GB2370370A

GB2370370A - Arrayed waveguide grating

Info

Publication number: GB2370370A
Application number: GB0031549A
Authority: GB
Inventors: Michael Lanker
Original assignee: Kymata Ltd; Alcatel Optronics UK Ltd
Current assignee: Kaiam Europe Ltd; Kymata Ltd
Priority date: 2000-12-22
Filing date: 2000-12-22
Publication date: 2002-06-26
Also published as: GB0031549D0; GB0106013D0

Abstract

An arrayed waveguide grating (AWG) device comprising: first and second optical interaction regions 3,4 between which an input optical signal propagates from a first position on a first side of the first optical interaction region to a second position on a second side of the second optical interaction region, a correspondence between said first and second positions depending upon a wavelength of the optical signal; a plurality of array waveguides 8 coupled between a second side of the first optical interaction region and a first side of the second optical interaction region; and a plurality of output waveguides 10' coupled at one end to the second side 5 of the second optical interaction region; wherein the output waveguides are arranged to fan-out from the second optical interaction region, in a fan-out region of the device which comprises an initial portion of the length of each output waveguide, and the waveguides in the fan-out region are substantially identically curved in at least a portion of the fan-out region. F is the length of the fan out region. The output waveguides may be tapered at 11.

Description

ARRAYED WAVEGUIDE GRATING

The present invention relates to dispersive optical devices. More specifically, but not exclusively, the invention relates to an arrayed waveguide grating device.

In order to meet the ever-increasing demand for transmission bandwidth in communication networks, operators are investing heavily in the development of techniques for Dense Wavelength Division Multiplexing (DWDM). DWDM employs many closely spaced carrier wavelengths, multiplexed together onto a single waveguide such as an optical fibre. The carrier wavelengths are spaced apart

by as little as 50GHz in a spacing arrangement designed in the style of an ITU 0 (International Telecommunications Union) channel"grid". Each carrier wavelength may be modulated to provide a respective data transmission channel. By using many channels, the data rate of each channel can be kept down to a manageable level.

Clearly, to utilize this available bandwidth it is necessary to be able to separate, or demultiplex, each channel at a receiver. New optical components for doing this have been designed for this purpose, one of these being the Arrayed Waveguide Grating (AWG). An arrayed waveguide grating is a planar structure comprising a number of arrayed waveguides which together act like a dif : traction gratmg in a spectrometer. AWGs can be used as multiplexers and as demultiplexers, and a single AWG design can commonly be used both as a multiplexer and demultiplexer. A typical AWG mux/demux 1 is illustrated in Figs. 1 (a) and (b) and comprises a substrate or"die"1 having provided thereon one or more input waveguides 2 for a multiplexed input signal, two slab couplers 3,4 connected to either end of an array 5 of transmission waveguides 8, only some of which are shown, and a plurality of output waveguides 10, which are commonly single mode or substantially single mode waveguides, for outputting respective wavelength channel outputs from the second (output) slab coupler 4 to the edge 12 of the die 1.

The array 5 of transmission waveguides exhibits dispersive imaging properties like that of a diffraction grating, so that input WDM signals are dispersed and focused C7 to respective ones of the output waveguides. Often, the input and/or output waveguides 2,10 are each tapered at a respective first end 7,11 thereof, where they are coupled to the first or second slab coupler respectively, the taper being such that the width of the waveguide increases towards the slab coupler 4,5. The output waveguides 10 are arranged to"fan-out"from their first ends 11, away from each other and from the coupler (i. e. the lateral spacing between the waveguides is increased), so as to achieve a desired physical spacing between the output waveguides, at an output edge of the device.

One known problem with such AWG devices is that the channel outputs from the output waveguides tends to deviate from the ideal device response in which the central frequencies of the channels are equally spaced apart, as illustrated in Fig. 2 (a). Instead, the output from each channel is often not centred on the respective desired frequency but is instead centred on a slightly different frequency. We believe these inaccuracies in channel spacing result, at least in part, from the shape and arrangement of the output waveguides, further influenced by manufacturing process aberrations introduced during manufacture of the device. In particular, we believe the radius of curvature of each of the output waveguides, particularly in the fan-out region where the output waveguides fan-out from the second slab coupler, is a significant contributing factor in the channel spacing inaccuracies experienced in the manufactured devices, particularly where tapers are used on said first ends 11 of the output waveguides.

