U.S. patent application number 13/541147 was filed with the patent office on 2012-10-25 for quantum interference device, atomic oscillator, and magnetic sensor.
This patent application is currently assigned to SEIKO EPSON CORPORATION. Invention is credited to Taku AOYAMA, Koji CHINDO.
Application Number | 20120267509 13/541147 |
Document ID | / |
Family ID | 42539941 |
Filed Date | 2012-10-25 |
United States Patent
Application |
20120267509 |
Kind Code |
A1 |
AOYAMA; Taku ; et
al. |
October 25, 2012 |
QUANTUM INTERFERENCE DEVICE, ATOMIC OSCILLATOR, AND MAGNETIC
SENSOR
Abstract
A quantum interference device includes: gaseous alkali metal
atoms; and a light source for causing a resonant light pair having
different frequencies that keep a frequency difference equivalent
to an energy difference between two ground states of the alkali
metal atoms, the quantum interference device causing the alkali
metal atoms and the resonant light pair to interact each other to
cause an electromagnetically induced transparency phenomenon (EIT),
wherein there are a plurality of the resonant light pairs, and
center frequencies of the respective resonant light pairs are
different from one another.
Inventors: |
AOYAMA; Taku; (Setagaya,
JP) ; CHINDO; Koji; (Kawasaki, JP) |
Assignee: |
SEIKO EPSON CORPORATION
Tokyo
JP
|
Family ID: |
42539941 |
Appl. No.: |
13/541147 |
Filed: |
July 3, 2012 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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13419789 |
Mar 14, 2012 |
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13541147 |
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|
12699350 |
Feb 3, 2010 |
8237514 |
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13419789 |
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Current U.S.
Class: |
250/201.1 ;
977/932 |
Current CPC
Class: |
G02F 2/02 20130101; H01S
5/06832 20130101; H03L 7/26 20130101; G02F 1/0126 20130101; H01S
5/0085 20130101; H01S 5/0687 20130101; H01S 5/0427 20130101 |
Class at
Publication: |
250/201.1 ;
977/932 |
International
Class: |
G01J 1/44 20060101
G01J001/44 |
Foreign Application Data
Date |
Code |
Application Number |
Feb 6, 2009 |
JP |
2009-025652 |
Jun 29, 2009 |
JP |
2009-153402 |
Sep 1, 2009 |
JP |
2009-201329 |
Claims
1. A quantum interference device comprising: gaseous alkali metal
atoms that causes an electromagnetically induced transparency
phenomenon by inputting a plurality of resonant light pairs in
which center frequencies of each of the plurality of resonant light
pairs are different from one another; a photo-detector that detects
the plurality of resonant light pairs passed through the alkali
metal atoms and that outputs a detection signal; a first oscillator
that outputs a first oscillation signal, the first oscillation
signal in which an oscillation frequency is controlled in
accordance with the detection signal; a second oscillator that
outputs a second oscillation signal; and a resonant light pair
generator that generate the plurality of the resonant light pairs
by inputting the first and second oscillation signals as a
modulation signal.
2. The quantum interference device according to claim 1, wherein
the resonant light pair generator further comprises: a mixer that
mixes the first and second oscillation signals and that output a
mixed signal; and a light source that outputs light including the
plurality of resonant light pairs by inputting the mixed signal as
the modulation signal.
3. The quantum interference device according to claim 1, wherein
the resonant light pair generator further comprises: a light source
that output first light; a mixer that mixes the first and second
oscillation signals and that output a mixed signal; and an
electro-optic modulation element that outputs second light
including the plurality of resonant light pairs by modulating the
first light in accordance with the mixed signal.
4. The quantum interference device according to claim 1, wherein
the resonant light pair generator further comprises: a light source
that output first light; a first electro-optic modulation element
that outputs modulated light by modulating the first light in
accordance with the first oscillation signal; and a second
electro-optic modulation element that outputs second light
including the plurality of resonant light pairs by modulating the
modulated light in accordance with the second oscillation
signal.
5. The quantum interference device according to claim 1, wherein
the resonant light pair generator further comprises: a light source
that output first light; a first electro-optic modulation element
that outputs modulated light by modulating the first light in
accordance with the second oscillation signal; and a second
electro-optic modulation element that outputs second light
including the plurality of resonant light pairs by modulating the
modulated light in accordance with the first oscillation signal.
Description
CROSS REFERENCE TO RELATED APPLICATIONS
[0001] This is a continuation application of U.S. application Ser.
No. 13/419,789 filed Mar. 14, 2012, which is a continuation
application of U.S. application Ser. No. 12/699,350 filed Feb. 3,
2010, which claims priority to Japanese Patent Application No.
2009-025652, filed Feb. 6, 2009, Japanese Patent Application No.
2009-153402, filed Jun. 29, 2009, and Japanese Patent Application
No. 2009-201329, filed Sep. 1, 2009 all of which are expressly
incorporated by reference herein.
BACKGROUND
[0002] 1. Technical Field
[0003] The present invention relates to a quantum interference
device, an atomic oscillator, and a magnetic sensor, and, more
particularly to a technique for efficiently causing an EIT
phenomenon.
[0004] 2. Related Art
[0005] An atomic oscillator employing an electromagnetically
induced transparency system (EIT system, which may be called CPT
system) is an oscillator that makes use of a phenomenon in which,
when two resonant lights having different wavelengths are
simultaneously irradiated on alkali metal atoms, absorption of the
two resonant lights stops (an EIT phenomenon). FIG. 24A is a
diagram of an energy state of one alkali metal atom. When first
resonant light having wavelength equivalent to an energy difference
between a first ground level 23 and an excitation level 21 or
second resonant light having wavelength equivalent to an energy
difference between a second ground level 24 and an excitation level
21 is independently irradiated on alkali metal atoms, as it is well
known, light absorption occurs. However, when the first resonant
light and the second resonant light are simultaneously irradiated
on the alkali metal atoms and a frequency difference between the
simultaneously-irradiated first and second resonant lights
precisely coincides with an energy difference (.DELTA.E12) between
the first ground level 23 and the second ground level 24, a system
shown in FIG. 24A changes to a superimposed state of the two ground
levels, i.e., a quantum interference state. Therefore, excitation
to the excitation level 21 stops and a transparency (EIT)
phenomenon occurs. It is possible to manufacture a highly-accurate
oscillator by detecting, as a signal, a steep change in light
absorption behavior at the time when a wavelength difference
between the first resonant light and the second resonant light
deviates from .DELTA.E12 and controlling the signal making use of
this phenomenon. Since .DELTA.E12 sensitively changes because of
the intensity or fluctuation of external magnetism, it is also
possible to manufacture a highly-sensitive magnetic sensor making
use of the EIT phenomenon.
[0006] To improve a signal-to-noise ratio (S/N) of an optical
output signal due to the EIT phenomenon, the number of atoms of
alkali metal, which interacts with resonant light, only has to be
increased. For example, JP-A-2004-96410 (Patent Document 1)
discloses, for the purpose of improving an S/N of an output signal
of an atomic oscillator, a method of increasing the thickness of a
cell in which gaseous alkali metal atoms are confined and a method
of increasing a beam diameter of a laser beam made incident on the
cell. In both the methods, to increase an area where the alkali
metal atoms come into contact with resonant light, the thickness or
the height of the cell is increased as shown in FIG. 24B or FIG.
24C. As the laser beam, only a pair of laser beams having two kinds
of wavelength that satisfy a development condition for the EIT
phenomenon are used.
[0007] U.S. Pat. No. 6,359,916 (Patent Document 2) discloses (1) a
technique concerning improvement of the sensitivity of an EIT (CPT)
system atomic oscillator. The technique has a characteristic that a
D1 line is used as a light source. Theoretically, EIT (CPT) signal
intensity can be improved compared with that in the case of a D2
line in the past. Consequently, sensitivity and frequency stability
is improved. (2) The signal intensity is further improved by using
a four-wave light source and causing P1/2 excitation levels
(hyperfine structure), which are energy-split into two, to
simultaneously interact in a double .LAMBDA. type transition. The
technique disclosed in the patent document relates to four wave
mixing, which is on the outside of the range of the technical field
related to the invention.
