FR2748328A1 - Outer cover construction for optical guide or Bragg network - Google Patents

Abstract

The outer cover construction forms part of an optical guide which may be fibre optics or planar, covering a section of interface with a Bragg network. A Silicon transmission strip (13) is placed on a layer of non-doped silicon (12) mounted on a substrate (11). The transmission strip is covered by a polymer material (14). The refractive index of the polymer material is close to that of the Silicon strip.

Description

 The present invention relates to optical guides with silica core and Bragg grating (x).

 Optical guide means here and throughout the text as well the optical fibers as the planar optical guides.

 FIGS. 1 and 2 show a planar Bragg grating guide according to a known prior art.

 The Bragg grating is made by photoinscription, for example by ultraviolet irradiation at a wavelength of 244 nm, on a component which conventionally comprises a substrate 1 made of silicon, a layer 2 of undoped silica, a ribbon 3 of doped silica which constitutes the guiding core, as well as an optical cladding 4 also of undoped silica.

 Because of their enormous potential in the field of optical components, Bragg gratings photoinscribed by ultraviolet irradiation in silica are currently the subject of numerous publications.

By way of illustration, for a general presentation of optical fibers with Bragg gratings, it will be possible in this respect advantageously to refer to
- "Photosensitive Optical Fibers: Devices and
Applications ", Raman Kashyap, Optical Fiber Technology 1, 17-34 (1994).

 However, as shown in FIG. 3, the spectra of the silica core optical guides and Bragg gratings known today have two reflection peaks, which are due to the birefringence in said guides.

 These two peaks are separated by a wavelength distance which is of the order of 0.8 nm and correspond respectively to the two modes of polarization TM and TE of the light which propagates in the optical guide.

 The invention proposes for its part an optical guide with a silica core whose Bragg wavelength is insensitive to polarization.

Moreover, the Bragg XB wavelength of the waveguides of the type of that of FIGS. 1 to 3 increases linearly with the temperature in the range -20 ° C. to more than 1000 ° C. with a coefficient of the order of 0, 01 nm / OC. This increase is mainly due to the increase of the optical index n of the silica with the temperature:
dn / dT = 10 * 10-6 / K
It has already been proposed by
- "Temperature-compensated optical-fiber Bragg
gratings ", GW YOFFE et al., OFC '95 Technical
Digest, p. 134-135, a solution for stabilizing the Bragg wavelength as a function of the temperature in this temperature range.

 This solution uses a mechanical system intended to expand the Bragg grating when the temperature of the latter decreases.

 We know that the wavelength KB at which a Bragg grating photoinscribed in an optical fiber or a planar optical guide reflects an incident beam is equal to = = 2 Neff * where Neff is the effective index of the optical guide , d is the step of the Bragg network.

 In the solution presented in the aforementioned publication, the Bragg grating optical fiber is mounted in tension on a mechanical support which makes it possible to release this mechanical stress when the temperature of the assembly increases. Thus, the pitch of the network d decreases with the temperature to compensate the increase of Neff with the temperature.

 This compensation method has several major disadvantages.

 By construction, it is limited to the case of optical fibers and can not be used for planar guides.

 In addition, its implementation is difficult because it assumes a fine adjustment of the tension of the fiber.

 Another object of the invention is therefore to propose a temperature stabilization of Bragg gratings that does not have these disadvantages.

 Moreover, to date, it is not possible to adjust or modulate the Bragg wavelength of a photoinscribed network other than by changing the pitch of this network.

 Another object of the invention is to provide a waveguide with silica core and Bragg grating whose Bragg wavelength can easily be adjusted or modulated.

 The invention proposes an optical guide such as an optical fiber or a planar optical guide, comprising a Bragg grating silica core, characterized in that it has an optical cladding of a polymer material.

 The inventors have indeed found that the transmission spectrum of such an optical guide has only one reflection peak.

 In addition, the Bragg length of such an optical guide is substantially temperature stabilized.

 The invention also relates to an optical guide of the aforementioned type in which the polymer material of the optical cladding is an electro-optical material and which comprises means for applying a modulation voltage to said optical cladding so as to modulate the Bragg wavelength. of that guide.

