Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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~ 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
. ~, .
:' .
.,' ~`,..... .
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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,
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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
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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
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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
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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.
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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
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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
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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
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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
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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
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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.
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