Patent Publication Number: US-2022229231-A1

Title: Integrated environmentally insensitive modulator for interferometric gyroscopes

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of U.S. aplication Ser. No. 16/869,425, filed May 7, 2020, and titled “INTEGRATED ENVIRONMENTALLY INSENSITIVE MODULATOR FOR INTERFEROMETRIC GYROSCOPES,” which is hereby incorporated herein by reference. 
    
    
     BACKGROUND 
     Typically, interferometric fiber optic gyroscopes include an integrated optical circuit that performs three functions. In particular, the integrated optical circuit operates as a 50/50 optical coupler, a phase modulator, and a polarizer. The integrated optical circuit receives a single beam of light, polarizes the light (for example, using annealed proton exchanged portions of a waveguide), splits the polarized light (for example, using a Y-junction), and passes the light through phase modulators. To prevent environmental instability, titanium indiffused waveguides can be used for the phase modulators and stitched together with the annealed proton exchanged portions of the waveguide. 
     Integrated optical circuits for interferometric gyroscope are generally monolithic lithium niobate devices because lithium niobate is particularly well suited for performing phase modulation. In order to reduce size, weight, and cost of interferometric gyroscopes, it is desirable to increase the integration of functions into a single device (for example, the integrated optical circuit). However, lithium niobate does not lend itself to large scale integration of other functions due to the physical properties of the material. 
     SUMMARY 
     In an example, an integrated optical circuit includes a first substrate formed of a first material and a first waveguide formed of a second material and positioned on the first substrate. The first waveguide includes a plurality of branches and is configured to polarize light beams that propagate through the first waveguide. The integrated optical circuit further includes a second substrate formed of a third material, the second substrate coupled to or positioned on the first substrate. The integrated optical circuit further includes a plurality of straight waveguides formed in the second substrate, each of the plurality of straight waveguides optically coupled to a respective branch of the plurality of branches of the first waveguide. The integrated optical circuit further includes a plurality of electrodes positioned proximate to the plurality of straight waveguides, the plurality of electrodes configured to modulate the phase of light beams that propagate through the plurality of straight waveguides. 
    
    
     
       DRAWINGS 
       Understanding that the drawings depict only some embodiments and are not therefore to be considered limiting in scope, the exemplary embodiments will be described with additional specificity and detail using the accompanying drawings, in which: 
         FIG. 1  is a top view of an example integrated optical circuit; 
         FIGS. 2A-2B  are cross-sectional, side views of example integrated optical circuits; 
         FIG. 3  is a top view of an example integrated optical circuit; 
         FIGS. 4A-4C  are cross-sectional, side views of example integrated optical circuits; 
         FIG. 5  is a diagram of an example interferometric gyroscope; 
         FIG. 6  is a flow diagram of an example method of manufacturing an integrated optical circuit; and 
         FIG. 7  is a flow diagram of an example method of manufacturing an integrated optical circuit. 
     
    
    
     In accordance with common practice, the various described features are not drawn to scale but are drawn to emphasize specific features relevant to the example embodiments. 
     DETAILED DESCRIPTION 
     In the following detailed description, reference is made to the accompanying drawings that form a part hereof, and in which is shown by way of illustration specific illustrative embodiments. However, it is to be understood that other embodiments may be utilized and that logical, mechanical, and electrical changes may be made. Furthermore, the method presented in the drawing figures and the specification is not to be construed as limiting the order in which the individual steps may be performed. The following detailed description is, therefore, not to be taken in a limiting sense. 
     Systems and methods for an integrated optical circuit for interferometric gyroscopes are provided herein. The integrated optical circuit utilizes a first substrate with a first waveguide for polarization and splitting functions and utilizes a second substrate and second waveguides for phase modulation. The first waveguide and the second waveguides are optically coupled and the second substrate is either deposited on the first substrate or otherwise physically coupled to the first substrate. In some examples, the first waveguide can be formed from a material that is transparent at the operating wavelength (for example, silicon nitride, titanium dioxide, or silicon oxynitride) and the second waveguides can be formed from a material that has a non-zero second-order nonlinear coefficient (for example, lithium niobate or lithium tantalate). By using different substrates rather than a single monolithic substrate, smaller size and/or better integration of functionality can be achieved than with previous designs while maintaining a high level of performance. 
       FIG. 1  is a top view of an example integrated optical circuit  100 . In the example shown in  FIG. 1 , the integrated optical circuit  100  includes a first substrate  102  and a second substrate  104  coupled to or positioned on the first substrate  102 . A first waveguide  106  is formed on the first substrate  102  and second waveguides  112  are formed in the second substrate  104  and positioned proximate to electrodes  114 . 
