CA1180795B - Laser gyro with phased dithered mirrors - Google Patents
Laser gyro with phased dithered mirrorsInfo
- Publication number
- CA1180795B CA1180795B CA000457634A CA457634A CA1180795B CA 1180795 B CA1180795 B CA 1180795B CA 000457634 A CA000457634 A CA 000457634A CA 457634 A CA457634 A CA 457634A CA 1180795 B CA1180795 B CA 1180795B
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- CA
- Canada
- Prior art keywords
- mirrors
- laser
- mirror
- vibrating
- ring laser
- Prior art date
- Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
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Classifications
-
- 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
- G01C19/66—Ring laser gyrometers
- G01C19/68—Lock-in prevention
- G01C19/70—Lock-in prevention by mechanical means
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- Physics & Mathematics (AREA)
- Engineering & Computer Science (AREA)
- Optics & Photonics (AREA)
- Electromagnetism (AREA)
- Power Engineering (AREA)
- General Physics & Mathematics (AREA)
- Radar, Positioning & Navigation (AREA)
- Remote Sensing (AREA)
- Gyroscopes (AREA)
- Lasers (AREA)
Abstract
ABSTRACT OF THE DISCLOSURE
Disclosed herein is a ring laser gyroscope having two oppositely traveling laser beams and is provided with mechanically dithered mirrors at its three or four reflection points. At least two of the mirrors are mounted for movement in and out as the result of the expansion and contraction of stacks of piezoelectric elements associated with these mirrors.
These mirrors are thus dithered, i.e. oscillated in and out, in phased relationship with one another so that the total length of the laser cavity is held at a fixed number of wave-lengths, but the laser beam translates back and forth across the surfaces of the mirrors. By this technique, the undesired phenomenon of lock-in at low rotation rates of the gyroscope is avoided, without the need for special optical or magnetic structures in the path of the laser beam.
Disclosed herein is a ring laser gyroscope having two oppositely traveling laser beams and is provided with mechanically dithered mirrors at its three or four reflection points. At least two of the mirrors are mounted for movement in and out as the result of the expansion and contraction of stacks of piezoelectric elements associated with these mirrors.
These mirrors are thus dithered, i.e. oscillated in and out, in phased relationship with one another so that the total length of the laser cavity is held at a fixed number of wave-lengths, but the laser beam translates back and forth across the surfaces of the mirrors. By this technique, the undesired phenomenon of lock-in at low rotation rates of the gyroscope is avoided, without the need for special optical or magnetic structures in the path of the laser beam.
Description
) 7 ~ 5 This invention rel.ate~s to ring laser gyroscopes which are known to have a closed-loop path optical cavity containing an active lasing medium for generating two light beams which travel in opposite dlrections along the path, the diEference between the frequencies of the light in the beams being a measure of the rate or rotation experienced by the ring laser gyroscope. In these instruments, the closed-loop path for the beams is formed by means of at least three mirrors which reilect the light beams. ; ;
~. 10 In the field of such ring laser gyros~opes, it is ~, ~
well known to use triangular:or rectangular~paths for the ~:; lase.r beams,~with mirrors located:at the~apeces oE the t:ri-angular path or at the corne:rs of the ;rectangular path. ~During operatlon, rotation of the gyroscope in the pla~ne of the :
:laser paths gives rise to a beat frequency between:the two ; oppositely dlrected laser ~bea~s, aDd thls beat~frequency~is used to determine the rotation associated with a cha:nge:in~
,, : ~ ~ .
orientatio~n of;the gyroscope. ~o.wever, at~ v~ery slow~rota~tion rate~s~, the two~beams tend to lock one~into the~other~ so; that 20; ~no~di:ference~f~requency is observed. Th~is ls~ ln part;:a result of back:scattering,:with some of the:energy~from each oL the~ lase~r beams being ref~lected bac:X~alDng~the path :of~the other~beam a~nd tendi:ng to make the~two beams lock~in ; step ~i.e. in phase.
Varicus techniques have been pro~osed for avoiding thls disturbi.ng phenomenon, frequently;called ring laser:
gyroscope lock-ln, and these have included~vibrating~ the : entire body of the laser gyroscope~ and the use of non- :
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reciprocal phase-shifting arrangements, such as Faraday rotating elements. Of course vibrating the entire body of the laser device is not the most elegant manner of handling the lock-in problem, and the use oE Faraday rotation or other magnetic or optical devices in the laser path makes for a more complex structure than might be desired.
In accordance with the invention, there is provided a ring laser comprising means forming a closed loop optical ~: .
cavity containing an active lasing medium for generating primary counterrotating laser light beams therein, the frequency difference bet~een the light beams havin~ a measure of the rate of rotation experienced by the ring laser, the cavity forminq means including a plurali-ty of mirrors for reflecting the light beams; and means for vibrating a plurality of the mirrors in translation of the same frequency in a direction only perpendicuIar to the surface of the mirror with the vibrating mirrors having ; nonze~ro amplltudes of vibration and phases of vibration to ~ -~cause the total distance around the closed loop to remain substantially constant.
Each mlrror may be vibrated at a phase difference - with respect to an adjacent mirror which is equal to 360 divided by the number of mirrors. Moreover, it is~possible to provide for substantially uniform amplitudes of vibratory motion for all mirrors. Additionally, the ; vibratory motion of all mirrors can be substantially sinusidal with respect to time. The translational displacement of each ~ dm: b\~'~
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~. lsn7~s ,, mirror is sui.tably of the order of ~ 0.19 sin ~ wherein ~
is the wavelength of the laser light and ~ is the angle formed between the direction of incidence oE the light beams on each mirror and a plane oriented perpendicularly to the ` , mirror surface.
In accordance with a speciic embodiment of the invention an individual transducer is used for driving each .;'~:~ .
of the mirrors into vibratory motion, with each transducer being preferably a stack of piezoelectric elements.
