WO2004099864A2

WO2004099864A2 - Electro-optical transducer

Info

Publication number: WO2004099864A2
Application number: PCT/NL2004/000308
Authority: WO
Inventors: Joseph Wilhelmus Maria Hoekstra; Marcel Hoekman; René Gerrit Heideman HEIDEMAN
Original assignee: LioniX BV
Current assignee: LioniX BV
Priority date: 2003-05-09
Filing date: 2004-05-10
Publication date: 2004-11-18
Anticipated expiration: 2005-11-09
Also published as: NL1023376C2; WO2004099864A3

Abstract

The invention relates to an electro-optical transducer, comprising at least two electrodes and at least one optical layer placed between the electrodes and manufactured from a light-transmitting, electro-optical material, the optical properties of this layer depending on the strength of an electric field applied by the electrodes over the layer, wherein the optical layer is divided into optical sub-layers by separating layers extending substantially transversely of the direction of the electric field generated by the electrodes. The transducer is found to be active only in the vicinty of one of the two boundary layers as a result of the presence of a depletion layer and an accumulation layer, which in any case only extend in the vicinity of one of the boundary surfaces. By providing additional separating layers the number of boundary surfaces increases, and therewith the number of depletion and accumulation layers. The desired effect will hereby extend through a greater part of or the whole volume of the optical layer.

Description

Electro-optical transducer

The invention relates to an electro-optical transducer, comprising at least two electrodes and at least one optical layer placed between the electrodes and manufactured from a light-transmitting, electro-optical material, the optical properties of this layer depending on the strength of an electric field applied by the electrodes over the layer.

Such a transducer is known from NL-A-1 006 323.

Such a transducer is adapted to conduct transversely of the direction of the electric field the light supplied to the transducer and to influence it subject to the strength of the electric field. In order to enable conduction of sufficient light, a minimal layer thickness of the optical layer is required. The inventors have found that the effectiveness of the transducer greatly decreases as the optical layer thickness increases, so that further increasing the layer thickness has scarcely any effect.

The object of the invention is to provide such a transducer wherein a good effectiveness of the layer is obtained over the whole thickness, so that larger amounts of light can be switched.

This objective is achieved in that the optical layer is divided into optical sub-layers by separating layers extending substantially transversely of the direction of the electric field generated by the electrodes.

The transducer is found to be active only in the vicinity of one of the two boundary layers. This is the result of the presence of a depletion layer and an accumulation layer, which in any case only extend in the vicinity of one of the boundary surfaces. By providing additional separating layers the number of boundary surfaces increases, and therewith the number of depletion and accumulation layers. Through a suitable choice of the number of separating layers the depletion and accumulation layers can extend through the whole thickness of the optical layer, whereby the desired effect will extend through a greater part of or the whole volume of the optical layer. The volume will of course decrease due to the presence of the separating layers. This effect is however much smaller than the advantage achieved. It is noted here that it is known from the journal article "Fabrication of Piezoelectric Thin Film Resonators with Acoustic Quarter Wave Multilayers", Japanese Journal of Applied Physics, May 2002, pp. 3455-3457, H. Kobayashi et al, to apply a layer structure which also comprises an alternation of two different types of layer. This article refers however to a piezoelectric transducer, i.e. a transducer for converting mechanical signals into electromagnetic signals, and vice versa. The alternation of the layers is intended to adjust the acoustic impedance; a quantity which plays no part in the present invention.

The number of separating layers depends of course on the dimensioning and the materials applied, but the inventors have found that a favourable effect is obtained when at least three optical sub-layers and at least two separating layers are applied.

The optimal effect is in general even more closely approximated when at least six optical sub-layers are applied and - consequently - five separating layers. The inventors have also found that the desired effect is optimal when an optical layer is applied with a thickness smaller than 200 nm. This is of course related to the width of the depletion or accumulation layer. This dimension thus depends to a certain extent on the type of material used. With some materials an optimum is however achieved when the thickness of the layer is smaller than 50 nm.

Another preferred embodiment provides the measure that the thickness of the separating layer is smaller than 20 nm. It will of course be apparent that, in order to make the adverse effect of the separating layer, i.e. screening of the light, as small as possible, the layer thickness must be as small as possible. Conversely, the thickness must be great enough that it must be possible to apply the layer reproducibly, controllably and with a uniform thickness using known techniques.

