Patent Application
20050163417
The present invention pertains to an optical switch with micromirror and its method of fabrication.
More precisely, it concerns an optical switch able to transfer a light wave conveyed by an input optical path towards a first or a second output path.
The invention finds applications in all areas which use optical switches, and in particular the area of optic telecommunications.
Prior Art
To enable the switching of a light beam from an input optical path to any output path, there currently exist two families of switches:
The invention concerns this latter family of switches.
The interposing of micromirrors in front of an optical beam is widely used in free space.
In
Therefore, when the micromirror is in a position in which it is not interposed between fibres 1 and 3 on the optical axis of said fibres, the light beam leaving fibre 1 is transmitted to fibre 3; and when the micromirror is in a position in which it is interposed between fibres 1 and 3 on the optical axis of said fibres, the light beam leaving fibre 1 is reflected by the mirror and is transmitted to fibre 5.
In the case shown
In the case shown
The rigid micromirrors used in these structures are difficult to transpose to integrated optics since the fabrication technologies for optical guides and for mirrors are different and hence not easily compatible.
In integrated optics, known switches using the principle of light beam transmission or reflection are obtained through the movement of two fluids (an air bubble in a liquid for example) in a recess arranged in a support comprising optical guides forming the input and output paths, one of the fluids enables transmission of the beam and the other fluid enables its reflection. These structures raise problems of reliability having regard in particular to the movement of a fluid in a recess of restricted volume with problems of pollution.
Also, the rigid micromirrors used in free space are generally controlled by electrostatic forces and the electrostatic voltages required to obtain translation or rotation of the mirror must be sufficient to move the whole mirror. The greater the size of the mirror, the higher the required forces.
The present invention sets out to propose an optical switch using a rigid micromirror which can be used both in integrated optics and in free space optics and therefore not having the prior art reliability problems of switches in integrated optics.
A further objective of the invention is to propose an optical switch using a micromirror able to be controlled by voltages which may be lower than those for previously described micromirrors.
Further objectives of the invention are to propose an optical switch using a micromirror minimizing optical losses and able to have the fastest access time possible, and which is insensitive to polarization and wavelength.
Finally, a further purpose of the invention is to put forward a method for fabricating a switch in integrated optics which is simple, easy to implement and hence offering good production yield.
More precisely, the invention concerns an optical switch comprising at least one input optical path, at least a first and a second output optical paths and a micromirror able to move between an output of the input optical path and inputs of the first and second output optical paths, the input optical path and the first output optical path having an identical optical axis, called first optical axis, and the second output optical path having an optical axis called second optical axis, the micromirror comprising a reflector part and an actuating part having an axis of rotation and able to drive the reflector part in rotation about a plane called a tilt plane, this tilt plane being perpendicular to a plane containing the axis of rotation, and said reflector part comprising at least one reflective face in a plane parallel to the tilt plane able to reflect a light wave derived from the input path towards the second output path, the first and second optical axes respectively forming an angle α relative to an axis of symmetry, the optical switch further including a control device to tilt the reflector part, this control device comprising a first set of electrodes arranged on the actuating part, a second set of electrodes arranged facing the first set, and means for applying a potential difference between the sets of electrodes.
According to one particular embodiment of the invention, the optical switch comprises a first input optical path associated with a first and a second output optical paths, and a second input optical path associated with a third and a fourth output optical paths, the micromirror being able to interpose itself either between an output of the first input optical path and the inputs of the first and second output optical paths, or between an output of the second input optical path and inputs of the third and fourth output optical paths.
According to the invention, the input and output optical paths are chosen independently from one another from among optical fibres or optical guides.
Advantageously, the input and output optical paths are respectively optical guides in a substrate, said substrate also including a recess able to allow rotation of the reflector part about the so-called tilt plane.
The fabrication of a switch in integrated optics using a rigid micromirror makes it possible to overcome the prior art reliability problems of switches in integrated optics.
Also, since the width of the recess is related to applied fabrication technologies, it can be narrow thereby minimizing the distance travelled by light waves outside the optical guides and hence minimizing optical losses.
