Abstract:
An arrayed waveguide grating device  1  has a substrate  1  with an array  41  of waveguides extending across the substrate between an input coupler  37  and an output coupler  39.  At least one input waveguide  31  extends between an input end for coupling to an input signal and the input coupler  37  and at least one output waveguide extends between an output end for coupling to an output signal and the output coupler  39 . The input end or ends of the at least one input waveguide and the output end or ends of the at least one output waveguide are brought to the same edge  35  of the substrate.

Description:
FIELD OF THE INVENTION  
         [0001]    The invention relates to an arrayed waveguide grating (AWG) and in particular to the geometry of an arrayed waveguide grating.  
         BACKGROUND OF THE INVENTION  
         [0002]    Optical Systems increasingly use wavelength division multiplex (WDM) in which a number of distinct optical signals are transmitted at different wavelengths, generally down an optical fiber. For example, optical communication in the so called “C” band may use 40 channels, or frequencies, at regular intervals. One optical signal can be transmitted at each frequency down a single optical fiber. There are other possibilities, for example, 56 channels may be used in the “L” band.  
           [0003]    A key component in WDM systems is the demultiplexer for splitting apart optical signals at a plurality of wavelengths into the individual channels at individual wavelengths. This may be done using a splitter and a number of different filters timed to the individual frequencies, by components that demultiplex the light directly, or a different combination of these components.  
           [0004]    One approach to filtering and demultiplexing is an arrayed waveguide grating (AWG), also known as a phased-array device. The operation and design of AWGs is described, for example, in “PHASAR-Based WDM-Devices: Principles, Design and Applications”, Meint K. Smit, IEEE Journal of Selected Topics in Quantum Electronic, Vol. 2, No. 2, June 1996.  
           [0005]    [0005]FIG. 1 illustrates a conventional AWG device. The arrayed waveguide device includes an array  11  of waveguides  3  arranged side by side on a substrate  1  and extending between an input star coupler  13  and an output star coupler  15 . The input and output star couplers  13 ,  15  may be defined by a wide core region in which light can travel freely in the two-dimensional plane of the substrate. This region is known as the free propagation region. Input  17  and output  19  optical waveguides are provided to feed input light into the array  11  of waveguides and to output light respectively. There may in particular be a plurality of input waveguides  17  or output waveguides  19 .  
           [0006]    As an example FIG. 2 illustrates the output s coupler of a system with a single input waveguide and a plurality of output waveguides. The ends  21  of the array of waveguides  11  are usually on a geometric circle  23  of radius r whose centre is at the centre  25  of an image plane  27 . The output waveguides  19  are arranged on the image plane, which also constitutes a circle. Note that the centres of the circles are not coincident, and need not have equal radii.  
           [0007]    The length of the individual waveguides  3  of the array  11  differ (see FIG. 1) and the shapes of the star couplers  13 ,  15  are chosen so that light input on the input optical waveguide  17  passes through the array of waveguides ad creates a diction pattern on the output waveguide or waveguides, such that light of a predetermined central wavelength creates a central interference peak at the centre  25  of the image plane. Light with frequencies slightly higher or lower than the predetermined central frequency is imaged with a central interference peak slightly above or below the centre of the image plate.  
           [0008]    In order to achieve this result the optical path length difference between adjacent waveguides of the array is chosen so that it is an integral multiple of the central wavelength. Accordingly, light at the central wavelength which enters the array of waveguides in phase will also leave in phase and thus will create the central diffraction spot at the centre of the image plane. Light with a slightly different frequency will arrive at the output star coupler with slight phase differences across the array, which will cause the light to be imaged to a spot on the image plane a little away from the central spot.  
           [0009]    Accordingly, the plurality of output waveguides arranged on the output plane receive light of slightly different frequencies. Equally spaced output waveguides correspond to equally spaced frequencies, to a first order of approximation.  
           [0010]    [0010]FIG. 2 shows the effect of one or more output waveguides connected to the output star coupler  15 . It is alternatively or additionally possible to arrange a plurality of input waveguides on the input star coupler with the same effect.  
           [0011]    An AWG filter has a number of properties. One important property is that the distance of the image spot along the image plane as a function of wavelength is substantially linear in wavelength, for wavelengths around the central wavelength. Accordingly, it is possible to separate signals with a given channel separation by positioning output waveguides at substantially regular intervals along the output plane.  