Thus, lt can be seen that in accordance with an : exemplary embodiment of the present invention, the mirrors .at all the refl.ection points of a ring Iaser gyroscope are dithered in and o~t, with the phase o o:c_1la~:ion of the~
mirrors being staggered around the periphery of the laser gyroscope structure such that the phase difference between two adja~cent mirrors is equal to 360 degrees divided~
by the number of mirrors, this condition resulting in a : constant path length for the oppositely~orlented beams, a~ccomp~anied ~by the point of incidence of each~laser beam : 20 sh~ifting across the surface of each:of the mirr:ors. This ~:, ~ : . . : ~
: i:s found to~shift the frequency of the back-scatter~ed;l~ight:, thu~s~reducing., if not avoiding, the coupllng between the laser beams:and~sub~st~antially preventing the l~ock-in phenomenon.
For a better understanding of the invention a~nd to ~
show how the same may be carried into effect, ~eference will-: now be~mad.e, by way of example, to the accompanying drawlngs,n whlch:
dm~ 3 -:
::: :
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7 ~ 5 Figure l is a schematic illustration of a ri.ng laser gyroscope;
- Figure 2 is a detail view of mirror-and-transducer assemblies oE which one is located at each reflection point of the laser gyroscope; and Figure 3 is a diagram schematically illustrating , , .
~ the movement of each of the three mirrors of Figure l 3~-:
with respect to the others.
With referenoe to the drawlngs, Figure l schem~atically illustrates a laser body 12 which may be made of quartz, for example. Three peripheral chaDnels 14, 16 and 18 forming the closed-loop path in the configuration oE a trian~ie, as shown, have been bored through ~he quartz body 12. Within the channels l4, 16 and~18 ~is a gaseous laser medium, such as~a mixture of gases suitable for laser ~
action. Mo~e specifically, the gas in one embodlment is~;
approximately 90~ helium and lO~ neon, and it may be~ at a~pressure of approximately 3 torr.
'In accorda~nce with known laser techn~ology two ~
20~ cathodes 20 and 22, and two anodes 24 and 26 are secure~ to the quartz body 12, so that a gas discharge can;be e~stablished~between cathode 20 and anode 24, as~well as between cathode 22 and anode 26, in channels 16~ and 18, r~espectiv~ely.-Mirror-a~nd-transducer assembl1és~ 28~, 30 and-3Z are mo~unted at the three re;flection points~of the~1llustrated ` triangular ring laser~gyroscope structure.
All of the~i~nte~rnal elements of ~the rin~ laser : :. ~
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,..., gyroscope assembly, as descrlbed above, including the mirrors, cathodes and anodes, are tigh-tly sealed into the quartz body 12, so thak the gas wi.thin the channels of the quartz body is maintained at the proper pressure and free ~ .
~ from contaminati.on. Laser action occurs in a single mode :~ 14 ~:~ at a frequency of approximately 5 x 10 ~z. This corresponds to a wavelength of approximately 0.633 microns, i.e. the resulting illumination is brilliant ~ light red in color.
:~ 10: The structure of one mirror-and-transducer ~ - assemblies 28, 30 or 32, mentioned above is shown in . ~ .
: detail in Figure 2. In Figure 2, the mirror-and- :
; trar.sducer assembly 2~ has a mirror 64 who~se`Ieflectlve, partially coated surface 34 faces the laser beams and reflects thelr light from one of the cha~nels 14,:16, 1~8 into another. The mirror 64 is fa:stened to the quartz : body 12 along its rim 36. The mirror is thinned down in an~ànnular zone 38 which extends around the mirror on its:
: ba~ck su:rface~just within the heavier outer rim 36. Secured ~20~ : to the rim 36 is a rigid housing 40, suitably cylindrical in shape. This:cylindrical housing 40 is provided with a : heavy bottom 42.
: Extend;ing between the bottom 42 of the housing and the central portion 46 of the mirror~is~a stack 44 :
:of piezoélectric:transducer elements. The piezoelectric tr~ansducer stack 44 is made up of a number of thin flat piezoelectric wafers. These wafers have the property that~ -: : :
~ when a voltage is applied across them, they become slightly ~ `:
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thicker or slightly thinner, depending on the polarity of the voltage. The stack 44 may be made up oE fifteen piezo-electric wafers, each oE which is about 10 mils thick. The :
`~ wafers have electrodes on their top and bottom surfaces and are connected "back-to-back"-, which means that alternate-;~ . ..
common electrodes forming one set of electrodes, are connected ` to the driving lead - of opposite polarity. With the wafers ; ~ .
being connected "back-to-back", when having oppositely directed e]ectric fields applied across alternate wafers, they expand and contract in thickness together, exerting substantial pressure on the mirror center portion 46 and causlng it to flex the thin annular section 38 of the mirror 64, in a manner ; similar to the displacement of a dia,~hrag.n secured to a frame along its edge.
; The stacks of piezoelectric transducer elements may be made up of piezoelectric wafers available from Gulton as Gulton Type No. 1408. The approximate level of voltage found to produce the magnitude of displacement dis-cussed below is approximately 160 volts peak-to-peak. This provides 20~ the dlsplacement of the proper order of magnitude to obtain the~amp~lltudes~needed for the practice~of the present~
invention, as developed below. ~ ~
Returnl~ng to Figure 1, the electrical driving circults include the two~ power supplies 52 and 54 of known types for ~
starting and maintaining the gas discharges between the anodes ~ ~:
a~nd cathodes of the laser device. ~
The mirror surfaces of the assemblies 28j 30~ and 32 are driven into vibratory motion by the transducers which are dm~ - 6 -' ~ i...'~1 ~. ;18()7~
,~, energized by the three-phase bias excita~,ion circui-t 56. Each of the piezoelectric transducer stacks, s~ch as stack 44 shown in Figure 2, is excited such that .its vibration is 120 out of phase with each of the others.
In Figure 2, the laser beams 57 and 58 are shown to ~ - . . . . ..
,~; pass through the partially coated mirror surface 34 and respectively through the apertures 59 and 60 in housing 40.
