Optical device for sustaining a radiant energy pulse which circulates within a monomode wave guide a gyrometer and a hydrophone equipped with said optical device

An interaction medium in which a photoinduced phase grating has been permanently recorded beforehand is coupled to the two ends of a monomode waveguide. A first wave which is a pulse travels through said waveguide and interferes with a second pumping wave in order to permit energy transfer from the second pumping wave to the first wave.

This invention relates to a device for sustaining a radiant energy pulse 
which travels within a monomode waveguide and is particularly concerned 
with the application of said device to a gyrometer and to a hydrophone. 
A device of this type finds an application in the field of measurement of 
small phase shifts between two interfering beams. Thus the waveguide 
employed in the case of an interferometric gyrometer is designed in the 
form of a loop which can be a coiled optical fiber having a predetermined 
number of turns. The angular velocity to be measured in this case is 
determined by interferometric measurement of a phase shift after one of 
the interfering beams has passed through the waveguide. 
In the field of acoustic detection in an underwater or ocean environment, a 
monomode optical-fiber hydrophone which operates by elastooptical effect 
and utilizes the effects of interaction between the acoustic wave to be 
detected and a monomode optical fiber on which said acoustic wave produces 
action comprises in particular a monomode optical fiber immersed in the 
water in which the sound wave propagates. Propagation of said sound wave 
produces variations in pressure within the propagation medium. As a result 
of the elastooptical effect, these pressure variations in turn produce 
variations in the geometrical and optical parameters of the fiber. A wave 
which propagates within the optical fiber undergoes phase variations which 
can be detected by interferometry by means of a second monomode optical 
fiber which forms a reference arm. 
However, losses arise at the time of propagation of the light waves within 
the waveguide considered and this explains the need to position a 
loss-compensating system on the path of the light wave. 
Thus the invention relates to a device for causing a radiant energy pulse 
produced by a coherent source to travel within an optical waveguide while 
compensating at each revolution for the losses introduced at the time of 
propagation. The pulse is thus regenerated at a constant level. By means 
of the device according to the invention, it is also possible to 
contemplate amplification of an incident radiant energy pulse by 
transferring the energy of a pumping beam to the signal. The device thus 
proposed has the function of causing a radiant energy pulse to travel 
within a fiber while maintaining the level of the signal irrespective of 
the number of revolutions which have been completed. In the case of an 
integration of N revolutions, the device induces larger phase shifts in 
the signal wave; these phase shifts are more readily measurable. 
The present invention is directed to an optical device for sustaining a 
radiant energy pulse within a monomode waveguide. Said device comprises a 
monomode optical waveguide in which is passed a first wave consisting of a 
radiant energy pulse produced by a coherent source. Optical connecting 
means are optically coupled to the end faces of said waveguide in order to 
form a circulation loop. The distinctive feature of the invention lies in 
the fact that the aforesaid optical connecting means consist of an 
interaction medium in which a phase grating has been photoinduced by the 
interference of two beams produced by said coherent source. Said 
interaction medium is coupled to the two end faces of said waveguide 
through the intermediary of optical focusing means.

The invention utilizes the phenomena of wave coupling at the time of 
reading of phase gratings. Consideration is therefore given to a phase 
grating having high diffraction efficiency and permanently recorded on a 
photosensitive substrate constituted by thick diffracting phase structures 
photoinduced by refractive index variation within electrooptical crystals, 
photopolymers or whitened argentic materials. 
Materials are in fact currently available for writing phase gratings or 
holograms having high diffraction efficiency and permitting permanent 
storage of information. These materials are "thick" with respect to the 
mean pitch of the photoinduced strata and their behavior at the time of 
writing and reading is described by the formalism of coupled waves 
governed by the Bragg relation. 
In the simple case of interference of two plane waves which arrive on the 
photosensitive substrate at the angles of incidence +.THETA. and -.THETA., 
the spacing .LAMBDA. of the diffraction planes within the substrate is 
related to the angle .THETA. and to the wavelength by the Bragg equation 
2.LAMBDA.sin .THETA.=.lambda.. Conversely, a given grating having a 
spacing .LAMBDA. will significantly diffract a reading beam of wavelength 
.lambda. if it arrives on the medium at the angle .THETA. which is related 
to .lambda. and .LAMBDA. by said Bragg equation. A given grating made up 
of parallel diffraction planes having a spacing .LAMBDA. can be re-read by 
a beam having a wavelength .lambda.'. It is only necessary to ensure that 
its angle of incidence .THETA.' is so adjusted as to ensure that the Bragg 
law 2.LAMBDA.sin .THETA.'=.lambda.' is satisfied. 
