CA1185333A - Selective directional coupler for guided waves - Google Patents
Selective directional coupler for guided wavesInfo
- Publication number
- CA1185333A CA1185333A CA000397779A CA397779A CA1185333A CA 1185333 A CA1185333 A CA 1185333A CA 000397779 A CA000397779 A CA 000397779A CA 397779 A CA397779 A CA 397779A CA 1185333 A CA1185333 A CA 1185333A
- Authority
- CA
- Canada
- Prior art keywords
- directional coupler
- waveguide
- waveguides
- dielectric
- film
- Prior art date
- Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
- Expired
Links
Classifications
-
- G—PHYSICS
- G02—OPTICS
- G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
- G02B6/00—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings
- G02B6/24—Coupling light guides
- G02B6/26—Optical coupling means
- G02B6/28—Optical coupling means having data bus means, i.e. plural waveguides interconnected and providing an inherently bidirectional system by mixing and splitting signals
- G02B6/293—Optical coupling means having data bus means, i.e. plural waveguides interconnected and providing an inherently bidirectional system by mixing and splitting signals with wavelength selective means
- G02B6/29331—Optical coupling means having data bus means, i.e. plural waveguides interconnected and providing an inherently bidirectional system by mixing and splitting signals with wavelength selective means operating by evanescent wave coupling
- G02B6/29332—Wavelength selective couplers, i.e. based on evanescent coupling between light guides, e.g. fused fibre couplers with transverse coupling between fibres having different propagation constant wavelength dependency
-
- G—PHYSICS
- G02—OPTICS
- G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
- G02B6/00—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings
- G02B6/24—Coupling light guides
- G02B6/26—Optical coupling means
- G02B6/28—Optical coupling means having data bus means, i.e. plural waveguides interconnected and providing an inherently bidirectional system by mixing and splitting signals
- G02B6/2804—Optical coupling means having data bus means, i.e. plural waveguides interconnected and providing an inherently bidirectional system by mixing and splitting signals forming multipart couplers without wavelength selective elements, e.g. "T" couplers, star couplers
- G02B6/2821—Optical coupling means having data bus means, i.e. plural waveguides interconnected and providing an inherently bidirectional system by mixing and splitting signals forming multipart couplers without wavelength selective elements, e.g. "T" couplers, star couplers using lateral coupling between contiguous fibres to split or combine optical signals
-
- G—PHYSICS
- G02—OPTICS
- G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
- G02B6/00—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings
- G02B6/24—Coupling light guides
- G02B6/26—Optical coupling means
- G02B6/28—Optical coupling means having data bus means, i.e. plural waveguides interconnected and providing an inherently bidirectional system by mixing and splitting signals
- G02B6/293—Optical coupling means having data bus means, i.e. plural waveguides interconnected and providing an inherently bidirectional system by mixing and splitting signals with wavelength selective means
- G02B6/29379—Optical coupling means having data bus means, i.e. plural waveguides interconnected and providing an inherently bidirectional system by mixing and splitting signals with wavelength selective means characterised by the function or use of the complete device
- G02B6/29395—Optical coupling means having data bus means, i.e. plural waveguides interconnected and providing an inherently bidirectional system by mixing and splitting signals with wavelength selective means characterised by the function or use of the complete device configurable, e.g. tunable or reconfigurable
-
- G—PHYSICS
- G02—OPTICS
- G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
- G02B6/00—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings
- G02B6/24—Coupling light guides
- G02B6/26—Optical coupling means
- G02B6/28—Optical coupling means having data bus means, i.e. plural waveguides interconnected and providing an inherently bidirectional system by mixing and splitting signals
- G02B6/2804—Optical coupling means having data bus means, i.e. plural waveguides interconnected and providing an inherently bidirectional system by mixing and splitting signals forming multipart couplers without wavelength selective elements, e.g. "T" couplers, star couplers
- G02B2006/2865—Optical coupling means having data bus means, i.e. plural waveguides interconnected and providing an inherently bidirectional system by mixing and splitting signals forming multipart couplers without wavelength selective elements, e.g. "T" couplers, star couplers couplers of the 3x3 type
Abstract
ABSTRACT OF THE DISCLOSURE
In a selective directional coupler composed of two outer waveguides, the outer waveguides are coupled together by an inter-mediate waveguide disposed between the two outer waveguides, the intermediate guide being constructed and positioned such that at a selected coupling frequency its coupling mode is in phase synchro-nism with the transmission modes in the two outer waveguides.
In a selective directional coupler composed of two outer waveguides, the outer waveguides are coupled together by an inter-mediate waveguide disposed between the two outer waveguides, the intermediate guide being constructed and positioned such that at a selected coupling frequency its coupling mode is in phase synchro-nism with the transmission modes in the two outer waveguides.
Description
i3~
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 3~
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 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.
J'~J~
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)
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 3~
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 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.
