Patent Publication Number: US-5834055-A

Title: Guided wave device and method of fabrication thereof

Description:
GOVERNMENT SUPPORT 
     The Government has rights in this invention pursuant to contract Number NOAA Contract No. 50-DKNA-4-00118, awarded by the Department of Commerce. 
    
    
     BACKGROUND OF THE INVENTION 
     A light waveguide is formed in a birefringent crystalline substrate such as lithium niobate by depositing a metal film followed by heating of the substrate in a flowing inert gas such as argon, nitrogen or oxygen at about 1,000° C. This results in a metal diffused layer with a higher refractive index near the surface. The metal film is typically selected from vanadium, titanium, nickel, and copper. Of these metals, titanium-diffused lithium niobate waveguides have exhibited preferred optical properties for many applications. 
     Though this type of surface channel waveguide can guide propagation modes efficiently, insertion loss is high because of surface scattering and mode mismatch between the waveguide and a coupled fiber. Propagation modes of such surface channel waveguides are inherently asymmetric with respect to the guiding axis. By burying the waveguide under the substrate surface, modes can be made more symmetric and better matched with the fiber. Magnesium oxide has been found useful for burying the waveguide. Both the ordinary and extraordinary refractive indices of lithium niobate have been found to be reduced by diffusion of magnesium. However, deposition of magnesium in lithium niobate is difficult as special precautions are required. Proton exchange in lithium niobate has also been used to bury waveguides below the surface; however, with proton exchange only one polarization mode can be guided by the waveguide. 
     A polarization beam splitter is a device that separates an incoming beam of light into orthogonal TE and TM modes at the output ports. Several approaches are known for achieving a polarization splitter, including mode sorting and modal interference. Other techniques include use of a directional coupler with metal loading on one of the waveguides to prevent the TM mode from coupling. Another approach employs electro-optical effects acting differently on TE and TM coupling lengths, allowing both to be tuned independently. However, most of these techniques require stringent fabrication tolerances. 
     A continuing need exists to provide improvements in optical waveguides to provide more efficient coupling of light and ease of manufacture. 
     SUMMARY OF THE INVENTION 
     In accordance with the present invention, the above-discussed problems are reduced or alleviated and an effective method and device are provided, characterized by ease of manufacture with reduced fabrication tolerance controls, while improving the performance of all integrated optic devices. 
     A preferred embodiment of the present invention is directed to a method for co-diffusing a first material and a second material into single-crystal lithium niobate (LiNbO 3 ), and to guided wave devices which are produced by such co-diffusion. A preferred embodiment uses titanium (Ti) as the first material. Other preferred embodiments can be formed using a metal selected from the group consisting of vanadium, nickel and copper. The second material must have a refractive index lower than the substrate into which it is diffused in order to reduce the index of refraction and it must have a diffusion temperature low enough so that it will not substantially alter the properties of the waveguides that have previously been formed. The second diffused material can also be used to alter the shape of the modes to match the modes of the optical fibers coupled to the waveguide to reduce insertion loss. A preferred embodiment utilizes aluminum as the second material. Other preferred embodiments utilize a metal or semiconductor material such as lithium, tin, zinc, or silicon. 
     According to the method of the present invention, titanium is diffused into an x-cut birefringent crystalline material, such as LiNbO 3  or LiTaO 3 , in a manner such as to define a light guiding region extending the length thereof from end-to-end. A layer of aluminum is deposited over at least the light guiding region and diffused into the birefringent crystalline material such that the light guiding region is buried below the surface of the crystal. Preferably, the initial titanium diffusion is done at a higher temperature, in the range of about 1000°-1100° C., while the diffusion of aluminum occurs at a lower temperature, in the range of about 900° to about 950° C. 
     One advantage of aluminum is that it is an easier material to deposit. Another advantage is that aluminum can be diffused into LiNbO 3  at a much lower temperature without affecting already diffused titanium waveguides. 
     An advantage of the present invention is the ability to alter or shape optical mode and path length characteristics of waveguides. Such mode shaping includes altering waveguide path length with aluminum diffusion to increase one path length relative to another path after optical measurement characterization of the already diffused titanium waveguides. 