If, as shown in FIG. 1, said grating is illuminated by two coherent plane 
waves R and S emerging from the same laser and having the same intensity, 
we have the relation .LAMBDA.=.lambda./(2 sin .THETA.). 
The spacing .LAMBDA. resulting from the interference of the beams R and S 
is strictly identical with the spacing of the phase grating. By way of 
example, the same beams R and S were used for the fabrication of the 
strata grating which has previously been written. 
Within the thickness of the medium considered, the object wave S has thus 
interfered with a reference wave R having a plane wave front. A pattern of 
interference fringes has thus been formed and has produced within the 
crystal a refractive-index strata grating. This strata grating diffracts 
part of the energy of the reference wave R so as to produce a diffracted 
wave S', the wave front of which is isomorphous with the object wave 
front. 
Thus, according to the formalism of coupled waves, redistribution of energy 
takes place between the two beams after these latter have passed through 
the crystal. This new energy distribution is a function of the relative 
phase shift .PSI. between the two waves. The following notations are thus 
adopted: 
I.sub.R,I.sub.S : the intensity of the two interfering waves; 
I.sub.R ',I.sub.S ': the intensity of the two waves at the exit of the 
crystal; 
.PSI.: the phase shift of the light intensity with respect to the 
photoinduced phase grating; 
.eta.: the diffraction efficiency of the structure. 
The intensities I.sub.S and I.sub.R are obtained respectively by means of 
the following relationships: 
##EQU1## 
In the particular case of the figure I.sub.R =I.sub.S =I.sub.O, hence: 
##EQU2## 
Optimum energy transfer is therefore achieved when the following conditions 
have been satisfied simultaneously: 
EQU .eta.=50% .PSI.=.pi./2 
In this case, the following values are obtained after traversal of the 
phase grating: 
EQU I.sub.R '=0; I.sub.S '=2I.sub.O 
The physical interpretation of the phenomena is as follows: the incident 
wave R is diffracted by the three-dimensional phase grating. The wave S' 
thus generated has a phase lag, for example, of .pi./2 with respect to the 
reading wave R. In a medium 1 in which two waves interfere, there is in 
fact a phase shift of .pi./2 between the interference pattern 31 of the 
two beams and the phase grating 30 as shown in FIG. 2. 
On the other hand, the wave R' which is generated by diffraction of the 
wave S would have a phase lead of .pi./2 with respect to said wave S. 
As a result of introduction of a phase shift .PSI.=.pi./2 between the two 
waves R and S which arrive in the medium 1, coherent phasing of the 
generated wave S' and of the incident wave S accordingly takes place. In 
fact, as shown in FIG. 3 in the case of the wave S', the resultant phase 
shift with respect to the wave S is -.pi./2+.pi./2; this phase shift is 
therefore zero. On the other hand, in the case of the wave R', the 
resultant phase shift with respect to the wave R is .pi./2+.pi./2=.pi.. In 
consequence, destructive interference takes place in the direction of the 
beam R and there is therefore a reduction in the energy transmitted 
directly along R'. 
The optimum conditions for energy transfer are satisfied under the 
following experimental conditions: 
EQU .eta.=50%; I.sub.R =I.sub.S =I.sub.O ; .PSI.=.pi./2 
and therefore the gain of I.sub.S '/I.sub.S is equivalent to 3 dB. 
These conditions are satisfied by means of a medium 1 formed of gelatin. 
The device in accordance with the invention and based on the wave coupling 
phenomena described in the foregoing is illustrated in FIG. 4. 
Thus in the case considered, the light intensity pattern of interference of 
the two beams R and S is phase-shifted by .pi./2 with respect to the phase 
grating. There is consequently obtained a diffracted wave of the wave R on 
the phase grating, said diffracted wave being superimposed on the 
transmitted wave S. The result thereby achieved is the same as if the 
transmitted wave S' had been enriched by a fraction of the energy of the 
reference wave. 
The beam S' which emerges from the grating is coupled to the propagation 
medium which can consist, for example, of a waveguide or a monomode fiber 
by means of conventional optical components. In the case considered in 
FIG. 4, the medium 1 is coupled to the fiber 16 by means of converging 
lenses 15 and 17. 
As shown in FIG. 4, the two beams S and R emerge from the same laser 21 and 
subsequently pass through a beam-splitter or separator 22. 
