Note: Descriptions are shown in the official language in which they were submitted.
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BACKGROUND OF THE I~IVENTIO~I
1. Field of the Invention. This invention relates to
improvements in interferometer gyros, and more particularly, to an
improved interferometer gyro in which phase detection between
counter-rotating light beams in a light path are combined with another
signal to facilitate phase detection therebetween.
2. Description of the Prior Art. Laser gyroscopes, or
gyros, known in the art of interest herein, are of two types, the
so-called rind laser gyro and the ring interferometer gyro. Both types
10 ~ of gyros are illustrated and described in United States Patent 4,013,365.
Briefly, the interferometer gyro, the gyro to which the
present invention pertains, can be represented by a structure in which
light from a laser light source is divided, each portion of the divided
beam being constrained to travel in a counter-rotating direction around a
path. If the path experiences a rotation, the apparent length of the
path in one direction traveled by one portion of the beam is longer than
the true path length while the apparent length of the path in the other
direction is shorter than the true path length. This is the so-called
Sagnac effect. The apparent path length change results in phase shifts
in the light traveling the paths ln each direction from that which would
be experienced if the path were merely at rest. The phase difference can
be measured, and from this measurement, the rotation of the path can be
determined.
In the prior art devices, the light beam portions which have
traversed the light path in counter-rotating directions are cornbined
after traversing the desired path. The combined waves add to or are
subtracted from each other, depending on the particular degree of phase
shift experienced durlng the traversal of the light path. The output,
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therefore, is ~erely 11ght o~ vari2~,1e intensjty, the intens1ty variation
indicating the sp~ed or rate of rotation of t~e uptical path. It can
be s~en, therefore, that the prior art devices ar~ singular1y light
sensitive. That is, if the light source generating the light ~/hich
is divided and introduced into this signal path e~periences a change in
intensity, the output li~e~lise will charlge in its intensity, which rnay
be interpreted as a rotation of the light path. Therefore, to assure
accuracy of the gyro, especially in inertial guidance uses and the like,
particular steps must be taken to carefully monitor the intensity of the
li~ht source.
BR7EF DESCRIPTIOi~l Or THE INVENTION
In light of the above, i~ is, therefore, an object of
the invention to provide an impro~ed interFerometer gyro.
It is another object of the ;nvention to provide an
interferometer gyro which ;s relatively ;nsensitive to variations in
the intensity of the source of light.
~ ' It is another object of the invention to provide 2n
interferometer gyro having a reference signal source for compar;son
against the light travel;ng in counter-rotating directions in an optical
path to determ;ne the phase shift thereof and the rate of rotation
of the opt;cal path.
lt is another object of the invention to provide an
inter~erometer gyro of the type described which can be fabricated usir,g
integrated optics and integrated electronics circuitry techniques.
These and other objects, features and advantages of the
present invention will be apparent to those skilled in the art from ~he
following detailed description when read in conjunction with the
accompanying dra~ing and appended claims.
The invention, in its broad aspect, presents an
lnter~eroneter gyro. The gyro includes a so~rce of liyht and an optical
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path ~ircumscribing an area into ~Jhich first and second portions of the
light are coupled to traYerse thè path in opposite directions. A
reference light source is provided ~lith ~Ihich the light coupled in the
optical path is combined after it has traversed the ~,th to produce
phase differences therebet~,/een ~or comparison to indicate the
rotation of the path. Means are also provided for measuring the phase
difference bet~leen the light after traversing the path in each direction
combined ~ith the light of the reference light source.
In another err,bodiment of the invention, the light of
the siqnal source is divided and modulated with the rirst and second
frequencies to provide respectiYely a s;gnal source and a reference
source. The sisnal source is divided and applied to an optical fiber
in counter-rotating directions. After the light has traversed the optical
fiber, it is compared ~Jith the light of the modulated reference source
and the phase difference therebet!~een,determined,,to indicate the rate of
rotation of the optical fiber about an axis,of sensitivity.
BRIEF DESCRIPTIO~I OF THE DRAWING
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The invention is illustrated in the accompanying
dra~ling ~herein
FIGURE 1 is a schematic diagram of a ring laser inter-
'' ~erometer gyro in accordance with the prior art.
