WO2004055566A1

WO2004055566A1 - Optical system based on optical fibers with sloping grating

Info

Publication number: WO2004055566A1
Application number: PCT/FR2003/003682
Authority: WO
Inventors: Pascal Joffre
Original assignee: Micro Module
Priority date: 2002-12-11
Filing date: 2003-12-11
Publication date: 2004-07-01
Also published as: FR2848749A1; EP1611467A1

Abstract

The invention concerns an optical transmission system comprising at least one optical fiber (1) wherein has been provided at least one first photo-induced diffraction grating (EZ1) in the fiber material. At least one first optical source (E1) emitting at a first wavelength or wavelength range is optically coupled to said diffraction grating. The pitch of the strata of indices of the diffraction grating and the inclination of said strata relative to the fiber axis are such that the light it receives from the optical source which is coupled thereto is diffracted in the optical fiber. Such a system operates also with an optical detector instead of an optical source. In that case, the diffraction grating serves to cause the light circulating in the fiber to be diffracted towards the detector.

Description

OPTICAL SYSTEM BASED ON OPTICAL FIBER NETWORK _INC LIN _E

The invention relates to an optical system and more particularly to a “Vampire” type optical transmission system on optical fiber, its application to optical measurement systems, communication systems and to illumination systems.

In optical fiber transmission systems known in the art, the injection of an optical signal into an optical fiber or its extraction is done by the ends of the fiber using couplers. For injection or extraction during

- fiber, we must therefore interrupt the fibers to access the fiber ends. Figure la represents the state of the art of a wavelength multiplexer / demultiplexer called "DWDM" as it is practiced in optical telecommunications. The invention provides a system for injecting and extracting an optical signal in the path of a fiber without having to interrupt it because the coupling points can be provided anywhere along the fiber. In addition, the optical transmission system according to the invention is more integrated because the spectral multiplexing / demultiplexing function is performed in situ in the optical fiber.

The invention also relates to optical instrumentation. Most optical instruments require a light source, optical couplers, a photo detector and an electronic signal preprocessing system to carry out the measurement either in transmission for the system in FIG. 1b, or in reflection for that in FIG. . The optical instruments thus concerned are photometers, polarimeters, goniometers, spectrophotometers ... The imaging systems also work on this principle (endoscope, microscope ...), the detector contains a large number of independent pixels to extract a image, ie two-dimensional information of the optical quantity measured.

The continuous light sources generally used in optical instruments are in fact hot bodies mounted at very high temperature, they are thermal sources made up of a metallic filament heated by the passage of an electric current

(Halogen lamps ...) or by an electric arc in a low pressure atmosphere (Vapor of Mercury, Xenon ...). These sources have the following disadvantages:

Significant power consumption and heat dissipation often requiring heavy means of cooling by ventilation or by liquid, - Low lifespan (100 to 100 h)

Temporal instability of light flux

Large emissive surface hence a large geometric extent

Difficulty in modulating the luminous flux, an electromechanical or electro-optical modulator must be added.

Large footprint.

The pulse light sources used in some optical instruments are also electric arc lamps in a low gas atmosphere pressure (Mercury vapor, Xenon ...). These sources have the following disadvantages:

- Poor lifespan (1,000 to 10,000 hrs)

Large emissive surface hence a large geometric extent

Difficulty adjusting the form factor of the pulse

Electromagnetic interference

Large footprint. Thus, it clearly appears that the usual systems are not optimized for use in optical instrumentation which requires the following ideal characteristics:

Light power of a few μW or mW coupled in an optical fiber (lOOμm core at 1mm),

Continuous spectrum or lines in UV, visible or near IR (0.3μm to 2μm),

Temporal stability and noise below 1%,

Triggering and modulation by an external high frequency electrical signal (100kHz to

1MHz).

Low electrical consumption requiring no cooling by ventilation or by liquid, - Long service life: 10 OOOh to 100 OOOh,

Low emissive surface for a small geometric range, between 1000μm ² .sr and 10 000μm ² .sr allowing efficient coupling in multimode optical fibers,

No electromagnetic interference on the immediate environment, - Small footprint.

The invention provides a system having these advantages and preferably using light-emitting diodes (LEDs), superluminescent diodes (SLD) or semiconductor lasers (LD) of small dimensions.

(chip of lOOμm x 300μm x 300μm typically) comparable to those of optical fibers.

