GB2384319A

GB2384319A - Polarisation converter for photonic crystal waveguide

Info

Publication number: GB2384319A
Application number: GB0201202A
Authority: GB
Inventors: Michael Charles Keog Wiltshire; Iman Khandaker
Original assignee: Marconi Optical Components Ltd; Bookham Technology PLC
Current assignee: Lumentum Technology UK Ltd
Priority date: 2002-01-19
Filing date: 2002-01-19
Publication date: 2003-07-23
Also published as: WO2003062909A2; WO2003062909A3; GB0201202D0; AU2003205818A1

Abstract

A polarisation converter is for use with a Photonic Crystal waveguide in which the waveguide comprises a host material having a first refractive index n 1 , bounded at least in part by host material (9) containing a plurality of inserts (10) each having a second different refractive index n 2 . The converter comprises a region (11a) of the waveguide in which incident radiation is resolved into different eigenmodes each of which propagates at such a different velocity as to introduce progressively a phase shift between the modes. The length of the region is such as to cause a predetermined phase shift, preferably of f . The host material may be InP/InGaAsP or GaAs/AlGaAs. InP substrate 12 and InP cladding layer 13 are shown. Inserts 10 may be rods or holes (n 2 =1 Ò 0).

Description

POLARISATION CONVERTER The present invention relates to a polarisation converter, more particularly a polarisation converter formed within a Photonic Crystal structure. Such devices are useful in compact optical communications systems. The invention is mainly described in the context of optical communications, however the invention has application for all electromagnetic radiation and is not constrained to optical communication wavelengths.

In optical communications systems, information is carried in the form of the modulation of a light beam. The wavelength of the light may be typically 1550nm (corresponding to a frequency of approximately 195 THz), whereas the modulation frequency may be many tens of GHz. A variety of components is necessary to generate and manipulate these optical signals and the efficiency of most of them depends on the polarisation state of the light. Even comparatively simple devices such as splitters are polarisation dependent. It is therefore essential that the required state of polarisation (SOP) should be presented to the input of each device. However, whether the light propagates through an optical sub-system such as a router or a receiver module, or through the optical fibre that forms the network, the SOP inevitably changes, either due to the effect of the components in the subsystem, or due to imperfections in the fibre. Therefore a device that restores the desired SOP is required; such a device is a polarisation converter or controller.

Polarisation converters or controllers are known. For example, US 5396365 describes an optical waveguide device on passage of which the polarisation of an input signal is

rotated. Many alternative devices have been described. Indeed, a simple half wave plate acts to rotate the SOP through 11 :/2, as is well known in the art.

However, such devices are bulky and unsuited for present day integrated optics applications. Hitherto optical sub-systems have consisted of discrete components interconnected by lengths of optical fibre, or by free space transmission paths. This approach suffers from the disadvantages of large size, lack of robustness, and restrictions in speed of operation. State of the art optical sub-systems ideally need to be integrated on a single substrate, to provide the optical equivalent of an electronic integrated circuit. Accordingly, a compact polarisation converter that can be integrated with other components on a common semiconductor substrate is required.

It is known to address this requirement using an asymmetric loaded buried or surface ridge waveguide such as described by Y Shani et al.,"Polarisation Rotation in Asymmetric Periodic Loaded Rib waveguides", Applied Physics Letters Volume 59, pages 1278-1280, September 9th 1991. The asymmetry in the guide substantially converts one input polarisation, say TE, to the other TM, and vice versa over a distance of some 300-400gm. In the context of optical semiconductor integration, this is a large device. There is also a significant, typically 3dB, intensity loss associated with the asymmetric loaded waveguide and when coupling losses are also taken into account, the loss may reach in excess of 11 IdB.

Other polarisation converters are known which use an asymmetric optical waveguide, that has a trapezoidal transverse section shape, as compared to the usual regular rectangular transverse section shape. Such a structure requires a long path length, typically 400-500il, in order to accommodate total polarisation conversion and suffers a high insertion loss as with the asymmetric loaded waveguide case. Thus such a polarisation converter device is not ideal for optical integration of many optical devices on a common substrate.