     The first substrate  102  is formed of a first material. In some examples, the first material is silicon. In other examples, other similar materials can be used for the first substrate  102 . In the example shown in  FIG. 1 , a first waveguide  106  is positioned on the first substrate  102 . The first waveguide  106  is formed from a second material that is different than the first material. In some examples, the first waveguide  106  is formed of silicon nitride. In some examples, the first waveguide  106  is formed of another material, such as, for example, titanium dioxide, silicon oxynitride, or another material that is transparent at an operating wavelength of the integrated optical circuit (for example, 1550 nm). The second material used for the first waveguide  106  has a higher refractive index than the surrounding material, which includes the first substrate  102  and any material used for cladding. 
     In the example shown in  FIG. 1 , the first waveguide  106  includes an input section  108  and multiple branches  110  that form a Y-junction. The first waveguide  106  functions as a splitter for light that is input into the first waveguide  106  at the input section  108 . In some examples, the first waveguide  106  is configured to polarize light beams that propagate through the first waveguide  106 . In some examples, the dimensions (for example, width or height) of the first waveguide  106  can be selected to achieve the desired polarization of light propagating through the first waveguide  106 . In some examples, the dimensions (for example, width and height) of the first waveguide  106  are varied over the length of the input section  108  and/or the branches  110  of the first waveguide  106 . 
     In the example shown in  FIG. 1 , the second substrate  104  is coupled to or positioned on the first substrate  102 . The second substrate  104  is formed of a third material that is different from the first material and the second material, and the third material has particular non-linear electro-optic properties that are suitable for phase modulation for an interferometric gyroscope. In some examples, the third material is a lithium niobate. In other examples, the second substrate  104  is formed of lithium tantalate substrate or another material that has a non-zero second-order nonlinear coefficient. 
     In the example shown in  FIG. 1 , two straight waveguides  112  are formed in the second substrate  104  and each of the straight waveguides  112  is optically coupled to a respective branch  110  of the first waveguide  106 . In some examples, there is an intermediate stage configured to facilitate the optical signal transition between the branches  110  of the first waveguide  106  and the straight waveguides  112 . In some examples, the straight waveguides  112  are coupled to the branches  110  of the first waveguide  106  via an adiabatic coupling, which can minimize optical loss. In some examples, the straight waveguides  112  are titanium indiffused waveguides formed in the second substrate  104 . Since titanium is not mobile inside the second substrate (for example, lithium niobate or lithium tantalate), titanium indiffused waveguides are more environmentally stable than other types of waveguides (for example, proton exchanged waveguides). In other examples, a different type of waveguide can be formed or patterned in the second substrate  104  (for example, by in-diffusion and etching). 
     In the example shown in  FIG. 1 , electrodes  114  are positioned proximate to the straight waveguides  112  in the second substrate  104 . The features of the second substrate  104  are configured to modulate the phase of light propagating through the straight waveguides  112 . In some examples, the electrodes  114  are configured to modulate the phase of light beams that propagate through the straight waveguides  112  of the second substrate  104 . 
     As discussed below with respect to  FIGS. 6-7 , the integrated optical circuit  100  can be manufactured in different ways, which result is some different physical characteristics for the integrated optical circuit  100 . However, the operation of the integrated optical circuit  100  is similar regardless of the manufacturing used to make the integrated optical circuit  100 . 
       FIG. 2A  is a cross-sectional, side view of an example of the integrated optical circuit  100  shown in  FIG. 1  where the second substrate  104  is coupled to the first substrate  102 . In such examples, the first substrate  102  and the second substrate  104  are formed separately and physically coupled together. In some examples, the first substrate  102  and the second substrate  104  are bonded or coupled together using an adhesive. The example integrated optical circuit  200  shown in  FIG. 2A  can be manufactured using the method described with respect to  FIG. 6 . 
       FIG. 2B  is a cross-sectional, side view of an example of the integrated optical circuit  100  shown in  FIG. 1  where the second substrate  104  is positioned on the first substrate  102 . In such examples, the first substrate  102  is a common substrate for the first waveguide  106  and the second substrate  104  with the straight waveguides  112 . In some examples, the second substrate  104  is deposited or grown on the first substrate  102  and the straight waveguides  112  are formed in the second substrate  104  (for example, by in-diffusion and etching). The example integrated optical circuit  250  shown in  FIG. 2B  can be manufactured using the method described with respect to  FIG. 7 , which can be easier to manufacture compared to the example shown in  FIG. 2A . 
       FIG. 3  is a top view of an example integrated optical circuit  300 . In the example shown in  FIG. 3 , the integrated optical circuit  300  includes a first substrate  302  and a second substrate  304  coupled to or positioned on the first substrate  302 . A first waveguide  306  is formed on the first substrate  302  and straight waveguides  312  are formed in the second substrate  304  and positioned proximate to electrodes  314 . In the example shown in  FIG. 3 , the integrated optical circuit  300  further includes second straight waveguides  316 , which are formed on the first substrate  302  or a third substrate  305 . 
