GB2369449A - Optical waveguide device with tapered branches - Google Patents

Abstract

An integrated optical device comprises a first waveguide 100 which branches into at least two second waveguides 110, 120, the branch comprising a structure in which the first waveguide 100 ends with a taper 102 and the second waveguides 110, 120 each start with a taper 112, 122 that is located adjacent the end taper 102 of the first waveguide 100. These can be located so as initially to accept the evanescent portion of the mode. The tapers of each waveguide can be symmetric or asymmetric. This structure of a waveguide branch can be employed in a variety of applications, such as a Mach-Zehnder interferometer, a switch, or any application where the waveguide must divide. It can also be used to equivalent effect where waveguides must combine. It is also possible to use the structure near the edge of a chip or a region thereof to assist in coupling to (for example) an optical fibre or the active region of an arrayed waveguide grating.

Description

integrated Optical Device The present invention relates to an integrated optical device.

It is often necessary to split a waveguide into two outputs, ie to create a Ybranch. This is typically done literally by forming the waveguide into a Yconfiguration, as shown schematically in Figure 1. An incoming waveguide 10 divides along its central axis at 12 into two departing waveguides 14,16. These initially run closely to each other with only a small divergence, to allow the optical mode to settle into the two branches. After a short distance, they then diverge to wherever intended.

However, there are difficulties in this structure. First, the split between the departing waveguides 14,16 is at the centre of the optical mode. At this location, it presents the maximum disturbance. Second, it is sometimes necessary to subject waveguide structures to thermal oxidation in order to reduce surface roughness. This effectively shrinks the structure by loss of a thin layer from the surface. The effect of this is shown schematically in Figure 1 in dotted lines.

Some exaggeration of the effect has been included to allow clear illustration. It will be seen that in the crucial concave formation at which the waveguides separate, the effect of thermal oxidation is to round off the formerly sharp division.

This creates a flat reflective surface at the centre of the optical mode, thus increasing scattering from the device and hence its propagation losses. That concave division is also difficult to form with a sharp point ab initio, and thus will be slightly blunt even before oxidation.

M. H. Hu et al report a Y-branch in"A low loss and compact waveguide Ybranch using refractive-index tapering", IEEE Photonics Technology Letters, Vol. 9, No. 2, February 1997. An input waveguide reduces in height adiabatically, eventually disappearing, between a pair of asymmetrically tapered output waveguides which are contiguous on either side. The optical mode is thus forced downwards in that region, and couples into the output waveguides. This arrangement can address the problems of the above. However, it is difficult to fabricate a smooth and accurate reduction in height, particularly in commercial volumes.

The present invention seeks to provide a Y-branch that alleviates these difficulties. It therefore provides an integrated optical device comprising a first waveguide which branches into at least two second waveguides, at least one of the second waveguides starting with a taper that is located adjacent the first waveguide.

It is preferred that the branch comprises a structure in which the first waveguide ends with a taper and the second waveguides each start with a taper that is located adjacent the end taper of the first waveguide.

Thus, the reducing width of the first waveguide again forces the optical mode out and into the second waveguides. These can be located so as initially to accept the evanescent portion of the mode. There need be no disturbance to the central axis of the mode until the tip of the taper of the first waveguide, at which point the optical mode will exist substantially in the second waveguides. Thermal oxidation will still affect the waveguides, but no concave structures are needed and hence the structure can undergo oxidation without the above difficulties. Indeed, oxidation will sharpen the tips of the (three) tapers and may be done deliberately.

The waveguides can be of the normal height at all points and hence the fabrication difficulties of Hu et al are avoided.

It is preferred that the tapers of each waveguide are symmetric. However, depending on the optical properties required, one or more may be asymmetric. Likewise, it is preferred that the start of the second waveguides are substantially aligned with the start of the tapering off of the first waveguide, and also that the end of the first waveguide is substantially aligned with the end of the taper of the second waveguides. Again, however, depending on the optical properties required, this may not be the case.

This structure of a waveguide branch can be employed in a variety of applications, such as a Mach-Zehnder interferometer, a switch, or any application where the waveguide must divide. It can also be used to equivalent effect where waveguides must combine.

It is also possible to use the structure near the edge of a chip or a region thereof to assist in coupling to (for example) an optical fibre or the active region of an arrayed waveguide grating. In such situations, the waveguide is split into two short waveguides which are parallel and closely adjacent; these end at the edge of the chip or region. The light leaving the pair will act as a conjoined mode if the waveguides are close enough, the conjoined mode being wider and having a greater dispersion. In some applications, such as arrayed waveguide gratings, this is desirable.