     According to another aspect of the invention, a first layer of aluminum is deposited onto a LiNbO 3  crystal. Using photolithography, a pattern is formed followed by etching of the aluminum layer. A layer of titanium is deposited into the pattern to define a light guiding region therein which is surrounded on two sides by the first layer of aluminum. A second layer of aluminum is deposited overlaying the first aluminum layer and the light guiding region. The first and second aluminum layers and the light guiding region are then diffused into the LiNbO 3  crystal. Preferably, the titanium diffuses into the LiNbO 3  at about 1000°-1100° C. while the aluminum diffuses further at a lower temperature in the range of about 900° to about 950° C. By this method, the mode shape can be optimized to match with an externally coupled fiber while simultaneously burying the waveguides. 
     In another aspect of the invention, a guided wave device is fabricated based on the method of the present invention. Accordingly, the guided wave device comprises a light guiding region formed within a substrate fabricated of an x-cut crystalline birefringent material such as LiNbO 3 . A burying region formed within the substrate overlaps the light guiding region such that the light guiding region is buried below the substrate surface. 
     In an alternate embodiment, a mode shaping region formed within the substrate surrounds the light guiding region on two sides. The mode shaping region has ordinary and extraordinary indices of refraction less than the substrate such that mode mismatch between the device and an externally coupled fiber is minimized. In the alternate embodiment, the burying region overlaps the light guiding region and the mode shaping region such that the light guiding region is buried below the substrate surface. 
     According to another aspect of the present invention, a polarization beam splitter is fabricated based on tuning the transfer length of one polarization, for example, the TE mode, independent of the transfer length of the other polarization, for example, the TM mode, by co-diffusing aluminum with titanium. Specifically, the polarization beam splitter comprises a pair of light guiding regions formed within a substrate fabricated of an x-cut crystalline birefringent material such as LiNbO 3 . Each light guiding region has an input port and an output port by which radiation of TE and TM modes can be coupled in and out of the substrate. In addition, each light guiding region includes a coupling section optically coupled to the other. A tuning region formed within the substrate surrounds the light guiding regions on two sides. The tuning region has ordinary and extraordinary indices of refraction which are less than the substrate such that odd number transfer lengths of the TE mode are substantially equal to even number transfer lengths of the TE mode, thereby yielding a device capable of separating the TE and TM modes at the output ports. 
     The light guiding regions are preferably formed by diffusing titanium into the substrate. The tuning region is formed by diffusing aluminum into the substrate. In a preferred embodiment, the tuning region surrounds only the coupling section of each light guiding region. A polarization beam splitter according to the present invention can achieve an extinction ratio of about 20 dB for both TE and TM modes. 
     In another aspect of the invention, a guided wave device comprises a light guiding region defining a Mach-Zehnder interferometer having two arms formed within a substrate fabricated of an x-cut crystalline birefringent material such as LiNbO 3 . A tuning region formed within the substrate surrounds one of the arms on two sides such that an optical path length differential between the arms is formed. Preferably, the light guiding region and the tuning region are formed by co-diffusing titanium and aluminum respectively into the substrate. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The above and other features of the invention including various novel details of construction and combinations of parts will now be more particularly described with reference to the accompanying drawings and pointed out in the claims. It will be understood that the particular guided wave devices embodying the invention are shown by way of illustration only and not as a limitation of the invention. The principles and features of this invention may be employed in varied and numerous embodiments without departing from the scope of the invention. 
     FIGS. 1a, 1b, 1c, 1d, 1e and 1f are diagrammatic sectional views of intermediate product stages in the method of making a guided wave device according to the present invention. 
     FIG. 2 is a diagrammatic perspective view of a guided wave device according to the present invention. 
     FIG. 3 is a diagrammatic top plan view of the device of FIG. 2 coupled to external fibers. 
     FIGS. 4a, 4b, 4c and 4d are diagrammatic sectional views of further product stages following the stages of FIGS. 1a-1f in the method of making the device of FIG. 2. 
     FIG. 5 is a diagrammatic perspective view of another guided wave device according to the present invention. 
     FIGS. 6a, 6b, and 6c are diagrammatic end views of further product stages following the stages of FIGS. 1a-1f in the method of making the device of FIG. 5. 