     The first substrate  302  is formed of a first material. In some examples, the first material is silicon. In other examples, other similar materials can be used for the first substrate  302 . In the example shown in  FIG. 3 , a first waveguide  306  is positioned on the first substrate  302 . The first waveguide  306  is formed from a second material that is different than the first material. In some examples, the first waveguide  306  is formed of silicon nitride. In some examples, the first waveguide  306  is formed of another material, such as, for example, titanium dioxide, silicon oxynitride, or another material that is transparent at an operating wavelength of the integrated optical circuit (for example,  1550  nm). The second material used for the first waveguide  306  has a higher refractive index than the surrounding material, which includes the first substrate  302  and any material used for cladding. 
     In the example shown in  FIG. 3 , the first waveguide  306  includes an input section  308  and multiple branches  310  that form a Y-junction. The first waveguide  306  functions as a splitter for light that is input into the first waveguide  306  at the input section  308 . In some examples, the first waveguide  306  is configured to polarize light beams that propagate through the first waveguide  306 . In some examples, the dimensions (for example, width or height) of the first waveguide  306  can be selected to achieve the desired polarization of light propagating through the first waveguide  306 . In some examples, the dimensions (for example, width and height) of the first waveguide  306  are varied over the length of the input section  308  and/or the branches  310  of the first waveguide  306 . 
     In the example shown in  FIG. 3 , the second substrate  304  is coupled to or positioned on the first substrate  302 . The second substrate  304  is formed of a third material that is different from the first material and the second material, and the third material has particular non-linear electro-optic properties that are suitable for phase modulation for an interferometric gyroscope. In some examples, the third material is a lithium niobate. In other examples, the second substrate  304  is formed from lithium tantalate substrate or another material that has a non-zero second-order nonlinear coefficient. 
     In the example shown in  FIG. 3 , two straight waveguides  312  are formed in the second substrate  304  and each of the straight waveguides  312  is optically coupled to a respective branch  310  of the first waveguide  306 . In some examples, the straight waveguides  312  are coupled to the branches  310  of the first waveguide  306  via an adiabatic coupling, which can minimize optical loss. In some examples, there is an intermediate stage configured to facilitate the optical signal transition between the branches  310  of the first waveguide  306  and the straight waveguides  312 . In some examples, the straight waveguides  312  are titanium indiffused waveguides formed in the second substrate  304 . Since titanium is not mobile inside the second substrate (for example, lithium niobate or lithium tantalate), titanium indiffused waveguides are more environmentally stable than other types of waveguides (for example, proton exchanged waveguides). In other examples, a different type of waveguide can be formed or patterned in the second substrate  304  (for example, by in-diffusion and etching). 
     In the example shown in  FIG. 3 , electrodes  314  are positioned proximate to the straight waveguides  312  in the second substrate  304 . The features of the second substrate  304  are configured to modulate the phase of light propagating through the straight waveguides  312 . In some examples, the electrodes  314  are configured to modulate the phase of light beams that propagate through the straight waveguides  312  of the second substrate  304 . 
     In the example shown in  FIG. 3 , the integrated optical circuit  300  includes additional straight waveguides  316  that are positioned on either the first substrate  302  or a third substrate  305  that is separate from the first substrate  302  and the second substrate  304 . Each of the additional straight waveguides  316  is optically coupled to a respective straight waveguide  312  formed in the second substrate  304 . In some examples, there is an intermediate stage configured to facilitate the optical signal transition between the straight waveguides  312  of the second substrate  304  and the additional straight waveguides  316 . The additional straight waveguides  316  are used to fine tune the polarization of the light after phase modulation. In some examples, the dimensions (for example, width or height) of the additional straight waveguides  316  can be selected to achieve the desired polarization of light propagating through the additional straight waveguides  316 . In some examples, the dimensions (for example, width and height) of the additional straight waveguides  316  are varied over the length of the additional straight waveguides  316 . 
     As discussed below with respect to  FIGS. 6-7 , the integrated optical circuit  300  can be manufactured in different ways, which result is some different physical characteristics for the integrated optical circuit  300 . However, the operation of the integrated optical circuit  300  is similar regardless of the manufacturing used to make the integrated optical circuit  300 . 
       FIG. 4A  is a cross-section view of an example of an integrated optical circuit  300  shown in  FIG. 3  where the first substrate  302  is coupled to the second substrate  304  and the second substrate  304  is coupled to a third substrate  305 . In such examples, the first substrate  302 , the second substrate  304 , and the third substrate  305  are formed separately and physically coupled together. In some examples, the first substrate  302 , the second substrate  304 , and the third substrate  305  are bonded or coupled together using an adhesive. The example integrated optical circuit  400  shown in  FIG. 4A  can be manufactured using the method described with respect to  FIG. 6 . 
