Note: Descriptions are shown in the official language in which they were submitted.
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The present. invention relates to selec-tive directional
couplers of the -type composed of two preferably parallel waveguides
which are to be coupled together.
The background of the invention and -the invention itself
will be better understood with reference -to the accompanying draw-
ings, in which:
Figure 1 is a pictorial diagram of the basic e]ements
of a direetional eoupler.
Figure 2 is a plan view of a basie embodiment of a coup-
ler according to the invention.
Figure 3 is a cross-sectional plan view of a coupler
similar to that of Figure 2.
Figure 4 is a phase constant vs. frequency diagram
illustrating the construction and operation of a coupler accord-
ing to the invention.
Figures 5-8 are cross-sectional views of preferred em-
bodiments of the coupler aecording to the invention.
Directional couplers whose coupling degree depends on
the frequency or wavelength of the electromagnetic oscillations
to be coupled out are required, for example, for carrier frequency
data transmission of several channels in frequency multiplex in
one and the same transmission medium. A typical application is
two-way voiee operation with optical signals on a glass fiber
employing wavelength multiplexing. In -this case, as well as in
other applieations with lower carrier frequencies transmission
in one direction takes place at a different light wavelength than
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than in -the opposite direction. At the end points of such a fiber
path, or at the repeaters, transmission takes place, as shown
in Figure 1 hereof, through a selec-tive directional coupler at
a wavelength ~0 and reception occurs from the opposite direction
at another wavelength ~e.
The selectivity of the directional coupler is such
-that the entire transmitting power at ~0 is fed into the fiber
connection and -the entire incoming power at ~e reaches the recei-
ver. The selectivity of the directional coupler, supported by
its directional effec-t, moreover reduces near-end crosstalk, so
that even with high transmitting power only a disappearingly small
portion of the transmitted power reaches the associated receiver.
As a Eurther advantage of such a transmitting-receiving
duplexer, the fundamental mode received from the fiber connection,
which for this application is a single mode fiber, can have any
desired polarization since it passes through the selective direc-
tional coupler without coupling.
It is an object of the present invention to simplify
the structure of a selective directional coupler of the above
described -type which can be realized in a simple manner.
The above and other objects are achieved, according
to the present invention, in a selective directional coupler
composed of two outer waveguides, by the provision of means for
coupling the waveguides together composed of an in-termediate
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3 ~ ~
waveguide disposed be-tween the two outer waveguiaes, the in-ter-
mediate guide being constructed and posi-tioned such that at a
selected coupling frequency its coupling mode is in phase synch-
ronism with the transmission modes in the two outer waveguides,
and wherein said -two outer waveguides are each coupled only to
said intermediate waveguide and signals can only be fed into or
out of said coupler via said ou-ter waveguides.
The basic struc-ture of a selective directional coupler
according to the invention for such and similar applications is
shown in Figure 2. Two continuous waveguides 1 and 3 of known
structure are coupled together via a waveguide section 2 disposed
therebetween and extending a]ong the transmission path z from
z = O to z = L.
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The continuous waveguides 1 and 3 may have identical
cross sections. For the purpose of calculation, it is here
assumed that the modes in waveguides 1 and 2 which are to be sel-
ectively coupled together have the same phase constants ~ 3 =
. In practice, this requirement need be met only for that fre-
quency fo, corresponding to the wavelength ~O, at which the
selective coupler is to couple all of the power from one waveguide
to the other.
Coupling of the modes between waveguides 1 and 3 is
effected via one of the modes carried by the intermediate wave-
guide 2. For the purpose of calculation, it is here assumed that
the mode in waveguide 1 couples with the mode in waveguide 2 just
as strongly as the mode in waveguide 3 couples with the mode in
waveguide 2. In practice, this requirement again need be met only
for the frequency fo, or the wavelength ~o, respectively.
~ ased on above conditions, and if losses in the coupler
can be neglecked, the following system of coupled differential
equations applies for the amplitudes A1, A~ and A3 of these modes;
normalized with respect to modal power.
dA1
dz = -i~A1 -jcA2
dA2
dz -jcA1 -i~2A2 -jcA3
dA3
dz -jcA2 -j~A3
where ~2 is the phase constant of ,he coupling mode
ln the intermediate waveguide 2t and c is the coupling
coefficient for the coupling modes in waveguides 1 and 3
with that of the intermediate waveguide.