FIGURE 2 is a schematic diagram illustrating ~he ring
laser interferometer gyro in accordance with one aspect of the invention,
and
FIGURE 3 is a schematic diagram of a preferred embodiment
of a ring laser interferometer g,yro in accordance with the principles
of the invention.
In the various figures of the drawings, light paths are
indicated by dashed lines and ~lectrical conduction and mechanical elements
are indicated by solid lines.
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DETAlLLr~ DES5RIPTi~ Of T~E P~EF~ E~ E~ODI!1.~ S
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Laser inter~erometer gyroscor1es, or gyros, are well
known in the art. A typical laser gyroscope of the prior art is shown
~iagramatically in FIGURE 1. The interferometer gyro 10 of the prior
art includes three reflective surfaces ll, 12 and 13, carried upon a
common inertial frame (not shown). Light frorn a laser light source is
reflected from each of the reflective surfaces 11-13 in counter-rota-tir.g
directions. This is achieved by allowing the light from the laser
` source 16 to impinge upon a bea~ splitter 17, which sends a first
- lO portion of the light beam from the laser 16 in the direction indicated
by the arrow 18 and a second portion in the direction of the arrow 19.
The light traveling in the direction of arrow 18 is reflected from,
in order, surfaces 11~ 12 and 13, then rè-rPflected from the beam splitter
17 to be detected by a detector 22. The second portion of the beam which
travels in the direction of the arrow 19 is reflected, in order, from
reflectiYe surfaces 13, 12 and 11, to thereafter pass through ~he beam
splitter-17 in the direction of the detector 22. The two beams~ after
traveling their respective counter-rotating paths, produce an interference
fringe shift or intensity change at the detector 22. The interference
fringe shift produced is of a form in which the intensity of the light
detected at the detector 22 varies in accordance with the rate of angular
rotation o~ inertial frame carrying the reflective surfaces 11-13. The -
rotation of the inertial frame is indicated by the c;rcular arrow 24.
. The interference pattern is produced by virtue of the apparent difference
in path length seen by the counter-rotatin~ beams of light due to the
angular rotation of the inertial frame, The interferometer gyro of the
pr;or art, however, suffers the disadvantages above discussed, and it is
to overcoming these disadvantages that the present invention is directed,
as presently discussed
In accordance with one aspect of the invention, an
interferometer gyro 30 is shown in FIGU2E 2. The interferometer gyro 30
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in a f~shion si~,ilar to that of ~he inJ~erfcrome-t~r g~ro 10 abo~/e
described ~ith reference to FIGURE 1, includes a light path or rir:g
34 de~ined at three corners by reflective surfaccs 31, 32, ar,d 33~
carried by an inertial frame (not sho~m). A beam split~er 35 is located
on the vertical frame at the fourth corner of the light path to complete
the ring to divide light from a source of light~ such as the laser source
36 illustrated, into t~o beams and to inject the light into the light
path in counter-rotating directions, denoted by arro~s 38 and 39. The
ring 34 is defined by the reflective surfaces 31-33 and beam splitter 35
forms an optical path enclosing an area having an axis (perpendicular
to the page) of sensitivity the rate of rotation of the optical path
about wh1ch is desired to be measured.
Two beam splitters 42 and 43 are proYided in the light
path to intercept the counter-rotating light from ~/ithin the li~ht path
to remove the counter-rotating light after it has completed, or substan-
tially comple~ed its travel around the ring 34. Thus, the portion of
the light Fro~, signal source 36 traveling in the direction of ~he arrow
. 38 leaves the beam splitter 35 to be passed by beam splitters 425 then
reflected, in order, from reflective surfaces 31, 32 and 33 to impinge
2Q upon the beam splitter 43 to be removed from the light path in the
. direction of the arrow 45. In l;ke fashion, the portion of the lignt
passing the beam splitter 35 in the direction of the arrow 39 is
passed by beam splitter 43, then reflected from reflective surfaces 33,
32, and 31, to be removed from the light path by the beam splitter 42,
in the direction of the arrow 46.
