Optical modulator

An optical modulator having a slit arrangement and an associated zone exhibiting both an electrooptical effect and an intrinsic polarization direction, includes an electrode arrangement to which an information signal is applied during operation. The slit arrangement is a double slit arrangement while the electrode arrangement includes three electrodes between which the slits are disposed. At one slit, the electrical field of the signal is oriented in the same direction as the intrinsic polarization direction and, at the other slit, the electrical field of the signal is oriented in a direction opposite to the intrinsic polarization direction.

CROSS REFERENCE TO RELATED APPLICATION 
This application claims the priority of application Ser. No. P 40 00 616.6, 
filed Jan. 11, 1990, in the Federal Republic of Germany, the subject 
matter of which is incorporated herein by reference. 
BACKGROUND OF THE INVENTION 
This invention relates to an optical modulator including a slit arrangement 
that is associated with a zone exhibiting an electrooptical effect as well 
as a preferred polarization direction. The modulator further includes an 
electrode arrangement to which a signal is applied for modulation 
operation. 
Such an arrangement is disclosed in the article, entitled "Organic Polymer 
Films For Nonlinear Optics," published in Br. Telecom Technol., Volume 6, 
1988. FIGS. 11 and 12 of this publication show an optical modulator which 
includes a glass substrate on which a polymer is disposed that is equipped 
with interdigital electrodes. The two electrodes are provided with two 
meshing "comb arrangements" and are each connected with a signal terminal. 
For manufacture of such an electrooptical modulator, the polymer is heated 
and simultaneously a direct voltage potential is applied to the signal 
terminals. The direct voltage potential remains in effect until the 
modulator has cooled to room temperature. The result is a polarization of 
the polymer. If now, during modulation operation, light, particularly 
laser light, is conducted through the optical grating formed by the 
electrodes and constituting a slit arrangement, diffraction lines are 
formed which can be made visible, for example, on a screen disposed behind 
the optical modulator. If a signal (alternating voltage signal) is applied 
to the signal terminals, a change in the refraction index of the described 
grating occurs which influences the arrangement of the diffraction lines. 
The latter thus change according to the information content of the signal. 
By means of a detector which scans the diffraction lines, the light 
modulation can be reconverted into an electrical signal. Since in the 
prior art arrangement the change in the index of refraction is the same 
for all individual slits of the grating, light modulation can be observed 
only in the near field. Therefore, the prior art arrangement has a 
relatively poor efficiency. 
SUMMARY OF THE INVENTION 
It is an object of the invention to provide an optical modulator of the 
above-mentioned type which has high efficiency and thus high 
effectiveness. 
This is accomplished by the present invention in that the slit arrangement 
is configured as a double slit arrangement including three electrodes 
between which the slits are located and which are configured in such a way 
that, at the first slit, the electrical field of the signal is oriented in 
the same direction as the polarization field and, at the other slit, the 
electrical field of the signal extends in the opposite direction to the 
polarization field. Because of the arrangement according to the invention, 
the signal supports the polarization field generated during the preceding 
polarization in the one slit of the double slit arrangement, while in the 
other slit the field of the signal and the polarization value extend in 
opposite directions. This opposition existing between the two slits has 
the result that the refractive indices in the two slits are changed in 
opposite directions so that the optical modulator according to the 
invention becomes very efficient and thus highly effective. Because of the 
change in index to the same extent but in the opposite direction in the 
two individual slits, the modulation can be observed not only in the near 
field but also in the remote field. 
According to a modification of the invention, the zone is intended to be 
composed of a non-linear optical polymer (NLO polymer). 
The zone may preferably be disposed on a substrate. 
According to another feature of the invention, its configuration is 
simplified in that the slit arrangement is formed by electrodes. 
The electrodes may be arranged in the region of the interface between 
substrate and zone. For example, the electrodes may be disposed on the 
surface of the substrate and may be covered by the zone. 
However, as an alternative, it is also possible to have a buffer layer 
disposed between the electrodes and the zone. This buffer layer prevents 
high interfering field intensities from occurring at the electrode edges 
in the zone. 
Preferably, the buffer zone may be made of polymethyl methacrylate (PMMA). 
If a larger irradiation area is to be created for the laser light, several 
double slit arrangements can be arranged next to one another with their 
ad]acent slits lying parallel to one another. This results in the 
formation of a grating. 
Also possible is the formation of a cascade for the production of logic 
linkages. In this case, several (e.g., two) double slit arrangements are 
arranged in tandem. The light coming from the optical modulator associated 
with the light source impinges on a further optical modulator and is there 
processed further, corresponding to the signal actuation. 
Finally, in order to form modulator arrays, it is possible to arrange 
several dual slit arrangements on top of one another with their slits 
being flush.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
According to FIGS. 1 and 2, the optical modulator 1 according to the 
present invention is composed of an electrode arrangement 2 that includes 
three electrodes. A center electrode 5 is arranged between two outer 
electrodes 3 and 4. 
Between outer electrode 3 and center electrode 5, there is formed a first 
slit 6 and between the outer electrode 4 and center electrode 5, there is 
formed a second slit 7. Slits 6 and 7 form a double slit arrangement 8. 
