Abstract:
The invention includes an integrated optical device having an embedded waveguide and an alignment groove. The waveguide is made by depositing waveguide material in a trench and then planarizing the chip. The alignment grooves can provide passive alignment for connecting the chip to other waveguides or optical fibers.

Description:
RELATED APPLICATIONS  
       [0001]    The present application claims the benefit of priority of copending provisional patent application No. 60/240,805 filed on Oct. 16, 2000, which is hereby incorporated by reference. 
     
    
     
       FIELD OF THE INVENTION  
         [0002]    The present invention relates generally to waveguide devices and mechanisms for aligning waveguides with other devices.  
         BACKGROUND OF THE INVENTION  
         [0003]    Integrated optical waveguides can be used for a number of signal processing tasks including switching, filtering, multiplexing, demultiplexing and the like. Integrated optical waveguides typically must be precisely aligned to optical fibers or other optical devices in order to be useful. Providing the precise alignment has often been difficult.  
           [0004]    Typically, alignment between fiber and waveguide has been provided by actively monitoring the alignment of the devices. This can be done by monitoring the coupling efficiency, for example. A problem with this technique is that it is slow and requires expensive alignment equipment.  
           [0005]    Alignment between fiber and waveguide has also been provided by forming mechanical features in the waveguide chip. Fibers or fiber holders are then fit into these features. This provides passive mechanical alignment between the fiber and the waveguide. Many such schemes are known in the art.  
         SUMMARY  
         [0006]    The present invention includes an integrated optical chip having a waveguide embedded in a substrate, and an alignment groove. The alignment groove is precisely located with respect to the waveguide. In one embodiment of the invention, the alignment groove and the waveguide are patterned in the same single mask step. The waveguide is formed in a trench by depositing waveguide core material, and, optionally, cladding material. The chip is planarized so that the waveguide core is isolated to the trench. 
       
    
    
     DESCRIPTION OF THE FIGURES  
       [0007]    [0007]FIG. 1 shows a perspective view of a waveguide chip according to the present invention.  
         [0008]    [0008]FIG. 2 shows a chip according to the present invention coupled to a fiber array. The connection is provided in a manner similar to a mechanical transfer (‘MT’) connector.  
         [0009]    [0009]FIG. 3 shows an alternative embodiment of the invention where the waveguide is covered with a top cladding layer.  
         [0010]    [0010]FIGS. 4 a - 4   e  Illustrate a method for making the waveguide chips according to the present invention.  
         [0011]    [0011]FIGS. 5 a - 5   e  Illustrate a second method for making the waveguide chips according to the present invention.  
         [0012]    [0012]FIGS. 6 a - 6   e  Illustrate a method for making the trench and alignment groove according to the present invention.  
         [0013]    [0013]FIG. 7 shows a chip where the alignment groove is in-line with the waveguide.  
         [0014]    [0014]FIG. 8 shows a chip where an optical fiber is disposed in the in-line alignment groove of FIG. 7. 
     