[0008] When attention is paid to respective atoms forming a group
of gaseous alkali metal atoms in a cell, the atoms have fixed
velocity distribution corresponding to motion states thereof. When
laser beams having only two kinds (a pair of) wavelengths are made
incident on the atom group, because of the influence of the Doppler
effect (Doppler shift) due to the motion of the atoms, atoms that
can actually interact one another are limited to only a part of
atoms having values of specific velocity components with respect to
a laser incident direction among a large number of atoms in the
cell. Therefore, a ratio of atoms contributing to the EIT
development is extremely low. The related art disclosed in Patent
Document 1 is the atomic oscillator configured in such a state of
low EIT development efficiency. Therefore, to obtain a desired
absorption spectrum with a large signal-to-noise ratio (S/N), the
thickness or the height of the cell has to be increased. It is
difficult to reduce the size of the cell while maintaining the
signal-to-noise ratio. In other words, the number of atoms
contributing to the EIT phenomenon per a unit volume in the cell
remains the same. The technique disclosed in Patent Document 2-(1)
has the same problem.
[0009] Specifically, in both Patent Document 1 and Patent Document
2-(1), only two light waves are used. Since the alkali metal atoms
in the cell have a velocity distribution, Doppler broadening of
energy involved in the velocity distribution is present. Therefore,
since only a part of atoms interact with one another in .LAMBDA.
type transition of only the two light waves, an EIT development
yield per a unit volume is extremely poor. Therefore, EIT signal
intensity is low.
[0010] An excitation level of existing alkali metal atoms has a
hyperfine structure and is split into levels having different
energies as shown in FIGS. 20A to 20C. Therefore, since the EIT
phenomenon targeting the alkali metal atoms cannot be explained in
a simple .LAMBDA. type three-level system shown in FIG. 24A, to
actually cause EIT efficiently, it is necessary to take into
account such multiple levels. However, so far, sufficient
examination has not been made by taking into account a relation
between the presence of the multiple level and the Doppler
broadening of energy involved in the atomic velocity
distribution.
[0011] In particular, as in the invention, when plural resonant
light pairs are used, it is important in terms of optimization of a
driving condition for a quantum interference device employing the
EIT phenomenon to determine a center frequency of a light source (a
laser) taking into account an energy state of an excitation level
and determine a modulation condition for the laser.
SUMMARY
[0012] An advantage of some aspects of the invention is to provide
a quantum interference device that efficiently causes the EIT
phenomenon for a larger number of gaseous alkali metal atoms in a
cell by generating plural resonant light pairs having different
wavelengths and provide a small atomic oscillator, a magnetic
sensor, or a quantum interference sensor by making use of the
quantum interference device.
[0013] The invention is devised to solve at least a part of the
problems and can be realized as embodiments or application examples
explained below.
Application Example 1
[0014] According to an aspect of the invention, there is provided a
quantum interference device according to the invention including at
least: gaseous alkali metal atoms; and a light source for causing a
resonant light pair having different frequencies that keep a
frequency difference equivalent to an energy difference between two
ground states of the alkali metal atoms, the quantum interference
device causing the alkali metal atoms and the resonant light pair
to interact each other to cause an electromagnetically induced
transparency phenomenon (EIT), wherein there are a plurality of the
resonant light pairs, and center frequencies of the respective
resonant light pairs are different from one another.
[0015] A characteristic of the quantum interference device
according to the invention is that the number of excitation laser
beam pairs is equal to or larger than two and center frequencies of
the respective laser beam pairs are set different from one another.
This makes it possible to cause the EIT phenomenon for a larger
number of gaseous alkali metal atoms per a unit volume.
Application Example 2
[0016] It is preferable that the resonant light pair caused to
interact with the alkali metal atoms be linearly polarized
light.
[0017] When a tip of an electric vector of light draws a straight
light in a plane perpendicular to a propagation direction of the
light, the light is called linearly polarized light. Therefore, the
resonant light pair emitted from the light source is linearly
polarized light unless being subjected to polarization. A
polarization state of the light can be considered as
superimposition of two linearly polarized lights orthogonal to each
other. Consequently, since the resonant light from the light source
is originally linearly polarized light and means for polarizing the
resonant light is unnecessary, it is possible to simplify the
configuration of the light source.
Application Example 3
[0018] It is preferable that the resonant light pair caused to
interact with the alkali metal atoms be circularly polarized
light.
[0019] When a tip of an electric vector of light draws a circle in
a plane perpendicular to a propagation direction of the light, the
light is called circularly polarized light. It is experimentally
confirmed that, when the resonant light pair is converted into the
circularly polarized light, light transmission intensity of
wavelength .lamda.0 becomes about six times as large as normal
light transmission intensity. Consequently, it is possible to
improve an S/N of an optical output signal due to the EIT
phenomenon.
Application Example 4
[0020] It is preferable that the resonant light pair caused to
interact with the alkali metal atoms be elliptically polarized
light.
[0021] When a tip of an electric vector of light draws an ellipse
in a plane perpendicular to a propagation direction of the light,
the light is called elliptically polarized light. It is seen that,
when a wave plate is placed on an optical path of the resonant
light pair to be orthogonal to the optical path and the surface of
the wave plate is rotated, a polarization state changes and
continuously changing elliptically polarized light is present
between the linearly polarized light and the circularly polarized
light. Therefore, even if the light is the elliptically polarized
light, it is possible to improve an S/N of an optical output signal
due to the EIT phenomenon.
Application Example 5
[0022] It is preferable that a wave plate be provided on an optical
path between the light source and a cell in which the alkali metal
atoms are encapsulated.
[0023] The wave plate means a birefringent element that causes a
phase difference between polarized light components orthogonal to
each other. A wave plate that causes a phase difference
.pi.(180.degree.) is called a .lamda./2 plate or a half-wave plate,
which is used for changing a polarization direction of the linearly
polarized light. A wave plate that causes a phase difference
.pi./2(90.degree.) is called a .lamda./4 plate or a quarter-wave
plate, which is used for converting the linearly polarized light
into the circularly polarized light (the elliptically polarized
light) and for converting the circularly polarized light (the
elliptically polarized light) into the linearly polarized light. In
the invention, since it is necessary to convert the linearly
polarized light into the circularly polarized light or the
elliptically polarized light, the .lamda./4 plate is used. It is
necessary to convert the resonant light pair of the linearly
polarized light emitted from the light source into the circularly
polarized light or the elliptically polarized light with the wave
plate and make the resonant light pair incident on a gas cell.
Consequently, it is possible to improve an S/N of an optical output
signal due to the EIT phenomenon with a simple configuration.
Application Example 6
[0024] It is preferable that the plural resonant light pairs
satisfy a development condition for the electromagnetically induced
transparency phenomenon and the light intensities of the respective
resonant light pairs be near a maximum P0 in an area in which an
EIT signal intensity linearly increases.
[0025] When such a light intensity distribution of the plural
resonant light pairs is adopted, it is possible to improve light
use efficiency.
Application Example 7
[0026] It is preferable that an intensity distribution of the
plural resonant light pairs be the Gaussian distribution with
respect to center frequencies of the respective pairs, the resonant
light pair corresponding to maximum light intensity satisfy a
development condition for the electromagnetically induced
transparency phenomenon corresponding to the atom group of alkali
metal, a velocity component in the light direction of which is near
0, and the intensity be the maximum P0 in a linear area.
[0027] Since the velocity distribution of the alkali metal atoms is
the Gaussian distribution, if a light intensity distribution of the
resonant light pair is set to the Gaussian distribution in advance,
it is possible to attain high light use efficiency with a simple
optical driving circuit.
Application Example 8
[0028] It is preferable that the quantum interference device
generate the plural resonant light pairs by combining amplitude
modulation and frequency modulation or phase modulation.
[0029] With such a modulation system, it is possible to control the
light intensity distribution of the resonant light pair at a high
degree of freedom.
Application Example 9
[0030] It is preferable that the quantum interference device
generate the plural resonant light pairs through modulation by a
signal having any one of a sine wave, a triangular wave, a saw
tooth wave, and a rectangular wave.
