GB2183866A - Optical fibre filter having tapered sections - Google Patents

Abstract

A filter with a tailored output characteristic for example useable as a comb filter is fabricated by producing two or more biconical tapered portions in an optical fibre. The manufacture is controlled so that the resulting concatenated couplers have specified pass-band characteristics.

Description

SPECIFICATION Concatenated coupler The present invention concerns coaxial waveguide optical fibres.

It has been discovered that a coaxial coupler can be produced from optical fibres of the type having a rod waveguide surrounded by a tubular waveguide by elongating a portion of the optical fibre to reduce the transverse dimensions of the portion. This is done by mounting the fibre in a driven clamping arrangement, launching light of a predetermined wavelength at one end of the fibre and detecting the intensity of the transmitted light at the other end whilst the clamped portion of the fibre is simultaneously elongated and heated.

The intensity of the detected radiation varies cyclically as the fibre is elongated and the procedure is stopped after a number of oscillations selected to provide the appropriate transmission characteristics.

The result of this procedure, which is described in the specification of our Patent Application No. 8519086, is that it is possible, starting from an optical fibre of approximately the correct refractive index profile and dimensions to produce a coupler which is operable at a required wavelength.

The present invention has for an object to utilise this technique to manufacture filters with output responses which are tailored to specific requirements such as comb filters.

Accordingly from one aspect the present invention consists in a filter comprising a coaxial optical fibre having at least two discrete tapered sections, caused by elongation of the fibre at two separate portions, the two sections having been produced in such a manner that the fibre has specified pass-band characteristics.

From another aspect the invention consists in a process of producing a filter from a coaxial optical fibre including the steps of elongating at least two portions of the fibre to provide two tapered sections.

In order that the present invention may be more readily understood, an embodiment thereof will now be described by way of example and with reference to the accompanying drawings, in which Figure 1 shows various refractive index profiles taken across the diameter of optical fibres suitable for use in the present invention, Figure 2 shows a diagrammatic representation of a rig for tapering optical fibres, Figure 3 shows a plot of the transmitted power against the extension of the fibre, Figure 4 is a diagram representing a section through a tapered portion of the waveguide for explaining the operation of the coupler, Figure 5 is a graph showing transmitted bandwidth against elongation for a single taper.

Figure 6 is a graph of the pass-band of a fibre which has been subjected to considerable elongation, and Figure 7 is a graph showing the pass-band of a fibre tapered in accordance with the invention.

Referring to the drawings, Fig. 1 shows an optical fibre which defines two waveguides, a central rod waveguide which is defined by the core 2 of the fibre; and a coaxial tubular waveguide 4. As illustrated in plot A of Fig. 1, the refractive indices n1, n'1, of the rod and tubular waveguides are elevated relative to the refractive indices n2, n3 of the intermediate cladding layer 6 and the outer coating layer 8. The dimensions and refractive indices n1 of the rod waveguide and the tubular waveguide are selected such that each will support at least one transmission mode at the design wavelength which may be, say, 1.33 or 1.55 micrometres.

Plots B and C in Fig. 1 show other typical refractive index profiles for optical fibre coaxial couplers.

Light energy at the design wavelength will only be coupled from the rod waveguide to the tubular waveguide if there is precise phase-matching at this wavelength between the modes propagated in the two waveguides. Phase-matching depends critically on the refractive indexes and dimensions of the fibre profile. In most cases a fibre produced to have the required profile will not, in fact, be precisely phase-matched at the design wavelength. In order to produce the required phase-matching at the desired wavelength the fibre 10 is placed in the apparatus illustrated in Fig. 2 to taper it over a portion of its length- to produce three regions of the fibre of opposite phase and mismatch at the design wavelength, so that light energy launched into one of the waveguides is substantially completely coupled to the other waveguide.

The apparatus of Fig. 2 comprises a laser light source 15 producing radiation at the design wavelength. This radiation is passed through a lens 14 to focus it onto the cleaved face of the core 2 of the fibre 10 so that light is propagated along the core of the fibre. Normally the fibre will be surrounded by an acrylate jacket which will strip any modes which start to propagate in the tubular waveguide. If such a jacket is not provided the fibre may pass through a bath of index matching fluid which will prevent light propagation along the tubular waveguide.

The fibre 10 is clamped at two spaced points along its length by clamps 16 and 18. Each clamp is provided with a motorised driver 20, 22 to enable the clamps to be pulled away from each other to taper the portion of the fibre between them. The drives are precisely controlled so that fibre extension of the order of 1 cm can be achieved and the extension stopped at a desired point to an accuracy of less than -20 microns. The clamps may be positioned vertically one above the other so that gravity can assist the tapering process. An oxybutane flame 23 (2-25 mm in diameter) is used to heat the fibre while the taper between the clamps is elongated.

Where the fibre is a depressed cladding fibre, that is, where n2 is less than n3 as shown in profile C of Fig. 1, the end of the fibre 10 is coupled to a power detector 24 which is connected to a chart recorder. If this is not the case then the power detection is provided by a microscope coupled to a vidicon camera. The power detector or microscope is focused so as to detect the power propagated along the core 2 of the optical fibre 10. If the acrylate jacket does not strip modes propagating in the tubular waveguide, a further bath of index matching fluid is provided after clamp 18 to avoid light propagating in the tubular waveguide affecting the power detector.

