Abstract:
An optical device including at least one first optical waveguide coupled to a second optical waveguide of smaller cross-section which penetrates into it on the side of a first end. The first optical waveguide is capable of being coupled with an optical fiber on the side of a second end. A surface of the first optical waveguide includes a diffraction grating capable of introducing-extracting-sending back light into and from the first optical waveguide.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an integrated optical circuit and, more specifically, to a method and a device for aligning an optical fiber and a waveguide formed at the surface of an integrated optical circuit. 
     2. Discussion of the Related Art 
     Integrated optical circuits are more and more used in telecommunications, especially for the transmission, the processing, or the storage of data. Integrated optical circuits may have many functions, such as multiplexing, demultiplexing, modulation, demodulation, spectral routing, amplification, storage, filtering, resonator functions, etc. 
     Integrated optical or optoelectronic circuits are generally formed in and on semiconductor wafers similar to those used in microelectronics. An integrated optical circuit comprises one or several elementary optical components processing one or several light beams, the light beams being conveyed between elementary optical components by optical waveguides. 
     The integration of an increasing number of functions on a same chip requires the miniaturization of integrated optical circuits and of the associated optical waveguides. When waveguides have dimensions below one micrometer, it is spoken of submicronic or nanometric waveguides. Currently, waveguides may have cross-sections on the order of 0.5×0.2 μm 2 . 
     For mid and long range transmissions, that is, within a range from a few meters to several kilometers, optical fibers are the preferred optical transportation means. An optical fiber usable in the visible and close infrared range currently has a diameter ranging between 10 μm and a few tens of micrometers. Accordingly, it is necessary to use light coupling systems between optical fibers and submicronic waveguides to compensate for the size mismatch imposed by such structures. 
       FIG. 1  is a perspective view illustrating a known structure for coupling an optical fiber and a submicronic waveguide associated with an integrated optical circuit. This anamorphotic structure is generally called “inverse taper” in the art, after its shape. 
     The structure of  FIG. 1  is formed on a silicon substrate  1  covered with an insulating layer  3 , for example made of silicon oxide. A wide waveguide  5 , for example made of silicon oxide SiOx, having an optical index ranging between 1.6 and 1.8, is formed on insulating layer  3 . Wide guide  5  typically has a cross-section with dimensions on the order of a few micrometers, for example, a 3-μm width and a 1-μm height, and is intended to be illuminated by an optical fiber (shown in  FIG. 1  by an arrow  7 ) at a first one of its ends, substantially above an edge of support  1 . 
     A submicronic optical waveguide  9 , formed at the surface of layer  3 , extends into wide waveguide  5  and progressively narrows therein to form a tip  11  on the side of the first end of wide waveguide  5 . Submicronic waveguide  9  and tip  11  may be made of silicon (having an optical index of 3.47). It should be noted that an insulating layer, not shown, for example made of stoichiometric silicon oxide of optical index equal to 1.44, extends on top of optical waveguides  5  and  9  to confine the light beams in these waveguides. 
     In normal operation, a light beam of adapted wavelength and polarization penetrating into wide waveguide  5  enters submicronic waveguide  9 . Conversely, a light beam conveyed by submicronic waveguide  9  penetrates into wide waveguide  5 . 
       FIG. 2  illustrates a simplified example of optical inputs/outputs of a chip comprising an integrated optical circuit  13 . Many wide waveguides  5  having their first ends located substantially above the chip edges extend on a silicon oxide layer  3  formed on a silicon support. Each wide waveguide  5  is coupled to a submicronic waveguide  9 . Submicronic waveguides  9  are connected to integrated optical circuit  13 , for example carrying out one or several of the above mentioned functions. As an example, integrated circuit chip  3  may have a surface area ranging between 1 mm 2  and 4 cm 2  and integrated optical circuit  13  may take up almost the entire surface area. 
     Wide waveguides  5  have cross-sections on the order of a few square millimeters (for example, with a side length between 1 and 4 μm). The coupling with an optical fiber typically having a diameter on the order of 10 μm is performed via an optical system comprising one or several lenses, another possibility being for the end of each optical fiber to be given a shape ensuring a lens effect. 
     For the circuit of  FIG. 2  to operate properly, each optical fiber must be perfectly aligned with the wide waveguide associated thereto. Several methods have been provided to form this alignment. For example, the integrated optical circuit may be provided to deliver a light beam at the output of wide waveguide  5  and the optical fiber is considered as being aligned when the amount of light received by said fiber is maximum. It is also possible to illuminate wide waveguide  5  with the optical fiber and to detect a light intensity maximum in the submicronic circuit. 
