Abstract:
A coupling module includes optical couplers that are coupled to waveguides. The optical couplers are configured to couple to cores of a multi-core optical fiber. The waveguides each include an external part extending from the module and an internal part extending into the module for connecting the external part to the associated optical coupler. The external part of some of the waveguides extends in a preferential direction, while the external part of others of the waveguides extends in a direction opposite to the preferential direction. The internal parts may include a curved portion configured for forming a turn-back.

Description:
PRIORITY CLAIM 
     This application claims priority to French Application for Patent No. 1657524 filed Aug. 3, 2016, the disclosure of which is incorporated by reference. 
     TECHNICAL FIELD 
     Embodiments of the invention relate to photonic integrated circuits and in particular to photonic integrated circuits comprising coupling means allowing for example optical signal exchanges with input/output devices such as for example, but not in a limiting way, optical fibers. 
     BACKGROUND 
     Conventionally, in order to couple an optical fiber to a photonic integrated circuit, an array of optical fibers (well known as a “fiber array” by those skilled in the art) comprising a plurality of single-core fibers aligned along an axis is used. This fiber array is conventionally secured on the upper face of the photonic circuit in such a way that each fiber of the device is opposite an optical array coupler formed in the photonic circuit. The device is inclined by a few degrees with respect to the normal to the upper surface of the photonic circuit in order to improve the coupling. Optical array couplers comprise at least one diffracting periodic array formed by an alternation of two thicknesses of semiconductor film, wherein one film is partially etched. 
     Thus, the light rays coming from the optical fibers arrive onto the photonic device in a preferential direction, which means that the waveguides coupled to the optical couplers extend in that direction. 
     Optical couplers are therefore conventionally produced on the edges of the circuit, the waveguides extending towards the inside of the circuit. 
     In order to increase the data rate of data that can be transmitted in an optical fiber, it is possible to use multi-core optical fibers. However, the coupling of such a fiber to a photonic integrated circuit by means of array couplers is difficult because each of the couplers have to be disposed opposite one of the cores of the fiber and therefore very close to each other. Each coupler comprises at its output one or two waveguides formed in a semiconductor film of the photonic circuit and whose routing complies with constraints, notably with regard to radius of curvature. In fact, these waveguides, whether they are strip waveguides or rib waveguides, are formed from two thicknesses of semiconductor film used for forming the array structure of the array couplers, these thicknesses being optimized for the performance of the couplers but which impose constraints regarding the minimum radius of curvature to be complied with. 
     Unless the photonic circuit comprises several layers of semiconductor film, which is complex and costly to produce, the routing of the light signals is carried out in a single plane and the waveguides generally cannot intersect, which gives rise to strict topological constraints. In the case of an assembly of array couplers intended for coupling the circuit to a multi-core fiber, the disposition of the waveguides at the input/output of the couplers therefore imposes constraints on the arrangement of the optical signal transmitting and receiving circuits. 
     An arrangement that is as symmetrical as possible is generally sought for the microwave electronic devices coupled to the optoelectronic transmitting and receiving circuits disposed at the ends of the waveguides coupled to the optical fibers, which is not always possible when a multi-core fiber is used. 
     There is therefore a need to be able to couple the optical fiber to different places in the photonic circuit, irrespective of the preferential direction, notably in order to increase flexibility and compactness in the design of the circuits. 
     SUMMARY 
     Thus, according to one embodiment, a particularly compact photonic integrated circuit is proposed, comprising optical couplers whose location does not depend on the preferential direction. 
     According to one aspect, a photonic integrated device is proposed comprising at least one coupling module configured for receiving and/or transmitting optical signals from and/or to an input-output device, disposed in such a way as to deliver and/or receive optical signals in a preferential direction, and waveguides coupled to the at least one coupling module. 
     According to a general feature of this aspect, the at least one module comprises optical couplers situated on the same level of a semiconductor film and aligned along several axes and each one coupled to a waveguide, the input-output device comprising a multi-core optical fiber and each core of the multi-core optical fiber is coupled to a coupler of the at least one module, each waveguide comprising an external part extending from the module and an internal part extending into the module and connecting the external part to the associated coupler, the external part of at least one of the waveguides extending in the preferential direction and the external part of at least one other waveguide extending in a direction opposite to the preferential direction, its internal part having a curved portion configured for forming a turn-back. 
     “Turn-back” refers to a turn of one hundred and eighty degrees to within a tolerance which depends on the manufacturing constraints of the device. 
     Thus, with the combination of a multi-core optical fiber, an arrangement of couplers on the same level and according to several axes and waveguides, some of which comprise curved turning sections, it is possible to obtain a compact coupling module making it possible to redirect light in several directions and which can therefore advantageously be placed at different locations in the photonic circuit. 