     In the example shown in  FIG. 4A , the additional straight waveguides  316  are positioned on the third substrate  305  that is separate from the first substrate  302  and the second substrate  304 . In such examples, the third substrate  305  can be formed from the same material as the first substrate  302  (first material) or a fourth material suitable as a substrate that is different than the other materials discussed above. In some such examples, the additional straight waveguides  316  are formed from the same material as the first waveguide  306  (second material) or from a different material that has a higher refractive index than surrounding materials including the third substrate  305  and any cladding material. In some examples, the additional straight waveguides  316  can be formed from silicon nitride, titanium dioxide, silicon oxynitride, or another material that is transparent at an operating wavelength of the integrated optical circuit (for example, 1550 nm). 
       FIG. 4B  is a cross-section view of an alternative example of the integrated optical circuit  300  shown in  FIG. 3  where the second substrate  304  is positioned on the first substrate  302  and the first substrate  302  is a common substrate for the first waveguide  306 , the second substrate  304  with the straight waveguides  312 , and the additional straight waveguides  316 . In some examples, the second substrate  304  is deposited or grown on the first substrate  302  and the first set of straight waveguides  312  are formed in the second substrate  304  (for example, by in-diffusion and etching). The example integrated optical circuit  425  shown in  FIG. 4B  can be manufactured using the method described with respect to  FIG. 7 , which can be easier to manufacture compared to the example shown in  FIG. 4A . 
     In the example shown in  FIG. 4B , the additional straight waveguides  316  are positioned on the first substrate  302 . In such examples, the additional straight waveguides  316  can be formed from the same material as the first waveguide  306  (the second material) or from a different material that has a higher refractive index than surrounding materials including the first substrate  302  and any cladding material. In some examples, the additional straight waveguides  316  can be formed from silicon nitride, titanium dioxide, silicon oxynitride, or another material that is transparent at an operating wavelength of the integrated optical circuit (for example, 1550 nm). 
       FIG. 4C  is a cross-section view of an alternative example of the integrated optical circuit  300  shown in  FIG. 3  where the second substrate  304  is positioned on the first substrate  302 , but the first substrate  302  is not a common substrate for the additional straight waveguides  316 . In some examples, the second substrate  304  is deposited or grown on the first substrate  302  and the first set of straight waveguides  312  are formed in the second substrate  304  (for example, by in-diffusion and etching). In such examples, the first substrate  302  and the third substrate  305  are formed separately and physically coupled together. In some examples, the first substrate  302  and the third substrate  305  are bonded or coupled together using an adhesive. The example integrated optical circuit  450  shown in  FIG. 4B  can be manufactured using a hybrid of the methods described with respect to  FIGS. 6-7 . 
     In the example shown in  FIG. 4C , the additional straight waveguides  316  are positioned on the third substrate  305  that is separate from the first substrate  302  and the second substrate  304 . In such examples, the third substrate  305  can be formed from the same material as the first substrate  302  (first material) or a fourth material suitable as a substrate that is different than the other materials discussed above. In some such examples, the additional straight waveguides  316  are formed from the same material as the first waveguide  306  (second material) or from a different material that has a higher refractive index than surrounding materials including the third substrate  305  and any cladding material. In some examples, the additional straight waveguides  316  can be formed from silicon nitride, titanium dioxide, silicon oxynitride, or another material that is transparent at an operating wavelength of the integrated optical circuit (for example, 1550 nm). 
     Typically, the front-end of an interferometric fiber optic gyroscope includes a light source that is separate from the integrated optical circuit and an optical fiber is used to couple the light into the waveguides of the integrated optical circuit. Further, additional components that are beneficial for monitoring and controlling operation of the integrated optical circuit and interferometric gyroscope are also separate from the integrated optical circuit. By having these additional components separate from the integrated optical circuit, the previous designs can have significant disadvantages from a size, weight, and cost perspective. 
     By using multiple substrates and different materials for the waveguides as discussed above, the integrated optical circuits  100 ,  300  can include a greater amount of integration on the integrated optical circuits  100 ,  300  (for example, more components on the integrated optical circuit) and reduce the size, weight, and cost of the integrated optical circuits  100 ,  300  and/or an interferometric gyroscope that includes the integrated optical circuits  100 ,  300 . For example, by using a different material to form the first waveguide  106 ,  306  (for example, silicon nitride) than the material used for phase modulation (for example, lithium niobate or lithium tantalate), the first waveguide  106 ,  306  can have significantly better mode confinement than previous designs and the size of the first waveguide  106 ,  306  can be reduced without degraded performance. In some examples, the length of the input section  108 ,  308  of the first waveguide  106 ,  306  could be shortened and the angle of the Y-junction can be made steeper. 