It is an object of the present invention to avoid or minimize one or more of the

foregoing disadvantages. m According to a first aspect of the invention there is provided an arrayed waveguide grating device comprising: first and second optical interaction regions between

which an input optical signal propagates from a first position on a first side of the first optical interaction region to a second position on a second side of the second optical interaction region, a correspondence between said first and second positions depending upon a wavelength of the optical signal; a plurality of array waveguides coupled between a second side of the first optical interaction region and a first side of the second optical interaction region; and a plurality of output waveguides coupled at one end to the second side of the second optical interaction region; wherein the output waveguides are arranged to fan-out from the second optical interaction region, in a fan-out region of the device which comprises an initial portion of the length of each output waveguide, and the waveguides in the fan-out region are substantially identically curved in at least a portion of the fan-out region.

The device according to the invention has the advantage of improved channel ., e of improved channel spacing accuracy i. e. avoiding, or at least minimizing, deviations in channel c spacing from the desired, ideal channel spacing which the device is designed to achieve.

The waveguides in the fan-out region preferably all have substantially the same radius of curvature in at least a portion of the fan-out region. Alternatively, the waveguides may each have a continuously varying radius of curvature, in the fanout region, in which case the radius of curvature of each waveguide varies substantially identically to that of the other waveguides, along corresponding portions of the lengths thereof, in at least a portion of the fan-out region.

Preferably, the waveguides are identically, or near identically, curved in at least a portion of the fan-out region, but alternatively there may be small variations in the radius of curvature between any two or more of the output waveguides, but preferably no two output waveguides differ in radius of curvature by more than 5mm, advantageously no more than Imm, and most preferably no more than 0. 5mm.

Preferably, the output waveguides are single mode, or substantially single mode, waveguides which are preferably tapered in width at the ends thereof which are coupled to the second optical interaction region, so as to increase m width towards the second optical interaction region.

Preferably, the portions of the waveguides having substantially equal radius of curvature, or substantially identically varying radius of curvature, are adjacent the respective tapered ends thereof. Preferably, the radius of curvature of these portions is kept sufficiently low to substantially filter out any psuedo higher order modes which may arise where the output waveguides are coupled to the second optical interaction region, while still being sufficiently high to avoid unacceptably high radiation losses in the output waveguides.

The waveguides preferably each comprise a core region having cladding material at least on either side of the core region. In our preferred embodiment, the difference between the refractive indices of the core region and cladding material is 0.01 and the core size is 6 x 6um in cross-section, and the radius of curvature of the waveguides in the portion of the fan-out region in which the waveguides are

substantially identically curved is preferably in the range of 16mm or less, preferably between 5. 5 and 10mm, most preferably substantially 8mm.

The device may conveniently be a planar waveguide-type device which is generally rectangular in shape, having an input edge, an output edge, and two side edges. A second end of each output waveguide may conveniently lie along an output edge of the device. Advantageously, the output waveguides may be substantially identically curved throughout a substantial portion of the fan-out region, preferably in the range of 3-20% of the length of the fan-out region, most preferably between 5 and 10%, as measured along a said side edge of the device which is perpendicular to the output edge of the device.

The device may further include at least one input waveguide coupled to a first side of the first optical interaction region. The or each input waveguide is preferably also a single mode, or substantially single mode, waveguide and may also be tapered in width at the end thereof which is coupled to the first optical interaction region, so as to increase in width towards the first optical interaction region. Where there are a plurality of input waveguides, the input waveguides are preferably arranged to fan-in towards the first optical interaction region, in a fan-in region which comprises a portion of the length of each input waveguide, and all the input waveguides may also be substantially identically curved in at least a portion of the fan-in region.

Preferred embodiments of the invention will now be described by way of example only and with reference to the accompanying drawings in which: Fig. l (a) is a schematic plan view of a known arrayed waveguide grating device; Fig. l (b) is a magnified schematic view of the ringed portion A of the device of Fig. l (a); Fig. 2 (a) illustrates an ideal channel spacing, in terms of frequency, of the output channels of the device of Fig. 1 ; Fig. 2 (b) illustrates a typical channel spacing, in terms of frequency, of the output channels of the device of Fig. 1 ; Fig. 3 (a) shows an arrayed waveguide grating according to the invention; Fig. 3 is a magnified schematic view of the ringed portion A'of the device of Fig.

3 (a).