[0031] With such a modulation system, it is possible to control a
light intensity distribution of the resonant light pair with a
simple optical driving circuit at a high degree of freedom.
Application Example 10
[0032] It is preferable that the quantum interference device
further include a driving circuit unit for modulating the light
source and the driving circuit unit be separated from other
components, and a constant of the driving circuit unit can be
arbitrarily controlled and set in a state in a manufacturing
process or after commercialization.
[0033] As the "quantum interference device" employing the EIT
phenomenon, various applied products such as high precision
oscillators, high precision measuring devices such as a clock, and
quantum interference sensors such as a magnetic sensor, a
particulate detection sensor for pollens and smoke are conceivable.
By adopting the structure explained above, it is possible to
acquire an optimum EIT signal profile corresponding to a
purpose.
Application Example 11
[0034] It is preferable that, when a nuclear spin quantum number of
the alkali metal atoms is represented as I, a quantum number of a
hyperfine structure of an excitation level of P1/2 or an excitation
level of P3/2 of the alkali metal atoms is represented as F', and
minimum energy in an area in which ranges of energies with Doppler
broadenings of F'=I-1/2 and F'=I+1/2 taken into account overlap
with each other is represented as E1 and maximum energy thereof is
represented as E2, excited energy Eend of any one of the plural
resonant light pairs, which cause the electromagnetically induced
transparency (EIT) phenomenon, satisfy E1<Eend<E2.
[0035] Atoms having velocity components in opposite directions can
be simultaneously subjected to EIT development for a resonant light
pair that satisfies the condition and corresponds to Eend.
Therefore, power broadening (a phenomenon in which, if optical
power is large, line width of an EIT signal increases) less easily
occurs. Therefore, a performance index (defined later) is improved
by increasing a Q value (an inverse of a half width of the EIT
signal).
Application Example 12
[0036] It is preferable that, when a nuclear spin quantum number of
the alkali metal atoms is represented as I and a quantum number of
a hyperfine structure of an excitation level of the alkali metal
atoms is represented as F', ranges of energies with Doppler
broadenings of F'=I-1/2 and F'=I+1/2 taken into account do not
overlap with each other, and, when a range of the energy of
F'=I-1/2 with the Doppler broadening taken into account is set to
E11 to E12 (E11<E12) and a range of the energy of F'=I+1/2 with
the Doppler broadening taken into account is set to E21 to E22
(E21<E22), excited energy Eend of any one of the plural resonant
light pairs, which cause the electromagnetically induced
transparency phenomenon, satisfy one of conditions
E11<Eend<E12 and E21<Eend<E22.
[0037] When the condition is satisfied, it is possible to realize
EIT by the plural resonant light pairs while maintaining pure
three-level system .LAMBDA. type transition. Therefore, it is
possible to increase an effect of enhancement of an EIT signal due
to a superimposition effect.
Application Example 13
[0038] It is preferable that the quantum interference device fold
the plural resonant light pairs once or plural times to cause the
plural resonant light pairs to pass through the alkali metal atoms
and detect the electromagnetically induced transparency phenomenon
from the alkali metal atoms, and, when energy of an excitation
level with Doppler width not taken into account is represented as
E10 and excited energy of the plural resonant light pairs is
represented as Eend0, the excited energy Eend0 satisfy E10<Eend0
or Eend0<E10.
[0039] In this case, in a forward path and a backward path of one
resonant light pair, it is possible to cause EIT with alkali metal
atom groups having velocity components in opposite directions in a
cell. Therefore, when the EIT is caused by the plural resonant
light pairs under such a condition, compared with that in a
non-reflection type, an equivalent effect can be obtained with a
half number of resonant light pairs for half light modulation
width.
Application Example 14
[0040] It is preferable that the quantum interference device fold
the plural resonant light pairs once or plural times to cause the
plural resonant light pairs to pass through the alkali metal atoms
and detect the electromagnetically induced transparency phenomenon
from the alkali metal atom, and, when excited energy of any one of
the plural resonant light pairs, which causes the
electromagnetically induced transparency phenomenon, is represented
as Eend, the excited energy Eend satisfy one of conditions
Eend<E10 and E10<Eend.
[0041] In this case, all the resonant light pairs contribute to the
EIT and are a reflection type. Therefore, compared with the
non-reflection type, efficiency is higher because half the resonant
light pairs only has to be used.
Application Example 15
[0042] It is preferable that the number of folds be an odd number
of times (total optical path length of the forward path and the
backward path be substantially equal).
[0043] When the optical path lengths of the forward path and the
backward path of light are set substantially equal, the numbers of
atoms contributing to the EIT in velocity groups different from
each other are substantially equal. This is advantageous in terms
of efficiency of EIT development.
Application Example 16
[0044] According to another aspect of the invention, there is
provided an atomic oscillator including the quantum interference
device according to the invention.
[0045] Since the quantum interference device according to the
invention is included in the atomic oscillator, it is possible to
develop the EIT phenomenon in a high S/N state. Therefore, it is
possible to reduce the size of the atomic oscillator.
Application Example 17
[0046] According to still another aspect of the invention, there is
provided a magnetic sensor including the quantum interference
device according to the invention.
[0047] An oscillator frequency of the atomic oscillator is set with
reference to an energy difference (.DELTA.E12) between two ground
levels of atoms. Since a value of .DELTA.E12 changes according to
the intensity or fluctuation of external magnetism, a magnetic
shield is applied to cells of the atomic oscillator to prevent the
cells from being affected by the external magnetism. It is possible
to manufacture a magnetic sensor that measures the intensity or
fluctuation of the external magnetism by removing the magnetic
shield and reading a change in .DELTA.E12 from a change in the
oscillation frequency. Since the magnetic sensor includes the
quantum interference device, it is possible to develop the EIT
phenomenon in a high S/N state. Therefore, it is possible to reduce
the size of the magnetic sensor.
Application Example 18
[0048] According to still another aspect of the invention, there is
provided a quantum interference sensor including the quantum
interference device according to the invention.
[0049] Since the quantum interference sensor includes the quantum
interference device according to the invention, it is possible to
improve the sensitivity and accuracy of every sensor that detects
disturbance, which affects an EIT signal profile, and reduce the
size of the sensor.
BRIEF DESCRIPTION OF THE DRAWINGS
[0050] The invention will be described with reference to the
accompanying drawings, wherein like numbers reference like
elements.
[0051] FIG. 1 is a schematic diagram of a velocity distribution of
alkali metal atoms in a gaseous state.
[0052] FIG. 2 is a block diagram of the configuration of an atomic
oscillator according to a first embodiment of the invention.
[0053] FIGS. 3A and 3B are diagrams of frequency spectra of
resonant light made incident on a gas cell.
[0054] FIG. 4 is a diagram of a state of the resonant light made
incident on the gas cell and moving directions of the gaseous
alkali metal atoms.
[0055] FIG. 5 is a schematic diagram for explaining a relation
between Doppler broadening of energy due to the motion of atoms and
resonant lights of the invention.
[0056] FIG. 6 is a block diagram of the configuration of an atomic
oscillator according to a second embodiment of the invention.
[0057] FIG. 7 is a block diagram of the configuration of an atomic
oscillator according to a third embodiment of the invention.
[0058] FIG. 8 is a block diagram of the configuration of a magnetic
sensor according to an embodiment of the invention.
[0059] FIG. 9A is a graph of light transmission intensity related
to an EIT phenomenon due to two resonant light pairs having
different wavelengths.
[0060] FIG. 9B is a graph of light transmission intensity related
to an EIT phenomenon that occurs when the two resonant light pairs
having different wavelengths are modulated.
[0061] FIG. 10 is a block diagram of the configuration of an atomic
oscillator according to a fourth embodiment of the invention.
[0062] FIG. 11 is a block diagram of the configuration of an atomic
oscillator according to a fifth embodiment of the invention.
[0063] FIG. 12A is a graph of a "velocity (one-dimensional
projection" distribution (a Maxwell-Boltzman distribution) of
atoms.
[0064] FIG. 12B is a graph of a "speed" distribution (a
Maxwell-Boltzman distribution) of atoms.