The beam R passes through a phase modulator 11 and then through a 
beam-widener composed of two lenses 13 and 14 after having previously been 
reflected from a mirror 34. The beam S consists of a radiant energy pulse 
which is initially entered by means of an amplitude modulator 33 
controlled by a generator 32. After passing through a beam-widener 23, 24, 
said pulse is entered into the loop via a splitter plate 25. The pulse is 
permitted to leave said splitter plate and to circulate within the loop 
formed by the fiber 16 which is coupled to the medium 1. Said plate may 
also be removed once the process has been initiated. 
Another possible arrangement consists in dispensing with the beam splitter 
22, the modulator 33 and the lenses 23 and 24. A radiant energy pulse is 
first transmitted in the beam R and then, by switching the phase of the 
laser by .pi./2, the operation is performed with the desired beam R. 
The signal which passes out of said fiber 16 is reinjected at the entrance 
of the grating in which it interferes coherently with the wave R delivered 
by the laser 21. The phase shift .PSI.=.pi./2 is accurately adjusted by 
means of the electrooptical phase modulator 11 which is placed on the beam 
R, for example. Said modulator is controlled by a voltage V.sub.1 and 
losses are adjusted by optical absorption and by coupling of the signal 
wave S at 3 dB. The losses arising from external coupling of the beam S 
through a splitter plate 18 are approximately 1 dB and the losses due to 
coupling between the fiber 16 and the medium 1 and the losses within said 
fiber 16 are approximately 2 dB. 
Under these conditions, the gain obtained by energy transfer of the pumping 
wave R towards the signal wave S compensates for the losses and the level 
of the signal pulse remains the same irrespective of the number of 
traversals through the propagation medium. Good operation of the system 
entails the need for stability in time of the initial phase condition 
.PSI.=.pi./2 and hence the need for very good stability of the mode of the 
laser source. In regard to the length of coherence of the monomode source, 
the requirements may be reduced if the transit time .tau. of the pulse 
within the fiber is a multiple of 2l/2, where l is the length of the laser 
cavity. In fact, 2l/c is the time interval between two maximum values of 
coherence in the case of a laser cavity. The result thereby achieved is 
good coherence of operation of the loop formed by the fiber 16 and the 
medium 1 with respect to the operation of the laser cavity. 
The device proposed therefore ensures circulation of an optical pulse 
within a fiber or monomode waveguide while maintaining a constant signal 
level irrespective of the number of turns completed by the pulse. The 
property just mentioned is particularly advantageous in the construction 
of fiber transducers such as gyrometers, hydrophones and the like, in 
which a very small modification of the wave phase is integrated over a 
length L within a propagation medium. Taking into account the compromise 
to be maintained between the level of detection and absorption of the 
signal within the propagation medium, it can be demonstrated that any 
conventional fiber interferometric transducer is limited to an optimum 
interaction length L deduced from the relation .varies.L.sub.op 
.perspectiveto.8.7, where .varies. is the coefficient of absorption of the 
waveguide in dB/km at the utilization wavelength. This condition 
corresponds to a reduction in optical power of 1/e.sup.2. 
The device proposed therefore makes it possible to remove this limitation 
and accordingly induces greater phase shifts in the signal wave. 
Equivalently, the device permits the use of a microguide of shorter length 
but having a higher absorption coefficient such as, for example, an 
annular guide fabricated from LiNbO.sub.3 : .varies..perspectiveto.1 
dB/cm.sup.-1. The complete optical structure thus presented can be 
fabricated by means of integrated optical techniques on a single substrate 
such as LiNbO.sub.3. By way of example, the phase grating can be 
photoinduced by a photorefractive effect in the electrooptical substrate. 
By using the device which is proposed with effective compensation for 
losses, that is, with .PSI.=.pi./2, the shape of the signal wave S' is 
thus represented in FIG. 5 by the light intensity curve I.sub.S ' as a 
function of time. During the time interval .DELTA.T, we have I.sub.S 
=2I.sub.o as explained earlier. On the other hand, during the remainder of 
the period, only that portion of the beam R which is diffracted on the 
phase grating is transmitted, which corresponds to an intensity of I.sub.o 
/2 by reason of the diffraction efficiency: .eta.=1/2. 
The signal I.sub.R ' is complementary to said signal I.sub.S '. In fact, 
during the time interval .DELTA.T, there is no transmission and therefore 
I.sub.R '=0. During the remainder of the period, only that portion which 
is derived from the beam R is transmitted and, by reason of the 
diffraction efficiency .eta.=0.5, we have I.sub.R '=I.sub.o /2. 