Figs. 3 (a) and (b) illustrate respectively an arrayed waveguide grating (AWG) device, and the arrangement of output waveguides 10'in an AWG, according to one embodiment of the invention. Like components to those of Fig. 1 (a) are referenced by like reference numerals. The AWG device is formed on a substrate or "die"1 and comprises at least one input waveguide 2, a first optical interaction

region in the form of a slab coupler 3, an array of waveguides 8 (only some shown) of different optical path lengths and arranged between the first slab coupler and a second slab coupler 4 (providing a second optical interaction region), and a plurality of output waveguides 10' (only seven shown). The output waveguides are single mode waveguides, or alternatively substantially single mode waveguides (in the sense that overall they effectively operate as if they were single mode). In generally known manner there is a constant predetermined optical path length difference between adjacent waveguides in the array, which determines the position of the wavelength output channels on an output face 5 of the second slab coupler 4.

The construction and operation of such AWGs is well known in the art and is described, for example, in"PHASAR-based WDM-devices : principles, design and applications", M K Smit and C. van Dam, J. Selected Topics in Quantum Electronics 2,1996, pp236-250, and in"An NxN optical multiplexer using a planar arrangement of two star couplers", C. Dragone, Photonics Technology Letters, 9, 1991, vol 3, pp812-815.

The output waveguides 10'are each tapered at a first end 11 thereof, where they are coupled to the output face 5 of the second slab coupler 4, in like manner to the device of Figs. 1 (a) and (b). The output waveguides 10'also"fan-out"from their first ends 11'towards the output edge 12 of the device, so as to achieve a desired physical separation of the output channels at the output edge 12 of the device. The area of the device in which the lateral separation of the waveguides increases, from their spacing at the output face 5 of the second slab coupler 4 to the final desired lateral spacing, is hereinafter referred to as the fan-out region. In the fan-out region the waveguides are curved. Output end portions 10a'of the output waveguides 10', adjacent the output edge 12, are substantially perpendicular to the output edge 12, in plan view of the AWG device. References made hereinafter to the length F of the fan-out region refer to the length of the fan-out region as measured along a bottom edge 14 of the device, which edge is perpendicular to the output edge 12, as shown in Fig. 3 (a).

The device is provided as a planar silica-on-silicon chip produced by, for example, Flame Hydrolysis Deposition (FHD) of Chemical Vapour Deposition (CVD). Each of the waveguides 2,8, 10'is of typical optical waveguide construction comprising a core region with a cladding material at least on either side of, and in our preferred

embodiment also covering, the core region. In generally known manner, the core 0 region is formed on a substrate of silicon, silica (silo2) or the like, which may (if not made of silica) have a silica buffer layer deposited thereon before the core and cladding regions are deposited.

Fig. 3 (b) illustrates the arrangement of output waveguides 10'in the fan-out region.

For clarity, only four of the output waveguides 10'are shown in Fig. 3 (b). In practice there would be many more, for example 40 output waveguides for a 40channel mux/demux. In a portion P of the fan-out region, this portion being adjacent the tapered ends 11 of the output waveguides, all the output waveguides have the same radius of curvature. In the illustrated embodiment, this"equal curvature"portion P comprises a substantial portion of the fan-out region, specifically in the range of 5 to 10% of the length F of the fan-out region. In the embodiment of Fig. 3 (which is not shown to scale), the length of the bottom edge 14 of the AWG device is 50mm, and the"equal curvature"portion of the output waveguides spans approximately 1mm of this length.

After these"equal curvature"sections of the waveguides, as illustrated in Fig. 3 (b), the waveguides diverge so that some have an opposite radius of curvature to others.

In fact, approximately half the output waveguides curve outwardly away from the other half of the waveguides, and vice versa. This particular arrangement is advantageous in that it allows greater compactness of the overall device to be achieved.

Beam Propagation Method (BPM) simulations have shown that the radius of curvature of the output waveguides, at the input ends 11 thereof, can affect the output channel spacing of the AWG device, in particular output waveguides with different radii of curvature can lead to inaccuracies in channel spacing. This is highly undesirable, as accurate channel spacing, in particular in terms of channel frequency, is a desired feature of any commercial AWG device. Our simulations also show that the detrimental affects due to different radii of curvature are greater the wider the tapers used on the input ends 11 of the output waveguides.

Additionally, it is known that output waveguides which are not exactly coupled to the second slab coupler 4 may excite higher order psuedo-modes in substantially single mode output waveguides which distort the optical field at the wide end of the taper. BPM simulations also show that this effect can be reduced by reducing the radius of curvature of the waveguides in the fan-out region. A narrower curve (i. e. smaller radius of curvature) can, at least to some extent, filter out such psuedo higher order modes, which a wider curve would not.

