FR2764398A1 - Electro=optic light beam deflector - Google Patents

Abstract

The deflector consists of an active layer (32) of an electro-optic material which is able to guide a luminous wave, first and second electrodes (36, 38) on either side of the active layer, the first electrode being at least partially transparent to the guided luminous wave (34), a variable tension applicator, and an uncoupler (39, 52) permitting the creation of a light wave (40) emerging from the first electrode and forming a variable angle with a perpendicular to it. The deflector also incorporates first and second buffer layers (36, 38) between the two electrodes, the first buffer layer being at least partially transparent to the guided light wave, and both layers having a refraction index below that of the active layer (32) and an electrical conductivity above that of the active layer, which is of an electro-optic polymer, e.g. PNA/PMMA (polymethylmethacrylate grafted with paranitroaniline).

Description

ELECTRO-OPTICAL LIGHT BEAM DEFLECTOR,
ESPECIALLY FOR MULTI-POINT OPTICAL ADDRESSING
DESCRIPTION
TECHNICAL AREA
The present application relates to an electro-optical deflector of a light beam.

 I1 is a device for deflecting a light beam quickly, inexpensively and reliably (this device having no moving part).

 The present invention has all the applications of known light beam deflectors, with the advantages that a deflection of a light beam has without moving part.

 Regarding these applications, we can consult document (1) which, like the other documents cited below, is mentioned at the end of this description.

 The optical deflector object of the invention makes it possible to send a light beam in a predetermined direction which is a function of the electric field applied to the electro-optical material that this deflector comprises.

The present invention applies more particularly - to optical addressing, - to the distribution of radar beams and - to optical interconnections in computers or
between the elements of electronic microstructures.

PRIOR STATE OF THE ART
The deflection of a light beam is an optical function which finds many applications in optical microsystems.

 This function concerns large markets such as optical radars, barcode readers, optical detectors as well as imaging devices such as printers and projectors.

In the systems currently marketed, the angular deflection of a light beam is carried out by means of mechanical systems
(rotating or vibrating mirrors), movable lenses or acousto-optical modulators.

 There are thus currently three main families of light beam deflectors.

 The first family includes electromechanical deflectors.

 In the latter, a movable mechanical part allows the deflection of a light beam.

 It is generally a rotating mirror or a vibrating mirror.

 In all cases, the bandwidth of such deflectors is limited to a few kilohertz for mechanical reasons.

 In addition, the reliability of these electromechanical deflectors is limited by the presence of mechanical actuators and moving parts.

 The second family includes acousto-optical deflectors.

 In these latter an acoustooptic transducer emits an acoustic wave to modulate the direction of propagation of an optical wave.

 This is a principle which has given rise to commercial achievements in volume optics or integrated optics.

 In both cases, the modulation speed is typically limited to a few megahertz by the propagation speed of the acoustic waves.

 In addition, the power consumed is relatively large (a few watts) and requires an adapted radio frequency source.

 This technique makes it possible to separate several hundred points with large light beams (several millimeters).

 Typical characteristics are 500 points, 8 mm and 100 kHz as indicated in the document (1).

 The third family includes electro-optical deflectors (see document (2)).

 The electro-optical effect is faster than the acousto-optical effect and in principle allows deflection at a rate of the order of a gigahertz.

 However, the variations in optical index are modest and the control voltages are high so that the electro-optical deflectors are not used in the systems currently marketed.

STATEMENT OF THE INVENTION
The present invention relates to an electro-optical deflector capable of having a low control voltage, of the order of a hundred volts, and a high modulation frequency, of the order of a few gigahertz, and of deflecting a beam. bright in a lot of different directions.

Specifically, the present invention relates to an electro-optical deflector, characterized in that it comprises - an active layer which is made of a material
electro-optic and able to guide a light wave, - first and second electrodes placed on the side
and other of this active layer, according to the
length thereof, the first electrode being at
less partially transparent to the light wave
guided, - a means of applying a variable voltage between
the first and second electrodes, and - a means for decoupling this light wave
guided, allowing the creation, starting from this one,
of a light wave emerging from the first
electrode and doing with one direction
perpendicular to the latter a variable angle in
function of said voltage applied between
first and second electrodes.