Embodiments of the present invention will now be described, by way of example, with reference to the accompanying drawings, in which; Figure 1, already described, shows a typical Y-branch; Figure 2 shows a plan view of a Y-branch according to the present invention; Figures 3,4 and 5 are sections on III-III, IV-IV and V-V, respectively, on Figure 2; Figures 6a, 6b and 6c show computed transverse mode profiles for the Ybranch of Figure 2 at locations A, B and C (Figure 2) along the structure; Figure 7 shows, schematically, a Mach-Zehnder interferometer employing the present invention; Figure 8 shows, schematically, a 1 x2 switch employing the present invention; Figure 9 shows, schematically, a combiner employing the present invention; Figure 10 shows, schematically, a 4 way divider employing the present invention; Figure 11 shows a band flattener employing the present invention; and Figure 12 shows a possible layout for the structure of Figure 11 in an arrayed waveguide grating.

Figure 1 is described above and will not therefore be described here.

Figure 2 shows a Y-branch according to the invention. An incoming waveguide 100 conveys an optical mode which is to be split so as to propagate in two waveguides. In the following, it is assumed that the mode is to be split equally and accordingly the described embodiments will be symmetric. Where this assumption does not hold, appropriate adjustment of the relative dimensions of the waveguides will be required.

At the point where the Y-branch is to take place, the incoming waveguide 100 is formed in a taper 102. Thus, the side walls 106 of the incoming waveguide 100 approach so as to reduce the lateral width of the waveguide 100, eventually to zero at a point 108.

Adjacent the taper102, first and second outgoing waveguides 110, 120 start. The starting point of each waveguide is again formed in a taper, 112 and 122 respectively. The tip 114 of the first outgoing waveguide 110 is approximately adjacent the start of the taper 102 of the incoming waveguide 100, and likewise the tip 124 of the second outgoing waveguide 120. The end of the tapers 112, 122 are approximately adjacent the tip 108 of the taper 102 of the incoming waveguide 100.

The tapers 112, 122 of the outgoing waveguides 110, 120 are located on either side of the taper 102 of the incoming waveguide, closely adjacent. A suitable spacing is between 0.25 and 2, um, which is achievable using known photolithographic techniques. Thus, at the start of the outgoing waveguides 110, 120, the evanescent part of the optical mode will exert an influence. As the incoming waveguide narrows, the optical mode will gradually be forced out of the incoming waveguide into the underlying substrate. The proximity of the outgoing waveguides then allows the mode to become associated with them, continuing to propagate under their guidance after the incoming waveguide 100 ceases.

Figures 3 to 5 show cross-sections of the structure. Initially, as in Figure 3, only the incoming waveguide 100 is present. Figure 4 shows the cross-section shortly after the incoming waveguide 100 begins to taper; the outgoing waveguides 110, 120 are present but narrow. Figure 5 shows the cross-section after the Ybranch, the incoming waveguide having ceased and the two outgoing waveguides 110, 120 running parallel.

Figures 6a, 6b and 6b show computed transverse mode profiles at points A, B and C on Figure 2 respectively. In Figure 6a, an optical mode of the usual type for an Si rib waveguide is shown, existing partly in the rib but mainly in the slab region below and on either side. As the incoming waveguide 100 narrows, the optical mode shifts downward slightly deeper into the slab region, from which it also begins to extend upward into the adjacent outgoing waveguides 110, 120.

As the incoming waveguide continues to narrow and the outgoing waveguides 110, 120 become more substantial (Figure 6c), the mode begins to divide-becoming bimodal-and occupies the outgoing waveguides more fully. It can be seen in Figure 6c that the remaining optical power in the incoming waveguide 100 is small.

Accordingly, losses caused in the known structure of Figure 1 by the disturbance to the central axis of the mode should in this arrangement be minimal.

The above description has been made in relation to a division of a propagating mode into two separate modes. However, the process is capable of operating in reverse using the same structure, allowing the combination of two propagating modes into a single waveguide. Each will be forced out of the tapering waveguides and will spread into the same adjacent developing waveguide.

Figure 7 shows a Mach-Zehnder interferometer (MZI) constructed using the Y-branch described above. The dimensions of the Y-branch compared to the dimensions of the MZI are small, so in Figure 7 (as with Figures 8-10) the Y branch has been illustrated schematically. An incoming waveguide 130 splits at a Ybranch 132 into two parallel waveguides 134,136. One waveguide is subjected to a perturbation by device 138 which is able to delay the signal slightly. Suitable devices include pn junctions, where the injected carriers affect the refractive index and locally change the light velocity. Both waveguides then recombine at a further Y-junction 140 and continue as an outgoing waveguide 142. The optical modes will interfere due to the delay to which one was subjected; this means that wavelengths where this delay results in a A/2 shift will be eliminated. As the number of injected carriers (and hence the refractive index change) can be varied by varying the current, this means that the wavelengths to be accepted or rejected can be selected.

The MZI structure is known, but through the present invention it can be made less lossy. This in turn permits more processing to be carried out on the light signal.

Figure 8 shows a 1 x2 switch-a digital optical switch or DOS switch. An incoming waveguide 150 divides at a Y-branch 152 into two outgoing waveguides 154,156. Close to the branch, one waveguide is perturbed by a device 158 similar to the device 138 of Figure 7, changing the index of that waveguide. As both of the waveguide branches are still quite close at this point, the branches interact. If the structure is suitably designed the light will tend to gradually migrate to the branch with the higher index as the branches separate. When the branches are sufficiently far apart most of light will be localise is the branch with the higher index.