     However, such methods pose several problems. First, they require providing, in the integrated optical circuit, elements dedicated to the alignment of the optical fibers, for example, light outputs or photodetectors. Further, in the alignment, the integrated optical circuit must be in operation, and thus, for example, electrically supplied. Finally, the wavelength of the light beams used for the alignment necessarily is that of the light beams used in the integrated optical circuit. 
     There is a need for a device and a method enabling to align an optical fiber on an optical waveguide, independently from the associated integrated optical circuit, from its operating mode, from its operating wavelengths, and from the light polarization states that it requires. 
     Patent application WO 2004/088801 provides a device comprising, at the surface of a support, a first optical waveguide coupled to a second waveguide of smaller size at one of its ends. A diffraction grating, formed at the surface of the first or second waveguides, is sized to filter beams exhibiting predetermined wavelengths. 
     SUMMARY OF THE INVENTION 
     An embodiment of the present invention provides a device and a method for aligning an optical fiber on an optical waveguide which overcomes at least some of the disadvantages of existing devices and methods. 
     Thus, an embodiment of the present invention provides an optical device comprising at least one first optical waveguide coupled to a second optical waveguide of smaller cross-section which penetrates into it on the side of a first end, the first optical waveguide being capable of being coupled with an optical fiber on the side of a second end, a surface of the first optical waveguide comprising a diffraction grating capable of introducing-extracting-sending back light into and from the first optical waveguide, independently from the second optical waveguide. 
     According to an embodiment of the present invention, the diffraction grating is formed in a portion remote from the coupling region between the first and second optical waveguides. 
     An embodiment of the present invention further provides a method for aligning an optical fiber on a free end of a first optical waveguide coupled to a second optical waveguide of smaller cross-section, the method comprising a step of introduction-extraction-sending back of light in a portion of the first optical waveguide, independently from the second optical waveguide. 
     According to an embodiment of the present invention, the step is a step of introduction of a light beam originating from a light source into the first optical waveguide by means of a light coupling device formed at the surface of the optical waveguide. 
     According to an embodiment of the present invention, the step is a step of extraction of a light beam from the first optical waveguide by means of a light coupling device formed at the surface of the first optical waveguide, the extracted light beam being detected by a photodetector. 
     According to an embodiment of the present invention, the step is a step of sending back, into the first optical waveguide, of a light beam originating from the optical fiber by means of a light coupling device formed at the surface of the optical waveguide. 
     According to an embodiment of the present invention, the light coupling device is a diffraction grating. 
     According to an embodiment of the present invention, the introduced-extracted-sent back light beam has any wavelength in the visible or close infrared range. 
     An embodiment of the present invention further provides a method for aligning several optical fibers on several optical waveguides, comprising a step of introduction of a light beam originating from a light source into a first optical waveguide by means of a light coupling device formed at the surface of the first optical waveguide and comprising a step of separation of the light beam into several light beams directed towards each of the optical waveguides. 
     According to an embodiment of the present invention, the alignment of each optical fiber on the associated optical waveguide is performed by detecting, on its output, a transmitted light power maximum. 
     The foregoing objects, features, and advantages of the present invention will be discussed in detail in the following non-limiting description of specific embodiments in connection with the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1 , previously described, illustrates a coupling device of inverse taper type; 
         FIG. 2 , previously described, is a simplified diagram illustrating the optical inputs/outputs of a chip on which an integrated optical circuit is formed; 
         FIG. 3  is a perspective view of a device according to an embodiment of the present invention; 
         FIGS. 4 to 6  illustrate different methods for aligning an optical fiber on a device according to an embodiment of the present invention; 
         FIG. 7  illustrates a variation of a method according to an embodiment of the present invention enabling to align two optical fibers in one step; and 
         FIG. 8  illustrates a variation of a method according to an embodiment of the present invention in which several optical fibers are aligned in a single step. 
     
    
    
     DETAILED DESCRIPTION 
     For clarity, the same elements have been designated with the same reference numerals in the different drawings and, further, as usual in the representation of integrated optical circuits, the various drawings are not to scale. 
       FIG. 3  is a perspective view of a device according to an embodiment of the present invention. 