[0065] FIG. 13A is a graph of a harmonic (+ component) distribution
during sine wave modulation.
[0066] FIG. 13B is a graph of a harmonic (+ component) distribution
during typical rectangular wave modulation.
[0067] FIG. 13C is a graph of a harmonic (+ component) distribution
during typical triangular wave modulation.
[0068] FIG. 14A is a graph of a linear to nonlinear branch point of
light intensity.
[0069] FIG. 14B is a graph of a laser frequency distribution.
[0070] FIG. 15A is a graph of dependency of EIT signal line width
on laser intensity.
[0071] FIG. 15B is a graph of comparison of a method in the past
and a method of the invention concerning a relation between EIT
signal intensity and the EIT signal line width.
[0072] FIG. 16 is a graph of a laser frequency distribution near a
CsD2 line.
[0073] FIG. 17 is a graph of a relation between the EIT signal
intensity and line width.
[0074] FIG. 18 is a graph of comparison of the EIT signal intensity
at the same line width.
[0075] FIG. 19 is a diagram of the configuration of an experiment
system.
[0076] FIG. 20A is an energy diagram corresponding to a D2
line.
[0077] FIG. 20B is an energy diagram corresponding to a D1
line.
[0078] FIG. 20C is an energy diagram near an excitation level with
Doppler broadening taken into account.
[0079] FIG. 21A is an energy diagram near the excitation level with
Doppler broadening taken into account.
[0080] FIG. 21B is an energy diagram near the excitation level with
Doppler broadening taken into account.
[0081] FIG. 22A is an energy diagram near the excitation level.
[0082] FIG. 22B is an energy diagram near the excitation level.
[0083] FIG. 22C is a diagram of an arrangement configuration of a
cell, in which alkali metal atoms are encapsulated, a light source,
an optical path, and a detector according to a sixth embodiment of
the invention.
[0084] FIG. 23A is an energy diagram near the excitation level.
[0085] FIG. 23B is an energy diagram near the excitation level.
[0086] FIG. 23C is a diagram of an arrangement configuration of a
cell, in which alkali metal atoms are encapsulated, a light source,
an optical path, and a detector according to a seventh embodiment
of the invention.
[0087] FIG. 24A is a diagram for explaining a principle of an EIT
system in the past.
[0088] FIGS. 24B and 24C are diagrams of a relation between a gas
cell and resonant light in the past.
DESCRIPTION OF EXEMPLARY EMBODIMENTS
[0089] The invention is explained in detail below with reference to
embodiments shown in the accompanying figures. However, components
and types, combinations, shapes, relative arrangement thereof, and
the like are not meant to limit a scope of the invention only
thereto and are merely explanation examples unless specifically
noted otherwise.
[0090] "Performance index" often referred to below is defined. The
performance index is defined as a product of an inverse (i.e., a Q
value) of line width of an EIT signal and an EIT signal-to-noise
ratio (i.e., S/N). For example, since the S/N is proportional to
EIT signal intensity, the performance index is improved if the EIT
signal intensity increases. It is the principal object of the
invention to improve the performance index.
[0091] FIG. 1 is a schematic diagram of a velocity distribution of
a gaseous alkali metal atom group confined in a container.
[0092] The abscissa of FIG. 1 indicates the velocity of gaseous
alkali metal atoms and the ordinate indicates a ratio of the number
of gaseous alkali metal atoms having the velocity. As shown in FIG.
1, gaseous alkali metal atoms have a fixed velocity distribution
corresponding to temperature with velocity 0 set in the center. The
velocity represents an atomic velocity component parallel to an
irradiation direction at the time when a laser beam is irradiated
on the gaseous alkali metal atom group. A value of the velocity of
the alkali metal atoms that is stationary relatively to the light
source is set to 0. The inventors paid attention to the fact that
the velocity of the gaseous alkali metal atoms substantially
affected the EIT phenomenon. When there is a distribution in the
velocity of the gaseous alkali metal atoms, because of Doppler
effect (Doppler shift) of light, a distribution occurs in apparent
wavelength of resonant light, i.e., wavelength of the resonant
light viewed from the gaseous alkali metal atom. Therefore, the
inventors paid attention to the fact that, in the group, there were
a considerable number of gaseous alkali metal atoms that remained
without causing the EIT phenomenon even if the resonant lights 1
and 2 were simultaneously irradiated as a pair. In the method in
the past, i.e., when the resonant lights 1 and 2 are simultaneously
irradiated on the alkali metal atom group as a pair, only a part of
alkali metal atoms can contribute to the EIT phenomenon among the
gaseous alkali metal atom group encapsulated in the cell.
Therefore, the inventors came up with ideas for causing gaseous
alkali metal atoms, which do not contribute to the EIT phenomenon
and are wasted in the past due to influence of Doppler effect, to
contribute to the EIT phenomenon. The invention is explained in
detail below.
[0093] FIG. 2 is a block diagram of the configuration of an atomic
oscillator according to a first embodiment of the invention. An
atomic oscillator 50 is an atomic oscillator that controls an
oscillation frequency making use of a light absorption
characteristic due to a quantum interference effect obtained when
two or more pairs (three pairs as explained later) of resonant
lights are made incident as coherent light pairs having different
wavelengths. The atomic oscillator 50 includes an LD (VCSEL)
(coherent light source) 2 that emits resonant lights, a center
wavelength generating unit 1 that generates center wavelength of
the LD 2, an oscillator 9 that oscillates a half (4.596 GHz) of a
frequency (9.2 GHz) equivalent to an energy difference (.DELTA.E12)
between two different ground states, an oscillator 10 that
oscillates a frequency of about 25 MHz, EOMs (electro-optical
modulation elements) 3 and 4 that apply frequency modulation to
resonant light 11 emitted from the LD 2 according to an electric
signal, a gas cell 5 encapsulated with gaseous cesium (Cs, as
alkali metal) atoms that change an absorption amount of light
according to wavelength of light 12 modulated by the EOM 4, a
photo-detector (a light detecting unit) 6 that detects light 13
transmitted from the gas cell 5, and a frequency control unit 7
that detects an EIT state of the gas cell 5 and controls output
voltage on the basis of an output of the photo-detector 6. The
oscillation frequency of the oscillator 10 is set to 25 MHz. This
frequency is a sufficiently small value with respect to typical
Doppler width (e.g., about 1 GHz at the room temperature) of cesium
atoms. The frequency can be changed as appropriate. An output
frequency of the oscillator 9 is set to 4.596 GHz because the
frequency equivalent to .DELTA.E12 of cesium is about 9.2 GHz
(4.596 GHz.times.2). The output frequency is generated by
multiplying a frequency obtained by controlling a voltage-control
quartz oscillator 8 with control voltage output from the frequency
control unit 7. The EOM 3 is modulated by the frequency (25 MHz) of
the oscillator 10 and the EOM 4 is modulated by the frequency
(4.596 GHz) of the oscillator 9. The EOM 3 and the EOM 4 are
arranged in series on an emission side of the LD 2. A combination
of the EOM 3 and the oscillator 10 and a combination of the EOM 4
and the oscillator 9 may be arranged in opposite order.
[0094] The configuration of the atomic oscillator 50 according to
this embodiment is different from the configuration of the atomic
oscillator in the past in that two or more pairs (three pairs) of
two resonant lights having different wavelengths are obtained by
modulating the resonant light 11 emitted from the LD 2 through the
EOM 3 as a modulating unit. In the atomic oscillator in the past,
only one pair of two resonant lights having different wavelengths
is prepared and a frequency is controlled such that a frequency
difference (a difference in wavelength) of two
simultaneously-irradiated resonant lights accurately coincides with
the energy difference .DELTA.E12 of ground levels. However, because
of the Doppler effect of resonant light due to the motion of atoms,
a distribution occurs in resonant light wavelengths of a cesium
atom group encapsulated in the gas cell 5. One pair of resonant
lights interact with only a part of the cesium atoms moving at
velocity that accidentally satisfies a resonant condition
corresponding to the wavelength. Therefore, efficiency of causing
the EIT phenomenon is low. Therefore, in this embodiment, the light
modulating unit is configured to generate at least four resonant
lights (two resonant light pairs) having different wavelengths to
interact with the gaseous cesium atoms encapsulated in the gas cell
5. Consequently, it is possible to increase the number of cesium
atoms contributing to the EIT phenomenon per a unit volume in the
gas cell 5 and efficiently acquire an EIT signal.