Figure 9 shows a combiner, as referred to above and employed in the MZI of Figure 7. A pair of incoming waveguides 160,162 meet at a Y-junction 164 and transfer their associated optical mode to an outgoing waveguide 166 in the manner described above.

Figure 10 shows a 4-way divider. An incoming waveguide 170 branches at a Y-junction 172 into first and second intermediate waveguides 174,176. The first intermediate waveguide 174 then branches again at a Y-junction 178 into first and second outgoing waveguides 180,182. The second intermediate waveguide 176 also branches at a Y-junction 184 into third and fourth outgoing waveguides 186, 188. In this way an incoming waveguide can be branched into 2"outgoing waveguides. It is also possible to arrange any number of outgoing waveguides, not necessarily 2 n, but the intensity in the outgoing waveguides will not then be identical.

Figure 11 shows a band flattener. A waveguide 200 is approaching an edge 202, which may be the edge of the substrate or the edge of an arbitrary region within a substrate. Near the edge of the substrate, for example, the waveguide may need to couple into an optical fibre. In an arrayed waveguide grating (AWG), a waveguide may need to end at the edge of a region within the substrate, so as to allow the optical mode to disperse and be coupled into a plurality of waveguides. Before the edge, the waveguide 200 divides in a Y-branch structure 204, as those described previously, into two stub waveguides 206,208. These extend parallel and close alongside each other for the short remaining distance to the edge 202 where they end.

At the edge, the proximity of the two stub waveguides 206,208 together with their short length means that the optical modes in each will still be associated.

Thus, when they end, a single conjoined mode 210 will be released into the substrate or fibre as appropriate. This mode will have different properties to the mode which would be released from a single waveguide; in particular it will have a greater dispersion which will be an advantage in many AWG designs. It will also be physically wider which may assist in coupling into a fibre.

Rib waveguides such as those illustrated above are typically formed (in practice) by etching trenches either side of the intended track of the waveguides so as to leave an upstanding rib. Figure 12 shows how the design of Figure 11 could be achieved for an AWG bearing this in mind. The waveguide 200 is defined by etched areas 212, 214 on either side. The edge 202 is defined by the end of the areas 212,214, and accordingly lies at the end of the waveguide 200. Near the edge, the areas 212,214 narrow to allow for the stub waveguides 206,208.

A pair of narrow trenches 216, 218 extend from the areas 212, 214 to cut off the waveguide 200, each extending at an angle to end the waveguide 200 with the necessary taper. They then join as trench 220, ending about at the edge 202, and separating the stub waveguides 206,208.

In this way, the required structure is achieved. The stub waveguides cease to be defined after the edge 202 and the optical mode that they carry is released into the substrate.

It will of course be appreciated that many variations may be made to the above described embodiments without departing from the scope of the present invention. Some such variations are described above; others will be apparent to the skilled person. For example, no other layers are shown in the devices, but these could be provided for unrelated reasons or to provide a level surface. Any low index material such as SiO2 could fill the gaps between the ribs without loss of optical performance. More than two branches could be formed, by providing further adjacent tapered waveguides.

Claims (12)

1. CLAIMS 1. An integrated optical device comprising a first waveguide which branches into at least two second waveguides, at least one of the second waveguides starting with a taper that is located adjacent the first waveguide.
2. 2. An integrated optical device according to claim 1 in which the branch comprises a structure in which the first waveguide ends with a taper and the second waveguides each start with a taper that is located adjacent the end taper of the first waveguide.
3. 3. An integrated optical device according to claim 2 in which the tapers of each waveguide are symmetric.
4. 4. An integrated optical device according to claim 2 or claim 3 in which the start of the second waveguides are substantially aligned with the start of the tapering off of the first waveguide.
5. 5. An integrated optical device according to any one of claims 2 to 4 in which the end of the first waveguide is substantially aligned with the end of the taper of the second waveguides.
6. 6. An integrated optical device according to any preceding claim including a Mach-Zehnder interferometer, at least one waveguide branch thereof being according to any one of the preceding claims.
7. 7. An integrated optical device according to any preceding claim including an optical switch, at least one waveguide branch thereof being according to any one of the preceding claims.
8. 8. An integrated optical device according to any one of claims 1 to 5 in which the waveguide branches into two short waveguides which are substantially parallel and adjacent, and end together at one of the edge of the chip.
9. 9. An integrated optical device according to any one of claims 1 to 5 in which the waveguide branches into two short waveguides which are substantially parallel and adjacent, and end together at a free space region.
10. 10. An integrated optical device according to claim 9 in which the free space region is part of an arrayed waveguide grating.
11. 11. An integrated optical device according to any preceding claim which has been subjected to a surface etch process after formation of the branch structure.
12. 12. An integrated optical device substantially as herein described with reference to and/or as illustrated in the accompanying drawings.