FR2815417A1 - Fibre optic telecommunications optical filter manufacture, having thin substrate deposited layer with inserted shaped sections forming pass band filter near normal incidence - Google Patents

Abstract

The optical component structure has a thin deposited layer (14) on a substrate (13). There is a structure or resonant network with a number of shaped sections (15,16) in the thin layer. The shaped sections form a pass band filter for a thin layer incidence near to the normal angle.

Description

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Optical filter, its manufacturing process and its use for a multiplex system ---------------
The present invention relates to an optical filter, to its manufacturing process and to its use for an optical multiplex system with wavelength division.

 The technical field of the invention is that of the manufacture of telecommunications systems by optical fibers.

 In order to increase the transport capacity of cable fiber-optic networks, it is known to multiplex several channels to make them transit simultaneously in a fiber, then to demultiplex them; when the multiplexing is carried out by division of the transmission band (in wavelength) of the fiber, one generally speaks of WDM system for Wavelenght Division Multiplex; in order to increase the number of channels capable of being multiplexed in order to increase the capacity of a fiber while remaining within a determined wavelength range, it is necessary to have a multiplexer (and a demultiplexer) capable of processing (mix or separate) several beams whose wavelengths are very close; there is therefore a need for optical filtration components having a very narrow bandwidth (of the order of 10-9 meters) and a very high rejection rate.

 US Patent 5,355,237 has proposed a multiplexer device comprising several input waveguides, a diffracting grating extending along a concave curve, and an output waveguide, one face of which is coupled to an optical fiber; the device comprises laser diodes, which extend orthogonally to the axis of the fiber, and to each of which is associated a surface diffraction grating adapted to the wavelength of the diode; in the case of a demultiplexer, the laser diodes and their associated diffraction gratings are replaced by photodiodes; the curved diffracting grating is produced by electron beam lithography, in the form of a narrow channel extending in a sawtooth fashion along a curved contour; the

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 disadvantages of this type of device are their high cost, their poor efficiency and their high sensitivity to temperature and to the polarization of the incident beam.

 US Patent 5,216,680 discloses a mathematical method for analyzing resonance phenomena in a waveguide and suggests applying the results thereof to the design of a resonant optical filter for laser, without however describing the filter; US Patent 6,035,089 explains that this method can be used to design a filter made up of bands hollowed out in a waveguide supported by a substrate, then filled with a material capable of obtaining a resonant structure, in particular at 533, 4 nm.

 It is also known to produce optical filters (passband in transmission) by stacking many thin layers of thickness and determined refractive index, to transmit a determined wavelength.

 It is also known to make Bragg filters photoinscribed in a fiber; these filters work in reflection.

 The present invention aims to provide an improved optical filter, a multiplexer (and a demultiplexer) incorporating this filter, and its manufacturing process.

 According to the invention, resonant structures are used at a determined wavelength and under a determined incidence; a resonance is obtained by the construction of a network of patterns on the surface and if necessary in the thickness of at least one waveguide constituted by a thin layer or a stack of thin layers; the resonance results from an electromagnetic coupling by the network between an incident wave substantially according to the normal to the surface of the waveguide, and a guided mode of the waveguide: the resonance is observed when temporal and spatial frequencies of l 'incident wave are substantially equal to those corresponding to a specific mode of the structure; for this purpose, patterns are generally provided

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dont les dimensions sont voisines de la longueur d'onde désirée, et généralement inférieures à celles-ci.

whose dimensions are close to the desired wavelength, and generally smaller than these.

According to the invention, said structure and / or said network is pluriperiodic: it (it) comprises a plurality of first identical salient patterns, in the form of studs, which are regularly spaced in two directions parallel to the waveguide, and according to a first step ; it (it) further comprises a plurality of second identical salient patterns which are regularly spaced in said directions and in said first pitch, which are arranged nested with said first salient patterns and which are of shape and / or dimensions different from those of first reasons. Thus, said first pads (or patterns) form a first sub-network which is nested with a second sub-network formed by said second pads (or patterns).

Generally, and as a result of this nesting, a first pattern and a second pattern adjacent to the first pattern will be separated by a distance equal to half of said pitch; in a preferred embodiment, the resonant network may include more than two subnetworks, in particular 3 or 4 nested subnetworks to form the pluriperiodic network.

Such a structure ensures bandpass filtering in reflection, which has a high rejection rate and which allows an inclination (of low value) of the incident wave relative to the normal to the surface of the guide. wave, this inclination being desirable to separate the reflected beam from the incident beam, without requiring the use of a semi-reflecting plate or an interference system.

