CA1294805C

CA1294805C - Optical fused couplers

Info

Publication number: CA1294805C
Application number: CA000514848A
Authority: CA
Inventors: David Bryan Mortimore
Original assignee: British Telecommunications PLC
Current assignee: British Telecommunications PLC
Priority date: 1985-07-30
Filing date: 1986-07-29
Publication date: 1992-01-28
Anticipated expiration: 2009-01-28
Also published as: AU588351B2; KR880700284A; WO1987000934A1; GB8519183D0; EP0231248B1; JP2711351B2; JPS63501527A; US4798436A; KR910006731B1; EP0231248A1; JPH0640167B2; AU6144486A; JPH08304662A; DE3673512D1; ES2000780A6

Abstract

OPTICAL FUSED COUPLERS

ABSTRACT

A fused optical fibre coupler is disclosed 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

This invention relates to optical fibre couplers. The invention has particular but not exclusive application to fused optical fibre couplers.
5A fused optical fibre coupler can be Eormed 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 10signals 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) which was published on 12th March, 151~86.
Fused optical fibre couplers are attractive for use in optical fibre communications networks because they have low loss, good temperature stability, mechanical rigidity and ease of manufacture. However, the coup]ing 20ratio of these devices is wavelength dependent. For example in a coupler where say one port couples to two poets, the splitting ratio between the two receiver fibres may be 50/50~ at 1.3 ~m and anywhere between 80 to 20% and 99 to 1% at 1.52 ~m depending upon whether the fibres are lightly fused or well fused. There is clearly a need for an optical fibre coupler in which the slitting 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 invention, there is provided an optical fibre coupler, and method of forming such an optical fibre coupler, in which light propagating in a first fibre is arranged to he coupled into one or more second fibres within a coupling region, and at a predetermined coupling ratio. The method of the invention comprises preselecting the second optical fibres such that the propagation constant of the first fibre, within the coupling region, is slightly different from the propagation constants of the second fibres. The length of the coupling region is increased, by applying tension to the fibres, while monitoring the coupler being - formed by launching optical radiation of one or both of the preselected wavelengths, into one of the fibres and monitoring the light transmi.tted through the coupling region. The increase in the length of the coupling region is stopped when the coupling ratios for the two preselected wavelengths are ~irst equal, the optical fibres being such that the coupling ratios at the two preselected wavelengths are equal to said predetermined coupling ratio.
According to another aspect of the invention, there is provided a broadband optical coupler, formed by the above described process, having a coupling reglon in which light propagating in one fibre i5 coupled into one or more other fibres. The propagation constant within coupling region of the first fibre is arranged to differ slightly from those of the other fibres. Additionally, the coupling region is arranged to have a predetermined length which is equal to the minimum distan e ~or which the coupling ratio at two preselected wavelengths is ~.

~P~

first equal during formation of the coupler.
The propagation constant of an optical fibre is a characteristic which can be defined, 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 appropr~ately 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 '15 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 oYer a relatively wide operating wavelength range.
The difference in propagation constant can be achieved by using fibres of 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.

' .

~(','' .;, ~ . .,..~, F R O M ~1 3 ~ 10 1.Z~ L 8 ~ , IJ ~ 16 . .

The invention will be described now by way of exa~ple only wlth particular reference to the accompanying drawings. In the drawings:-Figure 1 illustrates the spectral response of a typical known fusad optical fibre coupler;
~igure 2 is a series o~ curves showing a comparison of coupling power response curves for various fused optical fibre couplers, and Figure 3 shows spectral loss curves ~or couplers in accordance with the present invention~
The invention will be exemplified by reference to used optical fibre couplers although its application is not restricted to such couplers~
Figure 1 of the drawings shows the wave}ength response of a typical known ~used optical fibre coupler~ The curve applies to a coupling between tw~
optioal fibres which have the same propagation constant and in which at lts max.imum the coupled optical power is substantially 100~ Thi ~igure shows that the variation of co~pled powex with -wavelength ls a maximum when the coupler i8 fabricated to have approximately 50% coupling at a particular wavelength~ I~ can be seen from Figure 1 that a wavelength change ~ in the region of maximum coupled power produces a rela~ively s~all change ~ Pl in (~

.

