CA1057840A

CA1057840A - Electro-optical switch and modulator

Info

Publication number: CA1057840A
Application number: CA251,442A
Authority: CA
Inventors: Michel Papuchon
Original assignee: Thomson CSF SA
Current assignee: Thales SA
Priority date: 1975-04-30
Filing date: 1976-04-29
Publication date: 1979-07-03
Also published as: JPS51134643A; JPS5987B2; DE2619327C2; FR2309890A1; GB1551777A; FR2309890B1; DE2619327A1

Abstract

ELECTRO-OPTICAL SWITCH AND MODULATOR
Abstract of the Disclosure An electrically controlled light switch, which can also be used as a light modulator in integrated optical circuits is disclosed. The coupling length of a directional coupler is modified by applying a voltage between two electrodes positioned upon two rectilinear parallel waveguides made of an electro-optical material and arranged on a common substrate, thus generating a field distribution crossing the two waveguides in opposite directions and resulting in opposite signs refracti-ve index variations thereof.

Description

-Field of the invention- .
Thi~ invention relates to an electro-optical switch and modulator, and more particularly to an electro-optical :
switch ~nd modulator for integrated optical circuits.
-~ackground of the invention-The name 'lintegrated optical systems'l has become generic to monolithic thin-film structure~ designed to process light signals and obtained by techniques of depositing, dif~
fu~ion and etching, using ma~king operations and ~imilar to those employed in the manufacture of integrated electronic cir-cuits. It is possible in particular, u~ing these techniques, to build linear structure~ characteri.ed by a refractive index :
which is higher than that of the surrounding medium9 an~ forming wave guides along which light propagates in accordance with a ~eries of total reflexions or progressive refractions. .~
In the prior art, it i~ known to combine two ~uch ; :
waveguides by arranging them parallel to one another over part ~ . :
of their length in order to form directional couplers ; through the medium of the evane~cent wave phenomenon, the energy car- ;
ried in the first waveguide is transferred progressi~ely to the second waveguide and a maximum energy transfer is ob~erved at the end of a certain length known a~ the coupli.ng length, which depend~ upo;n the geometric and optical parameters of the struc-ture and in partioular upon the value of the refrac~ive indices of the materials constituting the two waveguides, as well as that of the medium ~eparating them ; subsequently, the energy -transfers progressively from the second waveguide to the first -:
and so on. It i~ also known, by utilizing an electro-optical material, for one of the material~ constituting the waveguide or the material which separate~ them, to vary the refractive index under the effect of an electric field, and thu~ by chan-ging the coupling length, to electrically control the energy ~ ' 1~5'~8~
proportion transferred from one waveguide to the other ; it i~
also possible, using this same principle, to form a light mo-dulator by arranging parallel to the waveguide which carries :;
the light-wave, a section of waveguide to which a greater or lesser proportion of said energy is transferred.
It has bee~ recognized that the solution which re-quires the minimum control voItage for a given coupler, is that which utilizes two identical rectilinear waveguides, one of them being imparted a given variation in refractive index and the other a variation of the same amplitude but opposite sign.
To achieve this condition, it has been proposed to arrange parallel to the waveguides, in the coupling ~one, three electrodes, one between the two waveguides and the two others at either side thereof ; it i~ thus possible to subject the two waveguides of the coupler to electric fields of the same value~ but oppos.ite directions. However, the need to reduce to some few wave lengths the interyal between the waveguide~
I and the coupling zone, imposes a vary narrow width and conse-quently higher re~i~tance, on the central eIectrode ; since the spread capacitance of the system constituted b~ the three ~ ~.
electrodes is not neg~igible, the long time constant of the oircuit limits the latter to ~witching or modulating frequen~
cles which are relatively low. Moreover, the presence of the central eleotrode, however narrow it may be, lead~ to an in-~ orease in the spacing between the waveguides and thi~, by re-I ~ducing the coupling effsciency, increases the length of the coupler~
It has also been proposed, again in order to achieve 1 opposite variations in refractive index in the two waveguides, i ~0 that the earlier mentioned device with three electrodes should .i ` ~
i ~ be used in order, at the time of manufacture of the coupler, ;;
I to prepolarize in two directions, perpendicular to the wave ;1 _3_ -. :

' i , ., .... , , . ~ , . ~ , ; , . . . .

