Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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Title: TRIAXIAL SPLIT-GAIN RING LASER GYROSCOPE
Inventor: GRAHAM JOHN MARTIN
LEO K. LAM
BACKGROVND
Field of the Invention
The present invention relates to systems for
sensing rotation that operate upon the interaction of
counterpropagating beams of light within a cavity. More
` particularly, this invention pertains to a system for
simultaneously sensing rotation ~bout three orthogonal
axes in which an axial magnetic field is imposed
simultaneously upon the gain region within each of three
lasing cavities.
Description of the Prior ~rt
United S-tates patent application Serial No.
115,018 of Graham J. Martin filed October 28, 1987
entitled "Split Gain Multimode Ring Laser Gyroscope and
Method" discloses a clear path, undithered multioscillator
ring laser gyroscope. To a large extent the device and
method of that invention provide an improvement over
previous multioscillator designs by utiliæing the concept
o~ selected mode repression applying an axial magnetic
field to the yain medium in a non planar cavity, thus
avoiding any intracavity elements.
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Multioscillators belong to a class of ring laser
gyroscopes in which stability problems are minimized by
allowing four modes to lase within the device's cavity.
This creates, in effect, two gyro beam pairs, one le-ft
circularly polarized and the other right circularly
polarized. The lasing modes of a conventional
multioscillator are configured so that two gyros, each
comprising a pair of counterpropagating beams,
simultaneously exists within the (single) cavity. The
resulting sum of the beat outputs provides a signal that
is doubly sensitive to input rotation and substantially
insensitive ko Faraday bias changes.
The type of multioscillator described in the
referenced patent application aomprises a clear path
Sagnac ring rotation sensor that includes means for
adjusting the gain medium to provide a frequency shift
between selected gain curve centers. Such a frequency
shift between the centers ~Isuppresses~ the lasing action
of selected modes in the cavity to prevent frequency
lockingO The actual lasing frequencies of the cavity
modes are not substantially changed by this frequency
shifting.
The described multioscillator concept provides,
in effect, a non-reciprocal bias in a ~our mode laser gyro
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by utilizing a large axial magnetic field without
incurring the disadvantayes o~ prior multioscillator
designs such as the well-known ZEELAG (Zeeman Laser Gyro).
Furthermore, such a device has such a large bias that back
scatter effects become secondary.
Navigation systems must measure space-dependent
varlables, such as rotakion, wlth respect to (or about) a
set of three orthogonal axes. The realization of the many
advantages of a multioscillator or any other rotation
sensor, ring laser or otherwise, must address problems
inherent in attempting to achieve a practical device that
i~ simult~neously sensitive to rotating about three
measuring or input axes. The des.lgn of a navigation
system that is sufficiently compact and reallzable in a
manufacturing sense is beset by numerous difficulties. In
thQ operation of a ring laser, the chosen fill yases must
interact wlth applied electrical ~ields to prod~lce lasing
action. Thus the design of any ring laser gyroscope must
provide for the positioning of anodes and ~athodes in
addition to properly locating mirror faces and internal
bores.
Additional design problems are posed by a device
whose operation relies upon the generation of current
flows in a gaseous medium. Unavoidable gas flows within a
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laser cavity can prove quite deleterious to the operation
of the device. So-called Lan~nuir flow effects can
degrade laser performance considerably, producing inter
31~ unwanted thermal bias. Such effects have been
compensated to varying extents in some single axis devi~es
by the symmetrical placement of a plurality of electrodes
about the body of the instrument. Generally, this implies
the use of numerous electrodes. See, for example, the
United States patents of Dorschner et al. (Serial No.
4,229,106) and 5mith et al. (Serial No. 4,585,501).
The United States patents of Stiles et al.,
(Serial No. 4,477,188) and Simms (Serial No. 4,407,583)
disclose the incorporation of three planar gyro cavities
into a single block. The expansion of a rin~ laser
concept to a unlt ~or measuring rotation about three
orthogonal axes necessarily complicates the problem of
providing a suitable arrangement of electrodes. The
Stiles et al. device utilizes six anodes and two cathodes
while the Simms apparatus includes six anodes and a single
cathode. The use of a considerable number of electrodes
substantially complicates instrument design. Each
electrode must be sealably secured to (or within) the gyro
frame in such a manner that the device remains airtight.
~his may add significant dif~iculties in fabrication.
