Abstract:
A single-mode planar nanophotonic waveguide includes an optical core, a cladding coating the optical core, and a structure for the optical coupling of the core of the waveguide with the core of a single-mode optical fibre. The coupling structure includes an adaptation element including a gradual broadening of the core of the waveguide ending in a broadened region of dimension adapted to the core of the optical fibre; and a light transmission element, optically connected to the broadened region, and defining a plane coupling surface by which light is transmitted to the optical fibre, the optical coupling ratio through the surface when the surface is in contact with the air being maximum for a predetermined coupling angle (θ) of the light relative to the coupling surface.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    The present application claims priority of pending French Patent Application No. 1056033, filed on Jul. 23, 2010, the content of which is incorporated herein by reference. 
       FIELD OF THE INVENTION 
       [0002]    The invention relates to the field of optoelectronic components, and particularly nanophotonic circuits defined by waveguides of less than one micrometre in size. 
         [0003]    More specifically, the invention relates to the coupling of such nanophotonic circuits with single-mode optical fibres, which have a core much more than one micrometre in size. 
       BACKGROUND OF THE INVENTION 
       [0004]    Planar, i.e. rectangular-geometry, optical waveguide technologies enable complex optical beam management functions, such as multiplexing, de-multiplexing, modulation and spectral routing for example, to be integrated very compactly into a single chip. 
         [0005]    Not only can planar optical waveguides be used to implement necessary functions, both for optical links over very short distances of about one millimetre, such as optical links internal to a chip for example, and over very long distances of several kilometres, such as links on a metropolitan communications network for example, but said waveguides also deliver very high data flow rates thereby constituting technologies of choice in response to the constant increase in demand for throughput. 
         [0006]    Furthermore, the integration of a growing number of optical functions onto one and the same chip surface requires an extreme miniaturisation of the optical circuits, thereby leading to a growing miniaturisation of the cross-section of the planar optical guides, down to submicronic dimensions, hence the reference to “nanophotonic” guides. It is therefore not unusual to have optical guide sections of 0.5 micrometres by 0.2 micrometres for rectangular-section waveguide cores. 
         [0007]    In fact, for medium- and long-distance applications, i.e. applications requiring data transmission over distances from a few metres to a plurality of kilometres, the only appropriate transmission medium is optical fibre, which has a section with a diameter of more than ten micrometres, or even more than a hundred micrometres. Consequently, it is necessary to provide adaptation or coupling systems, between the planar nanophotonic waveguides and the optical fibres, in order to compensate for this continually growing mismatch of dimensions, and to compensate for a difference in behaviour between waveguides and optical fibres in respect of light polarisation. 
         [0008]    Usually, the adaptation between a single-mode planar nanophotonic waveguide and a single-mode optical fibre is implemented by a surface coupling element that includes a “taper”, formed in the waveguide which adapts the dimensions of the guide to that of the fibre, and an optical element which makes the interface between the “taper” and the optical fibre, to advantage a diffraction grating formed of periodic slits. 
         [0009]      FIGS. 1 and 2  are a sectional view and a detailed view from above respectively of such an optical coupling structure  10  from the prior art, implemented in a single-mode planar nanophotonic waveguide  12 , and intended for optical coupling with a single-mode optical fibre  14 .  FIGS. 3A and 3B  are for their part perspective views of two alternative embodiments of a planar nanophotonic waveguide  12  from the prior art. 
         [0010]    A planar waveguide  12  comprises, for the transmission of light in a nanophotonic circuit, a guide core  16  of submicronic dimension, made for example of silicon, indium phosphide, silicon nitride, or silica doped with photophore, boron or germanium. The core  16  is inserted between plane lower  18  and upper  20  cladding, made for example of silica, silicon nitride, silica doped with photophore, boron or germanium, silicon oxide, or constituted by air. This stack is itself formed on a substrate  22 , made of silicon for example. 
         [0011]    The core  16  of the planar waveguide  12  may assume different geometries, and be for example of T-shaped section, as shown in  FIG. 3A  (a waveguide of the “ridge” type), or of rectangular section, as shown in  FIG. 3B  (waveguide of the “ribbon” type). 
         [0012]    Furthermore, the single-mode optical fibre  14 , cylindrical in shape, includes a cylindrical fibre core  24 , coated with cylindrical cladding  24 . The diameter of the core  24  of the fibre  14  is commonly less than 10 micrometres for single-mode fibres and the outer diameter of the cladding  24  is commonly about a hundred micrometres. 
         [0013]    The structure and the operation of the planar waveguide  12  and the single-mode optical fibre  14  that have just been described are well known from the prior art and will not therefore be described in further detail for reasons of conciseness. 
         [0014]    The waveguide  12  further comprises furthermore an optical coupling structure which allows the light to be transmitted from the core  16  of the waveguide  12 , with the submicronic dimensions, towards the core  24  of the optical fibre  14 , of diameter commonly of between 8 and 10 micrometres, and vice versa, which comprises:
   a region  28 , commonly known as a “taper”, which consists of a gradual broadening of the core  16  of the planar waveguide  12 , the region  28  ending in a broadened plane region  30  of dimensions compatible with the diameter of the core  24  of the fibre  14 ; and   a periodic grating of parallel slits  32 , perpendicular to the main direction of the planar waveguide  12 , engraved on the upper face of the broadened region  30 , of length L and of width I greater than or equal to the diameter of the core  24  of the fibre  14 .   
 
