Patent Application
20050232538
This invention relates to a multiplexer/demultiplexer in integrated optics comprising an optical cladding and its manufacturing method.
The invention is used for applications in all domains using wavelength multiplexers/demultiplexers, and particularly in the domain of optical telecommunications.
At the present time, multiplexers/demultiplexers with optical claddings are made from optical fibres. An example of this type of component is given in the document “Wavelength-selective coupler and add-drop multiplexer using long-period fiber gratings” by V. Grusby et al., published in the OFC2000 conference held from Mar. 5 to 10, 2000 in Baltimore, USA.
The two ends of these fibres are bonded to each other in a common coupling region 21 that is outside the interaction areas.
Thus, an initial light wave E comprising spectral bands with central wavelengths λj, λk introduced in the core 5 of the fibre 1 is transported by the core to the grating 9. This grating provides a means of coupling the guided mode of the wave symbolically shown by arrows, to one or several cladding modes propagating in the optical cladding 7, in the same direction as the guided mode. Coupling between the different modes takes place for wavelengths λj determined by the following known relation:
λj=Λ×(n0−nj) (1)
where:
This coupling results in an energy transfer between the guided mode and the cladding mode(s) for wavelengths λj. Coupled energy in the cladding modes is then transmitted through the coupling region 21 to the cladding 17 of the fibre 3, while energy which is not coupled is transported by the core 5 to supply a light wave T with central wavelengths λk, corresponding to the light wave E, from which a part corresponding to the central wavelengths λj has been extracted, to the output from the fibre 1.
Furthermore, the part of the light wave transmitted to the cladding 17 is then coupled by the grating 19 in the core 15 of the fibre 3 so as to obtain a light wave F output from the core 15, with central wavelengths λj.
In multiplexers/demultiplexers made with optical fibres, each core is dependent on the optical cladding in which it is formed; the cladding has a refraction index lower than the refraction index of the core, to enable propagation of a light wave in the core. The core cannot exist without the optical cladding; optical fibres have to be superposed, and therefore coupling regions between the fibres are necessary, before wavelengths may be multiplexed/demultiplexed.
Coupling regions are difficult to make. In particular, fibres in these regions have to be polished and glued while avoiding creating optical losses, to obtain good coupling.
The purpose of this invention is to divulge an integrated optics multiplexer/demultiplexer with at least one optical cladding, without the coupling problems of multiplexers/demultiplexers according to prior art.
Another purpose is to divulge a multiplexer/demultiplexer with at least one optical cladding that is independent of the guide cores to which it is associated. Independence of the core and the cladding means that they can exist in a substrate independently of each other.
More precisely, the invention relates to a multiplexer/demultiplexer including at least n guide cores in a substrate, where n is an integer greater than or equal to 2, and at least one optical cladding surrounding at least one portion of at least two distinct cores so as to define at least n interaction areas, each interaction area also comprising a grating enabling optical coupling between the core and the cladding in the interaction area, this multiplexer/demultiplexer comprising at least n inputs/outputs.
Thus, when this component is used as a demultiplexer, an input light wave with several spectral bands is added into one of the interaction areas through an input/output, the different interaction areas are such that each interaction area is suitable for providing a light wave with at least one of the spectral bands from the input wave, on an input/output.
Conversely, when this component is used as a multiplexer, p input light waves with determined spectral bands are introduced into p interaction areas through an input/output, a distinct light wave being introduced into each area, the different interaction areas are such that the input light waves are transported through different interaction areas so as to be coupled in at least one interaction area that then outputs a light wave with all or some of the spectral bands of the input waves on an input/output.
A multiplexer/demultiplexer can also be provided for which interaction areas have mixed operation, in other words they enable demultiplexing of spectral bands of a light wave and multiplexing of the spectral bands from several light waves.
A spectral band is a band with a set of wavelengths with determined central wavelength and band width.
Each light wave may comprise one or several spectral bands, the p light waves possibly having identical spectral bands.
The multiplexer/demultiplexer according to the invention can be used in particular, for example to make a spectral filter, but also to make an add and drop module, or a spectral coupler.
According to one preferred embodiment of the invention, each core comprises two ends, the n inputs/outputs are formed by at least one end of each core.
In the invention, the cladding is made artificially in the substrate, which enables the cores and the cladding to exist in the substrate independently of each other.
Independence of the cladding and the cores means that the n interaction areas can be made in the same cladding, so that it is possible to no longer have a coupling region between each optical cladding, as was the case for optical fibres according to prior art.
