GB2393264A

GB2393264A - Optical photonic crystal waveguide filter

Info

Publication number: GB2393264A
Application number: GB0221773A
Authority: GB
Inventors: Martin Cryan
Original assignee: University of Bristol
Current assignee: University of Bristol
Priority date: 2002-09-19
Filing date: 2002-09-19
Publication date: 2004-03-24
Also published as: AU2003269158A1; GB0221773D0; WO2004027470A1

Abstract

A wavelength filter for use in a photonic integrated circuit (PIC) comprising a photonic crystal (PC) waveguide structure where the width of a central light confining region is modified. The boundary of the central light confining region has a periodicity due the nature of the PC waveguide fabrication which causes coupling from a fundamental guided mode into a higher order mode. The width of the central light confining region can be precisely modified to result in a filtering action over a specific bands of wavelengths. The filter has a second photonic crystal waveguide located in series with the first photonic crystal waveguide.

Description

( FILTER DEVICE

Technical Field of the Invention

5 The present invention relates to an optical device, and in particular to a wavelength filter for use in a photonic integrated circuit (PIC).

Description of Related Art

Photonic crystals (PCs) have the ability to confine light strongly and thus have potential to be utilised in the fabrication of optical telecommunications systems. In particular, it is anticipated that 15 photonic crystals will be widely used to form waveguides and light sources in PICs. PC based PIC technology can provide the decrease in circuit size (and also fabrication and installation costs) and increase in integration levels and production volume, 20 which are currently sought by the optical telecommunications industry.

Ideally, waveguides comprising 3-dimensional photonic crystal (3D-PC) structures which completely confine 25 light would be fabricated, thus leading to ultra-low loss propagation and sub-micron radii bends within PICs. Current technology does not provide efficient means to create the 3dimensionally periodic structures in the sub-micron dimension range. An alternative 30 approach is to use PC boundaries to obtain lateral confinement and to use conventional refractive index confinement in the vertical direction. These structures are termed 2-dimensional photonic crystal (2D-PC) waveguides. Whilst the fabrication of such 35 structures is simpler to realise than 3D-PC structures,

( 2D-PC structures do not provide complete confinement and may therefore have higher losses.

The 2D-PC platform requires traditional signal 5 processing components, such as filters, add-drop multiplexers, switches and laser sources, to be implemented alongside the 2D-PC waveguides. Wavelength division multiplexing (WDM) technology relies on multiple different wavelengths to transmit data.

10 Therefore, it is vital to provide a wavelength filter component that is compatible with the 2D-PC platform.

Recent research into 2D-PC waveguides (see Phys Rev. B. VOL. 63 PP11331 Mar 2001) has encountered a problem 15 caused by the periodic nature of the waveguide boundary: not all signal frequencies are efficiently propagated through the waveguide.

In the field of optical filters known technologies

20 include thin film technology, fibre Bragg gratings, Arrayed Waveguide Gratings (AWGs) and planar waveguide Bragg gratings.

Thin film technology is the most common approach. Thin 25 layers of dielectric are' deposited on a substrate and the layers can be designed in such a way that light incident on the thin films is filtered on passing through the structure. This technology is very mature, but it is unclear whether it can compete in the high 30 volume, low cost telecommunications markets.

A fibre Bragg grating utilises a periodic Variation in the refractive index of the core of a photosensitive optical fibre in order to create a filtering action 35 within the fibre itself. Whilst very high performance

filters can be produced, this technology is not ideally suited for mass production.

An AWG is a more recent approach which combines both 5 filtering and multiplexing/demultiplexing functions in one component. The AWG acts to split the incoming signal into a number of output waveguides, with each waveguide propagating a certain portion of the bandwidth of the input signal. These planar structures 10 have a relatively large size and consequently present problems for mass production in terms of yield and cost. Planar waveguide Bragg gratings comprise gratings 15 etched into a waveguide structure such that a periodic variation in effective refractive index of the waveguide is created. In operation, planar waveguide Bragg gratings utilise a small perturbation in the effective refractive index of the waveguide, multiplied 20 over several hundred periods, to obtain effective filtering action. Therefore, these filters are necessarily large in size.