A typical plot produced on the chart recorder 26 is shown in Fig. 3. The plot comprises a level section 30 where a constant amount of power is being received. This is indicative of the power launched from the laser into the core being propagated along the core with little or no coupling of power into the tubular waveguide 4. After this level section 30 the received power oscillates increasingly rapidly between a series of minima 32, 34, 36 where little power is being received through the core. These minima correspond to extensions of the fibre at which the tapered portion of the fibre 10 between the clamps 16,18 is so dimensioned that there is complete energy exchange between the two waveguides. The plot produced in Fig. 3 has been shown for a considerable extension of the fibre between the clamps.However, in practice, to produce a coaxial coupler, the drive to the clamps is stopped when the recorder reaches one of the minima 34, 36, or maxima 37, 38, 39 or anywhere in between depending on the application.

If it is desired to produce a coupler which couples a desired proportion of the energy from one waveguide to the other, the taper can be stopped at other than one of the minima in order to produce the required ratio of energy transfer berween the waveguides.

The operation of the coaxial coupler produced by this tapered waveguide can be considered as analogous to a three section coupler where the mismatch (aft) between the propagation constants in each section alternates in sign. The three sections of the present coupler can be identified as shown in Fig. 4. The sign of the difference between the propagation constants of the two waveguides changes between the outer portions of the taper and the central, thinnest, portion of the tapered portion. A description of the mathematics of three section Afi couplers can be found in an article entitled "Switched Directional Couplers with Alternating Afi" by Herwig Kogelnik and Ronald V. Schmidt in IEEE Journal of Quantum Electronics, Volume QE-12, No. 7, July 1976.This article relates to couplers in which the mismatch is produced by applying electrodes with alternating potential differences across them to the coupled waveguides. However, the dimensional variation of the tapered portion can be considered to produce a similar effect.

Referring now to Fig. 5, this shows a graph of filter bandwidth versus the number of power oscillations during the fabrication of the coaxial coupler in which l=1.52 ,um and 1 oscillation=2 beatlengths.

It can be seen that the bandwidth reduces as the number of power oscillations used during fabrication increases. Thus by having a substantial number of oscillations a narrow bandwidth can be achieved. This can be seen in Fig. 6 where by subjecting a fibre to elongation for 20 power oscillations a pass bandwidth of 35 nm has been achieved. However, from the graph it will be appreciated that the pass bandwidth characteristics are not suitable for a spike or comb filter.

It has been discovered that the required characteristics can be achieved by concatenating together two or more tapered coaxial couplers. The spectral response of a tapered coaxial coupler is approximately sinusoidal with wavelengths of period Al. Thus the wavelength response of several concatenated coaxial couplers can be represented approximately as 2n 272 27 P([1 +Sin( +6] [1 +sin(-+6)] [1 +Sin(-+6)] M1 12 The wavelength response is the product of the wavelength responses of the individual couplers, for fixed 6. An indication of the desired wavelength response of the individual couplers can be obtained from the above equation. For example, for H1, 62, 63,=0, and when M,=2M2=3M,=4M,. . . =X.

Then a spike wavelength response is obtained within a wavelength range 2X. To obtain this result there should be successive tapered coaxial couplers each having bandwidth N (N= 1 for the first, N=2 for the second coupler, etc.) The tapering of the fibres is made in a manner identical to that previously described. The fibre is fixed onto a motorised jig, then tapered whilst an oxy-butane flame, for example, is applied.

The output power is plotted on a chart recorder. The first tapering is stopped after two oscillations and the second taper is made a few centimeters from the first one in the same manner. The second tapering is stopped when eight oscillations have occurred in the output power i.e. the bandwidth of the second taper is half that of the first.

Fig. 7 shows the wavelength response when two typical tapers have been made in the manner described above in a single fibre. Naturally differing degrees of tapering can be used, and more than two sets of taper can be imparted to a single fibre.

The wavelength response of the double taper just described is shown in Fig. 7. Clearly, a spike is present at 1=1430 nm and there are small suppressed sidelobes above and below this peak. This is not periodic as one would expect if the couplers were of identical fibres.

Further, the passband is narrower than the passband of the individual tapers there the passband is 70 nm as opposed to a passband of 100 nm for the second individual coaxial taper (as can be derived from the graph of Fig. 5).

For a maximum througput at the fabrication wavelength, it should be ensured that the tapering is stopped to a maximum power for both tapers.

Finally, the summary of the advantages of such a device is as follows: 1. A narrow spike or comb filter like wavelength response can be obtained which is narrower than the individual taper response.

2. The filter is of low loss, all fibre.

3. Simple fabrication on a standard coupler fabrication jib.

Filters of the type produced in accordance with the invention can be used in a number of different applications. For example, in a wavelength drop-off filter in a wavelength division, multiplexed communication link or to separate a single line from a multilongitudinal mode laser.

Claims (6)

1. A filter comprising a coaxial optical fibre having at least two discrete tapered sections caused by elongation of the fibre at two separate portions, the sections having been produced in such a manner that the fibre has specified pass-band characteristics.

2. A filter as claimed in Claim 1, wherein each tapered section is fabricated by heating the section whilst elongating the fibre, launching lifght down one end of the fibre, monitoring the light transmitted through the fibre and stopping the elongation after a selected number of power cycles of the monitored light.

3. A filter as claimed in Claim 2, wherein the fibre comprises a central rod waveguide, an intermediate cladding, a tubular waveguide and an outer coating layer.

4. A filter as claimed in Claim 3, wherein the refractive indices of the rod and tubular waveguides are higher than that of the intermediate cladding and the outer coating layer.

5. A filter as claimed in any one of Claims 2, 3 or 4, wherein each tapered section has a bandwidth determined during its manufacture by the number of power oscillations monitored during the fabrication of the taper, successive tapered sections having reduced bandwidths.

6. A filter as claimed in Claim 5, wherein there are more than two tapered sections.