Furthermore, independence of the cores and the cladding advantageously enables the cladding to surround a portion of the guide cores. Thus, the cladding only acts on the propagation of a light wave in each core only in the area of the core that it surrounds and the cladding can guide or transport light waves independently of the cores.
Furthermore, since the cladding is independent of the cores, the cladding and core parameters can easily be adapted to the required applications. Thus, the dimensions, the value of the refraction index and the position of the cladding can easily be varied, to differ from the dimensions and value of the refraction index for each guide core. Thus, at least one characteristic of the mode(s) propagating in a guide core and/or one of the propagation modes in the cladding, can be modified.
The cladding has a refraction index greater than the refraction index of the substrate at least in the said area, so that cladding propagation modes can be induced in an interaction area.
The grating of an interaction area may be formed in the corresponding core of the area and/or the cladding.
Thus, when a light wave is introduced into the interaction area through the guide core, the guide mode is then coupled to one or several cladding modes in the interaction area, and vice versa the cladding mode(s) is (are) coupled to the core guided mode in the interaction area when the light wave is introduced into the cladding.
The grating may be periodic or pseudo periodic, and it may also be composed of a sequence of gratings.
The gratings in each interaction area may be identical or different, depending on the target applications.
The cladding may surround several portions of the same core so as to form several interaction areas in series, with the same core.
In a given cladding, the interaction areas may be in series and/or in parallel.
The substrate used may obviously be made using a single material or by superposition of several layers of material. In the latter case, the refraction index of the cladding is not the same as the refraction index of the substrate, at least in layers adjacent to the cladding.
According to one preferred embodiment, the cladding and the cores are made from the substrate, by a modification of the refraction index of the substrate.
According to the invention, the guides may be plane guides when light is confined in a plane containing the direction of propagation of light or microguides, when light is confined in two directions transverse to the direction of propagation of light.
Many multiplexer/demultiplexer variants can be made by combining guide cores with optical claddings, each created interaction area comprising a grating.
According to a first embodiment, the multiplexer/demultiplexer comprises one optical cladding and n guide cores, the optical cladding surrounds each core in a distinct interaction area, so as to form n interaction areas in series.
According to a second embodiment, the multiplexer/demultiplexer comprises m optical claddings, each optical cladding i surrounding ni guide cores (where i is an integer from 1 to m) in distinct interaction areas so as to form ni interaction areas in series for each cladding i, each cladding comprising at least one interaction area with a core common to an interaction area of another cladding.
In this embodiment, the claddings are made in parallel in the substrate. Each cladding i may comprise the same number ni of interaction areas, or a different number as a function of the target applications.
According to a third embodiment, the multiplexer/demultiplexer comprises one optical cladding and n guide cores, the optical cladding surrounds each core in a distinct interaction area, so as to form n interaction areas in parallel.
According to a fourth embodiment, the multiplexer/demultiplexer comprises one optical cladding and n guide cores, the optical cladding surrounds each core so as to form one or several interaction areas in series with each core, and interaction areas in parallel with the different cores.
In this embodiment, the number of interaction areas is greater than the number n of cores, these interaction areas being arranged in series and in parallel.
Obviously many other embodiments are possible, particularly by combining the previous embodiments.
Advantageously, the substrate is made of glass.
Obviously, the substrate could also be made from other materials, for example such as crystalline materials such as KTP or LiNbO3, or LiTaO3.
Furthermore, the optical claddings and/or guide cores and/or gratings may be made by any type of technique capable of modifying the refraction index of the substrate. In particular, there are ion exchange, ionic implantation and/or radiation techniques, for example by laser insolation or laser photo writing.
More generally, gratings may be made by any technique capable of changing the effective index of the substrate. In addition to the above-mentioned techniques, in particular there are techniques for making gratings by etching the substrate close to interaction areas. This etching may be done above interaction areas or in the portion of the cladding in the interaction area and/or possibly in the portion of the cores in the interaction area.
Grating patterns may be obtained either by laser scanning if radiation is used, or by a mask. This mask may be the mask used to obtain cores and/or claddings, or a specific mask for making gratings.
The invention also relates to a process for making a multiplexer/demultiplexer in integrated optics, such as that previously defined including at least n guide cores in a substrate and at least one optical cladding, the cores, and the cladding being made by modifying the refraction index of the substrate such that the refraction index of the cladding is different from the refraction index of the substrate and is less than the refraction index of the cores, at least in the part of the cladding adjacent to the cores and at least in the interaction areas.
