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BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention is related to rotary motion sensors and, more
particularly, is directed towards a rotary motion sensor which employs a
multiple turn fiber interferometer.
2. Description of the Prior Art
Various devices have been developed which utilize a fundamental phenomenon,
sometimes referred to as the Sagnac interferometer effect, the early
exploratory work in this field having been accomplished by Sagnac,
Michaelson and others. Sagnac demonstrated that light traversing a closed
path experiences an apparent path length change when the closed path is
rotated about an axis perpendicular to the plane containing the closed
path. He demonstrated that the apparent path length increased in the
direction of rotation and decreased in the opposite direction.
With the advent of the laser and coherent optical radiation, devices known
as laser gyroscopes have been constructed based upon Sagnac's theory, and
have been developed into precision instruments for measuring rotary
motion. In a laser gyro, a closed optical circuit or ring is established
by the laser and mirrors about which coherent light is propagated in two
directions, clockwise and counter-clockwise. The path lengths about the
ring are frequency determining parameters of the clockwise and
counter-clockwise lasing. Increasing the path length by rotation causes a
decrease in frequency, while decreasing the path length causes a frequency
increase. The beat frequency between the clockwise and counter-clockwise
laser beams is directly proportional to the rotational rate of the
apparatus.
The laser gyro, however, has not been widely adopted as a result of several
serious problems. One such problem is known as the lock-in problem which
results from the tendency of the clockwise (CW) radiation and the
counter-clockwise (CCW) radiation to lock together in frequency as the
rotation of the gyro slows until it reaches a point where the radiation in
the CW direction scatters from imperfections in the path and mixes with
the CCW radiation, and vice versa. When lock-in occurs, the rotation rate
can no longer be determined as a frequency difference. Many proposals have
been advanced, as evidenced by, for example, U.S. Pat. Nos. 3,846,025;
3,841,758; and 3,714,607, whose primary objectives are to obviate the
frequency locking problems of ring laser rotary motion sensors.
Up until recently, the interferometer did not receive much interest as a
technique for measuring rotation, since it is not as sensitive as the
laser gyro. However, with the development of glass and single crystal
waveguides for use with optical and infrared coherent radiation, the
interferometer having multiple optical turns has become a viable solution
for precise rotation measurements. Experiments performed at the University
of Utah by Vali and Shorthill have demonstrated the validity of this
concept, as published in the June, 1975 issue of Laser Focus.
The concept as suggested by Vali and Shorthill, while demonstrating
feasibility, nevertheless made no suggestion as to the manner of adopting
the principle to perform as a practical, field worthy device which may be
utilized, for example, in missiles for measuring rotation rate and
attitude. Present techniques for accomplishing the latter, for example,
require very expensive optical systems for tracking the missile by taking
photographs which must later be manually reduced to obtain information as
to the missile attitude versus time. In addition to being labor intensive,
the manual data reduction takes many months to accomplish, and requires
expensive labor and equipment.
It is therefore clear from the foregoing that it would be quite
advantageous if the multiple turn laser interferometer principle could
somehow be adapted for use into a rugged, field worthy device for use in
missles, aircraft, and other vehicles with limited space and very rigorous
environments. If adapted for use, for example, in tracking missle attitude
versus time, such a device could achieve more accurate data, at longer
ranges, and be far less expensive than presently used optical systems. The
data could additionally be obtained far quicker than presently available.
One of the difficulties encountered in the utilization of a fiber
interferometer for rotary motion sensing are the extremely high laser
frequencies with which one must deal. In a missle-mounted system, it may
be appreciated that it would be extremely advantageous if ordinary
electronic instrumentation could be utilized to detect and process the
measurement signals generated by such a device. However, the prior art
does not suggest how this may be accomplished with lasing frequencies on
the order of 10.sup.14 Hz.
OBJECTS AND SUMMARY OF THE INVENTION
It is therefore a primary object of the present invention to provide a
fiber interferometer rotary motion sensor which overcomes all of the
problems noted above with respect to prior art systems and techniques.
Another object of the present invention is to provide a rotary motion
sensor which utilizes a multiple turn fiber interferometer which is
compact, rugged, and field worthy for use in missiles, aircraft, and other
vehicles with limited space and very rigorous environments.
An additional object of the present invention is to provide a fiber
interferometer rotary motion sensor whose output signals may be processed
by ordinary electronic instrumentation.
A still further object of the present invention is to provide a rotary
motion sensor which utilizes a fiber interferometer whose output is
accurate, which is inexpensive, may be quickly reduced, and easily
installed into remote environments where such measurements need to be
made.