The system of three coupled modes has three natural
waves, which travel independently of one another along the
coupling path. Their phase constants are respectively:
~; 3 + ~ + ~ 2 + 2c2; and ~ + ~ - ~ 2 ~ 2~2, where
~ 2)/2 is half the difference between the phase
1 10 constant of the modes in guides 1 and 3 and of the coupling
mode in guide 20
In the general solution for the amplitudes of the
modes in guides 1~ 2 and 3, the natural waves are superposed
as follows:
Al=-w1e~~Z~w2e-J(~+~ 2c )Z*w e~i(~ 2+2c2)z ,
A2= a I ~ w~e~~ 2c2)z
~-a W e ~i ( ~ 2~2 c2 ) z ,
A3= wle i~Z~w2e~i(~+~ 2c )Z~w e~i(~ a2+2c2)z~
If only the input of waveguide 1 is excited at z = 0,
the starting conditions are A1 = 1 and A2 = A3 = 0 at z = U.
With such excitation the amplitudes of the ~ energy of
~ 6
modes in guides l and 3 have the following absolute values,
or magnitudes, along the coupler:
IA1 1 = 1 ¦I`+e j~ (COS ~C z + ~ ~ sin ~ zl ¦
(,1~
¦A3 1 = 12 11-e; (COS ~ Z + ~Z Sin ~ +2C Z~I
Two borderline cases are of particular interest for
practical application:
1 ~2
The coupling mode in guide 2 has the same phase constant
as the two modes in guides l and 3. In this case ~ = O and
the amplitude absolute values areo
¦All = 2 ¦ 1~cos ( ~ cz31
IA3¦ 2 ¦ 1-co~ (r~cz)¦
With a phase synchronous coupling mode in guide 2 the
power thus swings back and forth between the guides l and 3
along the coupling path.
At the points:
z = (2m+1)~ c), where m = O, 1, 2....
it is carried completely by guide 3 and at the points:
z = 2m~ c) with m = O, 1, 2...
it is carried completely by guide 1. For full power transfer
from guide 1 to guide 3 the coupler is best given a length of
L = ~ ~c) (2)
2- ~ c:
Full power conversion from guide 1 to guide 3 is
possible only for ~ = O, i.e. with a phase synchronous
coupling mode. For ~ ~ O only part of the input power
from guide 1 is coupled to guide 3. In the second border-
line case, ¦~ ¦ c, only a very small amount of power is
coupled. Under this condition, the amplitude absolute
values result, in approximation from equations (1), as
follows:
... _ . ... .
¦A~ c2 ~in ~z e ~Z
¦A3 ¦ ~ c2 sin ~Z
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According to these approximations, at most c4/(4~4)
of the input power is coupled to guide 3; the remainder
remains mainly in guide 1, a small portion remains in
the coupling section 2. From guide 1 at most C2/~2 of its
input power is lost under this condition.
In order to now realize the desired selectivity, i.e.
full ~ower transfer at a frequency fo, or wavelength ~o,
respectively, and the least possible power transfer at certain
frequencies remote therefrom, the intermediate waveguide
0 section 2 should be selected whose coupling mode is phase
synchronous with the modes of guides 1 and 3 at f = fO,
but has a sufficient phase difference at the blocking
frequencies to there meet the condition:
1~ 1>~ c.
For light frequencies, all these requirements can be
met with dielectric films or strips as waveguides. These
optical film or strip waveguides are embedded, for example
as shown in Figure 3, in a transparent substance having an
index of refraction nO. In the embodiment of Figure 3,
the waveguides 1 and 3 have identical cross sections and the
same index of refraction nl > no. The intermediate
waveguide ~ has a larger index of refraction n2 > nl
and, depending on the requirement for selectivity, its cross
section should also be greater than the cross section of
each of waveguides 1 and 3.
