Multipurpose component with integrated optics and distribution network with optical amplification

A multi-purpose component with integrated optics includes an optical light guide (12) formed on one face of a substrate (10) made of a double refracting material having neutral axes orientated so that one electromagnetic wave extending into the guide (12) is split up into two waves having polarizations perpendicular to each other and along the neutral axes. A power source (18) applies an electric filed perpendicular to the optical light guide (12) and uniformly along the guide (12). Two rectilinear polarizers (20, 22) with polarization directions (d1, d2) parallel to each other are each disposed at one extremity of the guide (12). The component also includes focussing means (24, 26, 28, 30). The component has particular utility for tuneable optical filtering and optical telecommunications.

FIELD OF THE INVENTION 
The object of the invention is to provide a multi-purpose component with 
integrated optics and a distribution network with optical amplification 
for applying this component. The invention can be used in optical 
telecommunications applications and more particularly for selective 
wavelength-tuneable filtering or for coherence modulation. 
BACKGROUND OF THE INVENTION 
One of the purposes of optical fiber links is to increase the amount of 
information transmitted on a given support and in particular to increase 
the number of channels. Multiplexing methods, mainly optical, are 
currently being developed along these lines. Various components containing 
integrated optics are required to carry out these multiplexing techniques. 
The present invention relates to a multi-purpose component with integrated 
optics fully adapted to various types of multiplexing. 
SUMMARY OF THE INVENTION 
One first object of the invention is to allow for the embodiment of an 
integrated coherence modulator, that is a modulator assigned to the signal 
which traverses a variable delay. 
A further object of the invention is to allow for the embodiment in one 
fully integrated version of a multiplexing system by modulating an optical 
delay, such as the one described in the French patent application filed by 
the French State (represented by the Minister for Telecommunications and 
Postal Services) on the Dec. 23, 1986 and published under the number 2 608 
869. 
A further object of the invention is to make it possible to produce an 
integrated distribution network transmitting a signal modulated by optical 
delay and comprising various optical amplification stages. 
A further object of the invention is to allow for the embodiment of an 
integrated frequency-tuneable selective filter. 
The component of the invention only transmits the signal on one single 
guide. The propagation of the luminous wave in a double-refracting 
material introduces a delay between the two components extending along the 
neutral axes of the material. 
This delay may be modulated by electrically varying the birefringence or 
double refraction of the material. 
The choice of an applied continuous electric field also allows for the 
selective filtering, by means of interference between the components of 
the luminous wave, of the signal introduced into the component of the 
invention. 
More specifically, the present invention concerns a multi-purpose component 
with integrated optics including: 
an optical light guide formed on one face of a substrate made of a double 
refracting material having neutral axes orientated so that an 
electromagnetic wave extending into the guide is split up into two waves 
having polarizations perpendicular to each other and along the neutral 
axes; 
means to apply an electric field perpendicular to said face of the 
substrate supporting the optical light guide and uniformly along the 
optical light guide; 
a first rectilinear polarizer disposed at one extremity of the optical 
light guide; 
a second rectilinear polarizer disposed at the other extremity of the guide 
and exhibiting one polarization direction parallel to that of the first 
polarizer, and 
focussing means. 
The present invention also concerns a distribution network with optical 
amplifiers including a multiplexing system introducing optical delays 
comprising several components conforming to the component described above 
for creating these delays; 
a principal transmission line connected to the multiplexing system; 
at least one coupler connecting the principal transmission line to 
secondary transmission lines; 
at least one optical amplifier on one secondary transmission line; 
couplers connecting the secondary transmission lines to distribution 
channels; and 
each distribution channel comprising a detection system.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
FIG. 1 diagrammatically represents a component according to the invention. 
This component 1 includes a parallelpiped substrate 10 made of a double 
refracting material on which an optical light guide 12 is formed. The 
substrate 10 may, for example, have a thickness of 0.5 mm, a length of 
several tens of mm and a width of 8 mm. 
The guide is embodied by diffusing a metal in the double refracting 
material. Preferably, this metal may be either titanium (Ti) or magnesium 
(Mg) or nickel (Ni) or vanadium (Vn) or from other materials. The guide 12 
covers the length of the substrate 10 and has a width of 3 micrometers, 
for example. 
The double refracting material is preferably selected from lithium niobate 
(LiNbO.sub.3), lithium tantalate (LiTaO.sub.3) of from other materials. 
This material is cut to size so that an electromagnetic wave extending 
into the guide is split into two waves having polarizations perpendicular 
to each other and along the neutral axes of the material. 
On the example shown, the double refracting material is LiNbO.sub.3 and the 
neutral axis corresponding to the extraordinary index ne is orientated 
along an axis Z; the neutral axis corresponding to the ordinary index no 
is orientated along an axis Y; the propagation of the light in the guide 
is effected along an axis X. The axis X, Y and Z from one mark of the 
space. 