The foregoing and other objects are attained in accordance with one aspect
of the present invention through the provision of a rotary motion sensor,
which comprises means for generating a coherent optical signal at a first
frequency, a multiple turn fiber interferometer having first and second
ends for respectively transmitting the coherent optical signal in first
and second opposed directions therein, means for generating a reference
optical signal offset in a frequency from the first frequency by a second
frequency, means for mixing the coherent optical signals transmitted in
the first and second directions with the reference optical signal to
produce first and second signals each at the second frequency, and means
for detecting the relative phase shift between the first and second
signals.
More particularly, the means for generating the coherent optical signal
preferably comprises a fiber laser, and a first waveguide is also provided
for receiving the coherent optical signal from the fiber laser. The means
for generating the reference optical signal is preferably positioned
adjacent the first waveguide and also includes means for coupling a
portion of the coherent optical signal from the first waveguide into a
second waveguide also positioned adjacent the reference optical signal
generating means.
In accordance with yet other aspects of the present invention, the means
for generating the reference optical signal comprises an acousto-optical
waveguide having transducer means positioned at one end thereof for
generating traveling acoustic waves along the acousto-optic waveguide at
said second frequency. Means are also provided, preferably in the form of
a pair of directional couplers, for coupling the coherent optical signal
from the first waveguide into the first and second ends of the
multiple-turn fiber interferometer for producing a pair of signals
traveling in the first and second opposed directions in the fiber
interferometer. Means are also provided for extracting the pair of signals
from the multiple turn fiber interferometer after said signals have each
respectively traversed substantially the entire length of the fiber
interferometer.
More particularly, the extracting means comprises another pair of
directional couplers positioned respectively near the first and second
ends of the fiber interferometer for receiving the optical signals
traveling respectively in the second and first directions.
In accordance with another aspect of the present invention, the mixing
means preferably comprises first and second heterodyne detectors, which
may be diodes, each of which is connected to receive the reference optical
signal from the second waveguide, and which are connected to receive the
signals from the extracting pair of directional couplers, respectively.
BRIEF DESCRIPTION OF THE DRAWINGS
Various objects, features and attendant advantages of the present invention
will be more fully appreciated as the same becomes better understood from
the following detailed description of the present invention when
considered in connection with the accompanying drawings, in which:
FIG. 1 is a schematic representation which illustrates the main components
of a preferred embodiment of the fiber interferometer rotary motion sensor
of the present invention; and
FIG. 2 is an enlarged, detailed view which schematically illustrates one of
the components in the preferred embodiment of FIG. 1 and is helpful in
understanding the operation thereof.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
FIG. 1 is a schematic illustration of a preferred embodiment of the present
invention which consists essentially of a number of optical waveguides,
couplers, and other devices arranged in such a fashion so as to couple
optical energy into a multiple turn fiber interferometer, indicated by
reference numeral 30, and extracted therefrom in such a fashion that it
may be processed by standard electronic instrumentation to yield desired
information proportional to the rotary motion of the system.
The system includes a light source 10 which may, for example, comprise a
light emitting diode, for pumping a fiber laser 12 which produces a
coherent optical lasing signal at the frequency f. The fiber laser 12 may,
for example, comprise a miniaturized Nd:YAG laser such as, for example,
that developed by Chesler and Draegert of Bell Laboratories.
The fiber laser 12 drives a distribution optical waveguide 14 which
terminates in a directional coupler 16 and an absorber 18 for minimizing
back scattering. The waveguide 14 and directional coupler 16 are
preferably comprised of integrated optics which are becoming extremely
popular for use in miniaturized optical systems, as examplified by
Marcatili's U.S. Pat. No. 3,589,794.
The directional coupler 16 couples the coherent optical laser signal f into
one end 20 of a multiple turn fiber interferometer 30. The multiple turn
fiber interferometer 30 is comprised of an optical waveguide wound in a
helical configuration and has one end 20 terminated by an absorber 32 and
a second end 26 terminated by an absorber 34. The orientation of the
multiple turns of the interferometer 30 are not shown in FIG. 1 for the
sake of clarity, but it is understood that they lie in planes which are
substantially perpendicular to the axis of rotation of the system.
The signal coupled from directional coupler 16 into the end 20 of
interferometer 30 is designated by reference numeral 22 and is at the same
optical frequency as the initial laser signal f but is denoted by the term
f.sub.CCW to connote the counter-clockwise rotation thereof within the
fiber interferometer 30.