F F~ ~I M ~ a ~ . 0 7 . ~
. .~ ( coupled power whilst the same wavelength change in the region of S0~ coupled power produces a much larger variation in coupled power P2~ Thus provided the device is operating at oe in the reglon of the maximum S coupled power small changes of wavelength do not have a seriou~ e~fect~ This, however~ is not alwi3ys the case~ Consider a device which comprises t~o fi~res fuscd ~ogether to form a 2 x 2 port device~ The effect of wavelength changes can be consldered with reference to ~igure 2a which shows the variation o~
coupled power with coupler length~ It is desirable in a 2 x 2 port device to have approximately a 50/S0 split of power coupled to the two receptor ~lbre~. If the coupler has been drawn ln fabrication so that the S0/50 split occurs at 1~52 ~ then it can be seen ~rom : line 12 on ~iguce 2a that at 1.3 r the splitting ratlo would be considerable diÇferent~ This 1 s clearly not satisfactory for operat~on in a network which need~ to operate at both 1~52 ~ and 1.3 In the present technique an optical ibre coupler is fabricated by using the coupling region between two optical fibre~ whlch in the coupling region have sllghtly dif~rent propagation constants when ~easured at a given wavelength. The dif~erence in propagation constant can be achieved by arranging ~ ' .
:. :

the receptor fibres to have a different diameter, or a different profile, or by tapering one o~ two identical ~ibres more than the other. In an example of the present technique the coupler has been fabricated by pre-taperin~
one of the fibres. The actual fusing technique will not be described in detail but can be for example the method described in European Patent Application No. 174014 referred to previously herein which was published on 12th March, 1986. The difference in propagation constant can be selected to produce a selected degree oE coupling power, It is possible using this technique to produce for example, a 2 x 2 port device in which approximately 50/50%
split of optical power can be achieved with radiation of both 1.3 ~m and 1.52 ~m.
To illustrate the effects oE fabricating a 2 x 2 port fused fibre coupler with fibre of different propagation constants two diEferent tapers, taper A and taper B, were produced. Each taper had a gaussian type diameter variation described by D(Z) = DmaX - Do Exp (-aZ ) where D(Z) is the fibre diameter at a position Zmm, Dmax is the diameter of the untapered fibre ~125 ~m), 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 ~m and a equals ~' F l~ O ~ 6 . r~
:~,Z~

0~037 ~nd 0~035 mm~ respectively~
The tapers were twisted with untapered fibre of constant dlameter and the pairs heated and pulled in a ~anner similar to the method described in European.
Application No~ 174014 The coupling process w~s monitored throughou~ ~he pulling operation by launchin~ 1~3 ~ and 1~52 ~ radiaeion into an input por~ and by mea~uring the power a~ 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 th~n the normal stopping point to demonstrate the coupling behaviour over a large coupler length range~ It can be ~een that the difference in propagation constant produced by the taper leads to incomp}ete power transfer between the f~bres~ The larger this difference the smaller is the to~al power transferred as shown by comparison of Figures 2a and 2b~
It can be Been rom Figure 2b that at the firs~
region of intersection marked X of the~1~3 and 1~5 ~
curves the coupler has an equal coupling ratio at ~oth : wavelengths~ Furthermore ~hi~ coupling ca~io can be selected by appropeiately choosiny the magnltude of .