; As shown in Figure'1, they impinge upon external~mirrors 61 and 62. From these mirrors 61 and 62, the beams are directed 10: to a detector 63 which detects the beats between the two : oppositely directed laser beams, which beats occur wh~eD the structure rotates, in a manner known in the~art. ~:
In Figure 3, the three mirrors of the~ ring:laser gyroscope~ are~shown'schematically as mi:rror~64, wlth~
: additional mirrors 65 and 66. The neutraI position of each :~
of:the~mirrors i9 shown at:64', 65' an~d~66'. As indicated ~ ~
by~the arrows 68, each~of the mirrors moves in:and~out, toward : an:d:~away~ from ~the center of the laser gyroscope~assembly. In Fi~gure 3~ the~heavy line 70 represents the path of the laser 20~ ;gyro beam wlth the mirrors in the positian shown. More ~ :
specl~fically, ln that position the mirror 65 is~close tO~Its furthe:st retracted position,~whlle~ each of the~mlrrors~ 64 :and 66 is displaced~from the mirror 65 by successive 120~
increments and:therefore it lS more advanced in its posltlons.
Thus~ th~e~arrangement lS controlled to malntaln the~.tr~iangular~ :
lase~r beam~paths.substantially constant as the three mirrors progressivel~y move in and~out of thelr~phasea relationships of vibratory motion.
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For a four-mlrror ring laser gyroscope geometry, the four mirrors would be driven by transducers which are energized by a four phase supply, and the successive mirrors would be operated to perform vibratory motion 90 out ofphase with that of each adjacent mirror.
More generally, the phase dispIacement between adjacent mirrors should be equal to 360 dlvided by the-~; number of mirrors.
With regard to the frequency of the excitation source 56 of Figure 1, it is desirable that the frequency ~
be relatively high, suitably in the order of so~e tens of kilocycles up to several hundreds of kilocycles. However,~frequencies a~s low as l or 2 kilocycles may be employed.
Further, as illustrated in Figure 3, when the mirrors move~ln~and out~from thelr extreme retracted positlon to their~
extreme advanced positi~ons, the points of ~incidence of the laser beams move back~and~f~orth across the mir~ror surfaces~from;one extr~eme position,;;the~correspondlng;paths of~a~beam being~glven~
by lines 72, 74~and 76 to the other extreme position o these 20~ paths~g1ven by llnes 78,~ 80 an~d 82. Of~course~,~as ~indic~ated~by;
; the tr~iangular path 70, shown in Figure 3, the~beams reach the extreme p~ositions~ given by lines 72, 74 and 7.6! and subsequently reach the~inner boundaries~78~ 80 and~8~2 at~different~polnts in~ ;
time, always maintaining the~laser path precisely the~;same 1~eng~th.
A somewhat related prior art disclosure~is found in U.S. Patent No.~3,533,014, issued October 6! 1970-to Coccoli et al.
and entitled "Gas Ring Laser Using Oscillating Radiation Scattering Sources Within~the~Laser Cavity". This patent discloses an dm~
()7~5 an arrangemenk wherein the mirrors and other scattering sources are caused to oscillate parallel to their surfaces, ` rathex than perpendicular to their surfaces as taught by the present invention. It was found to be extremely difficult to ~-~ implement arrangements for moving the mirrors of a laser `~ gyroscope in their own plane, while concurrently keeping the laser cavities sealed and meeting the other necessary requirements.
~;~ In contrast, the present invention, using vibratory motlon of the mirrors perpendicular to their surfaces shifts the laser beams back and forth across the surfaces o the mirrors iD a manner which may be analyzed mathematically by the teohnique employed in accordance~with U.S. Patent No. 3,533,014. More specifically, the formula set forth in Column 6j lines 53 to 55 of that~patent is precisely applicable to the amplitude of the mirrors employed in the present invention. This surprising result arises from the fact~that the polnts or incide~nce of the laser~beams are translated back and forth across the surfaces of the mirrors ;2~0 ~by~ exactly the same distance that the mlrrors are osolllated inwardly and outwardly i.e. toward and away from the center of the assembly.
The formula glven ln U.S. Patent No. 3,533,014, me~ntioned above, in an abridged form, would read 4 ~ (sin ~) Wherein "A'i is the maximum displacement of each~
mirror in each direction from a neutral position, ~ is the wavelength ~f the laser light, ~ is the angle between the direction of incidence of the laser beams and a plane g dm~
:~;.-~ ' l) 7 ~ 5 perpendicular to the mirror surface, and ~ is any number ; which, when taken as the argument of the Bessel function ~ of zero order, yields a value of the Bessel function which ~:~
is zero.
~,:
For the embodiment of trlangular configuration, as shown in Figure 1, 9 is 30, and Sin ~ is equal to one-half. From mathematical tables it can be seen that ~ is ~1 equal to 2.405 of the lowest order Bessel function.
Substituting these values into equation (l) the following results are obtained =
0.191 ~ ~ (2) sin A= 0.382 ~ ; 13) When using the neon-helium gas~mixture mentioned above, the wavelength is 0.633 microns and the displacement amplitude "A" i9 equal to about 0.242 microns, i.e. 0.242 10- oentlmeters~, ln each dlrection from the neutral positlo~n~of the mlrrors.
Recapitulating, with the three~mirrors ~ibratinq 20~ ; exactly 120 out~of phase wlth respect to each other,;there~
is~no~chang~e in the instantaneous optical cavity length Thus,~laser operat~ion is~maintained at a frequency position~
at the center of~the gain curve with a m~inimum length perturbation due to the effect achieved by the invention whi~h may conveniently be referre~d to as "microdither" modulation.
:~ ~:~ :: : :
The microdither amplitude are of subwavelength~dimensions.
Geometrlcal analysis shows that the standing wave field assoc~ated wlth the triangular configuration shown in :: :
:: : `
~: :
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: .. , Figure 3 is caused to perform translational motion, bu-t not ; to rotate, in a manner such that the apeces of the triangIe, describe circles, as shown by the arrows in E'igure 3, while the points of incidence describe lines on the surfaces of the mirrors, inasmuch as the mirrors move in and out so that each laser beam moves back and forth across the mirror surfaces~
This results in the scatter centers being displaced with reepect to the translated i.e. position - shiftea standing wave field modes, hence satisfying the phase shif~ requirement : ~
of phase modulation. This displacement also results in the standing wave field being displaced wit-h respect to a body-fixed ~:.
aperture, as is well known in the art.