     FIG. 7 is a diagrammatic perspective view of a polarization beam splitter according to the present invention. 
     FIG. 8 is a diagrammatic end view of a further product stage following the stages of FIGS. 1a-1f in the method of making the splitter of FIG. 7. 
     FIG. 9 is a diagrammatic top plan view of the product stage of FIG. 8. 
     FIG. 9a is a diagrammatic top plan view of an additional product stage following the stage of FIG. 9 for making an alternative polarization beam splitter. 
     FIG. 10 is a diagrammatic end view of the splitter of FIG. 7. 
     FIG. 11 is a diagrammatic perspective view of a Mach-Zehnder interferometer according to the present invention. 
     FIG. 12 is a diagrammatic top plan view of a further product stage following the stages of FIGS. 1a-1f in the method of making the interferometer of FIG. 11. 
     FIG. 13 is a diagrammatic cross-sectional view of the product stage of FIG. 12. 
     FIG. 14 is a diagrammatic cross-sectional view of a further product stage following the stage of FIG. 13. 
     FIG. 15 is a diagrammatic top plan view of a further stage following the stage of FIG. 14. 
     FIGS. 16a and 16b show the principle of TE and TM mode separation. 
     FIGS. 17a and 17b illustrate the coupling ratio as a function of postbake time for 6 mm and 4 mm beam splitters respectively made according to FIG. 9. 
     FIGS. 18a and 18b illustrate extinction ratio as a function of postbake time for 6 mm and 4 mm beam splitters respectively. 
     FIG. 19 shows mode size as a function of postbake time for 6 mm and 4 mm beam splitters respectively. 
     FIG. 20 shows coupling ratio as a function of postbake time for a device made according to FIG. 9a. 
     FIG. 21 shows mode size as a function of postbake time for a device made according to FIG. 9a. 
     FIG. 22 shows insertion loss as a function of postbake time for a device made according to FIG. 9a. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The term &#34;co-diffusion&#34; as used herein refers to the diffusion of titanium and aluminum through the surface of a single-crystal lithium niobate substrate whether the titanium and the aluminum are diffused successively or concurrently. 
     It has now been discovered that co-diffusion of titanium with aluminum alters the shape of the propagation modes of guided wave devices to match with fiber modes for better insertion loss. Aluminum is patterned around titanium guides and diffused at a lower temperature compared to the titanium diffusion temperature. By use of controlled post diffusion, the mode match between the guided wave device and a coupled fiber is improved. Diffusion of aluminum to bury the titanium guides into the substrate also improves the device insertion loss. Aluminum has the advantage of being an easier material to deposit and it also can be diffused in lithium niobate at a much lower temperature without affecting already diffused titanium guides. 
     In the method of the present invention, a series of preparatory steps are employed followed by steps which are specific to fabrication of alternate embodiments. FIGS. 1a-1f illustrate the intermediate product stages resulting from these common preparatory steps. In general, an x-cut birefringent crystalline material such as lithium niobate forms a substrate 10 as shown in FIG. 1a. A layer of aluminum 12 is deposited on the substrate 10 as shown in FIG. 1b. Preferably, the aluminum layer 12 is deposited by well-known evaporation techniques. A layer of photoresist 14 is spun to overlay the aluminum layer 12 as shown in FIG. 1c. 
     The next product stage shown in FIG. 1d results from forming a pattern using standard photolithography techniques followed by development of the photoresist layer 14. The developed pattern yields channels 13. In the next step, the portion of the aluminum layer 12 which is exposed by the channels 13 is etched to a depth coincident with the top of the lithium niobate substrate 10. This etching step yields deeper channels 13&#39; as shown in FIG. 1e. The etching is performed by wet or dry etching. In the final preparatory step, a layer of titanium 16 is deposited over the photoresist layer 14 and also into the channels 13&#39; to define a light guiding region 17 therein as shown in FIG. 1f. 
     The above preparatory steps can be used in various methods according to the present invention to fabricate guided wave devices as described further below. 