Such a structure makes it possible to obtain improved performance for variations in the angle of incidence, the opening angle, or the polarization of the incident beam.

According to another aspect of the invention, an optical component is used comprising several thin layers superimposed on a

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 substrate and incorporating two such resonant structures, which are preferably arranged symmetrically with respect to a median plane, and which are identical and / or resonant at the same wavelength; a passive bandpass filter in transmission is thus obtained which has a high rejection rate, and which is simple to manufacture.

 The pattern network can be obtained by etching with an electron beam and / or by microlithography (or equivalent process).

 The production of a component incorporating two resonant networks can be carried out by etching the two networks on two portions of a thin layer resting on a common substrate; the substrate can then be cut into two parts which can be assembled to form the component; assembly can be carried out by bringing the two parts into contact through the face on which the network is provided; this assembly can be obtained by molecular adhesion, alternately the two parts can be assembled by their respective portions of the substrate, if necessary by bonding.

 According to an alternative embodiment, an etching can be carried out on the surface of each of two stacks of thin layers provided on each of the two faces of a substrate common to the two stacks.

 Preferably, the network is etched in an anti-reflection structure comprising at least one thin layer.

 Improved performances (narrow band and high rejection) are obtained by engraving the patterns of each network in a layer (called external) of index (for example close to 1.5) lower than that (for example close to 2) of the layer on which the outer layer is deposited, which leads to an anti-reflection structure.

 In addition, the use of circular patterns and / or with lateral faces inclined relative to normal to thin layers improves the resistance to opening.

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To increase the rejection rate of the optical filter obtained, it is also possible to provide several superimposed resonant networks.

 The components according to the invention have the advantage of requiring a reduced number of thin layers, compared with conventional thin layer filters.

 In addition, they make it possible to obtain narrower filters which are therefore better suited to dividing a transmission band into a large number of channels. In practice two or three thin layers may be sufficient to construct a bandpass filter having a high rejection rate for wavelengths far from the wavelength on which the filter is centered, having a spectral width less than one nanometer and reflecting or transmitting about 100% of the incident beam - outside the passband - even when the incident beam has an incidence (compared to normal) of up to several degrees and / or an opening of a few bn, in particular from 5 to 20 bn.

 Since the position of the central wavelength is essentially determined by the geometry of the network which extends over the external face (s) of the component, it is easy to shift and / or adjust the central wavelength after completion of the network: this offset can in particular be obtained by covering the network with a thin layer of suitable index and thickness, or else by modifying the geometry of the network, for example by a posterior step of electron beam etching.

 The thin layers can be produced by depositing a material chosen from metals such as AI, Ni, Ag, Au, oxides such as Tri02, Si02, Ta205, fluorides (such as MgF2), sulfides (such as ZnS ) or nitrides; each thin layer has a thickness generally ranging from 10 nm to 10 μm; the thickness of the thin layer covering the grating to offset the central wavelength may be less than these values: it can range from 1 to 10 nm (nanometer).

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 By comparison, the substrate has a greater thickness, for example of the order of 1 mm; it is made of a transparent material for the wavelengths considered, in particular the wavelengths close to 1.5 μm, such as glass.

 The etching of the network can be obtained by various techniques, such as the technique of dry etching, wet etching, reactive etching, etching by ion beam, optionally chemically assisted.

 Other advantages and characteristics of the invention will be understood through the following description which refers to the accompanying drawings, which illustrate without any limiting nature of the preferred embodiments of the invention.

 Figure 1 is a schematic perspective view of part of a component according to a first embodiment.

 Figure 2 is a schematic cross-sectional view of part of a component according to a second embodiment.

 Figure 3 is a schematic plan view of an eight-way demultiplexer incorporating bandpass filters in transmission according to the invention.

 FIG. 4 is a schematic perspective view of a component blank according to the invention, before it is cut, to form a component with a structure similar to that of the component illustrated in FIG. 1.

 Figure 5 is a schematic sectional view through a plane transverse to the planes of the layers, which shows a stud of a network whose sides are inclined.

 Figure 6 is a schematic plan view of a four-way demultiplexer incorporating four components forming reflection bandwidth filters.

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FIG. 7 illustrates in plan view the arrangement of the square studs of a pluriperiodic network. FIGS. 8 and 9 illustrate the performance of a filter consisting of a thin layer deposited on a substrate and on which the network of FIG. 7 is formed.

 With reference to Figures 3 and 6, the demultiplexer 1,2 comprises eight filters 3 (respectively four filters 4) bandpass centered on eight (respectively four) different wavelengths corresponding to the eight (respectively four) channels to be separated.