         [0017]    The grating  32  thus forms a diffraction grating that allows the light contained in the region  30  to escape, via the upper cladding  20 , towards the optical fibre  14 , and the light coming from the optical fibre  14  to enter into the region  30 , via the upper cladding  20 . The features of the grating  32  are determined for a wavelength for transmission between the guide  12  and the fibre  14 . 
         [0018]    The light is therefore exchanged between the waveguide  12  and the optical fibre  14  through a plane surface  34  of the upper cladding  20 , hereinafter referred to as the “coupling surface”. 
         [0019]    A major problem with this type of optical coupling structure  10  is the positioning in space of the optical fibre  14  in respect of the coupling surface  34 , or equivalently in respect of the slit-grating  32 . 
         [0020]    Indeed, the coupling ratio between the guide  12  and the fibre  14  (relationship between the amplitude of the light at the surface  34  in the direction of the optical fibre  14  and the amplitude of the light at the surface  34  in the direction of the waveguide  12 ), is optimum when the direction of propagation of the light forms an angle θ relative to the normal direction at the coupling surface  34 . This optimum angle is commonly known as the “coupling angle”. 
         [0021]    The coupling angle depends on the nature of the materials and on the dimensions of the adaptation structure  10 , on the refractive index of the medium with which the coupling surface  34  is in contact, and on the wavelength of the transmitted light, and is usually between 0° and 30°. If the direction of the light deviates from the coupling angle θ, a very substantial deterioration is observed in the coupling ratio, such a loss being able to render the coupling ineffective. 
         [0022]    It is therefore appropriate for the output section  36  of the optical fibre  14  to be as perpendicular as possible to the preferred direction of propagation of the light defined by the coupling angle θ. The angular positioning of the fibre  14  is thus optimum when its section  36  itself forms the coupling angle θ with the surface  34 . 
         [0023]      FIG. 4  shows this loss effect for a coupling set to the 1550 nanometre wavelength, an upper cladding  20  of silica with a refractive index n=1.44 and a thickness of 700 nanometres, a ribbon type guide core  12  of silicon with a refractive index equal to 3.47 and a thickness of 220 nanometres extending, via the “taper”  28 , by a broadened region  30  wherein a grating  32  is engraved 10 micrometres in width and in length, the periodicity and the depth of the slits being 630 nanometres and 70 nanometres respectively, and a lower cladding  18  of silica with a refractive index equal to 1.44 and a thickness of 2 micrometres. When the coupling surface  34  is in contact with the air, with a refractive index equal to 1, the coupling angle is then equal to 10°. 
         [0024]      FIG. 4  is a decibel curve of the coupling ratio as a function of the angular deviation in the output section  36  relative to the coupling angle θ. It can thus be seen that an angular deviation of 2.7° relative to the coupling angle θ brings about a loss of 20% of the coupling ratio. This reaches 40% when the angular deviation is equal to 4°. 
         [0025]    Not only therefore does the optical fibre  14  need to be positioned angularly with precision, but it is also necessary to position the core  24  of the fibre  14  and the coupling surface  34  on the same optical path. It will be noted in this respect that even a small misalignment of the core  24  and of the surface  34  also leads to significant transmission losses. Thus for example, an offset of 2 μm reduces the transmission by 15%. 
         [0026]    In fact, the relative positioning of micrometric elements is very tricky and requires a complicated procedure. This procedure comprises, during the positioning itself, measuring the light transmission between the fibre  14  and the waveguide  12 , and considering the positioning as correctly implemented when transmission is maximum. 
         [0027]    However, the optical fibre  14  also needs to be held in place once the positioning is correctly implemented. In fact, the only effective way hitherto to hold the fibre  14  in place is to secure it to the upper cladding  20  by means of an adhesive. 
         [0028]    As stated above, the coupling angle θ depends on the refractive index of the medium with which the coupling surface  34  is in contact, and therefore on the refractive index of the adhesive used. Ideally therefore, the positioning should be implemented in the adhesive in order to optimise the coupling angle. In fact, positioning directly in the adhesive requires the provision of extra adhesive so that adhesive can be retained between the fibre and the grating despite movements of the fibre when its positioning is being optimised. But above all, the viscosity of the adhesive hinders the movements of the optical fibre and prevents any accurate positioning. 
         [0029]    Usually, positioning is therefore carried out in the air, i.e. a positioning which is optimised for the coupling angle θ in the air, and the adhesive is then added in order to secure the fibre  14 . The result therefore is an alignment which is not achieved for the true coupling angle, namely the one that is a function of the refractive index of the adhesive, and consequently the result is a reduced coupling ratio once the adhesive is added. 
         [0030]      FIG. 5  shows the difference between the coupling angle in the air and the coupling angle in the adhesive with a refractive index equal to 1.45 for different wavelengths around the 1550 nanometre wavelength. As can be seen, a difference of several degrees is observed. For example, a coupling angle difference of 4° is seen between air and adhesive. With reference to  FIG. 5 , this is equivalent to a trimming of the coupling ratio of about 40%. 
         [0031]    Despite this significant observed loss, the solution comprising positioning in the air followed by adhesion is more often than not preferred because of the difficulty encountered when positioning the optical fibre in the presence of adhesive. 
       SUMMARY OF THE INVENTION 
       [0032]    The purpose of this invention is to propose a structure for optical coupling between a single-mode optical fibre and a single-mode planar nanophotonic waveguide of the aforementioned type, which allows positioning of the fibre in the air while reducing, or even eliminating, the transmission loss caused by the use of adhesive. 
         [0033]    To this end, the object of the invention is a single-mode planar nanophotonic waveguide, comprising an optical core, a cladding coating the optical core, and a structure for the optical coupling of the guide core with the core of a single-mode optical fibre, said coupling structure comprising:
   an adaptation element including a gradual broadening of the waveguide core ending in a broadened region of dimension adapted to the core of the optical fibre; and   a light transmission element, optically connected to the broadened region, and defining a plane coupling surface by which the light is transmitted to the optical fibre, the optical coupling ratio through said surface when said surface is in contact with the air being maximum for a predetermined coupling angle of the light relative to the coupling surface   
 