     In FIG. 2, a guided wave device according to the present invention is shown which can be fabricated using the preparatory steps of FIGS. 1a-1f and the specific steps of FIGS. 4a-4d as described herein. The guided wave device 20 in FIG. 2 is formed of an x-cut lithium niobate substrate 10 having light guiding regions (or guides) 17&#39; which are diffused into the substrate. A diffused burying region of aluminum 15&#39; serves to bury the guides 17&#39; into the substrate 10. The light guides 17&#39; of the device 20 can be coupled to external fibers 9, as shown in FIG. 3. 
     To fabricate the device 20 of FIG. 2, the specific steps of FIGS. 4a-4d are employed. With the intermediate product stage shown in FIG. 1f, a double layer liftoff is performed by first developing the photoresist layer 14 followed by etching of the aluminum layer 12. The double layer liftoff step results in a substrate 10 having titanium light guiding regions 17 on the surface thereof as shown in FIG. 4a. The titanium guides 17 are then diffused into the substrate 10 as shown in FIG. 4b. Preferably, the titanium diffusion is achieved by heating the intermediate product of FIG. 4a in the range 1000°-1100° C. for approximately 6-14 hours. A second aluminum layer 15, having a thickness in the range of 100 Å to 1000 Å, is deposited over the surface of the intermediate product of FIG. 4b as shown in FIG. 4c. 
     In the final stage, as shown in FIG. 4d, the aluminum is diffused into the substrate 10 to form a burying region 15&#39; which buries the titanium guides 17&#39;. The aluminum can be diffused at about 900° to about 950° C. At this temperature, only the aluminum is diffused without altering the electro-optical property of the previously diffused titanium. The titanium guides 17&#39; increase both the ordinary and extraordinary indices of refraction of the lithium niobate substrate, while the aluminum layer 15&#39; reduces both indices since the index of aluminum is less than lithium niobate. The result is a more symmetric modal power distribution with respect to the axis of the guided wave device 20 as well as improved coupling efficiency between the device 20 and an externally coupled fiber (as shown in FIG. 3). In addition, low surface scattering loss and low insertion loss are achieved. 
     FIG. 5 illustrates another guided wave device 22 fabricated by the method of the present invention and is characterized by having both mode shaping and buried guides. The titanium guides 17&#39; formed in the substrate 10 are surrounded on both sides by a first layer of diffused aluminum 12&#39;. The first diffused aluminum layer 12&#39; serves to further shape the modes to match with the fiber modes for better insertion loss. A second layer of diffused aluminum 15&#39; serves the burying function as described above. 
     To fabricate the device 22 shown in FIG. 5, specific steps in addition to the preparatory steps of FIGS. 1a-1f are employed. The specific stages are shown in FIGS. 6a-6c. Beginning with the intermediate product stage shown in FIG. 1f, a liftoff step is performed wherein the photoresist layer 14 is developed and removed. In FIG. 6a, the photoresist layer 14 has been removed along with the titanium layer 16, leaving an aluminum layer 12 surrounding titanium guides 17. A second aluminum layer 15 is then deposited as shown in the product stage of FIG. 6b. The second aluminum layer 15 overlays the first aluminum layer 12 and the titanium guides 17. 
     Following the aluminum deposition, the aluminum and titanium are co-diffused into the substrate. As shown in FIG. 6c, the final stage results in a device having a mode shaping region of diffused aluminum 12&#39; surrounding diffused titanium guides 17&#39; and overlayed with a diffused aluminum burying region 15&#39;. Initially, the titanium is diffused along with the aluminum at about 1000°-1100° C. for a period of time. This is followed by further diffusion of the aluminum at a lower temperature in the range of about 900° to about 950° C. Therefore, the initial titanium diffusion can be done at higher temperature and the mode shape can be optimized to match with the fiber by further diffusing aluminum at a lower temperature. 
     It should be noted that effective burying regions can also be formed by diffusing a metal or semiconductor material such as lithium, tin, zinc, or silicon, though aluminum is preferred due to the advantage of it being an easier material to handle for deposition and for its preferred optical properties. Aluminum is also preferred since it can be diffused in lithium niobate at a much lower temperature without affecting already diffused titanium guides. 
