Note: Descriptions are shown in the official language in which they were submitted.
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MULTIPOLE MAGNETIC GEOMETRY FOR A
RING LASER GYROSCOPE
BACKGROUND OF THE INVENTION
Field of the Invention
This invention relates to ring laser gyroscopes,
and more particularly to ring laser gyroscopes having
magnetic sources external to the bore cavity of the
closed path defining the gyroscope.
Description of the Related Art
Since its introduction in the early 1960's as a
laboratory experiment, the ring laser gyroscope has been
commercially developed as a logical replacement for the
mechanical gyroscope for use in all manner of inertial
guidance systems. Heretofore, the basic two mode ring
laser gyroscope has been developed which has two
independent electromagnetic wave modes oscillating in an
optical ring cavity. '~hen the ring is stationary, no
rotation is ideally indicated. As the ring cavity is
rotated about its central axis, the counter-rotating
waves interact with one another so that a beat frequency
is developed. A linear relationship between the beat
frequency and the rotation rate of the cavity with
respect to the inertial frame of reference may be
established. Ideally, the rotation rate is proportional
to the beat note. In this manner a gyroscope is
theoretically produced having no moving parts.
In practice, however, the two mode laser gyroscope
often must be mechanically dithered to keep the counter
rotating travelling waves from locking at low rotation
rates. For more information on planar gyroscope two
mode lock in, please see Laser Applications, edited by
Monte Ross, pp. 133-200 (1971). In an effort to solve
this lock-in problem, non-planar ring cavities have been
designed containing more than one pair of counter
rotating modes. These multi-oscillator ring laser
gyroscopes have been developed to achieve the goal of an
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accurate all optical gyroscope having no moving parts.
However, these multi-oscillator ring laser gyroscopes
require the use of a non-reciprocal polarization
rotation device (such as a Faraday rotator) to achieve
the splitting of the light within the ring cavity into
two pairs of counter rotating modes. Generally, the
multi-oscillator ring laser gyroscope is divided into a
pair of right circularly polarized and left circularly
polarized waves. The right circularly polarized waves
are split by the Faraday rotator into clockwise and
anti-clockwise modes. Likewise, the left circularly
polarized waves are split by the rotator into clockwise
and anti-clockwise modes. For a full discussion of the
multi-oscillator ring laser gyroscope, please see LASER
HANDBOOK (vol. IV) edited by M.L. Stitch (1985), pp.
229-332. A non-planar configuration comprising at least
four mirrors and a non-reciprocal Faraday rotator is
described in Smith, U.S. Patent 4,548,501 issued October
22, 1985. In such a non-planar configuration,
reciprocal rotation is accomplished by the non-planar
geometry of the multi-mode ring laser gyroscope. The
out-of-planeness geometry in a folded rhombus ring laser
gyroscopes provides the necessary the reciprocal
splitting into left and right circularly polarized
beams. However, the clockwise and anti-clockwise
component of each circularly polarized beam are
essentially locked at low rotation rates. In order to
further split the right and left circularly polarized
beams into their clockwise and anti-clockwise frequency
components, a non-reciprocal rotator means such as a
Faraday rotator is used. Since the left and right
circularly polarized sets of beam modes are widely
separated in frequency, the multi-mode ring laser
gyroscope avoids the problem of mode lock-in common to
two mode ring laser gyroscopes.
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Critical to the success of non-reciprocal splitting
in a multi-oscillator ring laser gyroscope is the need to
provide a uniform low gradient magnetic field inside the
Faraday rotator disk. Alternatively, an all optical out-
of-plane geometry ring laser gyroscope having no intra-
cavity elements for either reciprocal or non-reciprocal
splitting can be used. This alternative ring laser
gyroscope uses high and uniform magnetic fields to achieve
a split of the gain curve into Q and (Q+1) modes so as to
achieve a desired effect that is equivalent to Faraday
rotation. This split of the gain is achieved by the use
of high power, highly concentrated, magnetic fields
properly positioned along the bore cavity of the ring
laser gyroscope.
