CA1085499A - Ring lasers - Google Patents
Ring lasersInfo
- Publication number
- CA1085499A CA1085499A CA281,359A CA281359A CA1085499A CA 1085499 A CA1085499 A CA 1085499A CA 281359 A CA281359 A CA 281359A CA 1085499 A CA1085499 A CA 1085499A
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- H—ELECTRICITY
- H01—ELECTRIC ELEMENTS
- H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
- H01S3/00—Lasers, i.e. devices using stimulated emission of electromagnetic radiation in the infrared, visible or ultraviolet wave range
- H01S3/05—Construction or shape of optical resonators; Accommodation of active medium therein; Shape of active medium
- H01S3/08—Construction or shape of optical resonators or components thereof
- H01S3/081—Construction or shape of optical resonators or components thereof comprising three or more reflectors
- H01S3/083—Ring lasers
-
- 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
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- Engineering & Computer Science (AREA)
- Electromagnetism (AREA)
- Optics & Photonics (AREA)
- Plasma & Fusion (AREA)
- Power Engineering (AREA)
- General Physics & Mathematics (AREA)
- Radar, Positioning & Navigation (AREA)
- Remote Sensing (AREA)
- Lasers (AREA)
- Gyroscopes (AREA)
Abstract
ABSTRACT
A ring laser rotation measuring device in which, for reducing null-shift and frequency-pulling, the laser device generates radiation propagated in opposite directions round the loop and which is amplitude modulated to produce contradirectionally propagated pulses, the depth of modulation being such that all the radiation circulating in the closed loop is incorporated in the pulses. The modulation is preferably produced by two optical modulators disposed non-symmetrically with respect to a solid state laser (Nd:YRG), which modulators are operated in synchronism to define two points in the path at equal effective distances from the laser and at which the contra-circulating pulses cross one another, there being no optical component at said points and all the optical components in the loop being separated by distances not less than the pulse length.
A ring laser rotation measuring device in which, for reducing null-shift and frequency-pulling, the laser device generates radiation propagated in opposite directions round the loop and which is amplitude modulated to produce contradirectionally propagated pulses, the depth of modulation being such that all the radiation circulating in the closed loop is incorporated in the pulses. The modulation is preferably produced by two optical modulators disposed non-symmetrically with respect to a solid state laser (Nd:YRG), which modulators are operated in synchronism to define two points in the path at equal effective distances from the laser and at which the contra-circulating pulses cross one another, there being no optical component at said points and all the optical components in the loop being separated by distances not less than the pulse length.
Description
~ his invention relates to ring lasers and especially, but not e;clusively, -to such appara~us for use in a rotation measuring device.
r~ ring laser comprises ~n optical path in the form of a closed loop defirled by three or more reflective surfaces and containing an active laser device. If certain conditions are satisfied the laser device can sustain two continuous light beams travelling in opposite directions around the loop. ~ach beam is composed of light having a number of frequencies;
these are the resonant frequencies of the loop and are a function of the effective length of the closed path. Ihe number of these resonant frequencies is limited by the bandwidth over which the active lasing medium provides gain. If the system is isotropic with respect to the two contra-directional beams, and there is no rotation of the ring laser about an axis normal to the plane of the loop then the frequencies of the light contained in the two beams are identical~ If such rotation is present, however, the effective path length seen by each beam will be different. ~his difference in effective path length~ ~ i5 given to a first order by:-~ ~= 4 ~
: a where A is the area enclosed by the path of the beams, ~_ is the rotation rate about an axis as specified above~ and C is the velocity of light.
For a particular resonant mode such a path length change results in a frequency difference between the two . ~, .
:' .
.,' ~`,..... .
., .
49~
b~a~s~
~ , where~ i9 the w~velcng~h of the mode in question in the no~-rotating state.
~ his froquency difference may be detected to provide a measurement of rotation rate, or alternatively an interference pattern may be derived from the two beams which m~y be conti~uously sensed to provide a measure of the angle through which the ring lase~r has turned during a period of time~ Ring lasers used to measure rotation angle or rotation rate are oommonly referred to as ring laser gyroscopes.
~ aser gyroscopes are subject to a number of sources or error, particularly at low rotation rates. The principal sources of error being the phenomena of lock-in, null shift, and frequency pulling. The most difficult to deal with of these phenomena is lock-in which occurs when the rotation rate of the ring laser is reduced below some critical value known as the lock-in t~reshold. It is in effect a synchronisation of identical msdes in the two contra-direction beams to a common frequency, thus causing the laser gyroscope operating within this region to be unresponsi~e to rotations.