[0095] FIGS. 3A and 3B are diagrams of frequency spectra of
resonant light made incident on a gas cell. FIG. 4 is a diagram of
a state of the resonant light made incident on the gas cell and
moving directions of gaseous cesium atoms.
[0096] The operation according to this embodiment is explained
below with reference to FIGS. 3A and 3B and FIG. 4. The resonant
light 11 of the LD 2 is generated by the center wavelength
generating unit 1 to set center wavelength to X0 (a center
frequency f0). When the EOMs 3 and 4 apply frequency modulation to
the resonant light 11 of the LD 2, the resonant light 12 having
frequency spectra 30 to 32 shown in FIG. 3A is input to the gas
cell 5. In FIG. 3A, a frequency difference between A and A' is 9.2
GHz. For the pair of resonant lights, gaseous cesium atoms 15
having a small velocity component with respect to a direction of
the incident light 12 shown in FIG. 4 cause the EIT phenomenon when
.lamda.0 is set to an appropriate value. A frequency difference
between B and B' is also 9.2 GHz. For the pair of resonant lights,
gaseous cesium atoms 14 having a velocity component in a direction
opposite to the direction of the incident light 12 shown in FIG. 4
causes the EIT phenomenon. In FIG. 3A, a frequency difference
between C and C' is also 9.2 GHz. For the pair of resonant lights,
gaseous cesium atoms 16 having a velocity component in a direction
same as the direction of the incident light 12 shown in FIG. 4
cause the EIT phenomenon. In this way, the atoms in the gas cell 5
have various velocity distributions. Therefore, when the resonant
light 12, to which components of sidebands B, B', C, and C' are
given, is made incident on the gas cell 5 as explained above, all
the frequency differences between A and A', between B and B', and
between C and C' are 9.2 GHz. All the three pairs of laser beams
cause interaction with gaseous cesium atoms having velocity
components corresponding thereto. As a result, a ratio of cesium
atoms contributing to the EIT phenomenon increases. Consequently,
it is possible to obtain a desired EIT signal having a large
signal-to-noise ratio (S/N).
[0097] In this embodiment, the modulation frequency of the EOM 4 is
set to a half (4.596 GHz) of the frequency difference of the
gaseous cesium atoms. However, the modulation frequency may be set
to the frequency difference 9.2 GHz. Frequency spectra of the
resonant light in that case are as shown in FIG. 3B and frequency
spectra 33 to 35 are generated. However, for example, the frequency
spectrum 33 is not used and the frequency spectra 34 and 35 are
used (or the frequency spectrum 35 is not used and the frequency
spectra 33 and 34 are used). Specifically, a frequency difference
between A and .lamda.0 is 9.2 GHz. For the pair of resonant lights,
the gaseous cesium atoms 15 having the small velocity component
with respect to the direction of the incident light 12 shown in
FIG. 4 cause the EIT phenomenon when .lamda.0 is set to an
appropriate value. A frequency difference between B and .lamda.1 is
also 9.2 GHz. For the pair of resonant lights, the gaseous cesium
atoms 14 having the velocity components in the direction opposite
to the direction of the incident light 12 shown in FIG. 4 cause the
EIT phenomenon. A frequency difference between C and X2 is also 9.2
GHz. For the pair of resonant lights, the gaseous cesium atoms 16
having the velocity component in the direction same as the
direction of the incident light 12 shown in FIG. 4 causes the EIT
phenomenon. In this way, the atoms in the gas cell 5 have various
velocity distributions. Therefore, when the resonant light 12, to
which components of sidebands B, .lamda.1, C, and .lamda.2 are
given, is made incident on the gas cell 5 as explained above, all
the frequency differences between A and .lamda.0, between B and
.lamda.1, and between C and .lamda.2 are 9.2 GHz. All the three
pairs of laser beams cause interaction with gaseous cesium atoms
having velocity components corresponding thereto. As a result, a
ratio of cesium atoms contributing to the EIT phenomenon increases.
Consequently, it is possible to obtain a desired EIT signal having
a large signal-to-noise ratio (S/N).
[0098] To generate at least two resonant light pairs (three pairs),
it is conceivable to superimpose sidebands on resonant lights
emitted from the LD 2 to generate resonant light pairs and use
frequency spectra of the resonant light pairs. A frequency for
modulating the resonant light needs to be modulated by 4.596 GHz as
a half of the frequency (9.2 GHz) equivalent to the energy
difference (.DELTA.E12) of the two different ground states and a
frequency (25 MHz) that is a sufficiently small value compared with
the typical Doppler width (e.g., about 1 GHz at the room
temperature) of cesium atoms. An EOM for modulating light is used.
Therefore, in this embodiment, the oscillators 9 and 10 that
respectively oscillate two kinds of frequencies are prepared. The
EOMs 3 and 4 arranged in series on the emission side of the LD 2
are modulated by the respective frequencies. Consequently, it is
possible to generate resonant lights having three pairs of
frequency spectra, which maintain the frequency difference of 9.2
GHz, from the resonant light 11 emitted from the LD 2.
[0099] In this embodiment, one EOM 3 and one EOM 4 are provided.
However, the EOM 4 and at least two EOMs 3 may be arranged in
series on the emission side of the LD 2. Consequently, it is
possible to set the number of resonant light pairs to an arbitrary
number and generate the resonant lights at frequency intervals of a
comb tooth shape.
[0100] FIG. 5 is a schematic diagram for explaining a relation
between Doppler broadening of energy due to the motion of atoms and
the resonant lights of the invention. An energy state diagram of
the gaseous alkali metal atom group encapsulated in the container
can be represented by replacing the excitation level of the energy
state diagram for one atom shown in FIG. 24 with an energy band
equivalent to the Doppler broadening. Levels 20, 21, and 22 shown
in FIG. 5 are respectively excitation levels corresponding to the
atoms indicated by 16, 15, and 14 in FIG. 4. Consequently, it is
seen that a ratio of atoms contributing to the EIT phenomenon with
the plural resonant light pairs increases with respect to the
gaseous alkali metal atom group having the velocity distribution.
Therefore, for example, if power allocated to one pair of resonant
lights is set to be substantially equal to the power in the past,
since a saturation limit of absorption rises and total power
increases, it is possible to obtain an EIT signal having high
contrast. When total light irradiation power is substantially equal
to the power in the past, power per one pair of resonant lights of
the invention decreases. Therefore, power broadening of an EIT
signal (a phenomenon in which the line width of the EIT signal
increases when optical power is high) is suppressed. It is possible
to obtain a satisfactory EIT signal having narrow half width
compared with the EIT signal in the past. Therefore, when the EIT
signal is applied to an oscillator, it is possible to improve
frequency stability compared with the frequency stability in the
past.
[0101] FIG. 6 is a block diagram of the configuration of an atomic
oscillator according to a second embodiment of the invention.
Components same as those in the first embodiment are denoted by
reference numerals same as those shown in FIG. 2 and are explained
below. FIG. 6 is different from FIG. 2 in that the EOM 4 is
removed, a mixer 17 that mixes output signals of the oscillators 10
and 9 is provided, the EOM 3 is driven according to an output
signal 18 of the mixer 17, and the EOM 3 is arranged on the
emission side of the LD 2. Consequently, the resonant light 12
emitted from the EOM 3 generates frequency spectra same as those
shown in FIG. 3A.
[0102] Although the EOMs are used to modulate light, if the number
of frequency spectra is increased, the number of EOMs has to be
increased accordingly. As a result, cost of the atomic oscillator
increases and the number of components thereof increases.
Therefore, in this embodiment, signals for modulating the EOMs are
mixed by the mixer 17 in advance and one EOM 3 is modulated
according to the output signal 18 of the mixer 17. Consequently, it
is possible to minimize the number of EOMs and reduce the number of
components.
[0103] FIG. 7 is a block diagram of the configuration of an atomic
oscillator according to a third embodiment of the invention.
