CA2361394A1 - Fiber-optic homodyne gyroscope - Google Patents
Fiber-optic homodyne gyroscope Download PDFInfo
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- CA2361394A1 CA2361394A1 CA 2361394 CA2361394A CA2361394A1 CA 2361394 A1 CA2361394 A1 CA 2361394A1 CA 2361394 CA2361394 CA 2361394 CA 2361394 A CA2361394 A CA 2361394A CA 2361394 A1 CA2361394 A1 CA 2361394A1
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- G—PHYSICS
- G01—MEASURING; TESTING
- G01C—MEASURING DISTANCES, LEVELS OR BEARINGS; SURVEYING; NAVIGATION; GYROSCOPIC INSTRUMENTS; PHOTOGRAMMETRY OR VIDEOGRAMMETRY
- G01C19/00—Gyroscopes; Turn-sensitive devices using vibrating masses; Turn-sensitive devices without moving masses; Measuring angular rate using gyroscopic effects
- G01C19/58—Turn-sensitive devices without moving masses
- G01C19/64—Gyrometers using the Sagnac effect, i.e. rotation-induced shifts between counter-rotating electromagnetic beams
- G01C19/72—Gyrometers using the Sagnac effect, i.e. rotation-induced shifts between counter-rotating electromagnetic beams with counter-rotating light beams in a passive ring, e.g. fibre laser gyrometers
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Abstract
Most of the commercially available single mode fiber-optic gyroscopes work like this: a very short pulse of light from the laser is sent down the fiber. It first travels through the unidirectional coupler, then through the polarizer and then through the optical beam splitter/combiner which first splits the light beam and then recombines it after it passes clockwise and counterclockwise through one sensing loop. When the sensing loop is not rotating (moving) there is going to be no mutual phase offset of the clock and counterclockwise waves when they will recombine after passing through the beam splitter/combiner. When the sensing loop is rotating (moving) the above mentioned phases of clock and counterclockwise waves will be mutually offset. This mutual phase offset will produce a different interference pattern which will travel once again through the unidirectional coupler to the measuring equipment. This measuring equipment can then measure the degree of change in the interference pattern from which the speed at which the sensing loop (gyro) is rotating can be derived.
Description
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 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.
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.
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.
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.
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
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_ 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 g 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.
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.
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.
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~
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 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.
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.
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.
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.
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
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_ 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 g 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.
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.
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.
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~
Claims
What is claimed:
Claimed is the Fiber-optic homodyne gyroscope which consist of at least (see Figure 1):
The fixed wavelength laser of predetermined wavelength, coherence time (this coherent time must be longer than time 8t required by light to cover the length difference of clock and counter-clock fibers see section entitled Laser (2) in Specification section), power and frequency stability.
At the construction stage, the tunable laser shall be used to find the wavelength range (.lambda.qudrature +/- .delta.) at which the power-meters P1 and/or P2 exhibit large response to the varying wavelengths of the tunable laser source (the best readings will be close to the quadrature point). Once that highly responsive wavelength region is found one will determine at which wavelength within that interval (.lambda.qudrature +/-.delta.) the gyro under construction will operate see Figure 2. This will also depend on the availability of the fixed wavelength lasers at any particular wavelength within that interval (the closer to the (.lambda.qudrature) the better.
The polarizer which may or may not be used (its use shall improve the performance of this gyro). If used it will supply predetermined polarization state to the 3dB
sputter. The polarizer may not be used at the construction stage where tunable laser is used to find the most responsive wavelength region (close to the quadrature point). This is because the operating wavelength of the gyro is not known at this stage and the polarizer is the most wavelength sensitive part of this gyro.
The 3dB sputter which should have reasonably steady splitting ratio over the wavelength range where the operating wavelength of this gyro is looked for (due to the fiber type used).
Separate clockwise and counterclockwise fiber loops which will have slightly different length of at least .lambda.F/8 (ideally .lambda.F/4). This difference in length has to be considerably smaller than the coherence length of the laser-source.