In 2ddition to the light from the light source 36, an
additional or reference light source 50 is provided in the interferometer
gyro circuit 30. The light from the second light source 50 is divided
by a beam splitter 51 into two portions, one of which is directed to a
3Q beam splitter 52 and the other of ~hich is directed to a bea~ splitter 53.
.
Tl-~e beam splitters 5~ arld 53, in addition to receiving the divided
light beam from the source 50; are located in the light paths to
receive the light deflected by beam splitters 42 ar,d 43, respecti~ely.
The result, therefore, is tnat the light removed from the ring ir, the
counter-rotating directions, traveling in the direction of the arro~Js
46 and 45, is cGmbined and cornpared ~Jith the reference beams frotn the
light source 50. This comparison is done respectively by detecting the
combination of the light removed from the ring and the reference beams
on respective photodiodes 56 and 57. Again~ since the interferency
yyro produces light of changed phase due to the angular rotation of the
iight path, upon combination ~Jith the reference beam from the source 50,
- l~ght of intensity dependent upon the rate of rotation will be detected
upon photodiodes 56 and 57. The output from the photodiodes 55 and 57
jc then applied to ampllfiers 59 and 60, as electrical signals, and the
lS Fhase difference determined by a phase detector 61. The output of the phase
detector 61, therefore, represents the phase difference of the ,ight signals
removed from the ring, and, therefore, represents the angular rotation
of the ring. (In fact~ since the reference beam is constant, and the
iight removed from the ring in each direction experlences the same phase
shift, but with different directions or signs, the output from the phase
detector 61 ~lill represent t~rlice the phase difference produced by virtue
of the rotational rate of the ring.)
In the embcdiment shown in FIGURE 2, the signal light
source 36 is referred to as a "signal" source and the reference light
source is referred to as "L0" or local oscillator. This is to ;llustrate
the analogous nature of op~ration of the interference gyro 30 to 2
hetrodyne electrolllc pr~cessing operation That is, the light from the
signal source is "modulated" during its traversat of the pa~h around the
ring, and is then "mixed" ~Jith a reference signal or local oscillaton
signal, in ~ fashion similar to whcl~ is ordinarily thought of in term;
of hetrodyning one electronic signal upon another.
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Since a con~arison is madc bet~leen the p~ase dirterrlc~s
b~tween the light r~noved fr~n eac~l counter-rotating directiorl and the
re'erence wave, the interferor(leter ~yro enlbodiment shown in FIGU~ 2
is essentially intensity independent. For exarllple, if the intensity of
the light from the signal source 36 ~ere to ~e decreased for some reason,
the two outputs, each compared to a reference waYe, produces a phase
dlfference w!nich will be essentially the same even though the ;ntensity
cf the source were to be changed. Thus, the operation of the d~vice
need not be as closely monitored as that required of the prior art and,
there~ore, inertial grade interferometer gyros can be easily achieved.
In addition, since two output sisnals are deriYed, any ambiguity which i5
exhibited by an intensity change at the output can be resolved to determine ,
the direction of rotation of the device by develop;ng quadrature signals,
a techn;que known in the electrorlics and navigat;on arts.
A preferred embodiment of the r;ng laser interferometer
gyro embodying the principle; of the present invention is shown in FIGURE
- 3, and ;s des;gnated generally by the reference numeral 7C. Embodim~nt
. of FIGUR~ 3 is sho~n and descr;bed as being of an ;ntegrated electronic
circuit and integrated optics format, however~ the principles described
herein are equally achievable with discrete components, as will be
~pparent to those skilled in the art. ,~
A source of laser light 71, which can be a heiium neon 9dS
-l~ser or a d;stributed feedback g311;um arsen;de iniect;on laser, produces
an input beam ~Ih;ch is inc;dent upon a 'irst beam splitter 72. The beam
incident on t'ne beam splitter 72 is split ;ntb two portions, one of which
is directed in the direct;on ind;cated by the arrow 74 and the othér of
which is directed in the direction indicat~d by the arrow 75. The portion
of the beam directed in the direction of the arrow 74 impinges upon an ~¦
acousto-optic modulator 78, which can be, for example, a surface elastic
wave zinc oxide ~InO) interdigital transducer. It should be apparent to }
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those skilled in the art that other types of optic modulators can
be equally advantageously employed such as, for e~.ample~ electro-
optic modulators and the like.