F ~ l ! M ~U ~ R I~
~3~

~he propa~ation constant difference~ For a 2 x 2 por~
device the pulling step is stopped at a length corresponding to the region X~ ~uring fabrication the region X ls identified by monitoring rad~ation oE both 1~3 and 1~52 ~ . For a device produced using taper A
a coupling of 50% + g% can ~e achieved over wavelength range 1.23 to 1~57 ~ ;
To demonstrate the fabrication o~ a 50/SO~
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 po~ting in a slo~ted silica rod uslng a silicone rubber compound. Cutback measurements o~ coupling and exce~s loss were then made at the two wavelen~ths:

C = 49~9% C - S0.1~ Exces~ loss ~ 0~014 dB @ 1~3~m C = 52~2~ C = 47~8~ Excess loss ~ 0.11 d~ @ 1.52~m ~0 F igure 3a and 3b show cutback spectral loss measutements~from lnput port 1 to output:ports 3 and 4 : respectively~ The wavelength wlndow for an insertion loss of 3dB + 0.7dB i- over 300 nm~

Claims

1. A method of forming an optical fibre coupler in which light propagating in a first fibre is arranged to be coupled into one or more other fibres within a coupling region, and at a predetermined coupling ratio, comprising the steps of:
(a) preselecting said optical fibres such that the propagation constant of said first fibre, within said coupling region, is slightly different from the propagation constants of said other fibres;
(b) increasing the length of said coupling region;
(c) monitoring the coupler being formed during the increase in the length of the coupling region;
and (d) stopping the increase in the length when the coupling ratios for two preselected wavelengths are first equal, the optical fibres being such that the coupling ratios at said two preselected wavelengths is equal to said predetermined coupling ratio.

2. A method of forming an optical coupler as claimed in claim 1, wherein optical radiation at one of said preselected wavelengths is launched into at least one of said fibres and the optical radiation transmitted through the coupling region is monitored.

3. A method of forming an optical coupler as claimed in claim 1, wherein optical radiation at both of said preselected wavelengths is launched into at least one of said fibres and both wavelengths of the optical radiation transmitted through the coupling region are monitored.

4. A method of forming an optical coupler as claimed in claim 1, wherein two or more fibres are twisted together along a portion of their length, the fibres are heated, and said coupling length increased by pulling.

5. A method of forming an optical coupler as claimed in any of claims 1, 2, 3, or 4 wherein the difference in propagation constant is achieved by using fibres of different core diameters.

6. A method of forming an optical coupler as claimed in any of claims 1, 2, 3, or 4 wherein the difference in propagation constant is achieved by using fibres of different refractive index profiles.

7. A method of forming an optical coupler as claimed in claim 1, wherein the difference in propagation constant is achieved by tapering identical fibres, the amount of said taper being different for each of said fibres.

8. A method of forming an optical coupler as claimed in any of claims 1, 2, 3, or 4 wherein said predetermined coupling ratio is equal to or substantially less than 50:50.

9. A method of forming an optical coupler as claimed in claim 7, wherein each said taper has a gaussian type diameter variation given by D(Z) = Dmax-D0 Exp(-aZ2 where D(Z) is the fibre diameter at position Zmm; Dmax is the diameter of the untapered fibre; Do is the reduction in fibre diameter at the waist of the taper; and a is the taper length parameter.

10. A method of forming an optical coupler as claimed in claim 9, wherein the taper of one fibre has a D0=0 (untapered), and the other fibre has a D0=9 µm, a=0.37 mm and Dmax=125 µm, and wherein the predetermined coupling ratio is 50:50, whereby a coupler wavelength flattened between 1.3 µm and 1.52 µm is obtainable.

11. A method of forming an optical coupler as claimed in claim 9, wherein the taper of one fibre has a D0=0 (untapered), and the other fibre has a D0=15 µm, a=0.035 mm and Dmax=125 µm, and wherein the predetermined coupling ratio is 50:50, whereby a coupler wavelength flattened between 1.3 µm and 1.52 µm is obtainable.

12. An optical coupler formed by the method of forming an optical fibre coupler in which light propagating in a first fibre is arranged to be coupled into one or more other fibres within a coupling region, and at a predetermined coupling ratio, comprising the steps of:
(a) preselecting said optical fibres such that the propagation constant of said first fibre, within said coupling region, is slightly different from the propagation constant of the other fibres;
(b) increasing the length of said coupling region;
(c) monitoring the coupler being formed during the increase in the length of the coupling region;
and (d) stopping the increase in the length when the coupling ratios for two preselected wavelengths are first equal/ the optical fibres being such that the coupling ratios at said two preselected wavelengths is equal to said predetermined coupling ratio.