The invention therefore relates to an optical system comprising at least one optical fiber comprising at least a first and a second set of photo-induced diffraction gratings in the material of the fiber. The steps of the index strata of the networks of the first set of networks are adapted to diffract light at a determined wavelength and the steps of the index strata of the networks of the second set of networks are adapted to diffract light at a other determined wavelength. Each diffraction grating is inclined relative to the axis of the optical fiber so as to diffract light which circulates in the direction of the optical fiber towards the outside of the fiber.

The invention also relates to an optical transmission system applying the above optical system. This system comprises at least a first and a second photo-induced diffraction grating in the fiber material, at least a first and a second optical source emitting respectively at least a first and a second wavelength and being coupled respectively to the first and to the second diffraction grating. The steps of the index strata of the first and of the second diffraction grating are adapted to diffract the light respectively at the first and at the second wavelength, each diffraction grating being inclined relative to the axis of the optical fiber. so as to diffract the light it receives from the optical source which is coupled to it in the direction of the optical fiber. This transmission system also comprises at least a third and a fourth diffraction grating belonging respectively to the first and to the second set of gratings as well as at least a first and a second optical detector capable of detecting light respectively at said first and at said second wavelength and being coupled respectively to the third and fourth diffraction grids The steps of the index strata of the third and fourth diffraction grids are adapted to diffract light at the first and second wavelengths respectively. wave, each diffraction grating being inclined relative to the axis of the optical fiber so as to diffract light which circulates in the direction of the optical fiber towards the optical detector which is coupled to it.

In the foregoing, it has been provided that the diffraction gratings are individual, but they can also be provided in the form of a single grating such as a variable pitch grating. According to an alternative embodiment of the invention, a plurality of optical fibers is provided placed side by side, the optical sources being arranged in the form of a matrix and being each optically coupled to a diffraction grating of a fiber. In the case of a system comprising optical detectors, the same arrangement is provided.

The invention is applicable in optical communication systems. In this case, transmission systems having optical sources coupled to the fiber by the diffraction gratings are provided along the same fiber, as well as transmission systems having optical detectors also coupled to the fiber by the diffraction gratings. In such systems, the fiber can be provided in a ring, which makes it possible to compensate for possible cuts in the fiber. In this case, it is advantageous to provide additional optical sources and additional optical detectors arranged symmetrically from the previous optical sources and optical detectors with respect to the axis of the fiber and the associated diffraction gratings so as to transmit or receive the light whatever its direction of movement in the fiber.

The invention is also applicable to measurement systems. A first transmission system comprising optical sources and a second transmission system comprising optical detectors is then provided in an area of a fiber and arranged in series along this fiber. A first wavelength and a second wavelength of the first transmission system must be equal to respectively first wavelength and at a second wavelength of the second transmission system. One end of the fiber is oriented towards a medium to be measured.

According to an alternative embodiment, the first transmission system is produced in an area of a first fiber, one end of which is oriented towards a medium to be measured. The second transmission system is produced in an area of a second fiber, one end of which is also oriented towards said medium to be measured so as to collect light transmitted by the first fiber to the medium to be measured. The first wavelength and the second wavelength of the first transmission system are respectively equal to the first wavelength and to the second wavelength of the second transmission system for example.

The invention is also applicable to a system for measuring the level of a liquid. A first zone of a fiber comprises a first transmission system provided with optical sources as well as a second transmission system provided with optical detectors. A second zone of the fiber is intended to be immersed preferably vertically in a liquid contained in a tank comprising a series of at least two diffraction gratings of different pitches or a diffraction grating with variable pitch. A first wavelength and a second wavelength of the first transmission system are respectively equal to a first wavelength and a second wavelength of the second transmission system. The steps of a first grating (RMl) and a second grating (RMn) of the series of diffraction grids are adapted to diffract light respectively to the first and to the second wavelength emitted by a source (El, E2) or received from the tank.

In these measurement systems, there is provided a circuit for controlling transmission of optical sources, a circuit for reading optical detectors, a processing circuit allowing, depending on the time of control of the optical sources and results of detection of optical detectors, to calculate a measurement value corresponding for example to synchronous detection, to normalization with respect to a reference signal, filtering or averaging.

For goniometry applications, a first transmission system is provided, the emission ends of a sheet of fibers of which are arranged along a surface, such as a sphere or a plane, or along a curve, such as a straight line. or a plane, and are directed towards a reference point, and a transmission system whose receiving ends of a sheet of fibers are also arranged along a surface or a curve and are directed substantially towards the same reference point.