Traditional optical waveguides rely upon light being contained in a material of homogeneous refractive index, confined by homogeneous lower refractive index material or materials. So long as the propagation light hits the waveguide walls at angles below the critical angle, total internal reflection takes place and the light is

guided along the path of the waveguide. Such waveguides normally operate with single mode light.

Photonic Crystals are materials that will not allow electromagnetic radiation whose wavelength lies within a band of wavelengths to propagate in certain directions. These materials rely upon a dielectric, semiconductor or metal fabricated to have a one, two, or three-dimensional, spatially periodic refractive index contrast.

Electromagnetic radiation of a given wavelength range (the band gap) is substantially prevented from propagating through the material in a direction or directions in which the structure exhibits spatial periodicity. In certain Photonic Crystals, a band gap exists for all directions and for both polarisations of the radiation. Such a band gap is termed a full band gap, and the material is termed a Photonic Band Gap or PBG material. Although PBG materials as defined above are a subset of Photonic Crystals, it is common in the art to use the terms Photonic Crystal and PBG material interchangeably. Accordingly, the term PBG is hereafter to be taken to refer to both Photonic Crystals and Photonic Band Gap materials. So-called defects, breaks or changes within the regular periodic structure of the PBG material can couple to the radiation, or allow it to propagate within the defect. Such areas can form optical devices.

By its nature this approach allows some devices to be made much smaller than had previously been possible. For example a waveguide can be made by modifying one or two rows of the PBG structure, as described in US 5526449, to define a channel, which may be less than a micrometre (um) wide, through which light of appropriate wavelength (the band gap wavelengths) can propagate surrounded by the propagation inhibiting PBG structure. PBG waveguides can be bent through large angles because the radiation is rigorously excluded from the surrounding periodic PBG structure, and so cannot leak out of the guide. This light bending attribute makes it possible to fabricate compact integrated optical components. To maximise integration it is clearly desirable to construct components, for example splitters, combiners and active elements that are compatible with the PBG environment.

PBG structures can be two dimensional, or three-dimensional. A PBG structure may also act as a device in its own right, e. g. a band stop filter.

A two dimensional PBG structure in x, y, z space may be considered conceptually to comprise a plurality of rods, or similar shaped inserts, of material of refractive index nl in a host medium of refractive index n2. The rods are arranged, except where optical devices are defined, in a uniform array with regular spacing in, say, the x y plane and are long (effectively of infinite extent) in the z direction. Thus the periodic refractive index characteristic is found in, for example, the x y plane of the three dimensional material.

A three-dimensional PBG structure in x, y, z space may be considered conceptually to comprise spheres, or similar regular shaped inserts, of material of refractive index nl in a host medium of refractive index n2, the spheres, or other regularly shaped inserts, being arranged in a uniform matrix at regular spacing, except where optical devices are defined. Thus the periodic refractive index characteristic is found in, for example, the y z, x y and x z planes of the three dimensional material. The dimensions of the spheres, or other regularly shaped inserts, their spacing, the fill factor per unit volume, and the refractive indices ni and n2 of the contrasting materials determine the band gap centre wavelength and band gap width. As already established, such a PBG structure will not permit propagation of the band gap wavelengths in the direction through the exemplified y z, x y and x z planes.

PBG materials for optical communication applications can be fabricated using dielectric or semiconductor materials, or a combination of such materials. Other materials such as metals can also be used.

As disclosed in US 5526449 a waveguide in a two dimensional PBG material can be made by defining the waveguide pathway by removing a small number of adjacent rows of rods leaving a channel in the surrounding PBG medium. This channel constitutes a waveguide. It is also known that waveguides can be created using chains of coupled cavities. The normal modes of propagation for light passing through this waveguide are designated TE and TM. Here, the TE polarisation has the electric vector of the radiation parallel to the floor of the guide, whereas the orthogonal polarisation, TM, has the magnetic vector in this direction.