 According to a preferred embodiment of the deflector object of the invention, it further comprises first and second buffer layers which are respectively between the first and second electrodes, the first buffer layer being at least partially transparent to the guided light wave, the first and second buffer layers having a refractive index lower than that of the active layer and an electrical conductivity greater than that of this active layer.

 According to a particular embodiment, the active layer forms a ribbon, the buffer layers being provided to surround this ribbon, the deflector further comprising two other electrodes which laterally border the buffer layers so that, by applying another electrical voltage between these two other electrodes, the emerging light wave can be deflected laterally.

 The decoupling means can comprise a diffraction grating which is inserted or created in the active layer or a prism which is placed on the first electrode.

 The deflector which is the subject of the invention may further comprise a means for coupling the light wave in the active layer.

 According to a first particular embodiment, this coupling means comprises a diffraction grating which is inserted or created in the active layer, at one end of the latter.

 According to a second particular embodiment, this coupling means comprises a prism which is placed opposite one end of the active layer, next to the first electrode.

 According to a third particular embodiment, this coupling means is a coupling means by the edge (end fire coupling) provided for injecting the light wave into one end of the active layer.

 Said voltage applied between the first and second electrodes can vary continuously or discontinuously, the angle made by the emerging light wave with a direction perpendicular to the first electrode thus varying continuously or discontinuously.

 Preferably, the active layer is made of an electro-optical polymer.

BRIEF DESCRIPTION OF THE DRAWINGS
The present invention will be better understood on reading the description of exemplary embodiments given below, by way of purely indicative and in no way limiting, with reference to the appended drawings in which
Figure 1 is a longitudinal sectional view
schematic of a known structure provided with a
waveguide of electro-optical material,
Figure 2 is a schematic view of a
known device, intended to study this
structure,
Figure 3 is a longitudinal sectional view
schematic of a particular embodiment
of the electro-optical deflector object of
the invention,
Figure 4 is a cross-sectional view
schematic of the embodiment shown on
Figure 3, and
Figure 5 is a cross-sectional view
schematic of another embodiment
particular of the object electro-optical deflector
of the invention.

DETAILED DESCRIPTION OF PARTICULAR EMBODIMENTS
We begin by describing a structure known from document (3), which is useful for understanding the present invention.

 This known structure is schematically represented in longitudinal section in FIG. 1.

It comprises a substrate 2 of silica and various layers which are successively stacked on this substrate, namely - a thin layer 4 of SnO2 constituting a first
electrode, - a first buffer layer 6 of chiolite (Na5A11; F14), - a layer 8 of polymethylmethacrylate grafted with
paranitroaniline (PNA / PMMA), which constitutes a layer
light guide, - a second buffer layer 10 also in chiolite
and - another thin layer 12 of SnO2 constituting a
second electrode.

 Appropriate biasing means 14 make it possible to establish a voltage between the two electrodes 4 and 12, which makes it possible to create a transverse electric field in the light guide layer made of PNA / PMMA (electro-optical material).

 In addition, a diffraction grating 16 is formed in the guide layer 8.

As stated in the document
(3), a converging light beam FI is sent towards the electrode 12 of the structure and this results in a reflected beam FR, a transmitted beam FT and a beam FG guided in the layer.

 In this structure, the diffraction grating 16 constitutes a means of coupling the incident light beam FI, making it possible to obtain the guided beam from the latter.

 FIG. 2 schematically illustrates a known device, allowing the study of the structure of FIG. 1.

 This structure has the reference 18 in FIG. 2.

 It is mounted on a goniometer 20.

 A helium-neon laser 22, provided with a microscope objective 24, generates a light beam which is focused on the structure 18, thanks to a lens 26, and thus constitutes the incident beam FI of FIG. 1.

 In addition, the beam FI is polarized (polarized) by a polarizer 28.

 The reflected beam FR is detected by a CCD camera 30.

 When the voltage applied between the electrodes varies, the intensity of this reflected beam also varies but its inclination (relative to the electrode 12) remains constant.