Components same as those in the second embodiment are denoted by
reference numerals same as those shown in FIG. 6 and are explained
below. FIG. 7 is different from FIG. 6 in that the EOM 3 is removed
and the LD 2 is directly driven to be modulated according to an
output signal 19 of the mixer 17. Consequently, the resonant light
11 emitted from the LD 2 generates frequency spectra same as those
shown in FIG. 3A.
[0104] Specifically, the resonant light 11 emitted from the LD 2 is
generated by the center wavelength generating unit 1 to set center
wavelength to .lamda.0. To modulate the center wavelength, besides
the method of modulating the resonant light 11 emitted from the LD
2 with the EOMs, there is a method of modulating the LD 2 itself.
Therefore, in this embodiment, the LD 2 itself is driven to be
modulated according to the signal 19 mixed by the mixer 17 that
mixes the output frequencies of the oscillators 10 and 9.
Consequently, it is possible to make the EOMs unnecessary. The
output frequency of the oscillator 10 can also be generated from
the voltage-control quartz oscillator 8 via a PLL or the like (a
part of a circuit of the oscillator 9 can also be used). In that
case, the oscillator 10 is also unnecessary.
[0105] Although not shown in the figure, the LD included in the
atomic oscillator of the EIT system in the past may have a
configuration in which surface-emitting lasers having different
wavelengths are arranged in an array.
[0106] FIG. 8 is a block diagram of the configuration of a magnetic
sensor according to an embodiment of the invention. Components same
as those shown in FIG. 7 are denoted by reference numerals same as
those shown in FIG. 7 and are explained below. FIG. 8 is different
from FIG. 7 in that a measured-magnetism generating source 37 is
arranged near the gas cell 5 and a magnetic measuring device 36
that measures fluctuation in an output signal of the frequency
control unit is provided. An oscillation frequency of the atomic
oscillator is set with reference to the energy difference
(.DELTA.E12) between the two ground levels of atoms. Since a value
of .DELTA.E12 changes according to the intensity or fluctuation of
external magnetism, a magnetic shield is applied to cells of the
atomic oscillator to prevent the cells from being affected by the
external magnetism. It is possible to manufacture a magnetic sensor
that measures the intensity or fluctuation of the external
magnetism by removing the magnetic shield and reading a change in
.DELTA.E12 from a change in the oscillation frequency. By adopting
the configuration of the invention, it is possible to develop the
EIT phenomenon in a high S/N state. Therefore, it is possible to
reduce the size of the magnetic sensor.
[0107] FIG. 9A is a graph of light transmission intensity related
to the EIT phenomenon by two resonant light pairs having different
wavelengths. FIG. 9B is a graph of light transmission intensity
related to the EIT phenomenon that occurs when the two resonant
light pairs having different wavelength are modulated. In FIG. 9A,
a waveform 41 is a waveform of light transmission intensity of
linearly-polarized light from the VCSEL. Light transmission
intensity obtained when the resonant light pairs are caused to
further pass a wave plate and changed to circularly polarized light
is a waveform 42. It is seen that the level of the waveform 42 is
increased by about 20% from the level of the waveform 41. When the
resonant light pairs are modulated as shown in FIG. 9B, all the
plural resonant light pairs cause interaction with gaseous cesium
atoms having a velocity distribution corresponding thereto and a
waveform 43 having plural peaks is developed. In this embodiment,
for example, a wave plate 40 is arranged between the LD 2 and the
gas cell 5 to be orthogonal to an optical path as shown in FIG. 10
and a wave plate surface is gradually rotated. It was confirmed
that light transmission intensity had a maximum waveform 45 at
wavelength .lamda.0 when the resonant light pair 11 changed to
circularly polarized light. Therefore, it was confirmed that the
light transmission intensity changed to a waveform 43 (linearly
polarized light), a waveform 44 (elliptically polarized light), and
a waveform 45 (circularly polarized light) in a process of changing
the resonant light pair from the linearly polarized light to the
circularly polarized light.
[0108] When a tip of an electric vector of light draws a circle in
a plane perpendicular to a propagation direction of the light, the
light is called circularly polarized light. It is experimentally
confirmed that, when the resonant light pair is converted into the
circularly polarized light, light transmission intensity of
wavelength .lamda.0 becomes about six times as large as normal
light transmission intensity. Consequently, it is possible to
improve an S/N of an optical output signal due to the EIT
phenomenon.
[0109] When a tip of an electric vector of light draws an ellipse
in a plane perpendicular to a propagation direction of the light,
the light is called elliptically polarized light. It is seen that,
when a wave plate is placed on an optical path of the resonant
light pair to be orthogonal to the optical path and the surface of
the wave plate is rotated, a polarization state changes and
continuously changing elliptically polarized light is present
between the linearly polarized light and the circularly polarized
light. Therefore, even if the light is the elliptically polarized
light, it is possible to improve an S/N of an optical output signal
due to the EIT phenomenon.
[0110] FIG. 10 is a diagram of the configuration of an atomic
oscillator according to a fourth embodiment of the invention. In
the fourth embodiment, the wave plate 40 is added to the
configuration shown in FIG. 7. Specifically, the wave plate 40 is
arranged between the LD 2 and the cell 5 to be orthogonal to the
optical path. The resonant light pair 11 of the linearly polarized
light emitted from the LD 2 is made incident on the wave plate 40
and changes to circularly polarized light 11a with a phase thereof
polarized by 90 degrees. The wave plate 40 may be arranged anywhere
between the LD 2 and the cell 5 and may be arranged near an
emission surface of the LD 2 or near an incident port of the cell
5.
[0111] FIG. 11 is a block diagram of the configuration of an atomic
oscillator according to a fifth embodiment of the invention. In the
fifth embodiment, the wave plate 40 is added to the configuration
shown in FIG. 6. Specifically, the wave plate 40 is arranged
between the EOM 3 and the cell 5 to be orthogonal to the optical
path. The resonant light pair 11 of the linearly polarized light
emitted from the LD 2 is modulated by the EOM 3 into the resonant
light 12, made incident on the wave plate 40, and changes to
circularly polarized light 12a with a phase thereof polarized by 90
degrees. The wave plate 40 may be arranged anywhere between the EOM
3 and the cell 5 and may be arranged near an emission surface of
the EOM 3 and near the incident port of the cell 5.
[0112] The wave plate means a birefringent element that causes a
phase difference between polarized light components orthogonal to
each other. A wave plate that causes a phase difference
.pi.(180.degree.) is called a .lamda./2 plate or a half-wave plate,
which is used for changing a polarization direction of the linearly
polarized light. A wave plate that causes a phase difference
.pi./2(90.degree.) is called a .lamda./4 plate or a quarter-wave
plate, which is used for converting the linearly polarized light
into the circularly polarized light (the elliptically polarized
light) and for converting the circularly polarized light (the
elliptically polarized light) into the linearly polarized light. In
this embodiment, since it is necessary to convert the linearly
polarized light into the circularly polarized light or the
elliptically polarized light, the .lamda./4 plate is used. It is
necessary to convert the resonant light pair 11 of the linearly
polarized light emitted from the LD 2 into the circularly polarized
light or the elliptically polarized light with the wave plate 40
and make the resonant light pair 11 incident on the gas cell 5.
Consequently, it is possible to improve an S/N of an optical output
signal due to the EIT phenomenon with a simple configuration.
[0113] FIG. 14A is a graph of a relation between light intensity
(the abscissa) and EIT signal intensity (the ordinate) of a
two-light wave resonant light pair that satisfies a condition for
developing EIT. In an area where the light intensity is
sufficiently weak, the EIT signal intensity substantially linearly
changes while keeping a proportional relation with respect to the
light intensity. However, when the light intensity exceeds a
certain point (P0), the EIT signal intensity does not change much
even if the light intensity is increased (a saturation area). When
this point is taken into account and attention is paid to a group
(an ensemble) having specific velocity (which indicates a velocity
component parallel to incident light as in the past) among alkali
metal atoms in a cell, it is desirable in terms of light use
efficiency to set incident light intensity to maximum light
intensity P0 at which the EIT signal intensity does not saturate
with respect to incident light intensity (maximum light intensity
in an area in which intensity linearly increases).
