Fibre optic gyroscopes

A fibre optic gyroscope including a fibre optic loop, a radiation source for generating a broadband signal, splitting means for splitting radiation from the source into two components and inserting them into the loop in clockwise (CW) and counter clockwise (CCW) directions, recombining means for recombining the components interferometrically to give an indication of rotation rate of the loop, characterized in that there is further provided amplification means for amplifying both components travelling within the loop.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The invention relates to improvements in fibre optic gyroscopes. Such 
devices, otherwise known as a fibre resonator gyros, comprise a fibre or 
fibres defining a resonator loop through which clockwise (CW) and 
counter-clockwise (CCW) components of radiation are passed. Both CW and 
CCW directions are maintained at resonance by a suitable control and the 
difference in frequency between the resonances is a measure of the rate of 
rotation applied to the gyro. 
2. Discussion of Prior Art 
Ideally such devices will have high sensitivity to changes in rotation 
rate, low backscatter and minimal Kerr effect. However, in practice it has 
proved difficult or even impossible to satisfy all these criteria 
simultaneously. For example, a fibre optic gyroscope of the ring resonator 
type which is characterised by having a relatively short loop of between, 
say, 10 and 30 meters and in which the light from a narrow band source 
passes around the loop a relatively large number of times, has the 
disadvantage of significant backscatter throughout the light path, 
although the sensitivity is relatively high. Furthermore, a fibre optic 
gyroscope of the reciprocal interferometer type which is characterised by 
having a relatively long loop of between, say, 300 and 1000 meters and in 
which the light from a broad band source passes around the loop a 
relatively small number of times, has the disadvantage of relatively low 
sensitivity, but does not suffer significantly from backscatter or the 
Kerr effect. 
SUMMARY OF THE INVENTION 
An object of the present invention is to provide a fibre optic gyroscope in 
which sensitivity is enhance but backscatter and the Kerr effect are kept 
to a minimum. 
According to one aspect of the present invention there is provided a fibre 
optic gyroscope including a fibre optic loop, a radiation source for 
generating a broad band signal, splitting means for splitting radiation 
from the source into two components and inserting them into the loop in 
clockwise (CW) and counterclockwise (CCW) directions, re-combining means 
for re-combining the components interferometrically to give an indication 
of rotation rate of the loop, characterised in that there is further 
provided amplification means for amplifying both components travelling 
within the lop. 
Preferably the amplification means includes a laser radiation source and 
coupling means associated with the loop whereby radiation from the laser 
source provides gain in both the CW and CCW components.

DETAILED DISCUSSION OF PREFERRED EMBODIMENTS 
The illustrated gyroscope is shown generally at 1. A broad band source 2 
provides radiation, at say 1.06 .mu.m, which is transmitted through a 
first coupler arranged to split the radiation in to two halves 4 and 5, 
one of which, 4, is then passed through a mode filter 6 to a second 
coupler 7. Again the radiation is split into two halves 8 and 9 both of 
which are then passed through a third coupler 10 into a loop of say 
neodymium fibre 11. The loop may be any length between, say, 300 and 1000 
meters. 
After the radiation is split at coupler 10, two equal coherent components 
are caused to travel in CW and CCW around the loop. The two component then 
interferometrically re-combined at coupler 10 before being passed back to 
a detector 12 via the couplers 7 and 3. The components will only combine 
interferometrically where they have experienced the same number of 
transits around the fibre loop. Each interferometrical combination, for a 
given pair of coherent components, are added together to give any 
intensity dependance on rate .OMEGA. I (.OMEGA.) given by: 
##EQU1## 
Where .gamma.=Transmission coeficient of light 
r=Coupling ratio of coupler 11 
Io=Source intensity 
The sensitivity of the response to rate of the gyroscope is given by: 
##EQU2## 
Where 
.alpha.=2.gamma..sup.2 r.sup.2 
.beta.=.gamma..sup.4 r.sup.4 
.delta.=.gamma..sup.2 r.sup.2 
T=0.5 .gamma..sup.2 (1-r.sup.2).sup.2 
When using typical values for .gamma. and r and evaluating at the peak 
value, then it is found that there is a reduction in sensitivity of the 
gyroscope compared with that which would normally be expected. 