None of the above known filter technologies are 25 compatible with commercially produced PICs. Therefore, the present invention seeks to provide a wavelength filter device suitable for use in PICs.

Summary

According to a first aspect of the present invention, there is provided an optical filter having filtering characteristics and comprising a photonic crystal waveguide which defines therein a light confining 35 region, wherein dimensions of the light confining region define the filtering characteristics.

According to a second aspect of the present invention, there is provided a method of optical filtering, the method comprising fabricating a photonic crystal waveguide with a 5 light confining region of predetermined dimensions, inputting an optical signal into the photonic crystal waveguide and obtaining a filtered output having characteristics dependent upon the predetermined dimensions. According to a third aspect of the present invention, there is provided an optical communication system including the optical filter of the first aspect of the present invention.

Advantageously, the wavelength filter device of the present invention outputs an optical signal with good passband characteristics, high isolation and a desirable transmission response.

In particular the present invention provides especially favourable filtering action for wavelengths around 1550nm, around which wavelength modern telecommunications networks operate.

25; i Further advantage is gained from the multifunctional nature of the present invention. Namely, both a waveguiding action and a filtering action can be performed a single optical component, thus aiding the 30 component size reduction sought by the industry.

Brief Description of the Drawings,

For a better understanding of the present invention, 35 and to show how it may be put into effect, reference

will now be made, by way of example, to the accompanying drawings, in which: Figure 1 is a cross-section through an InP PC 5 waveguide; Figure 2 is a plan view of the InP PC waveguide of Figure 1; 10 Figure 3 shows a graph of a filter response of a first implementation of a first embodiment of the present invention; Figure 4 shows a graph of a filter response of a second 15 implementation of a first embodiment of the present invention; Figure 5 shows a graph of a filter response of a third implementation of a first embodiment of the present 20 invention; and Figure 6 shows a graph of filter response of an implementation of a second embodiment of the present invention. 25; Detailed Description of Preferred Embodiment

The asymmetrical 2-dimensional InP PC waveguide 10 of Figure 1 comprises a substrate layer 12, a buffer layer 30 14, a first spacer layer 16, a guiding layer 18 and a second spacer layer 20. An input light signal 11 enters the guiding layer 18 at one vertical edge of the PC waveguide 10 and an output light signal 13 leaves the PC waveguide 10 from the opposite vertical edge.

Figure 2 shows a hexagonal lattice structure of air cavities or holes 22 (where a is the lattice constant) which have been etched through the guiding layer 18 of the PC waveguide 10 of Figure 1 via a conventional 5 method utilising a mask. Width w, equivalent, in this example, to three rows of holes (typically 1300nm wide), has not been etched thus creating a defect or cavity-free region along which energy can propagate.

The cavity-free region can thus function as a light lo confining region 24 of the waveguide 10. The dimensions of the mask can be accurately calculated by using Finite Difference Time Domain (FDTD) method. The FDTD method is a full wave electromagnetic method which discretises Maxwell's curl equations in space and time 15 (Cryan. M. et al., "Analysis of ridge waveguides loaded with 2D lattice structures using the FDTD method", ECIO 2001, Paderborn, Germany, April 2001). The method proceeds by dividing the PC structure into a mesh of unit cells upon which E- and H-fields are defined.

20 This allows the partial derivatives in Maxwell's equations to be expressed as finite differences.

Following Yee's method (IEEE Trans. Antennas Propagat.

Vol.AP-14 pp. 302-306,1966) the E- and H-fields are

staggered in space and the fields can be solved

25 directly in the time domain via moving through time in discrete steps.

In operation, an input optical signal 11 is fed into the PC waveguide 10 and is vertically and laterally 30 confined as it propagates along the central light confining region 24. The periodic nature of the boundary between the etched and unetched Areas of the guiding layer 18 can cause coupling from the fundamental guided mode into a higher order mode. This 35 results in Bragg reflection in the reverse direction.

