OPTICAL FUSED COUPLERS

Abstract: A fused optical fibre coupler in which one of the fibres has a propagation constant which in the coupling region is slightly different from that of the other. The propagation constant difference can be arranged to provide selected splitting ratios over a relatively wide wavelength range.

Description: OPTICAL FUSED COUPLERS DESCRIPTION This invention relates to optical fibre couplers. The invention has particular but not exclusive application to fused optical fibre couplers. A fused optical fibre coupler can be formed by taking two or more optical fibres, twisting them around each other a few times, and heating the twisted portion while pulling so that the fibres along that portion taper and fuse together. In a coupler of. this type light signals propagating in say one of a number of optical fibres can be coupled into a number of other fibres. An example of the way in which fused optical fibre couplers can be formed is described in European Patent Application No. 174014 (Hitachi) Fused optical fibre couplers are attractive for use in optical fibre communications networks because they have low loss, good temperatu * re stability, mechanical rigidity and ease of manufacture- However the coupling ratio of these devices is wavelength dependent. For example in a coupler where say one port couples to two ports, che splitting ratio between the two receiver fibres may be 50/50% at 1.3 AUΠ and anywhere between 80 to 20% and 99 to 1% at 1-52 UΠ depending upon whether the fibres are lightly fused or well fused. There is clearly a need for an optical fibre coupler in which the splitting ratio is not critically wavelength dependent and it is an object of the present invention to provide such a coupler. According to one aspect of the present invention there is provided an optical fibre coupler in which light propagating in one fibre is arranged to be coupled into one or more other fibres, wherein the propagation constant of the receptor fibre or fibres in the coupling region is arranged to differ slightly from that of the one fibre. The propagation constant of an optical fibre is a characteristic which can be defined j for optical radiation of a given wavelength propagation in the fibre,as the " angular frequency divided by the velocity of a point of constant phase within the fibre. It has been found that in a coupling between two fibres of slightly different propagation constant the maximum coupled power from one fibre to the other can be made to have a value less than 100%. By appropriately selecting the difference in propagation constant the maximum coupled power can be arranged to have for example a value of 50%. Furthermore at the maximum value the variation of coupled power with wavelength is at its least sensitive so that the coupler will have that maximum coupling value over a relatively wide operating wavelength range. Thus it is possible to construct an optical fibre coupler in which light propagating in one fibre is split into two fibres with the splitting ratio between the two receptor fibres being essentially constant over a relatively wide operating wavelength range. The difference in propagation constant can be achieved by using fib.res o ' £ different diameter or fibres of different profile or by tapering one of two identical fibres more than the other. The coupler may be a fused fibre coupler. According to another aspect of the present invention there is provided a method of forming a fused optical fibre coupler in which two fibres are twisted together along a portion of their length, the propagation constants of the fibres in the twisted region being slightly different, optical radiation is launched into one or both of said fibres whilst the twisted region is heated and pulled, and the optical radiation transmitted through the coupling region is monitored. The invention will be described now by way of example only with particular reference to the accompanying drawings. In the drawings:- Figure 1 illustrates the spectral response of a typical known fused optical fibre coupler; Figure 2 is a series of curves showing a comparison of coupling power response curves for various fused optical fibre couplers, and Figure 3 shows spectral loss curves for couplers in accordance with the present invention. The invention will be exemplified by reference to fused optical fibre couplers although its application is not restricted to such couplers. Figure 1 of the drawings shows the wavelength response of a typical known fused optical fibre coupler. The curve applies to a coupling between two optical fibres which have the same propagation constant and in which at its maximum the coupled optical power is substantially 100%. This figure shows that the variation of coupled power with wavelength is a maximum when the coupler is fabricated to have approximately 50% coupling at a particular wavelength. It can be seen from Figure 1 that a wavelength change _ ^in the region of maximum coupled power produces a relatively small change Δ PI in coupled power whilst the same wavelength change in the region of 50% coupled power produces a much larger variation in coupled power Δ P2. Thus provided the device is operating at or in the region of the maximum coupled power small changes of wavelength do not have a serious effect. This, however, is not always the case. Consider a device which comprises two fibres fused together to form a 2 x 2 port device. The effect of wavelength changes can be considered with reference to Figure 2a which shows the variation of coupled power with coupler length. It is desirable in a 2 x 2 port device to have approximately a 50/50 split of power coupled to the two receptor fibres. If the coupler has been drawn in fabrication so that the 50/50 split occurs at 1.52 xϋn then it can be seen from line 12 on Figure 2a that at 1-3 JSD. the splitting ratio would be considerable different. This is clearly not satisfactory for operation in a network which needs to operate at both 1.52 Urn and 1.3 urn. In the present technique an optical fibre coupler is fabricated by fusing the coupling region between two optical -fibres which in the coupling region have slightly different propagation constants when measured ac a given wavelength. The difference in propagation constant can be achieved by arranging the receptor fibres to have a different diameter, or a di-fferent profile, or by tapering one of two identical fibres more than the other. In an example of the present technique the coupler has been fabricated by pre-tapering one of the fibres- The actual fusing technique will not be described in detail but can be for example the method described in Application No. 174014 The difference in propagation constant can be selected to produce a selected degree of coupling power. It is possible using this technique to produce for example, a 2 x 2 port device in which apprσximately 50/50% split of optical power can be achieved with radiation of both 1.3 urn and 1.52 urn. To illustrate the effects of fabricating a 2 x 2 port fused fibre coupler with fibre of different propagation constants two different tapers, taper A and taper B, were produced. Each taper had a gaussian type diameter variation described by D(Z) = D maχ - Do Exp (-aZ 2 ) where D(Z) is the fibre diameter at a position Zmm, Dmax is the diameter of the untapered fibre (125 iim) , Do is the reduction in fibre diameter at the waist of the taper and a is a taper length parameter. For the tapers A and B, Do equals 9 and 15 am and a equals 2 0-037 and 0.035 mm respectively. The tapers were twisted with untapered fibre of constant diameter and the pairs heated and pulled in a manner similar to the method described in European Application No. 174014 The coupling process was monitored throughout the pulling operation by launching 1.3 am and 1.52 urn radiation into an input port and by measuring the power at these wavelengths in both output ports. The coupling power response curves for the two tapers are shown in Figures 2b and 2c respectively. It will be seen that these curves show that the fibres were pulled further than the normal stopping point to demonstrate the coupling behaviour over a large coupler length range- It can be seen that the difference in propagation constant produced by the taper leads to incomplete power transfer between the fibres. The larger this difference the smaller is the total power transferred as shown by comparison of Figures 2a and 2b- It can be seen from Figure 2b that at the first region of intersection marked X of the 1.3 and 1.5 urn curves the coupler has an equal coupling ratio at both wavelengths. Furthermore this coupling ratio can be selected by appropriately choosing the magnitude of the propagation constant difference. For a 2 x 2 port device the pulling step is stopped at a length - corresponding to the region X. During fabrication the region X is identified by monitoring radiation of both 1.3 and 1.52 itm- For a device produced using taper A a coupling of 50% _+ 9% can be achieved over a wavelength range 1-23 to To demonstrate the fabrication of a 50/50% coupling device using this technique, a taper similar to A was produced and pulled with standard fibre until equal coupling at both wavelengths was obtained, — 50%. The fused region was then protected by potting in a slotted silica rod using a silicone rubber compound. Cutback measurements of coupling and excess loss were then made at the two wavelengths: C = 49.9% C = 50.1% Excess loss = 0.014 dB @ 1.3 ju C = 52.2% C = 47.8% Excess loss = 0.11 dB § 1.52 u Figure 3a and 3b show cutback spectral loss measurements from input port 1 to output ports 3 and 4 respectively. The wavelength window for an insertion loss of 3dB + 0.7dB is over 300 nm.