~ he phenomenon of lock-in is due to mutual coupling between backscattered energy from one bea~ with the other beam;
such scattering mainly occurring at the reflective surfaces ~nd in the active medium.
Mechanical biasing methods (spin or dither) have been used to allow a clear frequency region to be formed around the zero-rotation or null-point, but these methods are then subject to null-shift errors~ difficulties with high rotation rates, 1(~8~i499 and cause the loss of a main advantage of laser gyroscopes; that of having no moving parts. Polarization techniques have been proposed to diminish the effects of backscattering, but these tend to introduce anisotropic effects into the ring which again result in null-shift errors.
In general, null-shift and frequency-pulling errors are particular-ly serious in gas lasers. Most known laser gyroscopes suffer these particu-lar deleterious effects since they use a lle/Ne gas mixture as the active medium to obtain the gain chacteristics necessary for the single mode operation that they usually require. Attempts have been made to temporarily separate the two beams by pulsing the beams with weak modulation. The beams are then coincident for a limited region only in the cavity, and hence the effects of scattering are reduced. Such attempts have not significantly improved the performance of the laser gyroscope and have reduced lock-in thresholds only slightly. Particular failings of these attempts are that scat~ered and other radiation in the loop is not reduced sufficiently and that the points at which the pulses coincide vary with rotation rate.
It is an object of the present invention to provide a ring laser suitable for use in a rotation measuring device in which some of the fore-going difficulties are alleviated.
According to the present invention a ring laser comprises a closed loop optical path, a single solid state laser device disposed in said closed loop path for generating electromagnetic radiation in opposite direc-tions round said closed loop path, means for amplitude modulating said radiation comprising two optical modulators disposed in said closed loop path non-symmetrically with respect to the laser device and operable in synchronism to produce contradirectionally propagating pulses which cross at a point between said modulators and a corresponding diametrically opposite point, the depth of modulation imposed by said modulators being such as to eliminate substantially all radiation circulating in said loop outside said 3n contradirectionally propagating pulses, said points at which said contra-directionally propagating pulses cross being situated in the closed path at equal effective distances from said laser device to produce equal intervals i~54g9 between the arrival o~ said pulses at said lascrdevicc.
It will be understood that, in the arrangement of the invention, some radiation may llropagate along a region of the closed loop and not be contained in one of the pulses. However, in accordance with the invention such radiation will be attenuated and will not circulate re~etitively around the closed loo~. ~o achie~e this result the modulation ~epth must be sufficiently great to ensure that radiati.on not contained within the pulses is subject to a high loss, which effectively brings the system gain seen by such radiation to a value below unity.
The laser device may be a rod of neodymium doped yttrium aluminium garnet.
Preferably also, no optical component in the closed loop path is situated at a said cross-over point, and all optical components in the closed loop path are separated by a distance along said path which is not less than the length of 5_ __p . ~, .
3 08549~
said contradirectionally propagat:ing pulses.
In a p~rticular preferred embodiment said two optical modulators are surface acoustic wave devices.
In a preferred applicat:ion of the inven~ion there is is provided means for extracting a portion of the radiation contained witl~n said circulating pulses, a further closed loop optical path round which said extracted portions of said pulses propagate; ~nd detector means located at a point in said further closed loop optical path at which said extracted 1~ portions ~f said pulses cross one another, ~nd operable to detect the frequency di~ference between radiation contained in the portions of the respective pulses~ .
~ he invention will now be described, by way of example~ with re~erence to the accompanying drawings in which:-Figule 1 is a simplified schematic diagram of aring laser in accordance with the invention;
Figure 2 is a more detailed schematic diagram of the ring laser;
Figure 3 is a schematic illustration of the effect of scattered radiation in the ring laser;
Figure 4 is a further schematic illustration of the effect of scattered radiation in the ring laser; and Figure 5 is a schematic diagram of the ring laser having a detecting arrangement and being suitable for a rotation measuring device.
Referring to Figure 1, a ring laser comprises a closed loop optical path 1 in the form of an equilateral triangle b~unded at its apexes by three dielectric mirrors 2, 3 and 4. In one arm of the triangle is situated a Nd doped .
