GB2143394A

GB2143394A - Passive optical coupler

Info

Publication number: GB2143394A
Application number: GB08417523A
Authority: GB
Inventors: Thomas Huntington Wood
Original assignee: American Telephone and Telegraph Co Inc
Current assignee: AT&T Corp
Priority date: 1983-07-15
Filing date: 1984-07-10
Publication date: 1985-02-06
Also published as: NL8402232A; BE900162A; DE3425927A1; FR2549240A1; GB2143394B; JPS6042715A; GB8417523D0

Abstract

In a multiple-access local network taps are provided at each node for coupling a small fraction of the signal between the network (13) and a local receiver (14) and for coupling a local transmitter (15) to the network (13). To minimize the amount of power extracted while at the same time maximizing the coupling efficiency between the transmitter and the network, a mode selective coupler (11) is employed along with a mode scrambler (12). The transmitter (15), operating in the preferred mode, couples a large percentage of its signal to the network where it is scrambled among all of the modes capable of propagating along the network bus (13). Likewise, the mode selective coupler (11) directs a large fraction of one mode or a small group of modes travelling in the bus (13) to the local receiver (14). Thus, the arrangement permits very high coupling efficiency for injecting signal into the network while, at the same time, the fraction of the total signal removed at each node remains low. <IMAGE>

Description

SPECIFICATION Passive optical coupler This invention relates to passive optical couplers, for example taps for use in multiple-access networks.

Low-loss taps are an essential component of a multiple-access, local area distribution network. At each node in such networks, a tap intercepts a small portion of the through-signal and couples it to a local receiver. Simultaneously, the tap also serves to couple a local transmitter to the network. Advantageously, the coupling coefficient of the coupler is made low in an effort to minimize the losses along the through-path.

Too large a loss would severely limit the number of nodes that could be placed in the network. On the other hand, low coupling reduces the efficiency with which the local transmitter is coupled to the network. This, in turn, increases the output power requirements of the transmitter.

These conflicting requirements can be satisfied using active taps in which a portion of the power extracted at each node is amplified in a parallel path and then reinjected into the network. This makes it possible to use couplers having large coupling coefficients while still experiencing a low net loss at each node (see, for example, U.S. patent 4,310,217).

One problem with active taps is their expense. One would prefer to use passive taps if possible.

According to the present invention there is provided a passive optical coupler comprising a mode selective coupler comprising a first waveguide and a second waveguide arranged to couple wave energy of a preferred propagating mode or group of modes between the first waveguide and the second waveguide and a mode scrambler located in the first waveguide for coupling wave energy between the preferred mode or group of modes and other propagating modes supported by the first waveguide.

An embodiment of the invention will now be described by way of example with reference to the accompanying drawing in which: Figure 1 shows, in block diagram, one of the nodes of a multiple-access network including a tap embodying the invention; and Figure 2 shows a tap embodying the invention.

Figure 1 shows, in block diagram, one of the nodes of a multiple-access network including: a tap 10 comprising a mode selective coupler 11 and a mode scramble 12; a local receiver 14; and a local transmitter 15. In operation, a portion of the multimode signal propagating along through-path 13 is extracted by coupler 11 and coupled to receiver 14. Coupler 11 also serves to couple the output signal from transmitter 15 onto through-path 13.

For a conventional coupler, the signal power Pr delivered to the local receiver is related to the total signal power Ps on path 13 by Pr = Psk14 (1) where k14 is the coupling coefficient between couplers ports 1 and 4.

Similarly, the amount of power P1 coupled between transmitter 15 and path 13 is given by P = Pok23 (2) where P0 is the transmitter output power; and k23 is the coupling coefficient between coupler ports 2 and 3.

In known passive taps, k14 is advantageously much smaller than one. However, reciprocity dictates that k14 = k23 and, as a result, P0 must be large for a given level P of injected signal. To avoid this, in the illustrated tap, a mode selective coupler is used such that the coupling coefficients k14 and k23 are large for only one mode, or small group of modes, and small for all other modes. In such an arrangement, k14 and k23 can be relatively large. If, for example, k23 is 0.5, as much as one-half of the transmitter output power, in the preferred mode, is coupled to the through-path. Once it is in the through-path, it is diffused among all the modes supported within the path by means of mode scrambler 12.Since a typical multimode optical fibre can support up to several hundred modes, the injected power, Pj, is divided among the multiplicity of modes so that, on average, the power in each mode is only Pj/N, where N is the number of modes. Thus, at each node, the power extracted from the through-path is only.

P = (P1/N)mk14 (3) where m, the number of preferred modes, 3 1.