Yet another embodiment provides the measure that the total thickness of the assembled layer is smaller than 1 μm. An optimum is hereby achieved between the generally conflicting requirements arising from production technology and functionality. The advantages of the invention become particularly manifest when the optical sublayers substantially comprise zinc oxide. Zinc oxide has been found to be a material where the thickness of the depletion layer and of the accumulation layer is limited, so that the advantages of the invention manifest themselves well. Other materials wherein similar effects related to layer thickness occur are however not precluded.

The separating layers are preferably manufactured from silicon nitride or silicon oxide.

The above stated structure is suitable for application as modulator for modulating optical signals such as are used for instance in glass fibre communication. The electric signal applied to the electrodes is herein used to modulate the light signal before the light signal enters a glass fibre cable.

It is also possible to use the electro-optical transducer as a sensor for sensing and converting electric signals. By means of the transducer such electric signals can be converted into light signals, transported in this form over a great distance and subsequently detected.

In the construction of the structure according to the present invention, use is preferably made of a method wherein the following steps are performed of: providing a substrate; arranging a first optical layer in this substrate; applying a subsequent separating layer to this optical layer; applying a subsequent optical layer to the subsequent separating layer; repeating the latter two steps; and arranging an electrode on either side of the thus formed structure.

The electrodes are preferably arranged last on the structure. Use is preferably made for applying the layers of the following techniques: sputtering, vapour deposition or chemical vapour deposition (CVD).

Other attractive preferred embodiments are stated in the remaining claims.

The invention will be elucidated hereinbelow on the basis of two exemplary embodiments of the invention. In the figures: figure la shows a section through a first exemplary embodiment of a structure of a modulator according to the invention with a partial enlargement; and figure lb shows a section through a first, comparable single structure according to the prior art, with a partial enlargement. Figure la shows a structure of layers 22,23 applied (for instance by means of sputtering) to a substrate 24, for instance a silicon wafer, which together form an electro-optically active structure 2 according to the invention. Structure 2 is built up of alternating sublayers 22 of zinc oxide, of for instance 100 nm thickness, and separating layers 23 of for instance silicon oxide of for instance 5 nm thickness. By way of comparison, figure 1 shows a structure 2' grown onto the same type of substrate 24' built up of a single zinc oxide layer 22' grown on a silicon oxide substrate 23'. In the given conditions the zinc oxide can grow as crystalline columns 25,25' on a thin initial layer 20,20'. Columns 25,25' can again have depleted grain boundary regions 251,251' in addition to semiconducting cores 252,252'.

When an electric voltage is applied over structures 1,2, accumulation and depletion layers 211,213,21 T,213' will be created which are electro-optically active. The volume ratio of the active layer boundary regions 211,213,211',213' and the non-active layer parts 212,212' is much smaller in the single structure 2' than in the stacked structure 2. The active volume part is therefore much greater in the stacked structure 2 than in single structure 2'. The electro-optical behaviour of stacked structure 2 according to the invention will therefore be much stronger than that of single structure 2' at a comparable thickness and comparable total volume.

In structure 2 the sub-layers 12 each consist of a part of an initial layer 10'. The active volume part in the structure according to the invention 1 is therefore much greater than in the prior art single structure 1'. The electro-optical effect of structure 1 according to the invention will therefore be much stronger than that of the prior art single structure 1' at a comparable thickness and comparable total volume.

Figure 2 shows a schematic view of a modulator according to the invention.

The modulator according to the invention comprises a stack of layers 22 and 23. Layers 22 are the sub-layers of optically active material and layers 23 are the separating layers. In the present embodiment the optical sub-layers 22 are manufactured from zinc oxide and separating layers 23 are manufactured from silicon oxide. It will be apparent that both layers can be replaced by layers of other materials with the desired properties. The method by which the structure is manufactured will of course be a major consideration in the choice of the materials.

An electrode 24 is arranged on the top side of the structure and an electrode 25 is arranged on the bottom side of the stack. Both electrodes are manufactured from conductive material. A controllable voltage source 26 is connected between the two electrodes 24 and 25. The voltage source 26 functions here as signal source. This source supplies the modulating signal.

Placed on one side of the modulator is a light source, for instance a LED 27, which is adapted to generate light in the desired wavelength. The light is carried to the entry surface of the modulator by means of an entry element 28. During propagation of the light through the demodulator it is subjected to the electric field being generated by electrodes 24, 25.

