EP1346242A1

EP1346242A1 - Optically active waveguide device comprising a channel on an optical substrate

Info

Publication number: EP1346242A1
Application number: EP01990613A
Authority: EP
Inventors: Stéphane TISSERAND; Laurent Roux
Original assignee: Ion Beam Services SA
Current assignee: Ion Beam Services SA
Priority date: 2000-12-26
Filing date: 2001-12-21
Publication date: 2003-09-24
Also published as: US20040091225A1; CN1483151A; FR2818755B1; CN1264032C; FR2818755A1; WO2002052312A1; CA2432815A1

Abstract

The invention concerns an optically active device comprising an optical waveguide core on an optical substrate (11, 15, 20) and a control element (32-33, 37, 40). The core comprises a channel (12, 17, 25, 35-36, 38-39) and at least an active layer (13, 18, 22) arranged on said channel, the refractive index of the channel and that of the active layer being higher than that of the substrate. The optical substrate (11, 15, 20) has a mobile ion concentration less than 0.01 %. Advantageously, the device further comprises a covering layer (14, 19, 23) arranged on the active layer (13, 18, 22), the index of said covering layer being less than that of the active layer and of the channel. The invention also concerns a method for making said device.

Description

OPTICALLY ACTIVE OPTICAL WAVEGUIDE DEVICE HAVING A CHANNEL ON AN OPTICAL SUBSTRATE

The present invention relates to an optically active device comprising a channel on an optical substrate.

The field of the invention is that of integrated optics on a substrate, a field to which in particular the active devices which essentially provide an amplification, modulation or switching function of a light signal. Such devices include an active waveguide and a control element which modulates one of the characteristics of this signal conveyed by the waveguide, this characteristic generally being either the amplitude or the phase. Such a guide comprises a core which is produced on the substrate, this core having a higher refractive index than that of the surrounding medium.

Several methods have been proposed for manufacturing the core of the active waveguide. A first method uses the technology of thin layers. Generally, the substrate is either silica or silicon on which a thermal oxide has been grown, so that its upper face, the optical substrate, is made of silicon dioxide. A layer with an index higher than that of silicon dioxide is deposited on the optical substrate using any known technique such as flame hydrolysis deposition ("Flame Hydrolysis Deposition in English terminology) chemical vapor deposition high or low pressure and assisted or not by plasma, evaporation under vacuum, sputtering or deposition by centrifugation.

When it comes to making an amplifier, this layer is often silicon dioxide doped with a rare earth such as erbium (signal wavelength 1.55 microns) or neodymium (wavelength signal of 1, 3 microns). If on the other hand it is envisaged to produce a modulator or a switch, the layer is often made of a material having electro-optical properties which is in particular the case of certain polymers. This layer can also have thermo-optical properties, which is for example the case of silicon dioxide.

A mask defining the heart is then applied to the deposited layer by means of a photolithography technique. Then, the core is produced by a chemical etching or dry etching process such as plasma etching, reactive ion etching or ion beam etching. The mask is removed after etching and, commonly, a covering layer is deposited on the substrate to bury the heart. This covering layer, the refractive index of which is lower than that of the core, is intended to limit the disturbances exerted by the surrounding environment, in particular those due to humidity. Thus, the document GB 2 346 706 teaches a heart produced by means of two layers which are etched successively by means of a single mask. This heart therefore takes the form of two superimposed ribbons having the same dimensions in the plane of the substrate.

This method requires an engraving operation which is difficult to master both in terms of spatial resolution and in terms of the surface condition of the sides of the heart. Thus, the etching of silicon dioxide doped with erbium by means of a fluorinated reactive gas such as CHF ₃ produces erbium fluoride, a compound which appreciably increases the roughness of the etched surface. However, the surface condition and the geometry of the core directly condition the losses on propagation of the active waveguide.

A second method described in document US 4,834,480 uses ion exchange technology. In this case, the substrate is a glass having a high concentration of mobile ions (Na for example) at relatively low temperature. The substrate is again provided with a mask and it is then immersed in a bath containing active ions (K for example). The core is thus produced by increasing the refractive index following the exchange of the active ions of the bath with the mobile ions of the substrate. Then, generally, the heart is buried by application of an electric field perpendicular to the face of the substrate. This method is very simple. However, it requires the selection of a particular substrate which does not necessarily have all the desired characteristics. It is for example not possible to carry out an ion exchange from silicon while this material offers many advantages both in terms of cost, treatment methods which are those used in microelectronics, its thermal properties and its characterization. In addition, the ion exchange causes significant lateral diffusion of the active ions, so that the spatial resolution here is also seriously limited.

