Controllable directional coupler

A controllable directional coupler comprises two optical waveguides on the surface of a substrate of electrooptical material, the waveguides extending parallel for a predetermined distance. The coupler also comprises a control electrode structure including a pair of control electrodes formed on the surface of the substrate and covering the two parallel waveguides along their lengths. In order to achieve low control voltage and a low coupling loss, in the case of butt coupling to a monomode fiber, in the interstice between the two optical waveguides, the refractive index of the substrate is reduced to a specific depth of the substrate at which the coupling intensity becomes largely independent of the near-field expansion in the waveguides.

BACKGROUND OF THE INVENTICN 
1. Field of the Invention 
The present invention relates to a controllable directional coupler having 
two optical waveguides formed in a substrate of electro-optical material 
on a surface of the substrate and extending parallel for a distance, and a 
control electrode structure formed on the surface and comprising a pair of 
control electrodes, the control electrode structure covering the two 
parallel waveguides. 
2. Description of the Prior Art 
Controllable directional couplers of the type generally set forth above, 
for example, in a substrate of LiNbO.sub.3, LiTaO.sub.3, InGaAsP or 
GaAlAs, can be employed as rapid modulators and switches as disclosed, for 
example, by R. C. Alferness, L. L. Buhl and M. D. Divino in the article 
entitled "Low-Loss Fibre-Coupled Waveguide Directional Coupler Modulator", 
published in Electronics Letters 18 (1982), pp. 490-491. 
SUMMARY OF THE INVENTION 
It is the object of the present invention to provide a rapid 
electro-optical modulator or transfer switch in a waveguide embodiment 
according to the principle of the controllable directional coupler which 
is simultaneously optimized with respect to control voltage and coupling 
loss in the case of butt coupling to a monomode fiber. 
The above object is achieved, according to the present invention, in a 
controllable directional coupler of the type set forth above which is 
particularly characterized in that the refractive index of the substrate 
is lowered to a specific depth of the substrate in the interstice between 
the two parallel optical waveguides. 
The following considerations play a decisive role in the origin of the 
invention. In order to obtain a low control voltage with a given cutoff 
frequency, the maximum length L of the directional coupler i.e. of the 
parallel optical waveguides, should be selected, which is possible due to 
the transit time effect. Moreover, the coupling intensity between the 
parallel waveguides is to be selected such that precisely a complete 
transfer coupling is achieved over the length L of the directional 
coupler. 
In order to achieve low coupling losses in the case of butt coupling of a 
waveguide of the directional coupler to a monomode glass fiber, the 
near-field extension of the light guided in the optical waveguide of the 
directional coupler must correspond well to that in the glass fiber. For 
this near-field extension there then results a specific optimum distance 
between the parallel waveguides of the directional coupler and also the 
control voltages, which has the smallest control voltage as a consequence. 
The distance between the waveguides of the directional coupler and the 
near-field extension, however, also establish the coupling intensity 
between the parallel waveguides, and therefore also establish the coupling 
length L.sub.O for complete crossover coupling from one waveguide to the 
other, so that, in general, it is not possible to observe the optimum 
parameters for the operating voltage and insertion losses giving a 
specified cutoff frequency. If, by contrast, one reduces the refractive 
index between the parallel waveguides of the directional coupler between 
which the transfer or crossover coupling takes place, then the optimum 
coupling length L.sub.O =L can be observed, which results from the transit 
time limitation, and simultaneously the optimum distance between the 
parallel waveguides and also the control electrodes of the directional 
coupler for minimum control voltage can be selected while the near-field 
parameters for minimum coupling loss, in the case of butt coupling to 
glass fibers, can be adjusted. According to preferred and advantageous 
embodiments of the directional coupler of the present invention, the 
reduction of the refractive index can take place, for example, through 
etching of a channel by ion or plasma etching, through a suitable ion 
exchange process, or through ion implantation. Generally, for the 
individual, for example, for the above-captioned substrate materials, the 
wet or dry chemical etching processes conventional for this purpose can be 
employed. In the case of LiNbO.sub.3 components, for example, an ion 
implantation with He ions, or an ion etching in a fluorous molecular gas, 
for example, in a CHF.sub.3 gas, can be carried out. Advantageously, for 
the reduction of the refractive index, the electrode structure itself can 
serve as a mask, in particular, as an etching mask, so that a 
self-aligning process is provided. Only the regions outside of the 
electrode structure need then be covered by an additional auxiliary mask, 
the adjustment of which, however, is not critical. 
The directional coupler constructed in accordance with the present 
invention is suitable as a changeover switch for glass fiber systems.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Referring to the drawing, a controllable directional coupler is illustrated 
having optical waveguides and an electrode structure on the upper surface 
11 of a substrate 1, above which an auxiliary mask is arranged which, 
together with the electrodes, is employed as a mask in the production of 
the refractive index reduction between the waveguides and the control 
electrodes by way of ion implantation or ion etching. 
The controllable directional coupler, as seen on the drawing, is designed 
on the upper surface 11 of the substrate 1 in the form of a LiNbO.sub.3 
crystal and comprises two optical waveguides 2 and 3 designed in the 
substrate 1 on the upper surface 11. The waveguides 2 and 3 can have been 
produced, for example, through indiffusion of titanium or ion exchange or 
ion implantation in the substrate 1. They extend parallel for a distance 
of a length L and at a distance a from one another. 