The influencing of the light beam being propagated through the optical layers is expressed as a phase shift of the light beam. This depends on the strength of the local electric field. When the electric field changes, so will the phase of the light beam. The light signal being propagated through the modulator will hereby be modulated by the signal generated by voltage source 26 and applied to the electrodes.

On the exit surface of the modulator is placed an entry element 29 for entry of the modulated light coming from the modulator to a glass fibre cable 30 or other light conductor.

A linear structure has been discussed above; it will be apparent that such structures according to the invention can also be embodied in other spatial structures, such as cylindrical structures.

Another application of the structure according to the invention is the use as sensor. The sensor can herein be used to sense the voltage applied to the electrodes. It is however also possible to use the sensor to measure other quantities having an effect on the diffraction of the optical layers, such as the temperature.

Phase modulation is not easy to detect without reference signal. By applying a so-called Mach-Zender interferometer the obtained phase modulation can be converted into an amplitude modulation. In the application as sensor the measured signal can be converted into a change of intensity with the Mach-Zender interferometer. A configuration of a Mach-Zender interferometer can be readily implemented in the technology applied here. A schematic top view of such a configuration is shown in figure 3 and a cross-sectional view thereof is shown in figure 4. This configuration is applicable in the use of the transducer as modulator as well as in the use of the transducer as sensor.

Such a structure is arranged on a substrate 20 of for instance silicon. On this substrate an optical bridge structure is arranged with two branches, each incorporating a modulator according to the present invention. One of the sensors is herein screened from the immediate vicinity. This serves solely as reference and thereby as compensation for undesirable influences. Both optical branches are together connected to a light source, such as a glass fibre cable, while they are both also connected to a second glass fibre cable for receiving the modulated signal.

Reference is made to NL-C- 1006323 for the operation of such a Mach-Zender interferometer.

It will be apparent that the transducer according to the present invention can likewise be applied in other signal-processing components.

Claims

1. Electro-optical transducer, comprising at least two electrodes and at least one optical layer placed between the electrodes and manufactured from a light-transmitting, electro-optical material, the optical properties of this layer depending on the strength of an electric field applied by the electrodes over the layer, characterized in that the optical layer is divided into optical sub-layers by separating layers extending substantially transversely of the direction of the electric field generated by the electrodes.

2. Transducer as claimed in claim 1 , characterized in that the number of optical sub-layers is greater than three and preferably greater than six.

3. Transducer as claimed in claim 1 or 2, characterized in that the thickness of the optical layer is smaller than 200 nm.

4. Transducer as claimed in claim 3, characterized in that the thickness of the optical layer is smaller than 50 nm.

5. Transducer as claimed in any of the foregoing claims, characterized in that the total thickness of the assembly of the optical sub-layers and the separating layers placed therebetween is smaller than 10 μm.

6. Transducer as claimed in claim 5, characterized in that the total thickness of the assembly of the optical sub-layers and the separating layers placed therebetween is smaller than 1 μm.

7. Transducer as claimed in any of the foregoing claims, characterized in that the optical sub-layers substantially comprise zinc oxide.

8. Transducer as claimed in any of the foregoing claims, characterized in that at least one of the separating layers comprises silicon oxide.

9. Transducer as claimed in any of the foregoing claims, characterized in that at least one of the separating layers comprises silicon nitride.

10. Transducer as claimed in any of the foregoing claims, characterized in that the transducer is adapted to function as modulator.

11. Transducer as claimed in any of the claims 1 -9, characterized in that the transducer is adapted to function as sensor.

12. Combination of two transducers as claimed in claim 11, characterized in that the two transducers are accommodated in the configuration of a Mach-Zender interferometer.

13. Method for manufacturing a transducer as claimed in any of the foregoing claims, comprising the following steps of: providing a substrate; - arranging a first optical layer in this substrate; applying a subsequent separating layer to this optical layer; applying a subsequent optical layer to the subsequent separating layer; repeating the latter two steps; and arranging an electrode on either side of the thus formed structure.

14. Method as claimed in claim 13, characterized in that the electrodes are arranged last on the structure.

15. Method as claimed in claim 13 or 14, characterized in that at least one of the separating layers and the electro-optical layers are applied by means of one of the following techniques: sputtering, vapour deposition or chemical vapour deposition (CVD).