The refraction index of the substrate is modified particularly by radiation, for example by laser insolation or by laser photo writing and/or by introduction of ionic species.
According to one preferred embodiment, the process according to the invention includes the following steps:
Obviously the order of steps a) and b) could be reversed.
The first and/or second ionic species is advantageously introduced by ion exchange, or by ionic implantation.
The first and second ionic species may be the same or they may be different.
The first ionic species and/or the second ionic species may be introduced with application of an electric field.
In the case of an ion exchange, the substrate must contain ionic species that can be exchanged.
According to one preferred embodiment of the invention, the substrate is made of glass and contains previously introduced Na+ ions, and the first and second ionic species are Ag+ and/or K+ ions.
According to a first embodiment, step a) includes production of a first mask comprising a pattern that is suitable for producing the cladding, the first ionic species being introduced through this first mask, and step b) includes elimination of the first mask and production of a second mask containing a pattern suitable for producing cores, the second ionic species being introduced through this second mask.
The masks used in the invention may for example be made of aluminium, chromium, alumina or a dielectric material.
According to a first embodiment of step c), the first ionic species is buried at least partially before step b) and the second ionic species is buried at least partially after step b).
According to a second embodiment of step c), the first ionic species and the second ionic species are buried simultaneously after step b).
Advantageously, at least part of the burial is done including the application of an electric field.
According to a third embodiment of step c), burial includes a deposition of at least one layer of material with refraction index advantageously less than the refraction index of the cladding, on the surface of the substrate.
Obviously, this embodiment may be combined with the previous two embodiments.
In general, before burial under a field and/or deposition of a layer, the process according to the invention may also comprise burial by rediffusion in an ionic bath.
This rediffusion step may be done partly before step b) to rediffuse ions in the first ionic species and partly after step b) to rediffuse ions in the first and in the second ionic species. This rediffusion step may also be done entirely after step b), to rediffuse ions in the first and second ionic species.
For example, this rediffusion is obtained by dipping the substrate in a bath containing the same ionic species as that previously contained in the substrate.
Other characteristics and advantages of the invention will become clearer after reading the following description given with reference to the figures in the appended drawings. This description is given purely for illustrative purposes and is not limitative.
a to 10d illustrate a section through an example embodiment of a multiplexer/demultiplexer according to the invention,
a to 11d illustrate variant embodiments of a mask pattern for obtaining a grating in a core, and
Claddings with a constant refraction index are described for simplification reasons, but obviously it would be quite possible to use claddings with a variable index within the scope of this invention, provided that their indexes close to the core are smaller than the refraction index of the core.
Similarly, although the substrate may include one layer or several layers, all these figures represent a substrate with a single layer.
Furthermore, these figures only show the multiplexer/demultiplexer, but obviously other elements could be integrated in the same substrate.
The multiplexer demultiplexer shown diagrammatically in section in
In this Figure, the optical cladding 20 only surrounds the portion of the core 23 that includes the grating R1 and the portion of the core 25 that includes the grating R2. The substrate area that includes the cladding, the core 23 and the grating R1 is the interaction area I1 and the substrate area that includes the cladding, the core 25 and the grating R2 is the interaction area I2.
It is quite clear in these figures that the cores 23 and 25 exist independently of the cladding 20 since outside the interaction areas, the cores are no longer located in the cladding, but only in the substrate 18 that enables optical isolation of the cores.
The cladding is thus created artificially in the substrate, at least around a portion of the cores comprising the gratings and independently of the cores and the substrate.
In general, an artificial cladding refers to this type of cladding made according to the invention, and a grating with artificial cladding (ACG) when the interaction area comprises a grating.
In this example embodiment, and in the following embodiments, the cladding is made in the substrate so as to have a refraction index between the refraction index of the substrate and the refraction index of the guide cores, so that it is possible to have cladding modes due to the presence of the gratings (represented by arrows in the cladding).
The gratings made in the interaction areas are a sequence of periodic or pseudo-periodic patterns represented in these examples by segmentation of the cores.
Since the cladding is independent from the guide cores, it is possible to adapt cladding parameters (such as dimensions, the index and the position) to suit core parameters (such as the dimensions, the indexes and the positions) to target applications.
The coupling force between a guided mode and a mode of cladding j given on a spectral band with central wavelength defined by relation (1), is obtained by taking the product of the grating length and the coupling coefficient κ. This coupling coefficient is proportional to the overlap integral of the two coupled modes, weighted by the grating profile.