Finally, the invention is applicable to illumination systems providing in a first zone of at least one fiber, a transmission system provided with optical sources and in a second zone of said fiber several locations of diffusing points or of networks of diffraction of extracting light from the fiber in different directions.

An interesting embodiment can provide an alternation of locations of diffraction gratings making it possible to diffract light at lengths of different waves. A control system then allows sequential operation of the different optical sources.

The various characteristic objects of the invention will appear more clearly in the description which follows and in the appended figures which represent:

• Figure la, an optical transmission system by wavelength multiplexing according to the state of the art;

• Figures 1b and 1c, optical measurement systems known in the art and described above;

• Figures 2a to 2e, an optical transmission system according to the invention operating in transmission;

• Figure 2f shows a method of recording a diffraction grating.

• Figures 3a to 3c, optical transmission systems according to the invention operating in reception;

• Figures 4a and 4d, two-way optical transmission systems;

• Figures 5a to 5c, optical measurement systems of the characteristics of an environment or a material; Figure 6, an application of the invention to a liquid level measuring device.

• Figures 7a and 7b, an application of the invention to an online multi-channel spectrophotometer in transmission and reflection.

FIG. 8, an application of the invention to a gonio-spectro-photometer in reflection.

• Figures 9-a and 9b, an application of the invention to a multicolored light garland and propagation effect.

Referring to Figure 2a, we will first describe a transmission system according to the invention for its part operating in transmission. According to the invention, several diffraction gratings are produced by photo-inscription in the material of an optical fiber 1. For example, three gratings Ezl, Ez2 and Ez3 oriented at 45 ° are produced. Each network is optically coupled to an optical source E1, E2, E3 each emitting at a precise wavelength (laser) or in a relatively wide band (led, sld). Each diffraction grating Ezl to Ez3 is designed to diffract a certain spectral slice of the light which it receives from the source which is coupled to it ^~ towards the interior of the fiber and therefore substantially along the axis of the fiber.

Germanium-doped optical fibers then treated by hydrogenation are known to present some photosensitivity in UV. As shown in Figure 2f, each network is photoinduced using two collimated and coherent beams (UV1 and UV2) making between them a certain angle 2A to create a field of light interference IL of spatial period Λ = λuv / 2sin (A) whose fringes are flat and perpendicular to PI. The optical fiber 1 being exposed to this interference field transforms the light fringes IL into strata of indices of the same spatial period Λ. The inclination of approximately 45 ° of the plane of incidence PI with respect to the axis of the fiber y generates a Bragg grating whose vector of wave K at the same orientation of 45 ° allowing the injection, in the core of the fiber, light from the optical source associated with the diffraction grating. Such a system makes it possible to inject into the optical fiber in the direction of the arrow EF, several beams of different wavelengths or of disjoint spectral bands. It can be seen that the light emitted by the source El at the wavelength λl and diffracted towards the interior of the fiber 1 by the diffraction grating Ezl is not affected during its transmission through the diffraction grids Ez2 and EZ3. It is the same for the other wavelengths. To be more quantitative, we specify that with an Ezl diffraction grating oriented at 45 ° in a silica fiber (n = 1.45), the step Λl is approximately λl / 2 and the spectral bandwidth δλ is approximately 0.7λl ² / Ll if Ll is the length of the Ezl network projected on the axis of the fiber. Thus for an Ezl network of 350 μm in length L1, the spectral width δλl is about 2nm at a wavelength λl of lμm. This means that this network is completely transparent for the wavelengths λ outside the interval δλl centered around λl. FIG. 2b represents a method of fixing the fiber 1 and the optical sources. The fiber with these pre-recorded Ezl to Ez3 diffraction gratings is fixed, by gluing or welding for example, in a V-shaped groove 20 formed in a support part 2. On the upper face 21 of the support part are fixed, by gluing or welding, for example, the optical sources E1 to E3 in such a way that their emissive zone is directed substantially towards the axis of the fiber and more precisely towards their associated diffraction grating. For this, it is possible to provide a shim of suitable thickness, possibly conductive, under each source as shown in FIG. 2c. The face 21 can advantageously serve as a support for the electronic circuits for controlling the optical sources E1 to E3 as well as for their interconnections. We can also provide if necessary a micro-optic coupling LE1 (micro-lens, selfoc ... spherical or cylindrical) between the source and the optical fiber as shown in Figure 2c.