1~85499 Y.1L.G. lascr rod 5 arrange~d to be encrgised by a suitable pumping means 6 to giv~ con-tinuous wave emission. In another arm of tha tri~ngle are two optical modulators 7 and 8 controlled by suitable drive means 18~
In the absence o~ any activation of the modulators 7 and 8 the laser 5 sustains two continuous contradirectional light beams circulating around the triangular path 1, and being reflected in the process by mirrors 2, 3 and 4. ~hese beams are composed of a number of longitudinal modes having a frequency spacin~ of C/L, where ~ is the effective length of the triangular path 1. While the modulators 7 and 8 are inoperative there is no fixed phase relationship between the various modes, and hence the resultant beat frequency produced when the apparatus is subject to rotation will fluctuate.
However, in accordance with the invention the modulators 7 and 8 are operable at a modulation repetition rate approximately equal to the mode frequency spacing C/~, with the result thQt a periodic wave shape is impressed on the propagating light waves. ~he result of such amplitude modulation is to allow ol~y those propagating modes being harmonics of a fundamental mode having a wavelength equal to the effective closed path length to circulate around the loop~ ~he continuously propagating waves are thus locked in relative phase. ~ further effect of the amplitude modulation is to cause the propagating radiation to form pulses which circulate around the loop with a period equal to the modulation repetition rate C/~. Since this is also the transit time for radiation to circulc~te round the closed loop there will at any given time be two pulses propagating around the loop in opposite directions.
~7-Although phase locking ~ld pulse formation occur with a modulation depth which is quit~ sh~llow, the modulators 7 and 8 are operative to provide deep modulation, that is to say a hiKh loss to any radiation which arrives at the modulators out of step with the circulatin~ pulses. ~his loss must be sufficiently great -to ensur~ that the out of step radiation sees a system ~ain of less than one and hence will not circulate repetitively around the closed loop. Due to saturation of the gain medium the maximum system gain at the highest output level is unity~ and conse~uently the radiation in the pulses, which sees this maximum gain~ will be the only radiation which is continuously maintained in the closed loop.
Referri~g now to the more detamled ~igure 2~ the modulators 7 and 8 are surface acoustic wave devices and are separated from one another by a distance d. ~he respective distances between the various components in the closed loop path ~,as shown in Figure 2, are important. ~he necessary restrictions on these separation distances will be made clear during the following discussio~ of some of the situations in which scattered radiation is produced.
It will be appreciated that any scattered radiation which reaches the modulators 7 and 8 at the same time as pulses from either direction will not be attenuated by the modulators, and if such scattered radiation originates from the opposite pulse there will ensue as a result coupling between the contradirectional radiation leadin~ to an increase in the lock-in threshold.
Consider then, the effect of an optical component i.e. mirror, mcdulator, or laser material situated at either of the point~ 9 and 10 at which the pulses cross one another, as defined by the synchronous operation of the modulators.
Figure ~ shows the two scatter-free pulse~ 20 and 21 approaching such a component 22, As the pulses 20 and 21 cross one another (Fig. ~), light is reflected from scatter points in the component 22 causing scattered light to propagate in the opposite direction to the pulse 20 or 21 from which it originated and to be coincident with the contradirectional pulse (Fig, 3c). ~his process gives rise to a strong coupllng between the pulses l~hich cPnnot be removed by the modulators 7 and 8. Hence the first condition restricting the positioning of the optical components is that no component should be situated at either of the cross-over points 9 and 10. It will be appreciated that such a condition is impossible to satisfy with a single modulator since one cross-over point would then inevitably fall with;n the modulating material, A further source of scattered radiation which is capable of circulating with the pulses is the radiation produced by the scattering of previously scattered radiation. ~igure 4a shows one pulse 26 of spatial length 1 approaching two optical components 27 and 28 separated by a distance r, where r ~l/2. Radiation 29 scattered from the first component 27 (Fig. 4b~ propagates in the opposite direction to the pulse 26 and is attenuated at the modulators 7 and 8. Sim~,larly an amount of radiation 30 is scattered frc~ the second component 28 and is similarly attenuated at the modulators 7 and 8.
However, some of the radiation scattered from the second component 28 (Fig. 4c) is rescattered at the reverse side of the _g_ 49~
first compon(~nt ~7 and subscquerltly propagates with the pulse 26. Fig~e 4d sllows the result~nt shape of the pulso 26 whîch cont~ins a region ~1 wher~ the rescattered radiation ov~rlaps the pulse 26, ~nd a ~tail' region ~2 of scattcred radiation.
~he tail region 32 is a-ttenuated at the modulators 7 and 8, but the overlapping region 31 circulates with the pulse 26 and results in interfering noise~ Hence the second ~ondition restricting the positions of the optical co~ponents i5 that all components should be separated by a distance not less than half the spatial length of the pulses.