     In some examples, the integrated optical circuits  100 ,  300  include an integrated light source (not shown) mounted to the first substrate  102 ,  302 , and the integrated light source is optically coupled to the input section  108 ,  308  of the first waveguide  106 ,  306 . In some examples, the integrated light source is a semiconductor or solid-state laser light source configured to provide the light signal for the integrated optical circuit  100 ,  300 . Significant reductions in size, weight, and cost can be achieved over previous designs by using an integrated optical circuit with an integrated light source. 
     In some examples, the integrated optical circuits  100 ,  300  include one or more integrated tap couplers (not shown) configured to couple light from the first waveguide  106 ,  306 . In some examples, the one or more integrated tap couplers are configured to couple a portion of light from the first waveguide  106 ,  306  and provide the coupled light to another component on the integrated optical circuit  100 ,  300 . In some examples, the one or more integrated tap couplers are configured to provide the coupled light to one or more integrated photodetectors on the integrated optical circuit  100 ,  300 , which can be mounted or disposed on the first substrate  102 ,  302 . In some examples, the one or more integrated photodetectors are photodiodes. In some examples, one or more integrated tap couplers are configured to provide light to a rate detector. 
       FIG. 5  is a block diagram of an example interferometric gyroscope  500 . In the example shown in  FIG. 5 , the interferometric gyroscope  500  includes a light source  502 , a coupler  504 , an integrated optical circuit  506 , and a sensing coil  508 . While the integrated optical circuit  506  shown in  FIG. 5  corresponds to the integrated optical circuit  100  shown in  FIG. 1 , it should be understood that the integrated optical circuit  506  can comprise any of the integrated optical circuits  100 ,  300  discussed above with respect to  FIGS. 1-4C . 
     The light source  502  is configured to generate a light signal that is to be coupled into the sensing coil  508 . In some examples, the light source  502  is a broadband light source configured to generate a light signal that is comprised of many waves with different wavelengths and polarization states. In the example shown in  FIG. 5 , the light source  502  is separate from the integrated optical circuit  506 . In other examples, the light source  502  is integrated on the integrated optical circuit  506  as discussed above. 
     In the example shown in  FIG. 5 , the coupler  504  is included between the light source  502  and the integrated optical circuit  506 . In examples where the light source  502  is separate from the integrated optical circuit  506 , the light source  502  is optically coupled to the integrated optical circuit  506  using one or more optical fibers and the coupler  504 . For example, the optical fiber optically coupled to the light source  502  can extend through the coupler  504  and be optically coupled to a waveguide of the integrated optical circuit  506  (for example, the first waveguide  106 ,  306  as discussed above with respect to  FIGS. 1-4C ). In some examples, the coupler  504  includes both the optical fiber to optically couple the light source to the integrated optical circuit and an output optical fiber configured to carry a returned signal from the phase modulators to a rate detector that reads the returning signal from the sensing coil  508 . 
     In examples where the light source  502  is integrated on the integrated optical circuit  506 , the light source  502  itself is optically coupled to a waveguide of the integrated optical circuit  506  (for example, the first waveguide  106 ,  306  as discussed above with respect to  FIGS. 1-4C ). In such examples, the coupler  504  is also integrated on the integrated optical circuit  506 . In some examples, the integrated coupler  504  can be implemented using a tap coupler as discussed above. 
     In the example shown in  FIG. 5 , the sensing coil  508  is optically coupled to the integrated optical circuit  506 . In some examples, the sensing coil  508  is optically coupled to the straight waveguides (for example, waveguides  112 ,  312 , or  316  as discussed above with respect to  FIGS. 1-4C ) of the integrated optical circuit  506  using pigtail fibers. The sensing coil  508  is configured to receive light signals from the integrated optical circuit  506  and to output light signals to the integrated optical circuit  506 . 
       FIG. 6  is a flow diagram for a method  600  of manufacturing an integrated optical circuit. The functions, structures, and other description of liked-named elements for such examples described herein may apply to like-named elements described with reference to the method  600  and vice versa. 
     The method  600  includes depositing a waveguide material on a first substrate and patterning a first waveguide (block  602 ). The waveguide material can be deposited and patterned using known thin film techniques. In some examples, the first waveguide has an input section and multiple branches similar to the first waveguides discussed above with respect to  FIGS. 1-4C . In some examples, the first substrate is formed from a first material (for example, silicon) and the waveguide material is different than the first material. In some examples, the waveguide material is silicon nitride. In some examples, the waveguide material is formed of another material, such as, for example, titanium dioxide, silicon oxynitride, or other material that is transparent at an operating wavelength of the integrated optical circuit (for example, 1550 nm). The waveguide material has a higher refractive index than the surrounding material, which includes the first substrate and any material used for cladding. 