In addition, the distance from one scatter group ~:, , to the next group on the next mirror can be seen to be varying with tlme. It i~s the vector summation of the optically back-scattered light from all the scattering surfaces of the laser - cavity that determines the magnitude of the~ "lock-in" effect~
as well as the fin~al phase position. Thus,; the microdither of the mirrors~ causes~the net scatter vecto~r to be time-modulated.
~ This effect further reduces the "lock~in" effect.
For completeness, reference is also made~to U S.
Patent No. 3,581,227, issued May 25, 1971 to Podgorski, which is of~interest inasmuch as it shows a~stack of piezoeleotric elements and wherein the position of a mirror is shifted to accurately change the length of a laser cavity~ This is known technlque, and could be used in the apparatus of the present inve~ntion as a DC bias upon which the properly phased alternating current signals could be superimposed. Of course, U.S. Patent dm :\~J`~\~
: : :
No. 3,581,227 does not disclose a plurality of mirrors of whlch each has a pieæoelectric control, nor does it disclose the phased vibration, i.e. oscillation of the mirrors. It is evident that transducers o-ther than piezoelectric elements ' could be employed for oscillating the mirrors in the proper phase relationship. For example, magnetostrictive transducers ~could be employed. In addition, other lasi.ng materials and "~~laser cavlties having a different number or mirrors, such as four, could be used. Other minor changes from the disclosed ~structure are also considered to be within the scope of the ~,present invention`.
' . ~ i : ~
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dm~
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.
:
.
~. 10 In the field of such ring laser gyros~opes, it is ~, ~
well known to use triangular:or rectangular~paths for the ~:; lase.r beams,~with mirrors located:at the~apeces oE the t:ri-angular path or at the corne:rs of the ;rectangular path. ~During operatlon, rotation of the gyroscope in the pla~ne of the :
:laser paths gives rise to a beat frequency between:the two ; oppositely dlrected laser ~bea~s, aDd thls beat~frequency~is used to determine the rotation associated with a cha:nge:in~
,, : ~ ~ .
orientatio~n of;the gyroscope. ~o.wever, at~ v~ery slow~rota~tion rate~s~, the two~beams tend to lock one~into the~other~ so; that 20; ~no~di:ference~f~requency is observed. Th~is ls~ ln part;:a result of back:scattering,:with some of the:energy~from each oL the~ lase~r beams being ref~lected bac:X~alDng~the path :of~the other~beam a~nd tendi:ng to make the~two beams lock~in ; step ~i.e. in phase.
Varicus techniques have been pro~osed for avoiding thls disturbi.ng phenomenon, frequently;called ring laser:
gyroscope lock-ln, and these have included~vibrating~ the : entire body of the laser gyroscope~ and the use of non- :
:~ - dm ~
:
-,: : :
~: :
:
' :
-`"" I 18~)7~
reciprocal phase-shifting arrangements, such as Faraday rotating elements. Of course vibrating the entire body of the laser device is not the most elegant manner of handling the lock-in problem, and the use oE Faraday rotation or other magnetic or optical devices in the laser path makes for a more complex structure than might be desired.
In accordance with the invention, there is provided a ring laser comprising means forming a closed loop optical ~: .
cavity containing an active lasing medium for generating primary counterrotating laser light beams therein, the frequency difference bet~een the light beams havin~ a measure of the rate of rotation experienced by the ring laser, the cavity forminq means including a plurali-ty of mirrors for reflecting the light beams; and means for vibrating a plurality of the mirrors in translation of the same frequency in a direction only perpendicuIar to the surface of the mirror with the vibrating mirrors having ; nonze~ro amplltudes of vibration and phases of vibration to ~ -~cause the total distance around the closed loop to remain substantially constant.
Each mlrror may be vibrated at a phase difference - with respect to an adjacent mirror which is equal to 360 divided by the number of mirrors. Moreover, it is~possible to provide for substantially uniform amplitudes of vibratory motion for all mirrors. Additionally, the ; vibratory motion of all mirrors can be substantially sinusidal with respect to time. The translational displacement of each ~ dm: b\~'~
:::
. `~
.,~
.
~. lsn7~s ,, mirror is sui.tably of the order of ~ 0.19 sin ~ wherein ~
is the wavelength of the laser light and ~ is the angle formed between the direction of incidence oE the light beams on each mirror and a plane oriented perpendicularly to the ` , mirror surface.
In accordance with a speciic embodiment of the invention an individual transducer is used for driving each .;'~:~ .
of the mirrors into vibratory motion, with each transducer being preferably a stack of piezoelectric elements.
Thus, lt can be seen that in accordance with an : exemplary embodiment of the present invention, the mirrors .at all the refl.ection points of a ring Iaser gyroscope are dithered in and o~t, with the phase o o:c_1la~:ion of the~
mirrors being staggered around the periphery of the laser gyroscope structure such that the phase difference between two adja~cent mirrors is equal to 360 degrees divided~
by the number of mirrors, this condition resulting in a : constant path length for the oppositely~orlented beams, a~ccomp~anied ~by the point of incidence of each~laser beam : 20 sh~ifting across the surface of each:of the mirr:ors. This ~:, ~ : . . : ~
: i:s found to~shift the frequency of the back-scatter~ed;l~ight:, thu~s~reducing., if not avoiding, the coupllng between the laser beams:and~sub~st~antially preventing the l~ock-in phenomenon.
For a better understanding of the invention a~nd to ~
show how the same may be carried into effect, ~eference will-: now be~mad.e, by way of example, to the accompanying drawlngs,n whlch:
dm~ 3 -:
::: :
, ~ :
: :
~; :
7 ~ 5 Figure l is a schematic illustration of a ri.ng laser gyroscope;
- Figure 2 is a detail view of mirror-and-transducer assemblies oE which one is located at each reflection point of the laser gyroscope; and Figure 3 is a diagram schematically illustrating , , .
~ the movement of each of the three mirrors of Figure l 3~-:
with respect to the others.
With referenoe to the drawlngs, Figure l schem~atically illustrates a laser body 12 which may be made of quartz, for example. Three peripheral chaDnels 14, 16 and 18 forming the closed-loop path in the configuration oE a trian~ie, as shown, have been bored through ~he quartz body 12. Within the channels l4, 16 and~18 ~is a gaseous laser medium, such as~a mixture of gases suitable for laser ~
action. Mo~e specifically, the gas in one embodlment is~;
approximately 90~ helium and lO~ neon, and it may be~ at a~pressure of approximately 3 torr.
