A wavelength selective optical waveguide coupler

0001This invention relates to a wavelength selective optical waveguide coupler and particularly but not exclusively to a coupler formed from optical fibres having D-shaped cross-sections.

0002It is known to form a wavelength selective optical waveguide coupler by locating a pair of optical waveguides adjacent each other so as to provide a set of regularly spaced apart optical coupling regions with the optical path length between the coupling regions being greater in one waveguide than the other, which optical path lengths are a function of wavelengths of the light propagating in the waveguide. Light propagating in one waveguide will couple varying amounts of light into or out of the other waveguide at each coupling region dependent on the relative phase relationship of the light in the two waveguides. The aggregate effect for light propagating through the coupling regions will be additive only for a wavelength in which the optical path difference between coupling regions is such as to cause a relative phase shift of an integral number of 2π radians along the length of the waveguide between the each coupling region. Such a coupler will couple substantially all light of a preselected wavelength from one waveguide into another whilst leaving the other wavelengths substantially unaffected. Examples of such couplers are disclosed in Patent Application No. GB2152694A.

0003One of the couplers there discussed comprises first and second slab waveguides constructed in a semiconductor substrate. The distance between the first and second waveguides is varied periodically so as to provide the set of regularly spaced optical coupling regions.

0004The waveguides are formed in a semiconductor substrate. The position of the first waveguide relative to the other cannot be altered so it not only involves relatively complex manufacturing techniques it is also not tunable.

0005Another of the couplers there disclosed comprises two optical fibre waveguides. The fibres are wound in opposite senses on respective mandrels of slightly different radii for several turns. The path length of a turn is therefore different for each fibre. Optical coupling regions are formed when the turns of each optical fibre are brought together (after reducing the cladding width if necessary). The turns of each of the fibres when brought together, meet at tangents. Coupling occurs at these tangents, and tuning may be achieved by stretching the fibres on one mandrel by increasing the radius of that mandrel.

0006A disadvantage of this second known type of coupler is that due to the long path length around the mandrels, accidental path differences due to temperature fluctuations for example are difficult to eliminate, and fractional control of the radius of each mandrel must be achieved to a high level of accuracy in order to accurately tune the coupler to selectively couple the desired wavelength.

0007According to the present invention there is provided a wavelength selective optical waveguide coupler comprising a first optical waveguide and a second optical waveguide, said waveguides being arranged so as to pass selectively a signal of a desired wavelength from one of the optical waveguides to the other optical waveguide, characterised in that the first optical waveguide is in the form of a non-coiled undulatory optical fibre extending alongside the second optical waveguide to define a set of optical coupling regions spaced apart by a predetermined distance.

0008Thus invention provides a simple means of obtaining a set of regularly and accurately spaced optical coupling regions that is relatively temperature insensitive.

0009The path difference between the coupling regions can be set accurate the order of 1µm by relatively coarse macroscopic adjustment of the fibres.

0010Preferably, each of the optical fibre and second optical waveguide comprises a single mode optical fibre having a cross-section which has a linear portion in each coupling region. Thus the respective linear portions of the first and second waveguides will form substantially planar surfaces lying in substantially parallel planes in the coupling region.

0011Conveniently, the optical fibres are D-fibres, that is optical fibres having a D-shaped cross-section with the core located nearer to the linear portion of the "D" cross-section of the fibre, than to the curved portion. Such fibres are economical to make and easily manufacturable, and provide a convenient source of single optical fibres having the preferred linear portion in each coupling region. By placing the respective linear portions of the waveguides adjacent one another, the respective cores are close enough for coupling to occur across the coupling region.

0012The desired undulations in the optical fibre can be obtained in various ways.

0013The undulatory optical fibre may be an optical fibre mounted on an undulatory surface of a substrate. The substrate can be accurately machined to have the desired undulatory surface. When a fibre is mounted on the substrate it takes the form of the undulatory surface of the substrate. Conveniently, the surface of the substrate is made by forming V-grooves in the surface. This substrate may be made of silicon, and the V-grooves may then be formed by chemical etching processes.