It will be appreciated that scatter centres in the laser rod 5 or in the modulators 7 and 8 are unavoidably closer than half the spatial length of the pulses. ~urthermore~ the cross-coupling of the contradirectional radiation occurs principally in non-linear regions in the path 1 which are particularly the modulating and lasing mediums~ It is therefore essential that the laser rod 5 and modulators 7 and 8 be as short as possible both to minimize the number of scattering centres and to minimize the extent of non-linear regions of the path. In this respect, the solid s-tate, neodymium doped Y.~. lasing medium allows a significant reduction in the length of the lasing medium compared with a gas laser of comparable gain.
A further point to be considered in the positioning of the components i~ the closed path 1 is that the pulses should arrive at the lasing medium at equal intervals. ~his needs to be so or else the pulse circulating in one direction will see a more depleted gain region than the pulse circulating in the other direc~ion which may eventually result in the -~085499 extinction of on~ of the puls!~s. ~o satisfy this requircment the modulators .~e so position~d that the cross-over points 9 and 10 are e~ually opticall~ distant from the laser rod 5.
~hus the pulscs arrivc at the lasing medium at equal intervals of one half the tr~nsit time~
Referring once again to ~igure 2, the various separations of the components are shown in terms of the distance d separating the modulators 7 and 8. These separations satisfy all the require~ents outlined above. ~hus the length of an arm of the equilateral triangular loop is 4d, the modulators 7 and 8 are made effectively open (i.e. present a low loss) to radiation for a period of 2d/C, they are opened simultaneously, and as previously stated the repetition r~te is C/12d. ~hese dimensions give rise to a spatial pulse length of d.
Referring now to ~igure 5, a detecting arrangeme~t 11 is added to the ring laser to enable the ring laser to be used as a gyroscopeO ~he mirror 2 of Figure 2 is replaced by a partially reflective mirror 12 which allows a small percentage of the pulse radiation to pass into an equilaterally triangular path 13 having sides of length d and defined by the partially reflective mirror 12, a dielectric mirror 14 and a frequency difference detector 15. ~he contradirectional pulses passing through the mirror 12 cross one another at the point 16 at which the detector 15 is placed, thus enabling a beat frequency measurement to be obtained from which the rate of rotation, or upon integration the rotation angle~ can be derived.
It will be appreciated that the constancy of the 1~354~9 positions of the cross-over points 9, 10 and 16 is o~ prime iIlportance, since not onl-~ is this necessary ~or the require-ments for the avoidance of scattering to be known to be satisfied, but also it enables the position of -the detector 15 to be kept constant and accurat0.
In pr~ctice, when the apparatus is subject to rotation there is a difference between the apparent speeds of the con-tradirectional circulating pulses, and therefore a pulse from one direction will arrive at the modulators 7 and 8 before the corresponding pulse from the other direction with the result that the ~head' o~ the faster pulse or the 'tail' of the slower pulse will be attenuated at the modulators according to which pulse fre~uency -the modulation repetition rate is derived from. ~his attenuation is compensated automatically in -the system by an equivalent lengthening of~ respectively, the head of the pulse whose tail is attenuated or the tail of the pulse whose head is attenuated, due to the fixed period during which the modulators provide a loss low enough to be overcome by the gain medium. ~hus the cross-over points and the pulse lengths remain substantially constant even when the apparatus is subject to high rates of rotation.
lypically the length of one arm of the triangular path 1 would be 200 mm, and since the time taken for a laser beam to travel around the whole length o the triangular path 1 would be substantially 2 nanoseconds a suitable modulation fre~uency would be 500 MHz. ~he depth of modulation would be at least 9~/o. ~he maximum length of the laser rod 5 should be ~()8~99 less th~l onc quarter of the spac-ing bet~Jeen the mirrors 2 ~nd 3, i.c. the distance d should be less tha~ 50 mm. In practice a Y.f~.G. laser rod 5 ha~ing a length of 25 mm would be suitable with a distance of 45 mm between the modulators 7 and 8 using a pulse length of greater than 100 ps~ lcaving a margin for other Constraints. ~he diameter of the laser rod 5 would suitably be 3 mm which is a compromise between making the diameter as small as possible in order to minimise the threshold energy input required to produce laser oscillation and using a laser rod of larger diameter such as would facilitate optical coupling between a pumping lamp and the laser rod 5.
r~ ring laser comprises ~n optical path in the form of a closed loop defirled by three or more reflective surfaces and containing an active laser device. If certain conditions are satisfied the laser device can sustain two continuous light beams travelling in opposite directions around the loop. ~ach beam is composed of light having a number of frequencies;
these are the resonant frequencies of the loop and are a function of the effective length of the closed path. Ihe number of these resonant frequencies is limited by the bandwidth over which the active lasing medium provides gain. If the system is isotropic with respect to the two contra-directional beams, and there is no rotation of the ring laser about an axis normal to the plane of the loop then the frequencies of the light contained in the two beams are identical~ If such rotation is present, however, the effective path length seen by each beam will be different. ~his difference in effective path length~ ~ i5 given to a first order by:-~ ~= 4 ~
: a where A is the area enclosed by the path of the beams, ~_ is the rotation rate about an axis as specified above~ and C is the velocity of light.