FIGS. 2d and 2e represent respectively in top view and in section a sheet NF of optical fibers placed side by side, optically coupled to a matrix of emitters EM made up of elements such El, E2 ... arranged in lines and in columns. It is thus possible to control in transmission a large number of wavelengths on each fiber using networks such as Ezl, Ez2 ... associated with the transmitters El, E2 ... The latter can be identical light-emitting diodes (leds) of relatively wide spectrum or lasers

(Vcels), the Ezl, Ez2 networks allowing to take narrow lines at wavelengths λl, λ2 ...

Figure 3a shows an optical transmission system operating in reception. As before, the optical fiber includes diffraction gratings such as Rzl, Rz2, Rz3. To each network is optically coupled an optical detector RI, R2, R3 capable of detecting diffracted light. Each diffraction grating Rzl to Rz3 is designed to diffract the light transmitted over the fiber at a particular wavelength towards the optical detector which is coupled to it. This means that each network has been photoinduced so as to have the desired efficiency on the wavelength or the desired wavelength range on the associated detector according to the same technique as that described above for the emission. . The optical fiber thus treated allows the extraction without physical interruption of the fiber, and towards an optical detector, of light at said wavelengths or wavelength ranges. The networks Rzl, Rz2, Rz3 are designed to diffract light respectively at the wavelengths λl, λ2, λ3. We note, as before, that the light, at the wavelength λl, for example, circulating in the fiber according to the arrow RF is not disturbed by the diffraction gratings Rz2 and Rz3 and is diffracted by the grating Rzl towards the RI detector. It is the same for the other wavelengths. The fixing of the optical detectors facing the diffraction gratings can be done with arrangements similar to those of FIGS. 2b and 2c.

FIG. 3b represents a transmission system operating in reception in which the different diffraction gratings are produced in the form of a single variable pitch network RS1 (“shirped” network in English terminology). The light circulating according to the arrow RF is progressively diffracted in the diffraction grating according to the wavelengths which it contains. In this embodiment, the optical detectors are produced in the form of a strip RM1 which is suitably positioned with respect to the diffraction grating RS1 so that each detection element of the strip can receive an appropriate wavelength. A coupling micro-optic can allow a particular adaptation with an appropriate magnification between the network RS1 and the strip RM1 for example. A second network RS2 associated with a strip RM2 can also be implemented to extend the measurement area by increasing thus the number of measurement points or by combining two complementary detector technologies, for example Silicon for the spectral band 0.2μm to 0.9μm and GalnAs for the band 0.9μm to 1.9μm.

FIGS. 3c and 3d represent an exemplary embodiment in which optical fibers placed side by side form a sheet of NR fibers. The optical fibers are optically coupled to a matrix of optical detectors RM made up of elements such as RI, R2 ... arranged in rows and columns. It is thus possible to detect a large number of wavelengths on the different fibers of the sheet of fibers using networks such as Rzl, Rz2 ... associated with the receiving elements RI, R2 ... We will now describe different applications of l 'invention. We will first describe the application of the invention to optical communications systems.

FIG. 4a relates to a system comprising at least one optical fiber, a first and a second "Bragg" Ezl and Rzl diffraction grating photo-induced in the material of the optical fiber, a light emitter El and a detector Ri optically coupled respectively the first and second Bragg grids. The steps of the index strata of the first and second Bragg grids are identical to diffract the light respectively at the wavelength λl chosen for the transmission. Each Bragg grating is inclined at approximately 45 ° relative to the axis of the optical fiber so as to diffract light which circulates in the direction of the optical fiber towards the source or the optical detector which is coupled to it. Two diffraction gratings Ez2 and Rz2 are also provided making it possible to diffract light of wavelength λ2 and an emitter E2 and a receiver R2 associated with these diffraction gratings. A bidirectional transmission is thus obtained with 2 transmitter / receiver pairs operating at two distinct wavelengths to properly separate the two channels and with appropriately oriented diffraction gratings. It is possible to transmit as many channels of information as there are wavelengths discernible by the Bragg gratings. It is also possible to produce an optical loop as shown in FIG. 4b, by using transmitters and receivers ensuring bidirectional transmission. In this case, even if there is a break in the loop of the optical fiber, the network continues to operate. FIG. 4c shows a bidirectional transmission system using a section of optical fiber comprising two E / RA and E / RB transceivers. Channel 1 corresponds to a left-to-right transmission obtained using the El-A transmitter and the Rl-B receiver. Channel 2 corresponds to a right-to-left link obtained using the E2-B transmitter and the R2-A receiver.