           [0012]    A second important property is that the AWG has a repeat frequency. In other words, the interference properties as a function of frequency repeat with a period in the frequency domain. This period is known as the free spectral range (FSR). The free spectral range is a function of the difference in length between adjacent waveguides; a large length difference results in a small FSR and vice versa.  
           [0013]    The layout geometry shown in FIG. 1 is that normally used in an AWG filter. However, the exact geometry used for the coupler is limited by a number of parameters. One parameter is the minimum radius of curvature of the optical waveguides used. Another is the minimum separation between adjacent optical waveguides. Thirdly, the optical length difference between adjacent optical waveguides is a parameter. The space required for fan out is also relevant—the inputs and outputs have to be sufficiently spaced to be attached to input and output connectors, generally optical fibers. Other parameters include the refractive indices of the core, buffer and cladding.  
           [0014]    Although it is normally possible to implement a desired FSR and channel separation using the geometry shown in FIG. 1, the geometry is not suitable for small path differences between adjacent waveguides in the array. This is because a waveguide arranged around another waveguide in the geometry of FIG. 1 will need to be longer by at least a certain minimum distance in order to fit around the other waveguide. This means that for large FSRs, which require small length differences between adjacent waveguides, the geometry of FIG. 1 is not suitable. An alternative geometry, which might be described as an elongate S-shape, is proposed in Adar R, et al, “Broad-band array multiplexers made with silicon waveguides on silicon”, Journal of Lightwave Technology, vol. 11, no. 2, February 1993, pages 212 to 218.  
           [0015]    Arrayed waveguide gratings are often formed on silicon substrates. A number of gratings are generally patterned on a single wafer; the wafer is then sawn to split the wafer into individual gratings. In view of the high cost of semiconductor wafers, there is a general need to increase the number of individual gratings formed on a single wafer.  
           [0016]    Furthermore, a significant part of the cost of manufacturing modules including arrayed waveguide gratings is in attaching input and output optical fibers to the gratings. This generally requires accurate alignment carried out by skilled technicians. It would accordingly be desirable to simplify input and output from an arrayed waveguide grating.  
         SUMMARY OF INVENTION  
         [0017]    According to a first aspect of the invention there is provided an arrayed waveguide grating device comprising: a substrate having a plurality of edges; an array of waveguides extending across the substrate between a first coupler and a second coupler; at least one input/output waveguide extending from the first coupler, at least one input/output waveguide extending from the second coupler, wherein the opposite ends of the input/output waveguides to the first and second couplers are provided on are the same edge of the substrate to act as optical inputs and outputs.  
           [0018]    As far as the inventor is aware, all prior art AWG devices take the input and output ends of the input and output waveguides to opposite edges of a rectangular substrate. However, results will be presented later to show that by bringing the input and output waveguides to the same edge of the substrate in accordance with the invention it may unexpectedly be possible to reduce the area of an AWG device. This increases the number of AWG devices that it is possible to make on a single wafer.  
           [0019]    Arrayed waveguide gratings are generally formed on substantially flat substrates, the edges being the sides at the perimeter of the substrate.  
           [0020]    The ends of the waveguides extending from the first coupler may be arranged to have the same constant spacing as the ends of the waveguides extending from the second coupler. Furthermore, the provision of optical inputs and outputs at the same edge of the substrate can ease manufacture. The input and output waveguides may be brought together on the said edge of the substrate contiguously so that a single fiber ribbon connector may be brought to the edge for connecting input and outputs in a single operation In this way, only one optical connection needs to be made instead of two with conventional designs.  
           [0021]    Typically, the substrate may be rectangular.  
           [0022]    Electrical connections may be provided on the substrate on a different edge of the substrate to the optical inputs and outputs. The electrical connections may be used, for example, for Peltier effect cooling or temperature sensing on the chip. The provision of electrical and optical connections on different edges makes manufacture particularly easy by separating the electrical and optical connections. Preferably, opposite edges of the substrate may be used.  
           [0023]    The array of waveguides may bend on the substrate by more than 180° in order that the input and output waveguides can more readily be brought together.  
