GB2381877A

GB2381877A - Integrated optical device

Info

Publication number: GB2381877A
Application number: GB0126939A
Authority: GB
Inventors: Adrian Petru Vonsovici; Laurent Kling
Original assignee: Bookham Technology PLC
Current assignee: Lumentum Technology UK Ltd
Priority date: 2001-11-09
Filing date: 2001-11-09
Publication date: 2003-05-14
Also published as: GB0126939D0

Abstract

A novel geometry for a Mach-Zehnder interferometer (MZI) is proposed. The interferometer includes a pair of directional couplers 34, 44, acting on a pair of waveguides 30, 32. Each directional coupler comprises an approach region 36, 46, in which the separation of the waveguides decreases, and an exit region 38, 48, in which the separation of the waveguides increases. The geometry is characterised in that in at least one directional coupler (preferably both), a waveguide is substantially straight in at least the approach region. As a result the structure is more compact thereby reducing the demand for chipspace on the substrate and the design is more suitable for construction as part of an array of MZIs. In another aspect the problem of bringing two waveguides together is addressed, a directional coupler has a pair of waveguides which approach one another, at least one waveguide is curved in first and second S-bends. The first S-bend reduces the separation of the waveguides by a larger amount that the second.

Description

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

The Mach-Zender interferometer (MZ or MZI) is a well-know structure in the field of integrated optics. A directional coupler MZ (ie a DC-MZ) consists essentially of a pair of waveguides which approach sufficiently closely for the respective optical modes to interact and split the light intensity, then separate and run separately for a distance, before approaching again and interfering. A"path balanced"MZI has the same path length between the approach points whereas a "path imbalanced"MZI has a different length.

Active structures such as phase shifters can be formed around one or more of the paths to affect the optical length of the path. The phase shifters can (for example) be p-i-n diodes or heaters. It is known that high concentrations of charge carriers affect the refractive index of the substrate in which the optical mode propagates and thus by activating or deactivating the diode (s) the optical path

lengths of one or more arms can be varied. This can be used to modulate or filter the signal passing through the MZI, or to direct an optical mode to leave on one or other waveguide.

A typical DC-MZI structure is shown in figure 1. A pair of incoming waveguides 10, 12 curve in an approach region 14 so as to approach each other.

They stay close to allow optical modes therein to interact and/or spread from one waveguide to the other, a structure which is referred to as a directional coupler.

The waveguides then separate in an exit region 15 which is followed by a transit region 16 before entering another approach region 18. A pair of p-i-n diodes are formed by a p+ region 20 between the waveguides and a pair of n'regions 22,24, one either side of the waveguides. Thus, the p+ region 20 and n+ region 22 define a diode across waveguide 10 whilst the same p+ region 20 and the n'region 24 define a diode across waveguide 1 2. The central p+ region 20 is connected to a positive rail voltage and the n+ regions 22,24 are connected to positive and negative rails, or negative and positive rails, respectively, to create a flow of charge carriers across the waveguide 12 or the waveguide 10 respectively. This allows the MZI to influence the optical modes involved.

Instead of the p-i-n diodes, other phase shifters such as heaters can be employed. In general, pan diodes are shown and referred to herein for consistency, but it is to be understood that other phase shifters such as heaters, for example, can be employed instead.

The present invention proposes, in a first aspect, a novel geometry for such structures. Thus, the invention provides a Mach-Zender interferometer defined by a pair of directional couplers acting on a pair of waveguides, each directional coupler comprising an approach region in which the separation of the waveguides decreases and an exit region in which the separation of the waveguides increases, characterised in that in at least one directional coupler, a waveguide is substantially straight in at least the approach region.

It is preferred that a waveguide is substantially straight in both directional couplers.

In one design, it is preferred that the relevant waveguide is substantially straight through the whole directional coupler concerned. It is then particularly preferred that the other waveguide is substantially straight through the other directional coupler as this makes design of a path balanced MZI easier. However, path imbalance MZls are possible, and other means exist for balancing the optical path.

