Abstract:
A method of producing a PLC based optical component comprising: defining a waveguide; etching a notch thereby defining a facet of the waveguide; and cutting a trench in front of the notch, the cutting being accomplished subsequent to the etching, whereby at least a portion of the trench is displaced from the facet by the notch. The invention also provides for a PLC based optical component comprising: a substrate; a waveguide deposited above the substrate, the waveguide exhibiting a facet; a notch etched in front of the facet of the waveguide; and a trench cut in front of the notch.

Description:
This application claims priority from U.S. Provisional Patent Application Ser. No. 60/551,790 filed Mar. 11, 2004 entitled “Method of Coupling Fiber to Waveguide”; U.S. Provisional Patent Application Ser. No. 60/551,794 filed Mar. 11, 2004 entitled “Method of Connecting an Optical Element to a PLC”; and U.S. Provisional Patent Application Ser. No. 60/628,139 filed Nov. 17, 2004 entitled “Method of Connecting an Optical Element to a PLC”, the contents of each of which are incorporated herein by reference. 

   BACKGROUND OF THE INVENTION 
   The invention relates generally to the field of planar lightwave circuits and in particular to a method of defining a waveguide end in a planar lightwave circuit. 
   Optical fiber is commonly used in telecommunication equipment to carry an optical signal. Optical sub-components, devices and modules, hereinafter generally referred to as optical components typically comprise at least one element which operates on the optical signal. Such an operation may comprise conversion between an electrical signal and an optical signal. Advantageously, optical components are produced in the form of a planar lightwave circuit (PLC), thus allowing for consistent mass production and effective cost reduction. 
   A major difficulty in the production of optical components is in the stage of aligning an optical fiber with a waveguide of the PLC. Improper alignment leads to significant losses and depreciated performance of the PLC. One well developed technique for aligning an optical fiber with a waveguide involves the use of a v-groove etched into the substrate of the PLC. The fiber is secured in the v-groove, thus ensuring proper alignment in relation to the waveguide. In order to fully expose the end of the waveguide a dice line, or saw cut, is typically made thus also cleaning out the end of the v-groove facing the waveguide. Unfortunately, the facet of the waveguide exposed by a saw cut is not sufficiently clean, and thus polishing of the facet is typically required. 
     FIG. 1  shows a side view of a PLC based component  10  comprising a substrate  60  and a fiber  30  having core  50 . Substrate  60  exhibits v-groove  20 , waveguide  40  and trench  70  according to the prior art. V-groove  20  is etched into a portion of substrate  60  so as to accommodate fiber  30 . V-groove  20  is dimensioned so as to align core  50  of fiber  30  with waveguide  40 . Trench  70  is cut defining a facet of waveguide  40  and cleaning out the end of v-groove  20 . Fiber  30  is butt coupled to the facet of waveguide  40 . 
   In order to enable optimum operation of the optical component, the facet of waveguide  40  must be polished prior to bringing the waveguide to a butt coupling with optical fiber  30 . This polishing is an additional costly stage in the production of a PLC. 
   One prior art technique to avoid the extra polishing involves a combination dicing and polishing technique. In this technique, both polishing and dicing are accomplished simultaneously. Unfortunately this technique is somewhat difficult to accomplish in many situations. 
   Thus, there is a need for an improved method for coupling an optical fiber with a waveguide, and in particular a method that allows for clearing out the end of the v-groove exposing the waveguide end while ensuring a polished facet for the waveguide. Preferably, such a technique will allow for improved coupling of the waveguide with other optical elements used in a PLC. 
   SUMMARY OF THE INVENTION 
   Accordingly, it is a principal object of the present invention to overcome the disadvantages of prior art methods of coupling to a PLC based waveguide. This is provided in the present invention by first etching a notch defining a facet of the waveguide. Subsequently, a trench is cut in front of the notch. Butt coupling of the fiber is then accomplished with the fiber core being less than a mode diameter from the waveguide facet. 
   The invention provides for a method of producing a PLC based optical component comprising: defining a waveguide; etching a notch thereby defining a facet of the waveguide; and cutting a trench in front of the notch, the cutting being accomplished subsequent to the etching, whereby at least a portion of the trench is displaced from the facet by the notch. 
   In one embodiment the method further comprises depositing a mask layer, the depositing being accomplished before the defining a waveguide. In a further embodiment the method further comprises etching a v-groove. In one yet further embodiment the stage of v-groove is accomplished utilizing the deposited mask layer. In another yet further embodiment the method further comprises the stage of aligning an optical fiber placed in the etched v-groove with the defined facet of the waveguide. 
   In another embodiment the method further comprises the stage of aligning an optical fiber with the defined facet of the waveguide. In yet another embodiment the method further comprises coating the defined facet, the coating preceding the cutting. In an exemplary embodiment the coating is accomplished at least partially with Si 3 N 4 . 