     On a support  21 , for example made of silicon, is formed an insulating layer  23 , for example made of silicon oxide. On insulating layer  23  is formed a wide waveguide  25  for example having a cross-section of a few square micrometers and having one end intended to be illuminated by an optical fiber or to provide a light beam towards an optical fiber (direction of arrow  27  in  FIG. 3 ). In wide waveguide  25  and at the surface of insulating layer  23  is formed a submicronic waveguide  29  for example having a width of approximately 0.5 μm and a height of approximately 0.2 μm. Submicronic waveguide  29  ends with a tip  31  turned towards the end of waveguide  25  forming the interface with the optical fiber. The device further comprises, on top of wide waveguide  25 , a diffraction grating  33  allowing an optical coupling (introduction-extraction-sending back of light) with wide waveguide  25 . Diffraction grating  33  is formed, in the shown example, of parallel strips perpendicular to the direction of the light, dug at the surface of wide waveguide  25 . Diffraction grating  33  may also be formed by any other known technique, especially by forming of parallel metal strips perpendicular to the direction of the light at the surface of wide waveguide  25 . As an example, the diffraction grating may be formed of gold strips having a thickness of approximately 200 nm, a 10-μm length, and a 1-μm period for a 50% filling rate. Any other structure forming a network may also be used. 
     Diffraction grating  33  is preferably placed upstream of the tip of submicronic waveguide  29 , far from the coupling region between waveguides  25  and  29 . This avoids the generation of losses at the level of tip  31  where the light beam is confined or deconfined between wide waveguide  25  and submicronic waveguide  29 . Thus, in the shown example, diffraction grating  33  is located close to the end of waveguide  25  opposite to that intended to be coupled to the optical fiber. 
     In  FIG. 3 , the lateral and upper insulations of waveguides  25  and  29  have not been shown. Such insulations may be formed of an insulating layer, for example, made of silicon oxide, having a thickness on the order of 2 μm and surrounding waveguides  25  and  29 . It should also be noted that any known index matching device may be used between the optical fiber and the associated wide waveguide to limit light losses between these elements, for example, liquid structures. 
     The device of  FIG. 3  enables to align an optical fiber on waveguide  25  in several ways, some of which will be described hereafter in relation with  FIGS. 4 to 6 . 
       FIG. 4  is a cross-section view illustrating a first method enabling to align an optical fiber  35  and wide waveguide  25  of  FIG. 3 . A light source  37 , for example, a laser, provides a light beam  39  towards the surface of diffraction grating  33 . A portion of the light beam reaching diffraction grating  33  is transmitted by said network into wide waveguide  25 . Wide waveguide  25  transmits the light to its end where optical fiber  35  is desired to be aligned. Optical fiber  35  and wide waveguide  25  are aligned when the optical fiber conducts a maximum light intensity originating from the wide waveguide. Thus, to align the optical fiber, the light intensity that it conducts is detected at its output and the alignment is obtained when this light intensity is maximum. 
     Advantageously, to align optical fiber  35  on wide waveguide  25 , it is not necessary to accurately illuminate the diffraction grating. Indeed, it is sufficient for light source  37  to illuminate diffraction grating  33 , even partially, so that light comes out of wide waveguide  25  and enables to optimize the relative positioning of optical fiber  35 . Further, advantageously, light beam  37  may have any wavelength, for example, in the visible or infrared range. Indeed, light beam  37  does not need to have a specific wavelength to be at least partly coupled in waveguide  25  by the diffraction grating. 
       FIG. 5  illustrates another method for aligning an optical fiber  35 . In this method, optical fiber  35  illuminates wide waveguide  25 . When optical fiber  35  is aligned on wide waveguide  25 , said waveguide conducts light provided by the optical fiber towards diffraction grating  33 , which delivers a light beams to the outside, towards a photodetector  41 . Thus, the proper alignment of optical fiber  35  on wide waveguide  25  is detected by selecting the position of optical fiber  35  enabling the detection of a maximum light intensity at the level of photodetector  41 . It can be avoided for the beam provided by optical fiber  35  to the wide waveguide to be coupled in submicronic waveguide  29  via tip  31 . For this purpose, the optical fiber may provide a light beam having a wavelength external to the operating bandwidth of the anamorphotic coupling device. It may also be provided for the optical fiber to conduct a light beam with a polarization state which is not coupled by the anamorphotic device. 
       FIG. 6  illustrates another method for aligning optical fiber  35  on wide waveguide  25 . In this method, optical fiber  35  illuminates optical waveguide  25  and the light reflected by diffraction grating  33  is detected. Indeed, when a light beam penetrates into wide waveguide  25  towards diffraction grating  33 , part of this light beam is transmitted by the diffraction grating to the outside of the device, part of it follows its path into wide waveguide  25  and part of the light is reflected in wide waveguide  25  by diffraction grating  33 . Thus, optical fiber  35  is aligned when the light beam that it provides penetrates into wide waveguide  25 , partially reflects on diffraction grating  33 , returns into wide waveguide  25 , and is recovered in optical fiber  35 . In the same way as in the case of  FIG. 5 , in this method, the wavelength and/or the polarization of the light beam provided by optical fiber  35  may be provided to minimize the optical coupling in submicronic waveguide  29 . Any desired wavelength may also be used for the alignment. 