It also relates to a method for producing a silica core optical waveguide having a Bragg grating and sheathed in a polymer material, characterized in that it comprises a treatment for adjusting the optical index of the polymer. optical sheath, so as to obtain the wavelength of
Bragg desired.

 This treatment involves, for example, ultraviolet irradiation or annealing or a momentary heat supply.

 Such a treatment makes it possible, at very low cost, to adjust the Bragg wavelength of the guide with great finesse.

 The invention also relates to components incorporating at least one such optical guide.

Other features and advantages of the invention will become apparent from the description which follows. This description is purely illustrative and not limiting. It must be read in conjunction with the attached drawings on which
FIG. 1 is a schematic sectional representation of a Bragg grating planar optical waveguide according to a known prior art.
FIG. 2 illustrates the photo-inscription on the optical guide of FIG. 1
FIG. 3 is a representation of the spectrum of a Bragg grating optical waveguide according to the prior art.
FIG. 4 is a diagrammatic sectional representation of a Bragg grating planar optical waveguide in accordance with the invention;
FIG. 5 is a representation of the spectrum of an optical guide according to the invention;
FIGS. 6 and 7 are graphs on which the Bragg wavelength has been plotted as a function of temperature respectively for an optical guide according to the prior art and for an optical guide according to the invention
FIGS. 8a to 8c illustrate a possible embodiment for the guide of FIG. 4
FIG. 9 is a graph on which the Bragg wavelength of this guide as a function of the index of its optical cladding has been taken as an optical guide according to one possible embodiment for the invention;
FIGS. 10a and 10b are respectively diagrammatic views in plan view and in section of an optical guide according to a possible embodiment for the invention, which comprises means for modulating its Bragg wavelength.

 The planar optical waveguide according to the invention illustrated in FIG. 4 comprises a silicon substrate 11 on which a layer 12 of undoped silica is deposited, a ribbon 13 of silica doped with germanium oxide forming the core of the optical waveguide, and a layer 14 forming an optical cladding.

 The lower layer 12 of SiO 2 is, for example, 3 μm thick.

The ribbon 13 has a thickness and a width which are for example 6 μm
The GeO2 doping of this ribbon 13 forming an optical guide core is 7%. This doping increases the optical index of said ribbon 13 and allows an index difference of the order of 7.10-3. In addition, it photosensitizes the silica at the wavelength of 244 nm and thus allows the photo-inscription of a Bragg grating.

 The optical cladding layer 14 is made of a polymeric material.

 This polymeric material of the optical cladding 4 is for example a polymethyl methacrylate (PMMA). It is chosen so that its optical index is close to that of the silica (of the order of 1.45 at the wavelength of 1.5 pm) and less than this.

 Its thickness is for example of the order of 10 microns.

 The transmission spectrum of the optical optical sheath 14 which has just been described is of the type illustrated in FIG. 5 and has a single reflection peak at the Bragg wavelength.

 Moreover, the polymeric materials have the further advantage that their optical index variations as a function of temperature are negative (dn / dT s O).

 Their optical index therefore contributes to stabilizing the effective index of the optical guide.

 This is indeed a combined function of the optical index of the core 13 and the optical index of the optical cladding 14.

 In the case of silica, dn / dT = 10-5 K-1.

 For PMMA, dn / dT = -105 * 10-6 K - 1.

 FIG. 6 shows the variation of the Bragg XB wavelength as a function of temperature, in the case where the optical cladding is made of silica.

We see in this figure that the wavelength of
Bragg is particularly unstable in this case. It grows linearly with a coefficient of 0.013 nm / C.

 FIG. 7 shows the variation of the Bragg wavelength as a function of temperature in the case of the waveguide described with reference to FIG. 4. The PMMA corresponding to the curve on this FIG. of optical index equal to about 1.42 for a wavelength of 1.5 .mu.m.

 As can be seen in this FIG. 7, the Bragg wavelength varies according to an inverted bell curve as a function of temperature.