         [0036]    According to the invention, the planar waveguide further comprises a prism including a first and a second plane face, the first face being arranged on or in contact with the coupling surface and the second face being intended to engage optically with the optical fibre, the two faces being angularly separated from one another by an angle whereof the value is substantially equal to an angle of refraction in the prism at the coupling surface. 
         [0037]    Put another way, the prism defines a new coupling surface for the transmission of light between the waveguide and the optical fibre having the characteristic according to which the coupling angle is now zero, i.e. for a normal light at this surface. In fact, if the coupling angle in the air is zero, it is also zero for any other material, and particularly adhesive. Since the coupling angle is now invariant, it is therefore possible to implement the positioning in the air and then to add adhesive without the corresponding refractive index variation inducing an angle variation and therefore a transmission loss. 
         [0038]    It will be noted in this respect that the overall structure of the prior art waveguide is not modified and that the inventive prism may be considered as an optical element for the angular correction of the conventional coupling structure. In particular, the inventive prism involves no modification of the prior art transmission element, such as for example a periodic slit-grating. 
         [0039]    Additionally, there is generally an imprecision about the value of the coupling angle associated with the surface of the transmission element due, for example, to fabrication tolerances. However, even if the inventive prism does not exactly compensate for the coupling angle of the transmission element, it does nonetheless allow the angular variation caused by differences in refractive indices between the air and the adhesive to be minimized, as will be explained in further detail below. 
         [0040]    According to one inventive embodiment, the transmission element includes a periodic slit-grating formed on one face of the broadened region, the prism being arranged on the cladding flush with the periodic slit-grating. 
         [0041]    According to one embodiment, the second face of the prism inscribes an end section of the optical fibre with which the second face of the prism is intended to engage optically. 
         [0042]    According to one inventive embodiment, the prism and a portion of the cladding on which the prism is arranged have substantially the same refractive index value, and are in particular constituted by a single material. In particular, the prism and said portion are made of silica. 
         [0043]    A further object of the invention is a system for optical coupling between a nanophotonic circuit and a single-mode optical fibre, the optical fibre comprising a core ending in a plane face normal to the direction of propagation of the light in the fibre, which according to the invention, comprises:
   a planar nanophotonic waveguide of the aforementioned type; and   means for holding the optical fibre in such a way that the plane end face of the core of the optical fibre is opposite, and parallel, to the second face of the prism of the planar waveguide.   
 