The acousto-optical transducer 78 receives a signal
from signal source Z0 of, for example frequency ~2 Thus, the light
- output ~rom the acousto-optic modulator 78 is a light beam of frequency
egual to the frequency wO of the source 71 plus the frequency of the
modulating frequency source ~2 This signal is hereinafter referred to
as the "signal frequency".
The light beam at the signal frequency from the acousto-
optical modulator 78 is divided by a beam splitter 79 into t~o beams which
are directed by beam splitters 81 and 82 into a light path 84 in counter-
rotating directions. The light path 84 in the ernbodiment illustrated is
defined by an optical fiber. The optical fiber can be of the type
com,-nercially available and, although a multi-mode fiber can be used, the
fiber preferably should be a single mode-fiber for the most efficient
operation. The fiber can be of multiple turns to define a path about
which the si3nal can be applied to travel in counter-rotating directions
therearound. As in the case of prior art interferometer gyros, the
sensitiY;ty of the gyro is increased by increasing the number of turns
; of the fiber. It has been found, fpr example, that the optical fiber can
include a number of turns to utilize a length of approx;mately one
kilometer and exhibit satisfactory or suitable operation.
-The signal beam, after traversing the light path 84, is
~5 removed and directed onto photo-detectors such as photodiodes 86 and 87,
together with a reference ~eam presently to be described.
The-reference beam above referred to is generated by the
second portion of the signal source reflected from the beam splitter 72
along the pa+h indicated b~ the arrow 75. That beam is directed to a
s~cond acousto-optic modulator 88, of similar construction to the acousto-
optic modulalor 7~, and ~/hich receiYes a signal frorn a siynal
gellerator 89 at d frequency, for e~.3mple~ of ~1 Thus, the output
iro~ the second acousto-optic ~,odulator ~8 is a light beam of
frequency ~Jo ~ ~1 This reference beam is di~/ided by a beam
splitter 90 into ~.~o be~ms, each of which i5 directed to a respectiYe
one of photodiode 86 ar,d 87 by beam splitters 92 and 93. Thus, the
signal ~hich is applied to photodiode 86 is ~lo ~ and, like:lisef,
the signal which is applied to photodiode 87 is ~0 ~ wl f 0. The term
0 represents the phase difference undergone by the signal in intraYersing
the optical fiber 84 due to the rotation of the optical fiber about
its axis of sensitivity (an axis perpendicular ~o the plane in ~/hich
the optical fiber is configured).
The outputs from the photo diodes 86 and 87 are applied
respectively to amplifiers 95 and 96, the outputs of ~/hich are co~,pared
1~ by a phase detector 97 to produce an output indicative of the phase
~ifference, or as above stated, t~lo times the phase di~ference due to
the sum and difference thereof developed in the optical fiber.
As above stated, the electronics of the signal generators
` $0 and 89, the amplifiers 95 and 96, as well as the phase detector 97,
can be fabricated as a part of an integrated circuit ~hich can be
conveniently fabricated as a part of the optical circuit above described.
- In addition, since the output of the interFerometer gyro
70 appears as a voltage proportion~l to the phase difference bet~leen
the two lignt beams, numerous methods exist for electronically processir.g
2S the sign~l which may read out a function proportional to the phase
developed. For example, a~high frequency clock started by the phase in
one channel could be developed, the count being stopped by the phase ;n
the other channel at a zero crossing point. The high Frequency clock
could then be counted by a simple cfounter which ~Jould, as an output,
3~ yield a value equal to the time period representing the difFerential
phase.
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. Althougll the in~/ention has been descri~e~ and illus-
trated with a cer ~ in deyree of particularity, ;t ;s un~ers-tood th~t
numerous changes in ~he com~ination and arranyement of parts ma~ be r~sorted
. ~o by those skilled in the art without departing from the spirit an~ scope
of the inventions hereinafter claimed.
WHAT IS CLAII~,ED IS:
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