Electrodes 3 and 4 and center electrode 5 are each provided with a signal 
terminal 9 for the application of a polarization potential or a signal, 
respectively. 
Center electrode 5 has a width of 100 .mu.m. This dimension corresponds to 
the spacing between the first slit 6 and the second slit 7. The slit width 
in each case is 20 .mu.m. Electrode arrangement 2 is composed of CrAu. It 
is located on the surface 11 of a substrate 10 and is covered by a zone 12 
exhibiting an electrooptical effect as shown in FIG. 2. Zone 12 is 
composed of a non-linear optical polymer (NLO polymer) which is applied as 
a coating to substrate 10, preferably by spin coating. The layer thickness 
is a few microns (in particular, 3.7 .mu.m). 
For the manufacture of the modulator 1 according to the invention, zone 12 
is heated, with simultaneously the outer electrodes 3 and 4 being placed 
at the same potential (e.g., ground). The center electrode 5 is placed at 
another direct voltage potential (e.g., +600 Volt). This causes the 
polarization fields of the individual slits 6 and 7 to extend in opposite 
directions. 
During modulation operation, double slit arrangement 8 is irradiated with 
light, particularly laser light, with the position of the polarization 
plane of the incident light being significant. If the polarization P of 
the incident light lies in the plane of the drawing, the electrooptical 
coefficient r.sub.33 becomes effective. If the incident light is polarized 
perpendicularly to the plane of the drawing, the electrooptical 
coefficient r.sub.13 is decisive. If NLO polymers are employed as zone 12, 
the following applies according to previous examinations: 
EQU r.sub.33 &gt;&gt;r.sub.13 
FIG. 4 shows the above-described optical configuration, with the 
polarization P of the incident light L lying in the plane of the drawing. 
A single slit 13 is disposed behind optical modulatoer 1 and, behind it, a 
photodetector 14. 
The arrangement--polarized as already described above--is operated with a 
signal in such a manner that the outer electrodes 3 and 4 are at 
oppositely equal voltages relative to the center electrode 5 which is 
connected to ground. Thus, in the one slit (e.g., first slit 6), the 
electrical field of the signal is oriented in the same direction as the 
previously applied polarization field while, in the other slit (e.g., 
second slit 7), the field of the signal is oriented oppositely to the 
previously applied polarization field. This results in opposite identical 
changes in the refractive index in both slits 6 and 7. Application of a 
signal to electrodes 3 to 5 of the double slit arrangement 8 causes the 
diffraction image to be shifted so that photodetector 14 which is disposed 
behind single slit 13 is able to detect a fluctuation in intensity. 
Photodetector 14 is not part of modulator 1 but serves only to detect the 
modulation. 
Based on the arrangement according to the invention, it is possible, in 
principle, to realize very high intensity fluctuations which are the 
result of particularly high degrees of modulation. 
As an alternative to the already described polarization and signal 
application, the procedure may also be such that the two individual slits 
(first slit 6 and second slit 7) have polarization fields that extend in 
the same direction. For this purpose, the direct polarization voltage is 
applied accordingly to electrodes 3 to 5. For actuation by the signal, the 
center electrode is then connected, for example, to ground, while the two 
outer electrodes 3 and 4 are charged with the same signal potential. 
In contrast to the embodiment of FIG. 2, in the embodiment of FIG. 3, 
modulator 1 is provided with a buffer layer 15. The electrode arrangement 
2 resting on the surface 11 of substrate 10 is embedded in buffer layer 
15. It is composed of polymethyl methacrylate (PMMA). Its layer thickness 
is preferably 1.5 .mu.m. Then zone 12 composed of an NLO polymer is 
applied to buffer layer 15. Extremely high field intensities which may 
appear at the edges of the electrodes are prevented in zone 12 by buffer 
zone 15 so that these extreme field intensities will not have an 
interfering effect. 
FIG. 5 shows the relative light intensity change .DELTA.I according to the 
embodiment of FIG. 3. It is: 
##EQU1## 
where the individual slit width is 20 .mu.m, the layer thickness of zone 
12 is 3.7 .mu.m (NLO polymer) the layer thickness of PMMA buffer layer 15 
is 1.5 .mu.m, the spacing between slits is 100 .mu.m, the voltage is 
.+-.100 Volt and r.sub.33 =8.3 pm/V. 
FIG. 6 shows a cascade 16 which is formed by two modulators 1 placed one 
behind the other. In this way it is possible to realize, for example, 
logic linkages. The arrangement is such that the first double slit 
arrangement 17 facing the incident light is offset in such a way relative 
to the second double slit arrangement 18 behind it that the slits of the 
second double slit arrangement lie at the height of the edges of the 
intensity maxima of the Fraunhofer lines formed by the first double slit 
arrangement 17. This is shown in FIG. 6. 
Modulator 1 may also be configured as a modulator array 19, in which case 
several double slit arrangements 8 have a common center electrode 5 and 
are arranged one above the other with their slits 6 and 7, respectively 
being flush with one another. Cascades 16 can also be formed from such 
modulator arrays 19. 
It will be understood that the above description of the present invention 
is susceptible to various modifications, changes and adaptations, and the 
same are intended to be comprehended within the meaning and range of 
equivalents of the appended claims.