Patent Publication Number: US-6987895-B2

Title: Thermal compensation of waveguides by dual material core having positive thermo-optic coefficient inner core

Description:
RELATED APPLICATIONS 
   This application is related to co-pending application, filed Jul. 2, 2002, entitled “THERMAL COMPENSATION OF WAVEGUIDES BY DUAL MATERIAL CORE HAVING NEGATIVE THERMO-OPTIC COEFFICIENT INNER CORE,” and assigned to the Assignee of the present application. 

   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The described invention relates to the field of optical circuits. In particular, the invention relates to thermal compensation in an optical waveguide. 
   2. Description of Related Art 
   Optical circuits include, but are not limited to, light sources, detectors and/or waveguides that provide such functions as splitting, coupling, combining, multiplexing, demultiplexing, and switching. Planar lightwave circuits (PLCs) are optical circuits that are manufactured and operate in the plane of a wafer. PLC technology is advantageous because it can be used to form many different types of optical devices, such as array waveguide grating (AWG) filters, optical add/drop (de)multiplexers, optical switches, monolithic, as well as hybrid opto-electronic integrated devices. Such devices formed with optical fibers would typically be much larger or would not be feasible at all. Further, PLC structures may be mass produced on a silicon wafer. 
   PLCs often have been based on silica-on-silicon (SOS) technology, but may alternatively be implemented using other technologies such as, but not limited to, silicon-on-insulator (SOI), polymer on silicon, and so forth. 
   Thermal compensation for some optical circuits, such as phase-sensitive optical circuits, is important as devices may be operated in locations where temperatures cannot be assured. In some cases, optical circuits are combined with temperature regulating equipment. However, these configurations may be less than ideal, since the devices are prone to failure if there is a power outage, and temperature regulating equipment may require a large amount of power which may not be desirable. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIGS. 1A-1C  are schematic diagrams showing one embodiment of a cross-sectional view of a waveguide structure being modified to be thermally-compensating. 
       FIG. 2  is a flowchart showing one embodiment of a method for fabricating a thermally-compensating waveguide. 
       FIG. 3  is a schematic diagram showing one embodiment of an array waveguide grating (AWG) that makes use of the thermally-compensating waveguides. 
       FIG. 4  is a schematic diagram showing an embodiment of a PLC comprising an interferometric component that uses thermally-compensating waveguides in its coupler regions. 
       FIG. 5  is a graph illustrating the normalized mode field intensity in a cross section of a dual material waveguide. 
       FIG. 6  is a graph illustrating an aperture function for a dual material waveguide. 
       FIGS. 7A-7C  are schematic diagrams that illustrate another embodiment of a thermally compensated waveguide. 
       FIG. 7D  is a schematic diagram showing an enlargement of the core of the waveguide of  FIGS. 7A-7C . 
       FIG. 8  is a schematic diagram showing a cross sectional view of another embodiment of a waveguide having a dual material core. 
       FIG. 9  is a schematic diagram showing a cross section view of another embodiment of a waveguide having a dual material core. 
   

   DETAILED DESCRIPTION 
   A planar lightwave circuit comprises one or more waveguides that are thermally-compensating. The thermally-compensating waveguides comprise a cladding and a core that comprises two regions running lengthwise through the core. One region has a negative thermo-optic coefficient (“TOC”); the other region has a positive TOC. 
     FIG. 1A  is a schematic diagram showing one embodiment of a cross-sectional view of a waveguide structure  5 . In one embodiment, the structure is subsequently modified as described with respect to  FIGS. 1B and 1C  to be thermally-compensating. 
   As shown in  FIG. 1A , a layer of lower cladding  12  is typically deposited onto a substrate  10 . A waveguide core layer  20  is deposited over the lower cladding  12 , and an upper cladding  24  is deposited over the waveguide core layer  20 . In one example, the substrate  10  is silicon, the lower cladding  12  is SiO 2 , the core layer  20  is SiO 2  doped with Germanium, and the upper cladding  24  is a borophosphosilicate glass (BPSG). In one embodiment, the upper cladding  24  may form a thin layer of approximately 1-2 microns covering the core. 