'In accorda~nce with known laser techn~ology two ~
20~ cathodes 20 and 22, and two anodes 24 and 26 are secure~ to the quartz body 12, so that a gas discharge can;be e~stablished~between cathode 20 and anode 24, as~well as between cathode 22 and anode 26, in channels 16~ and 18, r~espectiv~ely.-Mirror-a~nd-transducer assembl1és~ 28~, 30 and-3Z are mo~unted at the three re;flection points~of the~1llustrated ` triangular ring laser~gyroscope structure.
All of the~i~nte~rnal elements of ~the rin~ laser : :. ~
~ dm~ 4 - ~
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.
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.,: ~ : : : :
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,..., gyroscope assembly, as descrlbed above, including the mirrors, cathodes and anodes, are tigh-tly sealed into the quartz body 12, so thak the gas wi.thin the channels of the quartz body is maintained at the proper pressure and free ~ .
~ from contaminati.on. Laser action occurs in a single mode :~ 14 ~:~ at a frequency of approximately 5 x 10 ~z. This corresponds to a wavelength of approximately 0.633 microns, i.e. the resulting illumination is brilliant ~ light red in color.
:~ 10: The structure of one mirror-and-transducer ~ - assemblies 28, 30 or 32, mentioned above is shown in . ~ .
: detail in Figure 2. In Figure 2, the mirror-and- :
; trar.sducer assembly 2~ has a mirror 64 who~se`Ieflectlve, partially coated surface 34 faces the laser beams and reflects thelr light from one of the cha~nels 14,:16, 1~8 into another. The mirror 64 is fa:stened to the quartz : body 12 along its rim 36. The mirror is thinned down in an~ànnular zone 38 which extends around the mirror on its:
: ba~ck su:rface~just within the heavier outer rim 36. Secured ~20~ : to the rim 36 is a rigid housing 40, suitably cylindrical in shape. This:cylindrical housing 40 is provided with a : heavy bottom 42.
: Extend;ing between the bottom 42 of the housing and the central portion 46 of the mirror~is~a stack 44 :
:of piezoélectric:transducer elements. The piezoelectric tr~ansducer stack 44 is made up of a number of thin flat piezoelectric wafers. These wafers have the property that~ -: : :
~ when a voltage is applied across them, they become slightly ~ `:
~ dm~ 5 -:~,: ~ : . ':
::
: :
.
~ 18()7~
thicker or slightly thinner, depending on the polarity of the voltage. The stack 44 may be made up oE fifteen piezo-electric wafers, each oE which is about 10 mils thick. The :
`~ wafers have electrodes on their top and bottom surfaces and are connected "back-to-back"-, which means that alternate-;~ . ..
common electrodes forming one set of electrodes, are connected ` to the driving lead - of opposite polarity. With the wafers ; ~ .
being connected "back-to-back", when having oppositely directed e]ectric fields applied across alternate wafers, they expand and contract in thickness together, exerting substantial pressure on the mirror center portion 46 and causlng it to flex the thin annular section 38 of the mirror 64, in a manner ; similar to the displacement of a dia,~hrag.n secured to a frame along its edge.
; The stacks of piezoelectric transducer elements may be made up of piezoelectric wafers available from Gulton as Gulton Type No. 1408. The approximate level of voltage found to produce the magnitude of displacement dis-cussed below is approximately 160 volts peak-to-peak. This provides 20~ the dlsplacement of the proper order of magnitude to obtain the~amp~lltudes~needed for the practice~of the present~
invention, as developed below. ~ ~
Returnl~ng to Figure 1, the electrical driving circults include the two~ power supplies 52 and 54 of known types for ~
starting and maintaining the gas discharges between the anodes ~ ~:
a~nd cathodes of the laser device. ~
The mirror surfaces of the assemblies 28j 30~ and 32 are driven into vibratory motion by the transducers which are dm~ - 6 -' ~ i...'~1 ~. ;18()7~
,~, energized by the three-phase bias excita~,ion circui-t 56. Each of the piezoelectric transducer stacks, s~ch as stack 44 shown in Figure 2, is excited such that .its vibration is 120 out of phase with each of the others.
In Figure 2, the laser beams 57 and 58 are shown to ~ - . . . . ..
,~; pass through the partially coated mirror surface 34 and respectively through the apertures 59 and 60 in housing 40.
; As shown in Figure'1, they impinge upon external~mirrors 61 and 62. From these mirrors 61 and 62, the beams are directed 10: to a detector 63 which detects the beats between the two : oppositely directed laser beams, which beats occur wh~eD the structure rotates, in a manner known in the~art. ~:
In Figure 3, the three mirrors of the~ ring:laser gyroscope~ are~shown'schematically as mi:rror~64, wlth~
: additional mirrors 65 and 66. The neutraI position of each :~
of:the~mirrors i9 shown at:64', 65' an~d~66'. As indicated ~ ~
by~the arrows 68, each~of the mirrors moves in:and~out, toward : an:d:~away~ from ~the center of the laser gyroscope~assembly. In Fi~gure 3~ the~heavy line 70 represents the path of the laser 20~ ;gyro beam wlth the mirrors in the positian shown. More ~ :
specl~fically, ln that position the mirror 65 is~close tO~Its furthe:st retracted position,~whlle~ each of the~mlrrors~ 64 :and 66 is displaced~from the mirror 65 by successive 120~
increments and:therefore it lS more advanced in its posltlons.
Thus~ th~e~arrangement lS controlled to malntaln the~.tr~iangular~ :
lase~r beam~paths.substantially constant as the three mirrors progressivel~y move in and~out of thelr~phasea relationships of vibratory motion.
: dm~
:
:
:
:
:~:: ~ :
.
i. lg()7~
For a four-mlrror ring laser gyroscope geometry, the four mirrors would be driven by transducers which are energized by a four phase supply, and the successive mirrors would be operated to perform vibratory motion 90 out ofphase with that of each adjacent mirror.
