FR3057675A1 - METHOD FOR PRODUCING A WAVEGUIDE - Google Patents

Abstract

The invention relates to a method of manufacturing an optical mirror in a glass plate (80), comprising the following successive steps: scanning a surface of the plate (80) by a laser beam (84) directed obliquely to at said surface, to form a trench (86) according to the drawing of the mirror to be formed, the duration of the pulses of this laser being between 1 and 500 femtoseconds; treat with hydrofluoric acid; and filling the trench (86) with a metal.

Description

Field

The present application relates to a method for manufacturing a waveguide and more particularly to a method for manufacturing a single-mode waveguide.

Presentation of the prior art

Optical signals can be used to transmit data, for example using an optical fiber. In order to increase the amount of data transmitted, it is known to transmit several optical signals of different wavelengths over the same optical fiber. Optical devices, such as that presented in connection with FIG. 1, make it possible to make the connection between an optical fiber and circuits for processing optical signals.

Figure 1 is a sectional view of an optical device 1 transmitting optical signals from a single-mode optical fiber 3 to, for example, optical signal processing circuits not shown in Figure 1. The device 1 comprises a plate glass 5 constituting an interposer between the optical fiber 3 and, for example, photonic integrated circuits, for example made of silicon, not shown in FIG. 1. The plate 5 has for example a rectangular shape or a

B15167 - 16-GR1-0258FR01 circular shape. Various elements are produced, in particular by etching, on the upper face of the plate 5, among which:

a waveguide 7 adapted to transmit a multifrequency optical signal received from a single-mode optical fiber 3; an optical signal demultiplexer 9 transmitting, on waveguides 11, single frequency optical signals obtained by frequency filtering from the multifrequency optical signal transmitted by the waveguide 7; and a mirror 13 interrupting each waveguide 11, adapted to reflect, towards the outside of the plate 1, the optical signal transmitted by the corresponding waveguide 11.

The end of the optical fiber 3 is disposed opposite one end of the waveguide 7. An index matching material 15, the refractive index of which is between that of the fiber and that of the material of the waveguide 7, is preferably disposed between the end of the optical fiber 3 and the end of the waveguide 7. The path of an optical signal sent by the optical fiber is represented in FIG. 1 by an arrowed line.

The guides wave 5 7 and 11 present the same characteristics. The guides wave 7 and 11 are made of a material having a higher index tall than the one of glass of the plate 5. Waveguides 7 and 11 have to to have some dimensions

transverse close to that of a single mode optical fiber, for example between 3 and 15 pm, for example of the order of 7 pm. These dimensions make it possible to minimize the signal losses at the input of the plate 5 and to dispense with the use of a coupler. The optical demultiplexer 9 is a device which separates on at least two output waveguides 11, the at least two wavelengths of the optical signal from the input guide 7.

Each mirror 13 is made of a material reflecting the wavelengths concerned, for example metal. The mirror 13 is formed in the plate 5 obliquely and forms with the axis of propagation of the waveguide 11 that it

B15167 - 16-GR1-0258FR01 interrupts an angle of the order of 42 degrees. The upper face of the plate 5 can be covered with an encapsulation layer 17 transparent to the wavelengths concerned.

To make this type of optical device, it is therefore desirable to manufacture waveguides in a glass plate whose dimensions coincide with those of a single-mode optical fiber. It is further desirable to be able to form inclined mirrors arranged in a desired location in the plate.

summary

Thus, one embodiment provides a method of manufacturing an optical mirror in a glass plate, comprising the following successive steps: scanning a surface of the plate with a laser beam directed obliquely to said surface, to form a trench according to the design of the mirror to be formed, the duration of the pulses of this laser being between 2 and 500 femtoseconds; treat with hydrofluoric acid; and fill the trench with metal.

According to one embodiment, the trench is filled with metal by sputtering.

According to one embodiment, the metal is copper, aluminum or an alloy of copper and aluminum.

According to one embodiment, the method comprises a step of subsequent deposition of an encapsulation layer on the surface of the structure.

According to one embodiment, the encapsulation layer is made of silicon oxide.

According to one embodiment, the trench has a rectangular or rectangular cross section with rounded angles.

According to one embodiment, the angle formed between the trench and the surface of the plate is between 30 and 50 degrees.

According to one embodiment, the laser emits pulses at a frequency between 10 and 500 kHz.