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The physical size of the electrodes also
complicatQ design. A large number of electrodes will
consume a correspondingly-large percentage of the frame's
surface mounting area. The size and shape of the block-
frame may not be sufficiently reducible to prevent arcingor other unwanted electrical interactions. Thus, the
design of a ring laser rotational rate~ sensor that is
sensitive to rotation about three orthogonal axes is
significantly complicated by unavoidable effects of gas
~low.
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In addition to the problems associated with
plaaement of electrodes, the realization of a triaxial
multioscillator in accordance with the teachings of the
above-referenced patent application is further complicated
by the requirement of an axial magnetic field ~or
adjusting the separation between the centers of the gain
media within each of the rotation-sensing cavities of the
multioscillator. The single axis device disclosed in the
referenced patent application alternately employs
difficult-to-machine frame cutout regions and six-post
magnet arrangements to encompass the gain region as
required. Such designs are complex in the case o~ a
single axis gyro. Their extension to three axes, even if
possible, would resu}t in a device o~ extreme complexity
and cost. Undoubtedly, the extrapolation of such concepts
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to a triax design would introduce interactions between the
axial ~ields ~or the three axes that could result in
error-causinq transverse components, an effect
particularly noticeable in smaller path length designs.
The capabilities (i.e. sensitivity) and price of
a triaxial rotation sensor are functions of the size of
the block-~rame. Any design that demands added surface
area for separation o~ electrodes necessarily adds to ~he
cost of the instrument. Such added cost partially defeats
the compactness advantages of a three axes-in-one block
device and can render the design inappropriate for single
use applications, such as guided missiles, where. th~e
premium is on economy and aacuracy is not critical.
SU~RY
The substantial task of designing an integrated
triaxial multioscillator ring laser rotation sensor of $he
type in which a predetermined axial magnetic field is
applied to each of the lasing cavities is addressed by the
present invention that provides apparatus for imposing
predetermined axial magnetic fields to three lasing
cavities arranged to measure rotation abou~ three
orthogonal axes so that the gain of each of said cavities
is manipulated in a predetermined manner. The device
provides an integral ~rame that includes three (3)
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internal cavities, each comprising four (4) intersecting
cavity segments. Each of the cavities is arxanged to
include a segment parallel to that of each o~ the other
two cavities. Such three parallel segments are equally
spaced about a circle whose center coincides with the
central axis of the ~rame.
A case is provided that includes means for
establishing a magnetic field within the case of
substantially parallel lines of flux. The case
additionally includes means for positioning the frame so
that the predetermined parallel cavity segments are
simultaneously aligned wlth the lines o~ magnetic flux.
The foregoing and additional features and
advantages of this invention will become further apparent
from the detailed description that follows. The written
description is accompanied by a set of drawing figures.
Numerals of the figures correspond to those of the written
description, like numerals referring to like features of
the invention throughoutO
BRIEF DESCRIPTION OF THE DRAWlNGS
Figure 1 is a perspective view of the rhombic
dodecahedral shaped frame for a triaxial multioscillator
illustrating the associated arrangements of lasing
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cavities and mirror faces;
Figure 2 is a perspective view of the
multioscillator frame illustrating the electrode
arrangements and the resultant gain regions within the
three lasing cavities;
Figure 3 is a cross-sectional view of the
multioscillator frame of the invention taken at line 3-3
o~ Figures 1 and 2;
Figure 4 is an exploded perspective view o~ the
mechanical casing of the multioscillator frame of the
invention;
Figure 5 is the simplified view of the assembled
casing for illustrating the resultant orientation of the
multioscillator frame therein;
Figure 6 is a schematic view that illustrates the
interaction between the imposed magnetic fields and the
multioscillator ~rame within the casing; and
Eigure 7 is a schematic view of the interaction
between the imposed magnetic fields and the
multioscillator frame in accordance with an alternative
em~odiment of the invention.
DETAILED DESCRIPTION
Figure 1 i5 a perspective view of a rhombic
dodecahedral-shaped frame 10 for a triaxial
multioscillator showing the associated arrangements of
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lasing cavities and mirrors. Such a basic geometry ~or a
multioscillator Prame is disclosed in United States patent
Serial No. 4,795,258 of Graham J. Martin entitled
"Nonplanar Three-Axis Ring Laser Gyro With Shared Mirror
Faces". That patent is the property of the Assignee
herein. The frame may be formed of an appropriate glass
ceramic (such as those commercially available under the
trademarks "CERVIT" and "ZERODUR") or the material sold
under the trademark "PYREX" is characterized by a total of
twelve (12) planar surfaces.