In another design, one waveguide is straight in the approach region of one or both directional couplers whilst the other waveguide is straight in the exit region of the directional coupler. The same waveguide can be substantially straight in the approach region of each directional coupler, to ease the design of a path balanced MZI, but again this is not essential.

In both designs, the structure is more compact thereby reducing the demand for chipspace on the substrate. Furthermore, the designs are more suitable for construction as part of an array of MZls. It is especially preferred that there are more than one such MZI structures alongside each other, with the transit regions thereof staggered. This allows phases shifters (such as p-i-n diode structures) to be laterally staggered and reduces overlap of their effects. Again, this allows a reduction in the separation and reduction or avoidance of blocking structures therebetween. These measures thus further reduce demand for chipspace.

In another aspect of the invention, the problem of bringing the two waveguides together is addressed. This aspect can be applied to the layouts of the first aspect, above, but it can also be applied to the design of otherwise conventional directional couplers such as those shown in figure 1.

Thus, in its second aspect, the present invention provides a directional coupler in which a pair of waveguides approach, wherein at least one waveguide is curved in first and second S-bends, the first S-bend serving to reduce the separation of the waveguides by a larger amount than the second.

It is also preferred that the radius of curvature of the second S-bend is less than that of the first.

In this way, the waveguides can approach (and depart) cleanly and crisply whilst maintaining adequate separation.

The above discussion has dealt with MZI structures formed with a pair of waveguides. However, the invention is applicable in a like manner to MZI structures including more waveguides, such as three, four or any number.

Embodiments of the present invention will now be described by way of example, with reference to the accompanying figures, in which; Figure 1, already described, shows a known MZI structure; Figure 2 shows a first MZI layout according to the present invention; Figure 3 shows a second MZI layout according to the present invention; Figure 4 shows a third MZI layout according to the present invention; Figures 5 and 6 show arrays of MZI structures according to the first and second layouts respectively ; Figure 7 shows a first design of approach curve according to the present invention; and

Figure 8 shows a second design of approach curve according to the present invention.

Referring to Figure 2, a pair of waveguides 30,32 enter a directional coupler 34 which includes an approach region 36 and an exit region 38. After the exit region 38, waveguides 30,32 travel through a transit region 40 in which their separation is maintained. In the transit region 40, a double p-i-n diode structure 42 is formed as per that of Figure 1. Subsequent to the transit region 40, there is a further directional coupler 44 which again comprises an approach region 46 and an exit region 48.

In the directional coupler 34, waveguide 30 remains substantially straight throughout, i. e. in the approach region 36 and the exit region 38. Likewise, in the directional coupler 44, waveguide 32 remains straight throughout, i. e. in the approach region 46 and the exit region 48. Thus, by providing symmetric patterns of straight and curved portions of the two waveguides 30,32, each has the same physical path difference and thus, in the absence of excitation of the p-i-n-diode 42, the two waveguides exhibit substantially the same optical path.

The use of a substantially straight portion of waveguide through the majority of the directional couplers reduces the creation of higher order modes in the optical signal, which are induced by sharp bends. Thus, the structure shown in Figure 2 should perform its function with less distortion of the optical mode or modes that pass through it. Furthermore, the absence of curved portions in one waveguide per directional coupler means that the overall width of the interferometer is approximately one half of that of the interferometer of Figure 1, assuming that other factors remain constant.

Figure 3 shows another design according to the same invention. In this design, a pair of incoming waveguides 50,52 enter a first directional coupler 54 which includes an approach region 56 and an exit region 58, before entering a

transit region 60 in which there is formed a double p-i-n diode 62, as before. As with the embodiment of Figure 2, the waveguide 50 is substantially straight in the approach region 56 of the first directional coupler 54, but (in this embodiment) then curves away from the waveguide 52 in the exit region 58. In region 58, the waveguide 52 remains substantially straight. After the transit region at 60 and p-in diode 62, there is a further directional coupler 64 again comprising an approach region 66 and an exit region 68. In this directional coupler 64, to maintain a balanced path length as between the two waveguides 50,52, waveguide 50 remains substantially straight in the approach region 66 whereas waveguide 52 remains substantially straight in the exit region 68.