     The method  600  further includes forming straight waveguides in a second substrate (block  604 ). In some examples, the second substrate is formed of a third material that is different from the first material and the second material, and the third material has particular non-linear electro-optic properties that are suitable for phase modulation for an interferometric gyroscope. In some examples, the third material is a lithium niobate. In other examples, the third material is lithium tantalate or another material that has a non-zero second-order nonlinear coefficient. In some examples, the straight waveguides are formed by diffusing titanium waveguide material into the second substrate. In some examples, the straight waveguides are formed by in-diffusion and etching (for example, deep ion etching). 
     The method  600  further includes physically coupling the first substrate and the second substrate and optically coupling the first waveguide and the straight waveguides (block  606 ). In some examples, the first substrate and the second substrate are bonded or coupled together using an adhesive. In some examples, the branches of the first waveguide are optically coupled to the straight waveguides formed in the second substrate via an adiabatic coupling. In some examples, there is an intermediate stage configured to facilitate the optical signal transition between the branches of the first waveguide and the straight waveguides. 
     The method  600  optionally includes depositing a waveguide material on a third substrate and patterning additional straight waveguides on the third substrate (block  608 ). The waveguide material can be deposited and patterned using known thin film techniques. In some examples, the third substrate is formed from the same material as the first substrate (first material) or a fourth material suitable as a substrate that is different than the other materials discussed above. In some such examples, the additional straight waveguides are formed from the same material as the first waveguide (second material) or from a different material that has a higher refractive index than surrounding materials including the third substrate and any cladding material. In some examples, the additional straight waveguides can be formed from silicon nitride, titanium dioxide, silicon oxynitride, or another material that is transparent at an operating wavelength of the integrated optical circuit (for example, 1550 nm). 
     The method  600  optionally includes physically coupling the second substrate and the third substrate and optically coupling the straight waveguides of the second substrate and the additional straight waveguides of the third substrate (block  610 ). In some examples, the second substrate and the third substrate are bonded or coupled together via an adhesive. In some examples, the straight waveguides of the second substrate and the additional straight waveguides of the third substrate are optically coupled via an adiabatic coupling. In some examples, there is an intermediate stage configured to facilitate the optical signal transition between the straight waveguides formed in the second substrate and the additional straight waveguides on the third substrate. 
       FIG. 7  is a flow diagram of a method  700  of manufacturing an integrated optical circuit. The functions, structures, and other description of liked-named elements for such examples described herein may apply to like-named elements described with reference to the method  700  and vice versa. 
     The method  700  includes depositing waveguide material on a first substrate and patterning a first waveguide (block  702 ). The waveguide material can be deposited and patterned using known thin film techniques. In some examples, the first waveguide has an input section and multiple branches similar to the first waveguides discussed above with respect to  FIGS. 1-4C . In some examples, the first substrate is formed from a first material (for example, silicon) and the waveguide material is different than the first material. In some examples, the waveguide material is silicon nitride. In some examples, the waveguide material is formed of another material, such as, for example, titanium dioxide, silicon oxynitride, or another material that is transparent at an operating wavelength of the integrated optical circuit (for example, 1550 nm). The waveguide material has a higher refractive index than the surrounding material, which includes the first substrate and any material used for cladding. 
     The method  700  further includes depositing or growing a second substrate on a portion of the first substrate (block  704 ). In some examples, the second substrate is grown on the first substrate using molecular beam epitaxy (MBE) or other thin film techniques. 
     The method  700  further includes forming straight waveguides in the second substrate (block  706 ). In some examples, the second substrate is formed of a third material that is different from the first material and the second material, and the third material has particular non-linear electro-optic properties that are suitable for phase modulation for an interferometric gyroscope. In some examples, the third material is a lithium niobate. In other examples, the third material is lithium tantalate or another material that has a non-zero second-order nonlinear coefficient. In some examples, the straight waveguides are formed by diffusing titanium waveguide material into the second substrate. In some examples, the straight waveguides are formed by in-diffusion and etching (for example, deep ion etching). 
     The method  700  optionally includes depositing waveguide material on the first substrate and patterning additional straight waveguides on the first substrate (block  708 ). The waveguide material can be deposited and patterned using known thin film techniques. In some examples, the additional straight waveguides are similar to the additional straight waveguides discussed above with respect to  FIGS. 3-4C . In some examples, the additional straight waveguides are formed from the same material as the first waveguide (second material) or from a different material that has a higher refractive index than the first substrate and any cladding material. In some examples, the additional straight waveguides can be formed from silicon nitride, titanium dioxide, silicon oxynitride, or other material that is transparent at an operating wavelength of the integrated optical circuit (for example, 1550 nm). 