           [0024]    The invention is particularly useful with small FSRs since the configuration of the array of waveguides tends to produce large path differences between adjacent waveguides which results in small FSRs. Typically, FSRs below 1000 GHz may be provided.  
           [0025]    In another aspect, the invention relates to a node of an optical telecommunications system comprising an arrayed waveguide grating device including a substrate having a plurality of edges, an array of waveguides extending across the substrate between a first coupler and a second coupler; at least one input/output waveguide extending from the first coupler; at least one input/output waveguide extending from the output coupler; wherein the opposite ends of the input/output waveguides to the first and second coupler are provided on a common edge of the substrate to constitute optical inputs and outputs.  
           [0026]    The ends of the input and output waveguides may be arranged to have the same constant spacing at the edge of the wafer, and a optical fiber ribbon with the same spacing may be connected in registration to the input and output waveguides.  
           [0027]    In another aspect, the invention relates to an optical system comprising a plurality of nodes, at least one of the nodes being an optical node having an arrayed waveguide grating on a substrate for which the input and output optical fibers are brought to the same edge.  
           [0028]    The invention also relates to a method of demultiplexing optical signals at a plurality of different frequencies, including inputting a multiplexed optical signal to at least one input arranged on an edge of a rectangular substrate; passing the multiplexed optical signal through an arrayed waveguide grating on the optical substrate to split the multiplexed signal according to wavelength into a plurality of demultiplexed signals passing along respective output waveguides; and outputting the demultiplexed signals from the plurality of output waveguides through a plurality of outputs arranged along the same edge of the rectangular substrate as the at least one input. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0029]    For a better understanding of the invention specific embodiments will now be described, purely by way of example, with reference to the accompanying drawings in which:  
         [0030]    [0030]FIG. 1 shows a known AWG device;  
         [0031]    [0031]FIG. 2 is a schematic drawing of the output star coupler of the AWG of FIG. 1;  
         [0032]    [0032]FIG. 3 is a schematic drawing of a first embodiment of the invention;  
         [0033]    [0033]FIG. 4 illustrates the cross section through a waveguide used in the AWG device of FIG. 3;  
         [0034]    [0034]FIG. 5 is a schematic of the wafer layout used in the embodiment of FIG. 3;  
         [0035]    [0035]FIG. 6 is a schematic drawing of a comparative example;  
         [0036]    [0036]FIG. 7 is a schematic of the wafer layout of the comparative example;  
         [0037]    [0037]FIG. 8 is a schematic diagram of an optical node incorporating an AWG according to FIG. 3; and  
         [0038]    [0038]FIG. 9 is a schematic diagram of an optical system according to the invention. 
     
    
     DETAILED DESCRIPTION  
       [0039]    Referring to FIGS.  3  to  5 , an arrayed waveguide grating (AWG) according to an embodiment of the invention provides a plurality of optical waveguides  3  defined on a rectangular substrate  1 . For example, to define the waveguides a buffer  5  may be deposited on the substrates a core  7  deposited along part of the buffer to define the waveguide  3  and a cladding layer  9  provided to cover the core and buffer. The refractive indices of the buffer  5 , core  7  and cladding  9  are selected so that light is guided along the waveguide in the region of the core. Thus, in the example the buffer and cladding have a refractive index a of 1.4464 and the core a refractive index of 1,4574. Since light travels partially in the core and partially in the buffer and cladding, light travelling down the waveguides experiences an effective refractive index, here around 1,452.  
         [0040]    The waveguides defined on the substrate include an input waveguide  31  and an output waveguide  33  which connect to input  37  and output  39  star couplers. An array  41  of waveguide  3  extends between the input and the output star couplers. In this embodiment, the array includes twenty five waveguides arranged side by side with a pitch of 6 μm at the star couplers. Each waveguide is longer than the adjacent waveguide by 253 μm so that, given the effective refractive index of about 1.452, a FSR of 800 GHz is achieved.  
         [0041]    The input and output waveguides  31 ,  33  extend from the respective star couplers  37 ,  39  to input and output ends  36  at adjacent positions on the same edge  35  of the substrate. This enables the input and output waveguides to be readily connected to optical fibers, by connecting correctly spaced optical fibers to the pair of waveguides in a single operation. In order that the input and output waveguides  31 ,  33  can readily be brought together in this way, the array of waveguides is bent through more a 180° so that it is not necessary to bend the input and output waveguides through narrow radii to bring them together.  