         [0046]    According to one embodiment, the holding means include adhesive arranged between the plane section of the fibre core and the second face of the prism. 
         [0047]    According to one embodiment, the prism, a portion of the cladding on which the prism is arranged, and the adhesive have substantially the same refractive index value. In particular, the prism and said portion are made of silica, and the adhesive is constituted by a polymer of the epoxide type. 
         [0048]    A further object of the invention is the use of a prism in a planar nanophotonic waveguide of the prior art to form a plane surface determining a coupling ratio in air through said surface that is maximum for a light that is normal at said surface. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0049]    The invention will be better understood from reading the following description, provided solely by way of example, and given in relation to the appended drawings, wherein identical references denote identical elements, and wherein: 
           [0050]      FIGS. 1 and 2  are a cross-section view and a detailed view from above respectively of a nanophotonic waveguide including a prior art optical coupling structure, as already described in the preamble; 
           [0051]      FIGS. 3A and 3B  are perspective views of two alternative embodiments of a planar nanophotonic waveguide, as already described in the preamble; 
           [0052]      FIG. 4  is a decibel curve of the coupling ratio of the coupling structure as a function of the angular deviation of an optical fibre relative to the coupling angle of the coupling structure; 
           [0053]      FIG. 5  is a plotting of the coupling angle of the structure in  FIGS. 1 and 2 , in the air and in an adhesive, as already described in the preamble; 
           [0054]      FIG. 6  is a cross-section view of a coupling structure according to the invention; 
           [0055]      FIG. 7  is a more detailed cross-section view of a slit-grating of the coupling structure; 
           [0056]      FIG. 8  is a plotting of the coupling angle difference between air and adhesive, with or without the inventive prism; 
           [0057]      FIGS. 9 and 10  are simplified perspective views respectively of a prism common to a plurality of planar waveguides and of a plurality of prisms each associated with one waveguide of a plurality of waveguides; and 
           [0058]      FIG. 11  is a broadened view of the coupling structure showing the angles of refraction in the general situation where the refractive indices of the waveguide and of the prism are different. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0059]    An inventive embodiment is described in relation to  FIG. 6  that implements the prior art optical coupling structure, for example the one described in relation to  FIGS. 1 to 3 , completed by a wedge-shaped prism  40 , having a first rectangular plane face  42  secured to the upper cladding  20  and covering at least the coupling surface  34 , and a second rectangular plane face  44  forming with the first plane face  42  an angle equal to the coupling angle θ in the air of the coupling surface  34 . The refractive index np of the prism  40  is furthermore equal to the refractive index ng of the upper cladding  20  for a selected wavelength, and to advantage for the central wavelength of a spectrum for transmission between the optical coupling structure  10  and the optical fibre  14 , this spectrum being defined in particular for the application under consideration of the structure  10  and the fibre  14 . The prism  40  and the upper cladding  20  are made for example out of the same material. 
         [0060]    The second face  44  of the prism  42  thus defines a new coupling surface for which the coupling angle is zero. The optical coupling ratio is thus maximum for a light that is normal at the plane of the face  44 , whatever the material is between the optical fibre  14  and the face  44  of the prism  40 . By placing the section  36  of the optical fibre  14  parallel to the face  44  and above the slit-grating  32 , the transmission between the optical fibre  14  and the waveguide  12  is then guaranteed to be optimum both in the air and in the adhesive. The fibre can thus be positioned in the air without the subsequent addition of adhesive impacting on the coupling ratio. 
         [0061]    The coupling angle in the air of the surface  34  may be easily calculated numerically using FDTD (finite-difference time-domain) numerical modelling or even analytically, as is described for example in the article by C. Kopp, A. Chelnokov, “Fiber grating couplers for silicon nanophotonic circuits: Design modeling methodology and fabrication tolerances”, Optics Communications, Volume 282, Issue 21, 4242-4248 (2009). 
         [0062]    With reference to  FIG. 7 , which is a more detailed view of the slit grating  32 , for a light beam to emerge from the diffraction grating  32  at an angle θp (propagative coupling, beam  46  towards beam  48 ) or at an angle θc (counter-propagative coupling, beam  49  towards beam  48 ), the phase agreement condition between points A and B has to be satisfied. Thus, the phases φA and φB, at points A and B respectively, must satisfy the condition: 
         [0000]      φ A−B= 2 π.mp  in the propagative case,
 