     In various aspects, system elements, method steps, or examples described throughout this disclosure may be implemented on one or more computer systems, field programmable gate array (FPGA), application specific integrated circuit (ASIC) or similar devices comprising hardware executing code to realize those elements, processes, or examples, said code stored on a non-transient data storage device. These devices include or function with software programs, firmware, or other computer readable instructions for carrying out various methods, process tasks, calculations, and control functions, used for synchronization and fault management in a distributed antenna system. 
     These instructions are typically stored on any appropriate computer readable medium used for storage of computer readable instructions or data structures. The computer readable medium can be implemented as any available media that can be accessed by a general purpose or special purpose computer or processor, or any programmable logic device. Suitable processor-readable media may include storage or memory media such as magnetic or optical media. For example, storage or memory media may include conventional hard disks, Compact Disk - Read Only Memory (CD-ROM), volatile or non-volatile media such as Random Access Memory (RAM) (including, but not limited to, Synchronous Dynamic Random Access Memory (SDRAM), Double Data Rate (DDR) RAM, RAIVIBUS Dynamic RAM (RDRAM), Static RAM (SRAM), etc.), Read Only Memory (ROM), Electrically Erasable Programmable ROM (EEPROM), and flash memory, etc. Suitable processor-readable media may also include transmission media such as electrical, electromagnetic, or digital signals, conveyed via a communication medium such as a network and/or a wireless link. 
     The methods and techniques described here may be implemented in digital electronic circuitry, or with a programmable processor (for example, a special-purpose processor or a general-purpose processor such as a computer) firmware, software, or in combinations of them. Apparatus embodying these techniques may include appropriate input and output devices, a programmable processor, and a storage medium tangibly embodying program instructions for execution by the programmable processor. A process embodying these techniques may be performed by a programmable processor executing a program of instructions to perform desired functions by operating on input data and generating appropriate output. The techniques may advantageously be implemented in one or more programs that are executable on a programmable system including at least one programmable processor coupled to receive data and instructions from, and to transmit data and instructions to, a data storage system, at least one input device, and at least one output device. Generally, a processor will receive instructions and data from a read-only memory and/or a random-access memory. Storage devices suitable for tangibly embodying computer program instructions and data include all forms of non-volatile memory, including by way of example semiconductor memory devices, such as EPROM, EEPROM, and flash memory devices; magnetic disks such as internal hard disks and removable disks; magneto-optical disks; and DVD disks. Any of the foregoing may be supplemented by, or incorporated in, specially-designed application-specific integrated circuits (ASICs). 
     Example Embodiments 
     Example 1 includes an integrated optical circuit, comprising: a first substrate formed of a first material; a first waveguide formed of a second material and positioned on the first substrate, wherein the first waveguide includes a plurality of branches and is configured to polarize light beams that propagate through the first waveguide; a second substrate formed of a third material, wherein the second substrate is coupled to or positioned on the first substrate; a plurality of first straight waveguides formed in the second substrate, wherein each of the plurality of first straight waveguides is optically coupled to a respective branch of the plurality of branches of the first waveguide; and a plurality of electrodes positioned proximate to the plurality of first straight waveguides, wherein the plurality of electrodes is configured to modulate the phase of light beams that propagate through the plurality of first straight waveguides. 
     Example 2 includes the integrated optical circuit of Example 1, wherein the second material is transparent at an operating wavelength of the integrated optical circuit. 
     Example 3 includes the integrated optical circuit of Example 2, wherein the second material is silicon nitride, titanium dioxide, or silicon oxynitride. 
     Example 4 includes the integrated optical circuit of any of Examples 1-3, wherein the third material has a non-zero second-order nonlinear coefficient. 
     Example 5 includes the integrated optical circuit of any of Examples 1-4, wherein the third material is lithium niobate or lithium tantalate. 
     Example 6 includes the integrated optical circuit of any of Examples 1-5, further comprising: a plurality of second straight waveguides positioned on the first substrate, wherein each second straight waveguide of the plurality of second straight waveguides is optically coupled to a respective first straight waveguide of the plurality of first straight waveguides, wherein the plurality of first straight waveguides is positioned between the first waveguide and the plurality of second straight waveguides; wherein each respective second straight waveguide of the plurality of second straight waveguides is configured to polarize light beams that propagate through the respective second straight waveguide. 
     Example 7 includes the integrated optical circuit of any of Examples 1-5, further comprising: a plurality of second straight waveguides positioned on a third substrate, wherein each second straight waveguide of the plurality of second straight waveguides is optically coupled to a respective first straight waveguide of the plurality of first straight waveguides, wherein the plurality of first straight waveguides is positioned between the first waveguide and the plurality of second straight waveguides; wherein each respective second straight waveguide of the plurality of second straight waveguides is configured to polarize light beams that propagate through the respective second straight waveguide. 
     Example 8 includes the integrated optical circuit of any of Examples 1-7, further comprising: a light source mounted on the first substrate, wherein the light source is optically coupled to the first waveguide and configured to generate a light signal. 