     FIG. 1B  is a schematic diagram showing one embodiment of a cross-section view of a waveguide after a trench  30  is created in the core layer  20 . In one embodiment, the trench  30  is formed to run along a length of the core of the waveguide. The trench may be formed by etching, ion beam milling, or other methods. In one embodiment, the trench has a depth of at least ⅔ of the depth of the core. However, the trench depth may extend down into the lower cladding  12 . The width of the trench is designed to be less than a wavelength of the optical signal to be propagated by the waveguide. 
     FIG. 1C  is a schematic diagram showing one embodiment of a cross-sectional view of  FIG. 1B  after a layer of material  50  having a negative TOC has been deposited. The negative TOC material  50  fills the trench to form a negative TOC center region  40  of the core. In one embodiment, a polymer, such as silicone, poly(methylmethacrylate) (“PMMA”), or benzocyclobutene (“BCB”), is used. However, various other materials may alternatively be used. 
   When an optical signal propagates within the waveguide  5 , a first portion of the optical field of the optical signal propagates in the negative TOC region  40 , and a second portion of the optical field propagates in the positive TOC region  42  of the core. In one embodiment, the first portion of the optical field in the negative TOC region  40  is substantially surrounded by the second portion of the optical field in the positive TOC region  42 . 
   In one embodiment, the refractive index difference between the negative TOC region  40  and the positive TOC region  42  is large enough to allow filling over the negative TOC region  40  with a layer of the same material that serves as an upper cladding. The structure described provides good compensation with low loss over a wide temperature range, and allows for convenient fabrication. 
     FIG. 2  is a flowchart showing one embodiment of a method for fabricating a thermally-compensating waveguide. The flowchart starts at block  100 , and continues with block  110 , at which a core of the waveguide is formed over an appropriate substrate structure. In one embodiment, the core is formed on a SOS structure and comprises SiO 2  doped with Germanium having a cross-sectional area of approximately 6 microns by 6 microns. Other positive TOC materials may alternatively be used. The flowchart continues at block  120  at which a trench is created in the core. In one embodiment, the trench is approximately 1 micron wide and runs an entire length of the waveguide. At block  130 , a negative thermo-optic coefficient material is deposited into the trench. In one embodiment, an optical signal of approximately 1550 nm propagates within both the materials making up the core, having both positive and negative TOC regions. The flowchart ends at block  140 . 
   In an alternate embodiment, after the trench is filled with the negative TOC material, another material having a positive TOC may be used to cover the negative TOC material. 
   The effective index of propagation in the core will have a close to linear response to compensate for the thermal expansion of the substrate, and allows for thermal compensation up to a range of approximately 100° C. Additionally, the described waveguide structure may be used for curved waveguides. A bend radius of down to 10 mm is achievable with losses on the order of approximately 0.3 db/cm. 
     FIG. 3  is a schematic diagram showing one embodiment of an array waveguide grating (AWG)  200  that makes use of thermally-compensating waveguides. In one embodiment, the waveguides  210   a - 210   x  are thermally-compensating as previously described, but the star couplers  220  and  222  and the input and output waveguides  230  and  232  are not thermally-compensated, allowing for easier alignment of the input and output waveguides  230  and  232  with other optical components. 
     FIG. 4  is a schematic diagram showing an embodiment of a PLC comprising an interferometric component  300  that uses thermally-compensating waveguides in coupler regions  310  and  312 . A temperature regulator  320  is used on a non-thermally-compensated waveguide portion to modify the phase of the optical signal. In one embodiment, an electrical component  350 , such as an optical-to-electrical converter and/or electrical-to-optical converter, is coupled to the thermally-compensated waveguide coupler  312 . One or more electrical connections  360  couple the electrical component  350  with power and other electrical signals. In an alternate embodiment, the phase modulation may be adjusted using other methods, such as mechanical. 
   In one embodiment, a temperature regulator  380  may be housed with a thermally-compensated optical circuit to keep the device within its thermally-compensating temperature range. 
   The thermally-compensating waveguides described compensate single mode waveguides independently. They may be used solely in a phase-sensitive portion or throughout an optical circuit. 
   A variety of different materials may be used for the thermal-compensation. For example, silicone has a TOC of −39×10−5/° C., PMMA has a TOC of −9×10−5/° C., and BPSG has a TOC of approximately 1.2×10−5/° C. The design of the trench may be altered to compensate for the use of various materials. 