When there is no loss compensation or in other words when .PSI.=0, the 
signal I.sub.S ' has the waveform shown in FIG. 7 and the envelope of the 
pulses has the shape e.sup.-.varies.L. 
Thus, when said device is employed in a gyrometer, the phase modulator is 
re-adjusted until the signal I.sub.S ' has the waveform shown in FIG. 5. 
This makes it possible to measure the phase shift resulting from rotation 
of the loop. 
In the case of very small induced phase shifts, if the signal pulse is 
subjected at each traversal to an elementary phase shift 
.DELTA..phi.=2.pi./.lambda..times.L.times..DELTA.n, the resultant phase 
shift in the case of an integration of N revolutions will be: 
EQU .DELTA..phi.=N.times.2.pi./.lambda..times.L.times..DELTA.n 
EQU .DELTA..phi.&lt;&lt;.pi./2 
.DELTA.n being the variation in the quantity to be measured, namely in this 
case a variation in refractive index, for example. 
Consideration will accordingly be given to FIG. 8. At the end of the time 
interval T=N.times..tau., where .tau. is the transit time within the 
fiber, there is a slight mismatch of the initial phase condition 
.PSI.=.pi./2 and the intensity transmitted in the beam I.sub.R increases. 
When .eta.=0.5, we have during the time intervals .DELTA.Ti: 
EQU I.sub.R =I.sub.O [1-sin(.pi./2+.phi.)] 
EQU I.sub.R =I.sub.O [1 +cos(.pi.+.DELTA..PSI.)] 
EQU I.sub.R .perspectiveto.I.sub.O /2.times..DELTA..PSI..sup.2 
A detector 20 is then interposed on the beam path after focusing by means 
of a lens 19. This detector may also be enabled only between the two 
leading edges of the pulse during the time interval .DELTA.T. Detection of 
abrupt positive and negative transitions then makes it possible to disable 
and to enable the detection signal during the time interval .DELTA.T. 
A linear detection on .DELTA..PSI. can be obtained by introducing a 
relative displacement .DELTA..PSI..sub.o with respect to the initial 
condition .PSI.=.pi./2-.DELTA..PSI..sub.o with .DELTA..PSI..sub.o 
&lt;&lt;.pi./2, and .DELTA..PSI..sub.o &gt;.DELTA..PSI.. 
During the time intervals .DELTA.Ti, we have : 
EQU I.sub.R =I.sub.O [1+cos(.pi.+.DELTA..PSI.+.DELTA..PSI..sub.o)] 
EQU I.sub.R .perspectiveto.(I.sub.O /2)[.DELTA..PSI..sub.o.sup.2 
+2.DELTA..PSI..sub.o .DELTA..PSI.] 
EQU .DELTA.I.sub.R .perspectiveto.I.sub.O .DELTA..PSI..sub.o .DELTA..PSI.(t) 
Thus the curve I.sub.R ' no longer passes through zero but has minimum 
values which vary as a function of .DELTA..PSI. about a straight line 
parallel to the x-axis having an ordinate .DELTA.I.sub.R resulting from 
said phase shift .DELTA..PSI..sub.o. 
EQU &lt;I.sub.R &gt;=(I.sub.O /2).DELTA..PSI..sub.o.sup.2. 
Under these conditions, the contribution to the noise current caused by 
photons in the photodetector remains at a low level. 
EQU &lt;i.sub.ph.sup.2 &gt;=2e.times..DELTA.f.times.&lt;I.sub.R &gt;. 
When the mean value &lt;i.sub.ph &gt; is non-zero, said detector 20 serves to 
detect mismatch of the initial phase condition .PSI.=.pi./2. 
Said detection signal can therefore be employed for computing the phase 
shift which exists within the loop of a gyrometer by reason of the 
rotation. In fact, by reducing it to zero, it is possible to measure said 
phase shift and consequently the rotational velocity. By virtue of the 
invention, said phase shift and said rotational velocity can be of very 
low value since it is possible to operate with N loop revolutions and thus 
to increase the resulting effect to a corresponding extent. 
In the case of the device of the invention as thus defined, consideration 
can be given to a length of fiber of 1000 meters, for example, which 
corresponds to a transit time of 5 milliseconds within said fiber 16. It 
can then be considered that the pulse generated by means of the amplitude 
modulator 33 has a time-duration of 100 nanoseconds, for example.