This process occurs over a narrow band of wavelengths

and results in a filtering action, and thus, a filtered output optical signal 13.

An important parameter for a filter component is the 5 transmission response. This describes the amount of power that can pass through the device at different wavelengths. A filter then shapes the transmission response to either pass or reject specific wavelengths.

In Figures 3-6, transmission of OdB corresponds to full 10 transmission and the more negative the transmission response, the higher the rejection of the filter at that specific wavelength.

Figure 3 illustrates a transmission response 26 of the 15 PC waveguide 10 of Figures 1 and 2. A dip 28 in the response 26, which indicates that filtering action is occurring, is recorded at approximately 1380nm.

By adjustment of the width, w, of the light confining 20 region 24 of the waveguide 10, for example narrowing it to llOOnm, a stronger filtering action is obtained (via the above described mode coupling effect).

Specifically, Figure 4 shows a transmission response 30 of such a structure, and filtering actions 32 34 are 25 recorded at approximately 1400nm and also approximately 1460nm. Figure 5 shows a transmission response 36 of a waveguide 10 with a light confining region 24 of 1160nm 30 width. Pronounced filtering action 38 is recorded at around 153Onm, with maximum depth of -16dB with a 3dB bandwidth of approximately 1.6nm at a Gentle wavelength of 1534nm.

35 It will be apparent to the skilled person that the PC waveguide 10, represented by the transmission response

/ 36 of Figure 5, is providing a filtering action near to the standard telecommunications window of 1550nm.

Further reference to 2D-PC waveguides displaying the filtering action described above, will subsequently be 5 referred to as PC filters in this description.

In order to further improve the performance of the PC filter (by increasing the rejection and pass-

bandwidth), several PC filter sections can be arranged 10 in series. Specifically, the graph of Figure 6 illustrates a transmission response 40 of a PC filter comprising first and second sections. The first section has a width, al of 1140nm and the second section has a width, w2, of 1160nm. The resultant 15 transmission response 40 is the transmission response of the first section superimposed onto the transmission response of the second section.

The performance of the series PC filter can be 20 optimised in several ways. Firstly, more than two filter sections of differing light confining width 24 can be used in series. Secondly, very precise dimensions of the light confining regions can be employed. This aspect is dependent upon the resolution 25 of;the means (eg. the mask) used to fabricate the lattice of air holes of the PC filter. In particular, the periodicity of the side walls and the waveguide width, w must be precisely engineered. Also the lateral PC sections and the waveguide vertical layer 30 structure must be engineered such that they form a low loss waveguide. Referring to Figure 6, an optimised transmission response would be illustrated where the two dips 42 44 in the response were brought closer together until a single rectangular-shaped dip were 35 recorded (in contrast to two peak-shaped dips 42 44 as illustrated). In order for the PC filter to be

( compatible with Coarse Wave Division Multiplexing (CWDM) standards, the PC filter would be configured to produce a pass bandwidth of around 10 nm and a rejection of -30dB at a 30 nm bandwidth.

It will be apparent to the skilled person that the above described embodiments of the invention are not exhaustive and variations on these structures may be employed to achieve a similar result whilst employing 10 the same inventive concept. Certain aspects of the described embodiments can be altered or interchanged with an entity providing equivalent function. In particular, the PC filter is suitable for use with electromagnetic signals lying within a range of 15 wavelengths. Therefore, the input light signal 11 of Figure 1 can have any wavelength from microwave to light and beyond.

For example, the PC filter can be fabricated from a 20 material other than InP. Also, 3-dimensional PC ! structures can be utilised instead of 2-dimensional PC structures. Further, the FDTD calculation method can be used where various combinations of metals and dielectric materials (having finite conductivity 2S values) are employed in the PC filter structure.

Furthermore, the light confining region within the guiding layer of the PC waveguide can be at any position (ie. not necessarily centrally located) within the guiding layer.

