Abstract:
An optical multiplexer/demultiplexer with an improved spectral characteristic is provided by two diffraction gratings ( 21, 20 ) arranged optically in tandem and one being configured to embrace the other, the gratings also being arranged to provide free spectral ranges differing by a factor of at least two, and having a coupling ( 22′ ) between them that carries over into the second grating information concerning the dispersion afforded by the first grating.

Description:
BACKGROUND TO THE INVENTION 
     Optical Wavelength Division Multiplexed (WDM) systems ideally require passive optical wavelength multiplexers and demultiplexers which have isolated pass-bands which are flat-topped so as to allow a measure of tolerance in the spectral positioning of the individual signals of the WDM system within these pass-bands. One method of multiplexing or demultiplexing channels in an optical WDM system relies upon the use of multilayer dielectric interference filters. Another relies upon Bragg reflection effects created in optical fibres. A third method, the method with which the present invention is particularly concerned, relies upon diffraction grating effects. 
     One form that such a diffraction grating can take for wavelength multiplexing/demultiplexing is the form described in EP 0 254 453, which also refers, with particular reference to its FIG. 5, to the possibility of having a tandem arrangement of two diffraction gratings arranged to provide a combined intensity transfer function that is the product of the intensity transfer function of its component diffraction grating 40 with that of its component diffraction grating 42. 
     An alternative form that such a diffraction grating can take is an optical waveguide grating that includes a set of optical waveguides in side-by-side array, each extending from one end of the array to the other, and being of uniformly incrementally greater optical path length from the shortest at one side of the array to the longest at the other. Such an optical grating constitutes a component of the multiplexer described by C Dragone et al., ‘Integrated Optics N×N Multiplexer on Silicon’, IEEE Photonics Technology Letters, Vol. 3, No. 10, Oct. 1991, pages 896-9. Referring to FIG. 1, the basic components of a 4-port version of such a multiplexer comprise an optical waveguide grating, indicated generally at  10 , where two ends are optically coupled by radiative stars, indicated schematically at  11  and  12 , respectively with input and output sets of waveguides  13  and  14 . Monochromatic light launched into one of the waveguides of set  13  spreads out in radiative star  11  to illuminate the input ends of all the waveguides of the grating  10 . At the far end of the grating  10  the field components of the emergent light interfere coherently in the far-field to produce a single bright spot at the far side of the radiative star  12 . Scanning the wavelength of the light causes a slip in the phase relationship of these field components, with the result that the bright spot traverses the inboard ends of the output set of waveguides  14  linearly with wavelengths as depicted at  15 . If the mode size of the waveguides  14  is well matched with the size of the bright spot, then efficient coupling occurs at each of the wavelengths at which the bright spot precisely registers with one of those waveguides  14 . Either side of these specific wavelengths the power falls off in a typically Gaussian manner as depicted at  15 . While this may allow acceptable extinction to be achieved between channels, it is far from the ideal of a flat-topped response. 
     A tandem arrangement of this alternative form of diffraction grating can also be constructed, an example of such an arrangement being described in EP 0 591 042 with particular reference to its FIG.  3 . This tandem arrangement similarly provides a combined intensity transfer function that is the product of the intensity transfer functions of its two component diffraction gratings. The response of this tandem arrangement also provides a typically Gaussian fall off in power that is similarly far from the ideal of a flat-topped response. 
     SUMMARY OF THE INVENTION 
     The present invention is directed to the provision of an optical multiplexer/demultiplexer that achieves a response that is more nearly flat-topped without introducing excessive insertion loss. In particular, it is directed to an improvement of the type of multiplexer/demultiplexer described in the specification of PCT Application GB 97/02051 corresponding to U.S. Pat. application No. 09/194,004, now U.S. Pat. No. 6,144,783, with particular reference to its FIG. 6, to which specification attention is specifically directed and its teachings incorporated herein by reference. 
     One of the features limiting the performance of such a multiplexer/demultiplexer is departures from uniformity within the area of the wafer from which the device is constructed, particularly departures from uniformity in the thickness and composition of the layers defining the optical waveguides. 
     An object of the present invention is to reduce the effects of such departures upon the performance so as to enable higher performance multiplexer/demultiplexer devices to be constructed from a given standard of wafer uniformity. 