More generally, the phase dispIacement between adjacent mirrors should be equal to 360 dlvided by the-~; number of mirrors.
With regard to the frequency of the excitation source 56 of Figure 1, it is desirable that the frequency ~
be relatively high, suitably in the order of so~e tens of kilocycles up to several hundreds of kilocycles. However,~frequencies a~s low as l or 2 kilocycles may be employed.
Further, as illustrated in Figure 3, when the mirrors move~ln~and out~from thelr extreme retracted positlon to their~
extreme advanced positi~ons, the points of ~incidence of the laser beams move back~and~f~orth across the mir~ror surfaces~from;one extr~eme position,;;the~correspondlng;paths of~a~beam being~glven~
by lines 72, 74~and 76 to the other extreme position o these 20~ paths~g1ven by llnes 78,~ 80 an~d 82. Of~course~,~as ~indic~ated~by;
; the tr~iangular path 70, shown in Figure 3, the~beams reach the extreme p~ositions~ given by lines 72, 74 and 7.6! and subsequently reach the~inner boundaries~78~ 80 and~8~2 at~different~polnts in~ ;
time, always maintaining the~laser path precisely the~;same 1~eng~th.
A somewhat related prior art disclosure~is found in U.S. Patent No.~3,533,014, issued October 6! 1970-to Coccoli et al.
and entitled "Gas Ring Laser Using Oscillating Radiation Scattering Sources Within~the~Laser Cavity". This patent discloses an dm~
()7~5 an arrangemenk wherein the mirrors and other scattering sources are caused to oscillate parallel to their surfaces, ` rathex than perpendicular to their surfaces as taught by the present invention. It was found to be extremely difficult to ~-~ implement arrangements for moving the mirrors of a laser `~ gyroscope in their own plane, while concurrently keeping the laser cavities sealed and meeting the other necessary requirements.
~;~ In contrast, the present invention, using vibratory motlon of the mirrors perpendicular to their surfaces shifts the laser beams back and forth across the surfaces o the mirrors iD a manner which may be analyzed mathematically by the teohnique employed in accordance~with U.S. Patent No. 3,533,014. More specifically, the formula set forth in Column 6j lines 53 to 55 of that~patent is precisely applicable to the amplitude of the mirrors employed in the present invention. This surprising result arises from the fact~that the polnts or incide~nce of the laser~beams are translated back and forth across the surfaces of the mirrors ;2~0 ~by~ exactly the same distance that the mlrrors are osolllated inwardly and outwardly i.e. toward and away from the center of the assembly.
The formula glven ln U.S. Patent No. 3,533,014, me~ntioned above, in an abridged form, would read 4 ~ (sin ~) Wherein "A'i is the maximum displacement of each~
mirror in each direction from a neutral position, ~ is the wavelength ~f the laser light, ~ is the angle between the direction of incidence of the laser beams and a plane g dm~
:~;.-~ ' l) 7 ~ 5 perpendicular to the mirror surface, and ~ is any number ; which, when taken as the argument of the Bessel function ~ of zero order, yields a value of the Bessel function which ~:~
is zero.
~,:
For the embodiment of trlangular configuration, as shown in Figure 1, 9 is 30, and Sin ~ is equal to one-half. From mathematical tables it can be seen that ~ is ~1 equal to 2.405 of the lowest order Bessel function.
Substituting these values into equation (l) the following results are obtained =
0.191 ~ ~ (2) sin A= 0.382 ~ ; 13) When using the neon-helium gas~mixture mentioned above, the wavelength is 0.633 microns and the displacement amplitude "A" i9 equal to about 0.242 microns, i.e. 0.242 10- oentlmeters~, ln each dlrection from the neutral positlo~n~of the mlrrors.
Recapitulating, with the three~mirrors ~ibratinq 20~ ; exactly 120 out~of phase wlth respect to each other,;there~
is~no~chang~e in the instantaneous optical cavity length Thus,~laser operat~ion is~maintained at a frequency position~
at the center of~the gain curve with a m~inimum length perturbation due to the effect achieved by the invention whi~h may conveniently be referre~d to as "microdither" modulation.
:~ ~:~ :: : :
The microdither amplitude are of subwavelength~dimensions.
Geometrlcal analysis shows that the standing wave field assoc~ated wlth the triangular configuration shown in :: :
:: : `
~: :
) 7 ~ ~
: .. , Figure 3 is caused to perform translational motion, bu-t not ; to rotate, in a manner such that the apeces of the triangIe, describe circles, as shown by the arrows in E'igure 3, while the points of incidence describe lines on the surfaces of the mirrors, inasmuch as the mirrors move in and out so that each laser beam moves back and forth across the mirror surfaces~
This results in the scatter centers being displaced with reepect to the translated i.e. position - shiftea standing wave field modes, hence satisfying the phase shif~ requirement : ~
of phase modulation. This displacement also results in the standing wave field being displaced wit-h respect to a body-fixed ~:.
aperture, as is well known in the art.
In addition, the distance from one scatter group ~:, , to the next group on the next mirror can be seen to be varying with tlme. It i~s the vector summation of the optically back-scattered light from all the scattering surfaces of the laser - cavity that determines the magnitude of the~ "lock-in" effect~
as well as the fin~al phase position. Thus,; the microdither of the mirrors~ causes~the net scatter vecto~r to be time-modulated.
~ This effect further reduces the "lock~in" effect.
For completeness, reference is also made~to U S.
Patent No. 3,581,227, issued May 25, 1971 to Podgorski, which is of~interest inasmuch as it shows a~stack of piezoeleotric elements and wherein the position of a mirror is shifted to accurately change the length of a laser cavity~ This is known technlque, and could be used in the apparatus of the present inve~ntion as a DC bias upon which the properly phased alternating current signals could be superimposed. Of course, U.S. Patent dm :\~J`~\~
: : :
No. 3,581,227 does not disclose a plurality of mirrors of whlch each has a pieæoelectric control, nor does it disclose the phased vibration, i.e. oscillation of the mirrors. It is evident that transducers o-ther than piezoelectric elements ' could be employed for oscillating the mirrors in the proper phase relationship. For example, magnetostrictive transducers ~could be employed. In addition, other lasi.ng materials and "~~laser cavlties having a different number or mirrors, such as four, could be used. Other minor changes from the disclosed ~structure are also considered to be within the scope of the ~,present invention`.