The faces of the frame are designated by
numerals A through L to facilltate the description of the
orientation of elements therein for the written
description. In the figure, dashed lines illustrate the
surfaces of the ~rame that face into the paper and away
from the viewer. Furthermore, the notations of those
surfaces are in parentheses. Accordingly, letters A, B,
C, D, E, and F are positioned inside surfaces bordered by
solid lines that face the reader while the letters G, H,
I, J, K, and L are lnternal ko surfaces of the frame 10 -
that are bordered by dashed lines and face away ~rom the
viewer.
Three indépendent, clear-path closed cavities
12, 14 and 16 are located within the frame 10. Each of
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these non~planar cavities comprises four straight segments
that intersect adjacent mirror faces fixed to the various
surfaces of the frame 10 to form continuous passageways
for pairs of counterpropagating beams.
A mixror is positioned adjacent each
intersection of cavity segments for re-directing the
counterpropagating beams within the cavities. A total o~
twelve (12) mirrors is employed, four (4) associated with
each of the three (3) cavities. Three of the surfaces
(surfaces A, F and H) provide faces for mounting a pair of
mirrors, each mirror of a mirror pair being associated
with a different i,nternal cavlty while six (6) surfaces
~C, D, E, I, J and K) serve for mounting a single mirror.
Three of the surfaces (B, G and L) are reserved for the
mounting of getters and an internal cathode.
Each of the cavity mirrors is denoted in Figure
1 by a lower ~ase letter that corresponds to its mounting
sur~ace. Furthermore, when a single surface serv~s for
mounting more than one mirror, each of those mirrors is
denoted by a subscript that corresponds to the numeral
identlfying the lasing cavity with which it is associated.
For example, the mirror "al2" is mounted to surface A and
serves to redirect light propagating about the lasing
cavity 12 while the mirror "a1~" is also mounted on
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sur~ace A but serves to direct light travelinq about the
cavity 14.
Figure 2 is a perspective view of the
multioscillator 10 that illustrates the arrangemsnt of
electrodes associated therewith and the resulting gain
regions within the lasing cavities 12, 14 and ~6. Fvr the
purposes of this figure, a glass ceramic plug 18 has been
removed, exposing the aluminum-coated sur~ace of the
internal cathode 20.
A total of six (6) anodes is located at the
surfaces of the rame lQ, pairs of which are in
communication through bores with preselected inclividual
segments of each of the three internal cavities. The
designated segments of the independent lasing cavities
also communicate (through bores) with the internal cathode
20. Such designated segments contain the gain for each of -
the cavities and are indicated in Figure 2 by the numerals
12', 14' and 16' corresponding to the cavity designations
previously provided. It is a property of the arrangement
of cavities within a rhombic dodecahedron as illustrated
in Figures 1 and 2 that the twelve light path segments may
be collected into four yroups of three parallel segments.
Furthermore, the segments of one of such groups are
equally spaced on the circumference of a circle whose
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center is coincident with t,he long axis 88 of the frame
10. The symmetry o~ such a bore set makes it an ideal
choice for imposition of a single magnetic field that
simultaneously satisfies the axial fie.ld reyuirement for
split gain operation in each of the three lasiny cavities.
Furthermore, as will become particularly apparent with
reference to Figure 3, the symmetrical placement of the
anode boras adjacent opposed ends of t:he cavity segments
~ 12', 14' and 16' result in a "no net flow" condition that
minimizes Fizeau-Fresnel effects otherwise occasioned by
gas flow within the gain region.
Each anode is fixed to the closest surface to
minimize the requlred bore lengths. ~The bores are shown
symbolically $n Figure 2 by straight lines.) An anode
notation is adopted to facilitate a complete understanding
of the electrode geometry. In it, each anode is denoted
by a numeral that corresponds to the lasing cavity with
which it communicates followed by a letter designating the
surface upon which it is fixed. ~he bores that connect
the ends of the three designated cavity segments to anodes
are not separately designated. As can be seen, gain
regions, indicated by shading, are established in the
designated cavity segments 12', 14' and 16' which, as
described above, are mutually parallel and equally spaced
on the circumference of a circle centered at the long axis
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of the ~rame 10~
The existence of the gain region within the
designated segments follows from the ~act that each of
said segments also communicates through a bore with the
internal cathode 20. Bores 22 (shown as a point in Figure
2), 24 and 26 (shown as lines) connect: the interior of the
cathode 20 with the mid-points of the cavity segments 12',
14' and 16', respectively. By tapping the mid-points of
the selected seqments, equal and opposite current flows
are established in the discharge regions within those
segments providing a cancellation of the Fresnal-Fizeau
effects associated with a net ion flow.
