WO2004021056A1

WO2004021056A1 - Improved photonic crystal device

Info

Publication number: WO2004021056A1
Application number: PCT/GB2003/003784
Authority: WO
Inventors: Richard Michael De La Rue; Pierre Michel Francois Pottier
Original assignee: The University Court Of The University Of Glasgow
Priority date: 2002-08-31
Filing date: 2003-08-29
Publication date: 2004-03-11
Also published as: GB0220314D0; AU2003269102A1

Abstract

There is disclosed an improved photonic crystal device (40), e.g. an optical coupler or coupling device, providing a photonic crystal waveguide means (44). Known taper coupling devices suffer from a number of problems such as excessive length and coupling losses. The invention seeks to provide an ultra-short taper, e.g. having a length of 5pm to l0pm or less. Accordingly the invention provides a photonic crystal device (40), providing a taper or optical transmission region (42) bounded by first and second photonic crystal (PhC) regions (43a,43b), first and second interfaces (46a, 46b) between the first and second photonic crystal regions (43a,43b) and the taper (42), respectively, being substantially continuous. By 'substantially continuous' is meant that each interface (46a,46b) does not include any discontinuities, e.g. steps in a lattice of the regions (43a,43b).

Description

IMPROVED PHOTONIC CRYSTAL DEVICE

FIELD OF INVENTION

The present invention relates to an improved device such as a photonic crystal or optical device, and in particular, though not exclusively, to an improved coupler or coupling device providing a photonic crystal waveguide means. The invention also concerns a related apparatus and method.

BACKGROUND TO INVENTION

In order to couple light between, for example, an optical fibre and an optically passive or active device, such as on a chip (e.g. an integrated photonic circuit), it is typically necessary to reduce an optical beam spot size of the fibre to an optical beam spot size of a waveguide of the device in order to avoid huge losses resulting from direct coupling.

Photonic band-gap (PBG) guiding is a mechanism for light guidance that is radically different from that used in traditional dielectric waveguides. Line defects made of air in photonic crystals (PhC's) can guide optical light in air, in certain conditions, because they do not rely on traditional index guiding. This means losses along a propagation direction due to material absorption can be greatly reduced. Due to the different physics of traditional index guiding and PBG guiding, coupling into and out of PhC waveguides using traditional coupling techniques can result in substantial reflection and scattering from PhC waveguides - which adversely affect transmission characteristics. Ridge tapers with weak confinement are used to reduce spot size, and ridge tapers having strong confinement are currently being developed. However, weak confinement tapers are long, often in the order of some millimetres, which is not suitable for use in integrated optics, such as integrated optics on semiconductor chips. Furthermore, strong confinement tapers, although shorter than weak confinement tapers - being in the order of 50μm to lOOμm - can suffer lateral losses due to irregularities of the side walls. It is an object of at least one aspect of the present invention to obviate or at least mitigate problems in the prior art .

It is a further object of at least one embodiment of at least one aspect of the present invention to provide an improved optical coupling means for use with integrated optical arrangements.

A further object of at least one embodiment of at least one aspect of the present invention is to create an ultra- short taper. In respect of the present invention it will be understood that the term "ultra-short" refers to a likely length range of typically around 5μm to lOμm or less.

A yet further object of at least one embodiment of at least one aspect of the present invention is to provide a novel hole-based photonic crystal (PhC) taper, based on continuous tapering and lattice distortion capable of transmitting typically up to 98.4% of' incident light

(computed value) for a beam reduction factor of 4:1 over a length of only 5μm into a single row (Wl) 2D PhC channel waveguide . A still further object of at least one embodiment of at least one aspect of the present invention is to provide a new approach which involves creation of a continuous taper by deforming a PhC lattice to provide a smooth size conversion of a beam over a short distance. Furthermore, a narrow end of such a taper can be naturally coupled into a PhC channel waveguide and an input end can be directly positioned at a facet of a chip, possibly suppressing a need for a ridge access guide and associated coupling losses. SUMMARY OF INVENTION

One or more objects of the present invention may be addressed by the general solution of providing a device such as an optical device with a taper, the taper having first and second sides adjacent first and second photonic crystal (PhC) regions, wherein a lattice of each of the first and second photonic crystal regions is arranged or distorted such that the first and second sides of the taper are substantially continuous or can be defined by substantially continuous surfaces.

According to a first aspect of the present invention there is provided a device such as an optical device providing a transmission region bounded by first and second photonic crystal (PhC) regions, first and second interfaces between the first and second photonic crystal regions and the transmission region respectively being substantially continuous .

By "substantially continuous" is meant that at least part of each interface does not include any discontinuities, e.g. steps in a lattice of the regions.

Preferably the device is an optical device, and is adapted to operate at optical wavelengths, the transmission region being an optical transmission region.

Preferably the optical device is an optical coupling device.

Preferably a width of the transmission region tapers or reduces between a first (e.g. input) end and a second (e.g. output) end thereof.

By such arrangement likely coupling or transmission efficiencies of as high as 98% can be expected for a single wavelength, as compared to 80% for prior art arrangements.

Preferably each PhC region comprises a plurality of rods or holes .

Conveniently the rods are prismatic objects. Preferably each rod is a dielectric rod, e.g. an air rod.

At least a part of each rod may be circular, elliptical, square, triangular or freeform in cross-section. Each rod in the device is typically provided with the same cross-sectional shape.

The cross-sectional shape or dimension of each rod may vary along the length of the rod.

The rods may have a higher, or preferably and most advantageously, a lower refractive index than a surrounding or carrier material of each PhC region.