For a particular resonant mode such a path length change results in a frequency difference between the two . ~, .
:' .
.,' ~`,..... .
., .
49~
b~a~s~
~ , where~ i9 the w~velcng~h of the mode in question in the no~-rotating state.
~ his froquency difference may be detected to provide a measurement of rotation rate, or alternatively an interference pattern may be derived from the two beams which m~y be conti~uously sensed to provide a measure of the angle through which the ring lase~r has turned during a period of time~ Ring lasers used to measure rotation angle or rotation rate are oommonly referred to as ring laser gyroscopes.
~ aser gyroscopes are subject to a number of sources or error, particularly at low rotation rates. The principal sources of error being the phenomena of lock-in, null shift, and frequency pulling. The most difficult to deal with of these phenomena is lock-in which occurs when the rotation rate of the ring laser is reduced below some critical value known as the lock-in t~reshold. It is in effect a synchronisation of identical msdes in the two contra-direction beams to a common frequency, thus causing the laser gyroscope operating within this region to be unresponsi~e to rotations.
~ he phenomenon of lock-in is due to mutual coupling between backscattered energy from one bea~ with the other beam;
such scattering mainly occurring at the reflective surfaces ~nd in the active medium.
Mechanical biasing methods (spin or dither) have been used to allow a clear frequency region to be formed around the zero-rotation or null-point, but these methods are then subject to null-shift errors~ difficulties with high rotation rates, 1(~8~i499 and cause the loss of a main advantage of laser gyroscopes; that of having no moving parts. Polarization techniques have been proposed to diminish the effects of backscattering, but these tend to introduce anisotropic effects into the ring which again result in null-shift errors.
In general, null-shift and frequency-pulling errors are particular-ly serious in gas lasers. Most known laser gyroscopes suffer these particu-lar deleterious effects since they use a lle/Ne gas mixture as the active medium to obtain the gain chacteristics necessary for the single mode operation that they usually require. Attempts have been made to temporarily separate the two beams by pulsing the beams with weak modulation. The beams are then coincident for a limited region only in the cavity, and hence the effects of scattering are reduced. Such attempts have not significantly improved the performance of the laser gyroscope and have reduced lock-in thresholds only slightly. Particular failings of these attempts are that scat~ered and other radiation in the loop is not reduced sufficiently and that the points at which the pulses coincide vary with rotation rate.
It is an object of the present invention to provide a ring laser suitable for use in a rotation measuring device in which some of the fore-going difficulties are alleviated.
According to the present invention a ring laser comprises a closed loop optical path, a single solid state laser device disposed in said closed loop path for generating electromagnetic radiation in opposite direc-tions round said closed loop path, means for amplitude modulating said radiation comprising two optical modulators disposed in said closed loop path non-symmetrically with respect to the laser device and operable in synchronism to produce contradirectionally propagating pulses which cross at a point between said modulators and a corresponding diametrically opposite point, the depth of modulation imposed by said modulators being such as to eliminate substantially all radiation circulating in said loop outside said 3n contradirectionally propagating pulses, said points at which said contra-directionally propagating pulses cross being situated in the closed path at equal effective distances from said laser device to produce equal intervals i~54g9 between the arrival o~ said pulses at said lascrdevicc.
It will be understood that, in the arrangement of the invention, some radiation may llropagate along a region of the closed loop and not be contained in one of the pulses. However, in accordance with the invention such radiation will be attenuated and will not circulate re~etitively around the closed loo~. ~o achie~e this result the modulation ~epth must be sufficiently great to ensure that radiati.on not contained within the pulses is subject to a high loss, which effectively brings the system gain seen by such radiation to a value below unity.
The laser device may be a rod of neodymium doped yttrium aluminium garnet.
Preferably also, no optical component in the closed loop path is situated at a said cross-over point, and all optical components in the closed loop path are separated by a distance along said path which is not less than the length of 5_ __p . ~, .