 As will be seen more clearly below, this known structure 18 can be used in the present invention no longer as a light coupler (which is illuminated by a convergent light beam) but as a decoupler.

 In fact, the authors of the present invention have very surprisingly found that, by sending a light beam into the guide layer 8 lengthwise thereof, instead of sending it obliquely over this layer, the structure of FIG. 1 then provided an emerging beam whose inclination varied as a function of the voltage applied between the electrodes and no longer a reflected beam having a constant inclination and a variable intensity as a function of the applied voltage.

The electro-optical deflector according to the invention, which is schematically represented in longitudinal section in FIG. 3, comprises - an active layer 32 made of an electro material
optical and capable of guiding a light wave 34, a first electrode or upper electrode 36 and
a second or lower electrode 38 which are
placed on either side of the active layer 32 and
extend along the length thereof, and - a means 39 for decoupling this light wave 34,
allowing the creation, from this, of a
light wave 40 emerging from the upper electrode
36 and doing with a direction perpendicular to
this one an angle O which varies according to a
voltage applied between electrodes 36 and 38 thanks
to suitable polarization means 42.

 The upper electrode 36 is completely or partially transparent to the light wave 34 so that the emerging wave 40 can exist.

 In the example shown in FIG. 3, the means for decoupling the guided light wave is a diffraction grating 39 formed at the lower longitudinal wall of the active layer 32, towards the middle of the latter, as is see it in figure 3.

 This diffraction grating defines a decoupling zone of length L.

 This length L of the decoupling zone limits the divergence of the emerging light wave 40.

 Due to the use of electrodes 36 and 38, which are capable of absorbing the guided light wave 34, it is preferable to have, between the active layer 32 and these electrodes 36 and 38, a first buffer layer or layer upper pad 46 and a second buffer layer or lower buffer layer 48 whose thicknesses are sufficient to avoid this absorption and whose refractive indices are lower than that of the active layer 32 so that the light can be effectively guided by this active layer.

 It is also necessary that the electrical conductivity of these buffer layers 46 and 48 be greater than that of the active layer 32 to ensure good distribution of the electric field (generated by application of the electrical voltage between the electrodes 36 and 38) through the structure shown in Figure 3.

 FIG. 3 also shows a substrate 50 on which are successively stacked the electrode 38, the buffer layer 48, the active layer 32, the buffer layer 46 and the electrode 36.

 Instead of the diffraction grating 39, a prism 52 placed on the upper electrode 36 could be used as decoupling means.

 Regarding such decoupling means, consult document (4).

In the example shown in FIG. 3, the substrate 50 is made of silica, the electrodes 36 and 38 are made of PA, the buffer layers 46 and 48 are made of chiolite and the active layer 32 is made of PNA / PMMA
(polymethylmethacrylate grafted with paranitroaniline).

 It is an electro-optical polymer which is advantageously usable in the present invention for a reason indicated below, but other materials having a non-zero electro-optical coefficient can be used in the invention.

 The chiolite could also be replaced by another material of good optical quality and slightly conductive to ensure good distribution of the electric field through the active layer.

 Similarly, the SnO2 electrodes could be replaced by electrodes made of another good conductive material, such as, for example, indium tin oxide or a conductive polymer, and transparent in whole or in part to allow the exit of the light wave guided through the electrode 36.

 The diffraction grating 39 can be etched in the substrate 50 on the upper face of the latter so that this grating is found on the lower wall of the active layer 32, the electrode 38 and the buffer layer 48 being sufficiently thin to allow the transfer of the network profile at this lower wall.

 Other techniques can of course be used to obtain this diffraction grating on the bottom wall of the active layer 32.

 One could for example deposit a photosensitive polymer layer on the lower buffer layer 48 and then etch the diffraction grating thereon by photobleaching using a mask, two light beams interfering with each other or a light beam of very small diameter that would be displaced relative to the buffer layer 48 or vice versa.

 On this subject, consult document (5).

 The structure which one sees on figure 3 is similar to that which one sees on figure 1 but an essential difference between the two lies in the functions of the diffraction gratings referenced 16 in figure 1 and 39 in the figure 3.