0014Alternatively, the first waveguide may be mounted on an elastic support and the second waveguide may be mounted on or in a substrate having a substantially planar surface. Spacers can be placed at regular intervals along the length of the second waveguide the undulations of the first waveguide being formed by urging the deformable, elastic support towards the second optical waveguide, thus forcing the second waveguide against the spacers and second waveguide.

0015Preferably, the optical fibre is embedded in the substrate with the planar surfaces of the coupling regions flush with the surface of the substrate. This provides protection to the fibre, whilst allowing the fibre to come into and out of contact with the second optical waveguide. Conveniently, the substrate is a thermo-plastic material so the D-fibre may be embedded in the substrate by heating the substrate, and at the same time, applying a former to the fibre. The substrate will soften as a result of the heating. The former is chosen to have the desired undulatory shape the fibre will be moulded into the substrate, and will take the shape of the former.

0016This method of supporting the fibre is described in more detail in the applicant's GB application filed the same day as this application.

0017These methods of defining the undulations result in a fixed wavelength optical coupler. Where a tunable filter is required a vibrating means coupled to the first fibre to provide an acoustic transverse wave passing along the optical fibre. The acoustic wave causes the first fibre to vibrate and thus take up an undulatory form. By providing the vibrating means with a variable frequency of vibration the coupler may be tuned to operate at a described wavelength by varying the frequency of the vibration.

0018Means may be included for moving the optical waveguides into and out of optical coupling relationship. The coupler may thus be switched off by moving the waveguides out of optical coupling.

0019This can be achieved by using an optical fibre for the second optical waveguide supported so that it can be moved electrostatically into contact with the undulatory fibre when coupling is required. Other means of achieving switching can be readily devised.

0020Conveniently, there is included means for bending the optical fibres in the plane containing both optical waveguides. By bending the fibres, the difference in optical path lengths between the coupling regions will alter, and the coupler will be tuned from one wavelength to another.

0021Embodiment of the invention will now further be described by way of example only with reference to the following drawings in which: <ul id="ul0001" list-style="none"><li>Figure 1 is a cross sectional view of a first embodiment an optical waveguide coupler made in accordance with the invention;</li><li>Figure 2 is a cross section of the device showing Figure 1 taken along line II - II of Figure 1;</li><li>Figure 3 is a cross sectional view of the device showed Figure 1 taken along the line III - III of Figure 1;</li><li>Figure 4 is a cross sectional view of another embodiment of the invention; and</li><li>Figure 5 is a cross sectional view of yet another embodiment of the invention.</li></ul>

