GB2367377A - Silicon rib waveguide with MMI device - Google Patents

Abstract

A silicon rib waveguide device has a tapered waveguide section 52 connected to a multimode interference (MMI) device 21, where the tapered waveguide section 52 is outwardly flared from a single mode transmission region 51 towards the input of the MMI device 21. The device may also have a mode filter section 40 which has an inwardly tapered section 50 and an outwardly tapered section 52 serially connected to the MMI device 21. The tapered section 52 may have a smaller taper angle than the tapered section 50 so as to avoid excitation of higher order modes. The waveguide device may also include optic fibres connected to on-chip or integral silicon waveguides. Figure 7 shows the modified multi-mode pattern which gives greater symmetry both transversely and vertically on passing through the MMI device 21.

Description

SILICON MMI DEVICE

The invention relates to a silicon multimode interference (MMI) device and particularly to such a device in combination with a silicon rib waveguide.

Silicon rib waveguide devices may conduct light in single mode or multimode.

They may be formed as part of an integrated silicon chip having one or more rib waveguide devices optically coupled to other optical circuitry including multimode interference devices. Such devices may require a single mode input with preferred configuration of field intensity entering the MMI device.

It is an object of the present invention to provide a silicon rib waveguide device which provides improved control of the field distribution of light entering an MMI device. In particular, it provides improved control of the transmission mode entering the MMI device.

The invention provides a silicon rib waveguide device including a tapered waveguide section connected to a multimode interference (MMI) device, said tapered waveguide section being outwardly flared from a single mode transmission region towards the input of the MMI device.

Preferably the device includes a mode filter section serially connected to the multimode interference (MMI) device, said mode filter section having first and second tapered waveguide regions at respective ends, the first tapered region tapering inwardly from a first wider rib width into a second narrower rib width and the second tapered region tapers outwardly to provide an input to said MMI device.

Preferably the taper at the input of the MMI device has a small taper angle so as to avoid excitation of higher order modes.

Preferably said second tapered region is arranged to increase symmetry of the single mode input from the straight waveguide into said MMI device.

The first rib width may be at least 50% greater than said second rib width.

The straight section may have a length between the tapered regions of at least 200 times the said second rib width.

The length of each tapered section is at least 80 times the said second rib width.

The second rib width may be approximately 4pM and the first rib width approximately 6pM.

The waveguide device may comprise an integral silicon chip device.

The waveguide device may have input and output optical fibres connected to onchip silicon waveguides.

An embodiment of the present invention will now be described by way of example and with reference to the accompanying drawings in which: Figure 1 is a schematic view of an integrated silicon waveguide chip in accordance with the present invention, Figure 2 shows a prior art construction of the type of silicon rib waveguide used on the chip of Figure 1, Figure 3 illustrates a prior art multiplexer and/or demultiplexer using an array of waveguides which may be incorporated on the chip of Figure 1,

Figure 4 illustrates a prior art use of a multimode interference (MMI) device which may be used on the chip of Figure 1, Figure 5 is a perspective view of a mode filter which is incorporated in the device of Figure 1, Figure 6 shows a transmission mode in the prior art device shown in Figure 2, and Figure 7 illustrates a modified transmission mode in the output of a mode filter of the type shown in Figure 5.

Figure 1 illustrates schematically an integrated silicon chip forming a waveguide device using silicon on insulator rib waveguides of the type illustrated in Figure 2.

Such waveguides are of a known type of ridge waveguide formed from silicon insulator. An upstanding rib 11 is formed on a silicon layer 12. A silicon substrate 13 is covered with a silicon dioxide layer 14 immediately below the silicon layer 12. A silicon dioxide coating 15 may be formed over the upper surface of the silicon 12 and over the rib 11. Optical signals are transmitted in a single mode through the silicon layer and each rib 30 as shown in Figure 6. The mode pattern is illustrated at 16 and is in this example single moded in the vertical and horizontal directions.

In Figure 1 the integrated silicon chip is indicated at 17 and has connected at its boundaries one or more input optical fibres 18 and a plurality of optical fibre outputs 19 and 20. On the chip are integrally formed a plurality of optical circuitry components 21 and 22 which may be in the form of prior art optical components such as are shown in Figures 3 and 4. In multiplexing and demultiplexing optical circuitry it may be desirable to use an array of curved waveguide paths arranged in parallel with each other as shown in the prior art arrangement of Figure 3. Such an array waveguide device 22 consists of a plurality of curved rib

waveguides 23 arranged side by side with straight input ends focussed at one end 25 of an input waveguide 26. The array has a plurality of straight output waveguides 27 focussed at 28 at one end of a plurality of output waveguides 29. The optical circuitry 21 and 22 in Figure 1 may include an array of the type that is shown in Figure 3.

A multimode interference device (MMI) of the type shown in Figure 4 and already known in the art may be included on the chip as part of the circuitry 21 or 22 in Figure 1. As shown in Figure 4, light which is being conducted on chip may have a field distribution of the type shown at 30 in Figure 4 and pass through an input guide 31 to an MMI coupler 33 such that the output field of the MMI coupler has the double peaked field distribution shown at 34. Such MMI couplers may be of use in providing input light to the input end of an array such as that shown in Figure 3. The input to the MMI coupler should be single mode and symmetrical with respect to the body of silicon forming the coupler.