     The device can moreover comprise an intermediate part which is not straight between the curved portion and the external part of the at least one other waveguide. 
     This intermediate part could be straight, but it is not generally so in practice because of the geometric constraints of the device. 
     According to one embodiment, the curved portion is U-shaped. 
     According to one embodiment, the waveguides comprise a first parallelepipedic part situated under a second parallelepipedic part of width less than or equal to that of the first parallelepipedic part, the U-shaped portion of the internal part of the at least one other waveguide has a radius of curvature less than or equal to five micrometers, and the first parallelepipedic part of the internal part of the at least one other waveguide has a height different from the height of the first parallelepipedic part of the external part of the at least one other waveguide. 
     The use of this type of waveguide makes it possible to minimize the optical losses and to do so despite the very small radius of curvature. 
     The thicknesses of the first parallelepipedic parts and of the second parallelepipedic parts of the internal parts of the waveguides correspond to the thicknesses of the semiconductor layers used for producing the couplers. 
     The external part of at least one of the waveguides can extend in a direction different from the preferential direction and from the direction opposite to the preferential direction. 
     It is also possible to transmit and/or to receive light in several different directions irrespective of the location of the module in the integrated photonic device. 
     According to one embodiment, the optical couplers coupled to a waveguide whose external part extends in the preferential direction are aligned along a first axis, and the optical couplers coupled to a waveguide whose external part extends in the direction opposite to the preferential direction are aligned along a second axis. 
     This notably makes it possible to avoid an intersection of the waveguides whilst using a single level of waveguides. 
     The waveguides can be coupled to optical modulators and the module is advantageously situated at least between two optical modulators. 
     The device can also comprise a plurality of modules each coupled to a separate input/output device. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Other advantages and features of the invention will become apparent on examination of the detailed description of embodiments, which are in no way limiting, and of the appended drawings in which: 
         FIG. 1  shows a coupling module; 
         FIG. 2  shows a cross-sectional view of a multi-core optical fiber; 
         FIG. 3  shows a cross-sectional view of a U-shaped portion of the waveguide; and 
         FIG. 4  shows two coupling modules produced within a photonic integrated circuit. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  shows a coupling module  1 . This module can conventionally be produced within a photonic integrated circuit CI, produced, for example, in and on a substrate of the silicon on insulator type, represented partially here for purposes of simplification. 
     The module  1  comprises a plurality of optical couplers. For example, in this case the module  1  comprises four couplers of the single polarization type  2 ,  4 ,  6 ,  8 , that is to say in this case couplers configured for receiving an optical signal and transmitting one of its components (“transverse magnetic” or “transverse electric” according to the terms well known to those skilled in the art) to a single waveguide in a first polarization mode, for example in the transverse electric mode. 
     The module also comprises four couplers of the polarization separation type  3 ,  5 ,  7 ,  9 , that is to say couplers configured for receiving an optical signal and transmitting its transverse electric component to a first waveguide and its transverse magnetic component to a second waveguide while converting it into a transverse electric signal. 
     The optical couplers  2  to  9  are disposed on a same level in such a way that each one is opposite a core  10  of a multi-core optical fiber  11  outside of the integrated circuit, a cross-sectional view of which is shown in  FIG. 2 . The optical fiber  11  can conventionally be secured to the upper face of the circuit CI by adhesion, for example using a resin. 
     Thus, the optical couplers  2  to  9  are capable of receiving signals from certain cores of the multi-core optical fiber  11 . 
     It is appropriate to note here that the disposition of the couplers and of the fiber cores is not limited to the example shown here, any configuration being conceivable. 
     The optical fiber  11  is conventionally placed above the module in an inclined manner, in order to form an angle of a few degrees with the normal to the surface of the integrated circuit comprising the module  1 , thus defining a preferential direction D 1  in which the fiber  11  transmits optical signals. The optical couplers are therefore produced in such a way as to transmit signals in this preferential direction D 1 . 
     The optical couplers are each connected to one or two waveguides making it possible to direct the optical signals coming from the optical fiber  11  to external of the module. 
     Four optical fibers  2 ,  3 ,  6 ,  7  are aligned with each other along a first axis Ax 1  and coupled to waveguides of which a part outside of the module  1  extends from the module  1  in the preferential direction D 1 . 
     Four other optical couplers  4 ,  5 ,  8 ,  9  are aligned with each other along a second axis Ax 2 , in this case parallel with the first axis Ax 1 , and coupled to waveguides of which a part outside of the module  1  extends from the module  1  in a direction different from the preferential direction D 1 . 
     In particular, two optical couplers  5  and  9  of the polarization separation type, aligned along the second axis Ax 2 , are each coupled to two waveguides  51 ,  52 ,  91 ,  92 , each comprising a part  510 ,  520 ,  910 ,  920  outside of the module and a part  511 ,  521 ,  911 ,  921  inside of the module. 