     According to the present invention there is provided an optical multiplexer/demultiplexer for the multiplexing/demultiplexing of optical signal channels at a substantially uniform optical frequency spacing, which multiplexer/demultiplexer includes a set of input/output ports optically coupled with an output/input port via a tandem arrangement of first and second optical waveguide diffraction gratings that provide multiple optical paths from each member of the set of input-output ports to the output/input port via different grating elements of the gratings, 
     wherein the difference in optical path length occasioned by paths via adjacent optical waveguide elements of the first grating is greater than that occasioned by paths via adjacent optical waveguide elements of the second grating, 
     wherein said difference in optical path length defines for its associated grating a frequency range, the Free Spectral Range, being the frequency range over which said optical path length difference produces a phase difference whose value ranges over 2π, 
     wherein the Free Spectral Range of the first diffraction grating is matched with the optical frequency spacing of the optical signal channels, 
     wherein the Free Spectral Range of the second diffraction grating is at least as great as the sum formed by the addition of the difference in frequency between adjacent frequency channels of the multiplexer/demultiplexer to the difference in frequency between the highest and lowest frequency channels of the multiplexer/demultiplexer, 
     wherein the portion of the optical coupling between the set of input/output ports and the output/input port that extends between the first and second diffraction gratings couples spatial information between the two gratings in addition to intensity information, 
     wherein the optical waveguide elements of each diffraction grating consist of a plurality of optical waveguides extending in side-by-side relationship in a set of arcuate optical paths and wherein the arcuate optical paths of the optical waveguides of one of said first and second diffraction gratings is configured to embrace the set of arcuate paths of the optical waveguides of the other of said first and second gratings. 
     Other features and advantages of the invention will be readily apparent from the following description of preferred embodiments of the invention, the drawings and the claims. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 (to which previous reference has already been made) schematically depicts a prior art optical multiplexer/demultiplexer employing an optical waveguide type diffraction grating, 
     FIG. 2 is a schematic diagram of the configuration of a practical implementation of a multiplexer/demultiplexer constructed in accordance with the teachings of the specification of PCT GB 97/02051, 
     FIG. 3 is a schematic diagram of a configuration of a multiplexer/demultiplexer constructed in accordance with the teachings of the present invention, and 
     FIG. 4 is a schematic perspective view of a portion of the multiplexer/demultiplexer of FIG.  3 . 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Whereas FIG. 6 of the specification of PCT GB 97/02051 (to which previous reference has already been made) shows a schematic diagram of a multiplexer/demultiplexer device with an in-line tandem arrangement of its two component optical waveguide diffraction gratings, a practical implementation may typically have a configuration more nearly like that depicted in FIG. 2 of this specification in which the waveguides of the gratings are arranged in a manner that is liable to be less prodigal with usage of integrated optics wafer area. In this arrangement of FIG. 2 the two optical waveguide diffraction gratings are depicted respectively at  20  and  21  on an integrated optics wafer  25 , these gratings exhibiting FSRs that differ in magnitude by a factor at least equal to the number of channels being separated. Grating  20  corresponds to grating  10  of FIG. 1 and, under the assumption that the left-hand side of the diagram of FIG. 1 is the input side, the output side to this grating is unchanged, comprising a coupling region  12  feeding a set of waveguides  14 . In this particular instance the outboard ends of the waveguides  14  have been arranged into two groups for facilitating the coupling of those ends with the ends of two optical fibre ribbons (not shown). On the input side of grating  20 , the radiative star coupling  11  is replaced by a coupling region  22  which provides optical coupling between grating  20  and grating  21 . On the input side of grating  21  is a further optical coupling region  23  which, optically couples a single waveguide  24  with the input end of the input of grating  21 . Each of these coupling regions functions as a device performing a Fourier Transform converting positional information into angular information, or vice versa. The coupling region  22  is in effect a tandem arrangement of two conventional type radiative stars arranged back-to-back and meeting in an ‘image plane’. The first radiative star component of coupling region  22  operates to convert angular information received from one of the gratings into positional information at the ‘image plane’, while the second converts it back into angular information again for launching into the other grating. Thus it is seen that this coupling region  22  couples both intensity and spatial information between the two gratings. (The two component radiative stars of coupling region  22  may normally be designed to meet in an ‘image plane’ that is indeed a planar surface, but for certain geometries it may be preferred for this surface to have a curvature.) 
     A multiplexer/demultiplexer of the type of configuration illustrated in FIG. 2 of this specification, and designed for operation in a WDM environment with a 200 GHz channel spacing, may typically occupy a silica-on-silica wafer area of about 30 mm by 30 mm. For satisfactory operation the two optical waveguide diffraction gratings  20  and  21  typically need to register to at least one tenth, and preferably to one fifteenth of a channel separation, or better; i.e. a tolerance of about ± 0.1 nm or better is called for. One of the limiting factors in achieving such tighter tolerances is control over the thickness and composition of the layers of wafer  25  that go to make up its optical waveguiding structure. Inspection of FIG. 2 of this specification reveals that in the case of a compositional or thickness gradient with a component extending in the direction of arrow  26 , that component is likely to be more troublesome than a component of equal magnitude extending in the direction of arrow  27 . This is because in the case of the component of the gradient extending in the direction of arrow  27 , the effects it produces in diffraction grating  20  are at least partially offset by those it produces in diffraction grating  21 . Using current wafer processing technology it has been found that these tolerance requirements impose at least somewhat of a yield problem so far as the provision of suitable wafers is concerned, and that this yield problem is liable to be significantly greater when attempting the construction of multiplexer/demultiplexer devices for operation in a WDM environment with a 100 GHz channel spacing. 