The PBG material or structure surrounding the waveguide described above may have a total band gap, that is it excludes both polarisations in all directions, or a partial gap that excludes only one polarisation or a gap for a limited range of directions.

However, a wider band gap is usually experienced for the TE than the TM radiation, so the PBG structure or material is polarisation sensitive. This difference in band gap characteristic for the TE and TM radiation stems in part from the way in which the electric field of the radiation interacts with the features of the structures (rods, holes or connecting material).

TE and TM are the approximate eigenmodes of the structure i. e. those modes in which the radiation propagates without conversion from one to another. However, it has now been appreciated that the propagation constant of each of the two polarisations or eigenmodes is different, because the PBG material responds differently to the two propagation polarisations. The propagation time of the TE mode is different to that of the TM mode and therefore, a length of waveguide within a suitably designed PBG structure can act as a polarisation converter.

According to the present invention, there is provided a polarisation converter for use with a Photonic Crystal waveguide comprising a region of that waveguide in which incident radiation is resolved into different eigenmodes each of which propagates at such a different velocity as to introduce progressively a phase shift between the modes.

Preferably the length of the aforesaid region is adapted to cause a predetermined phase shift, advantageously to introduce a phase shift of ? ! :.

The waveguide may comprise a host material having a first refractive index, bounded at least in part by host material containing a plurality of inserts each having a second refractive index different from said first refractive index.

In a preferred embodiment, the waveguide is defined at entry and exit regions between confinement layers and inserts in the form of rods extending normally to the confinement layers and arranged at regular spacing in an uniform array.

In this case, the aforesaid region of the waveguide may be defined at least in part by a plurality of said rods disposed to extend at an angle with respect to the longitudinal direction of the rods of the entry and exit regions.

Alternatively, the waveguide may be bounded by host material containing a plurality of regular shaped inserts arranged at regular spacing in an uniform array, the waveguide area defined by their absence being orientated differently in said median region from its orientation in the entry and exit regions.

Alternatively, the waveguide may be bounded by host material containing a plurality of substantially spherical inserts arranged at regular spacing in an uniform array, the waveguide area defined by their absence being orientated differently in said median region from its orientation in the entry and exit regions.

The preferred host material is InP/InGaAsP, which has a refractive index of the order of 3.3. Each insert is preferably a hole, which has a refractive index of 1.0.

A further preferred host material is GaAs/AlGaAs, which has a refractive index of the order of 3.0. Each insert is preferably a hole, which has a refractive index of 1.0 Ideally the holes are each of diameter between 250 and 300nm, and are arranged in a hexagonal array with a lattice spacing of between 350 and 450nm.

Embodiments of the present invention will now be more particularly described by way of example and with reference to the accompanying drawings, in which Figure 1 shows schematically a known two-dimensional photonic band gap (PBG) structure.

Figure 2 shows a traditional rectangular cross section waveguide to support an eigenmode of propagation TE; Figure 3 shows a traditional rectangular cross section waveguide to support an eigenmode of propagation TM;

Figure 4 shows a PBG structure sandwiched between two confinement layers ; Figure 5 shows a schematic plan view of a PBG structure including a region for obtaining polarisation shift ; Figure 6a is a cross sectional schematic view taken along the line A-A of Figure 5 ; and Figure 6b is a cross sectional schematic view taken along the line B-B of Figure 5.

Referring now to Figure 1, there is shown a two-dimensional PBG material as is known in the art. To avoid confusion only the hidden line structure of the closest rods are shown. The host material 1 has a refractive index of nu and the rods 2 have a refractive index of n2. The particular piece of material shown has a waveguide 3 which turns through 90 , as shown. The waveguide 3 is constructed by not implementing the rods 2 in the pathway that is to form the waveguide 3, and as a consequence such an area does not have the spatial periodic refractive index contrast and the band gap wavelengths will propagate through the host material 1 of refractive index nl bounded by the rods 2 of refractive index n2.