. ~ . . ~ - . . ~ , f.~3 guides, the material of which the~e latter are made ; with thi3 objective in mind, the device a~ a whole i~ r~ised to a temperature in excess of the Curie point of the ferroelectric material con~tituting the waveguide~ ; then, a voltage is ap-plied between the central electrodes and the lateral electrodes whilst the assembly i~ slowly cooled. l`he central electrode is then disgarded and the control voltage applied to the two ~ole remaining lateral electrode~, create~ an electric field which, passing through the two waveguides in the ~ame direction, is co-directional in one of them with the polarization vector and is opposi-tely directed thereto in the other; this field thus brings about oppositely directed variations in refractive index in the two waveguide~ of the coupler. However~ as in the device described earlier, the temporary presence o~ a central electrode dictates a certain spacing between the two waveguides.
Moreover, in order to manufacture this kind of coupler, it is necessary to carry out a high-temperature treatment on the o-verall integrated optical circuit, and this complicate~ the ~ design and may not be compatible with the presence of other J 20 elements in the circuit.
-Summary of the invention-It i~ therefore a primary object of the present in-vention to provide a novel electro-optical switch for integra-ted optical circuits.
It is another object of the invention to provide an electro~optical switch which can be controlled by low-power electrical signals, such as signals supplied by conventional ;l integrated electronic circuits.
It is a further object of the invention td provide an electro-optical ~witch in which rapid switching can be achieved over a minimum length.
It is yet a further object of the invention to provide _ ~

, . . . .. . ...

1~3~84~
an electro-optical modulator utilizing an electro~optical switch ~ ' ' as hereinabove.
These and other ob~ect and advantages are accomplished in accordance with the invention by providing an electro-optical modulator controlled by a control voltage and which includes two parallel waveguides arranged on a same sub~trate and made of a same electro-optical material, and two parallel and cop- ~
lanar control electrodes respectively covering said two wave- ~ ;
guides over the whole of the coupling length ; the control ;
voltage being applied between said two electrodes and genera-ting an electric field in the two waveguides and the substract ;
~aid electrio field passing through the two waveguides in suba~
tantially parallel but opposite directions, and thu~ giving rise to variations in refractive index which are of the same abso-lute value but opposite sign. The two waveguides may be ar-ranged upon the surface of or inside the substrate.
-Brief description of the drawings-For a better understanding of 'the invention as well '~; ~ as other objects and further features thereof, reference ia ~
made to the following detailed disclosure of variou~ preferred~ ' ~ embodiments there of taken in conjunction wlth the accompanying '~ drawings wherein -Fig~. 1 and 2,respectively illustrate a ~ectional view and a plan view of a fir~t embodiment o~ the switch in accordance with the invention ;
Fig. 3 illustrates a sectional view of a second embodiment of the switch in accordance with the invention~ ;
-Description of a first embodiment-In Figs. 1 and 2, which respectively illu~trate a sectional view and a plan vlew of the switch in accordance with the invention, there can be seen two light ~aveguides 1 and 2 deposited upon the substrate 3. On those of their faces . .

.:

.. . .. , : , ,, - :
: .

~ ~5 7 opposite to that in contact with the substrate, the two wave-guides 1 and 2 each carry a me-~al electrode, 10 and 20 respec-tively, insulated from the waveguide by a transparent dielectric layer, respectively 11 and 21. These two waveguide~ will pre-ferably have the same width and the same thickness an~, as Fig.

2 show~, are mutually parallel over a rectilinear zone of length which is a function of the so-called coupling length parameter which will be defined later ; the distance between the paral-, lel rectilinear parts, ha~ a value d whic.h should not exceed ; 10 some few wave lengths ~calculated in the medium separating the two waveguide3, air in the case of -the figure) of the light .
transmit-ted by the waveguides. The two waveguides are cons-titu-ted by the same electro-optical material which, when sub-jected~to an electric field, has a refrac-tive index which varies as a function of the value of the applied field. The refrac- ~ .
tive index of this material is thus chosen ~o that even in ~ ;
the presence of the appli.ed electric f.ield, it remains higher : ~.
., .
than the refrac-tive index of the material of which the subs- ~ trate 3 is made. ::
.t 20 When a voltage i9 applied bet~Jeen the electrodes 10 and 20, the potential distribution thus created gives rise to an electric field distribution of the kind shown b~ the refe~
.
rence 4 in ~ig. 1 where the lines of force can be ~een crossing :the two waveguides and the ~ubstrate. This ditribution is such l~ that the electric fie~d~ hl and h2 re~pectively crossing the ;J waveguides 1 and 2, are substantially perpendicular to the plan . ~ .
~, o~ the substrate 3, equal in absolute value, and of opposite ~:
3igns. Because of the electro-op-tical nature of the material of which the waveguides 1 a~d 2 are made~, this di~tribution of / 30 the electric field line~ in the waveguides produces within the :'~ latter variations of refractive index which are 3ubstantially ;~ equal in absolute value but of oppo3ite signs.

,. ..
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.: ~ , . .. . -. . : . , ,. .:- . ,: . . .
.. , .. . : ... .. : ... .- . ... .. : . ... . . . ..