In addition, according to the invention, the tilt plane of the reflector part and the axis of rotation of the actuating part are perpendicular. The reflector part which comprises the reflective face and the actuating part which generally comprises a set of electrodes and forms a zone of attraction are decoupled, which enables the micromirror of the invention to use a lever effect which reduces the movement of the reflector part.
The movement of the micromirror generally being obtained through the use of electrostatic forces generated by two sets of electrodes to which a potential difference is applied, and since the zone of attraction is independent from the reflector part, the surface of the electrodes of the actuating part can be a large surface allowing a reduction in the forces required to tilt the reflector part and hence in control voltages. The same applies to the inter-electrode space which may be reduced, which also enables a reduction in the forces required to tilt the reflector part.
Angle α is advantageously non-zero.
Each set of electrodes comprises at least one electrode.
The micromirror of the invention advantageously comprises at least one limit stop able to limit movement of the reflector part.
This limit stop, for switches with a single input path and two output paths for example, is in the form of a boss at one end of the reflector part, the width of said boss in a plane perpendicular to the tilt plane is greater than the width of the recess along the same plane.
The switch of the invention makes it possible to have rapid response time, in the order of a ms for example or a few dozen μs, due in particular to the dimensions of the micromirror which may be small. It provides for insensitivity to polarization and wave length on account of the use of a transmission effect or mirror reflection effect to achieve switching.
Evidently, the micromirror is not limited to total reflection. The reflector part of the micromirror may allow selective reflection of only one polarization or only some wave lengths and respectively transmit the other polarization or other wave lengths, the micromirror then acting as filter.
A further subject of the invention is a method for fabricating a switch of the invention in integrated optics.
This methods comprises the following steps:
Evidently, these steps may also comprise the fabrication of other elements depending upon intended applications.
Steps a), b) and c) may be performed in this order or in different order. Or they may be interlinked. In particular, the adding of the second substrate onto the first substrate may be performed before complete fabrication of the micromirror.
When the actuating part of the micromirror is conductive, this actuating part may then act as the first set of electrodes; the fabrication of said first set is then merged with the fabrication of the actuating part of the micromirror.
According to one first embodiment, the second substrate is a stack of a first carrier layer, a second layer and a third layer intended to form the micromirror.
According to one advantageous embodiment, the first carrier layer is a silicon layer, the second layer is a silicon oxide layer and the third layer is a silicon film, the micromirror being fabricated in said film.
Advantageously, the second substrate is a Silicon On Insulator (SOI) wafer obtained for example by adding a film of monocrystalline silicon onto a silicon carrier comprising a thermal oxide layer. This silicon film is optionally epitaxied to desired film thickness.
Step b) of the micromirror fabrication comprises the following steps:
If a reflector part with a limit stop is to be made, etching of the third layer is conducted so as to obtain a pattern for the reflector part comprising said limit stop.
Other characteristics and advantages of the invention will be more readily seen in the following description with reference to the appended drawings. This description is given solely for illustrative purposes and is non-restrictive.
a and 1b illustrate a first example of a known switch in free space,
a and 2b illustrate a second example of known switch in free space,
a, 3b and 3c illustrate an example of embodiment of a switch according to the invention in integrated optics,
a and 4b illustrate a variant of the preceding example in which the micromirror comprises a limit stop,
a to 6g shows an example of embodiment of the switch in
a, 3b and 3c illustrate an example of embodiment of a switch of the invention made in integrated optics.
a is an overhead view of said switch.
b is a cross-section of the switch along a plane containing the reflective face of the micromirror.
c is a perspective view of the micromirror used in this switch.
In a substrate S1, an input optical path 31 and two output optical paths 35 and 37 are fabricated. These optical paths are formed in this example by optical guides.
As a general rule, an optical guide consists of a central part generally called the core and surrounding media positioned all around the core which may be identical or different. To achieve confinement of the light in the core, the refractive index of the medium forming the core must be different to and in most cases greater than those of the surrounding media. The guide may be a planar guide when light confinement is made in a plane containing the direction of light propagation, or a microguide when light confinement is made in two directions transverse to the direction of light propagation.