FIG. 4d shows a two-way two-way transmission system intended for a fiber optic loop, for example according to the architecture presented in FIG. 4b. In this variant, transmitters El '-A, E2'-B and symmetrical detectors Rl'-B and R2'-A are provided respectively of transmitters El-A, E2-B and detectors Rl-B and R2-A with respect to the axis of the optical fiber. Each pair of transmitters can therefore transmit in both directions of the loop and each pair of detectors can detect light flowing in both directions of the loop. We will now describe different applications of these optical transmission systems.

FIG. 5a represents a measurement system in which a transmission system operating in transmission such as that of FIG. 2a and a transmission system operating in reception such as that of FIG. 3a are arranged on the same fiber or are coupled to the same portion of a fiber 1. One end of the fiber is oriented towards a medium or an OB material to be measured so that this medium or this material reflects towards the fiber from the light received from the fiber. Thus, an emitter EM consisting of optical sources E1, E2, E3 emitting at wavelengths λl, λ2, λ3 transmits a light beam containing these different wavelengths to the medium OB. Depending on the nature of the material, certain wavelengths are absorbed or scattered by the material, on the other hand certain wavelengths are reflected towards the fiber 1. The reflected wavelengths are transmitted by the fiber towards the diffraction gratings Rzl , Rz2, Rz3 which diffract these wavelengths towards the optical detectors RI, R2, R3. These then indicate the wavelengths reflected by the medium OB and indicate the light intensities reflected at these different wavelengths. Such a measurement system then makes it possible to analyze the nature of the medium OB.

FIG. 5b represents an alternative embodiment of the system of FIG. 5a in which the transmission system operating in reception is such as that of FIG. 3b with a diffraction grating with variable pitch RS and a strip of detectors RM. The operation of this system is similar to that of FIG. 5a.

FIG. 5c represents a measurement system in which a transmission system operating in emission illuminates an OB medium, by a fiber 1E and a collimation lens LE, with a light beam containing the wavelengths λl, λ2, λ3. This light in principle crosses the medium OB. Depending on the nature of the OB medium, certain wavelengths are more or less absorbed. The other wavelengths are more or less transmitted by the focusing lens LR and the fiber 1R to the transmission system RE operating in reception. The diffraction gratings Rzl to Rz3 diffract the appropriate wavelengths towards the optical detectors RI to R3. These therefore indicate the absorption by the medium OB at the different wavelengths. Such a system can therefore be applied to the analysis of materials. This is a measurement in transmission of the OB object in accordance with the principle recalled in FIG. 1b. FIG. 6 represents a system for measuring the level of a liquid.

A portion of fiber is provided with a transmission system operating in transmission and a transmission system operating in reception. One end of the fiber is placed in the tank 60 containing the liquid 61, the height of the level 62 of which is to be measured. The length of the fiber capable of being immersed in the liquid has from place to place diffraction gratings RM1 to RMn in lieu of "graduations" of a measurement gauge. These gratings diffract the different wavelengths they receive from the transmission system operating in emission towards the interface of the fiber with the liquid. The light diffracted by the RMl to RMn networks not immersed in the liquid is reflected with a coefficient of 4% corresponding to the silica-air interface, a significant part of this light returns to the RE receiver. The light diffracted by the submersible RMl to RMn networks and reflected by the fiber-liquid interface is much weaker, ie 1% in the case of water. Thus, depending on the level of liquid, the diffraction gratings RM1 to RMn will return to the transmission system operating in reception a certain distribution of wavelengths which will thus make it possible to know the level of the liquid in the tank. FIG. 7a shows an optical measurement system of the “multi-channel spectrophotometer” type consisting of an emission system such as that of FIGS. 2d and 2e and a reception system such as that of FIGS. 3c and 3d. The emitter EM can deliver the same distribution of wavelengths on each fiber FI to Fn of the emission layer (spectrometer mode) or allow to select some successively (monochromator mode). Each zone Z1 to Zn of the object OB is crossed by a collimated beam using mini-lenses ML associated with each end of the optical fibers of the emission sheet NE. Symmetrically, each beam is focused by a mini-lens in a fiber of the NR reception sheet. The RM receiver thus makes it possible to measure the optical transmission as a function of the wavelength on each of the n measurement channels.