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According to one embodiment, the mirror is concave or convex.

Brief description of the drawings

These characteristics and advantages, as well as others, will be explained in detail in the following description of particular embodiments made without implied limitation in relation to the attached figures, among which:

Figure 1, described above, illustrates an optical device;

FIGS. 2A to 2F illustrate steps of an embodiment of a method for manufacturing a waveguide;

Figures 3A and 3B illustrate various optical devices; and FIGS. 4A to 4C illustrate steps of an embodiment of a method for manufacturing an optical mirror. detailed description

The same elements have been designated by the same references to the different figures and, moreover, the various figures are not drawn to scale. For the sake of clarity, only the elements useful for understanding the described embodiments have been shown and are detailed.

In the following description, when referring to qualifiers of relative position, such as the terms above, top, etc., or to qualifiers of orientation, such as the terms horizontal, vertical, etc., it reference is made to the orientation of the figures. Unless otherwise specified, the expressions substantially and in the order of mean to the nearest 10%, preferably to the nearest 5%.

In order to produce waveguides on a glass substrate at low cost, it is known practice to use simple lithophotography equipment, that is to say of low resolution, such as that commonly available in a laser line. assembly of electronic components. However, such equipment only makes it possible to produce single-mode waveguides of dimensions greater than about 50 μm. These dimensions are

B15167 - 16-GR1-0258FR01 significantly larger than the dimensions of a single mode optical fiber, which are between 5 and 10 µm.

FIGS. 2A to 2E are a perspective view and sectional views illustrating steps of an embodiment of a method for manufacturing a single-mode waveguide with transverse dimensions between 5 and 10 μm, in a glass plate 30, the thickness of which is for example between 1 and 1.5 mm. We consider here a single mode waveguide having a form factor of the order of 1, that is to say that the cross section of the waveguide has a width / depth ratio of the order of 1. The straight section of the guide can then be round, square or square with rounded angles.

In the step of FIG. 2A, the plate 30 is scanned by a laser beam 32. The laser beam 32 scans the surface of the plate 30 along a path symbolized by an arrow 33 corresponding to the design of the waveguide to be formed. In this case, the waveguide to be formed is of rectilinear shape. The glass of the plate 30 is pulverized by the beam 32 and a trench 34 is thus formed. The laser beam 32 is produced by a pulse laser, each pulse of which has a duration of between 2 and 500 femtoseconds. For example, in the glass, the trench can be obtained for pulses having energies greater than 500 nJ and of duration on the order of 100 fs. This energy is calculated according to the dimensions of the cavity which it is desired to form with a pulse. The laser emits these pulses, for example at a frequency between 10 and 500 kHz. The laser will later be called femtolaser.

An advantage of using a femtolaser is that the duration of the pulses is quite short compared to the duration of thermal relaxation of the material. This property allows precise machining of the material and limits the thermal effects around the ablated area. The trench 34 has a cross section of semi-circular shape with a depth of between 5 and 10 μm. The width of the trench 34 is less than the

B15167 - 16-GR1-0258ER01 desired dimension. It can be seen that the glass of the walls 36 of the trench 34 has a certain roughness and cracks over a substantially constant thickness. This roughness can prevent the waveguide from working properly.

In the step of FIG. 2B, the plate 30 is subjected to etching with hydrofluoric acid. The cracked glass is dissolved by hydrofluoric acid over its entire thickness of between 1 and 2 μm. Consequently, the roughness exhibited by the walls 36 is reduced. The cracked glass is preferably etched with hydrofluoric acid, relative to the uncracked glass, so that the final shape of the trench 34 has a cross section of substantially square shape with rounded angles. Thus, the waveguide will have a substantially equal depth and width, between 5 and 10 μm. In other words, the waveguide will have a form factor of the order of 1 and will be single-mode for the wavelengths considered, namely between 1300 and 1500 nm. In addition, its dimensions will be adapted to those of a single-mode optical fiber.