Preferably the rods of each PhC region are regularly spaced one from the other so as to form a lattice.

The lattice may be square, parallelogram, triangular or hexagonal in shape, or of a more complex shape, e.g. Penrose tiling.

Each rod may be orientated in a same manner within the lattice .

The first interface and second interface may be disposed at a first acute angle and a second acute angle, respectively, with respect to an axis of the optical transmission region.

Rods at or adjacent each interface may be substantially identical one to the other. In each row of the lattice which forms a part of an interface adjacent rods may decrease in diameter towards the interface .

Alternatively the size of the rods can vary along a propagation or transmission direction and such variation may be coupled with a tuning angle to obtain particular propagation characteristics.

The first interface and the second interface may be substantially straight. The first acute angle and the second acute angles may be substantially equal.

The first acute angle and the second acute angle each may be around 0° to 10°, and preferably around 5°. Preferably one of the first or second acute angles may be greater than 0° to around 10°, and preferably 5°, while the other of the first or second acute angle may be around 0° to 10°, and preferably 5°, and most preferably greater than 0° to around 10°. In a first form of the invention each PhC region comprises a substantially triangular or hexagonal lattice, wherein an axis of each lattice is disposed at an acute angle to an axis of the optical transmission region.

In a second form of the invention each PhC region comprises a substantially parallelogram lattice, wherein an axis of each lattice is disposed at an acute angle to an axis of the optical transmission region.

Alternatively, in a third form of the invention each interface comprises a curve which is convex with respect to the optical transmission region.

The rods may comprise blind air holes.

In one form the device may advantageously be adapted for use in the C-, L- or S-band, e.g. most advantageously at a wavelength of around 1.55μm ± 60nm (total of C+L+S) , or alternatively in another form around 850nm ± 50nm. However, the device is scalable for use at other wavelengths, e.g. 1.3μm, 980nm, visible. Alternatively, the device may be adapted for use in the microwave or millimetre wavelength regions . • In said one form each rod may be around 0.3μm to 2.5μm in depth, and may preferably be 1.3μm in depth, or alternatively in said another form may be around 500nm to 900nm in depth, and may preferably be 700nm in depth. The same concepts apply for other technological approaches based on extended cylinders, e.g. deep etching, macroporous silicon.

In said one form each rod may be of substantially cylindrical form, and may advantageously have a diameter of around 0.2μm to 0.35μm, and preferably around 0.25μm, or alternatively in said another form may have a diameter of lOOnm to 150nm, and preferably around 130nm.

In said one form a period between centres of adjacent rods may be around 0.3μm to 0.55μm, and preferably around 0.4μm, or alternatively in said another form may be around 200nm to 230nm, and preferably around 215nm.

Preferably the optical transmission region of the tapered structure may be around lOμm or less in length.

Preferably in said one form a breadth of a first end of the transmission region may be around lμm to 5μm, and preferably around 2μm, or alternatively, in said another form may be around 0.5μm to 2μm, and preferably around lμm.

Preferably also, in said one form a breadth of the second end of the transmission region may be around 0.2μm to O.δμm, and preferably around 0.4μm, or alternatively in said another form may be around 0. lμm to 0.4μm, and preferably around 0.2μm.

The device may comprise a substrate upon which is provided a first cladding layer and a core (guiding) layer, and optionally a second cladding layer.

The device may alternatively comprise a substrate, a first cladding layer, a core layer, a second cladding layer and a superstrate .

The device may further comprise the core layer disposed upon a low refractive index material.

Alternatively the device may comprise a suspended layer or membrane .

The PhC regions may comprise optical waveguide confining means. In a first embodiment the PhC regions comprise a sole optical waveguide confining means. However, in a second embodiment the PhC regions comprise first optical waveguide coupling means, and a ridge waveguide comprising a second optical waveguide coupling means. The optical device may, in use, convert a spot size of an optical signal or optical mode input at a first end of the device and transmitted to a second end of the device, or vice versa.

The optical device may comprise an optical coupling device, and may be used as a spot size converter, e.g. in coupling between an optical fibre and a PhC passive device (e.g. waveguide) or PhC active device.

The optical coupling device may alternatively be used in coupling between a dielectric waveguide and a PhC passive or active device. The optical device may alternatively find use as a filter. The optical coupling device may alternatively comprise part of an active PhC device, e.g. an active optical source, e.g. laser.

According to a second aspect of the present invention there is provided a device such as an optical device comprising at least two photonic crystal (PhC) regions, each photonic crystal region including at least one edge, wherein the edge of each photonic crystal region is disposed at an acute angle or angles to an axis of optical transmission. According to a third aspect of the present invention there is provided a device such as an optical device comprising a taper formed of at least two regions of photonic crystal (PhC) each comprising a lattice structure, wherein the photonic crystal lattice is not parallel or orthogonal to an axis of optical transmission of the optical device.

According to a fourth aspect of the present invention there is provided a photonic device including at least one optical device according to the first, second or third aspects of the present invention. The optical device of the first, second or third aspects or the photonic device of the fourth aspect of the present invention may comprise a passive device, such as a spot size converter, Y-junction, mirror, reflector, coupler, or an active device, such as a light condenser, light speed reducer, optical source, laser, filter, or pulse compressor.

According to a fifth aspect of the present invention there is provided an apparatus or system including at least one optical device according to the first, second or third aspects of the present invention.