FIG. 7b describes a measurement system similar to that of FIG. 7a but operating in reflection on the object OB. The emitter EM and the receiver RM are placed on the same sheet of optical fibers NF.

This principle of the invention using plies of fiber optics and matrix transmitters or receivers allows for spatially and spectrally resolved optical measurements without any mechanical movement. It can be declined in many versions: transmission or reflection, with a linear distribution of the channels, two-dimensional or radial. In particular, the ends of the fibers carrying the lenses such as ML can be arranged in the form of matrices, for example, instead of sheets. FIG. 8 shows in particular a goniometric measurement head where the optical fibers FI to Fn of the NE emission or reception NR layers have their ends arranged according to a surface or a curve. For example, in Figure 8, they are arranged in a meridian plane of a half sphere DS. By electrically selecting one of the emitters Ei associated with each fiber FI to Fn, a particular angle of incidence AI of the beam illuminating the surface of the object OB and a wavelength or a spectral range is selected. Similarly, the angular distribution according to AR of the light flux reflected or scattered by the surface of the object OB is analyzed using the sheet of optical fibers NR and the associated detector RM for each wavelength or spectral range detectable . In addition, the processing unit associated with the detector calculates by linear interpolation of many intermediate directions. Thus with n optical fibers, N intensity levels per receiver, the angular resolution of the goniometric measurement is approximately 900 ° / (nN), for example 0.05 ° with n = 64 and N = 256. It is indeed a gonio-spectro-photometer type instrument without any mechanical movement. In a variant, it is also possible to place optical fibers in a meridian plane perpendicular to the plane of incidence to observe the diffusion for example.

In the case of FIGS. 7a, 7b and 8, the emission system EM, the reception system RM with their electronic control and processing circuits can be integrated in a single housing remote from the measurement head in optical connection by the 'between the NE and NR or NF layers.

FIG. 9a is an application of the invention for producing a light garland on a fiber optic support. A transmission device such as EM is coupled to one end of the optical fiber 1 along which Bragg gratings have been produced alternately Dzl, Dz2, Dz3, Dzl ... These gratings have the property of diffracting and / or to diffuse the light guided in the fiber perpendicular to this one and in a rather large cone of half angle (10 to 30 °) the wavelengths of Red, Green and Blue corresponding to those of sources El, E2 and E3. To make such arrays, the photo-registration method described above can be used by making arrays with high index modulation and juxtaposing them over a few millimeters. To ensure the symmetry of revolution of the radiation pattern of the Dzl network, the optical fiber can be rotated around its axis and translated between each exposure. It is also necessary to adapt the efficiency of these networks so that the light scattered as the light propagation in the fiber is homogeneous. Thus, less diffraction efficiency is required at the start of the propagation and better at the end. The electronic control circuit of the EM transmitter can generate all kinds of time sequences of the three colors Red, Green and Blue (RGB). Achromatic defects can also be produced in optical fiber by micromachining using a femtosecond laser which makes it possible to produce microstructures at the center of the fiber of a few microns in dimension without damaging the fiber too much mechanically. It does not need to be photosensitive and may even be made of plastic. A variant of the “light chain on optical fiber” application is described in FIG. 9b. It includes three identical RGB sources EM1, EM2 and EM3 each associated with a fiber FI, F2 and F3 respectively. These fibers are identical to fiber 1 described in Figure 9a and are joined together at 120 ° as shown in the section of Figure 9b. In this configuration, each source SI, S2 or S3 can take any color by composition of the 3 basic colors Red (El), Green (E2) or Blue (E3) and in particular the white color, ie 16 million colors with 8-bit source control. In addition, it is possible to simulate a light propagation by successively illuminating the sources Si, S2 then S3. Thus, any 3-phase chromato-temporal or spatio-temporal sequence can be generated. A control circuit, not shown, can control the sequential operation of the optical sources E1, E2, E3 to obtain this light propagation.

The advantages of such light garlands compared to those of the state of the art are numerous: - Elimination of unreliable and unsightly electrical connections because they are thick (a combination of three optical fibers can be less than 0.5mm in diameter) - A single light source thus improving the safety of people and equipment, making maintenance easy

Better performance and above all possibility of using LED or laser type sources with a long lifespan 10 000 to 100 OOOh.

- The length can be very large, several tens or hundreds of meters, well suited for outdoor use for decorating monuments, for example.

- The manufacturing cost can be very low because it is a very integrated technology on the fiber.