The other end 24 of the multiple turn fiber interferometer 30 terminates in
a directional coupler 26 which is positioned adjacent a portion of the
distribution waveguide 14 such that the optical laser signal 28 is coupled
thereinto from the waveguide 14. Note that the frequency of signal 28 is
the same as the frequency of the coherent optical signal generated by
laser 12, and that signal 28 is connoted by the term f.sub.CW to connote
its clockwise rotation within fiber interferometer 30.
Positioned adjacent a length of the distribution waveguide 14 is an
acousto-optic waveguide 36 down which an acousto-optic grating 74 moves as
driven by acoustic waves that are set up in the waveguide 36 by a
transducer 38 end-driven at an acoustic frequency F. The waveguide 36
terminates at a slight angle to minimize the effect of possible
reflections. An acoustic absorbing material 40 is also provided for the
same purpose.
The acousto-optic waveguide with the moving grating is provided as a means
for partially backward coupling the coherent optical signal f into an
adjacent directional coupler 42. The moving grating in the waveguide 36
also serves to offset the optical signal f by the acoustic drive frequency
F such that the signal received in the directional coupler 42, denoted by
f.sub.REF equals f-F. Directional coupler 42 also terminates in an
absorber 44 and leads to an optical waveguide 46 which is branched by a
power splitter 48 to deliver the optical reference signal f.sub.ref to a
pair of diode heterodyne detectors 50 and 60.
Means are also provided for coupling out the clockwise and
counter-clockwise optical signals from the multiple turn fiber
interferometer 30 after they have had an opportunity to substantially
completely traverse all the turns of the interferometer from one end to
the other. Reference numeral 56 indicates the counter-clockwise optical
frequency signal which originated at the other end of the optical
waveguide 20 as signal 22, while reference numeral 57 connotes the
clockwise optical signal in interferometer 30 after it has substantially
traversed the entire interferometer from its point of origination as
signal 28.
Another pair of directional couplers 52 and 54 are respectively provided
adjacent the ends of the fiber interferometer 30 as illustrated for
coupling out the signals 56 and 57, respectively, as signals 58 and 59.
Couplers 52 and 54, in turn, feed their respective signals 58 and 59 to a
pair of optical waveguides 62 and 64 which, in turn, deliver the signals
to diode heterodyne detectors 50 and 60, respectively. Couplers 52 and 54
also include absorbers 66 and 68, respectively, for minimizing
disturbances.
It may be appreciated that rotation of the multiple turn optical waveguide
fiber interferometer 30 about an axis normal to the plane of the turns of
the waveguide will cause the apparent length of the waveguide to increase
in the direction of the rotation and decrease in the opposite direction.
This will, in turn, impart a phase shift to the signals 56 and 57 in
proportion to this apparent path length change and, thus, the rotation
rate. Note that signals 56 and 57, having substantially fully traversed
the multiple turn fiber interferometer 30 from their respective initial
positions 22 and 26, have achieved a maximum phase shift due to rotation.
These phase-shifted signals 58 and 59, of the opposite sense, are each
heterodyned in detectors 50 and 60, respectively, with the reference
optical signal f.sub.ref which, it should be remembered, has not suffered
a phase shift due to rotation.
Therefore, the output frequency of heterodyne detector 50 conists of a
signal F.sub.CCW which is rotated in degrees the same amount as signal 58.
However, the frequency of F.sub.CCW is identical to the acousto-optic
drive frequency F impressed upon the acousto-optic grating 36.
Accordingly, F.sub.CCW is a relatively low frequency signal rotating in
phase the same as the high frequency signal f.sub.CCW. However, F.sub.CCW
is a much more manageable type of signal in terms of the capability of
detecting and processing same by conventional electronics.
Similarly, diode heterodyne detector 60 mixes the signal 59 with the
reference frequency signal f.sub.ref to produce the heterodyned difference
frequency F.sub.CW which is a relatively low, acoustic frequency signal
that is rotating in phase the same as the high frequency signal f.sub.CW.
The phase relationship between F.sub.CCW and F.sub.CW is therefore the
same as that between f.sub.CW and f.sub.CCW. The output from detectors 50
and 60 are therefore fed to a conventional electronic phase shift
measuring device 70 for detecting and measuring the relative phase shift
of one signal with respect to the other, which is a direct measure
proportional to the rotary motion of the system. The device 70 may
comprise, for example, a convention stop and start counter, a harmonic
measuring device, or the like.