 Between 25 and 1000C, the variation of XB is less than 0.3 nm, whereas in the same temperature range, the variation is 1 nm in the case of a silica optical cladding waveguide. prior art.

Of course, polymers other than polymers
PMMA may be envisaged, in particular polyimide, polystyrene, polymethacrylic, polyacrylic and poly-α-haloacrylate polymers, polysiloxanes, or alternatively ethervinyl or epoxidized polymers.

 Copolymers synthesized from these different polymers are also conceivable.

In particular, it is possible, for example, to use synthesized statistical copolymers based on methyl methacrylate (MMA) and trifluoroethyl methacrylate (MATRIFE). By adjusting the proportions of MMA and
MATRIFE, one can reach the desired index which is determined by the simulation.

It is known that the index np of such a copolymer is approximately given by
np = X.np ~ s + (1-X) npsTRIFE where X is the weight fraction of MMA reacted for the polymerization and n and npSTRIFE are respectively the refractive indices at the operating wavelength of the network of homopolymers of MMA and
TRIFEMA.

This copolymerization is made in conventional thermal radical (600C). Once the copolymer has been obtained, the index is readjusted because the above law is not sufficiently precise, by mixing this copolymer with one of the two homopolymers PMMA or PMATRIFE depending on whether the copolymer number is lower or higher. high than expected. For example, if the wavelength index of 0.6 ym for the superstrate is 1.45, we will make a 50/50 MMA / MATRIFE copolymer approximately and readjust after measuring np with either PMMA be from
PMATRIFE to obtain a mixture which is the desired index to 0.001 for example. The mixing is done in a solvent medium to give a solution that will be used during the step of placing the polymer sheath, as will be described later.

 For this purpose, the solution is diluted or concentrated, for example, to obtain concentrations compatible with the desired thicknesses after deposition on the photoinscribed network. They are generally between 50 and 600 g / l of solvent. The latter is, for example, 1,1,1-trichloroethane.

We can make the deposit
- By spreading with a pipette or by pouring the solution on the photo-inscribed network taking care not to damage the guide core. Once the spreading is done, the film is allowed to dry by heating at 1100C on a flat bed or by light heating <500C in a primary vacuum oven,
- We can also perform the deposit by spinning ("spin coating" according to the Anglo-Saxon terminology generally used by the skilled person) by adjusting the speed to obtain the desired thickness (500 to 3000 rpm). The film is then dried, in the same manner as before, in a vacuum oven.

 These two deposition techniques make it possible to obtain thicknesses greater than 2 μm, the thickness of the polymer of the optical cladding being typically of the order of about 10 μm.

 Referring now to FIGS. 8a to 8c, in which various steps of a method for producing the optical waveguide of FIG. 4 are illustrated.

 In a first step, the silica layer 12 and a doped silica layer are deposited on the substrate 11.

 The ribbon 13 is then etched by photolithography on the doped silica layer. The structure obtained is of the type illustrated in FIG. 8a.

In a second step, the network of
Bragg (15 in Figure 8b) by photoinscription on the previously obtained structure.

 The photo-inscription can be done by several techniques, especially by interference of two ultraviolet beams F1, F2 from the same source 5, as illustrated by Figure 8b.

 The duration of photoinscription is between a few minutes and several hours depending on the conditions of preparation of the sample, in particular high pressure molecular hydrogenation (> 150bars) considerably reduces this duration. The photoinscription wavelength (244 nm) chosen corresponds to the absorption bands due to the presence of germanium oxide in the core of the guide.

 Then, using the deposition techniques described above, the polymer sheath 14 is deposited on the layer 12 and the core ribbon 13 with a photo-written Bragg grating.

 Other embodiments are of course possible.

 For example, the Bragg grating 15 may be etched on a structure whose ribbon is between a lower silica layer and an undoped upper silica layer. The upper silica layer is, after photo inscription, suppressed or reduced by plasma etching.

 FIG. 9 shows the variation of the Bragg wavelength 4 as a function of the index of the optical cladding, in the case where the index of the core material is 1.454.