' . ~ i : ~
~, ~ : : :
:~ ~ ~' , : , , ::: : : ~ : :
dm~
:
,: ~
.
:
.
Claims (21)
1. A ring laser gyroscope having at least three mirrors for setting up a closed-loop path for one and the other of two light beams which, in operation of the gyroscope, propagate in mutually opposite directions in an active laser medium of the gyroscope, there being means for driving each of the mirrors into vibratory motion perpendicular to the reflective surface of the mirror, means for controlling the vibratory motions of the mirrors to maintain constant laser beam path lengths, and wherein each mirror can be vibrated at a phase difference with respect to an adjacent mirror which is equal to 360° divided by the number of mirrors.
2. A gyroscope according to claim 1, wherein all the mirrors have substantially uniform amplitudes of vibratory motion
3. A gyroscope according to claim 1, wherein the vibratory motion of all mirrors is substantially sinusoidal with respect to time.
4. A gyroscope according to claim 1, claim 2 or claim 3, wherein the translational displacement of each mirror is of the order of ? 0.19 , wherein .lambda. is the wavelength of the laser light and .THETA. is the angle formed between the direction of incidence of the light beams on each mirror and a plane oriented perpendicularly to the mirror surface.
5. A gyroscope according to claim 1, claim 2 or claim 3, and comprising an individual transducer for driving each of the mirrors into vibratory motion.
6. A gyroscope according to claim 1, claim 2 or claim 3, wherein each mirror is associated with a stack of piezo-electric elements constituting an individual transducer for driving a said mirror into vibratory motion.
7. A ring laser comprising:
means forming a closed loop optical cavity containing an active lasing medium for generating primary counterrotating laser light beams therein, the frequency difference between the light beams having a measure of the rate of rotation experienced by the ring laser, said cavity forming means including a plurality of mirrors for reflecting said light beams; and means for vibrating a plurality of said mirrors in translation at the same frequency in a direction only perpendicular to the surface of the mirror with said vibrating mirrors having nonzero amplitudes of vibration and phases of vibration to cause the total distance around said closed loop to remain substantially constant.
means forming a closed loop optical cavity containing an active lasing medium for generating primary counterrotating laser light beams therein, the frequency difference between the light beams having a measure of the rate of rotation experienced by the ring laser, said cavity forming means including a plurality of mirrors for reflecting said light beams; and means for vibrating a plurality of said mirrors in translation at the same frequency in a direction only perpendicular to the surface of the mirror with said vibrating mirrors having nonzero amplitudes of vibration and phases of vibration to cause the total distance around said closed loop to remain substantially constant.
8. A ring laser as recited in claim 7 wherein the amplitudes of vibration of said vibrating mirrors are substantially equal.
9. A ring laser as recited in claim 8 wherein the phase difference between the oscillation of said vibrating mirrors is substantially equal to 360 degrees divided by the number of vibrating mirrors.
10. A ring laser as defined in claims 7, 8, or 9 wherein said vibrating means comprises means for causing the displacement of said vibrating mirrors to be substantially sinusoidal with respect to time.
.
.
11. A ring laser as defined in claim 8 wherein the magnitude from a neutral position of the translational vibration of each "i"th vibrating mirror is in the order of:
Where .lambda. is the wavelength of the laser light beam, and .THETA.i is equal to the angle of incidence of the light beams on the "i"th vibrating mirror relative to the perpendicular from the mirror at the point of incidence, and .beta. is an argument of Bessel's function of the first kind and zero order which makes J0(.beta.) = 0.
Where .lambda. is the wavelength of the laser light beam, and .THETA.i is equal to the angle of incidence of the light beams on the "i"th vibrating mirror relative to the perpendicular from the mirror at the point of incidence, and .beta. is an argument of Bessel's function of the first kind and zero order which makes J0(.beta.) = 0.
12. A ring laser as defined in claim 7 wherein said vibrating means comprises a transducer.
13. A ring laser as defined in claim 12 wherein said transducer comprises a stack of piezoelectric elements.
14. A ring laser comprising:
a ring laser structure of the single mode type having two counterrotating laser beams and including at least three mirrors;
means for translating at least two of said mirrors in an oscillating mode at the same frequency substantially only in the direction of a line bisecting the beams incident on each said vibrating mirror; and means for phasing the movement of all of said vibrating mirrors to maintain constant primary laser beam path length as said vibrating mirrors are displaced.
a ring laser structure of the single mode type having two counterrotating laser beams and including at least three mirrors;
means for translating at least two of said mirrors in an oscillating mode at the same frequency substantially only in the direction of a line bisecting the beams incident on each said vibrating mirror; and means for phasing the movement of all of said vibrating mirrors to maintain constant primary laser beam path length as said vibrating mirrors are displaced.
15. A ring laser as defined in claim 14 wherein the magnitude from a neutral position of the translational oscillation of each "i"ith vibrating mirror is in the order of:
where .lambda. is the wavelength of the laser light beam and .THETA.i is equal to the angle of incidence of the light beams on the "i"th vibrating mirror relative to the perpendicular from such mirror at the point of incidence, and .beta. is an argument of Bessels function of the first kind of zero order which makes the function J0(.beta.) = 0.
where .lambda. is the wavelength of the laser light beam and .THETA.i is equal to the angle of incidence of the light beams on the "i"th vibrating mirror relative to the perpendicular from such mirror at the point of incidence, and .beta. is an argument of Bessels function of the first kind of zero order which makes the function J0(.beta.) = 0.
16. A ring laser as defined in claim 14 wherein the magnitude of the translational vibration of each "i"th vibrating mirror is in the order of:
where .lambda. is the wavelength of the laser light beam and .THETA.i is equal to the angle of incidence of the light beams on the "i"th vibrating mirror relative to the perpendicular from such mirror at the point of incidence.
where .lambda. is the wavelength of the laser light beam and .THETA.i is equal to the angle of incidence of the light beams on the "i"th vibrating mirror relative to the perpendicular from such mirror at the point of incidence.