We will denote the transverse profiles of guided and cladding modes as ξ0 and ξj respectively and the grating profile Δn, the coupling coefficient κ is then given by a relation of the following type:
κ∝∫∫ξ0,ξ*j.Δn.ds (2)
where ds is an integration element over the entire transverse surface of the substrate, in other words in a plane perpendicular to the propagation axis of the wave.
Thus, as the dimensions and index at the cladding increase, the number of cladding modes that could propagate will increase and therefore there will be a high possibility of coupling spectral bands. Conversely, if it is required to limit the number of cladding modes that can be coupled, it is useful to reduce the opto-geometric dimensions of the cladding.
The dimensions and index of the guide cores are independent and affect the characteristics of modes that propagate in them, and for example enable them to adapt to fibre modes in the case of coupling between the guide core and the fibre core.
Furthermore, as the differences between the indexes of the cores, the cladding and the substrate increase, the possibility of having couplings for short grating periods also increases, as shown by relation (1) (at a given resonance wavelength, the period is inversely proportional to the difference in index between guided and cladding modes).
Grating dimensions may also be adapted to target applications. Thus, it is possible to use gratings with long periods (for example a few tens of μm to several thousand μm) and gratings with shorter periods (for example less than a few μm) such as blazed gratings or gratings with inclined lines.
In the case shown in
The coupled part of the light wave in the cladding is then transmitted to the interaction area I2 and then, for all or part of its spectral bands, is coupled through the grating R2 with artificial cladding for which the parameters are adapted to this coupling, in the core 25. The coupled part S2 of the wave in the core 25 is transmitted through the core 25 to the output P2 of the component.
For example, if the wave E has two spectral bands with central wavelengths equal to λ1 and λ2 respectively, parameter settings are made for the grating R1 with artificial cladding such that the wave S1 has a spectral band with central wavelength λ1 and parameter settings are made for the grating R2 with artificial cladding such that the wave S2 has a spectral band with central wavelength λ2.
Conversely, when this component is used as multiplexer, two input light waves S1 and S2 with determined spectral bands are introduced through inputs P2 and P3 in interaction areas I1 and I2 respectively. All or some of the spectral bands of the wave S2 transported by the core 25 are coupled through the grating R2 with artificial cladding to the cladding mode(s), all or some of the spectral bands of these cladding modes in the core 23 are then coupled through the grating R1 with artificial cladding. Furthermore, the wave S1 transported by the core 23 passes the interaction area I1, at least some of its spectral bands are not coupled to the cladding mode(s) through the grating R1 such that at the output P1 from the component, the wave E comprises spectral bands of S1 and spectral bands of S2, parameter settings are made for the gratings R1 and R2 with artificial cladding so as to multiplex these spectral bands.
Thus, for example, if the wave S1 has a spectral band with central wavelength λ1 and wave S2 has a spectral band with central wavelength λ2, parameter settings are made for the gratings R1 and R2 with artificial cladding such that the wave E has spectral bands with central wavelengths λ1 and λ2.
For example, if the wave E introduced into the component has three spectral bands with central wavelengths equal to λ1, λ2, λ3 respectively, parameter settings are made for the grating R1 such that the wave S1 on the output P2 has a spectral band with central wavelength λ1, the grating R2 with artificial cladding is defined by parameters such that the wave S2 on the output P3 has a spectral band with central wavelength λ2 and parameter settings are made for the grating R3 with artificial cladding such that the wave S3 on the output P4 has a spectral band with central wavelength λ3. If three light waves S1 with a spectral band with central wavelength λ1, S2 with a spectral band with central wavelength λ2, and S3 with a spectral band with central wavelength λ3 are introduced, parameter settings are made for the gratings R1, R2, R3 with artificial cladding such that the wave E at the output P1 has spectral bands with central wavelengths λ1, λ2, λ3 respectively.
This component comprises four inputs outputs P1, P2, P3, P4, and is symmetric in its operation. With this type of component, two light waves can be introduced through P1, P4, and two light waves can be recovered at the output through P2, P3; similarly, two light waves can be introduced through P2, P3, and two light waves can be recovered at the output through P1, P4.
For example, if a light wave E1 with spectral bands with central wavelengths λ1, λ2 is introduced through P1, and a wave E2 with a spectral band with central wavelength λ3 is introduced through P4, parameter settings are made for the different gratings R1, R2, R3, R4 with artificial cladding such that at the output P2, the recovered wave S1 has spectral bands with central wavelengths λ2, λ3 and at output P3, the recovered wave S2 has a spectral band with central wavelength λ1.