         [0000]        et φA−φB= 2 π.mc  in the counter-propagative case, 
         [0000]    mp and mc being whole numbers. 
         [0063]    The phase shift between points A and B is equal to, in the case of propagative coupling (beam  46  towards  48 ): 
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         [0000]    and, in the case of counter-propagative coupling (beam  49  towards  48 ): 
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         [0000]    λ being the wavelength of the light beam under consideration, and neff 1  and neff 2  the actual indices of the guide portions of different thicknesses over one period of the grating  32 .
 
This is then equivalent to the following system:
 
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         [0064]    With all parameters being known, it is thus possible to calculate as a function thereof the angle θp or the angle θc according to the direction of propagation of the light in the waveguide  12  (mp and mc being fixed; if the diffraction order +1 is considered, then m=1). Since furthermore the refractive indices of the upper cladding  20  and of the air are known, the coupling angle θ at the surface  34  of the cladding  20  is then straightforwardly deduced therefrom. 
         [0065]    Clearly, when the coupling angle of the surface  34  is calculated as a function of a mathematical modelling which uses precise values for the dimensions and the indices of the different elements of the waveguide  12  and of its optical coupling structure  10 , the value so calculated may be different from the real value of the coupling angle for a particular waveguide. Indeed, even if the methods of fabrication of the waveguide elements and its coupling structure aim for said precise values, they are nonetheless of limited precision. Likewise, the method of fabrication of the prism  40  may also be of limited precision. 
         [0066]    Due to a lack of precision in fabrication, the prism  40  may not compensate perfectly for the real coupling angle of the coupling surface  34 . In fact, the problem related to the difference that exists between the coupling angle in the air and the coupling angle in the adhesive may subsist. 
         [0067]    However, even if the prism  40  is fabricated at an angle between the faces  42  and  44  different from the real coupling angle of the coupling structure  10  with which it is combined, the effect of the prism  40  is to advantage to reduce the losses caused by the difference between the coupling angle in the air and the coupling angle in the adhesive. 
         [0068]    Indeed, returning to the example of a coupling set to the 1550 nanometre wavelength, implementing:
   an upper cladding  20  of silica with a refractive index n=1.44 and a thickness of 700 nanometres,   a ribbon waveguide core  12  of silicon with a refractive index equal to 3.47 and a thickness of 220 nanometres   said waveguide extending, via the “taper”  28 , by a broadened region  30  wherein a grating  32  10 micrometres in width and length is engraved, the periodicity and the depth of the slits being 630 nanometres and 70 nanometres respectively,   and a lower cladding  18  of silica with a refractive index equal to 1.44 and a thickness of 2 micrometres;
 
the coupling angle in the air of the surface  34  is equal to 10°. With an adhesive of refractive index equal to 1.45, the coupling angle in the adhesive of the surface  34  is then equal to 6°.
   