     FIG. 5  is a graph illustrating the normalized mode field intensity in a cross section of a dual material waveguide.  FIG. 6  is a graph illustrating an aperture function for a dual material waveguide. In one approximation, the waveguide materials are chosen to satisfy the following relation:
 ∫Ψ A   PC   Ψ*·B   PC   +∫ΨA   GC   Ψ*·B   GC   +∫ΨA   CL   Ψ*·B   CL   =−nα   substrate           wherein   Ψ is the mode field intensity;   Ψ* is the complex conjugate of the mode field intensity;   α is the linear thermal expansion coefficient, which is dominated by the substrate;   B is the thermo-optic coefficient;   n is the effective index of propagation; and       
   A is an aperture function having the value 1 within the material and 0 outside the material, and wherein the subscript PC indicates within the polymer core, GC indicates within the Ge Silica core, and CL indicates within the cladding. 
   For those skilled in the art, it is relatively straight-forward to include effects of strain and polarization to improve the accuracy of the modeling. 
     FIGS. 7A-7C  are schematic diagrams that illustrate another embodiment of a thermally compensated waveguide  505 . In this embodiment, the core  520  has a central portion that has a positive TOC and an outer portion that has a negative TOC. 
     FIG. 7A  shows a first core portion  520   a  having a positive TOC. The first core portion  520   a  forms a spike running the length of a waveguide. In one embodiment the first core portion is formed on a lower cladding  512  over a substrate  510 , similar to that of FIG.  1 A. The first core portion may be deposited and then etched to form a spike having the desired dimensions. Support structures  524  may be formed on the lower cladding  512  as long as they are far enough away from the core  520  to prevent light from leaking from the core to the support structure. 
     FIG. 7B  shows a negative TOC material deposited over the positive TOC first core portion  520   a  to form a second core portion  520   b . The first core portion  520   a  and the second core portion  520   b  make up the core  520 . In one embodiment, the negative TOC core material is a polymer (“core polymer”). In one embodiment, the core polymer is formed by spinning accumulation. Alternatively, the core polymer may be applied by other lithography methods. In one embodiment, the core polymer has a refractive index of approximately 1.45 to 1.6. 
     FIG. 7C  shows a second negative TOC material deposited over the core  520  to form a cladding  530 . In one embodiment, the negative TOC material is a polymer (“cladding polymer”) and has a refractive index approximately 0.01 to 0.05 less than that of the core polymer  520   b . In one embodiment, the core polymer and the cladding polymer are related polymers. 
     FIG. 7D  is a schematic diagram showing an enlargement of the core  520  of the waveguide  505  of  FIGS. 7A-7C . In one embodiment, an undercladding  550  is deposited before applying the core polymer  520   a . This provides an undercladding of polymer under the core, which creates an interface under the core that substantially matches the core/cladding interface on top of the core to provide better performance. 
     FIG. 8  is a schematic diagram showing a cross sectional view of another embodiment of a waveguide having a dual material core. In this embodiment, an inner core  610  is completely surrounded by an outer core  612 . In one case, the inner core has a negative TOC and the outer core has a positive TOC. In an alternate embodiment, the inner core has a positive TOC and the outer core has a negative TOC. The inner and outer cores may comprise polymer or other suitable materials. 
     FIG. 9  is a schematic diagram showing a cross section view of another embodiment of a waveguide having a dual material core. In this embodiment, an inner core  620  is sandwiched between an outer core  622 . The inner core, however, lies substantially in the plane of the substrate of the PLC, and will not have as good optical confinement for PLC&#39;s with significant bend radii compared to the structures previously described with respect to  FIGS. 1C and 7C  having inner cores in a plane substantially perpendicular to the plane of the substrate of the PLC. 
   Thus, an apparatus and method for making thermally-compensating planar lightwave circuit is disclosed. However, the specific embodiments and methods described herein are merely illustrative. For example, although the techniques for thermally compensating waveguides were described in terms of an SOS structure, the techniques are not limited to SOS structures. Numerous modifications in form and detail may be made without departing from the scope of the invention as claimed below. The invention is limited only by the scope of the appended claims.