This arrangement retains the advantages of Figure 2 in that the total number of curved regions of waveguide remains the same. However, the waveguides 50, 52 exit the interferometer slightly displaced with respect to their approach directions. This may be useful according to the demands of other structures on the substrate.

Figure 4 shows a further layout. Again, the incoming waveguides 70,72 enter a first directional coupler 74 which includes an approach region 76 and an exit region 78. Waveguide 70 remains substantially straight throughout the directional coupler 74. Subsequent to the directional coupler 74, there is a transit region 80 in which is formed a double p-i-n diode structure 82 as with the previous examples. After the transit region 80, there is a further directional coupler 84 comprising an approach region 86 and an exit region 88. In this arrangement, the waveguide 70 which was substantially straight throughout the first directional coupler 74 is also substantially straight throughout the second directional coupler 84 including both its approach region 86 and exit region 88. Thus, all curvature is confined to the remaining waveguide 72 which therefore shows a longer optical path length in the absence of activation of the p-i-n diodes 82. This may be useful if the design parameters of the interferometer call for an initially unbalanced optical path. Meanwhile, the total number of curved regions of waveguide in this design

is the same as that of figures 2 and 3 and therefore the advantages thereof are retained.

Figure 4 shows a design which does correspond closely to Figure 2 in that one waveguide remains substantially straight throughout the whole of each directional coupler of the pair. However, it will be appreciated that a design analogous to that of Figure 3 could be provided in which one waveguide is substantially straight in the approach region and the other waveguide is substantially straight in the exit region of each directional coupler. This, again, would displace the exit paths of the waveguide 70,72 relative to their incoming paths, which may be advantageous depending on the design of the remainder of the structures on the same substrate.

Figure 5 shows how a large number of interferometers designed according to Figure 2 can be integrated onto the same substrate. An array of interferometers 90,92, 94 etc. are arranged in a substantially parallel arrangement. Each interferometer is substantially identical to that shown in Figure 2. Each includes a double p-i-n diode structure 96,98, 100 (respectively). Each has been designed such that the directional couplers, diode structures etc. are longitudinally displaced with respect to the adjacent waveguide. Thus, the first directional coupler 102 of the waveguide 92 is approximately adjacent the diode structure 96 of the adjacent interferometer 90. This displacement places the diode structure 98 of the interferometer 92 adjacent the second directional coupler of the adjacent waveguide 90 on one side and the first directional coupler of the adjacent interferometer 94 on the other side. This means that the diode structures 96,98, 100 etc. show a greater physical separation.

Hitherto, where such diode structures have been located close to each other, blocking structures have had to have been placed between the interferometers to prevent the spread of charge carriers from one diode structure to another. This would interfere with the transmission of optical modes through the adjacent diode

structure. By removing or alleviating the need for such blocking structures they can be eliminated or reduced in size, therefore further reducing the demand for space on the substrate.

Figure 6 shows an array of interferometers substantially as that shown in Figure 3. Again, the interferometers 102,104, 106 etc. are staggered such that the diode structures 108, 110, 102 etc. are not adjacent one another. It was noted in the description with respect to Figure 3 that the exits of the waveguides in the interferometer structures were displaced with regard to their approach, but in an array, each pair of waveguides can be allowed to shift to one side and occupy the space vacated by the adjacent waveguide.

Similar arrays can be envisaged for the pat-imbalance structure of Figure 4, or for a similar structure on the principles of Figure 3.

Figures 7 and 8 show embodiments of the second aspect of this invention.

As noted previously, in this aspect the problem of bringing the two waveguides together is addressed, and the solution shown can be applied to the interferometer shown in Figures 2-6 or to the interferometers shown in Figure 1 or to other interferometers.