 
         [0073]    With reference to  FIG. 8 , and assuming that the fabrication tolerances of the elements described above lead to a real coupling angle in the air of between 10° and 19°, the real coupling angle in the adhesive is then between 7° and 13°. There is therefore a maximum difference between the two angles in excess of 6°, which then corresponds in the absence of a prism to a possible trimming of the coupling ratio in excess of 5 dB (see  FIG. 5 ). 
         [0074]    By adding an inventive prism, whereof said first and second faces form between them an angle of 10°, the variation in the coupling angle in the air defined relative to the second face  42  of the prism is no more than from 5.5° to 14.5°. The angular variation, moving from air to adhesive, is thus at worst of 1.5°, which represents a trimming of the coupling ratio of less than 0.3 dB, as may be easily calculated from the refraction formula at the interface between two media of different refractive index. 
         [0075]    Thus, even if the prism is not actually optimised for the real coupling angle of the surface  34 , there does nonetheless ensue from using it a substantial reduction in the losses related to the introduction of the adhesive. 
         [0076]    To advantage, the materials constituting the prism  40 , the upper cladding  20  and the adhesive are selected to afford the best possible continuity of the refractive index in order to avoid any index deviation that might generate transmission losses by Fresnel reflection at the interfaces. Thus, the refractive indices of the upper cladding  20 , the prism  40  and the adhesive are selected to be as close as possible, and preferably identical, for a wavelength, and for example a central wavelength, of the spectrum for transmission between the optical coupling structure  10  and the optical fibre  14 . The optical transmission is in this case maximum since there is no loss by reflection at the interfaces and the optical path is rectilinear since there is no refraction effect at the interfaces. 
         [0077]    Thus, a preferred material for the upper cladding  20  and the prism  40  is silica with a refractive index equal to 1.44, and the adhesive is constituted by an epoxide polymer with a refractive index equal to 1.45. Clearly, other materials may be selected as a function of the fabrication method used, particularly to fabricate the prism  40 . 
         [0078]    To advantage also, the dimensions of the prism  40  are selected in such a way that the section  36  of the optical fibre  14  is inscribed in the second face  44 . For example, for a cladding  26  of the optical fibre  14  with an external diameter of 125 micrometres, the length and the width of the second face  44  of the prism  40  are greater than or equal to 125 micrometres. This in particular makes it easier subsequently to bond the optical fibre  14  to the surface  44  of the prism and to reduce the quantity of adhesive required. Preferably however, the dimensions of the prism will be selected to be as small as possible so that the prism does not have too significant a thickness, which may induce a transmission loss. Thus for example, the width and the length of the prism are selected to be substantially equal to 125 micrometres. 
         [0079]    As an alternative, the dimensions of the prism  40  are selected in such a way that the first face  42  has dimensions substantially equal to those of the periodic slit grating, or dimensions smaller than those of the grating  32 . The parts are thus smaller and therefore easier to fabricate. Additionally, gratings are commonly over-sized, typically 15 μm by 10 μm whereas their actually used surface is 6 μm by 6 μm, so as to be able to see them. Thus a prism of length equal to 125 μm, gives a factor  10 , advantageous for fabrication. 
         [0080]    An embodiment has been described wherein an optical coupling is implemented between a single optical fibre and a single waveguide. Clearly, as is known from the prior art, a nanophotonic circuit may contain a plurality of planar waveguides each needing to be connected to an optical fibre. 