Heretofore, the multi-oscillator ring laser
gyroscope and the split gain ring laser gyroscope have
applied axial magnetic fields along a segment of the
closed path formed by the bore cavity by use of
cylindrical, hollow magnets positioned parallel and
around the bore segment or within the segment. In the
multi-oscillator ring laser gyroscope, a Faraday rotator
glass was typically concentrically mounted within a
tubular axially directed magnet, the entire assembly
being "musket-loaded" into the bore cavity where the
Faraday rotator is aligned and positioned in the optical
pathway. This is a difficult and time-consuming
procedure. The "musket-loading" of the Faraday rotator
and magnet assembly must not scratch the side wall of
the bore cavity. Such "musket-loading" assembly of the
Faraday rotator of the multi-oscillator ring laser
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gyroscope was difficult to assemble. Also it is
difficult to place the magnets within the cavity bore
separate from the evacuated region.
In the case of the split gain multi-mode ring laser
gyroscope, an entire leg segment of the monolithic glass
block from which the ring laser gyroscope is
manufactured must be carved out to accommodate a hollow
cylindrical magnet which is positioned in parallel to
the closed pathway and around said pathway. This design
requires severe and costly machining of a segment of the
closed path and bore cavity of the ring laser gyroscope
in order to accommodate the placement of the 30
cylindrical magnet about the segment. The split gain
ring laser gyroscope requires precision machining in
order to accommodate the placement of a permanent magnet
of a cylindrical form around the outer portion of the
body of the ring laser gyroscope along a segment of its
closed path.
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Summary of the Invention
What is needed is a multi pole magnetic geometry
for a ring laser gyroscope which achieves the proper
positioning of a strong axially directed magnetic field
along a segment of the closed path of the ring laser
gyroscope without undue fabrication and machining of the
monolithic glass frame.
In order to solve the problems set forth in the
Background of the Invention, a ring laser gyroscope is
disclosed which has a closed pathway defined by a bore
cavity providing out-of-plane reciprocal image rotation
of a plurality of electromagnetic wave modes propagating
within the pathway. Non-reciprocal polarization
rotation is provided by a magnetic geometry system which
includes a plurality of primarily tranversely-directed
magnetic elements external to the bore cavity defining
the closed pathway. Thus, in a multi-oscillator ring
laser gyroscope, a plurality of magnetic elements in the
form of cylindrically-shaped posts may be positioned
along either side of the closed pathway. Each of the
magnetic elements may be cylindrically-shaped posts made
from a strong permanent magnetic material where each
magnetic element is poled along a diameter. Coarse
tuning of the magnetic elements may be achieved by
rotation of each element within its chamber about each
element's axis. Preferably,an equal number of magnetic
elements, each providing an equal magnetic strength, are
disposed along either side of the segment of the closed
pathway within the monolithic body of the ring laser
gyroscope. In this manner, the magnetic strength of the
field provided by the magnetic element is balanced along
a segment of the pathway. The magnetic elements may be
configured to form an octopole or a dipole impressed
upon an octopole.
For the split gain multi-mode ring laser gyroscope,
a resonator cavity may be defined having a closed
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optical pathway and a gain medium in the cavity. The
gain medium is excited for producing at least four
lasing modes in the cavity, such that the gain medium
provides a corresponding gain curve for each lasing
mode. Magnetic means are provided for adjusting the
gain medium to produce a frequency shift between
selected gain curves for supressing the lasing action of
pre-selected modes in the cavity. These magnetic
elements include a plurality of transversely directed
magnetic posts external to the cavity defining the
closed optical path. These magnetic elements, like
those disclosed for use with the multi-oscillator ring
laser gyroscope, may be coarsely tuned by turning each
of the posts within its particular transverse chamber
lS carved into the block and frame forming the ring laser
gyroscope. These and other advantages and invention
over existing magnetic geometry configurations for ring
laser gyroscopes will be seen by review of the brief
description of the drawings and the detailed description
of the preferred embodiment of this invention which
follows.
BRIEF DESCRIPTION OF THE DRAWINGS
Figure 1 is a perspective view of a preferred
embodiment of the multi-pole magnetic geometry for a
ring laser gyroscope which uses a Faraday rotator.
Figure 2 is a schematic diagram showing a top plan
view of a ring laser gyroscope using the multi-pole
magnetic geometry taught in this disclosure to apply a
magnetic field to a Faraday rotator element.
Figure 3 is a cross-sectional view taken along line
III-III of Figure 1.
Figure 4 is a front elevational view of an
alternative Faraday rotator assembly for use in
conjunction with a multi-pole magnetic geometry that is
suitable for radiation hardening conditions.
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Figure 5 shows a schematic perspective view of an
alternative embodiment of a carrier assembly holding a
glass Faraday rotator in conjunction with a multi-pole
magnetic geometry design for a ring laser gyroscope.