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Figure 4 presents a dispersion diagram which depicts
the phase constant ~ of the fundamental mode in the wave-
guides 1 and 2 and the phase constants ~2 f modes in the
intermediate waveguide which may serve as coupling modes, as
a function of frequency. All phase constant curves have
their origins at the respective limit frequency on line
no2~/cO, where co is the speed of light in vacuo. For
high frequencies, each curve approaches asymptotically the
wave number of the respective waveguide material. Aside from
the phase curve of the fundamental mode of the intermediate
waveguide 2, the phase curves of all higher modes of this
waveguide intersect the phase constant curve of the funda-
mental mode in waveguides 1 and 3, the latter approaching
asymptotically the line nl2~f/co. Therefore, they all
can serve as coupling modes between the fundamental modes in
waveguides 1 and 3. At the intersection frequencies with
the fundamental mode phase constant curve of waveguides 1
and 3 they provide full power transfer between the fundamental
modes in guides 1 and 3.
( 20 Which mode is selected as the coupling mode, and how
the waveguides 1 and 3 and the intermediate waveguide 2 are
designed, depends on the values of the frequencies which are
to be coupled or are to remain decoupled, respectively. If
these frequencies differ by a large amount, a coupling mode
of lower order is selected; for a small difference between
frequencies, requiring a correspondingly higher selectivity,
a coupling mode of higher order will be selected. Selectivity
can also be increased by increasing the index of refraction
-- 10 --
in the intermediate waveguide 2 and by enlarging its cross
section. Then the phase constant curves of the coupling
modes in the intermediate waveguide 2 intersect the phase
constant curve of the fundamental mode in waveguides 1 and 3
at an increasingly larger angle. The phase difference
between these modes then increases more rapidly beginning
with ~ = 0 at the point of intersection of the curves, with
increasing deviation of the frequeney from the intersection
fre~uency.
- 10 With the aid of the example of a selective directional
coupler made of dielectric strips which are placed, as shown
in Figure 5, on a dielectric substrate S, it will be shown
which dimensions should be selected with respect to the
wavelength ~ of the lightwaves or microwaves, respectively.
A simple directional coupler can consist of two parallel
strips Stl and St2 mounted on a substrate S. Each strip has
a width b = 3.5~, a height h = 1.75~, and an index of
refraction nl = 1.5. The strips are spaced apart by a
distance a = b. The substrate S has an index of refraction
20 no = nl/l~l and couples the fundamental modes of the
strips with the coupling coeffieient c - 0.002~.
If the same strips on the same substrate are selected
for a selective directional coupler aecording to the inven-
tion and a further strip ZWT~ two to four times the width b
and with an index of refraction n2 somewhat greater than
nl is used for the intermediate waveguide, the same
coupling coefficient can be set for coupling the fundamental
modes in the outer strips via a phase synchronous coupling mode in
the intermedlate waveguide when the distance, a, hetween the
strips is selected somewhat smaller than a = b. Condition (2) is
met if L, the length of strip ZWL in direction z, = lllO~. For
light wave]eng-ths this is of the order of magnitude oE one milli-
meter.
In order to be able to use even shorter couplers in
integrated optical systems, the strips must be moved even closer
together. ~ecause the coupling coefEicient depends exponentially
upon the distance between the strips, even a slight decrease in
the distance suffices to permit drastic shortening of the coupler.
The substrate and the films or strips of a selective
directional coupler for o~tical frequencies may be produced Erom
quartz glass or other silicate glasses. In order to increase the
index of refraction of the films or strips with respect to the
index of refraction oE the substrate and particularly in order to
realize a higher index of reEraction in the intermediate wavequide
than in the two outer waveguides, the quartz glass may be doped
with germanium oxide or phosphorus oxide. An exemplary value for
2~ such a doping is 15% molar concentration oE GeO2 in SiO2 in order
to raise the refractive index by nearly 1%.
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Even qreater differences in the indices of refraction
are realized ifl for example, a substrate glass wlth a low index
of refraction is employed, the outer waveguides (1 and 3) are made
of a transparent polymer, such as, for example, polyurethane, and
the intermediate waveguide (2) is made of ~inc sulfide. For such
selective directional couplers which are to operate at optical
frequenciesl many different materials can be employed. However,
care must always be taken that they are sufficiently transparent
to the light wavelengths to be transmitted so as to keep coupling
losses low.