The system may comprise a communications system. According to a sixth aspect of the present invention there is provided a method of manufacturing an optical device according to the first, second or third aspects of the present invention including the step of forming the PhC regions and the/an optical transmission region, thereby forming the/a first interface and the/a second interface. Preferably the PhC regions are formed by Reactive Ion Etching (RIE) . According to further aspect of the present invention there is provided a device, apparatus, system or method as recited in any of the appended claims.

BRIEF DESCRIPTION OF DRAWINGS These and other aspects of the present invention will become apparent from the following description, when taken in combination with the accompanying drawings, which are:

Figure 1 a schematic representation of a first prior art photonic crystal taper arrangement ; Figure 2 a schematic representation of a second prior art photonic crystal taper arrangement; Figure 3 (a) a schematic representation of a photonic crystal structure; Figure 3 (b) a schematic representation of a waveguide formed in a photonic crystal structure;

Figure 4 (a) a schematic representation of a photonic crystal device according to a first embodiment of the present invention;

Figure 4(b) a block diagram of the photonic crystal device of Figure 4(a);

Figure 5 (a) a schematic representation of a photonic crystal device according to a second embodiment of the present invention;

Figure 5 (b) a block diagram of the photonic crystal device of Figure 5 (a) ;

Figure 5(c) a schematic representation of a photonic crystal device according to a modification to the photonic crystal device of Figure 5 (a) ;

Figure 6 (a) a schematic representation of a photonic crystal device according to a third embodiment of the present invention;

Figure 6 (b) a block diagram of the photonic crystal device of Figure 6 (a) ;

Figure 7 a schematic representation of results of a finite difference time domain simulation of the photonic crystal device of Figure 5 (a) ;

Figure 8 a graphical representation of a spectral response of the photonic crystal device of Figure 5(a);

Figure 9 a schematic representation of a photonic crystal device according to a fourth embodiment of the invention;

Figure 10 a view of the photonic crystal device of Figure 9 taken along direction "A"; Figure 11 a cross-sectional view of any one of the photonic crystal devices of Figures

4(a) , 5(a) or 6(a) ;

Figure 12 a schematic representation of a photonic crystal device according to a fifth embodiment of the present invention;

Figure 13 a schematic representation of a photonic crystal device according to a sixth embodiment of the present invention;

Figure 14 a schematic representation of a photonic crystal device according to a seventh embodiment of the present invention; and

Figures 15 (a) to (g) schematic representations of photonic crystal devices according to eighth to fourteenth embodiments of the present invention.

DETAILED DESCRIPTION OF DRAWINGS

Referring initially to Figures 1 and 2 there are shown prior art photonic crystal tapers 10; 20 comprising photonic crystal waveguides 14; 24, which comprise line defects in a photonic crystal (PhC) structure, wherein a taper or optical transmission region 12,-22 is formed, bounded by first and second photonic crystal regions 15a, 15b; 25a, 25b adjacent to the waveguide 14; 24 to allow optical coupling to take place. In these arrangements, photonic crystal regions have a lattice structure arranged parallel to a direction of optical transmission along the taper 12;22 and waveguide 14,24. First and second photonic crystal regions have a first interface and a second interface, respectively, which form the waveguide 14; 24. The taper 12; 22 is formed by removal (or non- formation) of further lattice layers at each interface, the layers removed in increasing number from one end of the taper 12,-22 to another end so as to provide a wide aperture to allow coupling to the waveguide 14,-24 to take place. This provides a stepped taper 12;22 having discontinuities, e.g. steps in or at the interfaces adjacent the taper 12;22. Typically, such an arrangement is only able to provide a maximum transmission of 80% at a single wavelength. With reference now to Figure 3 (a) , there is shown a schematic representation of a photonic crystal (PhC) 30a which comprises a plurality of dielectric rods 31 of lattice constant a. The rods 31 have a circular cross-section of radius 0.2a, and a refractive index n = 3.4, appropriate for silicon at 1.55μm wavelength, and are disposed in a low index background, i.e. air, having n = 1. The rods may alternatively be air rods with a refractive index of n = 1 , the rods being surrounded with a background medium having a refractive index of n = 3.4. In such an arrangement having air rods, the PhC has a large band-gap for TE polarized modes.

Referring now to Figure 3 (b) , a waveguide is formed by a simple line defect 34 in the crystal 30a, created by removing a line of rods 31. The waveguide 34 is thus bounded by photonic crystal regions 35a, 35b, respectively. Turning now to Figure 4 (a) there is shown a photonic crystal device or optical device 40 according to a first embodiment of the present invention, comprising an optical transmission region or taper 42 bounded by first and second photonic crystal (PhC) regions 43a, 43b, respectively. The regions 43a, 43b form taper 42 which acts to facilitate optical coupling between a wide (e.g. input) end of the taper 42 and a PhC waveguide 44 formed between PhC regions 45a and 45b at a narrow (e.g. output) end of the taper 42. First and second interfaces 46a, 46b between the first and second PhC regions 43a, 43b and the taper 42 are substantially continuous and form "walls" of the taper 42.

As can be appreciated, herein by "substantially continuous" is meant that each interface 46a, 46b does not include any discontinuities, e.g. steps in a lattice of the regions 43a, 43b. A continuous smooth surface, e.g. planar surface, or in a modification curved surface, can therefore be defined between a first rod 31 on an interface 46a, 46b and adjacent rods 31 on either side of the first rod 31. It is envisaged that such an arrangement can provide coupling or transmission efficiencies of up to 98%.

