon transitions in at least one selected velocity class within the atomic population characterized by an average velocity component V z along an acceleration-measurement axis; and
the Raman laser or lasers are arranged to irradiate the atomic vapor on a path that has mirror symmetry along the acceleration-measurement axis so that two velocity classes are selected, said classes respectively characterized by the average velocity components V z and −V z .
7. The apparatus of claim 6 , wherein:
the detection system comprises one or more acceleration probe lasers arranged to emit optical pulses directed parallel to the acceleration-measurement axis and one or more acceleration photodetectors arranged to detect said optical pulses after they have interacted with the atomic vapor;
the detection system is arranged to separately detect changes in the population of the metastable optically excited state in each of the two velocity classes, leading to respective acceleration photodetector output signals; and
the apparatus further comprises a processing circuit arranged to combine the respective acceleration photodetector output signals so as to enhance said signals and to derive an acceleration measurement at least in part from said signals.
8. The apparatus of claim 7 , wherein:
the detection system further comprises one or more rotation probe lasers arranged to emit optical pulses directed transverse to the acceleration-measurement axis and one or more rotation photodetectors arranged to detect said optical pulses after they have interacted with the atomic vapor;
the rotation probe laser or lasers are tuned or tunable such that changes in the population of the metastable optically excited state are detected in at least some of the velocity subclasses (V z ,V x ), (V z ,−X x ), (−V z ,V x ), and (−V z ,−V x ), wherein said velocity subclasses are selected from the V z and −V z velocity classes and are further characterized by a velocity component V x or −V x directed transverse to the acceleration-measurement axis;
the detection system is arranged to separately detect changes in the population of the metastable optically excited state in each of at least two of the velocity subclasses, leading to respective rotation photodetector output signals; and
the processing circuit is further arranged to combine the respective rotation photodetector output signals so as to enhance said signals and to derive a rotation measurement at least in part from said signals.
9. The apparatus of claim 1 , comprising at least two detectors associated with the vapor cell, wherein one of the detectors is conformed to measure uniaxial acceleration and the other detector is conformed to measure uniaxial rotation.
10. The apparatus of claim 1 , comprising three vapor cells, wherein:
a vapor source is situated within each vapor cell;
at least two Raman lasers are arranged to irradiate a warm atomic vapor within each vapor cell;
each vapor cell has a detection system comprising at least one detector conformed to measure uniaxial acceleration and at least one detector conformed to measure uniaxial rotation; and
the three vapor cells are respectively oriented on three mutually perpendicular axes.
11. The apparatus of claim 1 , further comprising:
at least one auxiliary sensor situated externally to the vapor cell and selected from the group consisting of magnetic field sensors, temperature sensors, acceleration sensors, and rotation sensors; and
a processing circuit adapted to combine signal output from the detection system with signal output from at least one said auxiliary sensor and to provide acceleration measurements and/or rotation measurements from the combined signal outputs.
12. The apparatus of claim 1 , further comprising a spin anti-relaxation coating applied to an inner surface of the vapor cell.
13. The apparatus of claim 12 , wherein the spin anti-relaxation coating comprises an aminosilane.
14. The apparatus of claim 12 , wherein the spin anti-relaxation coating comprises tris(N,N-dimethylamino)octadecylsilane.
15. The apparatus of claim 1 , further comprising an optical cavity arranged so that each pulse from the Raman laser or lasers passes multiple times through the warm atomic vapor.
16. The apparatus of claim 1 , wherein the vapor cell formed of a glass having helium permeability low enough to permit steady-state operation without vacuum pumping or with vacuum pumping at a speed of at most 5 liters/s.
17. The apparatus of claim 1 , wherein:
the detection system is conformed to produce an on-resonance output signal responsive to Raman pulses tuned to populate the metastable optically excited state;
the detection system is further conformed to produce an off-resonance output signal responsive to laser pulses tuned to be ineffective for populating the metastable optically excited state; and
the apparatus further comprises a processing circuit adapted to perform background subtraction in which the off-resonance output signal is at least partially removed from the on-resonance output signal.
18. The apparatus of claim 17 , wherein the optical system is arranged such that the on-resonance Raman pulses and the off-resonance laser pulses are concurrent.
19. A method for making inertial measurements, comprising:
irradiating a warm atomic vapor with a sequence of laser pulses that are adapted to stimulate Raman or single-photon transitions in atoms of the warm atomic vapor and thereby to populate a metastable optically excited state; and
detecting changes in the population of the metastable optically excited state.
20. The method of claim 19 , wherein at least two Raman lasers are controlled so as to irradiate the warm atomic vapor with coincident optical pulses that stimulate Raman transitions in the atoms of the warm atomic vapor.
21. The method of claim 19 , further comprising transmitting pulses of light through the atomic vapor from one or more probe lasers, wherein the probe pulses are tuned to select for at least one particular velocity class within the atomic population, and wherein the detecting step comprises detecting the probe pulses.
22. The method of claim 19 , further comprising:
transmitting pulses of light from one or more acceleration probe lasers through the atomic vapor in a direction parallel to the acceleration-measurement axis; and
transmitting pulses of light from one or more rotation probe lasers through the atomic vapor in a direction transverse to the acceleration-measurement axis;
and wherein the detecting step comprises detecting the acceleration probe optical pulses and the rotation probe optical pulses after said pulses have interacted with the atomic vapor.