3 08549~
said contradirectionally propagat:ing pulses.
In a p~rticular preferred embodiment said two optical modulators are surface acoustic wave devices.
In a preferred applicat:ion of the inven~ion there is is provided means for extracting a portion of the radiation contained witl~n said circulating pulses, a further closed loop optical path round which said extracted portions of said pulses propagate; ~nd detector means located at a point in said further closed loop optical path at which said extracted 1~ portions ~f said pulses cross one another, ~nd operable to detect the frequency di~ference between radiation contained in the portions of the respective pulses~ .
~ he invention will now be described, by way of example~ with re~erence to the accompanying drawings in which:-Figule 1 is a simplified schematic diagram of aring laser in accordance with the invention;
Figure 2 is a more detailed schematic diagram of the ring laser;
Figure 3 is a schematic illustration of the effect of scattered radiation in the ring laser;
Figure 4 is a further schematic illustration of the effect of scattered radiation in the ring laser; and Figure 5 is a schematic diagram of the ring laser having a detecting arrangement and being suitable for a rotation measuring device.
Referring to Figure 1, a ring laser comprises a closed loop optical path 1 in the form of an equilateral triangle b~unded at its apexes by three dielectric mirrors 2, 3 and 4. In one arm of the triangle is situated a Nd doped .
1~85499 Y.1L.G. lascr rod 5 arrange~d to be encrgised by a suitable pumping means 6 to giv~ con-tinuous wave emission. In another arm of tha tri~ngle are two optical modulators 7 and 8 controlled by suitable drive means 18~
In the absence o~ any activation of the modulators 7 and 8 the laser 5 sustains two continuous contradirectional light beams circulating around the triangular path 1, and being reflected in the process by mirrors 2, 3 and 4. ~hese beams are composed of a number of longitudinal modes having a frequency spacin~ of C/L, where ~ is the effective length of the triangular path 1. While the modulators 7 and 8 are inoperative there is no fixed phase relationship between the various modes, and hence the resultant beat frequency produced when the apparatus is subject to rotation will fluctuate.
However, in accordance with the invention the modulators 7 and 8 are operable at a modulation repetition rate approximately equal to the mode frequency spacing C/~, with the result thQt a periodic wave shape is impressed on the propagating light waves. ~he result of such amplitude modulation is to allow ol~y those propagating modes being harmonics of a fundamental mode having a wavelength equal to the effective closed path length to circulate around the loop~ ~he continuously propagating waves are thus locked in relative phase. ~ further effect of the amplitude modulation is to cause the propagating radiation to form pulses which circulate around the loop with a period equal to the modulation repetition rate C/~. Since this is also the transit time for radiation to circulc~te round the closed loop there will at any given time be two pulses propagating around the loop in opposite directions.
~7-Although phase locking ~ld pulse formation occur with a modulation depth which is quit~ sh~llow, the modulators 7 and 8 are operative to provide deep modulation, that is to say a hiKh loss to any radiation which arrives at the modulators out of step with the circulatin~ pulses. ~his loss must be sufficiently great -to ensur~ that the out of step radiation sees a system ~ain of less than one and hence will not circulate repetitively around the closed loop. Due to saturation of the gain medium the maximum system gain at the highest output level is unity~ and conse~uently the radiation in the pulses, which sees this maximum gain~ will be the only radiation which is continuously maintained in the closed loop.
Referri~g now to the more detamled ~igure 2~ the modulators 7 and 8 are surface acoustic wave devices and are separated from one another by a distance d. ~he respective distances between the various components in the closed loop path ~,as shown in Figure 2, are important. ~he necessary restrictions on these separation distances will be made clear during the following discussio~ of some of the situations in which scattered radiation is produced.
It will be appreciated that any scattered radiation which reaches the modulators 7 and 8 at the same time as pulses from either direction will not be attenuated by the modulators, and if such scattered radiation originates from the opposite pulse there will ensue as a result coupling between the contradirectional radiation leadin~ to an increase in the lock-in threshold.
Consider then, the effect of an optical component i.e. mirror, mcdulator, or laser material situated at either of the point~ 9 and 10 at which the pulses cross one another, as defined by the synchronous operation of the modulators.