     In FIG. 7, a polarization beam splitter 24 is shown which is fabricated in accordance with the method of the present invention. The operation of the beam splitter 24 will be described further below. The beam splitter 24 includes diffused light guiding regions or titanium guides 17&#39; formed in a coupler pattern in substrate 10. A diffused aluminum layer 12&#39; laterally surrounds the titanium guides 17&#39;. The titanium guides 17&#39; at one edge of the device 24 form input ports 19a and 19b and at the opposite edge form output ports 19c and 19d. 
     Fabrication of the beam splitter 24 is accomplished by the preparatory steps of FIGS. 1a-1f together with the steps shown in FIGS. 8-10. The intermediate product of FIG. 1f undergoes a liftoff operation whereby the photoresist layer 14 is developed to remove the titanium layer 16. In FIG. 8 is shown the resulting product stage in which the deposited aluminum layer 12 remains and surrounds the titanium guides 17. In FIG. 9 is shown a top plan view illustrating the specific coupler pattern forming the titanium guides 17. The aluminum layer 12 and the titanium guides 17 are co-diffused to yield diffused titanium guides 17&#39; surrounded by tuning region 12&#39; in the end product as shown in FIG. 10. 
     The co-diffusion of titanium and aluminum is performed initially at higher temperature at about 1000°-1100° C. for a period in the range of 6-14 hours followed by post diffusion of the aluminum only at a lower temperature in the range of 900° to 950° C. In an alternate embodiment of the beam splitter 24, a portion of the aluminum layer 12 is etched, leaving an aluminum layer 12b only in the coupling section around the titanium guides 17. This step is performed prior to the co-diffusion stage shown in FIG. 10. 
     In either beam splitter embodiments, the diffused aluminum tuning region 12&#39; performs a particular mode shaping function whereby odd number transfer lengths of the TE mode are substantially equal to even number transfer lengths of the TM mode. This results in a device which can operate to separate the TE and TM modes at the output ports 19c and 19d (FIG. 7). Thus, a mixed TE/TM beam input to input 19a is separated into TM and TE modes at outputs 19c and 19d respectively. 
     Referring now to FIG. 11, a Mach-Zehnder interferometer 26 is shown which is fabricated according to the method of the present invention. The Mach-Zehnder device 26 is formed of a titanium light guiding region 17&#39; diffused into a lithium niobate substrate 10. A diffused aluminum region 12a&#39; is formed around arm 21a of the Mach-Zehnder pattern. 
     To fabricate the Mach-Zehnder device 26, specific steps are performed in addition to the preparatory steps outlined in FIGS. 1a-1f. The stages resulting from these specific steps are shown in FIGS. 12-15. Beginning with the intermediate stage of FIG. 1f, the first specific step is to lift off the photoresist layer 14 to remove the titanium layer 16. 
     The titanium light guiding region 17 is then formed as a Mach-Zehnder pattern as shown in FIG. 12. The deposited aluminum layer 12 surrounds the titanium light guiding region 17 and overlays the lithium niobate substrate 10. This is also seen in the cross-sectional view of FIG. 13. A second photoresist layer 18 is spun to overlay the aluminum layer 12 and the titanium region 17, as shown in FIG. 14. The second photoresist layer 18 is patterned around the titanium guide 17 in arm 21a of the Mach-Zehnder. The aluminum layer 12 is then etched, yielding aluminum region 12a surrounding arm 21a as shown in FIG. 15. The aluminum and titanium are then co-diffused according to the co-diffusion process discussed above. Referring again to FIG. 11, the diffused aluminum region 12a&#39; performs a tuning function such that an optical path length differential between arms 21a and 21b is formed. 