A third method used for the production of passive components implements ion implantation technology. The document “Channel waveguides formed in fused silica and silica on silicon by Si, P and Ge ion implantation - LEECH PW et al - IEEE Proceedings: Optoelectronics, Institution of Electrical Engineers, Stevenage GB - Volume 1 3 n ° 5, pages 281 to 286 ”teaches a device deposited on an optical substrate of silicon dioxide. A germanium doped layer is deposited on the substrate, then a mask is applied and the channel is produced by ion implantation of the deposited layer. This layer produces mechanical stresses which cause deformation of the substrate. The deformation, the greater the thicker the layer, is detrimental to the optical specifications of the waveguide and poses difficulties during the photolithography step. The present invention thus relates to an optically active device having a suitable spatial resolution and a good surface condition.

According to the invention, this device comprises a core on an optical substrate and a control element, this core comprising a channel and at least one active layer arranged on the channel, the refractive index of this channel and that of the active layer being greater than that of the substrate; the optical substrate is practically free of mobile ions.

On a suitable substrate, the geometric definition of the core only depends on that of the channel because the active layer is not etched.

Preferably, the device comprises at least one covering layer disposed on the active layer, the index of this covering layer being lower than that of the active layer and that of the channel.

According to a first embodiment, the channel is integrated into the substrate.

According to a second embodiment, the channel projects from the substrate.

Advantageously, the index of the active layer is that of the substrate multiplied by a factor greater than 1.001.

For example, the thickness of all of the active layers is between 1 and 20 microns. According to a privileged characteristic, the channel results from an ion implantation in the substrate.

In addition, it is recommended that the face of the substrate on which the ion implantation is carried out is made of silicon dioxide.

The active layer is for example made of silicon dioxide doped with a rare earth, or else in a material which has either electrooptic or thermo-optical properties, this depending on the function of the device. The invention also relates to a method of manufacturing an active device on an optical substrate.

According to a first variant, this method comprises the following steps: production of a mask on the optical substrate to define the pattern of a channel,

- ion implantation of the masked substrate,

- removal of the mask,

depositing at least one active layer on the substrate, the refractive index of this active layer being greater than that of the substrate. According to a second variant, the method comprises the following steps:

- ion implantation of the substrate,

- production of a mask on the substrate to define the pattern of a channel,

- etching of the substrate to a depth at least equal to the implantation depth,

- removal of the mask,

depositing at least one active layer on the substrate, the refractive index of this active layer being greater than that of the substrate.

Advantageously, the method comprises a step of annealing the substrate which follows the step of ion implantation.

This method is also adapted to the realization of the various characteristics of the device mentioned above.

The present invention will now appear in more detail in the context of the following description of exemplary embodiments given by way of illustration with reference to the appended figures which represent:

FIG. 1, a sectional diagram of an active waveguide core,

FIG. 2, the manufacture of the heart according to a first variant,

FIG. 3, the manufacture of the heart according to a second variant, and

- Figure 4, a set of active devices seen from above. Initially, in order to simplify the presentation of the invention, only the production of the core of the active waveguide will be exposed.

With reference to FIG. 1a, according to a first variant, the substrate is made of silicon on which an insulation layer has been produced, either by growth of a thermal oxide, or by deposition of a layer of silicon dioxide SiO ₂ or another material such as Si ₃ N ₄ , AI ₂ O ₃ , or SiON. These are dielectric materials commonly used in both electronics and optics. opposition to glasses charged with mobile ions. However, it is impossible to guarantee that these materials have a zero concentration of mobile ions. We can only specify that this concentration is very low, less than 0.01% for example. The substrate thus has an upper face or optical substrate 11, commonly made of silicon dioxide, with a thickness of 5 to 20 microns, for example. The channel 12 produced by ion implantation is here integrated into the optical substrate which is itself covered with an active layer 13. The refractive index of the channel is naturally higher than that of silicon dioxide. The active layer 5 microns thick, for example, is made of erbium-doped silicon dioxide and has a refractive index higher than that of the optical substrate, for example 0.3%. It can possibly result from a stack of thin layers. Preferably, a covering layer 14 which may also consist of a stack of thin layers is provided on the active layer 13. This covering layer, also 5 microns thick, has a lower index than that of the active layer and to that of the canal; in this case it is made of undoped silicon dioxide.

According to a second variant, the substrate does not have an insulation layer so that it merges with the optical substrate. It is, for example, a III-V type semiconductor compound, for example InP, GaAs, AIGaAs or InGaAsP. The channel is implanted before the deposition of the active layer obtained with a doped material similar to that of the substrate. Naturally, the various materials commonly used in optics such as silica or lithium niobate can be suitable as an optical substrate. Whatever the variant adopted, the core formed by the association of the channel 12 and the active layer 13 can support one or more propagation modes whose properties are a function of the optical and geometric characteristics adopted.

