Note: Descriptions are shown in the official language in which they were submitted.
CA 02361394 2001-10-22
Field of the Invention - fiber-optic gyroscope:
It is generally known that fiber-optic gyroscopes are based on the constant
speed of light
to any of the referential measuring point. That is the light propagating in
the fiber with
the velocity c will keep propagating at that speed even if the fiber will
start to move
forward (in the direction of traveling light) or backwards. Based on this
principle
rotational speed, as well as straight line acceleration can be measured by the
help of light.
The fiber-optic gyroscopes are currently used on large ships, big planes and
some
to luxurious cars to complement GPS system. But they did not manage to
penetrate these
markets very deeply (smaller ships, planes and cars) due to the fact that they
are quite
expensive.
I believe that my Fiber-optic homodyne gyroscope will be considerably cheaper
to make
is as well as more accurate than other currently available commercial
gyroscopes. As soon
as they will become cheaper, their usage will increase considerably. At first
it will
complement the currently cheaper GPS system (as a general nature of people -
people
like to compare results from more than one source to be sure of its
reliability/accuracy)
and eventually it may even supplement it.
2s
CA 02361394 2001-10-22
Many designs of fiber-optic gyroscopes based on the above description have
been
suggested. Nevertheless they seem to be quite complicated and expensive to
make. What
makes them so is the fact that they have only one sensing loop for clock and
2o counterclockwise propagating waves. This has a couple of consequences:
~ The laser has to send a very short pulses or the modulator has to be used to
generate
these short pulses.
~ After traversing the sensing loop the two light beams recombine in the same
phase
(when the gyro is not rotating).
25 ~ When the gyro is rotating a mutual phase shift of clock and
counterclockwise waives
is going to occur. Unfortunately the mutual phase shifting of the two coherent
waves
around the same phase point produces a very small interference pattern
changes. The
various modulating methods had been suggested to overcome this problem.
~ Gyroscopes with only one sensing loop are quite difficult to construct.
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CA 02361394 2001-10-22
I have found that the above mentioned disadvantages can be overcome by
spliting the
light into two beams which propagate through separate clockwise and
counterclockwise
fibers. The clockwise and counterclockwise fibers are of different length so
that for one
particular wavelength (that wavelength is called ~,'~'~) the difference is
just ~,'~'"'~
*(n~1/4) - (n being a whole number). The tunable laser is used to find that
~,'~"~'"~
wavelength. Tunable laser is then replaced by the fixed wavelength laser at
wavelength
~,F which should be as close as possible to that of ~,'i°a~'"re. It
does not have to be exactly
the same wavelength. The consequences of ~,F being different from ~,'""~ will
be
discussed in the interference section under Specifications part.
When this gyroscope is not rotating, the split light from the 3dB sputter will
traverse the
clockwise and counterclockwise fibers and arnve at the interference region
(see Figure 1 )
with a certain amount of the mutual phase offset ideally by ~,F* (n~t/4).
These two offset
coherent light signals will then interfere with each other within the
interference region so
that when leaving the interference region their intensity is going to fall to
about one half
of its pre-interference region value. This will be reflected on the readings
on power-meter
P 1 (and/or P2) (see Figure 1 ).
When this gyroscope will rotate, the above mentioned mutual phase offset of
light
2o coming from clockwise and counterclockwise fibers will get either bigger or
smaller.
When the mutual phase offset gets smaller, the power on P1 (and/or P2) will
increase and
when the mutual phase offset gets bigger the power on P1 (and/or P2) will
decrease.
The gyroscope is then calibrated on a rotational table. The gyro is rotated
while the
values on PI (and/or P2) are read in certain intervals into the computer which
will then
analyze these readings. The calibration process will also determine the proper
length of
the reading intervals.
Note: for certain rotational speeds (common for certain applications) there
are going to be
corresponding ideal reading intervals.
4
CA 02361394 2001-10-22
To sum up the advantages of this Fiber-optic homodyne gyroscope:
~ This gyro allows for continuous operation of the Laser.
~ High sensitivity to rotation of this gyro comes from the fact that the
signals from
clock and counterclockwise fibers will meet in the interference region offset
by about
~.F* (n~1/4).
~ The process to construct this gyroscope is very easy and therefore cheap.
~ It does not require overly accurate optics - the design of this gyroscope
eliminates the
use of expensive optical parts.
to ~ The degree of accuracy for certain rotational speeds of this is gyro can
be optimized
by the appropriate reading speed on Pl (and/or P2) power-meters.
The embodiment of this Fiber-optic homodyne gyroscope can be seen on Figure 1
on the
following page. It consists of a laser (with predetermined power/wavelength
stability and
is coherence time), polarizer (to produce predetermined polarization state),
fused 3dB
sputter, circular mandrel of predetermined diameter around which the fibers
are wound
clockwise and counterclockwise in predetermined number of times, interference
region
(consisting of fused fibers fabricated as described in specifications) and at
least one
power-meter P 1 and/or P2 (photo-detector) to measure the intensity of light.