Therefore, the radius of curvature of the"equal curvature"portions of the output waveguides is chosen to be relatively small, while still within acceptable radiation losses for the output waveguides. In the described embodiment, we chose a radius of curvature of 8mm, for a waveguide core size of 6 x 6 pm, and where the difference, An, of the refractive indices of the waveguide core and cladding is 0. 01.

We have found that for this core size, with this value of An, the radiation losses in the output guides are acceptable. For this embodiment, if a radius of curvature of less than 8mm is used, the radiation losses become higher, particularly for below 5.5mm where we have found these losses are unacceptably high. It will thus be

a generally understood that for waveguide designs with different core size and/or An value to the described embodiment, the radius of curvature of the"equal curvature" sections should ideally be chosen so as to be high enough to avoid unacceptable radiation losses, but also low enough that the waveguides do function to filter out,

or at least partially filter out, the psuedo higher order modes. This can be done empirically and/or via simulation. A working value for the maximum tolerable additional power loss attributable to radiation losses (i. e. over and above power loss due to other factors) in an output waveguide, due to bending of the waveguide, is generally accepted to be about O. ldB for a ninety degree bend in the waveguide.

In general, the longer the length of the"equal curvature"portion of the fan-out

region (measured along the bottom edge 14), the larger will need to be the overall reaion (measured along the bottom ed.-e size of the AWG device. As it is usually desirable to make the AWG device as small as possible, the designer must therefore balance the benefits of using a larger "equal curvature"portion (potentially better filtering out of pseudo higher order

rr n modes and/or improved channel spacing accuracy) against the corresponding increase in overall size of the device.

As a further improvement, the input waveguides 2 may also be designed so that a portion of each input waveguide adjacent the first slab coupler 3 is substantially identically curved to a corresponding portion of all the other input waveguides, the radius of curvature again being chosen to be as low as possible while still high enough to avoid undesirably high radiation losses. This can be beneficial to filter out, or at least partially filter out, any pseudo higher order modes which may be present in the input waveguides. In the embodiment of Fig. 3 (a) the input waveguides fan-in towards the first slab coupler 3, and the input waveguides are all substantially identically curved for at least a portion of the fan-in region.

It will be appreciated that further variations and modifications to the abovedescribed embodiment are possible without departing from the scope of the invention. For example, instead of the output waveguides having one, equal, radius of curvature along at least a portion of the length thereof in the fan-out region, the radius of curvature of the waveguides may vary continuously along the length of each waveguide, or along at least a portion thereof. (For example, the path of each

waveguide might be defined by a polynomial function.) In this case, according to the invention the radius of curvature of each waveguide would be varied identically to that of the other waveguides, along at least a portion of the length of the waveguides in the fan-out region. i. e. the waveguides would all have the same curvature along corresponding portions thereof, in at least a portion of the fan-out region.

Although the output waveguides are ideally identically curved in the portion P of the fan-out region, as described with reference to the embodiment of Figs. 3 (a) and (b), small variations in the radius of curvature are tolerable, within limits, while still achieving at least some of the benefit of the invention. We believe that variations in radius of curvature of no more than 5mm between any two of the output waveguides is tolerable, with variation of no more than Imm, ideally no more than O. Smm, being most preferable.

In a further possibility instead of, or in addition to, tapers on the first ends 11 of the output waveguides, there may be a multi-mode interferometer (MOI) disposed between the second slab coupler 4 and the input ends 11 of each of the output waveguides, the slab coupler and MMIs together forming a desired"optical interaction region"between the output waveguides 10'and the array waveguides 8.

In another possible modified embodiment, there may be no tapers (or MMIs) on the input ends 11 of the output waveguides 10 (i. e. adjacent the second slab coupler 4).

For example, in this embodiment the output waveguides may be multi-mode waveguides, for example two-mode waveguides transmitting both the fundamental and first order modes. While an embodiment with no tapers on the output waveguides may not be so susceptible to channel inaccuracies introduced due to

unequal curvature of the output waveguides in the fan-out region, we envisage that the use of"equal curvature"waveguide sections according to the invention will still provide some benefit.

In a yet further embodiment there could be more than one single-mode output waveguide allocated for each output channel. For example, a respective pair of single mode waveguides may be provided to output each channel.

Also, it will be appreciated that the terms"input"and"output", as used above to describe the waveguides, are used with reference to the use of the AWG as a demultiplexer. However, the same AWG could equally be used as a multiplexer in which case the input and output waveguides function as output and input waveguides, respectively. Therefore the accompanying claims shall be read as covering an AWG suitable for use as a demultiplexer, a multiplexer or a device which can function as both (i. e. a mux/demux device).