 In the case of FIG. 1, the diffraction grating 16 allows the coupling of an incident light wave (light beam FI) whereas, in the case of FIG. 3, the diffraction grating 39 on the contrary allows the decoupling of a guided light wave 34.

 FIG. 3 also shows a means of coupling a light wave in the active layer 32.

 In the example shown, this coupling means is a diffraction grating 54 which is formed at the level of the lower face of the active layer 32, at one end of the latter.

 A light wave 56 which arrives towards this end is thus coupled into the active layer 32 in order then to propagate therein and to be guided therein.

 The diffraction grating 54 could be replaced by a coupling means by the edge 58 making it possible to inject the light wave 56 into the active layer 32 and placed on the side of the same end of the active layer.

 One could also use, as a coupling means, a prism 60 placed on the side of this end of the active layer, above the latter and more precisely, in the example shown, on the upper buffer layer 46 , next to the upper electrode 36 which, as seen in Figure 3, does not completely cover the active layer.

 Regarding these coupling means, consult document (4).

 When the voltage is applied between the electrodes 36 and 38 by the polarization means 42, the electric field generated in the active layer 32 modifies the refractive index thereof and consequently modifies the angle O of the light wave emerging 40.

 By varying this voltage continuously or discontinuously, this angle is also varied continuously or discontinuously.

 The angular deflection function of a light beam is thus well realized.

 This allows application of the invention to addressing.

 For example, we see in Figure 3 a row of photodetectors 62 (or any other elements that we want to address) and, by applying an appropriate electrical voltage between the electrodes, any of these photodetectors can be reached by the emerging light wave 40.

The advantages of a deflector of the kind of that of FIG. 3 compared to known deflectors are indicated below
1) This deflector has a large bandwidth.

 This results from the electro-optical effect which is rapid and allows the devices which use it to have large bandwidths as shown by the intensity modulators commercially available in integrated optics.

 Frequencies of several gigahertz are typical and frequencies of several tens of gigahertz can be reached if special phase tuning conditions are met.

 2) This deflector is likely to have a large number of resolvable points.

 The use of guided optics eliminates diffraction.

 In fact, in a guided mode, the light wave remains confined in a small volume over the length L of the decoupling zone.

 The divergence of the emerging wave is little different from AO1 = X / (L), X being the wavelength of this emerging wave, while the amplitude of the deflection is little different from AO2 = An / cosO, An being the difference between the refractive index of the active layer and the refractive index of the buffer layers.

 Therefore, the number N of resolvable points, which is equal to the ratio AO2 / AO1, can become as large as we want if we increase L.

We can for example obtain N = 180 with
L = 3 cm, k = 1 µm and An = 0.001.

 3) This deflector is also likely to have a low control voltage.

 Indeed, if the light is confined in an active layer of small size (a few micrometers), the control voltage is considerably reduced compared to the case of a modulator in volume optics and is typically worth a few hundred volts.

 4) This deflector also has great reliability.

 This results from the use of the electro-optical effect which makes it possible to produce an electro-optical deflector without a moving part.

 5) This deflector also has a low cost.

 The use of electro-optical polymers makes it possible to obtain excellent characteristics at low cost.

 Indeed, these materials have a high electro-optical coefficient (greater than or equal to 10 μm / V) and a high breakdown electric field (greater than or equal to 100 V /> m) which allow large variations in refractive index.

 In addition, these materials are malleable and allow inexpensive manufacturing of devices, in particular by the technique of printing (embossing) a matrix.

 In an alternative embodiment, not shown, of the deflector of FIG. 3, the lower electrode 38 is eliminated and the silica substrate 50 is replaced by an electrically conductive substrate, for example made of silicon, on which the lower buffer layer 48 rests directly.

 In this case, the electrical voltage is applied between this silicon substrate and the upper electrode 36.

 The deflector according to the invention which is schematically shown in longitudinal section in FIG. 3 can use a planar light guide.

 A cross-sectional view of this deflector is schematically represented in FIG. 4.

 In this case, the active layer 32 extends from one edge to the other of the device and is not laterally bordered with buffer layers.

 The invention can also be implemented with a lateral confinement light guide.

 An example corresponding to this case is schematically represented in cross section in FIG. 5.