The first and second interfaces 46a, 6b of the optical device 40 are disposed at a first and second acute angle θi_fθ₂/ respectively, to an axis of the optical transmission region or taper 42; in this case each acute angle θι,θ₂ being 5°. θχ,θ₂ can be described as angles of tilt of the lattice. It can also be seen that in this embodiment the first and second interfaces 43a, 43b are substantially straight or linear. In Figure 4 (b) there is shown a schematic representation of the optical device 40 of Figure 4 (a) from which it can be seen that each side of the taper 42 is formed by offsetting a rectangular section of the PhC structure or regions 43a, 43b, having a substantially triangular lattice at an angle of 5° from the axis of the taper 42. By offsetting the regions 43a, 43b from the regions 45a, 45b (which having a substantially triangular lattice, are arranged to provide the waveguide 44) , the taper 42 is provided with first and second interfaces 46a, 46b which are graded inwards smoothly in view of the planar nature of the edge of the PhC regions 43a, 43b. This arrangement enables a reduced size taper 42 to be formed, having the compactness of known strong confinement tapers but the transmission capability of known weak confinement tapers.

In Figure 5 (a) there is shown a photonic crystal device or optical device 50 according to a second embodiment of the present invention, having optical transmission region or taper 52 formed in a PhC lattice structure, and which facilitates optical coupling to a PhC waveguide 54.

A schematic representation of the optical device 50 is shown in Figure 5 (b) . In this case it can be seen that each of the PhC regions 53a, 53b - which act to form the taper 52 - is formed as a substantially parallelogram lattice. A first end of each parallelogram lattice is abutted against a first end of the respective section or region 55a, 55b of the PhC structure forming the waveguide 54. The regions 55a, 55b have a triangular lattice structure. This results in inner edges 56a, 56b of the parallelogram sections 53a, 53b forming first and second interfaces or taper walls, which are provided with substantially planar edges. As detailed previously such a taper 52 provides the benefits of a transmission capability of known weak confinement tapers, while having the compactness of dimensions of known strong confinement tapers. Turning now to Figure 5(c) , there is shown an optical device 50' according to a modification to the second embodiment of Figure 5(a). The device 50' consists of a Wl PhC channel waveguide 54 ' preceded by a taper 52 ' where a width of the PhC channel waveguide 54 ' is varied progressively . A linear variation is represented here. For this purpose, a PhC lattice is deformed by shifting each row of holes 31 laterally (i.e. in a direction perpendicular to the taper axis), by the amount a V3. tilt with respect to the previous row, where a is the lattice period of the PhC lattice, and tilt is a parameter introduced to describe this deformation. The angle x of the tilted lattice is given by a = arctan (V3.tilt) .

The taper 52 ' can also be obtained by rotating a PhC lattice, which preserves the form of the original PBG properties, but creates disorder at the interface with the PhC channel waveguide 54 ' . The disorder is limited, however, if the angle is small. Extending the generality of the situation, the lattice tilt can be varied locally and it can be either compressed or dilated by bending, allowing freely chosen taper profiles (e.g. exponential, cosine or parabolic) to be obtained.

A photonic crystal device or optical device 60 according to a third embodiment of the present invention is shown in Figure 6(a) . The optical device 60 has an optical transmission region or taper 62, formed in a PhC structure to facilitate optical coupling to a PhC waveguide 64.

In this third embodiment each PhC region 63a, 63b of the taper 62 is formed having a curved interface 66a, 66b, respectively. In this case, each interface 66a, 66b is curved convexly with respect to the optical transmission region 62, thus providing the taper 62 with smooth inside surfaces .

A schematic representation of the taper 62 of Figure 6 (a) is shown in Figure 6 (b) .

In each of the above embodiments deformation of the regions 43a, 43b; 53a, 53b; 63a, 63b in the optical transmission region or taper 42,-52,-62 provides a high transmission device

40,-50,-60 , having a short taper length, which typically is in the region of 5μm to lOμ .

Results obtained using two dimensional (2D) Finite Difference Time Domain (FDTD) simulations of the tapers show a tapering mechanism able to shrink a beam width by a factor of 5 (e.g. from 2μm to 0.4μm) over a distance of lOμm with a transmission of 98% for a particular wavelength. Furthermore, such an optical device 40; 50; 60 can transmit more than 90% of light on a bandwidth representing 3.8% of the spectrum which covers the C-band or L-band normally used in WDM transmission systems.

Turning to Figure 7, an FDTD simulation of the device 50 of Figure 5(a) is shown at a wavelength, λ = 1.55μm. This illustrates a transmission of 98% achieved by the device 50. The grey scale provided represents an amplitude of magnetic field recorded across the device 50. The transmission of the device 50 is calculated by: power of EM wave through the section at the end of the taper 52 divided by power of EM wave injected at the entry of the taper 52.

In Figure 8 a spectral response of the device 50 of Figure 5 (a) is graphically illustrated with transmission and reflection plotted against wavelength. The upper (solid) line represents the transmission of the device 50. The lower

(dotted) line represents light reflected by the device 50.

As can be seen at a wavelength of 1.518μm and at a wavelength of 1.586μm, the device 50 selectively reflects a substantial part of an input beam, and therefore, this feature allows the device 50 to be further used as a filter. The wavelengths which are filtered by the device 50 are determined by the angles θχ,θ₂, that is the angles at which the taper 52 is formed. The device 50 of Figure 5(a) shows reflection of around 50%; however this device 50 was not optimised for the reflection. When the device is optimised for reflection, reflection of much more than 50% can be achieved.