[0114] An alkali metal atom (e.g., cesium Cs) group in the cell as
an EIT development area has a distribution (a profile) of velocity
as shown in FIG. 12B. The profile changes because of environmental
factors such as pressure and temperature. When attention is paid to
a distribution of velocity components only in a fixed direction,
the distribution is a substantial Gaussian distribution as shown in
FIG. 12A. When a two-light wave resonant light pair is made
incident in this system to develop EIT, since Doppler broadening of
energy is caused by this velocity distribution, an EIT signal
intensity distribution with respect to a center frequency in a
frequency domain for EIT development is also the Gaussian
distribution (typically having broadening of about 1 [GHz] in terms
of frequency). Therefore, when attention is paid to the light use
efficiency, if light intensities of the respective plural resonant
light pairs are set to be near P0, a distribution of the resonant
light pairs has a shape close to a velocity distribution of atoms,
i.e., the Gaussian distribution as shown in FIG. 14B.
[0115] A semiconductor laser or the like emits, when a DC current
is applied thereto, single-color light (coherent light) having a
frequency (wavelength) corresponding to a current value (Ivias) of
the DC current. When center wavelength is set to about 852 [nm] and
"modulation" of 4.6 [GHz] is applied to Ivias (Imod (1)=4.6 [GHz]),
sidebands having an interval (4.6.times.2=9.2 [GHz]) are formed on
both sides of the center wavelength. When the two light waves are
made incident on Cs atoms in the cell as the resonant light pair,
quantum interference is caused and the EIT phenomenon develops.
When the Doppler broadening explained above is recalled, it is seen
that the number of Cs atoms in the cell contributing to the EIT
phenomenon in the resonant light pair (one pair) by the two light
waves is extremely small. In other words, EIT development
efficiency is low in the past.
[0116] A state of an applied current for driving the semiconductor
laser and a frequency distribution of a laser are explained in
detail with reference to the drawings. FIG. 16 is a graph of a
frequency distribution observed when frequency modulation is
applied to a single-color semiconductor laser beam having center
wavelength of about 852 [nm]. To develop EIT with Cs as alkali
metal atoms set as target atoms, Ivias (a DC bias current) is set
such that a center frequency is set to about 852 [nm] equivalent to
excitation energy. Sidebands are generated when frequency
modulation Imod(1) of 4.6 [GHz] is applied directly to Ivias or by
use of an EOM (an electro-optic modulation element). One resonant
light pair by two light waves having a frequency difference of 9.2
GHz can be generated. When modulation Imod(2) of an arbitrary
frequency (e.g., 15 [MHz]) is further superimposed on the resonant
light pair (superimposed modulation), each of the two light waves
is modulated by a superimposed frequency 15 [MHz] and a frequency
distribution of a comb tooth shape having intervals of the
superimposed frequency 15 [MHz] is generated. Since the resonant
light pair can be regarded as plural pairs, if the original two
light waves respectively having the frequency distribution of the
comb tooth shape are caused to interact with the Cs atoms in the
cell, it is possible to develop EIT simultaneously with a CS atom
group moving at different velocities. Therefore, EIT development
efficiency is remarkably improved (the invention).
[0117] (a) of FIG. 16 indicates one of the two light waves not
subjected to the superimposed modulation as in the past. (b) and
(c) of FIG. 16 indicate spectra obtained when Imod(2) is
superimposed as a sine wave. Both modulated frequencies are equal
at 15 [MHz] but amplitude conditions for modulation are different
in (b) and (c). It is seen that both the spectra show frequency
distributions of a comb tooth shape and a range of frequency spread
is larger in (c) in which modulation amplitude is 1.0 [V] than in
(b) in which modulation amplitude is 0.2 [V].
[0118] FIG. 17 is a graph of a relation between intensity (the
ordinate) and line width (the abscissa) of an EIT signal of Cs
obtained by irradiating plural resonant light pairs according to
laser driving with the superimposed modulation Imod(2) of the
invention taken into account. The relation is compared with a
relation in the method in the past. Data of the graph is acquired
by changing laser power irradiated on Cs. (a), (b), and (c) of FIG.
17 respectively correspond to (a), (b), and (c) of FIG. 16. It is
seen that, at the same line width, the EIT signal intensity is far
larger in the invention compared with the method in the past and
the "performance index" (=Q.times.(S/N)) defined above is improved.
In the method of the invention, the EIT signal intensity is larger
in (c) than (b). This is understood to be because, as it is seen
from the laser spectrum distributions shown in FIG. 16, efficiency
of interaction with a resonant light pair is improved by capturing
Cs atoms of a larger number of velocity distributions in the cell
and the Cs atoms and the resonant light pair can contribute to the
EIT development. It was confirmed that, in the method in the past
(a), since the EIT signal intensity was not obtained, it was
difficult to set the EIT line width to be equal to or smaller than
120 [kHz] and improve a Q value (an inverse of the EIT signal line
width) but, in (b) and (c) of the invention, it was possible to
further reduce line width and substantially improve the performance
index.
[0119] FIG. 18 is a graph of comparison of EIT signals at full
width at half maximum (line width) of 127 [kHz]. It was confirmed
that, the EIT signal intensity was about 14 times as large as that
in (a) of the method in the past.
[0120] The following points are clarified by summarizing the
results obtained above. When laser power is reduced to narrow line
width by power broadening (FIG. 15A), the EIT signal intensity
weakens in proportion to reduction in the laser power (FIG. 15B).
In the method in the past, the EIT signal intensity falls to 0 at a
point A. In other words, signal line width narrower than signal
line width at the point A cannot be acquired.
[0121] However, in the method of the invention, since the number of
atoms (density) in the cell contributing to the EIT signal
development substantially increases, sufficient signal intensity
can be obtained at the EIT signal width at which signal intensity
disappears in the method in the past (a point B). In other words, a
value obtained by dividing the EIT signal intensity at the point B
by the EIT signal intensity at the point A represents a maximum
amplification ratio of the method of the invention to the method in
the past and is an index of an effect of improvement of the S/N.
When the S/N is improved, since the performance index is improved,
it is possible to improve, in proportion to a level of the
performance index, performance of every device that makes use of
the EIT phenomenon. For example, in the atomic oscillator that
makes use of the EIT phenomenon, frequency stability is improved in
proportion to the S/N. If the atomic oscillator is applied to a
quantum interference sensor such as a magnetic sensor (using a
characteristic that a frequency of an EIT atomic oscillator changes
sensitively in response to an external magnetic field), it is
evident that an effect such as an increase in sensitivity is
realized. In the invention, since the S/N is improved, signal
intensity equivalent to that in the past can be obtained even if
the size of the cell that causes the EIT phenomenon is reduced.
Therefore, there is an effect that it is possible to further reduce
the size of the device.
[0122] As shown in FIG. 15B, if sufficient EIT signal intensity is
obtained at the point B, it is possible to further narrow the
signal line width by further reducing the laser intensity
(exclusion of the influence of the power broadening). For example,
when a target minimum signal intensity line is indicated by an
alternate long and short dash line, in the method of the invention,
it is possible to attain signal line width at a point C. As
explained above concerning the S/N, as a value of the line width
decreases, the Q value increases and, therefore, a value of the
performance index increases. Therefore, it is possible to improve
performance of every device that makes use of the EIT phenomenon.
For example, in the atomic oscillator that makes use of the EIT
phenomenon, frequency stability is improved by narrowing the EIT
signal line width. If the atomic oscillator is applied to a quantum
interference sensor such as a magnetic sensor (using a
characteristic that a frequency of an EIT atomic oscillator changes
sensitively in response to an external magnetic field), an effect
such as an increase in accuracy is realized.
[0123] Consequently, according to the invention, it is possible to
obtain EIT signal intensity and EIT line width, which cannot be
attained by the method in the past, by appropriately selecting a
method of modulating a laser. Therefore, there is an advantage that
it is possible to extensively determine an EIT signal profile
matching a purpose to which the invention is applied. If this
advantage is utilized, for example, if means that can control
parameters for the laser modulation (a modulated waveform,
intensity, and the like including modulation on and off) at stages
of EIT device design and manufacturing is independently provided
integrally with a laser driving circuit IC or the like and a
considerable number of other components are common parts, it is
possible to easily manufacture an EIT device exclusively used for a
purpose. There is also an effect such as a reduction in cost.