 The demultiplexer comprises an interface 5 for optical coupling with an optical fiber 6 transporting the radiation to be divided; the incident beam 7 delivered by the fiber forms with the normal 8 at the face 9 of the filter an angle 10 of incidence, the value of which is as low as possible in order to limit the disturbances liable to result therefrom.

 The demultiplexer 2 in FIG. 6 comprises four output collimators 11 respectively associated with the four filters 4, which serve to direct towards a sensor (such as a photodiode) - not shown - the part 12 of the incident beam 7 corresponds to the wavelength associated with the corresponding channel.

 Similarly, the demultiplexer 1 in FIG. 3 comprises eight output collimators 11; each demultiplexer 1,2 may include an integrated input collimator to limit the opening of the incident beam 7 coming from the fiber. Filters 3 and 4 have a plate structure such as that respectively illustrated in FIGS. 1 and 2.

 The filter 3 in FIG. 1 extends in a symmetry with respect to its median plane 12: it comprises, on either side of this plane, a substrate 13 covered with a thin layer 14 in part of the thickness of which were formed by etching pads 15,16 forming a resonant network.

 This network comprises on the one hand a plurality of first studs 15, of cylindrical shape along axes parallel to the normal 8 to the upper external face 9 and to the lower external face 17 of the component 3,

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 and of circular cross section (or contour); the network also comprises a plurality of second studs 16, also cylindrical in shape and parallel to the normal 8 to the plane 12.

 The studs 15 are of identical shape and size and are regularly spaced in two orthogonal directions perpendicular to the normal 8 to form a first square mesh.

 The studs 16 are also of identical shape and size and are spaced along said orthogonal directions perpendicular to the normal 8 to form a second square mesh, which is nested with the first square mesh.

 The pads 15, 16 thus form a two-dimensional two-dimensional network which extends over the surface of the layer 14.

 For this purpose, the longitudinal 21 and transverse steps 18 of the two meshes are identical, but the diameter 19.20 and / or the respective height of the studs 15.16 is (are) different.

 Component 3 thus comprises two identical networks respectively provided on its upper external face 9 and on its external lower face 17, so as to form a bandpass filter in transmission; the central frequency (wavelength) of this filter is calculated by the usual methods in this area, according to the optical and geometric characteristics of the pads 15, 16 and of the layer 14 on which these pads are formed; this layer constitutes a waveguide endowed with its own frequencies (temporal and spatial) of resonance.

 According to the alternative embodiment illustrated in FIG. 2, the passband filter 4 in reflection can consist of several resonant structures stacked along the axis 8 (of normal incidence) which is perpendicular to the faces of the layers: this filter comprises a substrate 13 covered with 'a thin layer 14 in a part of the thickness of which is engraved a first resonant network; a layer 22 forming a substrate (or spacer spacer) covers the layer 14 and is itself

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 even coated with a thin layer 23 comprising a second resonant network; a layer 24 similar to layer 22 covers layer 23 and is coated with a thin layer 25 comprising a third resonant network.

 With reference to FIG. 4, a structural component similar to that illustrated in FIG. 1 can be obtained by bonding or adhesion of two portions obtained by cutting along lines 26, 27 of the blank illustrated in FIG. 4; this blank comprises a substrate 28 covered with two thin layers 29 and 30; in a part of the layer 30 (or in a layer covering it) have been formed a plurality of pyramidal patterns 31,32 of square section, of identical shape and size; the patterns 31 form a first network extending in the first portion 33 of the blank, while the patterns 32 form a second network (of characteristics identical to the first) in a second portion 34 of the component blank.

 The two portions 33, 34 can be separated by a section along the plane defined by the lines 26, 27, and are then joined by the face 35 of their substrate 28 respectively, to form a similar structure (symmetrical with respect to this face 35). to that of the component illustrated in FIG. 1.

 With reference to FIG. 5, the lateral faces (or flanks) 36, 37 of the stud 38 of a network formed on the surface 9 of the thin layer 40, are inclined, with respect to a normal 8a at the surface 9, d 'an angle 41.

 In the case of a demultiplexer 2 using reflecting filters 4 (FIG. 6), the filters can be aligned along the axis 50, their respective planes 12a being arranged parallel to each other and regularly spaced apart by a pitch 51.

 In the demultiplexer 1 illustrated in FIG. 3, the transmitter filters 3 extend in two planes 12b, 12c parallel and are regularly spaced apart by a step 52.

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 With reference to FIG. 7, the network 60 comprises a plurality of square meshes 61 of width 62 which are arranged side by side and extend in two orthogonal directions contained in the plane of the figure.