~7~
However, those skill~d in the art will appreciate that when a wave is being transmitted through a waveguide, part of ~he energy propagates outside the waveguide in the surroun-ding medium, in the form of an evane~cent wave ; the amplitude of this wave decreases exponentially considered in the direction extending away from the walls of the waveguide. If a second waveguide is disposed parallel to the fir~lt, then through the agency of this evanescent wave it progressively picks up the energy transmitted in the first wa~eguide) this the fa~ter the energy transmitted in the first waveguide, thi~ the faster the closer the two waveguides are together. At the end of a given distance known as the coupling length, thi~ depending both on the geometric and the optical parameters of the two waveguides and the medium separating them (and in particular upon the re~
fractive indices?, a maximum of energy will have been trans~
ferred from the first waveguide to the second ; beyond this coupling length, the reverse phenomenon takes place; the energy transfers progres~ively from the second waveguide to the first until the minimum value i~ reached in the second waveguide ;
any modification in~;the refractive index of one of the media, obviously acts in one way or another, on the coupling length.
In the device shown in Figs. 1 and 2, the length can be chosen equal to the coupling length in -the absence of any applied electric field. Because of the perfect symmetry of the two waveguides in the coupling zone, the transfer of I energy from the first waveguide to the second (or from the se-i cond to the first), is total. ~he application of a voltage between the electrodes 20 and 21 reduces the coupling length ~l and part of the energy is retransferred from the second wave-`~ ~0 guide to the first (or from the first to the second). The ove- ;
rall result is then that as the voltage increases the energy transferred from the first waveguide to the second (or from the second to the first), as measured at the end of the coupling .. . . . . . . .
...
:. , .
:

57~
zone, diminishes until it reaches ~ero. The coupling between the two waveguidei3 thus decreases from lOO~o to 0~0 as the applied voltage on the electrodes increases. The result would be the same if the length L had a value equal to an old multiple of the coupling length at zero field.
It is also possible to give the length ~ a value equal to an even multiple of the coupling length at zero field.
The coupling then increases from zero as the voltage applied between the electrodes increases from zero.
Thus, a device has been created which, under the con-trol of an electrical signal, makes it possible to switch part or all of the energy carried by a waveguide to the other wave-guide associated with it in the coupling zone.
It then goes without saying that if one of the wave-guides is limited to a section having as its minimum length the length ~ of the coupling ~one, the aforedescribed device makes it possible to lOO~o modulate the energy carried by the other waveguide.
-Description of a second embodiment-Fig. 3 describes a second embodiment of the invention - in which the waveguides are inserted into the substrate ; the material through which coupling is effected, is then no longer air but that of which the substra~e is made.
Considering the sectional view shown in ~ig. 3, there ; cian be seen the waveguides 1 and 2 which are disposed parallel to one another in the substrate 3 in such a fashion that one of their faces is flush with the surface of the substrate.
Two metal electrodes 10 and 20, also mutually parallel, are ar-- ranged upon the waveguides at the surface of the 3ubstrate ~, through the medium of insulating liayers 11 and 21.
To implant the waveguides 1 iand 2 in the substrate ~, the following method can be adopted:

.-~ I . .
. . .

... ~ - . - . .. - " ,.. . .: , . ,,:, . . .... . . .. . . . . . . ..

, 7~
In order to manufacture the substrate 3, a monocrys-talline wafer of lithium niobate (~i Nb 03), this being a fer-roelectric material having a rhombohedric crystalline ~tructure, i9 used ; the wafer is cut ~o that the axis of the rhombohedron constituting the cry~talline lattice, i9 disposed parallel to the direction marked C in Fig. 3, that i~ to say perpendicu-larly to the surface 30 of the substrate. Then, at the sur~
face 30 of the sub~-trate 3 (using techniques of deposition, marking and etching of a thin titanium ~ilm), two thin titanium ~ -~ilms are produced forming two bands with parallel edges, the path followed by which reproduces the path which the waveguides 1 and 2 are to conform to. ~he wafer is then heated in order to diffuse the titanium into the lithium niobate ; the titanlum, in the diffusion zone, partially substitutes the niobium in order to produce a mixed compound of the formula ~ Nbl x3 which is also ferroelectric in nature and rhomboedric in struc-ture, and has a refractive index hlgher than that of the pure niobate ; these diffused zones, having a higher refractive ;1~ index than that of the substrate, constitute the waveguides 1 and 2. If the diffusion temperature is in excess of the Curie point of the material, then the ensuing cooling phase is utili-zed in order to subject the wafer to a uniform electric field in order to uniformly polarize the wafer and thus create a :, .
mono-domain structure.
When a voltage is applied between the electrodes 10 and 20, a distribution in the electric field i9 produced9 which corresponds to that indicated by the reference 4 in Fig. 3.
~he component of the field which is in the direction C perpen~

3 dicular to the surface 30, has the same ab:olute value but 1s oppositely directed, in the two waveguides, bringing about va-~ rlations in refractive index o~ the same absolute value but 'i .