Different
2o parts of this gyro are described in passages in Specifications part
following Figure 1.
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Specifications
The Fiber (1) and the optical signal wavelength:
Regular 9/125/900pm single mode optical fiber may be used. However the use of
fibers
with smaller cladding diameter (such as Flexcore fibers which have cladding
diameter of
250pm) may be advantageous for the construction of interference region.
Nevertheless,
Flexcore fibers are more fragile to handle. The tradeoff between the fibers
should be
considered before choosing one for the construction of this gyro.
0
The wavelength at which this gyro will operate will depend on many factors
such as fiber
used for the construction of this gyro, as well as other factors which will
become apparent
from the specifications described below.
15 Note: In general the shorter the wavelength the more sensitive the gyro is
going to be to
turning. On the other hand, longer wavelengths are less sensitive to
imperfections and
have therefore steadier interfering characteristics.
Note: The type of fiber will most likely determine at which wavelength we will
want the
2o gyroscope to operate, and therefore at which wavelength range we are going
to look for
appropriate wavelength with the help of tunable laser.
Laser (2):
25 Tunable laser shall be used at the construction stage of this gyroscope.
Tunable laser is
used to find a quadrature point of this gyro - at this point a mutual
wavelength offset
from light signals entering the interference region from clockwise and
counterclockwise
fibers is about ~, /4. When the quadrature point is found (at wavelength
~,'~"~'"'~ ) and the
wavelengths range (~,'~'"'~ +/- b) within which the wavelengths are reasonably
close to
o the qudrature point - and therefore about ~, /4 offset - tunable laser may
be replaced with
~O
CA 02361394 2001-10-22
regular fixed wavelength laser which will be operating at the wavelength which
will fall
within the (~,'"'~'~ +/- 8) wavelength range (see Figure 2).
Note: it does not have to be exactly at ~,'~~ wavelength. Tunable laser's main
function
is to find either a ,wavelength or a wavelength range within which the power-
meter readings on P1 (and/or P2) are the most sensitive to the change of the
wavelength.
Polarizer (3)
Will considerably improve the performance of this gyro for many reasons. Some
of
which are:
~ The 3dB sputter will split the light more steadily.
~ The 3dB splitter and the interference region will give better mutual
performance -
more stable readings on P 1 (and/or P2 ) power-meters.
~ It can also be used as an optimization element at construction stage by
slightly
turning it in order to adjust readings on Pl (and/or P2) power-meters.
Note: The polarizer does not have to be used when looking for ~,by the help of
2o tunable laser. Once the ~,is found (or simply the wavelength range at which
the
readings on P1 andlor P2 are not flat) then the corresponding ~,F is chosen
and the proper
polarizer for ~,F is determined.
3dB sputter region (4)
It is highly recommended that the fused 3dB splitter be used so that the two
split light
signals are as close to each other (form-wise, coherent-wise) as possible.
Note: When one is using only Pl or P2 power-meter one has to take into account
slight
3o variations in splitting ratio for difl'erent wavelengths at the
construction stage when
tunable laser is used_
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Fiber-loop A (5) and Fiber-loop B (5)
One fiber from the 3dB sputter is wound clockwise around the mandrel Fiber-
loop A
while the other fiber is wound counter-clockwise around the mandrel Fiber-loop
B (see
Figure 1 ). Both fibers are then fi~sed together in the interference region.
As the (Figure 1 ) shows the Fiber-loop A has a slightly larger diameter than
the Fiber-
loop B which is done only for better illustration so that one can better see
the actual
o arrangement of the fibers. In reality these two fibers are wound around the
same
mandrel.
The only requirement on the length of clockwise and counterclockwise fibers is
that they
must be of slightly different length (at least by as much as ~,F l8 - optimum
is ~.F /4 ).
1 s The wavelength ~,F is the operational wavelength of the gyroscope. The ~,F
/8 is such a
small distance that in reality one has to pay more attention to the fact that
these two
fibers must be of as much as possible the same length, but for the required
difference of
the above mentioned ~,F /8. This difference in length will mutually offset the
light phases
from clockwise and counterclockwise fibers entering the interference region.
Within the
2o interference region these two light signals will mutually interfere so that
at the end of the
interference region their intensity is going to be about one half of their pre-
interference
region value.
When this gyroscope will start to rotate, the above mentioned mutual phase
offset at ~,F
2s will either increase or decrease which will move the readings on P1 (or P2)
correspondingly lower (if the phase difference moves closer to ~,F(n+1l2)) or
higher (if
the phase difference is moved closer to t>a,F).