 It can be seen in FIG. 5 that the active layer 32 then forms a ribbon which is surrounded by all of the buffer layers 46 and 48.

 Two additional electrodes 64 and 66 can be added to the deflector of FIG. 5 which laterally border all of the buffer layers and extend along the active layer 32.

By establishing a voltage between these electrodes 64 and 66 by means of polarization
(biasing) appropriate 68 and by varying this voltage, we are then able to vary the angle (p of the emerging light wave 40 relative to the median longitudinal plane P of the active layer 32 (FIG. 5).

 By applying appropriate voltages by means 42 and 68, we are then able to carry out two-dimensional addressing.

 The emerging light wave 40 is then capable of reaching any of the photodetectors of a matrix of photodetectors 70 arranged transversely to the active layer 32 as seen in FIG. 5.

 The documents cited in this application are the following (1) Mini catalog Crystal Technology Inc.

(2) A. Yariv, Quantum Electronics, 2nd edition,
John Wiley and Sons.

(3) G. Blau, M. Cairone, PA Chollet, F. Kajzar,
Electro-optic modulation and SHG through grating
induced resonant excitation of a guided mode,
Nonlinear Optical Properties of Organic Materials IX,
SPIE, Denver August 4-9 (1996).

(4) RG Hunsperger, Integrated Optics: Theory and
Technology, Springer-Verlag, Berlin 1991, Chapter 6.

(5) JT Yardley, Design and Characterization of
Organic Waveguides for Passive and Active Optical
Devices, in Organic Thin Films or Waveguiding
Nonlinear Optics, F. Kajzar and J. Swalen Eds.,
Gordon & Breach Publ., Amsterdam 1996, Chapter 11.

Claims (11)

 1. Electro-optical deflector, characterized in that it comprises - an active layer (32) which is made of a material  electro-optic and able to guide a light wave, - first and second electrodes (36, 38) placed  on either side of this active layer, according to the  length thereof, the first electrode (36)  being at least partially transparent to the wave  guided light (34), - means (42) for applying a variable voltage  between the first and second electrodes, and - a means (39, 52) for decoupling this wave  guided light, allowing the creation, from  this, of a light wave (40) emerging from the  first electrode and doing with one direction  perpendicular to the latter an angle (0)  variable according to said applied voltage  between the first and second electrodes.  2. The deflector according to claim 1, further comprising first and second buffer layers (36, 38) which are respectively between the first and second electrodes, the first buffer layer (36) being at least partially transparent to the light wave. guided (34), the first and second buffer layers having a refractive index lower than that of the active layer (32) and an electrical conductivity higher than that of this active layer.  3. Deflector according to claim 2, in which the active layer forms a strip (32), the buffer layers (46, 48) being provided to surround this strip, the deflector further comprising two other electrodes (64, 66) which the buffer layers are bordered laterally so that, by applying another electrical voltage between these two other electrodes, the emerging light wave can be deflected laterally.  4. Deflector according to any one of claims 1 to 3, in which the decoupling means comprises a diffraction grating (39) which is inserted or created in the active layer (32).  5. Deflector according to any one of claims 1 to 3, in which the decoupling means comprises a prism (52) which is placed on the first electrode (36).  6. Deflector according to any one of claims 1 to 5, further comprising means (54, 58, 60) for coupling the light wave (56) in the active layer (32).  7. A deflector according to claim 6, in which this coupling means comprises a diffraction grating (54) which is inserted or created in the active layer, at one end of the latter.  8. Deflector according to claim 6, in which this coupling means comprises a prism (60) which is placed opposite one end of the active layer, next to the first electrode (36).  9. A deflector according to claim 6, in which this coupling means is a wafer coupling means (58) provided for injecting the light wave (56) into one end of the active layer (32).  10. Deflector according to any one of claims 1 to 9, in which said voltage applied between the first and second electrodes varies continuously or discontinuously, the angle (0) made by the emerging light wave (40) with a direction perpendicular to the first electrode thus varying continuously or discontinuously.  11. Deflector according to any one of claims 1 to 10, in which the active layer  (32) is made of an electro-optical polymer.