All of the components illustrated in FIG. 1 are preferably encapsulated in
a matrix having an index of refraction n.sub.2 which is slightly less than
the index of refraction n.sub.1 of the optical waveguides and whose
acoustic velocity is slightly faster than that of the acousto-optic
grating 36. These characteristics, along with transparency make the
waveguides 14, 46, 30, 62 and 64 optical waveguides, while grating 36 is
an acoustic waveguide. The characteristic of an optical waveguide is that
energy traveling at a small angle to the center line of the guide leaves
the core of the guide but is deflected by the faster (lower index of
refraction) matrix surrounding the core of the guide back into the core.
The region of the matrix next to the core of the guide, therefore, carries
radiation with small angular excursions. The acousto-optic grating 36
similarly guides acoustic waves which are generated by transducer 38.
Acoustic waves and optical waves, therefore, both occupy the region
between the guides. The optical waves are scattered backward by the
acoustic waves according to: .lambda./.LAMBDA. = sin.theta.+sin.theta.'
where .lambda.is the wavelength of the laser signal f in the matrix,
.LAMBDA. is the wavelength of the acoustic signal F, .theta. is the angle
of propagation of the radiation f with respect to the center line of
grating 74, and .theta.' is the angle of backward deflection of the
radiation f as it is scattered by the grating 74.
FIG. 2 shows an enlarged sketch of the backscattering technique. Radiation
f is moving at a small angle into the matrix surrounding guide 14 where it
scatters off of the acoustic waves 74 which are moving into the matrix
surrounding the waveguide 36. The optical guides in FIGS. 1 and 2 are
considered to be single mode which means that the angle of the radiation
with respect to core center line never exceeds some very small angle such
as 2.degree.. The acousto-optic driving frequency F can be determined from
the equation .lambda./.LAMBDA. = sin.theta. + sin.theta.'. As an example,
the waveguide 36 might be made of YAG (Yttrium Aluminum Garnet; .eta. =
1.83, velocity of acoustic propagation = 8.6 .times. 10.sup.3
meters/sec.). With .theta. = 2.degree. and .theta.' = 2.degree., the
wavelength is 8.295 microns. The acoustic drive frequency F would,
therefore, be 8.6 .times. 10.sup.3 /16.590 .times. 10.sup.-6 or 0.518377
.times. 10.sup.9 Hz.
The optical waveguides operate single mode when (2.pi.a/.lambda.)
(n.sub.1.sup.2 - n.sub.2.sup.2)1/2 is less than 2.4; where a is the radius
of the waveguide core, .lambda. is the freespace wavelength of radiation,
n.sub.1 is the index of the core, and n.sub.2 is the index of the matrix.
Using YAG as an example of an optical waveguide and a core radius of 20
microns, the matrix index of refraction n.sub.2 would have to be 0.0001122
less than the core for single mode operation.
The fringe shaft, S, of a multiple turn interferometer is equal to
4.pi.R.sup.2 N.OMEGA./.lambda. C, where R is the radius of the turns, N is
the number of turns, .pi. is the rotation rate, .lambda. is freespace
coherent radiation wavelength, and C is the freespace velocity of light.
This can be rewritten to 2 RL.OMEGA./.lambda. C where L is the total
length of the multiturn interferometer 30. For greatest sensitivity, L and
R should both be maximized. The length L is normally limited by laser
signal level, losses, and waveguide imperfections to a few thousand
meters. The radius R needs to be very large for high sensitivity but very
small for a compact field use interferometer. The radius is, therefore, a
compromise.
Measuring fringe shifts at the laser frequency is difficult and/or
cumbersome but it becomes much easier at frequency F where F is 518.377
MHz as in the above example. The angular phase shift between F.sub.CW and
F.sub.CCW is the same as between f.sub.CW and f.sub.CCW. The periods,
however, differ by F/f. In the example, the ratio is 2.830 .times.
10.sup.14 /0.518 .times. 10.sup.9 or 5.459712 .times. 10.sup.5. The time
phase shift at the lower frequency is, therefore, about 5.46 .times.
10.sup.5 times as long at the higher frequency. The frequency F (518.377
MHz) driving the acousto-optic waveguide 36 may, if desired, be divided
down by a factor such as 2.sup.20 to obtain a still lower reference
frequency of 494.3628 Hz. This low frequency may be added to the 518.377
MHz reference and heterodyned with the frequency F detected by D.sub.2 and
D.sub.3 of FIG. 1. The time phase shift measured at 494.3628 Hz is longer
by 2.sup.20 than that measured at frequency F. The present invention thus
lends itself to electronic processing to greatly enhance the resolution of
the multiturn fiber interferometer.
Obviously, numerous modifications and variations of the present invention
are possible in light of the above teachings. It is therefore to be
understood that within the scope of the appended claims, the invention may
be practiced otherwise than as specifically described herein.
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