 As can be understood from reading this FIG. 9, it is possible to adjust or modulate the Bragg wavelength of the optical guide which has just been described by varying the index of the polymer of the optical cladding 14. .

 The variation of possible wavelength is of the order of 1 nm.

 For example, the Bragg wavelength of the optical waveguide can be permanently adjusted by annealing or ultraviolet irradiation, by crosslinking the polymer or by chemical modification, in particular by heat input, or by isomerization of a lateral group. of the polymer.

 Irradiation with an ultraviolet beam chemically modifies the polymer material. The consecutive index variation on the optical cladding can reach some 10-2.

 The wavelengths used to obtain such a modification depend on the reactive side group.

They are generally between 240 and 400 nm.

 The irradiation powers are of the order of a few mW / cm 2 and are applied for a few minutes (i.e. maximum energy of a few joules / cm 2).

 Advantageously used to adjust the index of the optical cladding polymer materials with cinnamon groups that can be grafted onto polymers. These groups react with each other with radiation between 254 and 300 nm and cause a variation of index.

 For example: vinylcinnamates react at a wavelength of 254 nm. With an energy of 4 joules / cm 2, it is possible to reach a decrease in index of 10 -2 without the polymer being degraded. With an energy of 15 joules / cm 2, an index decrease of 2 * 10 -2 can be achieved without the polymer being degraded.

 Moreover, it is also possible to modulate the Bragg wavelength of the waveguide, for example by electrically modulating the optical index of its sheath.

 Electro-optical cladding polymers are used for this purpose.

 As illustrated in FIGS. 10a and 10b, a modulation voltage is imposed on the optical cladding by means of two metal electrodes 16 disposed on either side of the core ribbon 13, on said optical cladding 14.

 These electrodes 16 can also be used during the manufacture of the optical guide to guide the polymer in field to make it electroactive.

The modulation frequency can reach a few
GHz.

 Finally, it is also possible in the case of an electrooptical polymer material to adjust its index by annealing or ultraviolet irradiation, so as to adjust the Bragg wavelength around which the filter will be modulated electrooptically.

Claims (9)

 1. An optical guide, in particular an optical fiber or planar optical guide, comprising a core (13) made of silica with a Bragg grating, characterized in that it has an optical cladding (14) made of a polymer material.  2. optical guide according to claim 1, characterized in that the material of the sheath is selected from polyimides, polystyrenes, polymethacrylics, polyacrylics, poly-ahaloacrylates, polysiloxanes, ethervinyl polymers or epoxides or is a mixture or a copolymer of these materials.  3. optical guide according to one of claims 1 and 2, characterized in that the polymer material of the optical sheath (14) is an electro-optical material and in that it comprises means (16) for applying a modulation voltage on said optical sheath (14) for modulating the Bragg wavelength of said guide.  4. Method for producing an optical guide according to one of the preceding claims, characterized in that it comprises a treatment for adjusting the optical index of the polymer optical sheath (14), so as to obtain the desired Bragg wavelength.  5. Method according to claim 4, characterized in that this treatment uses ultraviolet irradiation or annealing or a momentary heat input.  6. Method according to one of claims 4 and 5, characterized in that before said treatment - is deposited on a substrate (11) silicon an undoped silica layer (12) and a doped silica layer, - engraves the core t13) of the guide by photolithography on the doped silica layer, - the Bragg grating (15) is engraved by photo-inscription on the structure thus obtained, - the optical polymer cladding (14) is deposited on the structure thus obtained.  7. Method according to one of claims 4 and 5, characterized in that before said treatment - is deposited on a substrate (11) silicon an undoped silica layer (12) and a doped silica layer, - engraves the core (13) of the guide by photolithography on the doped silica layer, - is deposited on the structure thus obtained an undoped silica layer, - the Bragg grating (15) is etched by photoinscription on the structure thus obtained, the upper silica layer is removed or reduced by plasma etching; the optical polymer sheath (14) is deposited on the structure thus obtained.  Optical fiber according to one of claims 1 to 3.  9. Optical component, characterized in that it incorporates at least one planar guide according to claims 1 to 3.