Figure 6 shows a schematic diagram including the
magnetic field 30 lines illustrating the interaction
between six different magnets being used in conjunction
with a radiation hardened Faraday rotator for a ring
laser gyroscope.
Figure 7 shows a perspective view of a DC discharge
Split Gain gyroscope having post magnets symmetrically
positioned at both anodes.
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Figure 8 is a schematic view of the Split Gain
gyroscope which uses the multi-pole magnetic geometry of
this invention showing where the high field strength
magnets are positioned with respect to the discharge.
Figure 9 is a schematic diagram showing magnetic
field line interactions of the six post magnets
positioned along a single leg of the optical path of the
Split Gain Gyroscope of Figure 8.
Figure 10 shows experimental data indicating that
the multi-pole magnetic geometry achieves a relatively
uniform magnetic field at a designated value in its
middle section by proper rotation and alignment of the
cylindrical posts.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
The multi-pole magnetic geometry of this invention
has application both to the multi-oscillator ring laser
gyroscope which requires a Faraday rotator element for
non-reciprocal splitting of counter-rotating beams, as
well as the split gain multi-oscillator ring laser
gyroscope which contains no intra-cavity elements.
With reference to Figures 1 and 3, an out-of-plane
multi-oscillator ring laser gyroscope is shown at 10
containing a plurality of transversely loaded magnetic
poles 32, 36, 38, and 40 about a bore 30 (which contains
a Faraday rotator assembly). This multi-oscillator ring
laser gyroscope 10 has a monolithic hard glass frame 11,
normally made from a ceramic glass material under the
registered trademark Zerodur, manufactured by Schott,
optics Division, of Mainz, West Germany. The optical
pathway 20 of the multi-oscillator ring laser gyroscope
10 shown in Figure 1 defines a path connecting corner
mirrors 18, 22, 24, and 26. Positioned along one leg of
the optical path 20 is a metallized cathode 12 and a
pair of side-positioned anodes 14 and 16. The
monolithic ring laser gyroscope 30 may be mounted on a
central cylinder 28 made from the metallic material
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Invar, a registered trademark. The Invar 28 assists in
isolating the magnetic fields produced by posts 32, 36,
38, and 40 so that minimal far field effects are
generated in the gain bore region bound by anodes 14 and
16 and passing by cathode 12.
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As is heretofore known in the ring laser gyroscope
art, where a multi-oscillator (such as that shown in
Figure 1) has an out-of-plane configuration, reciprocal
splitting of two sets of right and left circularly
polarized light is achieved due to the out-of-plane
geometry of the mirrors 26, 24, 22, and 18 which define
the optical pathway 20. Non-reciprocal splitting
between counter propagating clockwise and anti-clockwise
modes is achieved by use of a Faraday rotator placed
within the bore 30 shown in Figure 1. In order for the
Faraday rotator to achieve non-reciprocal rotation, a
magnetic field must be applied to the glass rotator 43
(Figure 3), to cause non-reciprocal splitting of the
left and right circularly polarized light. Unlike the
"musket loading" design that was used in the prior art,
a Faraday rotator 43 may be positioned through side
insertion from the bottom of the glass frame into the
center of the magnetic field produced by the multi-pole
design. The Faraday rotator assembly may be loaded into
the glass frame 11 by use of pillar 53 which holds the
glass Faraday rotator 43. The rotator 43 and the
support pillar 53 may be inserted by use of an optically
sealed plug 45. Each of the magnets, such as 32 and 38,
may be loaded into their respective bores outside the
optical path 20 to provide an axially directed magnetic
field having a low gradient, which is normal to the face
of the Faraday rotator glass 43. The Invar support bar
28 at the center of the multi oscillator ring laser
gyroscope 10 may be used to prevent the magnetic field
from interfering with the gain medium discharge
positioned between the 25 cathode 12 and each of the
anodes 14 and 16.
An example of the magnetic field line distribution
which occurs when using the magnetic posts in
conjunction with the Faraday glass is shown at Figure 2.
The Faraday rotator glass 52 is positioned within the
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optical pathway 54 at a slight angle. An octopole
construction comprising magnets 44, 46, 48, and 56 is
positioned outside but along the optical pathway in
order to provide an axially directed magnetic field
through the Faraday rotator 52.