In the arrangement of Figure 1 various circuitry of the type shown in Figures 3 and 4 together with a plurality of straight and curved rib waveguide sections may be formed to provide the most compact arrangement of circuitry on a single integrated silicon chip. In the example of Figure 1 a plurality of waveguide mode filter sections are provided as indicated at 40,41, 42,43 and 44. Each of these consists of a straight narrow rib waveguide located between tapered sections at opposite ends flared outwardly from the narrow straight waveguide section and connected to adjacent optical paths of greater transverse width than the narrow rib waveguide and incorporating one or more curved rib waveguide sections as indicated at 45 and 46.

In this example the curved rib waveguide 45 and mode filter 40 will be described with more detail with reference to Figures 1 and 5.

In this particular example the straight narrow rib waveguides have a narrow rib width such as 4um which supports only single mode transmission across its width. When forming a curved waveguide on such a silicon chip, losses occur in transmission around a curved section and the losses increase with smaller radius of curvature. For a 4um rib width the radius should not less than 20mm. In order to produce a compact arrangement, it may be desirable to increase the rib width to 6pm around the curve thereby enabling the radius to be reduced to 12mm with approximately the same light loss as 20mm radius for a 4um rib width. However, for a silicon rib width of 6pm it is possible for multimode propagation to occur across the width of the rib in a straight waveguide section. Consequently the present embodiment includes a mode filter 40 following the curved waveguide section 45. The structure of this is shown more clearly in Figure 5. The curved section 45 has in this example a rib width W1 of 6pm and this is joined by an inwardly tapered section 50 to the narrow straight rib waveguide 51 of uniform width along its length. The straight region 51 is then connected by an outwardly flared tapered region 52 into the optical circuitry 21 which in this example is a planar slab of silicon forming an MMI coupler of the type shown at 33 in Figure 4.

It will be understood that in Figure 5 the curved rib 45, straight rib 51 and the planar slab 21 are all formed on a silicon substrate 53 so as to form upstanding regions from that planar substrate. In the particular example shown, the transverse width W2 of the straight waveguide 51 is 4um. The length of the straight waveguide section 51 is shown at L1 and this example is 1000un, that is at least 200 times the width W2 of the straight rib section. The radius of curvature of the curved section 45 is less than 20mm and in this example is approximately 12mm. The length L2 of the tapered section 50 is in this example 340um which is at least 80 times the width W2 of the straight waveguide section.

The length of the tapered section 52 is in this preferred example the same as the length of the tapered section 50.

Preferably the length of the straight section L1 before the tapered section 52 is between 1 and 500 times the width W2 of the straight section.

It will however be understood that other dimensions may be used. The angle of taper of the outer walls of the tapered sections 50 and 52 in relation to the axis of the straight rib waveguide 51 may for example be 0. 170. The angle may be less, for example 0. 1 . In each case the taper is so small that the taper is adiabatic and does not excite higher order modes from the single mode transmission in section 51.

In use of the device shown in Figures 1 and 5, light which is input from the input fibre 18 may include multimode across the fibre and these will enter the chip waveguides and pass through the filter section 44 which will transmit only a single fundamental mode. Light which passes into the wider curved waveguide section 45 may include multimode transmission with some scattering on passing around the curved waveguide 45. Any modes other than the fundamental which are output by the wider waveguide 45 will be filtered by the mode filter 40 such that only the single fundamental mode across the width of the waveguide is transmitted into the optical circuitry 21. In the case of coupling the filter 40 to an MMI coupler providing the circuitry 21 of Figures 1 and 5, the symmetry of that single mode will be improved by the outwardly tapered section 52. The normal single mode pattern shown in Figure 6 is modified by the widening tapered section 52 so as to change into a pattern substantially as shown in Figure 7 thereby giving greater symmetry both transversely and vertically on passing through the multimode interference device 21. Such field patterns may be multimode in depth as well as transversely on forming the output of the MMI device 21.

It will be understood that the device of Figure 1 may incorporate a variety of optical components interconnected by curved and straight rib waveguide sections. The width of the curved sections may be increased to allow the compact formation obtainable by using wider waveguide sections and the mode control may be effected by incorporating mode filter sections having straight rib waveguides with end tapered sections as described above.

The invention is not limited to the details of the foregoing example.

Claims (11)

CLAIMS :

1. A silicon rib waveguide device including a tapered waveguide section connected to a multimode interference (MMI) device, said tapered waveguide section being outwardly flared from a single mode transmission region towards the input of the MMI device.

2. A silicon rib waveguide device according to claim 1 including a mode filter section serially connected to the multimode interference (MMI) device, said mode filter section having first and second tapered waveguide regions at respective ends, the first tapered region tapering inwardly from a first wider rib width into a second narrower rib width and the second tapered region tapers outwardly to provide an input to said MMI device.

3. A silicon rib waveguide device according to claim 1 or claim 2 in which the taper at the input of the MMI device has a small taper angle so as to avoid excitation of higher order modes.

4. A waveguide device according to any of claims 1 to 3 in which said second tapered region is arranged to increase symmetry of the single mode input from the straight waveguide into said MMI device.

5. A waveguide device according to any one of the preceding claims in which the first rib width is at least 50% greater than said second rib width.

6. A waveguide device according to any one of the preceding claims in which the straight section has a length between the tapered regions of at least 200 times the said second rib width.

7. A waveguide device according to any one of the preceding claims in which the length of each tapered section is at least 80 times the said second rib width.

8. A waveguide device according to any one of the preceding claims in which the second rib width is approximately 4uM and the first rib width is approximately

6pM.

9. A waveguide device according to any one of the preceding claims comprising an integral silicon chip device.

10. A waveguide device according to claim 9 in which input and output optical fibres are connected to on-chip silicon waveguides.

11. A waveguide device substantially as hereinbefore described with reference to Figures 1 and 7 of the accompanying drawings.