In the step of FIG. 2C, the trench 34 is filled with a material 38 having a refractive index greater than the refractive index of the glass. The material 38 is for example a polymer, for example used in the form of a dry stretch film (that is to say one which does not require a solvent, known by the English designation dry film). The index difference between the glass and the material 38 is between 10 _"e ^ t 10" 2, for example of the order of 5x1O ^- ^. a suitable polymer material is that distributed by the company Elga Europe in the form of dry stretch film, under the trade name Ordyl SY 317. The trench is for example filled by a rolling technique. The material 38 is initially deposited on the surface of the structure. The material 38 is then spread out, then laminated on the glass in order to make it penetrate into the trench and to obtain a flat upper surface. In the case where the material 38 is a polymer, the rolling step is followed

B15167 - 16-GR1-0258FR01 of a step of crosslinking the polymeric material, for example annealing or exposure to ultraviolet radiation. The trench 34 filled with the material 38 constitutes the heart of the waveguide to be formed.

In the step of FIG. 2D, the excess material 38 has been removed from the surface of the structure, for example by polishing. The polishing is for example a mechanochemical polishing or CMP step (from the English Chemical-mechanical polishing). This polishing also makes it possible to smooth any roughness of the upper surface of the material 38.

FIG. 2E illustrates an alternative embodiment of the step of FIG. 2D, in which a layer 40 of the material 38 is left above the structure. The waveguide form factor is then equal to the ratio of the width L of the trench 34 and the sum of the depth 1 of the trench and the thickness h of the layer 40. In a first case, the trench has, without the layer 40, a form factor of the order of 1. The layer 40 must then have a thickness thin enough not to alter the form factor of the waveguide. In a second case, the trench has, without the layer 40, a form factor not equal to 1 and the thickness of the layer 40 is calculated to correct this form factor so that the waveguide has a form factor of the order of 1.

In the step of FIG. 2F, the waveguide 41 is completed. A cladding layer 42 is deposited on the surface of the structure. The layer 42 is made of a material having a lower refractive index than that of the material 38. The difference in index between the glass and the material of the layer 42 is of the order of the difference in index between the glass and the material 38. The layer 42 is for example made of silicon oxide or of a polymer. The layer 42 has a thickness between 5 and 50 μm, for example 10 μm.

An advantage of this embodiment is that it makes it possible to form guides of different shapes and different sizes with a femtolaser. Guides can be trained

B15167 - 16-GR1-0258FR01 of curved waves, waveguides of different depths or cavities by scanning the glass plate adequately. It is also possible to form various types of optical devices such as those presented in connection with FIGS. 3A and 3B.

FIG. 3A is a top view of an optical device 60 produced according to the manufacturing method presented in relation to FIGS. 2A to 2F. The device 60 is a directional optical coupler. The device 60 includes two waveguides 62 and 64 and has two inputs and two outputs. A portion 62A of the waveguide 62 and a portion 64A of the waveguide are parallel. The portions 62A, 64A are arranged so that the portion 62A, respectively 64A, can receive an evanescent wave created by an optical signal propagating in the other portion 64A, respectively 62A. The light power of the signal propagating in the portion 62A, respectively 64A, is transmitted progressively in the portion 64A, respectively 62A. The coupling depends on the distance between the portions 62A and 64A and on their length. The device 60 can be used interchangeably in both directions of light. This device can be used as a power splitter or polarizer. To make the device 60, a laser beam from a femtolaser scans a glass plate following the drawings of the waveguides 62 and 64.

FIG. 3B is a top view of an optical device 70 produced by the manufacturing method presented in relation to FIGS. 2A to 2F. The device 70 is an optical power splitter, commonly called a Y-junction. The device 70 comprises an input waveguide 72 and two output waveguides 74 and 76. The input waveguide 72 and the output waveguides 74,76 form a fork which separates an optical input signal transmitted by the input waveguide 72 into two optical output signals each transmitted on one of the waveguides of exit 74 or 76. It is also

B15167 - 16-GR1-0258EN01 possible to use the device 70 in the opposite direction in which the output waveguides 74 and 76 are input waveguides and the input waveguide 72 is an output waveguide. In this case, the device 50 is an optical power combiner. To make the device 70, a laser beam from a femtolaser scans the surface of the glass following the drawings of the waveguides 72, 74 and 76.

Of course, other passive optical devices comprising waveguides and resonant cavities can be produced according to the manufacturing method described in relation to FIGS. 2A to 2F.

To produce inclined optical mirrors in a glass plate, it is known to eliminate the edge of a glass plate so as to obtain an inclined wall. The wall is then covered with a reflective material, for example a metal. A disadvantage of this process is that it is not possible to form inclined optical mirrors other than at the edge of the plate.