Figure 3 is a cross-sectional view of the
multioscillator frame 10 taken at line 3-3 of Figure 1 and
of Figure 2. The orientation and shape of the internal
cathode 20 are apparent in this view. Furthermore, the
angular orientations of the bores 22, 24 and 26 are shown.
In addition to the features described with
reference to the preceding figures, getters 28 and 3Q
comprising barium-coated spring-mounted frames, are
enclosed within glass ceramic caps 32 and 34 respectively
that have been omitted ~rom the prior ~igures, are shown.
The getter caps 32 and 34 are fixed to the sur~aces L and
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G of the frame 10 respectively. Bores 36 and 38 connect
the getters with the cathode 20. The bores are collinear
with the cathode bores 26 and 24 and consequently may be
machined therewith. The nature of the geometry of the
internal bore structure accordingly minimizes
manufacturing complexity.
Figure 4 is an exploded perspective view of a
mechanical fixture and casing for mounting the
multioscillator frame 10. The fixture comprises an
assemblage that includes an upper magnetic field
generation assembly 39 that includes equally spaced
permanent magnets 40, 42 and 44, each of which has an
assoaiatQd field coil indicated by the corresponding
primed numeral. ThP elements of the upper magnetic field
generation assembly 39 are aligned with those of a lower
magnetic field generation assembly 45 that comprises
equally spaced lower permanent magnets 46, 48 and 50, each
again having an associated field magnet indicated by
primed numeral. In combination, the aligned upper and
lower assemblies provide a substantially uniform magnetic
field for shifting the centers of the gain curves in
frequency, as required for operation of a multioscillator
with mode suppression. The bulk of the magnitude of the
~ield (about 413 Gauss for a 20 cm path length) is
provided i~ each of the three cases by the per~anent
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magnet while the associated field coil produces a
relatively small, adjustable field for "fine tuning".
The gyro ~rame 10 is held and aligned between a
pair of support posts 52 and 52' that grasp it by the
periphery of the cathode plug 18. (As discussed supra,
the frame 10 i5 aligned so that the axis 88 thereo~ is
vertical.) The getter caps 32, 34 are held somewhat more
loosely by aluminum support posts 54, 54' and 55, 55'. As
mentioned, the upper and lower magnetic ~ield assemblies
39 and 45 are comprised of symmetrically spaced elements.
That is, there is an angular separation of 120 degrees
between each of the adjacent permanent magnet-and-field
coil arrangements of each assembly. This spaaing is
secured by means of an upper yoke 56 and a lower yoke 58,
each formed of a non-magnetic material such as aluminum
and including three generally-radially, equally-spaced
arms. Arcuate indentations span the regions of
intersection between the arms and provide a means for
maintaining the desired angular spacings between the
cyllndrical permanent magnet-and filed coil assemblies.
Screws ~0 fix the arms o~ the upper yoke 56 to
the support posts 52, 54 and 55 while screws 60' similarly
fix the arms of the lower yoke 58 to the lower support
posts 52~ 54' and 55' respectively. Spacers 62 and 62'
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rest atop the upper yoke 56 and the lower yoke 58
respectively providing separations between an upper disk-
like plate 64 and a lower disk-like plate 64'. Each of
such plates is fabricated of 50ft iron for accommodating
the large magnetic flux generated by the permanent magnet-
and-field coil arrangements. The plates 64 and 65 are
secured to the fixture by means of upper screws 66 and
lower screws 66' respectively and the fixture as assembled
is enclosed and contained within a cylindrical casing 68
of soft iron fabrication that provides return path of
large magnetic susceptibility for the flux generated by
the upper and lower magnet field generation assemblies
39 and 45.