With reference to FIG. 1 b, when the refractive index of the channel is relatively low, 1.56 for example, the extended propagation mode GM extends widely in the active layer 13. The width of the channel, 7.5 microns for example, and the thickness of this active layer are chosen so that the GM propagation mode is as close as possible to that of optical fibers. We can then obtain a fiber coupling coefficient of a value of 90%. The effective guide mode index is lower than the refraction of the active layer and that of the channel; it is greater than the refractive index of the upper face 11 and that of the covering layer 14. With reference to FIG. 1c, it should be noted that the core can also support a reduced propagation mode PM which extends much less in the active layer 13. The channel index should then be relatively high, 1.90 for example. The width of the channel can be significantly reduced.

The effective index of the guided mode is here higher than that of the active layer and lower than that of the channel. The lateral confinement of the reduced PM mode is very important. The technique of ion implantation was chosen because it makes it possible to precisely define a very thin channel, of the order of a few hundred nanometers.

Furthermore, this technique now benefits from very high precision on the doses of implanted ions, typically 1%. The silicon dioxide optical substrate has a refractive index which exhibits little or no variation, it follows that very high accuracy can be obtained on the index of the channel. For example, for an implanted dose of titanium of 10 / cm respectively 10 ¹⁷ / cm ² , the precision on the refractive index reaches 10 ^{~ 4} respectively 10 ^"3. This precision is particularly important when looking for the extended propagation mode GM because the channel index is a parameter which very significantly affects the coupling to the optical fibers. With reference to FIG. 2a, a first method of manufacturing the core comprises a first step which consists in carrying out a mask 16 on the optical substrate 15, this by means of a conventional photolithography process The mask 16 is made of resin, metal or any other material capable of constituting an impassable barrier for ions during implantation.

Optionally, the mask can be obtained by a direct writing process.

With reference to FIG. 2b, the channel 17 is produced by ion implantation of the masked substrate. For example, for a titanium implantation, the implantation dose is between 10 ¹⁶ / cm ² and 10 ¹⁸ / cm ² and the energy is between a few tens and a few hundred KeV.

Referring to Figure 2c, the mask is removed, for example by means of a chemical etching process. The substrate is then annealed to reduce losses from propagation within the heart. Annealing notably makes it possible to eliminate the defects of the crystal structure and the absorbent colored centers, to stabilize the new chemical compounds and to restore the canal stoichiometry. For example, the temperature is between 400 and 500 ^β C, the atmosphere is controlled or it is open air, while the duration is of the order of a few tens of hours.

With reference to FIG. 2d, the active layer 18 is then deposited on the substrate 15 by means of any of the known techniques provided that this leads to a low loss material whose refractive index can be easily controlled . Finally, the covering layer 19 is optionally deposited on the active layer 18.

Note that this first method has the advantage of defining an active waveguide whose structure is perfectly planar since it does not include an etching step.

With reference to FIG. 3a, a second method of manufacturing the wave core comprises a first step which consists in implanting the entire optical substrate 20. The dose and the implantation energy can be identical to the values mentioned in relation to the first method.

Referring to Figure 3b, the next step is to make a mask 21 on the substrate 20. This mask has the same pattern as that used during the first method but it must not undergo the implantation step.

Referring to Figure 3c, the channel 25 is obtained by etching the optical substrate to a depth at least equal to the implantation depth. Any of the known etching techniques is suitable provided that this leads to acceptable geometrical characteristics of the channel, in particular the profile and the surface condition of its sides.

With reference to FIG. 3d, the mask is removed and then the substrate is here also subjected to annealing. The active layer 22 and possibly the covering layer 23 are then deposited in accordance with the first method.

According to this second method, the drawbacks associated with etching are considerably limited because the channel has a small thickness.

It is now necessary to explain how the invention makes it possible to produce optically active devices.

With reference to FIG. 4a, an amplifier comprises a first rectilinear channel 31 which, associated with the active layer, constitutes the heart of the active waveguide. The control element here takes the form of a second curved channel 32 having a rectilinear coupling section 33 arranged in the immediate vicinity of the first channel 31 and parallel to the latter. The second channel 32 is provided for convey an optical pumping signal. It is produced at the same time as the first channel by means of the mask which in fact defines the two channels.

With reference to FIG. 4b, a modulator consists of an so-called "Mach Zehnder" interferometer. The mask now delimits a guide 34 which is subdivided into a first 35 and a second 36 channels, these two channels coming together to reform a single guide. A section of the second channel 36 is surrounded by a pair of elongate electrodes 37 whose connections are not shown in the figure. These electrodes are for example deposited by means of a thin film technology on the active layer. This layer is here made of a material provided with electro-optical properties, that is to say a material whose refractive index is a function of an electric field which is applied to it. The control element consists of the combination of the second channel 36 and the pair of electrodes 37.