       FIG. 7  is a top view illustrating a variation of the above methods in which a single light introduction-extraction diffraction grating is used to align an input optical fiber and an output optical fiber of an integrated optical circuit. The device comprises an input optical fiber  43  which is coupled, via a wide waveguide  45  and a submicronic tip  47  formed on a support  48 , to a submicronic waveguide  49  formed on this same support. Submicronic waveguide  49  is partially shown and comprises, at the surface of support  48 , a curved portion which brings it towards an integrated optical circuit, not shown. An output of the integrated optical circuit is connected to the input of a submicronic optical waveguide  51  which is coupled, via a submicronic tip  53  and a wide waveguide  55  formed on support  48 , to an output optical fiber  57 . 
     Wide waveguides  45  and  55  join on support  48  and a diffraction grating  59 , of any known type enabling to introduce light into wide waveguides  45  and  55 , is formed, at the surface of wide waveguides  45  and  55 , at the intersection thereof. Diffraction grating  59  is illuminated by a light beam  61  originating from a light source, not shown, and light beam  61  penetrates, via the diffraction grating, into wide waveguides  45  and  55  towards optical fibers  43  and  57 . To align optical fibers  43  and  57  on wide waveguides  45  and  55 , the position in which the optical fibers receive a maximum light intensity is detected. It should be noted that, as in the case of  FIG. 4 , light beam  61  does not need to be perfectly aligned on the diffraction grating for the alignment so that it is enough for part of this beam to be coupled in wide waveguides  45  and  55  to enable the alignment of optical fibers  43 ,  57 . Further, the wavelength of beam  61  may be different from that used in the integrated optical circuit. 
       FIG. 8  illustrates, in top view, a variation enabling to align several optical fibers on several wide waveguides in a single step. Four optical fibers  81 ,  83 ,  85 ,  87  intended to be aligned on first ends of wide waveguides, respectively  89 ,  91 ,  93 ,  95  are formed on a support  80 . In the shown example, each wide waveguide  89 ,  91 ,  93 ,  95  is associated with a submicronic waveguide, respectively  97 ,  99 ,  101 ,  103 , via adapted tips (inverse tapers). The second ends of wide waveguides  89 ,  91 ,  93 ,  95  are connected, at the surface of insulating support  80 , to a beam-dividing device  105 . An additional wide waveguide  107 , formed on support  80 , is connected to the input of divider  105 . It should be noted that divider device  105  is not shown in detail in  FIGS. 8 and 9  and that any type of known beam dividing device may be used to enabling the coupling between waveguide  107  and waveguides  89 ,  91 ,  93 ,  95 . 
     A diffraction grating  109 , formed at the surface of wide waveguide  107 , enables to couple light originating from a light source (arrow  111 ) towards wide waveguide  107 . To align optical fibers  81 ,  83 ,  85 ,  87 , the diffraction grating is illuminated (arrow  111 ) which transmits part of this light to wide waveguide  107 . The light beam then penetrates into dividing device  105  which conveys it to each of wide waveguides  89 ,  91 ,  93 ,  95 . The alignment of optical fibers  81 ,  83 ,  85 ,  87  is obtained in the same way as in the case of  FIG. 7 , when the optical fibers conduct a maximum light intensity. 
     Thus, the device of  FIG. 8  enables to align several optical fibers. It should be noted that a combination of the devices of  FIGS. 7 and 8  can enable to align all the input/output optical fibers of an integrated optical circuit, for example, of a circuit such as that in  FIG. 2 . 
     Various specific embodiments of the present invention have been described. Various alterations and modifications will occur to those skilled in the art. In particular, a specific type of wide waveguide has been described herein. It should be understood by those skilled in the art that the present invention applies to the alignment of an optical fiber on any type of wide waveguide. The present invention also applies to any system for coupling a wide waveguide and a submicronic waveguide. 
     Further, a system of introduction-extraction-sending back of light in the form of a diffraction grating formed at the surface of the wide waveguide has been discussed herein. It should be noted that any other known device enabling to introduce-extract-send back light in a wide waveguide, independently from the presence of a submicronic waveguide formed in the wide waveguide, may also be used instead of the diffraction grating. It should however be noted that the use of the diffraction grating provides a decreased bulk and an optimized coupling. 
     Such alterations, modifications, and improvements are intended to be part of this disclosure, and are intended to be within the spirit and the scope of the present invention. Accordingly, the foregoing description is by way of example only and is not intended to be limiting. The present invention is limited only as defined in the following claims and the equivalents thereto.