Figure ~ shows the two scatter-free pulse~ 20 and 21 approaching such a component 22, As the pulses 20 and 21 cross one another (Fig. ~), light is reflected from scatter points in the component 22 causing scattered light to propagate in the opposite direction to the pulse 20 or 21 from which it originated and to be coincident with the contradirectional pulse (Fig, 3c). ~his process gives rise to a strong coupllng between the pulses l~hich cPnnot be removed by the modulators 7 and 8. Hence the first condition restricting the positioning of the optical components is that no component should be situated at either of the cross-over points 9 and 10. It will be appreciated that such a condition is impossible to satisfy with a single modulator since one cross-over point would then inevitably fall with;n the modulating material, A further source of scattered radiation which is capable of circulating with the pulses is the radiation produced by the scattering of previously scattered radiation. ~igure 4a shows one pulse 26 of spatial length 1 approaching two optical components 27 and 28 separated by a distance r, where r ~l/2. Radiation 29 scattered from the first component 27 (Fig. 4b~ propagates in the opposite direction to the pulse 26 and is attenuated at the modulators 7 and 8. Sim~,larly an amount of radiation 30 is scattered frc~ the second component 28 and is similarly attenuated at the modulators 7 and 8.
However, some of the radiation scattered from the second component 28 (Fig. 4c) is rescattered at the reverse side of the _g_ 49~
first compon(~nt ~7 and subscquerltly propagates with the pulse 26. Fig~e 4d sllows the result~nt shape of the pulso 26 whîch cont~ins a region ~1 wher~ the rescattered radiation ov~rlaps the pulse 26, ~nd a ~tail' region ~2 of scattcred radiation.
~he tail region 32 is a-ttenuated at the modulators 7 and 8, but the overlapping region 31 circulates with the pulse 26 and results in interfering noise~ Hence the second ~ondition restricting the positions of the optical co~ponents i5 that all components should be separated by a distance not less than half the spatial length of the pulses.
It will be appreciated that scatter centres in the laser rod 5 or in the modulators 7 and 8 are unavoidably closer than half the spatial length of the pulses. ~urthermore~ the cross-coupling of the contradirectional radiation occurs principally in non-linear regions in the path 1 which are particularly the modulating and lasing mediums~ It is therefore essential that the laser rod 5 and modulators 7 and 8 be as short as possible both to minimize the number of scattering centres and to minimize the extent of non-linear regions of the path. In this respect, the solid s-tate, neodymium doped Y.~. lasing medium allows a significant reduction in the length of the lasing medium compared with a gas laser of comparable gain.
A further point to be considered in the positioning of the components i~ the closed path 1 is that the pulses should arrive at the lasing medium at equal intervals. ~his needs to be so or else the pulse circulating in one direction will see a more depleted gain region than the pulse circulating in the other direc~ion which may eventually result in the -~085499 extinction of on~ of the puls!~s. ~o satisfy this requircment the modulators .~e so position~d that the cross-over points 9 and 10 are e~ually opticall~ distant from the laser rod 5.
~hus the pulscs arrivc at the lasing medium at equal intervals of one half the tr~nsit time~
Referring once again to ~igure 2, the various separations of the components are shown in terms of the distance d separating the modulators 7 and 8. These separations satisfy all the require~ents outlined above. ~hus the length of an arm of the equilateral triangular loop is 4d, the modulators 7 and 8 are made effectively open (i.e. present a low loss) to radiation for a period of 2d/C, they are opened simultaneously, and as previously stated the repetition r~te is C/12d. ~hese dimensions give rise to a spatial pulse length of d.
Referring now to ~igure 5, a detecting arrangeme~t 11 is added to the ring laser to enable the ring laser to be used as a gyroscopeO ~he mirror 2 of Figure 2 is replaced by a partially reflective mirror 12 which allows a small percentage of the pulse radiation to pass into an equilaterally triangular path 13 having sides of length d and defined by the partially reflective mirror 12, a dielectric mirror 14 and a frequency difference detector 15. ~he contradirectional pulses passing through the mirror 12 cross one another at the point 16 at which the detector 15 is placed, thus enabling a beat frequency measurement to be obtained from which the rate of rotation, or upon integration the rotation angle~ can be derived.
It will be appreciated that the constancy of the 1~354~9 positions of the cross-over points 9, 10 and 16 is o~ prime iIlportance, since not onl-~ is this necessary ~or the require-ments for the avoidance of scattering to be known to be satisfied, but also it enables the position of -the detector 15 to be kept constant and accurat0.