Figure 5 is a simpliied view of the
multioscillator ~rame 10 within the casing 68 for
illustrating its orientation relative to the upper and
lower magnetic field generation assemblies 39 and 45. The
casing 68 includes a top 70, a cylindrical wall 72 and
disk-shaped bottom 74, each fabricated of soft iron that
form, in combination, a continuous return path for the
lines of magnetic flux generated by the assemblies 39 and
45. (The fixture for preferentially supporting the frame
10 within the casing is omitted in Figure 5 for purposes
of clarity.) By comparing the orientation o the frame 10
as shown in Figure 5 with that of Figure 2, it is seen
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that the selected cavity segments 12', 14' and 16' are
mutually aligned vertically within the casing as the frame
10 is held therein by the fixture so that its axis 88 is
vertical. Furthermore, since these predetermined cavity
segments are equilaterally disposed about the axis 88 of
the dodecahedral frame 10 (i.e. spacecl by 120 degrees
about a circle perpendicular to the axis 88), the fixture
is easily configured as in Figure 4 to hold the frame 10
so that each of the three predetermined cavity segments is
simultaneously aligned with vertically-diracted lines of
magnetic flux generated by one of the three sets of upper
and lower permanent magnet-and-field coil pairs that is
held in like orientation by the fixture.
Figure 6 is a schematic view that il:Lustrates
the field interactions that take place within the casing
between the imposed magnetic fields generated by the sets
of permanent magnet-and-field coil assemblies and the
multioscillator frame 10. Each of the magnet-and-field
coil arrangements of the upper and lower magnetic field :
generator assemblies 39 and 45 is aligned as shown so that
vertically oriented groupings of lines of magnetic flux 76
in free space are simultaneously aligned with the
predetermined cavity segments 12', 14' and 16'.
Accordingly, the requisite imposition of a uniform axial
magnetic ~ield upon the gain region is simultaneously
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achieved in each of the lasing cavities 12, 14 and 16.
The soft iron composition of the casing 68 which confines
flux line continuations 78 assures that the axial magnetic
fields are strong and uniform in the gyro frame region.
Figure 7 illustrates the interaction between
the magnetic field within the casing 68 and the
multioscillator frame 10 in accordance with an alternative
embodiment of the invention. In the embodiment of Figure
7, the upper and lower magnetic fielcl generation
assemblies comprise ring-like magnet-and-field coil
assemblies 80 and 82 fixed adjacent the top and bottom of
the casing 68. The ring geometry of the alternative
emhodiment functions is substantially the same way as the
assembly of the prior embodiment. However, due to the
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$ the arrangement of Figure 7, the frame 10 of the
multioscillator requires no particular orientation in the
horizontal plane. Rather it need only be aligned so that
the axis 88 is vertical to assure that axial magnetic
~ields are aligned with the gain regions of each of the
cavities 12, 14 and 16. As a consequence, the design of a
fixture for properly positioning the frame within the
container is somewhat simplified. This type of design is
more amenable to shorter path length frames~ Of course,
while the embodiments of Figures 6 and 7 will function to
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a greater or lesser extent even when the segments 12', 14'
and 16' are not precisely aligned with the three pairs of
magnet-and-field coil assemblies (i.e. the axis 88 is not
exactly vertical), optimum operation and control for
shifting the multioscillator gain curves in frequency is
only achieved when the selected segments are precisely
aligned.
Thus, it is seen that the present invention
provides a compact and relatively-simple apparatus and
method for measuring rotations about three orthogonal axes
in accordance with the mode of operation of a
multioscillator such as that disclosed in pending United
States patent application Serial No. 115,018 that requires
the imposition of a substantially uni~orm axial magnetic
field on the gain region of the ring cavity. By employing
tha teachings of this invention, one can simultaneously
impose substantially uniform axial magnetic fields on the
gain regions of each of three independent lasing cavities
so that the necessary shifting of the centers of the gain
curves in frequency to effect mode suppression is
independently achieved. By employing the invention, the
operational crlterion o~ a uniform axial magnetic ~ield is
simultaneously satisfied with respect to each of the three
non-planar multioscillator cavities. The invention avoids
the use o~ cut out regions in the frame and six-post
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magnet arrangements for encompassing the gain region with
khe correct ~ield.
While this invention has been described with
reference to its presently preferred embodiment, it is not
limited thereto. Rather, this invention is limited only
insofar as defined by the ~ollowing set of claims and
includes all equivalents thereof.
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