GB2096790A

GB2096790A - Selective directional coupler

Info

Publication number: GB2096790A
Application number: GB8206427A
Authority: GB
Original assignee: Licentia Patent Verwaltungs GmbH; Licentia Oy
Current assignee: Licentia Patent Verwaltungs GmbH; Licentia Oy
Priority date: 1981-03-07
Filing date: 1982-03-04
Publication date: 1982-10-20
Also published as: FR2501383A1; GB2096790B; FR2501383B1; JPS57161706A; DE3108742C2; DE3108742A1; CA1185333A; JPH0522209B2

Abstract

A selective directional coupler comprises two optical wave guides (RL1, RL2) to be coupled together, and a further optical wave guide (EWL) arranged therebetween in such a manner that its coupling wave at the desired coupling frequency is synchronous in phase with the waves in the two other wave guides. <IMAGE>

Description

SPECIFICATION Selective directional coupler The present invention relates to a selective directional coupler and has particular reference to a coupler comprising two wave guides to be coupled together and preferably extending parallelly.

Directional couplers, the degree of coupling of which depends on the frequency or wave length of the electromagnetic oscillation to be coupled out, are needed in, for example, the carrier frequency communications transmission of several channels in frequency multiplex in one and the same transmission medium. A typical application is in the duplex speech operation by optical signals over a glass fibre in wave length multipiex. In this case, transmitting in one direction is done by a light wave length different to that in the opposite direction. At the end points of such a fibre path or at repeaters, transmitting is done at a wave length A1 and receiving at another wave length 2 from the opposite direction through the selective directional coupler, as in Fig. 1.

The selectivity of the directional coupler ensures that the entire transmitting power at RO is fed into the fibre and the entire arriving power at Re reaches the receiver. The selectively of the directional coupler supported by its directional effect also reduces the near-end crosstalk so that only a negligibly small part reaches the receiver even from a high transmitting power.

As a further advantage of this send-receive duplexer, the fundamental waver which is received from the single-wave fibre for this application, can have any desired polarisation, since it passes through the selective directional coupler without any coupling.

According to the present invention there is provided a selective directional coupler for intercoupling optical waves, comprising two first wave guides for conducting optical waves and a second wave guide so constructed and arranged between the two first wave guides as to conduct an optical wave which at a predetermined coupling frequency is synchronous in phase with the waves conducted in the two first wave guides thereby to couple with such waves.

Embodiments of the present invention will now be more particularly described by way of example and with reference to the accompanying drawings, in which: Fig. 1 is a diagram showing wave length relationships; Fig. 2 is a schematic view of the general form of a selective directional coupler embodying the present invention; Fig. 3 is a schematic view of a coupler according to a first embodiment of the invention; Fig. 4 is a diagram showing the phase and frequency relationships of waves conducted in a coupler embodying the present invention; Fig. 5 is a schematic cross-section of a coupler according to a second embodiment of the invention; Fig. 6 is a schematic cross-section of a coupler according to a third embodiment of the invention; Fig. 7 is a schematic cross-section of a coupler according to a fourth embodiment of the invention; and Fig. 8 is a schematic cross-section of a coupler according to a fifth embodiment of the invention.

Referring now to the drawings, there is shown in Fig. 2 the basic form of a selective directional coupler embodying the present invention, the coupler comprising two continuous wave guides 1 and 3 coupled one with the other through a wave guide portion 2 lying therebetween over the path from z = 0 to z = L.

The continuous wave guides 1 and 3 can have an identical cross-section. For the purpose of the following calculations, it is presupposed only that the waves in the guides 1 and 3, which are to be selectively coupled together, have the same phase constants p 3 P, = /3. This condition need in practice be fulfilled only for that frequency f0 or wave length RO at which the entire power is to be coupled over from one to the other wave guide by selective coupling.