17. A ring laser as defined in claim 16 wherein said vibration is substantially sinusoidal.
18. A ring laser as defined in claim 14 wherein all of the trigonometric Fourier components of said vibration are phased the same as the fundamental component.
19. A ring laser as defined in claim 14 in which said vibrating mirrors vibrate open-loop.
20. A ring laser as defined in claim 14 in which all of said vibrating mirrors are closed-loop servoed to control their amplitude and phasing.
21. A ring laser as recited in claim 14 wherein the phases of the n vibrating mirrors of a generalized closed laser path are related one to another by 2 .pi./n radians and the amplitudes of the vibrating mirror displacements, x, are described by where .beta.m is a value of the argument of the zero order Bessel function of the first kind, J0(.beta.m)=0 for m=1, 2, 3...., .THETA.i=angle of incidence at the "i"th vibrating mirror, .omega.=the dither angular frequency which is large compared to typical lock-in frequencies of said laser.
Applications Claiming Priority (2)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| US73976776A | 1976-11-08 | 1976-11-08 | |
| US739,767 | 1976-11-08 |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA1180795B true CA1180795B (en) | 1985-01-08 |
Family
ID=24973708
Family Applications (2)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA285,902A Expired CA1085031A (en) | 1976-11-08 | 1977-08-31 | Laser gyro with phased dithered mirrors |
| CA000457634A Expired CA1180795B (en) | 1976-11-08 | 1984-06-27 | Laser gyro with phased dithered mirrors |
Family Applications Before (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA285,902A Expired CA1085031A (en) | 1976-11-08 | 1977-08-31 | Laser gyro with phased dithered mirrors |
Country Status (6)
| Country | Link |
|---|---|
| JP (1) | JPS5357793A (en) |
| CA (2) | CA1085031A (en) |
| DE (1) | DE2749157C3 (en) |
| FR (1) | FR2370262A1 (en) |
| GB (1) | GB1536077A (en) |
| IL (1) | IL52920A (en) |
Families Citing this family (16)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| CA1125895A (en) * | 1978-05-26 | 1982-06-15 | Virgil E. Sanders | Laser gyro mode locking reduction scheme |
| IL57936A (en) * | 1978-10-02 | 1982-07-30 | Litton Systems Inc | Ring laser with adjustable mirrors |
| US4243324A (en) * | 1979-03-21 | 1981-01-06 | The Singer Company | Feedback system for controlling lock-in in spring suspended ring laser gyroscope |
| US4383763A (en) * | 1979-09-12 | 1983-05-17 | Litton Systems, Inc. | Controllable mirrors |
| CA1154851A (en) * | 1980-03-24 | 1983-10-04 | Litton Systems, Inc. | Acoustically dithered ring laser gyro |
| US4410276A (en) * | 1981-06-15 | 1983-10-18 | The Singer Company | Ring laser gyroscope with doppler mirrors |
| US4653919A (en) * | 1982-02-08 | 1987-03-31 | Sundstrand Optical Technologies, Inc. | Laser gyro with dithered mirrors and current dither |
| US4526469A (en) * | 1982-03-01 | 1985-07-02 | Honeywell Inc. | Discriminant apparatus for laser gyros |
| US4551021A (en) * | 1982-03-01 | 1985-11-05 | Honeywell Inc. | Discriminant apparatus for laser gyros |
| IL70499A (en) * | 1982-12-27 | 1988-06-30 | Litton Systems Inc | Laser cathode |
| DE3320345A1 (en) * | 1983-06-04 | 1984-12-06 | Litton Systems, Inc., Beverly Hills, Calif. | Ring laser gyroscope |
| US4755057A (en) * | 1984-10-02 | 1988-07-05 | Litton Systems, Inc. | Path length control method for ring laser gyroscope |
| US4801206A (en) * | 1986-06-04 | 1989-01-31 | Litton Systems, Inc. | Simplified ring laser gyroscope dither control and method |
| US4779985A (en) * | 1986-12-22 | 1988-10-25 | Litton Systems, Inc. | Dither suspension for ring laser gyroscope and method |
| US5442442A (en) * | 1987-10-28 | 1995-08-15 | Litton Systems, Inc. | Ring laser gyroscope scale factor error control apparatus and method control apparatus and method |
| CN115406468B (en) * | 2022-11-01 | 2023-01-17 | 天津集智航宇科技有限公司 | Device and method for adjusting back scattering of laser gyroscope based on mechanical nano stepping |
Family Cites Families (2)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| US3581227A (en) * | 1968-04-18 | 1971-05-25 | Honeywell Inc | Adjustable, thin membrane mirror for use in the stabilization of ring lasers |
| US3533014A (en) * | 1968-06-04 | 1970-10-06 | Massachusetts Inst Technology | Gas ring laser using oscillating radiation scattering sources within the laser cavity |
-
1977
- 1977-08-31 CA CA285,902A patent/CA1085031A/en not_active Expired
- 1977-09-11 IL IL52920A patent/IL52920A/en unknown
- 1977-10-06 GB GB41708/77A patent/GB1536077A/en not_active Expired
- 1977-10-11 FR FR7730553A patent/FR2370262A1/en active Granted
- 1977-10-12 JP JP12161377A patent/JPS5357793A/en active Granted
- 1977-11-03 DE DE2749157A patent/DE2749157C3/en not_active Expired
-
1984
- 1984-06-27 CA CA000457634A patent/CA1180795B/en not_active Expired
Also Published As
| Publication number | Publication date |
|---|---|
| IL52920A (en) | 1979-09-30 |
| DE2749157C3 (en) | 1980-10-16 |
| FR2370262A1 (en) | 1978-06-02 |
| JPS5357793A (en) | 1978-05-25 |
| GB1536077A (en) | 1978-12-20 |
| DE2749157B2 (en) | 1980-02-28 |
| DE2749157A1 (en) | 1978-05-11 |
| FR2370262B1 (en) | 1980-05-16 |
| JPS5738194B2 (en) | 1982-08-13 |
| CA1085031A (en) | 1980-09-02 |
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| Date | Code | Title | Description |
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| NARE | Reissued | ||
| MKEX | Expiry |