In pr~ctice, when the apparatus is subject to rotation there is a difference between the apparent speeds of the con-tradirectional circulating pulses, and therefore a pulse from one direction will arrive at the modulators 7 and 8 before the corresponding pulse from the other direction with the result that the ~head' o~ the faster pulse or the 'tail' of the slower pulse will be attenuated at the modulators according to which pulse fre~uency -the modulation repetition rate is derived from. ~his attenuation is compensated automatically in -the system by an equivalent lengthening of~ respectively, the head of the pulse whose tail is attenuated or the tail of the pulse whose head is attenuated, due to the fixed period during which the modulators provide a loss low enough to be overcome by the gain medium. ~hus the cross-over points and the pulse lengths remain substantially constant even when the apparatus is subject to high rates of rotation.
lypically the length of one arm of the triangular path 1 would be 200 mm, and since the time taken for a laser beam to travel around the whole length o the triangular path 1 would be substantially 2 nanoseconds a suitable modulation fre~uency would be 500 MHz. ~he depth of modulation would be at least 9~/o. ~he maximum length of the laser rod 5 should be ~()8~99 less th~l onc quarter of the spac-ing bet~Jeen the mirrors 2 ~nd 3, i.c. the distance d should be less tha~ 50 mm. In practice a Y.f~.G. laser rod 5 ha~ing a length of 25 mm would be suitable with a distance of 45 mm between the modulators 7 and 8 using a pulse length of greater than 100 ps~ lcaving a margin for other Constraints. ~he diameter of the laser rod 5 would suitably be 3 mm which is a compromise between making the diameter as small as possible in order to minimise the threshold energy input required to produce laser oscillation and using a laser rod of larger diameter such as would facilitate optical coupling between a pumping lamp and the laser rod 5.
Claims (2)
1. A ring laser comprising a closed loop optical path, a single solid state laser device disposed in said closed loop path for generating electromagnetic radiation in opposite directions round said closed loop path, means for amplitude modulating said radiation comprising two optical modulators disposed in said closed loop path non-symmetrically with respect to the laser device and operable in synchronism to produce contradirectionally propagating pulses which cross at a point between said modulators and a corresponding diametrically opposite point, the depth of modulation imposed by said modulators being such as to eliminate substantially all radiation circulating in said loop outside said contradirectionally propagating pulses, said points at which said contra-directionally propagating pulses cross being situated in the closed path at equal effective distances from said laser device to produce equal intervals between the arrival of said pulses at said laser device.
2. A ring laser as claimed in Claim 1, wherein the laser device is a rod of neodymium doped yttrium aluminium garnet.
Applications Claiming Priority (2)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| GB28291/76 | 1976-07-07 | ||
| GB2829176A GB1542723A (en) | 1976-07-07 | 1976-07-07 | Ring lasers |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA1085499A true CA1085499A (en) | 1980-09-09 |
Family
ID=10273335
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA281,359A Expired CA1085499A (en) | 1976-07-07 | 1977-06-24 | Ring lasers |
Country Status (4)
| Country | Link |
|---|---|
| CA (1) | CA1085499A (en) |
| DE (1) | DE2730427A1 (en) |
| FR (1) | FR2358038A1 (en) |
| GB (1) | GB1542723A (en) |
Cited By (1)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| US7446879B2 (en) * | 2003-05-16 | 2008-11-04 | Thales | Solid-state gyrolaser stabilised by acousto-optic devices |
Families Citing this family (1)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| DE3921424A1 (en) * | 1989-06-29 | 1991-01-03 | Max Planck Gesellschaft | RINGLASER-GIRLS |
Family Cites Families (3)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| DE1292899B (en) * | 1965-04-02 | 1969-04-17 | Honeywell Inc | Device for measuring a rotary movement |
| US3627422A (en) * | 1965-06-21 | 1971-12-14 | Varian Associates | Means for avoiding locking in ring lasers |
| US3714607A (en) * | 1970-08-07 | 1973-01-30 | Hewlett Packard Co | Acousto optic method and apparatus for mode decoupling a ring laser |
-
1976
- 1976-07-07 GB GB2829176A patent/GB1542723A/en not_active Expired
-
1977
- 1977-06-24 CA CA281,359A patent/CA1085499A/en not_active Expired
- 1977-07-06 DE DE19772730427 patent/DE2730427A1/en not_active Ceased
- 1977-07-06 FR FR7720867A patent/FR2358038A1/en active Granted
Cited By (1)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| US7446879B2 (en) * | 2003-05-16 | 2008-11-04 | Thales | Solid-state gyrolaser stabilised by acousto-optic devices |
Also Published As
| Publication number | Publication date |
|---|---|
| DE2730427A1 (en) | 1978-02-16 |
| GB1542723A (en) | 1979-03-21 |
| FR2358038A1 (en) | 1978-02-03 |
| FR2358038B1 (en) | 1984-04-20 |
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