On the upper surface 11 of the substrate 1, above the parallel waveguides 2 
and 3, a control electrode structure is provided which consist of a pair 
of control electrodes 4 and 5, arranged at a distance from one another, 
which cover the parallel waveguides 2 and 3. The two control electrodes 4 
and 5 are separated from one another above the interstices 6 between the 
parallel waveguides by a continuous gap 7 having a width b, extending in 
the longitudinal direction of the parallel waveguides 2 and 3, which gap 7 
corresponds approximately to the distance n between the parallel 
waveguides 2 and 3. 
The refractive index of the substrate 1 is reduced in the interstices 6 
between the parallel waveguides 2 and 3 beneath the gap 7 of the control 
electrodes 4 and 5, which can be employed as a mask, so that a 
self-adjusting method is provided. In order that the regions of the upper 
surface 11 of the substrates 1 outside of the gap 7, which are not covered 
by the control electrodes 4 and 5, are not exposed to any action reducing 
the refractive index, an auxiliary mask 8 having a window 81 is 
additionally arranged above the control electrodes 4 and 5, the window 81 
being so dimensioned that the auxiliary mask 8 leaves the gap 7 between 
the control electrodes 4 and 5 exposed, but the remainder of the mask 
provides a covering for the remaining regions of the upper surface 11 of 
the substrate 1. The action lowering the refractive index, for example, an 
ion beam directed at the window 81 of the auxiliary mask 8, which ion beam 
effects an etching or implantation of the surface 1, can act in this 
manner only on the portion of the substrate exposed in the gap 7. A 
refractive index reduction thereby results in the interstice 6 between the 
parallel optical waveguides 2 and 3, which reduction is to be carried out 
up to a specific depth of the substrate. 
The following is provided for an improved understanding of the meaning and 
purpose of the refractive index reduction in the interstice 6 between the 
parallel optical waveguides 2 and 3. In order to reach very high 
bandwidths of 5 GHz and more with low control voltages, the directional 
coupler must be so dimensioned that the length L of the coupler is equal 
to the coupling length L.sub.O, which corresponds to the length of the 
distance over which complete crossover coupling takes place. If typical 
monomode glass fibers, for example, for the wavelength .lambda.=1.3-1.5 
.mu.m are to be butt coupled to the optical waveguides 2 and 3 of the 
directional coupler with low coupling loss, then the near field of the 
waveguide 2 or 3 in the substrate 1 of the electro-optical material must 
be well matched to that of the glass fiber. Typical spot radii of the 
intensity profile of the light guided in such glass fibers are 
approximately 4-5 .mu.m. 
In the case of large near-field extensions or expansions of this type in 
the waveguides 2 and 3 of the directional coupler, the coupling between 
the parallel waveguides 2 and 3 in the case of distances a in the 
micrometer range is relatively strong. This has as a consequence the fact 
that the distance a of the parallel waveguides 2 and 3 must likewise be 
selected to be relatively great in order that the coupling intensity 
becomes sufficiently small and, hence, the coupling length L.sub.O becomes 
sufficiently great. A great length L of the directional coupler and, 
because of the condition L=L.sub.O, a great coupling length L.sub.O, is 
also desired because the necessary control voltage is inversely 
proportional to the length L of the directional coupler. In the case of 
rapid directional couplers, the maximum length L is limited by the 
required 3 dB cutoff frequency fg due to the transit time effect; in the 
case of LiNbO.sub.3 modulators it amounts to L.sup.mm .apprxeq.100-150 GHz 
mm/fg.sup.GHz. 
The great distance a between the parallel waveguides 2 and 3, however, also 
requires a great width b of the gap 7 between the control electrodes 4 and 
5, whereby the required control voltage increases in relation to the value 
for optimum width b of the gap 7. 
However, if it is possible to adjust the coupling intensity largely 
independently of the near field expansion in the optical waveguides 2 and 
3, it is then possible to observe the optimum distance a between the 
parallel waveguides 2 and 3, which optimum distance a is necessary for the 
lowest operating voltage, and therefore simultaneously obtain low 
insertion or coupling loss and the lowest control voltage for the given 
cutoff frequency. This can be achieved in that, in the interstice 6 
between the two parallel optical waveguides 2 and 3, the refractive index 
of the substrate 1 is lowered to a specific depth at which the coupling 
intensity becomes largely independent of the near-field expansion in the 
waveguides 2 and 3. 
As already mentioned, the reduction of the refractive index can be 
achieved, for example, through etching of a channel by way of ion or 
plasma etching, through a suitable ion exchange process, or through an ion 
implantation. In the case of LiNbO.sub.3 components, for example, an ion 
implantation of He-ions or an ion etching in a CHF.sub.3 gas can be 
carried out. In the case of GaAlAs or InGaAsP components, the wet or dry 
chemical etching processes conventional for these materials can likewise 
be employed. 
Although I have described my invention by reference to particular 
illustrative embodiments thereof, many changes and modifications of the 
invention may become apparent to those skilled in the art without 
departing from the spirit and scope of the invention. I therefore intend 
to include within the patent warranted hereon all such changes and 
modifications as may reasonably and properly be included within the scope 
of my contribution to the art.