    
     DETAILED DESCRIPTION  
       [0015]    The present invention provides a waveguide aligned with a groove. The waveguide is formed below the surface of a substrate in a Damascene-type process. The groove can be used to aligned optical fibers or other optical devices to the waveguide. For example, the groove can be used to hold and passively align optical fibers, or fiber arrays to the waveguide. The groove can be formed in the same lithographic process as the waveguide, so that alignment of the groove and waveguide is highly accurate.  
         [0016]    [0016]FIG. 1 shows an integrated optical device  18  according to the present invention. The integrated optical device has two alignment grooves  20 ,  22  disposed adjacent to a waveguide  24 . The waveguide  24  includes a core  26  and a cladding  28 . The alignment grooves  20 ,  22  and the waveguide  24  are formed in a substrate  30  which may comprise silicon or other material such as metal, ceramic, semiconductor, or polymer. The integrated optical device has a front face  32  and a back face  34 . The front face  32  can be aligned and coupled to other optical devices such as optical fibers, waveguides, lenses or the like (not shown). The back face  34  may extend beyond what is illustrated to include waveguide devices. For example, the integrated optical device may extend to include arrayed waveguide gratings, couplers, filters switches or other optical devices (not shown).  
         [0017]    It is important to note that the cladding material can be absent if the substrate is made of a material that can function as a cladding. For example, if the substrate is made of glass with a refractive index less than the waveguide core, then the cladding layer is optional. In this case, waveguide core  26  is in direct contact with the substrate.  
         [0018]    If the substrate is made of (100) silicon, the alignment grooves can be V-grooves or U-grooves made by anisotropic wet etching of silicon, for example by potassium hydroxide.  
         [0019]    The front face  32  of the present integrated optical device can be polished.  
         [0020]    [0020]FIG. 2 shows the integrated optical device  18  coupled to a fiber array  38  according to an exemplary embodiment of the present invention. The fiber array  38  and the integrated optical device  18  are coupled by pins  40 . Pins  40  are disposed in the alignment grooves  20 ,  22  and in alignment grooves  42 ,  44  in the fiber array  38 . An optical fiber  46  is disposed in the fiber array  38 . The mechanical connections between the integrated optical device  18 , pins  40 , and fiber array  38  assure that the optical fiber and the waveguide  24  are passively aligned. The coupling between the integrated optical device  18  and the fiber array  38  is similar to the connection in a mechanical transfer (‘MT’) style optical fiber connector.  
         [0021]    [0021]FIG. 3 shows an alternative embodiment where the waveguide is covered with a top cladding layer  28   a.  In this embodiment, the waveguide core  26  is illustratively shown to be flush with a top surface  45  of the substrate  30 .  
         [0022]    In the present invention, the embedded waveguide  24  is made by a Damascene-type process where a trench is filled with the waveguide cladding (optional) and the waveguide core (essential), and then the substrate is planarized. The planarization process generally removes the waveguide core material from all areas of the substrate outside the trench. When complete, the waveguide is in the trench below the top surface of the substrate.  
         [0023]    Embedded waveguides have some advantages over waveguides deposited over a substrate. Embedded waveguide tend to have lower scattering losses because the core-cladding boundary is extremely smooth. Also, substrates with embedded waveguides tend to have lower stress because the waveguide material (typically oxide) is not deposited over the entire surface of the substrate.  
         [0024]    [0024]FIGS. 4 a - 4   e  Illustrate a method for making the waveguide integrated optical device of the present invention. FIGS. 4 a - 4   e  Are front views of the present integrated optical device. A process according to the present invention is described below:  
         [0025]    [0025]FIG. 4 a:  A trench  48  is etched using reactive ion etching (RIE), wet etching, or a combination of RIE and wet etching. If desired, the sidewalls of the trench can be polished by a polishing etch or, in the case of a silicon substrate, a thermal oxidation followed by an oxide etch.  
         [0026]    [0026]FIG. 4 b:  If a cladding is desired, a cladding layer  28  is deposited. The cladding layer  28  can be formed by CVD oxide, or thermal oxide if the substrate is made of silicon. Polymer materials can also be used for the cladding layer  28 .  
         [0027]    [0027]FIG. 4 c:  Optionally, (as shown) the cladding layer is removed by planarization (e.g. chemical-mechanical polishing). Waveguide core material is then deposited into the trench. The waveguide core material may fill the trench above the level of the substrate top surface. Alternatively, the waveguide core material does not fill above the substrate top surface.  