J'~J~
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
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
3~
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.
g 331~
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 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%.
'`'~
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 5~
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.
-~, 1 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.
, ~.L~
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.
g 331~
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 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%.
'`'~
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 5~
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.
-~, 1 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.
, ~.L~
Claims (21)
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:
1. In a selective directional coupler composed of two outer waveguides, the improvement comprising means for coupling said waveguides together composed of an intermediate waveguide disposed between said two outer waveguides, said intermediate guide being constructed and positioned such that at a selected coupling frequency its coupling mode is in phase synchronism with the trans-mission modes in said two outer waveguides, and wherein said two outer wave-guides are each coupled only to said intermediate waveguide and signals can only be fed into or out of said coupler via said outer waveguides.
2. Directional coupler as defined in claim 1 wherein said two outer wave-guides are constituted by dielectric films and said intermediate waveguide is constituted by a further dielectric film disposed between said films con-stituting said outer waveguides.
3. Directional coupler as defined in claim 1 wherein said two outer waveguides are dielectric strip conductors, and said intermediate waveguide is constituted by a dielectric film.
3. Directional coupler as defined in claim 1 wherein said two outer waveguides are dielectric strip conductors, and said intermediate waveguide is constituted by a dielectric film.
3. Directional coupler as defined in claim 1 wherein each of said waveguides is constituted by a respective strip of dielectric material.
17
5. Directional coupler as defined in claim 4 further com-prising a substrate of dielectric material on which said strips are mounted and having a lower index of refraction than said strips.
6. Directional coupler as defined in claim 5 wherein the index of refraction of at least one said material is variable.
7. Directional coupler as defined in claim 4 further comprising a substrate of dielectric material in which said strips are embedded and having a lower index of refraction than said strips.
8. Directional coupler as defined in claim 7 wherein the index of refraction of at least one said material is variable.
9. Directional coupler as defined in claim 1 wherein said outer waveguides comprise ribs of dielectric material on a film of dielectric material, and said intermediate waveguide comprises a rib of dielectric material on a film of dielectric material.
10. Directional coupler as defined in claim 9 further comp-rising a substrate of dielectric material on which said film asso-ciated with said outer waveguides is mounted and having a lower index of refraction than said waveguides.
11. Directional coupler as defined in claim 9 or 10 wherein the index of refraction of at least one said material is variable.
12. Directional coupler as defined in claim 1 further com-prising a dielectric film, and wherein said two outer waveguides are constituted by ribs on said dielectric film and said inter-mediate waveguide is constituted by a strip forming a strip loaded film waveguide mounted on said film.
13. Directional coupler as defined in claim 12 wherein said outer waveguides are dielectric rib and further comprising a diel-ectric substrate on which said waveguides are supported via said film and having a lower index of refraction than said waveguides or said film.
14. Directional coupler as defined in claim 1 wherein each said waveguide is an image line, and further comprising a metal plate on which said waveguides are mounted.
15. Directional coupler as defined in claim 1 wherein the coupling distance between each said outer waveguide and said in-termediate waveguide is variable.
16. Directional coupler as defined in claim 1 wherein each said waveguide is a hollow waveguide, and further comprising two partitions each interposed between said intermediate waveguide and a respective outer waveguide and provided with a row of holes via which said respective outer waveguide is coupled to said inter-mediate waveguide.
17. Directional coupler as defined in claim 16 wherein each said waveguide is a rectangular waveguide.
18. Directional coupler as defined in claim 16 or 17 further comprising a dielectric insert at least partially filling the cross section of said inter-mediate waveguide and extending along the entire length of said coupler.