13. An optical coupler as claimed in claim 12, wherein two or more fibres are twisted together along a portion of their length, the fibres are heated, and said coupling length increased by pulling.

14. An optical coupler as claimed in claim 12, wherein the difference in propagation constant is achieved by using fibres of different core diameters.

15. An optical coupler as claimed in claim 12, wherein the difference in propagation constant is achieved by using fibres of different refractive index profiles.

16. An optical coupler as claimed in claim 12, wherein the difference in propagation constant is achieved by tapering identical fibres, the amount of said taper being different for each of said fibres.

17. An optical coupler as claimed in any of claims 12, 13, 14, 15, or 16 wherein said predetermined coupling ratio is equal to or substantially less than 50:50.

18. An optical coupler as claimed in claim 16, wherein each said taper has a gaussian type diameter variation given by D(Z) = Dmax-D0 Exp(-aZ2) where D(Z) is the fibre diameter at position Zmm; Dmax is the diameter of the untapered fibre; D0 is the reduction in fibre diameter at the waist of the taper; and a is the taper length parameter.

19. An optical coupler as claimed in claim 18, wherein the taper of one fibre has a D0=0 (untapered), and the other fibre has a D0=9 µm, a=0.37 mm and Dmax=125 µm, and wherein the predetermined coupling ratio is 50:50, whereby a coupler wavelength flattened between 1.3 µm and 1.52 µm is obtainable.

20. An optical coupler as claimed in claim 18, wherein the taper of one fibre has a D0=0(untapered), and the other fibre has a D0=15 µm, a=0.035 mm and Dmax=125 µm, and wherein the predetermined coupling ratio is 50:50, whereby a coupler wavelength flattened between 1.3 µm and 1.52 µm is obtainable.

21. A broadband optical coupler having a coupling region in which light propagating in one fibre is coupled into one or more other fibres, the propagation constant within said coupling region of said fibre differing slightly from those of the other fibres, said coupling region having a predetermined length, said predetermined length being equal to the minimum distance for which the coupling ratio at two preselected wavelengths is first equal during formation of the coupler.

22. An optical coupler as claimed in claim 21, wherein the difference in propagation constant is achieved by using fibres of different core diameters.

23. An optical coupler as claimed in claim 21, wherein the difference in propagation constant is achieved by using fibres of different refractive index profiles.

24. An optical coupler as claimed in claim 21, wherein the difference in propagation constant is achieved by tapering identical fibres, the amount of said taper being different for each of said fibres.

25. An optical coupler as claimed in any of claims 21, 22, 23, or 24 wherein said predetermined coupling ratio is equal to or substantially less than 50:50.

26. An optical coupler as claimed in claim 24, wherein each said taper has a gaussian type diameter variation given by D(Z) = Dmax-D0 Exp(-aZ2) where D(Z) is the fibre diameter at position Zmm; Dmax is the diameter of the untapered fibre; D0 is the reduction in fibre diameter at the waist of the taper; and a is the taper length parameter.

27. An optical coupler as claimed in claim 26, wherein the taper of one fibre has a D0=0 (untapered), and the other fibre has a D0=9 µm, a=0.37 mm and Dmax-125 µm, and wherein the predetermined coupling ratio is 50:50, whereby a coupler wavelength flattened between 1.3 µm and 1.52 µm is obtainable.

28. An optical coupler as claimed in claim 26, wherein the taper of one fibre has a D0=0 (untapered), and the other fibre has a D0-15 µm, a=0.035 mm and Dmax=125 µm, and wherein the predetermined coupling ratio is 50:50, whereby a coupler wavelength flattened between 1.3 µm and 1.52 µm is obtainable.

29. An optical fibre coupler as claimed in any of claims 12 to 16, 18 to 24, or 26 to 28, wherein the optical fibre coupler is a fused fibre coupler.