     Example 9 includes the integrated optical circuit of any of Examples 1-8, wherein the second substrate is deposited or grown on the first substrate, wherein the plurality of first straight waveguides is formed by in-diffusion and etching. 
     Example 10 includes the integrated optical circuit of any of Examples 1-9, wherein the second substrate is bonded or coupled to the first substrate via an adhesive. 
     Example 11 includes an interferometric gyroscope, comprising: an integrated optical circuit, comprising: a first substrate formed of a first material; a first waveguide formed of a second material and positioned on the first substrate, wherein the first waveguide includes a plurality of branches and is configured to polarize light beams that propagate through the first waveguide; a second substrate formed of a third material, wherein the second substrate is coupled to or positioned on the first substrate; a plurality of first straight waveguides formed in the second substrate, wherein each of the plurality of first straight waveguides is optically coupled to a respective branch of the plurality of branches of the first waveguide; and a plurality of electrodes positioned proximate to the plurality of first straight waveguides, wherein the plurality of electrodes is configured to modulate the phase of light beams that propagate through the plurality of first straight waveguides; a light source configured to generate a light signal, wherein the light source is optically communicatively coupled to the first waveguide of the integrated optical circuit; and a sensing coil configured to receive signals from the integrated optical circuit and output signals to the integrated optical circuit. 
     Example 12 includes the interferometric gyroscope of Example 11, wherein the second material is silicon nitride, titanium dioxide, or silicon oxynitride. 
     Example 13 includes the interferometric gyroscope of any of Examples 11-12, wherein the third material is lithium niobate or lithium tantalate. 
     Example 14 includes the interferometric gyroscope of any of Examples 11-13, wherein the integrated optical circuit further comprises: a plurality of second straight waveguides positioned on the first substrate, wherein each second straight waveguide of the plurality of second straight waveguides is optically coupled to a respective first straight waveguide of the plurality of first straight waveguides, wherein the plurality of first straight waveguides is positioned between the first waveguide and the plurality of second straight waveguides; wherein each respective second straight waveguide of the plurality of second straight waveguides is configured to polarize light beams that propagate through the respective second straight waveguide. 
     Example 15 includes the interferometric gyroscope of any of Examples 11-13, further comprising: a plurality of second straight waveguides positioned on a third substrate, wherein each second straight waveguide of the plurality of second straight waveguides is optically coupled to a respective first straight waveguide of the plurality of first straight waveguides, wherein the plurality of first straight waveguides is positioned between the first waveguide and the plurality of second straight waveguides; wherein each respective second straight waveguide of the plurality of second straight waveguides is configured to polarize light beams that propagate through the respective second straight waveguide. 
     Example 16 includes the interferometric gyroscope of any of Examples 11-15, wherein the light source is mounted on the first substrate. 
     Example 17 includes the interferometric gyroscope of any of Examples 11-16, wherein the second substrate is deposited or grown on the first substrate, wherein the plurality of first straight waveguides is formed by in-diffusion and etching. 
     Example 18 includes the interferometric gyroscope of any of Examples 11-17, wherein the second substrate is bonded or coupled to the first substrate via an adhesive. 
     Example 19 includes an integrated optical circuit, comprising: a first substrate formed of a first material; a first waveguide formed of a second material and positioned on the first substrate, wherein the first waveguide includes a plurality of branches and is configured to polarize light beams that propagate through the first waveguide; a second substrate formed of a third material, wherein the second substrate is coupled to or positioned on the first substrate; a plurality of first straight waveguides formed in the second substrate, wherein each of the plurality of first straight waveguides is optically coupled to a respective branch of the plurality of branches of the first waveguide; a plurality of electrodes positioned proximate to the plurality of first straight waveguides, wherein the plurality of electrodes is configured to modulate the phase of light beams that propagate through the plurality of first straight waveguides; and a plurality of second straight waveguides formed of the second material and positioned on the first substrate or a third substrate, wherein each second straight waveguide of the plurality of second straight waveguides is optically coupled to a respective first straight waveguide of the plurality of first straight waveguides, wherein the plurality of first straight waveguides is positioned between the first waveguide and the plurality of second straight waveguides, wherein each respective second straight waveguide of the plurality of second straight waveguides is configured to polarize light beams that propagate through the respective second straight waveguide. 
     Example 20 includes the integrated optical circuit of Example 19, wherein the second material is silicon nitride, titanium dioxide, or silicon oxynitride; and wherein the third material is lithium niobate or lithium tantalate. 
     Although specific embodiments have been illustrated and described herein, it will be appreciated by those of ordinary skill in the art that any arrangement, which is calculated to achieve the same purpose, may be substituted for the specific embodiments shown. Therefore, it is manifestly intended that this invention be limited only by the claims and the equivalents thereof.