19. Directional coupler as defined in claim 1, 9 or 10 wherein said outer waveguides extend parallel to one another.
20. Directional coupler as defined in claim 12, 13 or 14 wherein said outer waveguides extend parallel to one another.
21. Directional coupler as defined in claim 16 or 17 wherein said outer waveguides extend parallel to one another.
Applications Claiming Priority (2)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| DE19813108742 DE3108742C2 (en) | 1981-03-07 | 1981-03-07 | Selective directional coupler |
| DEP3108742.6 | 1981-03-07 |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA1185333A true CA1185333A (en) | 1985-04-09 |
Family
ID=6126627
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA000397779A Expired CA1185333A (en) | 1981-03-07 | 1982-03-08 | Selective directional coupler for guided waves |
Country Status (5)
| Country | Link |
|---|---|
| JP (1) | JPS57161706A (en) |
| CA (1) | CA1185333A (en) |
| DE (1) | DE3108742C2 (en) |
| FR (1) | FR2501383B1 (en) |
| GB (1) | GB2096790B (en) |
Families Citing this family (9)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| AU567022B2 (en) * | 1981-12-16 | 1987-11-05 | Polaroid Corporation | Optical resonant cavity filters |
| EP0130223B1 (en) * | 1983-06-25 | 1988-11-02 | ANT Nachrichtentechnik GmbH | Symmetrical waveguide coupler |
| CA1273515A (en) * | 1985-12-30 | 1990-09-04 | Colin Anderson Millar | Optical coupling |
| DE3806824A1 (en) * | 1988-03-03 | 1989-09-14 | Ant Nachrichtentech | Electro-optical transmission system |
| DE4111095C1 (en) * | 1991-04-05 | 1992-05-27 | Ant Nachrichtentechnik Gmbh, 7150 Backnang, De | |
| DE4234486C1 (en) * | 1992-10-13 | 1993-09-30 | Ant Nachrichtentech | Arrangement for splitting an optical input signal into two signals with mutually orthogonal polarization |
| DE4234485C1 (en) * | 1992-10-13 | 1993-09-30 | Ant Nachrichtentech | Device with two optical inputs and four optical outputs and polarization diversity receiver for optical heterodyne reception |
| GB9301052D0 (en) * | 1993-01-20 | 1993-03-10 | Marconi Gec Ltd | Optical devices |
| DE19532470B4 (en) * | 1995-09-02 | 2005-03-03 | Eads Radio Communication Systems Gmbh & Co.Kg | Selective multikopter |
Family Cites Families (20)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| US2532317A (en) * | 1947-08-25 | 1950-12-05 | Sperry Corp | Ultra high frequency coupler |
| BE549131A (en) * | 1955-06-30 | |||
| BE561886A (en) * | 1956-11-19 | |||
| US2951218A (en) * | 1957-02-19 | 1960-08-30 | Itt | Directional couplings |
| US3518576A (en) * | 1967-06-27 | 1970-06-30 | North American Rockwell | Crossed guide directional coupler |
| DE1591704A1 (en) * | 1967-07-21 | 1971-03-04 | Telefunken Patent | Directional coupler in stripline technology |
| US3512110A (en) * | 1968-05-06 | 1970-05-12 | Motorola Inc | Microstrip-microwave coupler |
| DE1964291C3 (en) * | 1968-08-07 | 1979-11-29 | Western Electric Co., Inc., New York, N.Y. (V.St.A.) | Band stop filter for optical waves |
| US3589794A (en) * | 1968-08-07 | 1971-06-29 | Bell Telephone Labor Inc | Optical circuits |
| GB1294502A (en) * | 1969-05-24 | 1972-11-01 | Hitachi Electronics | Frequency variable directional filter |
| US3798575A (en) * | 1972-12-14 | 1974-03-19 | Rca Corp | Microwave transmission line and devices using multiple coplanar conductors |
| US3806226A (en) * | 1973-03-16 | 1974-04-23 | Bell Telephone Labor Inc | Light-guiding interconnections for thin-film optical circuitry |
| DE2423448A1 (en) * | 1974-05-14 | 1975-11-27 | Siemens Ag | DIRECTIONAL COUPLER |
| DD118753A1 (en) * | 1975-03-24 | 1976-03-12 | ||
| US4091343A (en) * | 1975-06-30 | 1978-05-23 | Epsilon Lambda Electronics Corp. | Insular waveguide directional coupler |
| US4019051A (en) * | 1975-12-24 | 1977-04-19 | Bell Telephone Laboratories, Incorporated | Directional optical waveguide couplers |
| JPS53137160A (en) * | 1977-05-06 | 1978-11-30 | Nippon Telegr & Teleph Corp <Ntt> | Optical filter of directivity coupler type |
| JPS54104359A (en) * | 1978-02-03 | 1979-08-16 | Hitachi Ltd | Photo coupling circuit |
| US4210881A (en) * | 1978-11-09 | 1980-07-01 | The United States Of America As Represented By The Secretary Of The Navy | Millimeter wave microstrip triplexer |
| JPS55103509A (en) * | 1979-02-05 | 1980-08-07 | Nippon Telegr & Teleph Corp <Ntt> | Photo coupler |
-
1981
- 1981-03-07 DE DE19813108742 patent/DE3108742C2/en not_active Expired
-
1982
- 1982-03-04 JP JP3316282A patent/JPS57161706A/en active Granted
- 1982-03-04 FR FR8203638A patent/FR2501383B1/en not_active Expired
- 1982-03-04 GB GB8206427A patent/GB2096790B/en not_active Expired
- 1982-03-08 CA CA000397779A patent/CA1185333A/en not_active Expired
Also Published As
| Publication number | Publication date |
|---|---|
| GB2096790A (en) | 1982-10-20 |
| FR2501383B1 (en) | 1986-04-04 |
| GB2096790B (en) | 1984-08-01 |
| JPH0522209B2 (en) | 1993-03-26 |
| FR2501383A1 (en) | 1982-09-10 |
| JPS57161706A (en) | 1982-10-05 |
| DE3108742A1 (en) | 1982-09-23 |
| DE3108742C2 (en) | 1985-11-14 |
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