     The parts  510 ,  520 ,  910 ,  920  outside of the module extend from the module  1  in a direction D 2  opposite to the preferential direction D 1 . In order to do this, the parts  511 ,  521 ,  911 ,  921  inside the module of the waveguides  51 ,  52 ,  91 ,  92  each comprise a U-shaped curved portion, configured for producing a turn-back, and an intermediate portion  513 ,  523 ,  913 ,  923  which is not straight forming the connection between the U-shaped portions  512 ,  522 ,  912 ,  922  and the couplers and between the U-shaped portions and the external parts  510 ,  520 ,  910 ,  920  of the waveguide. 
     The intermediate portion could be straight, but it is not so in this case because of the geometric constraints of the device. 
     The U-shaped portions  512 ,  522 ,  912 ,  922  in this case have a radius of curvature of the order of 5 micrometers so as to direct the waveguide outside of the module in the direction D 2  opposite to the preferential direction D 1  without intersecting the other waveguides which extend from the other optical couplers. 
     The other two optical couplers  4  and  8  aligned along the axis Ax 2  are coupled to waveguides  41  and  81  of which the parts  410  and  810  outside of the module  1  extend from the module in directions different from the preferential direction D 1  and from its opposite direction D 2 . 
       FIG. 3  shows a cross-sectional view of the U-shaped portion  512  of the waveguide  51  through the axis in  FIG. 1 . As the waveguides are produced in a similar manner, this U-shaped part  512  is in this case identical to the U-shaped parts of the other waveguides  52 ,  91 ,  92 . 
     The waveguide is conventionally produced in a silicon film  12  situated above a buried insulator layer  13  (a buried oxide “BOX” as well known to those skilled in the art) which is itself situated above a silicon supporting layer  14 . 
     The inventors have observed that in order to limit losses in the U-shaped portion  512  of the waveguide  51 , and also in the intermediate part  513 , which is not straight, it is advantageous for the internal part  511  to have a strip waveguide structure or, as is the case here, rib waveguides having a structure close to that of a strip waveguide, that is to say a slab of low height. 
     Thus, in the U-shaped portion  512  of the waveguide  51  the first part  120 , or slab, is of low height, fifty nanometers in this case for example, and surmounted with the second part  121  which is narrower but is of greater height, for example 300 nanometers in this case. 
     The structure of the intermediate parts  513  of the internal part  511  of the waveguide  51  is in this case the same as the structure of the U-shaped part. 
     The height of the first part  120  and the height of the second part  121  are equal to the thicknesses of the semiconductor layers used for producing the couplers. This is particularly advantageous for the method of producing the device. 
     In the external part  510  of the waveguide  51 , which is straight and also of the rib type, the first part  120  has a more conventional height, for example in this case a height of one hundred and sixty three nanometers, and the second part  121  has the same height as in the internal portion  511 , that is to say three hundred nanometers. 
     The internal part  511  and the external part  510  of each waveguide are conventionally coupled by the intermediary of a transition region (not shown) making it possible to join the two parts  510  and  511  having different thicknesses while limiting the optical losses. 
     Thus, although it is possible to have an internal part  511  in strip form, that is to say an internal part  511  of which the first part  120  and the second part  121  have the same width, the choice of a rib waveguide such as the one described previously allows a less sudden transition between the internal curved part  511  and the external straight part  510  of the waveguide  51 , which contributes to reducing the losses due to the coupling between the two parts of the waveguide. 
     The module  1  therefore makes it possible to redirect the signals coming from the optical fiber  11  in all directions around the module, and to do so irrespective of the preferential direction D 1 . 
       FIG. 4  shows an embodiment in which two coupling modules  15  and  16  are advantageously produced within a photonic integrated circuit CI. 
     The photonic integrated circuit CI comprises micro-cases ME comprising lasers coupled to several modulators M 1  to M 8  configured for modulating an optical signal and transmitting it to two modules  15  and  16  such as the one described above and shown in  FIG. 1 . 
     The integrated circuit CI comprises moreover photodiodes P 1  to P 8  coupled to the modules  15  and  16  and capable of converting the light signals coming from the module into electric signals. 
     Thus, the use of coupling modules such as those described above advantageously makes it possible not to have to position the coupling modules on the edges of the integrated circuit, and therefore to increase flexibility in the design of the circuits. 
     It is appropriate to note that the arrangement described here by way of example is in no way limiting and that any other configuration incorporating coupling modules similar to the one described above and shown in  FIGS. 1 to 3  can be envisaged. 
     Similarly, although coupling modules comprising eight optical couplers aligned along two axes Ax 1  and Ax 2  have been described here, it is possible to envisage coupling modules comprising a different number of optical couplers disposed along two or more axes.