The form of the wavequides between which electromagnetic
wave energy is to be selectively converted as well as the form of
the intermediate wavequide is by no means limited to simple films
or strips in or on substrates; rib or ridge waveguides as well as
strip loaded film waveguides can also be used.
Figure 6 showsl as a representative example onlyl a
cross-sectional view of a selective directional coupler according
to the invention, particularly for optical frequencies, in which
the two outer wavequides are rib waveguides RL1 and RL2 and a
strip loaded film waveguide EWL serves as the intermediate wave-
guide. The base of the intermediate waveguide is formed by a die-
lectric film which is integral with the outer rib waveguides. ~he
index of refraction n1 f the fi]m and rib waveguides must be
somewhat hiqher than the index of refraction no oE the
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su~strate S and the intermediate waveguide EWL should have
an index of refraction n2 which is even higher than nl.
Selective directional couplers for microwaves can also
be constructed of dielectric strip waveguides, particularly
if millimeter waves are involved because in that case the
dielectric strips still have a relatively small cross
section. However, dielectric image guides and hollow
waveguides can also be used for this purpose. Figure 7
is a cross-sectional view of a selective directional
coupler employing image guides. Its three image guides BL1,
BL2 and BL3 are parallel to one another on a common metal
plate P. The two outer image guides BL1 and BL3 have the
same cross-sectional dimensions and the same index of
refraction nl, while the inner image guide BL2, serving as
the intermediate waveguide, has a larger cross section and
also an index of refraction n2 which is higher than nl.
Figure 8 is a cross-sectional view of a selective
directional coupler according to the invention composed of
hollow rectangular waveguides Hl, H2 and H~. The inter-
mediate waveguide H2 is coupled with the outer waveguides Hl
and H2, for example, by rows of holes Ll and L2 in the
common partitions between adjacent waveguides. The inter-
mediate waveguide H2 has a larger cross section than the
outer wave~uides, in that H2 is wider than Hl or H3, or is
partially or completel~ filled with a dielectric material.
In the embodiment shown in Figure 8, both measures, i.e. a
broader cross section for waveguide H2 than for the outer
waveguides as well as filllng with a dielectric material, are
provided for the intermediate waveguide H2. These two measures
support one another in their effect to increase selectivity.
The waveguide sections shown in Figures 5, 6 and 7 do
not need any cladding layers around the dielectric material for
operation according to the invention. The region above the dielec-
tric material should rather be air or vacuum. Any housing for
protection and handling could be placed directly below the sub-
strates S in Figures 5 and 6 or the metallic ground plane P in
Figure 7. Sideways from the dielectric material and above it any
housing should however have sufficient distance in order not to
interfere with the evanescent fields of the waves on the dielec-
tric material. These distances must be in the range of the crGss-
sectional dimensions of the waveguides or larger.
When the waveguide sections in Figures 5 and 6 are -to be
used for microwaves a metallic ground plane directly below their
substrates S can improve their performance and give more mechan-
ical strength to the structures.
The waveguide section in Figure 6 should consist of the
same or similar dielectric materials as that in Figure 5 and as
described above. Its dimensions are comparable to those disclosed
relative to Figure 5~ When designed for and applied to the trans-
mission of light waves exemplary values for the wavelengths are
~o=1.3 ~m where the material dispersion of silica fibres is
minimal and ~e=1~5 where silica fibres can have the lowest
transmission loss.
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In this example the selective coupler at the other end of a
two-way transmission system would of course have ~ ~1.5/um
and ~e-1.3~um.
In millimeter-wave applications of the waveguide section in
Figures 5, 6 and 7 their cross-sectional dimensions are
correspondingly larger. Exemplary values for the wavelength
for ~0 and ~ are near 4 mm or near 8 mm where for radio
transmission the atmosphere has transmission windows.
For still lower frequencies of the microwave spectrum the
structure in Fig.8 will usually be preferred with exemplary
values of ~0 and A e ln the range of 2 to lo cm.
It will be understood that the above description of
the present invention is susceptible to various modifications,
changes and adaptationsJ and the same are intended to be
comprehended within the meaning and range of equivalents
of the appended claims.
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