Referring next to Figure 9, there is shown a photonic crystal device or optical device 90 according to a fourth embodiment of the present invention, having optical transmission region or taper 92. In this device 90 there is provided a first optical confining means, in this case the PhC taper 92, and a second optical confinement means 98, in this case a ridge taper. This combined arrangement provides the advantages of smooth borders of tapers provided by the ridge taper 98, and strong lateral confinement without lateral losses, provided by the PhC taper 92. In Figure 10 there is shown a side view of the optical device 90 of Figure 9. As can be seen, the ridge taper 98 is raised with respect to the PhC regions 93a, 93b. In one implementation the ridge taper 98 can be a continuation of a ridge formed outwith the optical device 90. In Figure 11 there is shown a cross-section of a device

40; 50,-60 such as that illustrated in Figures 4(a), 5(a) or 6(a) . As can be seen, the device 40,-50; 60 comprises a substrate 111 upon which is provided a first (lower) , cladding layer 112 of around 3μm thick and a core (guiding) layer 113 of around 0.5μm thick, a second (upper) cladding layer 114 of around 0.2 μm thick and a superstrate 115 (usually air) . Dielectric rods 116, having a lattice period of around 0.4μm, and a length of around 1.3μm extend through the upper cladding layer 114 and core layer 113 to cladding layer 112. As shown, the dielectric rods 116 may comprise air rods .

In Figure 12 there is shown a photonic crystal device or optical device 120 according to a fifth embodiment of the present invention, wherein rods have been removed from the PhC lattice leaving dielectric rods 121a, 121b at interfaces 126a, 126b of an extended transmission region or taper 122. The feature of the extended transmission region or taper 122 can be utilised to provide a broadened transmission region. The removal of rods around the interfaces 126a, 126b modifies the transmission properties, in this case increasing the spectral bandwidth of transmission.

In Figure 13 there is shown a photonic crystal device or optical device 130 according to a sixth embodment of the present invention. In this device 130, dielectric rods are removed from the PhC lattice on each side of the transmission region 132 resulting in blank areas 138a, 138b. This arrangement allows the device 130 to laterally extract light at a particular frequency, thus acting as a drop filter with, as is effective for a filter arrangement, the waveguide being a single row defect only. Furthermore, this arrangement can be used to modify the transmission spectrum of taper 132 of the device 130.

Turning now to Figure 14, there is illustrated a photonic crystal device or optical device 140 according to a seventh embodiment of the present invention in which the taper 142 is formed in two sections 142a, 142b, respectively, by the voluntary shifting of the lattice structure of the photonic crystal - meaning that the interfaces 146a, 146b are disposed at, in this case, two different angles from an axis of optical transmission 147. This results in the interfaces 146a, 146b having a "free-form" shape. This device 140 may be used, for example, to modify the spectral response of taper 142. Each of the embodiments illustrated hereinbefore perform full photonic crystal integration, or hybrid integration, for example, coupling between a dielectric waveguide and a PhC passive or active device, or indeed between PhC passive and/or active devices. The tapers may be connected upstream or downstream. The entry, or exit, of these tapers can also be connected to classical wide ridge waveguides, and the coupling made easier by the width of the entry. Alternatively, the tapers may be connected to no particular element with the beam entering freely, or continuing uninterrupted. The tapers may alternatively be positioned at an end or beginning of a chip or facet, and in this case an optical fibre can be directly coupled to the taper. Further alternatives include a connection to a further focusing element, or a combination of elements such as those detailed above.

Furthermore the "continuous" taper may be connected to other tapers whether continuous or as in the prior art having discontinuities. Such an arrangement can be used to capitalise on particular transmission or reflection characteristics, or for an active device by creating a cavity.

Another arrangement may include two tapers arranged side by side, e.g. with the entrance of each taper disposed adjacent one another. Such an arrangement can be used to compensate for misalignment or stiching errors occurring in fabrication between two parts of a PhC device, in particular waveguides. One taper in such an arrangement is typically much narrower than a second taper, and the arrangement may be such that a pure waveguide is disposed adjacent to a taper.

It should be understood that one or more holes or dielectric rods of the PhC taper can be modified in shape or position or removed or added, particularly in the transmission region, to engineer or tune transmission spectrum characteristics of the taper. Examples of this can be seen in the embodiments shown in Figures 12 and 13.

An optical device, comprising an optical coupling means, as illustrated in the embodiments described hereinabove, can act as a spot size converter, converting the spot size of an optical signal or optical input mode at a first end of the device and transmitted to a second end of the device, or vice versa . The optical device can be used in coupling between other optical devices or transmission means, e.g. an optical fibre and a PhC passive device (such as a waveguide) , or a PhC active device.

Furthermore an optical device according to the present invention, including a photonic crystal taper as described hereinbefore may alternatively be used for filtering or light generation (in the WDM field) utilising frequency selective characteristics .

An optical device according to the present invention including an optical transmission region or taper acting, in use, as an optical coupling means can form part of a passive PhC device or an active PhC device, such as an active optical source, e.g. a laser. Furthermore, the optical coupling device can be incorporated into a photonic device or an optical apparatus or optical system. The optical system in which the device can be included can be a communications system.

The optical device of the present invention can be manufactured utilising a method wherein the PhC regions are formed, thus forming the optical transmission region or taper, wherein the PhC regions are formed by Reactive Ion Etching. It will however be appreciated that other techniques which are known in the art may be used.

Photonic crystal devices according to the present invention may find use in a number of applications, including applications where reflective properties of the photonic crystal device can be exploited.