         [0081]    To advantage, as is shown in  FIG. 9 , the coupling structures  10  of the different single-mode planar nanophotonic waveguides are identical and their periodic slit gratings  32  end on one and the same line  50 . A single prism  52  is then made to simplify fabrication. 
         [0082]    As an alternative, as shown in  FIG. 10 , one prism  60   a,    60   b,    60   c,    60   d  may be used per waveguide, which allows different waveguides to be designed which may have different dimensions. 
         [0083]    The prism or prisms are fabricated by micro-replication, for example by hot embossing or by UV cast embossing or UV cast imprint. To advantage, a plurality of prisms of a single face of a nanophotonic circuit are fabricated collectively in a single micro-replication operation. 
         [0084]    Micro-replication comprises dispensing a fluid organic compound, and typically an epoxy adhesive or a transparent thermoplastic material in the vitreous state, on the surface of the circuit, in the form of a layer whereof the thickness is close to the intended height of the prism. A structured mould whereof the hollow patterns correspond to the complementary of the patterns to be made, in this case the prism or prisms, is then applied. 
         [0085]    Once the mould is applied, the structured layer of organic compound is cross-link hardened by heating or by application of ultra-violet rays. These techniques are described for example in the documents by Becker et al. “Hot embossing as a method for the fabrication of polymer high aspect ratio structures”, J. Sensors and Actuators A: Physical, Volume 83, Issues 1-3, 22 May 2000, Pages 130-135, and by Rudman et al. “Design and fabrication technologies for ultraviolet replicated micro-optics”, Opt. Eng. 43(11), 2004. 
         [0086]    The organic compound used for the prisms is to advantage a polymer. For example in the case of hot embossing, an optical adhesive is preferred, and for example epo-tek 353ND fabricated by the company Epoxy Technology, Inc. or a thermoplastic polymer, such as PMMA for example. The optical adhesive allows patterns to be obtained that are particularly stable in temperature, through its high vitreous transition temperature (Tg=120° C.) and its degradation temperature (Tmax=400° C.). 
         [0087]    In the case of UV cast embossing, an optical adhesive that will crosslink under UV is preferred, such as for example ChemOptics CO150, fabricated by the company Chemoptics Inc., or epotek OG142 fabricated by the company Epoxy Technology, Inc. These two materials have the advantage of having a relatively fast ultra-violet cross-linking time (of about a minute) relative to polymers with thermal cross-linking, which leads to a shorter prism fabrication cycle. 
         [0088]    Embodiments have been described wherein the refractive index np of the prism  40  is equal to the refractive index ng of the upper cladding  20  for a given wavelength of a spectrum for transmission between these two elements, and in a preferred alternative, an identity between the refractive indices of the prism, the upper cladding and the adhesive. 
         [0089]    Some applications may however compel the use of different refractive indices for the prism  40  and the upper cladding  20 , for example in the situation where the materials constituting same are imposed. To obtain an optimum coupling between the optical structure  10  and the optical fibre  14  in a situation such as this, the angle of the prism  40  is also calculated to compensate for the deviation in refractive indices between the prism  40  and the upper cladding  20  and therefore to compensate for the angular refraction at the interface between the prism  40  and the cladding  20 . 
         [0090]    With reference to  FIG. 11 , which is a broadened view of the coupling structure  10  and of the prism  40 , if we denote in a general way:
   θ g  the coupling angle in the upper cladding  20  relative to the vertical at the plane of the core  16 , and ng the refractive index of the upper cladding  20 ;   θ p  the coupling angle in the prism  40  relative to the vertical at the plane surface  34  of the cladding  20 , and np the refractive index of the prism  40 ; and   θ the angle of the prism  40  with the surface  34 
 