   In another embodiment the method further comprises placing an element in the trench. In one further embodiment the element is a photodiode. In another embodiment the method further comprises aligning the placed element with the defined facet of the waveguide. Preferably, the element is a photodiode. 
   In one embodiment the cutting comprises one of sawing and dicing. In another embodiment the notch exhibits a width less than the optical mode width of light constrained within the waveguide. 
   The invention also provides for a PLC based optical component produced in accordance with any of the above methods. 
   The invention also provides for a PLC based optical component comprising: a substrate; a waveguide deposited above the substrate, a notch etched to the depth of the waveguide, the notch defining a facet of the waveguide; and a trench cut in front of the notch, the trench being displaced from the defined waveguide facet by the notch. 
   In one embodiment the PLC based optical component further comprises an optical component placed within the trench, the optical component being aligned with the waveguide facet. In another embodiment the trench exhibits a wall opposing the waveguide facet, the wall presenting an oblique angle to an axis of the waveguide. In a further embodiment the PLC based optical component further comprises an optical component placed within the trench against the wall, the optical component being aligned with the waveguide facet. 
   In one embodiment the notch exhibits a width less than the optical mode width of light constrained within the waveguide. In another embodiment the PLC based optical component further comprises an optical fiber exhibiting a core, the optical fiber core being optically aligned with the waveguide. In one further embodiment the PLC based optical component further comprises a v-groove, the optical fiber being placed in the v-groove. In another further embodiment the notch exhibits a width less than the optical mode width of light constrained within the optical fiber 
   Additional features and advantages of the invention will become apparent from the following drawings and description. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     For a better understanding of the invention and to show how the same may be carried into effect, reference will now be made, purely by way of example, to the accompanying drawings in which like numerals designate corresponding elements or sections throughout. 
     With specific reference now to the drawings in detail, it is stressed that the particulars shown are by way of example and for purposes of illustrative discussion of the preferred embodiments of the present invention only, and are presented in the cause of providing what is believed to be the most useful and readily understood description of the principles and conceptual aspects of the invention. In this regard, no attempt is made to show structural details of the invention in more detail than is necessary for a fundamental understanding of the invention, the description taken with the drawings making apparent to those skilled in the art how the several forms of the invention may be embodied in practice. In the accompanying drawings: 
       FIG. 1  illustrates a side view of a PLC based optical component according to the prior art; 
       FIG. 2   a  illustrates a side view of a PLC based optical component according to the principle of the invention; 
       FIG. 2   b  illustrates a top view of a PLC based optical component according to the principle of the invention; 
       FIG. 3  is a high level flow chart of the stages in production of a PLC based optical component having an optical fiber aligned with a waveguide in accordance with the principle of the invention; 
       FIG. 4   a  illustrates a side view of a PLC based optical component having an element placed within a trench according to the principle of the current invention; 
       FIG. 4   b  illustrates a top view of a PLC based optical component having an element placed within a trench according to the principle of the current invention; 
       FIG. 5  illustrates a side view of a PLC based optical component having an element placed with a sloped trench according to the principle of the current invention; and 
       FIG. 6  is a high level flow chart of the stages in production of a PLC based optical component having an element aligned with a waveguide in accordance with the principle of the invention. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   The present embodiments enable an improved method of coupling an element such as an optical fiber or photodiode to a planar waveguide, providing for a polished facet defining the waveguide end at a reduced cost. Optionally the waveguide facet is angled. This is accomplished by etching a notch defining the waveguide facet, and subsequently saw cutting or dicing a trench in front of the notch. The trench cut thus does not come into contact with the waveguide facet. 
   Before explaining at least one embodiment of the invention in detail, it is to be understood that the invention is not limited in its application to the details of construction and the arrangement of the components set forth in the following description or illustrated in the drawings. The invention is applicable to other embodiments or of being practiced or carried out in various ways. Also, it is to be understood that the phraseology and terminology employed herein is for the purpose of description and should not be regarded as limiting. 
     FIG. 2   a  illustrates a side view of a PLC based component  100  according to the principle of the invention comprising substrate  60  and fiber  30  exhibiting core  50 . The top of substrate  60  is generally shown as line  80 . A waveguide  40  is defined in layers deposited above substrate  60 , and a V-groove  20  is defined in substrate  60  as known in the prior art to secure fiber  30  in optical alignment with waveguide  40 . V-groove  20  is dimensioned so as to ensure optical alignment of core  50  of fiber  30  with waveguide  40 . A notch  110  having a depth y and a width z is etched to define a facet  120  of waveguide  40 . Trench  70  is subsequently cut cleaning out the end of v-groove  20 . A unique feature of the invention is that facet  120  is defined by chemical etching in advance of, and independently of, the cutting of trench  70 . Facet  120 , defined by chemical etching may be perpendicular to waveguide  40 , or may be angled to reduce back reflection without exceeding the scope of the invention. 