For example, the photonic crystal device of the invention can be used as a light condenser. In such application the photonic crystal device can be used to shrink a beam down to a narrower than Wl waveguide, (e.g. 50nm at 0.85μm). Herein Wl means that one (1) row of rods or holes is removed (see Figure 3 (b) ) . High concentration of power is then obtained which can facilitte reaching nonlinear effects. A periodic corrugation of a narrow photonic crystal waveguide can be used for phase matching for 2nd harmonic generation purposes, for example.

Further, for example, the photonic crystal device of the invention can be used as a light speed (velocity) reducer. In such application the photonic crystal device can be used to slow down the light. The continuous variation of width of the taper (or waveguide) can be used to progressively modify the group velocity of a given wavelength down to zero. Such can be useful to couple to slow group velocity waveguides, e.g. for delay lines or for dispersive waveguides. Such can also be used as an optical memory by trapping light. The light can then be released by modifying the index, by electro-optic effect or all optically by non-linear effects, for example. Other possible applications include quantum computing, ultra-sensitive magnetometry, acousto-optics, or the like. It should be noted that high density of energy per volume can be obtained by the longitudinal spatial compression of light.

Yet further, for example, the photonic crystal device of the present invention including a taper can be formed to be an optical source, e.g. by use of the reflective properties of the taper for particular wavelengths. By inserting a gain medium (quantum dots, quantum well or the like) in the taper region where reflection occurs, a laser source can be created. The wavelength of lasing is determined by the wavelength at which the reflection phenomenon occurs, itself determined by the angle and filling factor of the taper. The "open" cavity that the taper constitutes has the advantage that emitted light is directly coupled into a photonic crystal waveguide. This concerns the case of in-plane emission, although out-of-plane emission is also likely to be possible, depending on how the taper is configured.

Different layouts can be considered. A first device 200 according to an eighth embodiment of the invention (see Figure 15 (a) ) is a simple taper, where light may escape in both directions. A second device 210 according to a ninth embodiment of the invention (see Figure 15b) comprises a PhC wall at one extremity forcing emission in one direction only.

A more sophisticated device according to a tenth embodiment of the invention (see Figure 15(c)) comprise two tapers 222,222' which are facing each other, providing reflection at each side, and constituting then a more complex cavity or cavities .

Tunability of these devices for the emitted wavelength can be obtained by modifying the index locally using electro- optic effect by applying electrodes around the region of emission. Thermo-optic effect can also be used by locally heating this region (with thin resistor wire) . This effect can be significantly increased in case of suspended membranes. Finally, nonlinear effects can be used to optically tune the source.

Further still, for example, the photonic crystal device of the present invention including a taper can be used as a filter by using the regularly spaced reflection peaks. Add- drop functionality can even be established by partially breaking the resonsance by removing holes around this region.

Here also, tunability can be envisaged by electro-optic, thermo-optic or nonlinear effects in a similar way as for sources. In this case, thermo-optic effects can also be achieved by focusing light on the surface.

Referring now to Figure 15 (d) , there is illustrated an optical device 230 according to a tenth embodiment of the present invention. The optical device 230 comprises a Y- junction. In such, the angles θι,θ₂ may be increased (more acute) than in the simple tapers of other embodiments.

Referring now to Figure 15 (e) there is illustrated an optical device 240 according to an eleventh embodiment of the present invention. The device 240 provides a taper 242 and ridge waveguide 249 in optical communication. Referring to Figure 15(f) there is illustrated an optical device 250 according to a twelfth embodiment of the present invention. The device 250 provides a taper 252 and a ridge waveguide 259 formed between etched regions.

Referring to Figure 15 (g) there is illustrated an optical device 260 according to a thirteenth embodiment of the present invention. The device 260 comprises a photonic crystal mode-locked or short pulse laser with internal dispersion compensation. The laser is realised in a photonic crystal channel waveguide (e.g. Wl) . At one end one or more holes or rods are used as a mirror 269. At another end a taper 262 reflects light at different locations, depending upon wavelength so as to allow dispersion compensation. Care should be taken regarding dispersion occurring in the taper 262 before reflection.

It will be appreciated that the embodiments of the present invention hereinbefore described are given by way of example only, and are not meant to limit the scope of the invention in any way. Indeed, various modifications may be made to the devices hereinbefore described without departing from the scope of the present invention. For example in the arrangement shown in Figure 11, the device is illustrated as being arranged to include an upper cladding layer 114, however this need not be included in the device. Alternatively, the device may be formed including a suspended membrane or layer, using the core layer 113, with or without the cladding layers 112 and 114, by replacing the substrate layer 111 (or at least the top of the substrate layer 111) by air or another lower refractive index dielectric. The same principles can be demonstrated in alternate systems, e.g. silicon-on-insulator .

A further modification which may be made, is that the angles θi, θ₂ can be arranged in an asymmetric manner, wherein θi does not equal θ₂. It should also be noted that although throughout the description the lattice distortion which forms the taper is linear, this need not necessarily be the case, and the taper may be formed by a lattice structure which is distorted in a non-linear manner. Furthermore, in the arrangement in which the interfaces forming the taper are curved, the curve described was a convex curve. However, the interfaces may alternatively be curved concavely, or in a combination of both convex and concave, or may be a freeform shape, e.g. exp, cos, x². The PhC deformation providing a freeform interface can be obtained by bending, shifting or tilting the PhC lattice.

The device according to the present invention may also allow connection of different kinds of PhC waveguides or other • passive or active PhC devices together in a more complex device, as well as the waveguides being of different widths as detailed previously. In particular such devices allow an adaptation of mode between two PhC waveguides.

It should be noted that small angle tapers can be used for reflecting a wide range of wavelengths, the position at which reflection occurs being dependent upon wavelength.