To simplify the description, the guide will be likened to its central part or core, and only the cores of these guides are shown in all the figures.
Also, all or part of the surrounding media shall be called “substrate”, it being understood that when the guide is not or only scarcely buried one of the surrounding media may be outside the substrate being air for example.
Depending upon the type of technique used, the substrate may be monolayer or multilayer.
Also, depending upon applications, an optical guide in a substrate may be more or less buried in this substrate and may in particular comprise portions of guide buried at varying depths. This is particularly the case in ion exchange technology in glass. To simplify the description, the guides are shown at a constant depth in the substrate.
In
The output of guide 31 and the input of guide 35 firstly and the input of guide 37 secondly are separated by a recess 39 able to allow tilting of a micromirror 41 about a tilt plane B.
The micromirror 41 comprises a reflector part 13 and an actuating part 15 having an axis of rotation 17 parallel to the axis of symmetry S; the reflector part and the actuating part being integral with one another, the actuating part is able to drive the reflector part in rotation about a plane called a tilt plane. The tilt plane of the reflector part is perpendicular to a plane containing the axis of rotation.
The reflector part comprises at least one reflective face R in a plane parallel to the tilt plane of the reflector part. This face R is able to reflect a light wave derived from guide 31 towards guide 35.
In the figures, the reflective face is shown as a dotted line.
Therefore, when the reflector part of mirror 41 is interposed between the optical axis of guide 31, the face R which then faces the output of guide 31 and the input of guide 35 reflects the light wave derived from guide 31 towards guide 35.
On the contrary, when the reflector part of mirror 41 is not interposed in the optical axis of guide 31, the light wave derived from guide 31 is transmitted directly via recess 9 to guide 37.
The switch further includes a control device controlling rotation of the actuating part so that the latter induces tilting of the reflector part which can then be interposed or not in the optical axis. This control device includes for example as shown in
Each set of electrodes comprises at least one electrode. In this example, set J1 comprises a single electrode and set J2 comprises two electrodes J21 and J22 facing the electrode of set J1. Therefore, the application of a different potential difference between each of the electrodes of set J2 and the electrode of set J1 makes it possible tilt the reflector part towards the electrode of set J2 for which the potential difference is the greatest.
Hence two positions may be defined:
The reflector part of the micromirror has a side face which is fully or partly reflective; the part of the side face able to reflect is the reflective face. In
The actuating part (see
In this example of embodiment of a switch in integrated optics, the rigid structure to which the mobile part is joined consists of a second substrate S2 arranged on substrate 1.
In the invention, the reflector part is able to move along the tilt plane perpendicular to a plane containing the axis of rotation 17 of the actuating part. The latter enables tilting of the reflector part under a lever effect. The effective part of the reflective face may, on this account, be distanced away from the axis of rotation and the inter-electrode space may be small (for example a few μm).
a and 4b show a variant of embodiment of a micromirror of a switch in integrated optics,
This micromirror, as previously, comprises an actuating part 15 and a reflector part 13. These parts are the same as those described with reference to
This limit stop limits the movement of the reflector part outside the recess. In this way, it particularly enables locking of the micromirror in a position in which the reflector part is not interposed in front of the optical beam.
The limit stop consists for example of a boss at the end of the reflector part; the width of said boss in a plane perpendicular to the tilt plane is greater than the width of the recess along this same plane.
By way of indication, the form of recess 49 along this plane is shown as a dotted line.
The reflector part 13 of the micromirror is able to interpose itself either between the output of the first input optical guide and the inputs of the first and second output optical guides, or between the output of the second input optical guide and the inputs of the third and fourth output optical guides.
Therefore, when the light beam conveyed by guide 31 is reflected towards guide 35, the light beam conveyed by guide 31′ is transmitted to guide 37′. Similarly, when the light beam conveyed by guide 31 is transmitted to guide 37, the light beam conveyed by guide 31′ is reflected towards guide 35′.
a to 6g illustrate an example of embodiment of the switch shown
In
Etching was performed in silicon layer 50 then in silica layer 51 to obtain an opening 33. Etching of layer 50 may be made along preferential crystallographic planes using the silica layer as stop layer; this etching is anisotropic chemical etching for example of KOH type so as to obtain an opening of conical shape, and etching of layer 51 may be performed using dry anisotropic etching of reactive ion etching type using fluorinated gases.