   Fiber  30  is butt coupled as close as possible to facet  120  of waveguide  40 . In a preferred embodiment width z of notch  110  is less than the optical mode width of light constrained in core  50 . In an exemplary preferred embodiment width z is on the order of 5 microns. The end of fiber  30  is thus separated from facet  40  approximately by width z of notch  110 . In a preferred embodiment depth y of notch  110  is cut to a depth sufficient to reveal waveguide  40  and thus define facet  120 , and even further preferably is larger than the optical mode width of light constrained in core  50 . In an exemplary embodiment, notch  110  does not extend to the silicon of substrate  60 . 
     FIG. 2   b  illustrates a top view of a PLC based component  100 , with any layers above a waveguide  40  removed, according to the principle of the invention. PLC based component  100  comprises substrate  60  and fiber  30 . Waveguide  40  is defined in layers deposited on substrate  60 , and fiber  30  is secured in optical alignment with waveguide  40 . Notch  110  having length x and width z is etched to define facet  120  of waveguide  40 . Trench  70  is subsequently cut. A unique feature of the invention is that facet  120  is defined by chemical etching in advance of, and independently of, the cutting of trench  70 . Facet  120 , defined by chemical etching, may be perpendicular to waveguide  40  or may be angled to reduce back reflection without exceeding the scope of the invention. 
   Fiber  30  is butt coupled as close as possible to facet  120  of waveguide  40 . In a preferred embodiment length x of notch  110  is greater than the optical mode width, and in an exemplary preferred embodiment is on the order of 20 microns. The end of fiber  30  is separated from facet  40  approximately by width z of notch  110 . 
     FIG. 3  is a high level flow chart of the stages in production of a PLC based component having an optical fiber aligned with a waveguide in accordance with the principle of the invention. In stage  1000  a waveguide is defined, typically in a layer above the substrate. Optionally, a mask layer is deposited and etched for use in defining a v-groove. In stage  1010  additional layers, including a layer burying the waveguide are deposited. 
   In stage  1020  a notch is etched defining a facet of the waveguide. The facet defined by etching the notch may be perpendicular to the layers deposited or angled without exceeding the scope of the invention. Advantageously, an etched facet requires no further polishing. Optionally, the facet is coated to protect it during subsequent etching of the v-groove in the silicon substrate. In an exemplary embodiment, the facet is coated with Si 3 N 4 . 
   In stage  1030  a v-groove is etched is etched in the silicon substrate to ensure proper placement of an optical fiber in relation to the waveguide defined in stage  1000  above. In a preferred embodiment, the notch is etched utilizing the optional mask layer deposited and etched in stage  1000  above. In one embodiment, the optional mask layer and the facet coating is removed after the v-groove is etched. 
   In stage  1040  a trench is sawed, cut or diced, denoted generally as cut, in front of the notch etched in stage  1020 . Cutting the trench cleans out any remnants of the v-groove etched in stage  1030  allowing the optical fiber clear access up to the end of the notch etched in stage  1020 . 
   In stage  1050 , an optical fiber is placed, optically aligned and secured in the v-groove defined in stage  1030  above, thus creating a PLC based optical component having a waveguide with a polished facet in optical alignment with an optical fiber in accordance with the principle of the invention. 
     FIG. 4   a  illustrates a side view of a PLC based component  200  according to the principle of the invention comprising a substrate  60  and an optical component  220 . The top of substrate  60  is generally shown as line  80 . Waveguide  40  is defined in layers deposited on substrate  60 , and notch  110  having a depth y and a width z is etched to define facet  120  of waveguide  40 . Trench  70  is subsequently cut defining a location for optical component  220 . Trench  70  may be cut perpendicular to substrate  60 , or angled without exceeding the scope of the invention. 
   A unique feature of the invention is that facet  120  is defined by chemical etching in advance of, and independently of, the cutting of trench  70 . Facet  120 , defined by chemical etching, may be perpendicular to waveguide  40  or may be angled to reduce back reflection, without exceeding the scope of the invention. 
   Optical component  220 , which in an exemplary embodiment is a photodiode, is placed in trench  70 . Optical component  220  is attached using flying connections, or in any other manner known to those skilled in the art and is aligned with waveguide  40 . Width z of notch  110  may be greater than or less than the optical mode width of light constrained in waveguide  40 , in accordance with the requirements of optical component  220  and waveguide  40 . In a preferred embodiment depth y of notch  110  is cut to a depth sufficient to reveal waveguide  40  and thus define facet  120 , and even further preferably is larger than the optical mode. In an exemplary embodiment, notch  110  does not extend to the silicon of substrate  60 . 