In the devices of the present invention detailed hereinbefore, lateral tapering of the photonic crystal has been described. However, this lateral tapering may also be combined with vertical tapering.

Furthermore, the specifically disclosed most advantageous embodiments of the present invention have been described with reference to photonic crystal material where the rods are of a dielectric material, such as air, having a lower refractive index than the surrounding material, which is, for example, a semiconductor such as GaAs based materials (e.g. AlGaAs) or InP based materials (e.g. InGaP, InGaAsP, etc) . However, the rods and/or surrounding material may alternatively be formed of metals, which is metallic photonic crystal material. Additionally, the refractive index of the rods may be higher than the refractive index of the surrounding material .

It should be further understood that light can be extracted, or inserted in the third dimension, that is the vertical dimension or out-of-plane, in particular for filtering purposes or light generation.

It should be noted that the disclosed embodiments have been described as operating at optical wavelengths. However, it should be understood that such devices may operate over all or any part of the electromagnetic spectrum, and are particularly useful at microwave wavelengths. Furthermore, the principle of such devices can also be effective when used with other types of waves, such as mechanical waves, and in particular sound or acoustic waves. However in such latter devices, the device will not generally be formed of dielectric rods on a semiconductor slab, instead being formed of materials with differing sound wave velocities and acoustic waves impedances. It should be clear tht the band-gap reduces as the angle of the interfaces increases, which is intuitively.

It will be appreciated that the present invention provides a novel and efficient kind of PhC taper for coupling guided light into planar (2D) PhC channel waveguides. High transmission factors have been predicted and validated experimentally. The tapers also show potentially useful spectral selection properties and structures of this type could be important in augmenting the functionality and implementation possibilities for PhC-based integrated circuits.

While devices of the present invention may comprise rods in air, it has been found particularly beneficial to provide "rods" or "holes" in a carrier material.

Claims

1. A device such as an optical device providing a transmission region bounded by first and second photonic crystal (PhC) regions, first and second interfaces between the first and second photonic crystal regions and the transmission region, respectively, being substantially continuous .

2. A device such as an optical device as claimed in claim 1, wherein the optical device is an optical coupling device.

3. A device such as an optical device as claimed in either of claims 1 or 2, wherein a width of the transmission region tapers or reduces between a first end and a second end thereof .

4. A device such as an optical device as claimed in any of claims 1 to 3 , wherein each PhC region comprises a plurality of rods or holes.

5. A device such as an optical device as claimed in claim 4, wherein the rods are prismatic objects.

6. A device such as an optical device as claimed in either of claims 4 or 5, wherein each rod is a dielectric rod.

7. A device such as an optical device as claimed in any of claims 4, 5 or 6, wherein each rod is an air rod.

8. A device such as an optical device as claimed in any of claims 4 to 7, wherein at least a part of each rod is circular, elliptical, square, triangular or freeform in cross-section.

9. A device such as an optical device as claimed in any of claims 4 to 8, wherein each rod is of a same cross-sectional shape .

10. A device such as an optical device as claimed in any of claims 4 to 9, wherein a cross-sectional shape or dimension of each rod varies along a length of the rod.

11. A device such as an optical device as claimed in any of claims 4 to 10, wherein the rods have a higher or a lower refractive index than a surrounding or carrier material of each PhC region.

12. A device such as an optical device as claimed in any of claims 4 to 11, wherein the rods of each PhC region are regularly spaced one from the other so as to form a lattice.

13. A device such as an optical device as claimed in claim 12, wherein the lattice is square, parallelogram, triangular or hexagonal in shape or of a Penrose tiling shape.

14. A device such as an optical device as claimed in any of claims 4 to 13, wherein each rod is oriented in a same manner within the lattice.

15. A device such as an optical device as claimed in any of claims 4 to 14, wherein the first interface and second interface are disposed at at least one first acute angle and at at least one second acute angle, respectively, with respect to an axis of the transmission region.

16. A device such as an optical device as claimed in any of claims 4 to 15, wherein rods at or adjacent each interface are substantially identical one to the other.

17. A device such as an optical device as claimed in any of claims 4 to 15, wherein in each row of the lattice which forms a part of an interface adjacent rods decrease in diameter towards the interface.

18. A device such as an optical device as claimed in any of claims 4 to 15, wherein a size of the rods varies along a transmission direction and such variation coupled with a tuning angle provides selected propagation characteristics.

19. A device such as an optical device as claimed in any of claims 1 to 18, wherein the first interface and the second interface are substantially straight or linear.

20. A device such as an optical device as claimed in claim 15, wherein the first acute angle and the second acute angle are substantially equal.

21. A device such as an optical device as claimed in either of claims 15 or 20, wherein the first acute angle and the second acute angle are each around 0° to 10°, or one of the first or second acute angles is greater than 0° to around 10°, and the other of the first or second acute angle is around 0° to 10°, or each of the first and second acute angles are greater than 0° to around 10°.

22. A device such as an optical device as claimed in claim 21, wherein the first acute angle and the second acute angle are each around 5°.

23. A device such as an optical device as claimed in any of claims 1 to 22, wherein each PhC region comprises a substantially triangular or hexagonal lattice, wherein an axis of each lattice is disposed at an acute angle to an axis of the transmission region.

24. A device such as an optical device as claimed in any of claims 1 to 22, wherein each PhC region comprises a substantially parallelogram lattice, wherein an axis of each lattice is disposed at an acute angle to an axis of the transmission region.

25. A device such as an optical device as claimed in any of claims 1 to 22, wherein each interface comprises a curve which is convex with respect to the transmission region.