Claims

1. An arrayed waveguide grating (AWG) device comprising : first and second optical interaction regions between which an input optical signal propagates from a first position on a first side of the first optical interaction region to a second position on a second side of the second optical interaction region, a correspondence

between said first and second positions depending upon a wavelength of the optical between said c signal; a plurality of array waveguides coupled between a second side of the first optical interaction region and a first side of the second optical interaction region; and a plurality of output waveguides coupled at one end to the second side of the second optical interaction region; wherein the output waveguides are arranged to fan-out from the second optical interaction region, in a fan-out region of the device which comprises an initial portion of the length of each output waveguide, and the waveguides in the fan-out region are substantially identically curved in at least a portion of the fan-out region.

2. An AWG device according to claim 1, wherein the output waveguides all have substantially the same radius of curvature in at least a portion of the fan-out region.

3. An AWG device according to claim 1, wherein the output waveguides each have a continuously varying radius of curvature, in the fan-out region, and the radius of curvature of each waveguide varies substantially identically to that of the other waveguides, along corresponding portions of the lengths thereof, in at least a portion of the fan-out region.

4. An AWG according to any preceding claim, wherein the output waveguides are substantially single mode waveguides which are tapered in width at the ends thereof which are coupled to the second optical interaction region, so as to increase in width towards the second optical interaction region.

5. An AWG according to claim 4, wherein said substantially identically curved portions of the waveguides are adjacent the respective tapered ends thereof.

6. An AWG according to claim 5, wherein the radius of curvature of said substantially identically curved portions is kept sufficiently low to substantially filter out any psuedo higher order modes which arise where the output waveguides are coupled to the second optical interaction region, while still being sufficiently high to avoid radiation losses above a predetermined maximum level, in the output waveguides.

7. An AWG according to claim 6, wherein the output waveguides each comprise a core region having cladding material at least on either side of the core region, the difference between the refractive indices of the core region and the cladding material is 0.01, and the size of the core region is 6 x 6um in cross-section, and wherein the radius of curvature of the waveguides in the portion of the fan-out region in which the waveguides are substantially identically curved is in the range of 5. 5 to 16mm.

8. An AWG according to claim 7, wherein the radius of curvature of the output waveguides in the portion of the fan-out region in which the waveguides are substantially identically curved is substantially 8nun.

9. An AWG according to any preceding claim, wherein the output waveguides are I substantially identically curved throughout a substantial portion of the fan-out region.

10. An AWG device according to claim 9, wherein the device is of generally planar rectangular shape, having an input edge, an output edge, and two side edges, a second end of each output waveguide lies along the output edge of the device, and the portion of the fan-out region in which the output waveguides are substantially

identically curved has a length in the range of 3-20% of the length of the fan-out region, as measured along a said side edge of the device which is perpendicular to the output edge of the device. z

11. An AWG device according to claim 10, wherein the portion of the fan-out region in which the output waveguides are substantially identically curved has a length in the range of 5-10% of the length of the fan-out region, as measured along said side edge of the device which is perpendicular to the output edge of the device.

12. An AWG device according to any preceding claim, wherein the waveguides are identically curved in at least a portion of the fan-out region.

13. An AWG device according to any of claims I to 11, wherein each of said 0 substantially identically curved portions of the waveguides differs in curvature by no more than 5mm radius of curvature from any other said substantially identically curved waveguide portion.

14. An AWG device according to claim 13, wherein each of said substantially identically curved portions of the waveguides differs in curvature by no more than 1mm radius of curvature from any other said substantially identically curved waveguide portion.

15. An AWG device according claim 14, wherein each of said substantially identically curved portions of the waveguides differs in curvature by no more than 0. 5mm radius of curvature from any other said substantially identically curved waveguide portion.

16. An AWG device according to any preceding claim, further including a plurality

of input waveguides coupled to a first side of the first optical interaction region, z each input waveguide is tapered in width at the end thereof which is coupled to the

first optical interaction region, so as to increase in width towards the first optical interaction region, and the input waveguides are arranged to fan-in towards the first rti e lth optical interaction region, in a fan-in region which comprises a portion of the length of each input waveguide, and wherein all the input waveguides are substantially identically curved in at least a portion of the fan-in region.

17. A multiplexer/demultiplexer comprising an AWG device according to any preceding claim.

18. A communications system incorporating at least one AWG device according to any of claims 1 to 16.

19. An AWG device substantially as described herein with reference to Fig. 3 (b).