As is apparent from equation (3), whereas the coupling coefficients k14, k23 for the preferred modes are relatively larger, thus providing efficient coupling for the transmitter, the fraction of the total power extracted from the network at each node is relatively small, being equal to only (m/N)k214, where N m.

Figure 2, now to be considered, shows a tap embodying the present invention. The mode selective coupler 20 comprises two dissimilarwaveguiding structures 21 and 22 which are in coupling relationship over an interval L. These can be integrated optical waveguiding strips embedded in a common substrate or, as illustrated, they can be optical fibres. While, in general, both can be multimode fibres, for purposes of explanation it is assumed that the through-path fibre 22 is a multimode fibre whereas fibre 21, which couples to the local transmitter, is a single-mode fibre.The latter supports a single propagating mode having a propagation constant P,. The multimode fibre 22, on the other hand, is supportive of a multiplicity of modes having different propagation constants Pi, P2 P3 Pm The parameters of the two fibres are chosen so that the propagation constant of the single mode fibre is equal to the propagation constant of only one (or a small group) of the modes supported by the multimode fibre, and is different from the propagation constants of all the other modes.For example, a multimode fibre, having a parabolic index distribution given by

nlm (1Am(~)2) r < am nm ={ am (4) nOm r > am; has a propagation constant Pm given by

where: r is the radial distance from the centre of the fibre; nim is the refractive index at r=0; am is the core radius; nom is the fibre index for ram, (i.e., the cladding index); Am = (nim - nom)nim, Xis the free-space wavelength of the signal; p and vary the mode numbers;

For a single mode fibre having a step index distribution given by

n = n1S r < as; (6) nos r > as; the propagation constant P, is given by

The preferred modes (p, v) are those for which Pm=Ps.

For a numerical example, the following parameters are assumed for the multimode fibre: am = 1 5 X = 1.3 nim = 1.5 nom = 1.4925.

Such a fibre is supportive of approximately 30 modes. It can also be shown that the maximum value of (2p+v+1) for the fibre is 5.4. If we arbitrarily let (2p+v+1 )=5, all the modes whose mode numbers and v satisfy this relationship will have a phase constant ssm=7.2165 To satisfy the desired mode selectivity, P5 for the single mode fibre must also be 7.2165. If we assume a single mode fibre having a core radius aS=3.5 , and, for convenience, a cladding index nO5= 1.4925, we find that the propagation constants are equal for n15=1.4952.

Clearly, other fibre designs can be devised by selecting different fibre parameters and mode numbers.

The signal coupled into the through-path is diffused among all the propagating modes by the mode scrambler 23 which can be a length of fibre specifically fabricated to have enhanced mode coupling as described, for example, in U.S. patent 3,687,514. Such fibres are deliberately designed to include variations in their transmission characteristics produced by changes in their physical dimensions and/or electrical parameters. The length L1 of the mode scrambler is selected to produce the desired level of mode mixing.

It is an advantage of the invention that the signal loss at each node can be made small. As a result, the number of passive taps that can be included in the network can be correspondingly increased and/or the distance between taps can be greater. Thus, the local loop can be larger and the number of subscribers increased.

While the invention has been described using an evanescent field coupler, it will be recognized that other types of mode selective couplers can just as readily be employed. See, for example, U.S. Patent 4.060,308 which describes an angle selective coupler for use with optical fibres.

Claims

1. A passive optical coupler comprising a mode selective coupler comprising a first waveguide and a second waveguide arranged to couple wave energy of a preferred propagating mode or group of modes between the first waveguide and the second waveguide and a mode scrambler located in the first waveguide for coupling wave energy between the preferred mode or group of modes and other propagating modes supported by the first waveguide.

2. A passive optical coupler as claimed in claim 1, wherein the first waveguide is dissimilar from the second waveguide and the first and second waveguides are in coupling relationship over an interval, the first waveguide is adapted to support a plurality of propagating modes having different propagation constants and the second waveguide is adapted to support a propagating mode having a propagation constant substantially equal to the propagation constant of the preferred modes or group of modes from the plurality of propagating modes supported by the first waveguide and different from the propagation constants of all of the other said modes,

3. A passive optical coupler as claimed in claim 2, wherein the second waveguide is adapted to support only the one said propagating mode.

4. A passive optical coupler as claimed in any of the preceding claims wherein the first and second waveguides are optical fibres and the mode scrambler comprises a section of mulimode optical fibre whose transmission characteristic varies along its length.

5. A passive optical coupler substantially as herein described with reference to the accompanying drawing.