0022Referring to Figure 1, an optical waveguide coupler 1 comprises a first D-fibre 2 which is mounted in a first substrate 4 and a second D-fibre 6 mounted in a second substrate 8. The substrates 4 and 8 are held together by a pair of end-clamps 10. Fibre 2 is an undulatory fibre and is mounted in substrate 4 the surface 12 of D-fibre 2 is substantially flush with the surface of the substrate 3. The fibre 6 is also mounted in substrate 8 such that the its surface 14 is substantially flush with the surface of the substrate 8. The surfaces of fibres 2, 6 thus come into and out of contact with another to form coupling region. In the intervening regions, where the surface of fibre 2 is separated from the surface of substrate 4, the distance between fibres 2 and 6 is too great to allow coupling to occur. Decoupled regions are therefore formed. Light of a pre-selected wavelength entering fibre 6 at port 16 will be substantially completely coupled into fibre 2 and will be output at port 18 of fibre 2. If an optical signal containing a plurality of wavelengths including the pre-selected wavelength is input at port 16 of fibre 6, light of the pre-selected wavelength will be filtered out of fibre 6 and will be output at port 10 of fibre 2. If the distance between coupling regions is P and the amplitude of the ripple or undulation is h, then the path difference between optical coupling regions, for small h, is approximately d = h²/8.P. The longest wavelength passed by the filter is where the propagation constant β = <maths id="math0001"><math display="inline"><mrow><mfrac><mrow><mtext>2π</mtext></mrow><mrow><mtext> d</mtext></mrow></mfrac></mrow></math><img file="EP0346024B1_D0001.tif" /></maths>. Subsequent wavelengths are spaced such that Δβ = <maths id="math0002"><math display="inline"><mrow><mfrac><mrow><mtext>2π</mtext></mrow><mrow><mtext> d</mtext></mrow></mfrac></mrow></math><img file="EP0346024B1_D0002.tif" /></maths> and the width of each pass band is of the order of <maths id="math0003"><math display="inline"><mrow><mfrac><mrow><mtext>2π</mtext></mrow><mrow><mtext>dN</mtext></mrow></mfrac></mrow></math><img file="EP0346024B1_D0003.tif" /></maths> where N is the total number of coupling points in the coupler. Light of all other wavelengths contained in the input signal will be output at port 20 of fibre 6. Bending the coupler 1 in the plane containing the fibres 2 and 6 causes the coupler to be tuned to a different wavelength because this varies p and h. In the embodiment of Figure 1 a 3mm ripple wavelength with an amplitude of 65 microns gives a path difference of 0.88 microns between coupling regions for a pass wavelength in the 0.6 to 1.6 micron window. A bend radius of 2m would give about a 10% wavelength shift.

0023Referring to figures 2 and 3, the relative positions of the fibres 2 and 4 are shown in the decoupling region and the coupling region respectively.

0024Referring to figure 4, a second embodiment of the present invention is shown. A D-fibre fibre 22, is located adjacent the fibre 6 mounted in a substrate 8 as in the Figure 1 embodiment such that their flat surfaces are facing. Vibrating means 24, for example a piezo-electric transducer, is coupled to the fibre 2 such that an acoustic transverse wave may pass along the fibre 22. The acoustic wave passing along fibre 2 causes the fibre 22 to vibrate and thus take up an undulatory form. By varying the acoustic signal passing down the fibre 22 by means of a controller 26, the coupler 1 may be tuned to different wavelengths. For example, a one KHz signal will give a three centimetre wavelength ripple and 100 KHz wave will give a 3 millimetre ripple approximately.

0025Referring now to Figure 5 there is shown an alternative method of providing the undulatory optical fibre. The optical D-fibre 28 is mounted on a support 30 having deformable portion support 31 which was then pressed by clamp 10 against the firmly supported D-fibre 6 between which spacers 32, for example silica cylinders, have previously been placed. The portion 3 deforms as the D-fibre 28 forms undulations of period and height determined by the spacers.

0026A wavelength selective optical coupler according to the present invention arranged to be operated as a switch finds application in various network architectures. For example, an optical fibre may carry a wavelength multiplex to a section of D-fibre to which the fibre is spliced to form a common second optical waveguide on which undulating D-fibres cross. Each cross-point forms a wavelength selective optical switch according to the present invention arranged to couple a pre-selected wavelength from the D-fibre when the fibres are urged into coupling relationship, by mechanical or other means such as electrostatically. This arrangement can also be used to form a wavelength multiplex for transmission down an optical fibre to which the D-fibre is spliced. When any one of the undulating D-fibres is urged into contact with the D-fibre, light of a preselected wavelength can be coupled into the fibre.

0027It will be appreciated other methods of forming the undulations may be devised in order to make a coupler according to the present invention.

0028The coupler of the present invention is not in any way restricted to those formed using D-fibres and other optical fibres able to form coupling regions with another optical waveguide may be used, for example a fibre having a rectangular cross-section and on offset core. The second waveguide is not restricted to those formed by an optical fibre. It may be a planar waveguide formed on a substrate, for example.