     As discussed above, according to a preferred embodiment of the present invention, a polarization beam splitter is fabricated based on tuning the transfer length of one polarization, for example, the TE mode, independent of the transfer length of the other polarization, e.g. the TM mode, by co-diffusing aluminum and titanium in a guided wave device structure. FIGS. 16a and 16b illustrate the basic principle of TE/TM mode separation. When power is launched in waveguide 1, complete power transfer occurs at the exit port of the waveguide 2 after a distance L s  =(2 m±1) L c . Similarly, for L s  =2 m L c , the power transfers back to port 1 where m is an integer and L c  is the coupling length defined as the length which provides complete power transfer. For the design of a TE/TM polarization beam splitter, launching both TE and TM polarizations at the input of waveguide 1 should yield separation of the two polarizations at the output. Under these conditions, L s  should satisfy the following condition: 
     
         L.sub.s =(2m-1)L.sub.TE =2m L.sub.TM                       Eq. (1) 
    
     For example, for m=1, L TE  =2L TM , which means that the transfer length of TE polarization is twice the TM polarization or vice-versa. When the condition of (1) is satisfied, one polarization is in the &#34;cross&#34; state and the other is in the &#34;bar&#34; state. In contrast, the case of a polarization insensitive coupler requires that L TE  =m L TM . In that case, the transfer length of one polarization is equal to or a multiple of the transfer length of the other polarization, where m is an odd integer. 
     Examples of the alternate embodiments of the beam splitter 24 (FIG. 7) were carried out for two configurations: 
     EXAMPLE I 
     In the first case, the aluminum is diffused fully across the substrate except for the titanium guides (FIG. 9). 
     EXAMPLE II 
     In the second case, the aluminum is diffused only in the coupling region of the coupler except for the titanium guides (FIG. 9a). 
     Results 
     EXAMPLE I 
     FIGS. 17a and 17b illustrate the coupling ratio `R` as a function of postbake time for 6 m and 4 mm beam splitters respectively. The coupling ratio `R` is defined as R=P b  /P a  where P a  and P b  are the measured powers at the output ports 19c and 19d respectively (FIG. 7). 
     The initial postbake was performed at 1030° C. for 8 hours followed by further postbaking at 950° C. At this temperature only aluminum is diffused further. 
     FIGS. 18a and 18b illustrate the extinction ratio in dB as a function of postbake hours for both TE and TM modes for 6 mm and 4 mm coupler respectively. From FIGS. 17a and 17b and 18a and 18b, it is seen, as postbaking continues, the coupling ratio of TM polarization increases rapidly and the coupling ratio of TE polarization decreases rapidly. For the 6 mm coupler, at 12.5 hours postbake time, the TE and TM modes are well separated with an extinction ratio of nearly 20 dB for both TE and TM polarizations. The maximum polarization splitting for the 4 mm coupler occurs in the region 10-11 hours postbake time. At this point, odd number transfer lengths of one polarization are equal to the even number transfer lengths of the other polarization. 
     FIG. 19 gives the mode size as a function of postbake time for TE and TM mode size both in the horizontal and vertical directions, as well as their Geometric Mean. Also shown is the Geometric Mean mode size of the input fiber. Optical confinement in the titanium waveguide is observed to be much better with aluminum diffusion. 
     EXAMPLE II 
     For this configuration, the initial postbake was also performed at 1030° C. for 8 hours and further postbake was done at 950° C., where only aluminum is diffused further. This is important so as not to impact mode matching conditions while fine tuning the coupler. FIG. 20 illustrates coupling ratio as a function of postbake time where aluminum is diffused only in the coupling region. The TE and TM modes behave in a similar way as in the previous case. The coupling ratio of the TM mode increases rapidly with postbake time and the coupling ratio of TE mode decreases rapidly, with a high extinction ratio of 15 dB for both TE and TM modes at 12 hours postbake time. 
     FIG. 21 shows the mode size as a function of postbake time for both TE and TM modes in the vertical and horizontal direction along with input fiber Geometric Mean. FIG. 22 gives the insertion loss in dB as a function of postbake time for both the cases where aluminum is diffused fully and as a strip. The insertion loss remains reasonably constant even up to 12 hours postbake time, indicating titanium guides were not affected during the postbake at 950° C., and good mode matching exists between the input fiber and the guides. 
     The extinction ratio can be further improved in different ways: 
     1) Incorporating polarizers in the output branches such as proton exchange polarizers and dielectric film polarizers can be used to extinguish TE and TM modes respectively. Incorporation of such polarizers can effectively produce 50 dB pure linear polarized outputs. 
     2) Pigtailing a few meters of polarizing fiber at the output port in orthogonal orientation can improve the extinction ratio well above 30 dB. 
     3) Combining both the techniques. 
     Equivalents 
     While this invention has been particularly shown and described with references to preferred embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the invention as defined by the appended claims.