26. A device such as an optical device as claimed in any of claims 4 to 18 or claim 20, wherein the rods comprise blind air holes .

27. A device such as an optical device as claimed in claim 1 , or claim 2 or any of claims 3 to 26 when dependant upon claim 2, wherein the device is adapted for use in the C-, L- or S-band, at a wavelength or wavelengths of around 1.55μm ± 60nm (total of C+L+S) , around 850nm ± 50nm, or at around 1.3 μm, 980nm, or visible wavelengths or in the microwave or millimetre wavelength regions.

28. A device such as an optical device as claimed in claim 4 or any of claims 5 to 27 when dependant upon claim 4, wherein each rod is around 0.3μm to 2.5μm in depth, or alternatively is around 500nm to 900nm in depth.

29. A device such as an optical device as claimed in claim 4 or any of claims 5 to 28 when dependant upon claim 4, wherein each rod is of substantially cylindrical form having a diameter of around 0.2μm to 0.35μm, or alternatively having a diameter of lOOnm to 150nm.

30. A device such as an optical device as claimed in claim 4 or any of claims 5 to 29 when dependant upon claim 4, wherein a period between centres of adjacent rods is around 0.3μm to 0.55μm, or alternatively is around 200nm to 230nm.

31. A device such as an optical device as claimed in any preceding claim, wherein the optical transmission region of the tapered structure is around lOμm or less in length, a breadth of a first end of the transmission region is around lμm to 5μm, or alternatively is around 0.5μm to 2μm, and a breadth of a second end of the transmission region is around 0.2μm to 0.8μm, or alternatively is around 0. lμm to 0.4μm.

32. A device such as an optical device as claimed in any preceding claim, wherein the device comprises a substrate upon which is provided a first cladding layer and a core (guiding) layer, and optionally a second cladding layer.

33. A device such as an optical device as claimed in any of claims 1 to 31, wherein the device comprises a substrate, a first cladding layer, a core (guiding) layer, a second cladding layer and a superstrate.

34. A device such as an optical device as claimed in either of claims 32 or 33, wherein the device further comprises the core layer disposed upon a lower refractive index material.

35. A device such as an optical device as claimed in any of claims 1 to 33, wherein the device comprises a suspended layer or membrane.

36. A device such as an optical device as claimed in any of claims 1 to 35, wherein the PhC regions comprise optical waveguide confining means, the PhC regions comprising a sole lateral optical waveguide guiding or confining means, or the PhC regions comprising first lateral optical waveguide guiding or confining means and a ridge waveguide comprising a second lateral optical waveguide or confining means.

37. A device such as an optical device as claimed in any of claims 1 to 36, wherein the optical device, in use, converts a spot size of an optical signal or optical mode input at a first (wide) end of the device and transmitted to a second (narrow) end of the device, or vice versa, the optical device comprising an optical coupling device used as a spot size converter, in coupling between an optical fibre and a PhC passive device or PhC active device, or in coupling between a dielectric waveguide and a PhC passive device or PhC active device or between active and/or passive PhC devices.

38. A device such as an optical device as claimed in any of claims 11 to 37, wherein the rods are provided in or on a surrounding or carrier material .

39. A device such as an optical device as claimed in claim 38, wherein the surrounding or carrier material comprises a

III-V semiconductor based material such as a binary material or a tertiary or quaternary alloy.

40. A device such as an optical device as claimed in either of claims 38 or 39, wherein the rods comprise holes formed in the surrounding or carrier material .

41. A device such as an optical device as claimed in any of claims 1 to 40, wherein no photonic crystal regions or rods or holes are provided between the first and second interfaces or within the transmission region.

42. A device such as an optical device as claimed in any of claims 1 to 41, wherein the interfaces define lateral waveguide guiding means, the device also including transverse waveguide guiding means .

43. A device such as an optical device having a taper transmission region, the taper having first and second sides adjacent first and second photonic crystal (PhC) regions, wherein the lattice of each of the first and second photonic crystal regions at least adjacent the first and second sides is arranged or distorted such that the first and second sides of the taper are substantially continuous or define substantially continuous surfaces.

44. A device such as an optical device comprising at least two photonic crystal (PhC) regions, each photonic crystal region including at least one edge, wherein the edge of each photonic crystal region is disposed at an acute angle or angles to an axis of transmission.

45. A device such as an optical device comprising a taper formed of at least two regions of photonic crystal (PhC) each comprising a lattice structure, wherein the photonic crystal lattice structures are not parallel or orthogonal to an axis of transmission of the device.

46. A device such as an optical device as claimed in any of claims 1 to 42, claim 43, claim 44 or claim 45, wherein the device/optical device comprises a passive device or an active device, such as a spot size converter, waveguide, Y-junction, mirror, reflector, coupler, light condenser, light speed reducer, optical source, laser, filter, pulse compressor.

47. A photonic device including at least one optical device according to any of claims 1 to 42, claim 43, claim 44 or claim 45.

48. An apparatus or system including at least one device such as an optical device according to any of claims 1 to 42, claim 43, claim 44, or claim 45.

49. An apparatus or system as claimed in claim 48, wherein the system comprises a communications system.

50. A method of manufacturing a device such as an optical device according to any of claims 1 to 42, claim 43, claim

44, or claim 45 including the step of forming the PhC regions and the/an optical transmission region, thereby forming the/a first interface and the/a second interface.

51. A method of manufacturing a device such as an optical device as claimed in claim 50, wherein the PhC regions are formed by Reactive Ion Etching (RIE) .