     FIG. 4   b  illustrates a top view of a PLC based component  200 , with any layers above a waveguide  40  removed, according to the principle of the invention. PLC based component  200  comprises a substrate  60  and an optical component  220 , which in an exemplary embodiment comprises a photodiode. Waveguide  40  is defined on substrate  60 , and a notch  110  having a length x and a width z is etched to define a facet  120  of waveguide  40 . Trench  70  is subsequently cut to accommodate optical component  220 . A unique feature of the invention is that facet  120  is defined by chemical etching in advance of, and independently of, the cutting of trench  70 . Facet  120 , defined by chemical etching may be perpendicular to waveguide  40 , or may be angled to reduce back reflection without exceeding the scope of the invention. 
   Optical component  220  is placed into trench  70  and attached using flying connections, or in any other manner known to those skilled in the art and is aligned with waveguide  40 . In a preferred embodiment length x of notch  110  is greater than the optical mode width, and in an exemplary preferred embodiment is on the order of 20 microns. 
     FIG. 5  illustrates a side view of a PLC based component  250  exhibiting a sloped trench according to the principle of the invention comprising a substrate  60  and an optical component  220 . The top of substrate  60  is generally shown as line  80 . Waveguide  40  is defined in layers deposited on substrate  60 , and a notch  110  having a depth y and a width z is etched to define facet  120  of waveguide  40 . Trench  70  is subsequently cut defining a location for optical component  220 . Trench  70  exhibits a sloped surface  260  opposing facet  120  of waveguide  40 . In an exemplary embodiment, sloped surface  260  defines an oblique angle between a plane of optical component  220  and the axis of light proceeding from waveguide  40 . In an exemplary embodiment the angle is between 5° and 85°, preferably between 30° and 70°. 
   A unique feature of the invention is that facet  120  is defined by chemical etching in advance of, and independently of, the cutting of trench  70 . Facet  120 , defined by chemical etching, may be perpendicular to waveguide  40  or may be angled to reduce back reflection, without exceeding the scope of the invention. 
   Optical component  220 , which in an exemplary embodiment is a photodiode, is placed in trench  70  against wall  260 . Optical component  220  is attached using flying connections, or in any other manner known to those skilled in the art and is aligned with waveguide  40 . Width z of notch  110  may be greater than or less than the optical mode width of light constrained in waveguide  40 , in accordance with the requirements of optical component  220  and waveguide  40 . In a preferred embodiment depth y of notch  110  is cut to a depth sufficient to reveal waveguide  40  and thus define facet  120 , and even further preferably is larger than the optical mode. In an exemplary embodiment, notch  110  does not extend to the silicon of substrate  60 . 
     FIG. 6  is a high level flow chart of the stages in production of a PLC based optical component having an element aligned with a waveguide in accordance with the principle of the invention. In stage  2000 , a waveguide is defined. In an exemplary embodiment the waveguide is defined in a layer above the substrate. In stage  2010  additional layers, including a layer burying the waveguide are deposited. In stage  2020  a notch is etched at the end of the waveguide. The facet defined by etching the notch may be perpendicular to the layers deposited or angled without exceeding the scope of the invention. Advantageously, an etched facet requires no further polishing. 
   In stage  2030  a trench is sawed, cut or diced, denoted generally as cut, in front of the notch etched in stage  2020 . Cutting the trench defines a location for placement of an optical component to be aligned with the waveguide defined in stage  2000  above. 
   In stage  2040 , an element such as a photodiode, is placed within the trench cut above in stage  2030 . In stage  2050 , the element is electrically connected to the PLC based component and aligned with the waveguide defined in stage  2000  above, thus creating a PLC based component having a waveguide with a polished facet in optical alignment with an element in accordance with the principle of the invention. 
   It is appreciated that certain features of the invention, which are, for clarity, described in the context of separate embodiments, may also be provided in combination in a single embodiment. Conversely, various features of the invention which are, for brevity, described in the context of a single embodiment, may also be provided separately or in any suitable sub-combination. 
   Unless otherwise defined, all technical and scientific terms used herein have the same meanings as are commonly understood by one of ordinary skill in the art to which this invention belongs. Although methods similar or equivalent to those described herein can be used in the practice or testing of the present invention, suitable methods are described herein. 
   All publications, patent applications, patents, and other references mentioned herein are incorporated by reference in their entirety. In case of conflict, the patent specification, including definitions, will prevail. In addition, the materials, methods, and examples are illustrative only and not intended to be limiting. 
   It will be appreciated by persons skilled in the art that the present invention is not limited to what has been particularly shown and described hereinabove. Rather the scope of the present invention is defined by the appended claims and includes both combinations and sub-combinations of the various features described hereinabove as well as variations and modifications thereof, which would occur to persons skilled in the art upon reading the foregoing description.