The interference region (see Figure 3) at which the mutually phase offset (ideally by .lambda.F/4) light waves from clockwise and counterclockwise fibers interfere so that past the interference region the intensity of these waves is about one half of the pre-interference region level (see Figure 3).
Must have at least one power-meter P1 (and/or P2). The reading speed (the reading intervals) of the electrical current may be determined by the experimental calibration.
The P3 power-meter may or may not be used. Its use shall improve the accuracy of this gyro. It can eliminate the source power fluctuation from the gyro readings.
Claimed is the Fiber-optic homodyne gyroscope which consist of at least (see Figure 1):
The fixed wavelength laser of predetermined wavelength, coherence time (this coherent time must be longer than time 8t required by light to cover the length difference of clock and counter-clock fibers see section entitled Laser (2) in Specification section), power and frequency stability.
At the construction stage, the tunable laser shall be used to find the wavelength range (.lambda.qudrature +/- .delta.) at which the power-meters P1 and/or P2 exhibit large response to the varying wavelengths of the tunable laser source (the best readings will be close to the quadrature point). Once that highly responsive wavelength region is found one will determine at which wavelength within that interval (.lambda.qudrature +/-.delta.) the gyro under construction will operate see Figure 2. This will also depend on the availability of the fixed wavelength lasers at any particular wavelength within that interval (the closer to the (.lambda.qudrature) the better.
The polarizer which may or may not be used (its use shall improve the performance of this gyro). If used it will supply predetermined polarization state to the 3dB
sputter. The polarizer may not be used at the construction stage where tunable laser is used to find the most responsive wavelength region (close to the quadrature point). This is because the operating wavelength of the gyro is not known at this stage and the polarizer is the most wavelength sensitive part of this gyro.
The 3dB sputter which should have reasonably steady splitting ratio over the wavelength range where the operating wavelength of this gyro is looked for (due to the fiber type used).
Separate clockwise and counterclockwise fiber loops which will have slightly different length of at least .lambda.F/8 (ideally .lambda.F/4). This difference in length has to be considerably smaller than the coherence length of the laser-source.
The interference region (see Figure 3) at which the mutually phase offset (ideally by .lambda.F/4) light waves from clockwise and counterclockwise fibers interfere so that past the interference region the intensity of these waves is about one half of the pre-interference region level (see Figure 3).
Must have at least one power-meter P1 (and/or P2). The reading speed (the reading intervals) of the electrical current may be determined by the experimental calibration.
The P3 power-meter may or may not be used. Its use shall improve the accuracy of this gyro. It can eliminate the source power fluctuation from the gyro readings.
Priority Applications (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| CA 2361394 CA2361394A1 (en) | 2001-10-22 | 2001-10-22 | Fiber-optic homodyne gyroscope |
Applications Claiming Priority (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| CA 2361394 CA2361394A1 (en) | 2001-10-22 | 2001-10-22 | Fiber-optic homodyne gyroscope |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA2361394A1 true CA2361394A1 (en) | 2003-04-22 |
Family
ID=4170451
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA 2361394 Abandoned CA2361394A1 (en) | 2001-10-22 | 2001-10-22 | Fiber-optic homodyne gyroscope |
Country Status (1)
| Country | Link |
|---|---|
| CA (1) | CA2361394A1 (en) |
Cited By (1)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| CN102878991A (en) * | 2012-10-19 | 2013-01-16 | 重庆华渝电气仪表总厂 | Method for inhibiting front polarizing noise of Y-wave guide in optical fiber gyro |
-
2001
- 2001-10-22 CA CA 2361394 patent/CA2361394A1/en not_active Abandoned
Cited By (2)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| CN102878991A (en) * | 2012-10-19 | 2013-01-16 | 重庆华渝电气仪表总厂 | Method for inhibiting front polarizing noise of Y-wave guide in optical fiber gyro |
| CN102878991B (en) * | 2012-10-19 | 2015-04-22 | 重庆华渝电气仪表总厂 | Method for inhibiting front polarizing noise of Y-wave guide in optical fiber gyro |
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