FIGS. 4A to 4C are a perspective view and sectional views illustrating an embodiment of a method for manufacturing an optical mirror inclined in a glass plate 80.

In the step of FIG. 4A, the plate 80 is positioned on a support making it possible to tilt it at an angle c <] _ relative to the horizontal. The angle c <] _ is, for example, between 30 and 50 degrees, for example of the order of 35 degrees. A vertical laser beam 84 scans the plate 80 following the design of the mirror and forms a trench 86. The laser beam 84 is produced by a femtolaser, the advantages of which have been presented in relation to FIG. 2A. The laser beam 84 is adapted to produce a cylindrical cavity at each pulse, the bottom of which is semi-spherical. The trench 8 6 therefore has a cross section with a rounded bottom. It can be seen that the glass of the walls 88 of the trench 86 has a certain roughness and cracks over a thickness

B15167 - 16-GR1-0258FR01 substantially constant. This roughness can prevent the proper functioning of the optical mirror.

In the step of FIG. 4B, the plate 80 is subjected to etching with hydrofluoric acid of the same type as that presented in relation to FIG. 2B. The cracked glass is dissolved by hydrofluoric acid over its entire thickness of between 1 and 2 μm. Thus the roughness exhibited by the walls 88 is reduced. Trench 86 retains its shape but widens slightly. The trench 86 is inclined relative to the plate at an angle ag. The angles ag and ag are complementary.

In the step of FIG. 4C, the trench 86 is filled with a reflective material 90. The trench 86 is for example filled by spraying. The reflective material 90 is for example a metal, for example, copper, aluminum or an alloy of copper and aluminum, deposited for example by sputtering or PVD (in English spray vapor deposition). The excess of reflective material 90, deposited on the surface of the structure, is removed for example by etching or by chemical mechanical polishing. The surface of the material 90 deposited in the trench 86 is further flattened by the same method.

An optional step of depositing a protective layer on the upper face of the structure, similar to the step presented in relation to FIG. 2F, can complete this process. The protective layer has the same index as glass and for example silicon oxide.

An advantage of this embodiment is to allow the production of one or more optical mirrors directly at the desired location on a glass plate.

FIG. 4C shows a plane mirror formed in a straight trench. An advantage of this embodiment is to be able to form curved trenches in the glass plate. In this case, concave or convex mirrors would be produced.

B15167 - 16-GR1-0258FR01

It is possible to combine the methods described in relation to FIGS. 2A to 2F and FIGS. 4A to 4C in order to produce an optical device of the type presented in relation to FIG. 1. By way of example, the following successive steps:

forming the trenches intended to form the waveguides; fill these trenches with adequate material to form the core of the waveguides;

forming the trenches intended to form the mirrors, these trenches being deeper than the trenches intended to form the waveguides;

fill these trenches with reflective material; and deposit an encapsulation layer on the surface of the structure.

The cores of the waveguides will not be damaged during the formation of the trenches intended to form the mirrors because the femtolaser has negligible thermal effects.

B15167 - 16-GR1-0258FR01

Claims (9)

1. Method for manufacturing an optical mirror in a glass plate (80), comprising the following successive steps: scanning a surface of the plate (80) with a laser beam (84) directed obliquely to said surface, to form a trench (86) according to the design of the mirror to be formed, the duration of the pulses of this laser being included between 2 and 500 femtoseconds; treat with hydrofluoric acid; and fill the trench (86) with metal (90). 2. The method of claim 1, wherein the trench (86) is filled with metal by sputtering. 3. Method according to claim 1 or 2, wherein the metal (90) is copper, aluminum or an alloy of copper and aluminum. 4. Method according to any one of claims 1 to 3, comprising a step of subsequent deposition of an encapsulation layer on the surface of the structure. 5. Method according to claim 4, wherein the encapsulation layer is made of silicon oxide. 6. Method according to any one of claims 1 to 5, wherein the trench (86) has a rectangular or rectangular cross section with rounded angles. 7. The method of claim 6, wherein the angle formed between the trench (86) and the surface of the plate (80) is between 30 and 50 degrees. 8. Method according to any one of claims 1 to 7, wherein the laser emits pulses at a frequency between 10 and 500 kHz. 9. Method according to any one of claims 1 to 8, wherein the mirror is concave or convex. B 15 166.815 167 1/4