Claims

1. Optical system comprising at least one optical fiber (1), characterized in that it comprises at least a first and a second set of photo-induced diffraction gratings in the material of the fiber, the steps of the index strata gratings of the first set of gratings being adapted to diffract light at a determined wavelength and the steps of the index strata of the gratings of the second set of gratings being adapted to diffract light at another determined wavelength, each diffraction grating being inclined relative to the axis of the optical fiber so as to diffract light which circulates in the direction of the optical fiber towards the outside of the fiber.

2. Optical transmission system applying the optical system according to claim 1, characterized in that it comprises at least a first and a second diffraction grating (EZl to EZ2) photo induced in the material of the fiber, at least a first and a second optical source (E1 to E2) respectively emitting at least a first and a second wavelength and being coupled respectively to the first and to the second diffraction grating, the steps of the index strata of the first and of the second diffraction grating being adapted to diffract light at the first and second wavelength respectively, each diffraction grating being inclined relative to the axis of the optical fiber so as to diffract the light which it receives from the optical source which is coupled to it in the direction of the optical fiber; as well as at least a third and a fourth diffraction grating (RZ1 to RZ2) belonging respectively to the first and to the second set of gratings, at least a first and a second optical detector (RI to R2) capable of detecting light respectively at said first and said second length of wave and being coupled respectively to the third and to the fourth diffraction grating, the steps of the index strata of the third and of the fourth diffraction grating being adapted to diffract the light respectively at the first and at the second wavelength, each grating diffraction being inclined with respect to the axis of the optical fiber so as to diffract light which circulates in the direction of the optical fiber towards the optical detector which is coupled to it.

3. Transmission system according to claim 2, characterized in that said first and second diffraction gratings on the one hand and said third and fourth diffraction gratings on the other hand are produced in a single grating with variable pitch.

4. Transmission system according to one of claims 2 or 3, characterized in that it comprises a plurality of optical fibers placed side by side, the optical sources being arranged in the form of a matrix and being optically coupled each to a photo diffraction grating induced in one of said fibers.

5. Transmission system according to one of claims 2 or 3, characterized in that it comprises a plurality of optical fibers placed side by side, the optical detectors being arranged in the form of a matrix and being optically coupled each to a diffraction grating.

6. Communication system according to claim 4, characterized in that it comprises optical sources (El '-A, E2'-B) and optical detectors (R2'-A, Rl'-B) arranged symmetrically from said sources optical (El-A, E2-B) and said optical detectors (R2-A, 1-B) respectively with respect to the axis of the optical fiber and their associated diffraction gratings so as to emit or receive light whatever or its direction of circulation in the fiber.

7. Measuring system applying the transmission systems according to one of the preceding claims, characterized in that one end of the fiber (1) is oriented towards a medium to be measured.

8. Measuring system applying the transmission systems according to one of the preceding claims, characterized in that it comprises in a zone of a first fiber (1E) a first transmission system comprising transmitters coupled to the first and to the second photo diffraction grating induced in said first fiber, one end of said first fiber being oriented towards a medium to be measured, and, in an area of a second fiber (1R), a second transmission system comprising detectors coupled to the third and to the fourth photo diffraction grating induced in said second fiber, one end of said second fiber being oriented towards said medium to be measured so as to collect the light transmitted by the first fiber to the medium to be measured.

9. liquid level measurement system applying the transmission systems according to one of claims 2 to 5, characterized in that it comprises in a first zone a fiber (1) and arranged in series along of this fiber a first transmission system comprising transmitters coupled to the first and second photo diffraction gratings induced in said first fiber and a second transmission system comprising detectors coupled to the third and fourth photo diffraction gratings induced in said second fiber, the first length d wave and the second wavelength of the first transmission system being respectively equal to the first wavelength and the second wavelength of the second transmission system, a second region of the fiber (1) intended to be immersed in a liquid (61) contained in a tank (60) comprising a series of at least two diffraction gratings (RMl to RMn) of different pitches or a diffraction grating with variable pitch, the pitches of a first network ( RMl) and a second grating (RMn) of the series of diffraction grids being adapted to diffract light respectively at the first and at the a second wavelength emitted by a source (E1, E2) or received from the tank.

10. Measuring system according to any one of claims 7 to 9, characterized in that it comprises a circuit for controlling the emission of optical sources, a processing circuit allowing, depending on the switching time, to optical sources and detection results of optical detectors to calculate measured values.