The face of the substrate 10 on which the guide 12 is formed is coated with 
a fine dielectric layer 14, such as SiO, having a thickness of about 400 
nm. 
The dielectric layer 14 is covered by two electrodes 15 and 16 made of 
aluminium, for example, and having a thickness of about 500 nm. 
Preferably, these electrodes are rectangular and have a length of 20 mm, 
for example. 
The electrode 15 is disposed so as to cover the guide 12 over its length. 
With the guide 12 dividing the face of the substrate 10 into two sections, 
the electrode 15 mostly covers one of the sections and projects over the 
other by about 6 micrometers. 
The other electrode 16 covers another section of the substrate: the space 
between the electrodes is 7 micrometers, for example. 
The electrodes 15 and 16 are connected to an electric power source 18. The 
unit formed by the dielectric layer 14, the electrodes 15 and 16 and the 
power unit 18 make it possible to apply an electric field in the substrate 
10 perpendicular to the guide 12 and uniformly along the guide 12. 
The voltages delivered by the power source 18 amount to about ten volts. 
The component 1 also includes a first and a second rectilinear polarizer 
respectively 20 and 22 disposed at each extremity of the optical light 
guide 12. These polarizers 20 and 22 exhibit polarization directions d1 
and d2 preferably disposed parallel 45.degree. from the axes Y and Z, that 
is 45.degree. from the neutral axes of the double refracting material (d1 
and d2 being symbolized by arrows on FIG. 1). 
The component 1 further comprises focusing means 24, 26, 28 and 30 which 
allow for crossing of the polarizers and a focussing adapted to the 
introduction of light into the guide 12 or light being introduced into an 
outgoing fiber (not shown). 
The polarizers 20 and 22 and the focusing unit 24, 26, 28 and 30 may also 
be embodied by optical fibers. 
Any polarization luminous beam penetrating into the component 1 is 
polarized rectilinearly and focused in the optical light guide 12. When 
extending into the guide 12 and with no electric field being applied, the 
electromagnetic wave is divided into two waves with polarizations 
orientated along the neutral axes of the double refracting material and 
extending at different speeds. 
The application of an electric field makes it possible to vary the double 
refraction of the material and thus to modify the delay between the two 
waves. The phase shift introduced by the electric field is proportional to 
its amplitude. 
At the guide outlet, the two waves are again polarized parallel. 
If the coherence length L of the luminous beam (L=.lambda..sup.2 / 
.lambda.; .lambda.=wavelength of the luminous beam; .lambda.=spectral 
width of the beam) is less than the delay introduced (or rather its 
equivalent in terms of length, in other words this delay is multiplied by 
the speed of the light), the two waves shall not interfere with each other 
at the guide outlet and the information constituted by the period of the 
delay is preserved. 
If the coherence length L of the luminous beam is greater than the 
introduced delay, the component 1 acts as an interferometer. 
There now follows a description on how these two situations may be best 
exploited. 
FIG. 2 diagrammatically represents a coherence modulation transmission 
system including a component according to the invention. By means of an 
optical fiber 34, a luminous source 32 delivers a luminous beam to the 
inlet of a component 1, as described earlier. This may, for example, be a 
laser diode emitting a luminous beam with a wavelength .lambda.=1300 nm 
and a coherence length Lc=200 .mu.m or a superluminescent diode with a 
coherence length Lc=40 .mu.m. 
The component 1 acts here as a coherence modulator. In fact, the power 
source 18 makes it possible to apply a variable electric field introducing 
variations in the delay between the two waves extending inside the optical 
light guide. The power source 18 may consist of a video amplifier 
connected to a video camera (not shown) and whose output voltage is 
proportional to the signal delivered by the camera. The information 
contained in this signal is transferred in a delay form by the luminous 
beam inside a monomode optical fiber 36. 
For such a device to function, the delay introduced into the double 
refracting material needs to be greater than the coherence length L of the 
luminous beam. In this way, the waves produced in the modulator from the 
luminous beam do not interfere at the outlet of the modulator. 
Demodulation is ensured by a Mach-Zehnder type integrated optical fiber 
interferometer tuned to the delay introduced by the component 1 where no 
field is applied. 
A HgCdTe type detector photodiode 40 makes it possible to transform the 
demodulated luminous beam into an electric signal. This signal is 
amplified by an amplifier 42 and allows for display of the images recorded 
by the video camera. 
This type for carrying information lends itself particularly well to 
multiplexing and, by means of the component of the invention, makes it 
possible to construct a multiplexing system of the type described in the 
previously mentioned French patent published under the number 2 608 869 in 
the fully integrated version. 
Said component lends itself particularly well to the embodiment of a 
network for distributing a signal. With reference to FIG. 3, a 
multiplexing system 50 delivers multiplexed signals on a principal 
transmission line 52. 
The multiplexing system 50 combines a series architecture with a parallel 
architecture. The system 50 is made up of a set of luminous sources and 
modulators embodied by components 1 according to the invention allowing 
for modulation by optical delays of the luminous signals delivered by the 
sources 54. 