Figure 7 shows a directional coupler for use in an interferometer substantially as per the illustration in Figure 1. A pair of waveguides 200,202 are initially substantially parallel and separated by a transverse distance d. They then both enter a first S-bend 204 to bring them together and reduce the separation to a smaller transverse distance d2. The distance d2 is typically at least 1 Opm, a distance at which there is substantially no interaction between optical modes travelling in the two waveguides.

Each waveguide then enters a second S-bend 206 which reduces the distance still further to a small distance d3 (at 208), which is sufficiently small for

optical modes in the two waveguides 200,202 to interact and form the directional coupler. After a short distance 208 in which the waveguides 200,202 remain close, they then separate at 210 via an S-bend which is essentially a mirror image of the second S-bend 206.

In this design, the first S-bend 204 serves to remove most of the separation between the waveguides 200,202. Thus, d-dz > d-dg.) t is preferred that d1 - d2 is more than several times dz-da.) n addition, the radius of curvature employed in the second S-bend 206 is less than the radius of curvature employed in the first S-bend 204. Thus, much of the distance between the waveguides 200,202 is removed by a gently curving S-bend which will limit the production of higher order modes (as noted above). However, as d is greater than the (typically 10jJm) distance beyond which there is little or no interaction between optical modes, the final approach between the two waveguides in the second S-bend 206 can be made at a tighter radius of curvature thereby giving a sharp and clean approach (206) and break (210) of the optical modes.

Figure 8 shows the same principle applied to a directional coupler as used in the interferometers of Figures 2-6. This is essentially the same as that of Figure 7 except that whereas one waveguide 212 shows the same pattern of curvatures, the second waveguide 214 remains substantially straight throughout. As noted previously, it could of course be that waveguide 212 is curved as necessary in the approach to the directional coupler, whereas waveguide 214 is curved on the exit, waveguide 212 then being substantially straight during the exit region.

It will of course be understood that whilst a very short approximately parallel region is shown in Figures 7 and 8 between the two S-bends, this region could if required be longer or shorter, perhaps vanishingly short with the curves of the two S-bends flowing into each other.

In this design, the rapid separation of waveguides ensures that the contribution of the bends to the coupling in between the optical modes is minimised as much as possible. Experimental observation shows that it is at this bend that the maximum variation in etch depth is encountered in practice, and this is difficult to model and account for.

It will be appreciated that many variations can be made to the abovedescribed embodiments without departing from the present invention.

Claims

1. A Mach-Zender interferometer defined by a pair of directional couplers acting on a pair of waveguides, each directional coupler comprising an approach region in which the separation of the waveguides decreases and an exit region in which the separation of the waveguides increases, characterised in that in at least one directional coupler, a waveguide is substantially straight in at least the approach region.

2. An interferometer according to claim 1 in which a waveguide is substantially straight in both directional couplers.

3. An interferometer according to claim 1 or claim 2 in which the relevant waveguide is substantially straight through the whole directional coupler concerned.

4. An interferometer according to claim 3 in which the other waveguide is substantially straight through the other directional coupler.

5. An interferometer according to claim 1 or claim 2 in which one waveguide is straight in the approach region of one or both directional couplers whilst the other waveguide is straight in the exit region.

6. An interferometer according to claim 5 in which the same waveguide is substantially straight in the approach region of both directional couplers.

7. An integrated optical device comprising an array of adjacent interferometers, each being according to any one of the preceding claims.

8. An integrated optical device according to claim 7 in which the interferomters are staggered such that their transit regions are not aligned.

9. A directional coupler in which a pair of waveguides approach, wherein at least one waveguide is curved in first and second S-bends, the first S-bend serving to reduce the separation of the waveguides by a larger amount than the second.

10. A directional coupler according to claim 9 in which the radius of curvature of the second S-bend is less than that of the first.

11. An integrated optical device substantially as herein described with reference to and/or as illustrated in the accompanying figures.