The intercoupling of the waves in the guides 1 and 3 takes place through one of the waves conducted by the intermediate wave guide 2. It is presupposed for the purpose of the calculations that the wave 1 couples with the wave 2 just as strongly as the wave 3 with the wave 2. This condition in practice again needs to be fulfilled only for the frequency f0 or wave length SO.

With the named presumptions and if losses in the coupler are ignored, the following system of linked differential equations applies for the amplitudes A1, A2 and A3 of the named waves: dA, = jpA, jcA2 dz dA2 = jcA1 ig32A2 jcA3 dz dA3 -jcA2 jpA3 dz In that case, p2 is the phase constant of the coupling wave 2 in the intermediate wave guide and c the coupling co-efficient for the coupling of the waves 1 and 3 by this coupling wave.

The system of three coupled waves has 3 natural wave lengths which wander indeDendentiv nf one another along the coupling path Their phase constants are

wherein S = (ssss2)/2 designates half the frequency between the phase constants of the waves 1 and 3 and the coupling wave 2.

In the general solution for the amplitudes of the waves 1,2 and 3, the natural wave lengths superimpose in the following manner:

If only the entry of the wave guide 1 is excited at z = o, the initial conditions read A1 = 1 and A2 = A3 = O at z = O. With this excitation, the waves 1 and 3 have the following amplitude amounts along the coupler:

Two limiting cases are of particular interest for the practical application: 1 P2 = P The coupling wave 2 has the same phase constant as both the waves 1 and 3. In this case, 8 = 0, and the amplitude amounts read

With the coupling wave 2 synchronous in phase, the power oscillates to and fro along the coupling path between the waves 1 and 3.

When

withm=0, 1,2..., it is conducted entirely from the wave 3 and when

with m=0, 1,2..., entirely from the wave 1. For full power conversion from wave 1 to wave 3, the coupler best has the length of

2. 161 c: Full power conversion from wave 1 to wave 3 is thus possible with the coupling wave synchronous in phase only for 8 = o. For S + 0 only a part of the input power of the wave 1 is converted each time into the wave 3. In the second limiting case of 181 c, even only very little is transmitted.

Under this condition, there namely follows from the equations (1) approximately for the amplitude amounts

According to this approximation, at most still only the part C4/(4S4) of the input power is converted into the wave 3; the rest remains mainly in wave 1 , with a small part also in the coupling wave 3. The wave 1 under this condition, however, loses only at most the part C2/82 of its input power.

In order to achieve the desired selectivity, i.e. full power conversion at a frequency f0 or wave length A0 and as little power conversion as possible at certain frequencies apart therefrom, the intermediate wave guide is selected so that the coupling wave 2 thereof is synchronous in phase with the waves 1 and 3 at f = fO, but has sufficient phase difference at the blocking frequencies in order to fuifill the condition |S| 181 c.

For optical frequencies, these requirements can be fulfilled by dielectric foils or strips as wave guides. These optical foil or strip wave guides can be, for example as shown in Fig. 3, embedded in a transparent substance with a refractive index nO. The wave guides 1 and 3 in the example of the Fig. 3 have the same cross-section and the same refractive index n, > nO. The intermediate wave guide 2 has a greater refractive index n2 > n1 and also its cross-section must, in correspondence with the selectivity requirement, be greater than the cross-section of the wave guides 1 and 3.

Fig. 4 shows the phase constants of the fundamental wave ss in the wave guides 1 and 3 and of waves in the intermediate wave guide 2, which can serve as coupling waves, as a function of the frequency in the dispersion diagram. All phase curves have their origin on the line n,2rr/c, at the respective fundamental wave for the wave number in the surrounding medium with the refractive index nO. In that case, c0 is the speed of light in vacuum. For high frequencies, they asymptotically approach the wave length of the respective wave guide substance. Apart from the phase curve of the fundamental wave of the intermediate wave guide, the phase curves of all higher waves of this wave guide intersect the phase curve of the fundamental wave in the wave guides 1 and 3.They can therefore all serve as coupling waves between the fundamental waves in wave guides 1 and 3. At the frequencies of insection with the fundamental wave phase curve of wave guides 1 and 3, they make possible a full power conversion between the fundamental waves in 1 and 3.