In the special case in which λ1=λ3, the component of
This example embodiment is also symmetrical, and comprises three inputs (for example P1, P4, P6) and three outputs (for example P2, P3, P5).
For example, if a wave E1 with spectral bands with central wavelengths λ1, λ2, λ3 is introduced through P1, and a wave E2 with a spectral band with central wavelength λ4 is introduced through P4, and a wave E3 with spectral band with central wavelength λ5 is introduced through P6, parameter settings are made for the different gratings with artificial cladding R1 to R8 such that the wave S1 recovered at output P2 has a spectral band with central wavelengths λ4, λ2, λ5, the wave S2 recovered at output P3 has a spectral band with central wavelength λ1 and the wave S3 recovered at output P5 has a spectral band with central wavelength λ3.
This example with four claddings may be made with a larger number m of claddings. The dashed lines in
This component as shown is also symmetrical. It comprises four inputs/outputs, including two inputs (for example P1, P4) and two outputs (for example P2, P3).
In operation with a single input, for example P1, if a wave E1 is introduced with spectral bands with central wavelengths λ1, λ2, waves S1 are S2 are recovered through gratings R1, R2 with artificial cladding on outputs P3, P2 respectively, wave S1 having a spectral band with central wavelength λ1 and wave S2 having a spectral band with central wavelength λ2.
Use of the input P4 makes it possible to introduce a wave E2 into the core 62, for which the spectral band(s) may be added onto the output P2 at the spectral band with central wavelength λ2 of the wave S2. Therefore this special case enables operation both in multiplexer and demultiplexer.
In this example, the grating R1 and the grating R2 have at least one of the common spectral bands.
This component comprises three inputs/outputs. Only one of the ends of each core forms an input/output, the other ends of the cores face inwards into the cladding 60.
For example in operation as demultiplexer, a wave E1 with spectral bands with central wavelengths λ1, λ2 is introduced through input P1, and a wave S1 with a spectral band with central wavelength λ1 is recovered at the output P3 from this component, and a wave S2 with a spectral band with a spectral band with central wavelength λ2 is recovered at the output P2.
In the special case in which λ1=λ2, the component in
For example, considering operation as demultiplexer, the use of additional gratings R3 and R4 makes it possible to also couple in the core 61 all or some of the light wave E2 introduced into the core 62 through the input P4, which was not the case in
Finally,
Thus, if the gratings A1 and B1 can be used to couple a determined spectral band and gratings A2 and B2 can be used to couple another determined spectral band, and if a wave E1 with spectral bands with central wavelengths λ1, λ2, λ3 are introduced at input P1, it is possible to recover a wave S1 with spectral band with central wavelength λ1 on output P3, and to recover a wave S2 with spectral band with central wavelengths λ2, λ3 on output P2, through grating R1 (composed of A1 and A2) and grating R2 (composed of B1 and B2).
Obviously, other example multiplexer/demultiplexer embodiments could be produced particularly by combining these variants. Furthermore, operation of the above examples of components has been presented essentially to demultiplex spectral bands of a light wave, but these components could also operate in the reverse direction, in other words to multiplex spectral bands of several light waves. Furthermore, the gratings used have been shown in cores, although these gratings can be made in claddings and/or in the cores.
An example embodiment of a multiplexer/demultiplexer will now be described. To simplify the description, we will describe the production of a single cladding, a single core and a single grating, to produce a single interaction area, for example I1, those skilled in the art will easily be able to make all types of multiplexer/demultiplexer, and particularly those shown in the previous figures, from this description.
Thus,
a shows the substrate 18 containing ions B.
A first mask 81 is made, for example by photolithography on one of the faces of the substrate; this mask comprises a determined opening as a function of the dimensions (width, length) of the cladding that is to be obtained. Furthermore, when the grating is made in the cladding, the patterns of the mask 85 may be adapted to the patterns of the grating to be formed.
A first ion exchange is then made between ions A and ions B contained in the substrate, in a substrate area located close to the opening of mask 81. This exchange is obtained for example by dipping the substrate with the mask into a bath containing ions A and possibly applying an electric field between the face of the substrate on which the mask is located and the opposite face. The substrate area in which this ion exchange takes place forms the cladding 20.
This cladding is buried by carrying out an ion rediffusion step A with or without the assistance of an electrical field applied as above.