         [0042]    In the embodiment, the core  7  is 5 μm wide and 7 μm high. The pitch of input and outputs at the edge  35  of the substrate is 250 μm, to match a particular fiber ribbon pitch. As will be appreciated, the pitch may be varied to suit different input and output connectors.  
         [0043]    The separation of the waveguides  3  of the array  41  is 6 μm at the star couplers  37 ,  39 . The output waveguides have a slightly larger pitch, 12 μm at the star couplers. The skilled person will appreciate that these dimensions may be varied as required.  
         [0044]    The package which incorporates the AWG filter also includes a temperature sensor  43  connected to a plurality of electrical connections  45  arranged on the opposite edge  47  of the substrate to the edge  35  used to which the input and output optical waveguides  31 ,  33  are brought.  
         [0045]    A plurality of such AWGs can be made on a single wafer  51  as illustrated in FIG. 5, which shows  12  substrates  1  as shown in FIG. 3 arranged on a wafer. Saw lines  53  separate the filter chips on the wafers. After fabrication, the wafer  51  is split along the saw lines  53  to form a plurality of separate AWGs on individual substrates  1 .  
         [0046]    For comparison, FIG. 6 illustrates a conventional arrangement to produce the same AWG properties. In this arrangement, the input  31  and output  33  optical waveguides are brought to respective opposed edges  61 ,  63 . Electrical connections  45  are brought to an edge  47  adjacent to the opposed edges. The requirement to connect at three different edges makes integration of the AWG of FIG. 6 more difficult; in particular it is necessary to make optical connections on each of the opposed edges  61 ,  63 .  
         [0047]    Moreover, as illustrated in FIG. 7 only 8 of the AWGs according to FIG. 6 fit onto a wafer of the same size as that shown in FIG. 5. That is to say, the number of AWGs according to the invention that fit on a substrate is larger than the number of conventional AWGs with the same FSR. The size reduction using the invention creases with reducing FSR; above a threshold FSR there is no size advantage. The threshold will vary depending on the various parameters of the AWG. This is because for large FSR the optical length difference between adjacent waveguides of the AWG is too small to be able to loop the AWG round sufficiently to get both ends of the AWG to the same edge without difficulty.  
         [0048]    Referring to FIG. 8, an optical node  80  includes an input optical connector  87  connected via an input fiber  89  to a switch  91  which directs some of the light, for example input signals in a particular range of channels, through part of an optical fiber ribbon  85  to a connector  81  with a plurality of optical fibers arranged in registration with the input  31  and output waveguides  33  of an AWG as described above in which input and output waveguides are brought to a common edge. The output of the AWG is in the example taken through the optical fiber ribbon to an array of detectors  93  for fisher processing. The output may instead be taken an alternative optical processing device.  
         [0049]    Referring to FIG. 9, an optical network includes a plurality of transmitters  95  connected by optical fibers  99  and optical switches  97  to a plurality of optical nodes  80 . Some or all of the optical nodes may be as described with reference to FIG. 8, including an AWG  1  in which input and output waveguides are brought to the same edge of the substrate. The skilled person will realise that the network may be implemented in a large number of ways.  
         [0050]    The invention has been described with reference to a number of specific embodiments, However, the skilled person will realise that the embodiments are not limiting and that the invention may be implemented in a number of different ways.  
         [0051]    The skilled person will appreciate that the AWG may, purely by way of example, be used to demultiplex a group of WDM channels into a plurality of separate output signals, one in each channel. Alternatively, the reciprocity of the device allows the device to also carry out the reverse process of multiplexing.  
         [0052]    There are a number of alternative approaches for defining waveguides on a substrate and any of these may be used in an AWG according to the invention. The substrate need not be rectangular. The invention is not just applicable to conventional glass waveguides, but also to polymer waveguides.  
         [0053]    Moreover, although the invention has been described with reference to “input” and “outputs” waveguides these may in practice be reversible. Accordingly, the terms “input” and “output” pay be considered as nothing more than labels indicating opposite ends of the AWG.