         [0028]    [0028]FIG. 4 d:  The substrate is planarized. Optionally, after this step, the waveguide core material can be selectively etched, so that the core is below the level of the substrate top surface.  
         [0029]    [0029]FIG. 4 e:  V-grooves  20 ,  22  are formed in the substrate. The V-grooves can be located precisely with respect to the waveguide  24  using lithographic techniques (e.g. using an edge of the waveguide as a fiduciary for aligning the V-grooves). The V-grooves can be formed by anisotropic wet etching if the substrate is made of single crystal silicon. The V-grooves can instead be grooves having other shapes such as a U-shape or rectangular shape.  
         [0030]    [0030]FIGS. 5 a - 5   e  describe an alternative method for making the integrated optical device according to the present invention. Figs. 5 a - 5   e  are front views.  
         [0031]    [0031]FIG. 5 a:  The trench  48  and V-grooves  20 ,  22  are formed in the substrate. The trench  48  and the V-grooves  20 ,  22  can be made by the same or different etch processes. The V-grooves  20 ,  22  and trench  48  are preferably patterned in the same mask step, so that they are accurately aligned with respect to one another.  
         [0032]    [0032]FIG. 5 b:  Cladding material  28  and core material are deposited on the substrate  30 , covering the trench  48 , and V-grooves  20 ,  22 .  
         [0033]    [0033]FIG. 5 c:  The integrated optical device is planarized, for example to the level of the substrate  30 . The V-grooves  20 ,  22  may be filled with remnant material  47  (cladding material and, optionally, core material), as shown.  
         [0034]    [0034]FIG. 5 d:  Optionally, a top cladding layer  28   a  is deposited on the substrate  30 . The top cladding layer  28   a  can be SiO2 deposited by chemical vapor deposition or spin-on-glass, for example. It can also be a polymer layer.  
         [0035]    [0035]FIG. 5 e:  The top cladding layer  28   a  is masked and etched so that the remnant material is removed from the V-grooves  20 ,  22 . The top cladding layer  28   a  is preserved over the waveguide  24 . The top cladding layer  28   a  can be spin-on-glass, CVD oxide, polymer or other materials. The top cladding layer  28   a  can be selected to etch slower than the remnant material  47 .  
         [0036]    [0036]FIGS. 6 a - 6   e  illustrate how to form the trench  48  and V-grooves  20 ,  22  according to a single mask step. The method is related to a method described in copending U.S. patent application Ser. No. 09/519,165, incorporated herein by reference.  
         [0037]    [0037]FIG. 6 a:  A substrate  30  is patterned with a metal layer  50  on a dielectric layer  52  (e.g. SiO2 or silicon nitride). The substrate  30  is (100) single crystal silicon. All the patterns in the metal layer  50  can be made in the same mask step, so that all the metal layer patterns are accurately located with respect to one another.  
         [0038]    [0038]FIG. 6 b:  The substrate  30  is masked with a mask layer  54 , and the trench  48  is formed by RIE or RIE combined with wet etching, for example. The trench location and shape are defined by the patterns in the metal layer  50 .  
         [0039]    [0039]FIG. 6 c:  The substrate  30  is remasked with a second mask layer  54   a  so that the trench  48  is protected. Then, the dielectric layer  52  is removed in an area defined by the metal layer  50 , exposing the substrate. The dielectric layer  52  can be removed by wet or dry etching, for example. The area of the dielectric layer removed is defined by the pattern in the metal layer  50 .  
         [0040]    [0040]FIG. 6 d:  The substrate  30  is etched. In the specific embodiment shown, the etch is an anisotropic wet etch, forming a V-groove  20 . The V-groove can be one of the V-grooves  20 ,  22  in the integrated optical device of FIG. 1.  
         [0041]    [0041]FIG. 6 e:  The second mask  54   a  is removed, and, optionally, the dielectric layer  52  is removed. The trench  48  and V-groove  20  are precisely aligned because they were defined by the same ask step. The substrate  30  is ready to have waveguide material formed in the trench  48  as described above.  
         [0042]    [0042]FIG. 7 shows another embodiment of the present invention where the alignment groove  20  is in-line with the embedded waveguide  24 . A dicing saw cut  55  can be provided so that an optical fiber (not shown) disposed in the alignment groove  20  can be butted against the waveguide  24 .  
         [0043]    [0043]FIG. 8 shows the integrated optical device of FIG. 7 with an optical fiber  46 .  
         [0044]    Also, the pins  40  can be bonded to the grooves  20 ,  22 . The pins  40  can be bonded to the grooves  20 ,  22  using solder, epoxy or other materials.  
         [0045]    The material of the waveguide can be CVD or thermal SiO2 or other low loss materials such as polymers.  
         [0046]    It will be clear to one skilled in the art that the above embodiment may be altered in many ways without departing from the scope of the invention. Accordingly, the scope of the invention should be determined by the following claims and their legal equivalents.