The silica layer could have been maintained in opening 33.
b shows an epitaxy step of silicon film 52; this step enables adaptation of the thickness of the silicon layer to the desired thickness of the micromirror to be fabricated. Evidently, if the initial thickness of film 52 is sufficient, this epitaxy is not necessary.
By way of example, the thickness of silicon layer 54 obtained after epitaxy lies between 5 and 50 μm for example depending upon the mechanical characteristics and the reflective surface involved.
c shows the fabrication of the micromirror by etching layer 54 in an appropriate pattern.
To achieve this two etchings are conducted, for example:
The first etching must be conducted starting from the face of film 54 opposite the face present in opening 33. This etching is made through an appropriate mask (not shown) and in particular enables thinning of film 54 outside the zones intended to form the two ends E1 and E2 of the reflector part.
The second etching can be made starting from either one of the faces of layer 54. The mask (not shown) used for this etching must allow etching of layer 54 over its entire remaining thickness so as to obtain the contour of the micromirror, i.e. the reflector part and the mobile part such as shown in the overhead view in
The first and second etchings are chosen independently from one another from among anisotropic chemical etching for example with a KOH solution or dry anisotropic etching, for example reactive ion etching using SF6 fluorinated gases.
With these etchings it is possible to obtain good surface quality since they are used to fabricate the side face of the micromirror.
After this step, as shown
If layer 54 is itself conductive as is the case with silicon, then this conductive deposit is not necessary and that part of layer 54 corresponding to the actuating part itself forms the electrode.
e shows a cross-section of substrate S1 along a plane containing input guide 31 and output guide 37. The optical guides may be made in the substrate using any integrated optics technique, and in particular using ion exchange techniques in glass, or silica depositing techniques on silicon or on glass or on fused silica.
A recess 39 is also made in the substrate, with a glass substrate for example this recess may be obtained by chemical type etching using hydrofluoric acid through a mask (not shown).
For a silica or semiconductor substrate, this recess is preferably made using dry anisotropic etching so as to obtain etch flanks having very good perpendicularity relative to the surface of the substrate.
This recess may also be made by mechanical sawing such as polishing-sawing.
Also, on the surface of S1 (before or after forming the recess) a conductive deposit is made which is etched to obtain electrodes J12 and J22 of set J2.
This deposit is a layer of metallic material for example such as aluminium or gold, chromium deposited by cathode vapour or sputtering and etched by chemical etching or reactive ion etching so as to obtain the two electrodes J21 and J22. At the same time as this electrode is made, the electric connections (not shown) of these electrodes to supply means are also made.
f and 6g illustrate the switch of the invention after adding substrate S2 onto substrate S1 so that the micromirror lies opposite the recess and in particular so that the reflector part may have a tilting movement within this recess.
In
In
Adding substrate S2 onto substrate S1 may be made using any known technique, in particular by molecular bonding or any appropriate cementing technique (a bead of polymer cement for example) or further by brazing.
A stack of substrate S2 such as shown in
In this example of embodiment, substrate S2 is added onto substrate S1 after fabrication of the micromirror; evidently, substrate S2 may be added onto substrate S1 before the fabrication of said micromirror or at least before its release so that this addition can be conducted with a mechanically more rigid structure.
The examples of embodiment previously described concern switches in integrated optics using optical guides. Evidently, as seen above, the switch of the invention may be made in free space. In this case, the input and output guides are optical fibres which may be arranged in a substrate in which rails have been cut (“V” grooves for example) to hold said fibres in position. A recess for movement of the micromirror may also be provided between the ends of the fibres. The micromirror may, as for the case in which optical guides are arranged on an independent substrate, be added onto the substrate of fibres.
| Number | Date | Country | Kind |
|---|---|---|---|
| 02/00598 | Jan 2002 | FR | national |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/FR03/00130 | 1/16/2003 | WO |