Further, means with which a product user can appropriately control
and set the laser modulation parameters according to an environment
of use or the like may be provided.
[0124] FIGS. 13A to 13C are graphs of a relation between a method
of modulating a laser and a Fourier component. In FIG. 13A, a
Fourier component obtained in amplitude modulation (AM) by a sine
wave is shown. In FIG. 13B, a Fourier component in amplitude
modulation (AM) by a rectangular wave is shown. In FIG. 13C, a
Fourier component in amplitude modulation (AM) by a triangular wave
is shown. The abscissa indicates a frequency. In rectangular wave
modulation, a Fourier component relatively higher in order than
that in triangular wave modulation is present. If a combined wave
of these waves is subjected to superimposed modulation and a laser
beam is subjected to superimposed modulation as Imod(2) in
frequency modulation (FM) and phase modulation (PM), it is possible
to obtain an arbitrary modulated waveform. Therefore, it is
possible to control an intensity distribution of plural resonant
light pairs and intervals of adjacent frequencies at a high degree
of freedom. Consequently, it is possible to realize an effect that
it is easy to perform EIT signal control necessary for device
performance required for each application and accuracy is
improved.
[0125] FIG. 19 is a diagram of the configuration of an experiment
system according to the invention. This configuration is an example
in which a laser is not modulated in Imod(1) and an EOM (an
electro-optical modulation element) is used.
[0126] FIGS. 20A to 20C are energy diagrams of an electron state of
alkali metal. FIG. 20A is an energy diagram of an excitation level
P3/2 and corresponds to a so-called D2 line. FIG. 20B is an energy
diagram of an excitation level P1/2 and corresponds to a so-called
D1 line. In FIG. 20C, interaction between one resonant light pair
by two light waves in the past or plural light pairs of the
invention and alkali metal atoms with the Doppler broadening taken
into account is shown. FIG. 20C is an energy diagram near an
excitation level that satisfies a condition for causing the EIT
phenomenon.
[0127] The excitation level P3/2 has a hyperfine structure. In a
normal operating temperature range of the device using the EIT
phenomenon, F'=I+1/2, I-1/2 involved in the EIT development has
overlapping energies because of the Doppler broadening. (FIG. 20C).
In a high-temperature region, overlapping of energies due to the
Doppler broadening may occur in the hyperfine structure of the
excitation level P1/2. A laser center frequency (a center
wavelength) is set such that excited energy Fend of as many
resonant light pairs as possible among the plural resonant light
pairs of the invention is made incident in the overlapping region.
In other words, as shown in FIG. 20C, the excited energy Eend is
set to satisfy E1<Eend<E2. F' represents a quantum number of
the hyperfine structure and I represents a nuclear spin quantum
number.
[0128] One resonant light pair made incident in the energy
overlapping region causes the EIT phenomenon for two kinds of
alkali metal atoms corresponding to different quantum numbers of
the hyperfine structure (F'). In other words, EIT simultaneously
occurs in alkali metal atoms in different two kinds of velocity
groups (ensembles) having velocity components in opposite
directions. When such a condition is satisfied, since light
intensity (a photon number) of a resonant light pair is dispersed
to the respective ensembles, EIT signal intensity less easily
saturates, a stronger laser beam can be irradiated, and the S/N is
improved. In particular, the effect is more conspicuous when it is
necessary to reduce the size of the cell and enhance the EIT signal
intensity. If total light intensity to be irradiated is the same,
the photon number is dispersed such that the alkali metal atoms of
the different two kinds of velocity groups and photons interact
with each other in the overlapping region. As a result, power
broadening is suppressed and line width of an EIT signal is
narrowed for one velocity group (an increase in the Q value). In
other words, it is possible to improve the performance index.
[0129] FIG. 21 is an energy diagram of a typical P1/2 level. In
general, hyperfine structure energy split width of the D1 line
(typically 0.5 to 1 GHz) is large compared with that of the D2
line. Two kinds of energy bands due to the Doppler broadening do
not overlap. As explained above, in the case of the D2 line (the
excitation end level is P3/2), since the energy split width of the
hyperfine structure is small, the energy bands overlap because of
the Doppler broadening. Plural resonant light pairs could
simultaneously cause interaction with the same atoms. In this case,
four-light wave mixing occurs, the pure three-level system .LAMBDA.
type transition fails, and the EIT efficiency falls. However, in
general, hyperfine structure energy split width of the D1 line
(typically 0.5 to 1 GHz) is large compared with that of the D2
line. Two kinds of energy bands due to the Doppler broadening do
not overlap. Therefore, since it is possible to realize EIT by the
plural resonant light pairs while maintaining the pure three-level
system .LAMBDA. type transition if the D1 line is used, it is
possible to increase the effect of enhancement of the EIT signal
due to the superimposition effect. In this case, there are two
kinds of methods; a method of setting the excited energy Eend to
satisfy E11<Eend<E12 (FIG. 21A) and a method of setting the
excited energy Eend to satisfy E21<Eend<E22 (FIG. 21B).
[0130] FIG. 22C is a diagram of an arrangement configuration of a
cell, in which alkali metal atoms are encapsulated, a light source,
an optical path, and a detector according to a sixth embodiment of
the invention. Light emitted from the laser light source is made
incident on the cell and causes the EIT phenomenon with the alkali
metal atoms. Then, the light folded via means such as reflection
travels in the opposite direction to cause the EIT phenomenon with
the alkali metal atoms in the cell again and is guided to the
photodetector. This is a so-called reflection type. As shown in
FIGS. 22A and 22B, when energy of an excitation level with Doppler
width not taken into account is represented as E10, if excited
energy Eend0 of single color light of the light source is selected
not to be equal to E10 (E10<Eend0 or Eend0<E10), one resonant
light pair can cause, in a forward path and a backward path, EIT
with an alkali metal atom group having velocity components in
opposite directions in the cell. Therefore, when plural resonant
light pairs are configured to cause EIT under such a condition,
compared with a type other than the reflection type, the equivalent
effect of the invention can be obtained with half the resonant
light pairs or half modulation width of light. Therefore, in this
configuration, a mechanism for generating plural resonant light
pairs of a laser driver or the like is designed more easily and
power consumption during device driving is reduced, which
contributes to energy saving.
[0131] FIG. 23C is a diagram of an arrangement configuration of a
cell, in which alkali metal atoms are encapsulated, a light source,
an optical path, and a detector according to a seventh embodiment
of the invention. Light emitted from the laser light source is made
incident on the cell and causes the EIT phenomenon with the alkali
metal atoms. Then, the light passes through the cell plural times
via means such as reflection, causes the EIT phenomenon every time
the light passes through the cell, and is led to the photodetector.
This is a so-called multiple reflection type. As shown in FIGS. 23A
and 23B, if excited energy Eend of all plural resonant light pairs,
which could cause the EIT phenomenon, is selected to satisfy only
one of conditions Eend<E10 and E10<Eend, one resonant light
pair can cause, in a forward path and a backward path, EIT with an
alkali metal atom group having velocity components in opposite
directions in the cell. Since optical path length is longer in the
multiple reflection type, coherent time increases, intensity of an
EIT signal increases, and line width narrows. This leads to
improvement of the performance index. When the number of times of
reflection of the light is set to an odd number of times and
optical path lengths in the forward path and the backward path of
the light are set substantially equal, the numbers of atoms
contributing to EIT in different velocity groups are substantially
equal. This is advantageous in terms of EIT development efficiency.
Therefore, when plural resonant light pairs are configured to cause
EIT under such a condition, compared with a type other than the
reflection type, the equivalent effect can be obtained with half
the resonant light pairs or half modulation width of light.
Therefore, in this configuration, a mechanism for generating plural
resonant light pairs of a laser driver or the like is designed more
easily and power consumption during device driving is reduced,
which contributes to energy saving.
* * * * *