to obtain a light beam  70  orthogonal to the surface  44  of the prism  40  and therefore a zero coupling angle with the optical fibre  14 , the angle θ of the prism  40  is selected to be equal to the angle θ p , in which case the Fresnel relationship at the interface between the prism  40  and the cladding  20 , n p ×sin(θ p )=n g ×sin(θ g ), is verified.
   
 
         [0094]    It will be noted that for identical refractive indices for the prism  40  and the upper cladding  20 , n g =n p , we get θ g =θ p , and therefore θ=θ p =θ g  of the embodiments described previously. 
         [0095]    The table below describes a few numerical examples of refractive indices and angles. 
         [0000]    
       
         
               
               
               
               
               
             
               
               
               
               
               
             
           
               
                   
               
               
                 n g   
                 θ g (°) 
                 n p   
                 θ p (°) 
                 θ 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 1.45 
                 10 
                 1.3 
                 11.2 
                 11.2 
               
               
                 1.45 
                 10 
                 1.4 
                 10.4 
                 10.4 
               
               
                 1.45 
                 10 
                 1.45 
                 10.0 
                 10.0 
               
               
                 1.45 
                 10 
                 1.6 
                 9.1 
                 9.1 
               
               
                 1.45 
                 10 
                 1.7 
                 8.5 
                 8.5 
               
               
                   
               
             
          
         
       
     
         [0096]    The following table describes examples of materials for the prism  40  with their respective refractive indices. 
         [0000]    
       
         
               
               
               
               
             
               
               
               
               
             
           
               
                   
               
               
                 Material 
                 Refractive index 
                 Chemical base 
                 Cross-linking type 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 ChemOptics 
                 1.514 
                 Acrylate 
                 UV 
               
               
                 CO150 
               
               
                 Epotek 353ND 
                 1.5694 
                 Epoxy 
                 Thermal 
               
               
                 Epotek OG142 
                 1.5692 
                 Epoxy 
                 UV 
               
               
                 PMMA 
                 1.4914 
                 Polymethyl- 
                 (thermoplastic) 
               
               
                   
                   
                 methacrylate 
               
               
                   
               
             
          
         
       
     
         [0097]    The indices of the materials described above are those of the marketed products, the composition of the materials being able to be modified in order to obtain different refractive indices. For example, the composition of ChemOptics CO150 may be modified to obtain a refractive index that may reach the value of 1.628. 
         [0098]    Embodiments have been described wherein the transmission element used to transmit light into or out of the core  16  of the optical guide  12  is a periodic slit grating  32  providing a diffraction function. Clearly, the invention applies to any type of diffraction element, provided a coupling surface is defined at the surface of the waveguide and which has a non-zero coupling angle.