 In each mesh are provided four pads of square section: - two first pads 631. 632 whose square section has a side of width 64, - a second pad 65 whose square section has a side of width 66, - a third pad 67 whose square section has a side of width 68, the values of said widths being different while being close.

 The studs 63 are regularly spaced apart by a pitch whose value is equal to the width 62 of the mesh; the studs 632 are also spaced apart by the same pitch, as well as the studs 65 between them on the one hand and the studs 67 between them on the other hand.

 The four studs of each mesh are arranged so that their respective centers are arranged at the top of a square whose side 70 corresponds to half of the side 62 of the mesh 61.

 By etching the network illustrated in Figure 7 in a layer of Ta2 05 of index n = 2.023 (at 1550 nm), and choosing the dimensions of the following pads: side 62 = 1010 nm, side 64 = 202 nm, side 66 = 236 nm, side 68 = 337 nm height of each pad = 770 nm, thickness of the remaining layer of Ta2 05 = 335 nm, a filter was obtained whose reflection curve as a function of the angle of incidence of the beam is illustrated FIG. 8, and the reflection curve of which as a function of the wavelength is illustrated in FIG. 9.

Claims (17)

1. Optical component (3,4) comprising at least one thin layer (14,23, 25,29, 30,40) deposited on a substrate (13,28), characterized in that it comprises at least one structure or a pluriperiodic resonant network comprising a plurality of studs (15,16, 31,32, 38,63, 65, 67) or patterns regularly spaced in two directions, the structure or network being etched in at least one thin layer, so as to form a bandpass filter whose passband is centered on the wavelength corresponding to the resonance of said resonant structure.  2. Component according to claim 1, in which the resonant structure or network (60) comprises: - a plurality of first studs (15) or patterns having a first shape common to all of said first studs or patterns, - a plurality second studs (16) or patterns having a second shape common to all of said second studs or patterns, said second shape being different from said first shape.  3. Component according to claim 1 or 2, which comprises: - a first network (15,16, 31,32, 38) pluriperiodic resonant etched in at least a first thin layer, - a second pluriperiodic resonant network etched in at least one second thin layer, - at least a third thin layer forming a first coupling and decoupling region associated with the first network.  4. Component according to claim 3, which further comprises: - at least a fourth thin layer forming a second coupling and decoupling region associated with the second network,

 - At least one thick layer forming said substrate. <Desc / Clms Page number 12>  5. Component according to any one of claims 1 to 4 which comprises two structures or resonant networks arranged symmetrically with respect to the median plane (12, 12b, 12c) of the component, so as to form a bandpass filter (3) transmission.  6. Component according to any one of claims 1 to 5, wherein said studs or patterns have a substantially circular or square section.  7. Component according to any one of claims 1 to 6, wherein the pads or patterns of the network are formed in an outer thin layer whose index is lower than that of the layer supporting said outer thin layer.  8. Component according to any one of claims 1 to 7, the resonant structure of which comprises a network of studs (38) whose side faces (36,37) are inclined.  9. Component according to any one of claims 1 to 8, which comprises at least three resonant networks provided in three thin layers (14, 23, 25) superimposed.  10. Component according to any one of claims 1 to 9, which comprises less than ten thin layers, in particular one, two or three thin layers associated (s) with each resonant structure, and whose bandwidth is centered on a length wave close to 1.5 micron.  11. Wavelength division optical multiplexer or demultiplexer (1,2), which comprises a plurality of components, in particular from 8 or 16 to 256 components (3,4) according to any one of claims 1 to 10 .  12. Multiplexer or demultiplexer according to claim 11, wherein said components (3,4) are mechanically integral with each other, whose respective central wavelengths are different, which extend substantially parallel to each other <Desc / Clms Page number 13>  others, and which further comprises a collimator (11) associated with each component (3,4), and if necessary a collimator (5) or waveguide suitable for the optical coupling of the multiplexer or demultiplexer with the end piece '' an optical fiber (6).  13. Multiplexer or demultiplexer according to claim 11 or 12, wherein said components forming reflection filters are arranged aligned along an axis (50) common to all components.  14. Multiplexer or demultiplexer according to claim 11 or 12, wherein said components forming transmission filters are arranged in at least two planes (12b, 12c) substantially parallel to each other.  15. A method of manufacturing a component according to any one of claims 1 to 10, in which 1 to 10 thin layers are deposited on a substrate, and in which an array is etched on a thin layer by electron beam or by lithography.  16. The method of claim 15, wherein one stacks and assembles at least two components whose networks are identical.  17. The method of claim 15 or 16, wherein the central wavelength of the filter is adjusted by depositing a thin layer on the network, or by etching.