Figure 2 shows a traditional rectangular cross section waveguide 4. So long as dimension"b"is greater than half the wavelength of the wavelengths of interest X, the waveguide will support an eigenmode of propagation TE wherein the electric field is parallel to the substrate and the waveguide floor and ceiling 6, and the magnetic field is parallel to the walls of the waveguide 5. The direction of wave propagation is as shown in Figure 2.

Figure 3 shows the same waveguide as in Figure 2, but now with an eigenmode of propagation TM wherein the magnetic field is parallel to the substrate and the waveguide floor and ceiling 6, and the electric field is parallel to the walls of the waveguide 5.

As shown in Figure 4, in practice a PBG structure, be it two or three dimensional, will be sandwiched between two confinement layers 7 and 8. The PBG structure may be that of Figure 1. In this arrangement, light of wavelength in the waveguide 3 is trapped by the confinement layers 7 and 8 and by the PBG material.

The two polarisations (TE and TM) of radiation propagating through such a waveguide will posses different propagation velocities. Thus there will be a phase shift between the two polarisations of the light as they propagate through the guide.

On exit, they will recombine to provide a different overall polarisation from that which entered the guide.

The key requirement for the device to act as a polarisation converter is that the eigenmodes of the first and last sections, the input and output respectively, are not eigenmodes of the central region. Then incoming radiation in one of the eigenmodes of the input section will be resolved into the eigenmodes of the converter section, and propagate through the converter section according to the local, different propagation constants. On emerging from the converter section to the output section, the radiation is once again resolved into new polarisations that are the eigenmodes of the following waveguide. By suitably designing the length and propagation constant of the converter, it can rotate the input polarisation by n/2, i. e. it acts as a half wave plate does in bulk optics.

A preferred means of obtaining the polarisation shift is shown in plan view in Figure 5, and in cross section in Figures 6a and 6b.

The input section, as shown in Figures 5 and 6a is a regular PBG waveguide section lie comprising host material 9 of refractive index nl and rods 10 of refractive index n2, sandwiched between a substrate 12 and a cladding layer 13. A waveguide 11 is formed by omitting a number of lines of the rods 10. Radiation 14 propagates along the waveguide lie in the TE and TM polarisations in the manner shown in Figure 6a.

The input section lie is followed by a different PBG structure region lla that supports neither the TE nor TM modes of the previous section but has eigenmodes TE'and TM'. This causes the wave to undergo a resolving into components a in the

TE'wave, and in the TM'wave, that match the new waveguide conditions. The new a and waves will have different propagation velocities so that when they emerge from the region lla there has been a 7t phase shift between the two components. The phase shift will be in dependence upon the length of time the energy stays in its a and form, that is upon the length of the region 11a. It should also be appreciated that the phase shift will be dependent upon the radiation wavelength.

An example of such a different PBG structure 11a comprises angled rods lOa of material of refractive index n2, defining a waveguide region lla, as shown in Figure 5 and in cross section in Figure 6b.

The different PBG structure 11a is followed by a further section of regular PBG structure lib that causes the wave to resolve itself back into the original TE and TM eigenmodes. The proportions of TE and TM in this second section of regular waveguide lib will be different from those that were present in the initial section of waveguide lie, because of the effect of the different section of waveguide 11a between them.

By suitably choosing the length of this different section lla, a conversion from TE to TM and vice versa can be effected.