It is well known in the art that the degree of
Faraday rotation on non-reciprocal splitting of counter-
propogating beams that may be achieved is dependent upon
the axial length of the Faraday glass rotator glass 52;
the magnetic field intensity provided by each of the
magnets 44, 46, 48, and 56, acting together in an
octopole configuration; and, the Verdet constant, which
is related to the particular properties of the glass
material selected for use as a Faraday rotator 52.
Glasses such as SF 57 or FR 5 are used for the thin
Faraday rotator 52 shown in Figure 2. The configuration
of Figure is suitable to achieve the desired results of
an axially directed magnetic field of appropriate
strength.
It will be noted that each of the magnets 44
through 56 shown in Figure 2 are not conventional
dipoles (where the north and south poles are at either
longitudinal end of the magnets), but rather, the
magnetic north and south poles are diametrically
opposite one another along the entire axial length of
the pole. An example of a radially directed magnet
which was used as a bearing is described in U.S. Patent
4,451,811 to Hoffman, which issued May 29, 1984, and is
assigned to the common assignee of this invention. The
Hoffman patent, however, shows a radially directed
magnetic field, while the applicants have chosen
diametrically opposed poles for each magnet such that a
magnetic field arises transverse to both the axial
length of each of the magnets 44 through 56, and the
front and back surfaces of the Faraday rotator 52. In
this manner, a high far field magnetic flux is
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established for use with the Faraday rotator assembly of
a multi-oscillator ring laser gyroscope.
Figure 5 shows the assembly of Figures 1 and 3 for
positioning a glass Faraday rotator 43 along the central
axis of the optical path within the four post magnetic
geometry. Each of the magnets 32, 36, 38, and 40, are
spaced along the central axis of the optical path
outside the bore cavity. This assembly is comprised of
a pillar 53 supported by a base 45 which may be bottom
side mounted into the monolithic frame of a multi-
oscillator ring laser gyroscope, like the one shown in
Figure 1. The pillar 53 defines a bore 27, which is
axially directed (as shown in Figure 5) in order that
the Faraday rotator 43 be exposed at both its faces to
the optical pathway of the ring laser gyroscope.
Figures 4 and 6 show an alternate embodiment of a
Faraday rotator assembly which is suitable for use in
high radiation environment. The monolithic frame 13
defines a chamber 57 into which a mounting pillar 56 may
be inserted from the bottom of the frame 13. The pillar
56, seated on optically contacted plug 55, supports a
substantially cube-shaped fused silica Faraday rotator
58. Such a Faraday rotator meets current requirements
for operation of the ring laser gyroscope in a nuclear
hardened environment. The Faraday rotator 58 is
suggested to be shaped as a cube for ease of
manufacture. When nuclear hardening is required, and
fused silica material is used to make a rotator 58
meeting this requirement, the multi-pole magnetic
geometry design must take into account that the Verdet
constant for fused silica glass is approximately 1/5
that of the SF 57 glass used in the thin glass Faraday
rotator shown in Figures 1-3 and Figure 5. The fused
silica Faraday rotator S8 has a substantially lower
Verdet constant than the SF 57 glass heretofore used.
The Faraday rotator 58, therefore, is about 3-5 times
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thicker than the conventional glass rotator, in order to
provide a comparable degree of non-reciprocal splitting
of counter-propogating beams within the optical path of
multi-oscillator ring laser gyroscope. The magnetic
fields which are used in the fused silica Faraday
rotator assembly may be better tuned using a six post
design as shown in Figure 6. Figure 6 shows a fused
silica rotator 58 positioned within an optical pathway
and bore 15. The Faraday rotator 58 has been rotatably
mounted within the 25 chamber 57 defined by the glass
frame 13 (see Figure 4). The rotator 58 is also
positioned (at an angle beta) off a straight axial line
to eliminate retro-reflection within the ring laser
cavity. The configuration shown in Figure 6 depicts an
octapole formed by magnets 60, 62, 64, and 66. These
magnets provide an axially directed field through the
Faraday rotator 58, as depicted in particular by field
lines 72 and 74. Since the Faraday rotator 58 is so
much thicker than the conventional rotator, 51, in
Figure 5, the field lines 72 and 74 will tend to curve
considerably if only the octapole configuration of
magnets 60, 62, 66, and 64 were used. Therefore, a
dipole 35 configuration, made from the two smaller
dipole magnets 70 and 68, have been positioned as shown
at Figure 6 in order to arrange the magnetic flux
through the Faraday rotator 58 in a more uniform and
flat configuration as the magnetic flux 72 and 74 passes
through the fused silica Faraday rotator 58.