Which wave is chosen as the coupling wave and how the wave guides 1 and 3 and the intermediate wave guide 2 are correspondingly structured depends on the position of the frequencies which are to be coupled or remain decoupled. With greater spacing between these frequencies, a coupling wave df low order is chosen, whereas with smaller frequency spacing and correspondingly higher sharpness of separation a coupling wave of higher order is chosen. The sharpness of separation can be increased by increasing the refractive index in the intermediate wave guide and by increasing its cross-section. The phase curves of the coupling waves in the intermediate wave guide then intersect the phase curve of the fundamental waves in the wave guides 1 and 3 at an increasingly larger angle.The phase difference between these waves then increases, starting from 8 = O at the intersection of the curves, with increasing deviation of the frequency from the synchronous frequency.

By reference to an example of a selective directional coupler with dielectric strips, which can, as shown in Fig. 5, rest on a dielectric substrate S, it will be shown which dimensions are to be chosen in relation to the wave length A of the light waves or microwaves. A simple directional coupler of two parallel strips Stl and St2 of the width b = 3.55 and of the height h = 1 .75R with the refractive index n1 = 1.5 at a mutual spacing a = b on a substrate S of the refractive index nO = n,/1. 1 couples the ground waves of the strips with the coupling co-efficient c = 0.002 ;1.When the same strips on the same substrate are chosen for the selective directional coupler and a strip two to four times as wide with somewhat greater refractive index than n, for the intermediate wave guide ZWL, the same coupling co-efficient for the coupling of the fundamental wave in the outer strips with a phasesynchronous coupling wave in the intermediate wave guide ZWL can be set in that the spacing between the strips is chosen to be somewhat smaller than a = b. The condition (2) is then fulfilled when L = 111 0A. In the case of light wave lengths, that is in the order of magnitude of one millimetre. In order to manage with still shorter couplers in an integrated optical system, the strips must be moyed even closer together. Because the coupling co-efficient depends exponentially on the spacing of the strips, very little reduction in spacing is sufficient to substantially shorten the coupler.

The substrate and the foils or strips of a selective directional coupler for optical frequencies can be made of quartz glass or other silicate glasses. In order to increase the refractive index of the foils or strips compared with the refractive index of the substrate, and particularly in the intermediate wave guide to provide a higher refractive index than in both the outer wave guides, it is possible to dope the quartz glass with germanium or phosphoroxide.

An even greater refractive index difference can be achieved if, for example, a low-refracting substrate glass is used and the outer wave guides are made from a transparent polymer, for example polyurethane, and the intermediate wave guide from zinc sulphide. A wide variety of materials are usable for such selective directional couplers which are to operate at optical frequencies. It is, however, always necessary to ensure that they are sufficiently transparent to the light wave lengths to be transmitted in order to keep the coupling losses low.

In addition, the shape of the wave guides between which electromagnetic wave energy is to be selectively converted, and the shape of the intermediate wave guide, are not restricted to simple foils or strips in or on substrates. Rib and bead guides as well as strip-loaded foil wave guides can be used. Fig.

6 shows by way of a representative example, the cross-section through a selective directional coupler, particularly for optical frequencies, in which both outer wave guides are rib guides RL1 and RL2, respectively and a strip-loaded foil wave guide EWL serves as the intermediate wave guide, the base of which is formed by the same dielectric foil as that of the outer rib guides. The refractive index n1 of foil and ribs must be somewhat greater than the refractive index nO of the substrate S and the strip should have a refractive index n2 which is greater than n1.

Selective directional couplers for microwaves can also be constructed with dielectric strip wave guides, particularly when millimetre waves are concerned, because the dielectric strips for this still have a relatively small cross-section. However, dielectric image guides and hollow guides are also usable.

