Note: Descriptions are shown in the official language in which they were submitted.
-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
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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
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. ~ . . ~ - . . ~ ,
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
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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
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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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.. , .. . : ... .. : ... .- . ... .. : . ... . . . ..
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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
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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:
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... ~ - . - . .. - " ,.. . .: , . ,,:, . . .... . . .. . . . . . . ..
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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_
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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-