It will be noted in all the designs discussed thus
far, that each of the magnet posts is of a cylindrical
shape. This allows the easy coarse tuning of each of
the magnets to properly position their respective
diametrically opposed north-south poles to achieve an
optimum field condition.
Turning to Figures 7 and 8, a DC discharge split
gain multi-oscillator ring laser gyroscope is shown.
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14
This split gain configuration as taught in US Patent
application serial no. 115,081, filed October 28, 1987,
and assigned to the common assignee of this invention,
discloses a split gain out-of-plane multi-oscillator
configuration which requires no intra-cavity elements.
By splitting the gain curves to achieve both Q and Q+l
modes, four active modes, two of which are counter-
propagating, may be achieved without the need for a
Faraday rotator element. However, to achieve the split
gain desired in order for this multi-oscillator ring
laser gyroscope to properly operate, strong but
localized magnetic fields are needed which surround the
gain medium in order to cause the split gain effect to
arise and match the split gain curves with lasing mode
frequencies. In the DC discharge design illustrated in
both Figures 7 and 8, two sets of bores 96' and 97'
surround each of the anodes 78 and 79 to achieve high
magnetic fields along the only portion of the discharge
pathway which overlaps the optical pathway 82, between
the out-of-plane mirrors 84, 86, 88, and 91.
With reference to Figure 7, post magnets 94A, 94B,
and 94C are each inserted into the lower post position
under the anode receiving mount 99. Post magnets 96A,
96B and 96C may be positioned above the optical pathway
82 at the anode mount position 99. The monolithic frame
81 of the multi-oscillator split gain ring laser
gyroscope 80 is carved to form a set of grooves, such
as 93, to accomodate the positioning of DC wire coil 92
and 90 about either side of the anode mount 99, between
the posts of the magnetic geometry configuration formed
by magnets 94A, 94B, 94C, 96A, 96B and 96C. These
coils 92 and 90 are close to a Helmholtz pair
configuration and may be used to electronically fine
tune the magnetic field produced by the magnetic posts
94A, 94B, 94C, 96A, 96B and 96C, so as to precisely
match the split gain curves with the frequencies of the
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lasing modes. With reference to Figure 9, this view is
taken along that portion of the optical pathway 82A
between mirrors 86 and 84. The fine tuning coil 92 and
sO are shown containing a portion of the DC gain lOOA
within the axial distance between magnet 96A and 96C and
magnets 94A and 94C. The magnetic flux lines 102 and
104 are substantially uniform through the gain medium
lOOA in order to provide a strong and symmetrical gain
curve for each lasing mode.
The experimental results of magnetic fields
produced by the multi-pole magnetic geometry (6 posts)
disclosed in this application is shown at curve 106 in
Figure 10. It will be noted that the curve 106 crosses
the axis line 108 at either side of the curve, at about
10 and30 mm.
In this manner, a multi-oscillator ring laser
gyroscope using a Faraday element such as shown in
Figure 1 may provide a uniform magnetic field at the
Faraday rotator with minimal far field effects, thereby
averting unintentional Zeeman effects upon the glow
discharge medium defined between the cathode 12 and each
of the anodes 14 and 16. The test results shown in
Figure 10 indicate that the multi-pole magnetic geometry
of this disclosure provides the high strength isolated
magnetic field. Such uniformity of field strength also
allows the split gain ring laser gyroscope to achieve
maximum field strength where the excited medium lOOA
(Figure 9) is positioned along the optical pathway 82A,
while preventing far field interference with the optical
pathway outside the gain medium.
Therefore an optimum and simply constructed multi-
pole geometry is disclosed for use in a ring laser
gyroscope. This geometry is preferably made from
cylindrical posts which may be oriented at the time of
construction to provide coarse tuning and direction for
the magnetic field. While four and six post designs
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have been disclosed, other combinations of magnetic
posts may be envisioned which provide the balanced and
uniform low far field magnetic field needed for both
multi-oscillator ring laser gyroscopes disclosed herein.
Therefore, it is desired that the appended claims be
construed to cover not only the preferred and
alternative embodiments disclosed herein, but also
equivalent multi-pole magnetic geometry configurations
which may also be used in multi-oscillator and split
gain ring laser gyroscopes.