Therefore, the production of the cladding according to the invention is similar to the production of a guide core, but the dimensions are different.
The next step shown in
A second ion exchange is then made between the B ions of the substrate and the C ions that may be the same as or different from the A ions. This ion exchange may be made as above by dipping the substrate in a bath containing C ions and possibly applying an electrical field.
Finally,
Conditions for the first and second ion exchanges are defined so as to obtain the required differences in refraction indexes between the substrate, the cladding and the core. The adjustment parameters of these differences are particularly the exchange time, the bath temperature, the ion concentration in the bath and whether or not there is an electric field present.
As an example embodiment, the substrate 18 is made of glass containing Na+ ions, the mask 81 is made of aluminium and has an opening about 30 μm wide (the length of the opening depends on the required cladding length for the target application).
The first ion exchange is made with a bath containing Ag+ ions at a concentration of approximately 20%, a temperature of about 300° C. and for an exchange time of about 5 minutes. Rediffusion of ions takes place firstly in free air at a temperature of about 330° C. for 30 s, then the cladding thus formed in the glass is partially buried. This burial is done by rediffusion in a sodium bath at a temperature of about 260° C. for 3 minutes.
The mask 85 is also made of aluminium and has an opening pattern approximately about 3 μm wide (the pattern length depends on the required core length for the target application).
The second ion exchange is made with a bath also comprising Ag+ ions at a concentration of about 20%, a temperature of about 330° C. and for an exchange time of about 5 minutes, rediffusion of ions firstly in free air at a temperature of about 330° C. and for 30 s. The core thus formed in the glass is then partially buried by rediffusion in a sodium bath at a temperature of about 260° C. for 3 minutes.
Burial of the cladding and the core is finalized under an electric field, with the two opposite faces of the substrate being in contact with the two baths (in this example sodium), so that a potential difference between these two baths can be obtained.
Many variants of the process described above can be produced. In particular, burial steps of the cladding and the core may be performed as described above during two successive steps, but they may also be done simultaneously because the core gets buried faster than the cladding because it has a higher ionic concentration than the cladding, and this also enables centring of the core in the cladding.
The difference in concentration between the core and the cladding is usually obtained by rediffusion of ions forming the cladding in a bath, or by a difference in the concentration of ions introduced in steps a) and b).
As we have already seen, one variant of the process for burying the cladding and the core consists of depositing a layer of material 71, shown in dashed lines in
The component according to the invention is produced not only using the ion exchange technique. The component according to the invention may obviously be made using any technique that can be used to modify the refraction index of the substrate.
Moreover, the period, size and position of the grating with respect to the core and the guide are parameters that may be adapted as a function of the application.
The grating pattern may be defined on the mask to produce the cladding and/or the mask so that the core can be produced, or on a specific mask for production of the grating only.
a to 11d illustrate examples of variant embodiments of masks M1, M2, M3, M4 used to obtain a grating, for example R1. These Figures show top views of masks and only show the part of the masks used to obtain the grating. White areas in the pattern of masks correspond to openings in the masks.
These masks can be used to obtain a periodic grating with period Λ.
For example, these masks may be specific masks for making the grating in the core and/or in the cladding, or part of the masks that can be used to obtain the core and/or the cladding, the grating then being made at the same time as the core and/or the cladding.
Thus, in this Figure, the grating R is formed in the cladding by an alternation with period A of areas 95 with variable width considered in the direction of propagation of a light wave. These areas have an effective index different from the effective index of the remainder of the cladding due to a change in the refraction index in these areas. Moreover, the core is included in the cladding at least in the interaction area, the grating is also inscribed in the core, in other words the core also comprises areas with refraction index different from the refraction index for the rest of the core.
The gratings may be formed by any conventional technique for locally modifying the effective index of the substrate in the core and/or in the cladding.
Therefore, it can be done as described above, during ion exchanges used to make the core and/or the cladding or during a specific ion exchange. But, it may also be done by etching the substrate at the interaction area or by radiation. In particular, the grating may be obtained by insolation of the core and/or the cladding with a CO2 type laser. The laser can locally rediffuse ions by creating local temperature rises, and thus inscribe the grating pattern.
For example, the substrate can be scanned with a laser beam, for example an amplitude modulated laser beam, so as to introduce a modulation of the grating at the required pitch.
The grating pattern depends on target applications. In particular, the grating may have a variable period (chirped grating), or variable efficiency (apodised grating).
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/FR03/01441 | 5/13/2002 | WO | 6/15/2005 |