Fig. 7 shows a selective directional coupler of image conductors in cross-section. The three image conductors BL1, BL2 and BL3 thereof run parallel to each other on a common metallic plate P. Both outer image conductors BL1 and BL3 have the same cross-sectional dimensions and the same refractive index, while the inner image conductor BL2, serving as the intermediate wave guide, has a greater cross-section and also a refractive index n2 which is greater than n Fig. 8 shows a selective directional coupler with rectangular hollow guides H1, H2, and H3 in cross-section. The intermediate hollow guide H2 is coupled with the outer hollow guides H1 and H3, for example through rows of holes L1 and L2 in the common separating walls. The intermediate hollow guide H2 has a wider cross-section than the outer hollow guides H1 and H3 or is partially or entirely filled by a dielectric. In the embodiment of the Fig. 7, the measures of wider cross-section than the outer hollow guides as well as filling of the intermediate hollow guide H2 by a dielectric are both present.

Both measures enhance each other in their effect to increase the selectivity.

Claims

1. A selective directional coupler for intercoupling optical waves, comprising two first wave guides for conducting optical waves and 9 second wave guide so constructed and arranged between the two first wave guides as to conduct an optical wave which at a predetermined coupling frequency is synchronous in phase with the waves conducted in the two first wave guides thereby to couple with such waves.

2. A coupler as claimed in claim 1, wherein the two first wave guides extend substantially parallel to each other.

3. A coupler as claimed in either claim 1 or claim 2, wherein the first and second wave guides are intercoupled exclusively by the second wave guide,

4. A coupler as claimed in any one of the preceding claims, wherein each of the wave guides comprises a respective dielectric foil.

5. A coupler as claimed in any one of claims 1 to 3, wherein each of the first wave guides comprises a respective dielectric strip conductor and the second wave guide comprises a dielectric foil.

6. A coupler as claimed in any one of claims 1 to 3, wherein each of the wave guides comprises a respective dielectric strip.

7. A coupler as claimed in claim 6, wherein the dielectric strips are mounted upon a dielectric substrate having a low refractive index.

8. A coupler as claimed in claim 6, wherein the dielectric strips are rebated into a dielectric substrate having a low refractive index.

9. A coupler as claimed in either claim 1 or claim 2, wherein each of the wave guides is formed by a respective rib projecting from a dielectric foil.

1 0. A coupler as claimed in claim 9, wherein the dielectric foil is mounted upon a dielectric substrate having a low refractive index.

11. A coupler as claimed in either claim 1 or claim 2, wherein each of the first wave guides is formed by a respective rib projecting from a dielectric foil and the second wave guide comprises a striploaded foil wave guide mounted on the dielectric foil.

12. A coupler as claimed in claim 1 wherein the dielectric foil is mounted upon a substrate having a low refractive index.

1 3. A coupler as claimed in either claim 1 or claim 2, wherein the wave guides comprise respective image conductors mounted on a common metal plate.

14. A coupler as claimed in any one of the preceding claims, wherein the spacing of the second wave guide from the first wave guides is adjustable.

1 5. A coupler as claimed in any one of claims 7, 8, and 1 0, wherein the refractive index of the dielectric substrate is adjustable.

1 6. A coupler as claimed in either claim 1 or claim 2, wherein each of the wave guides comprises a hollow member, the hollow member of the second wave guide being separated from the hollow members of the first wave guide by common apertured walls.

17. A coupler as claimed in claim 16, wherein each of the hollow members is of rectangular crosssection.

18. A coupler as claimed in claim 16, wherein the hollow member of the second wave guide is provided in its interior with a dielectric insert which at least partially fills the interior cross-section of the member and which extends along substantially the entire length of the coupler.

1 9. A selective directional coupler substantially as hereinbefore described with reference to Figs.

1, 2 and 4 of the accompanying drawings.

20. A selective directional coupler substantially as hereinbefore described with reference to any one of Figs. 3, 5, 6, 7 and 8 of the accompanying drawings.