When under construction and when tunable laser is used the above mentioned
length
3o difference of clockwise and counterclockwise fibers will insure that when
not rotating (as
gyro) the different wavelength from the tunable laser will be reaching the
interference
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CA 02361394 2001-10-22
region in different offsets. The ~,(n+1/2) shifted wavelength will be the most
attenuated
and no shifted (or n7~, shifted) wavelength will be the least attenuated. The
n(~.+1/4)
wavelength shifted will be in quadrature point (~,'~'"'~ ) - medium
attenuation - so that
slight increase or decrease in the wavelength will cause the readings on PI
(or P2) to go
up or down correspondingly. The operational wavelength ~.F of this gyroscope
should be
as close as possible to ~,.
On the other hand, the clockwise and counterclockwise fibers must be as close
to each
other in length but for above mentioned ~,F /8 difference for three particular
reasons:
i0
One: The Free Spectral Range - shortly FSR (explained in the section for
interference
region) of 3dB sputter and interference region is going to be very large and
therefore the
quadrature point is going to be stretched out. In other words the larger
wavelength range
is going to satisfy the condition that the light signal in clockwise and
counterclockwise
is fibers will be mutually offset by about aJ4 (see Figure 2).
Two: The effect of the wavelength drift of the laser on the accuracy of this
gyro is going
to be minimized.
2o Three: Will insure that the interfering light beams are going to be
mutually coherent.
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CA 02361394 2001-10-22
Interference region (6):
As one can see from (Figure I ) the interference region is located further
away from the
loops than the 3dB splitter. This is only for better illustration purpose. In
reality they can
be placed more conveniently above one another.
This region should be fabricated as follow (see the Figure 3). the clock and
counter
clockwise fibers will be wounded around each other:
to The tunable laser shall start sweeping over its wavelength range and before
the heating
process is started it should be noted what power at which wavelength we are
reading on
the power-meter Pl and/or P2. The interference region should decrease that
reading
(power-meter measure the intensity of light) by up to 50% at ~,'~'~
wavelength.
~ 5 The heating process should then start at the end where the fibers are
entering the
interference region from the fiber spools and in a sweep-like manner continue
from this
end towards the other end As the build up of interference region continues the
power-
meters P1 and/or P2 will start to show sinusoidal type of power drops (see
Figure 2). This
will go on until the readings on power-meter P 1 and/or P2 will drop to about
one half at
2o any of the quadrature points - may be only one (see Figure 2).
At this point the tunable laser can be replaced by the fixed wavelength laser
at operating
wavelength ~,F.
Why one half. What is happening within the interference region is that signal
Il see
25 Figure 3 will start leaking over to the fiber carrying I2 signal and vice
versa. So that
farther down the interference region ~/2 of I1 will have leaked to the fiber
carrying I2
while'/Z of I~ will have leaked to the Il fiber. Since the Il and I2 signal
are coherent and
about ~,/4 offset their electrical vectors are at right angle. So that at the
beginning of the
interference region we have: IT (total intensity) = E12 = I l . At the end of
the interference
o region we get total intensity at IT/2 since: (E1/2) ~ + (E2/2) Z = E12/4+
Ez2/4 (since ~ El ~ _
~E2~)=E12/2=II/2=IT/2.
CA 02361394 2001-10-22
Power-meters P1 (7), P2 (7) and P3 (7)
At the construction stage especially when looking for ~.'~''~ with the tunable
laser both
of the power-meters P1 and P2 may be used in order to better differentiate
interference
erects from the 3dB sputter dependence on the wavelength length.
Once the wavelength ~,'~°a~'"'~ is found and wavelength ~,F selected
the power-meter P 1
or P2 with better readings can be selected. The Loose end of the fiber from
the unused Pl
0 or P2 power-meters can be wound up around a small mandrel to prevent
backreflection
from this loose end to occur.
In the case the enhanced reliability andlor accuracy of this gyroscope is
required the third
power-meter P3 (see Figure 1 ) can be used As shown in (Figure 1 ) the Tap (8)
(about
~s 5%) is placed between the laser source and the polarizer so that the fiber
length between
the 5% tap heading for P3 and 95% of the untapped signal heading for P l
and/or P2 are
of approximately the same length. This is accomplished by the help of Fiber
loop (9) see
Figure 1. Through calculations the readings on P3 can then reduce the effect
of power
fluctuation of the source on the readings on Pl or P2.
Since this gyroscope allows for continuous operation of the laser the easiest
way would
be to read the electrical current coming out of the power-meters P1 and/or P2
and/or P3 at
predetermined intervals. The ideal reading frequency would be based on the
experimental
measurement. The gyro would be simply calibrated on the rotational table of
known
rotational speed. The speed of readings (for any particular rotational speed)
would be
based on the comparison of the measurement results from the rotational table
with the
readings from the gyro. For certain rotational speed, the optimum reading
speed will be
found.
CA 02361394 2001-10-22
The reading speed found from experimental optimization will remove in the best
way the
effects of (random power fluctuation of the laser source, frequency shifts of
the laser
source, all the falling in and out of coherence of the laser source and
others... .).
i~