With reference to Figure 1, it may be desirable to introduce an air layer into the lower spader layer, in order to minimise leakage of light into the substrate and thus minimise attenuation.

It can therefore be seen that the present invention provides a filter device which is suitable for use in PICs.

Claims

1. An optical filter having filtering characteristics and comprising a photonic crystal 5 waveguide which defines therein a light confining region, wherein dimensions of the light confining region define the filtering characteristics.

2. The optical filter as claimed in claim 1, wherein 10 the photonic crystal waveguide is a 2-dimensional photonic crystal.

The optical filter as claimed in claim 1, wherein the photonic crystal waveguide is a 3-dimensional 15 photonic crystal.

4. The optical filter as claimed in claim 1, wherein the photonic crystal waveguide defines an arrangement of cavities therein outside of the 20 light confining region, the arrangement defining at least one boundary of the light confining region.

5. The optical filter as claimed in claim 4, wherein 25 the arrangement defining the at least one boundary is a lattice of air holes.

6. The optical filter as claimed in claim 5, wherein the lattice of air holes has a periodicity.

7. The optical filter as claimed in any preceding claim, further comprising a Second photonic crystal waveguide which defines therein a second light confining region, wherein the second 35 photonic crystal waveguide is located in series with the first photonic crystal waveguide,

( wherein dimensions of both light confining regions together define the filtering characteristics. 5

8. The optical filter as claimed in claim 7, wherein the second photonic crystal waveguide is a 2 dimensional photonic crystal.

9. The optical filter as claimed in claim 7, wherein 10 the second photonic crystal waveguide is a 3 dimensional photonic crystal.

10. The optical filter as claimed in claim 7, wherein the second photonic crystal waveguide defines an 15 arrangement of cavities therein outside of the light confining region, the arrangement defining at least one boundary of the light confining region. 20

11. The optical filter as claimed in claim 10, wherein the arrangement defining the at least one boundary is a lattice of air holes.

12. The optical filter as claimed in claim 11, 25 wherein the lattice of air holes has a periodicity.

13. The optical filter as claimed in any of claims 7 to 12, wherein the dimensions of the first and 30 second light confining regions are different.

14. The optical filter as claimed inland of claims 7 to 13, wherein more than two photonic crystal waveguide sections are aligned in series.

(

15. A method of optical filtering, the method comprising: fabricating a photonic crystal waveguide with a light confining region of predetermined 5dimensions; inputting an optical signal into the photonic crystal waveguide; and obtaining a filtered output having characteristics dependent upon the predetermined 10dimensions.

16. The method of optical filtering as claimed in claim 15, wherein the photonic crystal waveguide is a 2-dimensional photonic crystal.

17. The method of optical filtering as claimed in claim 15, wherein the photonic crystal waveguide is a 3-dimensional photonic crystal.

2018. The method of optical filtering as claimed in any of claims 15 to 17, the method further comprising: fabricating a second photonic crystal waveguide with a light confining region of 25predetermined dimensions; positioning the first and second photonic crystal waveguide sections in series; inputting an optical signal into the first photonic crystal waveguide; and 30obtaining a filtered output having characteristics dependent upon the first and second predetermined dimensions.

19. The method of optical filtering as claimed in 35claim 18, wherein the second photonic crystal waveguide is a 2-dimensional photonic crystal.

20. The method of optical filtering as claimed in claim 18, wherein the second photonic crystal waveguide is a 3-dimensional photonic crystal.

21. The method of optical filtering as claimed in any of claims 18 to 20, wherein more than two photonic crystal wavequide sections are positioned in series.

1022. The method of optical filtering as claimed in any of claims 15 to 21, wherein the filtered output results from a coupling from a fundamental guided mode of the optical signal into a higher order mode I of the optical signal.

23. The method of optical filtering as claimed in claim 22, wherein the coupling results from a periodicity in structure of a boundary of the light confining region.

24. An optical communication system including an optical filter as claimed in any of claims 1 to 14.