If gain is introduced into the fibre loop 11, then the transmission of the 
coupler 10 and the transmission loss to the fibre itself can be 
compensated, as long as .gamma.r.ltoreq.1. 
The reason for this is that if .gamma.r&gt;1 then undesired self-sustained 
laser oscillation will result. However, by having .gamma.r&lt;1, but always 
close to 1, there can be a considerable improvement in sensitivity over a 
conventional gyroscope system. 
The gain is introduced by providing a dichroic coupler 13 in association 
with the loop 11. The dichroic coupler is fed with pump radiation at say 
820 nm from a laser source 14, and causes the radiation to enter the loop 
with a high efficiency. The radiation at wavelength 1.06 .mu.m (i.e., the 
broadband source) is transmitted through the coupler 13 without 
attenuation i.e. r.fwdarw.1 at 820 nm and r.fwdarw.0 at 1.06 .mu.m. A 
neodymium doped fibre with gain at 1.06 .mu.m is spliced into the loop so 
that the pump light at 820 nm gives rise to a controlled amount of gain 
within the loop. 
The user of the neodymium doped fibre in such a scheme gives rise to a 
broad band gain which will be reciprocal. Further advantages are as 
follows: 
a) A broadband source can be used for the gyro with a thermal wavelength 
distribution to eliminate Kerr effects. Also polarisation errors are 
eliminated and backscatter effects are significantly reduced. These are 
effects which are shared with the fibre optic gyro. 
b) The gain will tend to be polarisation sensitive depending on the 
orientation of the polarisation of the pump. If the gain depends on 
polarisation and this is aligned to the correct fibre axis the effect will 
be to suppress the unwanted polarisation which will not seen the 
sensitivity increase. 
c) There is no need for path length control (as in the ring resonant gyro) 
as successive round trips of light are incoherent with respect to one 
another. 
d) As lasing action does not occur the gain will not saturate so there 
should be no mode competition effects between the two directions, as the 
system is working in a linear region of gain. The gain should be matched 
in the two directions, which will eliminate the effects of differential 
mode pulling (causing a gyro bias) due to different refractive indexes in 
the two directions. 
FIGS. 2 and 3 show typical fibre optic gyroscope intensity and sensitivity 
characteristics for two different .gamma. r values, for FIG. 2 
.gamma.r=0.81 and for FIG. 3 .gamma.r=0.045. On comparison of the graphs, 
we can see that for a typical fibre optic gyroscope by increasing .gamma.r 
there is narrowing of the peaks, but a reduction in sensitivity and 
intensity. 
By introducing a gain as described above the characteristics are as shown 
in FIGS. 4 and 5. For a high value of .gamma.r, namely .gamma.r=0.88 the 
intensity is increased by, about, a factor of 4, and more significantly 
the sensitivity of the gyroscope is increased by a factor of over 100. 
A value of gain of about 30% per round trip is required to maintain the 
value of .gamma.r=1 however the exact value will vary depending upon the 
values of .alpha. and r chosen. Different situations will dictate that 
different values of .alpha. and r be chosen. 
The pump laser may require regulation during operation of the gyroscope to 
inhibit lasing occurring. This may be achieved by regulating the pump in 
response to the detectors responses by means of a feedback loop. 
It should be noted that the pump laser and dichroic coupler may be replaced 
by other suitable index matched gain media as long as there is no 
likelihood of back reflection. 
It should further be noted that for different broadband sources, different 
pumped lasers may be required, e.g. it is possible to use Erbuim doped 
fibre to give gain in a source at 1.550 nm by using a pump at 988 nm.