~ oppo:ite sign. Nevertheless, the existence in a direction per-_g_ ' ! :
~, .
: ' , . , , .. , ... .. . . - , . . :

pendicular to the direction C, of a non-zero field component, as well a~ the fact as the applied electric field algo causes the value of the refractive index, in that part of the ~ubs-trate 31 located between the two waveguides, to vary, produces a certain assymetry in the phenomenon ; the coupling obtained varies in accordance with the polarity of the voltage applied between the electrodes 20 and 21. The polari-ty of the voltage fu~ni~hin~ the maxim~m coupling can be deduced from the crystal-lographic orientation of the material of which the ~ub~trate is made. If this orientation is not kl10Wn9 it is a very simple matter to determine the optimum polarity experimentally by mea-suring the light intensity tran~mitted by one of the waveguides for two polarities o~ opposite signs.
--- If the metal electrodes are ~eposited directly upon the surface of the waveguides, the existence of an evane~cent wave propagating in the metal medium which i~ relatively absor-bent, can givé rise to energy losses in the coupler. To prevent this happening, it is possible, as Figs. 1 and 3 show, to inter-pose a tran~parent dielectric layer 11 and 21 between the wave-guide~ 1 and 2 and the electrodes 10 and 20, respectively.
Thi~ insulating layer is made of a material having good tran~-missivity at the wave length of the light carried by the wave-guideJ and a refractive index le~s than that of the waveguide.
Silica ~SiO2) constitute~ an ideal material in the earlier des-cribed case in which the substrate i~ made of li-thium niobate.
Still by way of non-limitative example, it is al~a possible ln the embodiment described in Fig. 3, to utilize for the substrate, lithium tantalate (LiTaO3) in which the niobium is partially substituted, again by a`diffusion operation.

' '` -10-

Claims

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. An electro-optical switch for switching radiant energy between a first and a second optical circuit under the control of a voltage, comprising:
a substrate;
a first and a second waveguide positioned over a given length in close proximity to, but spaced from one ano-ther and respectively arranged in series in said first and second optical circuits; each said waveguide being made of an electro-optical material exhibiting a refractive index higher than that of the material of said substrate and having a first portion of its lateral surface in contact with said substrate along at least said given length ;
a first and a second electrodes; at least part of said first and said second electrode respectively covering, along at least part of said given length, a second portion of the late-ral surface of said first and said second waveguide; said first and second portions being distinct; and said voltage being ap-plied between said first and second electrodes.

2. A switch as claimed in claim 1, wherein said given length is equal to the coupling length at zero field mul-tiplied by an integer at least equal to unity.

3. A switch as claimed in claim 1, wherein said first and second waveguides are rectilininear and have substantially the same size; said electro-optical material being substantially identical and having, in the absence of said voltage, substan-tially the same refractive index in both waveguides.

4. A switch as claimed in claim 1, wherein a trans-parent dielectric layer having a refractive index lower than that of said electro-optical material is arranged between res-pectively said first and second waveguides and said first and second electrodes.

5. A switch as claimed in claim 1, wherein said first and second waveguides are arranged upon the surface of said substrate and separated by air from each other.

6. A switch as claimed in claim 1, wherein said first and second waveguides are arranged inside said substrate;
said second portion of the lateral surface of said waveguides being flush with the surface of said substrate.

7. A switch as claimed in claim 1, wherein said electro-optical materials and the material of said substrate are ferroelectric materials.

8. A switch as claimed in claim 7, wherein the ma-terial constituting said substrate being lithium tantalate, niobium is partially substituted for the tantalum in said tan talate to form the material constituting said waveguides.

9. A switch as claimed in claim 7, wherein the ma-terial constituting said substrate being lithium niobate, tita-nium is partially substituted for the niobium in said niobate to form the material constituting said waveguides.

10. An electro-optical modulator for modulating the radiant energy propagating in an optical circuit under the con-trol of a voltage, comprising a substrate;
a first and a second waveguide positioned over a gi-ven length in close proximity to, but spaced from one another;
said first waveguide being arranged in series in said optical circuit; each said waveguide being made of an electro-optical material exhibiting a refractive index higher than that of the material of said substrate and having a first portion of its lateral surface in contact with said substrate along at least said given length;
a first and a second electrodes; at least part of said first and said second electrode respectively covering, along at least part of said given length, a second portion of the lateral surface of said first and said second waveguide;
said first and second portions being distinct, and said voltage being applied between said first and second electrodes.