Referring to Figure 4c, a switch consists of a coupler having a first 38 and a second 39 parallel channels which approach in a coupling section to move away again. These two channels made with the same mask are coated with the active layer. By way of example, this layer is made of a material provided with thermooptical properties, ie a material whose refractive index is a function of temperature. At the coupling section, above the second channel 39, an electrode 40 is deposited on the active layer, an electrode whose function is the local heating of this layer. The electrode 40 constitutes the control element.

The embodiments of the invention presented above have been chosen for their specific nature. However, it would not be possible to exhaustively list all the embodiments covered by this invention. In particular, any step or any means described may be replaced by a step or equivalent means without departing from the scope of the present invention.

Claims

1) Optically active device comprising a core on an optical substrate

(11, 15, 20) and a control element (32-33, 37, 40), this core comprising a channel (12, 17, 25, 31, 35-36, 38-39) and at least one active layer (13, 18, 22) arranged on said channel, the refractive index of this channel and that of the active layer being greater than that of the substrate, characterized in that said optical substrate (11, 15, 20) has a concentration in mobile ions less than 0.01%.

2) Device according to claim 1, characterized in that it comprises at least one covering layer (14, 19, 23) disposed on said active layer (13, 18, 22), the index of this covering layer being lower than that of the active layer and that of the channel (12, 17, 25, 31, 35-36, 38-39).

3) Device according to any one of claims 1 or 2, characterized in that said channel (12, 17) is integrated in said substrate (11, 15). 4) Device according to any one of claims 1 or 2, characterized in that said channel (25) projects from said substrate (20). 5) Device according to any one of the preceding claims, characterized in that the index of said active layer (13, 18, 22) is equal to that of the substrate (11, 15, 20) multiplied by a factor greater than 1, 001 . 6) Device according to any one of the preceding claims, characterized in that the thickness of all the active layers (13, 18, 22) is between 1 and 20 microns.

7) Device according to any one of the preceding claims, characterized in that said channel (12, 17, 25, 31, 35-36, 38-39) results from an ion implantation in said substrate (11, 15, 20 ).

8) Device according to any one of the preceding claims, characterized in that the face of the substrate (11, 15, 20) on which the ion implantation is carried out is made of silicon dioxide.

9) Device according to any one of the preceding claims, characterized in that said active layer (13, 18, 22) is made of silicon dioxide doped with a rare earth.

10) Device according to any one of the preceding claims, characterized in that said active layer (13, 18, 22) has electro-optical properties. 11) Device according to any one of the preceding claims, characterized in that said active layer (13, 18, 22) has thermo-optical properties.

12) Method for manufacturing an active device on an optical substrate comprising a step of producing at least one control element (32-

33, 37, 40), characterized in that it further comprises the following steps:

- production of a mask (16) on said substrate (15) to define the pattern of a channel (17),

- ion implantation of the masked substrate, - removal of said mask,

- Deposition of at least one active layer (18) on the substrate, the refractive index of this active layer being greater than that of the substrate.

13) Method for manufacturing an active device on an optical substrate comprising a step of producing at least one control element (32- 33, 37, 40), characterized in that it further comprises the following steps:

- ion implantation of the substrate (20),

- production of a mask (21) on said substrate to define the pattern of a channel (25),

- etching of the substrate to a depth at least equal to the implantation depth,

- removal of said mask,

- Deposition of at least one active layer (22) on the substrate, the refractive index of this active layer being greater than that of the substrate.

14) Method according to any one of claims 12 or 13, characterized in that it comprises a step of annealing the substrate (15, 20) which follows the step of ion implantation.

15) Method according to any one of claims 12 or 13, characterized in that it comprises a step of depositing a covering layer (19, 23) on said active layer (18, 22), the index of this covering layer being lower than that of the active layer and that of the channel (17, 25).

16) Method according to any one of claims 12 or 13, characterized in that the index of said active layer (18, 22) is that of the substrate (15, 20) multiplied by a factor greater than 1, 001. 17) Method according to any one of claims 12 or 13, characterized in that the thickness of all the active layers (18, 22) is between 1 and 20 microns.

18) Method according to any one of claims 12 or 13, characterized in that the face (15, 20) of the substrate on which the ion implantation is carried out is made of silicon dioxide.

19) Method according to any one of claims 12 or 13, characterized in that the material of said active layer (18, 22) is silicon dioxide doped with a rare earth. 20) Method according to any one of claims 12 or 13, characterized in that the material of said active layer (18, 22) has electro-optical properties.

21) Method according to any one of claims 12 or 13, characterized in that the material of said active layer (18, 22) has thermo-optical properties.