Claims: CLAIMS 1- An optical fibre coupler in which light propagating in one fibre is arranged to be coupled into one or more other fibres, wherein the propagation constant of the receptor fibre or fibres in the coupling region is arranged to differ slightly from that of the one fibre. 2- An optical fibre coupler as claimed in claim 1 wherein the difference in propagation constant is achieved using fibres of different diameter- 3- An optical fibre coupler as claimed in claim 1 wherein the difference in propagation constant is achieved by using fibres of different profile- 4- An optical fibre coupler as claimed in claim 1 wherein the difference in propagation constant is achieved by tapering one of two identical fibres more than the other. 5. An optical fibre coupler as claimed in any preceding claim wherein the coupler is a fused fibre coupler. 6. A method of forming a fused optical fibre coupler in which two fibres are twisted together along a portion of of their length, the propagation constants of the fibres in the twisted region being slightly different, optical radiation is launched into one or both of said fibres whilst the twisted region is heated and pulled, and the optical radiation transmitted through the coupling region is monitored. 7. A method as claimed in claim 6 wherein optical radiation of more than one wavelength is launched into said fibre or fibres.

Inventors: MORTIMORE DAVID BRYAN, MORTIMORE, DAVID, BRYAN
Applicants: BRITISH TELECOMM, BRITISH TELECOMMUNICATIONS PUBLIC LIMITED COMPANY
Application year: 1986
Main classification: G02B6
IPC codes: G02B6