     Attention is now turned to FIG. 3 of this specification which depicts a configuration of multiplexer/demultiplexer embodying the present invention in a preferred form. Most of its components have their counterparts in the multiplexer/demultiplexer of FIG. 2 of this specification, and these components have been identified with the same index numerals as those FIG. 2 counterparts. Thus the device of FIG. 3 similarly has an arrangement of two optical waveguide diffraction gratings optically in tandem. These gratings are optically coupled by means of a coupling region  22 ′, that is very similar to the coupling region  22  of FIG. 2, but additionally contains a reflector  32  to provide an optical path that is folded back almost upon itself. Light that is launched into the device by way of the single waveguide  24  is coupled by coupling region  23  into grating  21 . The coupling region  22 ′, with its reflector  32 , then couples this light into grating  20 , from where it is coupled by coupling region  12  into the waveguides  14 . 
     In relation to its multiplexer/demultiplexer of its FIG. 6, the specification of PCT Application GB 97/02051 explains that, at least in certain configurations, it is desirable to include some form of field stop to block light of unwanted diffracted orders from being coupled between the two optical waveguide diffraction gratings, and in relation to its multiplexer/demultiplexer of FIG. 7 explains that the function of that field stop could be performed by the appropriate dimensioning of the reflector that couples the two optical waveguide diffraction gratings of the device. This also applies to multiplexer/demultiplexers of the present invention. FIG. 4 of this specification depicts in greater detail one manner in which this may be accomplished. 
     FIG. 4 depicts a schematic perspective view of that portion of the multiplexer/demultiplexer of FIG. 3 that contains its reflector  32 , and shows the basic layer structure of the integrated optics wafer  25 , which consists of a silicon substrate  40  upon which has been formed three glassy layers  41   a  to  41   c . Layer  41   a  is a buffer layer of silica. Layer  41   b  is a core-glass layer of doped silica having a refractive index greater than that of the buffer layer  41   a . Layer  41   c  is a cladding-glass layer, typically also a layer of doped silica, but doped in a manner providing a lower refractive index than that of the core-glass layer  41   b,  and typically doped so as to make its refractive index substantially matched with that of the silica buffer layer  41   a . 
     In principle, where the coupling region  22 ′ meets the edge of the wafer in face  42 , what is wanted for a field stop is for that face to be specularly reflective only over a certain specific width ‘a’. Beyond the confines of width ‘a’ there should preferably be substantially no specular reflection. Specular reflection outside the confines of width ‘a’ is however acceptable provided that its specular reflection plane is aligned at a sufficiently large angle with respect to the reflection plane within the confines of width ‘a’ for that specularly reflected light (outside the confines of width ‘a’) to be deviated enough to fail to couple into either of the optical waveguide diffraction gratings  20  and  21 . 
     Instead of attempting to provide a high quality specularly reflecting surface at face  42  itself, it is preferred for this reflecting surface to be provided by metallisation  43  deposited upon a cleaved surface of a crystal chip  44  which is subsequently cemented with an index-matching adhesive (not shown) to face  42 . By this means, a face  42  prepared by sawing the wafer can be of acceptable quality. 
     In principle, the field stop can be provided on chip  44  either by masking off all but a width ‘a’ central stripe of the metallisation  43  with non-reflective material, or by confining the metallisation to a stripe of width ‘a’ flanked by regions of non-reflective material. Having regard, however, to the fact that width ‘a’ is typically only a few μm wide, it is preferred to avoid the registration problems involved in bonding such a chip to wafer  25  by instead creating the field stop in the wafer  25  itself. To this end, two wells  45  are etched through cladding- and core-glass layers  41   c  and  41   b , and into buffer layer  41   a  before the wafer is sawn to produce the face  42 . These wells have side-walls  46  that are aligned at a sufficiently large angle with respect to face  42  for any light specularly reflected by these side-walls to be deviated enough to fail to couple into either of the optical waveguide diffraction gratings  20  and  21 . A light-absorbing or light-reflecting coating  47  is applied to the interiors of the wells  45  so that the side-walls  46  cannot be contacted by the index-matching adhesive subsequently used to secure chip  44  in position on face  42 .