The luminous sources 54 may be monomode or multimode laser diodes, 
electroluminescent diodes or superluminescent diodes or any combination of 
these various types of sources. The modulators 1 introduce optical delays 
greater than the coherence lengths of the sources; more generally, such 
modulators ensure that no modulation of the outgoing luminous intensity 
appears when the optical delay is modulated by the signal to be 
transmitted. 
The various stages of the system 50 are connected to the inputs of a 
coupler 56 connected to the principal transmission line 52. 
Couplers 58 connected to the transmission line 52 shunt one part of the 
multiplexed signals to secondary transmission lines 60. The output of each 
coupler 58 is connected to the input of an optical amplifier 62 whose 
pass-band is adapted to the signals to be transmitted. 
It is possible to use fiber, stimulated Raman effect or other similar 
amplifiers, semiconductive amplifiers or any other amplifier whose 
pass-band is sufficient. 
Couplers 64 connect the secondary transmission lines to distribution 
channels 66. In a program transmission system, these channels 66 serve the 
users. They are connected to detection systems 68 to demultiplex or 
demodulate the signal transmitted in the distribution channels 66. At its 
output, each coupler 64 has as many distribution channels 66 as users. 
The signals arriving at the input of the detection system are tainted by an 
intensity noise inherent to the luminous sources used. Considering the 
information transmitted coded in a delay form and extracted by 
constructive interference, the noise brought by the source occurs as a 
multiplicative term of the signals. 
The detection system 68 shown on FIG. 3 makes it possible to be freed of 
the intensity noise generated by the luminous sources and the optical 
amplifier 62. It includes a device 70 for separating the signal 
transmitted on the distribution channel 66 to which this signal is 
connected. This signal is thus divided into one first signal and one 
second signal. A photodiode type detector 72 delivers an electric signal 
proportional to the first signal. 
The second signal is transmitted to a demodulation device 74, for example a 
Mach Zehnder type interferometer tuned to the delay introduced by one of 
the components 1 of the multiplexing system 50. This device 74 is 
connected to a second photodiode type detector 76 delivering an electric 
signal proportional to the second demodulated signal. 
A divider 78 divides the signal derived from the delivered second detector 
76 by the signal delivered by the first detector 72. At the output of the 
divider 72, a demodulated signal is obtained freed of the noise due to the 
luminous sources 54 and the optical amplifier 62. 
The component of the invention thus provides the function of the selective 
tuneable integrated filter. 
FIG. 4 diagrammatically represents the transmission curve of a component 
according to the invention according to the frequency of the guided 
luminous wave inside the component. When the luminous source associated 
with the component delivers a luminous beam whose coherence length is 
greater than the delay introduced at the time of extending the luminous 
beam inside the optical light guide, the components of the beam polarized 
along the two neutral axes of the double refracting material interfere at 
the output of the component, this component constituting an 
interferometer. 
The transmission T of the component then has the aspect of a sinosoid 
according to the frequency of the luminous beam. The interference is 
constructive for the frequencies f.sub.o, f.sub.2 , . . . and T is 1, 
whereas they are destructive for the frequencies f.sub.1 , f.sub.3 , . . . 
and T is 0. The values of f.sub.o , f.sub.1 , . . . depend on the 
characteristics of the component (double refracting material used, guide 
length, value of the voltage applied . . .). 
Only certain frequencies may extend inside the guide: the component in this 
case is thus a selective filter. The value of these frequencies depends on 
the electric field applied: the filter is tuneable. 
FIG. 5 diagrammatically represents a wavelength multiplexing and 
demultiplexing system including the component of the invention. In this 
example, only four luminous sources 80, 82, 84 and 86 are represented, but 
such a device may equally comprise others. The luminous sources 80, 82, 84 
and 86 emit luminous beams on different wavelengths, respectively 
.lambda.1, .lambda.2, .lambda.3 and .lambda.4. These luminous beams are 
delivered onto the inputs of a coupler 88 which multiplexes them and which 
delivers the multiplexed beams onto the optical transmission fibers 90. 
These fibers are connected to demultiplexing systems including a component 
1 according to the invention and a photodetector 92. 
The luminous beams on the wavelengths .lambda.1, .lambda.2, .lambda.3 and 
.lambda.4 have coherence lengths greater than the delay introduced by the 
component 1 which here acts as a tuneable selective filter. In fact, 
depending on the electric field applied, it is possible to select the 
wavelength which is transmitted by the component and to filter the others. 
So as to improve the selection of such a device, it is possible to connect 
in series several components whose transmissions T are all centered on the 
same wavelength, but whose transmission curve periodicity is double or 
triple that of the first component. 
In the current state of the art, with the aid of a single component, it is 
possible to separate wavelengths distant by about 0.3 nm. 
The preceding descriptions are given solely by way of non-restictive 
examples. However, a large number of variants is possible without 
departing from the context of the invention.