A particular example will now be described, provided with a waveguide layer 9 of InGaAsP, whose refractive index ni is approximately 3.3, on an InP substrate 12 with a further InP cladding layer 13. For the input section, there is provided a PBG structure comprising a hexagonal array of holes 10 (so that n2 = 1.0) of diameter 273nm with a lattice spacing of 382nm. The waveguide is along one of the hexagonal symmetry directions, conventionally denoted FM, and is 1*4 rows wide. This transmits both TE and TM polarised radiation, but we assume, for example, TM radiation in the input. The second region lla is composed of the same structure, but with the holes lOa etched at an angle of 45 to the substrate, as shown in Figure 6b. Then the incident radiation (TM in section He) will be resolved into equal amounts of the new eigenmodes TE'and TM'. These experience different refractive indices, and hence propagate at different velocities. For this example, the difference between the refractive indices or birefringence, An, is approximately 0.08.

Accordingly, whereas the TE'and TM'modes started in phase with each other (because they arose from the resolving of a single input wave), a phase shift of magnitude 27LAnd/k is introduced after a distance d in this section where is the wavelength of the radiation. For a wavelength X, of 1550nm after a distance of 9. 7 p. m, the phase shift has reached n. At this point, the radiation leaves section lla, and returns to the original structure 11b. The modes TE'and TM'are further resolved back onto the TE and TM eigenmodes of the final section, but taking account of the phase shift introduced by section lla. This leads to the cancellation of the TM component and a complete conversion into the TE component, as required. By designing appropriate alternative lengths for the region lla, so other shifts may be obtained, and thus other SOP obtained.

Sections lie, 11a and 11b of the preferred polarisation converter can be manufactured, for example, using Focussed Ion Beam Milling techniques.

As may be seen the invention provides a polarisation converter which is compatible with existing PBG structures, which has a compact size and which gives repeatable performance. The converter is able to operate with definable characteristics and with low loss at a high efficiency.

Claims

CLAIMS 1. A polarisation converter for use with a Photonic Crystal waveguide comprising a region of that waveguide in which incident radiation is resolved into different eigenmodes each of which propagates at such a different velocity as to introduce progressively a phase shift between the modes.
2. A polarisation converter as claimed in claim 1 wherein the length of the region is adapted to cause a predetermined phase shift.
3. A polarisation converter as claimed in either claim 1 or claim 2 wherein a phase shift of is introduced.
4. A polarisation converter as claimed in any one of the preceding claims, wherein the waveguide comprises a host material having a first refractive index, bounded at least in part by host material containing a plurality of inserts each having a second refractive index different from said first refractive index.
5. A polarisation converter as claimed in claim 4, wherein the waveguide is defined at entry and exit regions between confinement layers and inserts in the form of rods extending normally to the confinement layers and arranged at regular spacing in an uniform array.
6. A polarisation converter as claimed in claim 5, wherein the region of the waveguide is defined at least in part by a plurality of said rods disposed to extend at an angle with respect to the longitudinal direction of the rods of the entry and exit regions.
7. A polarisation converter as claimed in claim 4, wherein the waveguide is bounded by host material containing a plurality of regular shaped inserts arranged at regular spacing in an uniform array, the waveguide area defined by their absence being orientated differently in said region from its orientation in the entry and exit regions.

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8. A polarisation converter as claimed in claim 7, wherein the waveguide is bounded by host material containing a plurality of substantially spherical inserts arranged at regular spacing in an uniform array, the waveguide area defined by their absence being orientated differently in said region from its orientation in the entry and exit regions.
9. A polarisation converter as claimed in any one of the preceding claims, wherein the host material is InP/InGaAsP having a refractive index of the order of 3.3
10. A polarisation converter as claimed in any one of claims I to 8, wherein the host material is GaAs/AIGaAs having a refractive index of the order of 3.0.
11. A polarisation converter as claimed in any one of the preceding claims, wherein each insert is a hole of refractive index 1.0.
12. A polarisation converter as claimed in claim 11, wherein the holes are each of diameter between 250 and 300nm, and are arranged in a hexagonal array with a lattice spacing of between 350 and 450nm.
13. A polarisation converter, for use with a Photonic Crystal waveguide, substantially as described herein with reference to the Figures of the accompanying drawings.