Patent Publication Number: US-2005123244-A1

Title: Embedded optical waveguide coupler

Description:
All rights in connection with this application are assigned to Intel Corporation.  
      This application relates to devices having optical waveguides, and more particularly, to integrated devices and circuits having optical waveguides fabricated on substrates such as semiconductor substrates.  
      Optical waveguides are optical devices for spatially confining and guiding optical signals. An optical waveguide may be formed, for example, by surrounding a high-index waveguide core with one or more low-index waveguide cladding regions, to guide the light along the waveguide core. For example, optical fiber is a waveguide with a cylindrical fiber core surrounded by cylindrical fiber cladding.  
      Optical waveguides may be used in a wide range of devices and applications. For example, an integrated optical or opto-electronic device may be constructed by integrating optical waveguides and other device components on a substrate.  
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       FIGS. 1, 2A ,  2 B, and  2 C are various views of one exemplary waveguide coupler according to one implementation.  
       FIG. 3  shows another view of the waveguide coupler shown in  FIGS. 1, 2A ,  2 B, and  3 C and illustrates the mode transformation.  
       FIG. 4  shows a waveguide coupler according to another implementation and corresponding mode transformation.  
       FIGS. 5A, 5B ,  5 C,  5 D, and  5 E show an exemplary fabrication flow for fabricating the waveguide couplers  100  and  400  in  FIGS. 1 and 4 .  
       FIG. 6  illustrates a photonic chip that implements waveguide couplers described in this application.  
    
    
     DETAILED DESCRIPTION  
      Techniques and devices described in this application include waveguide couplers to efficiently couple light from one waveguide to another with different cross sections, including but not limited to waveguides in compact integrated packages fabricated on substrates. Such coupling between different waveguides may be generally used as an optical interface between optical devices having waveguides of different cross sections or as an optical focusing mechanism to change the cross section of light.  
      As a specific example, many photonic integrated circuits (ICs) use ridge or embedded channel waveguides on a substrate to guide light between different components integrated on the same substrate. Waveguides with different cross sections may be used in such a photonic IC and a waveguide coupler described in this application may be used to connect two different waveguides. Also, an on-chip waveguide at an input/output (I/O) port may have a cross section different from that of a waveguide external to the photonic IC that either supplies an optical input to the IC or receives an optical output from the IC. A waveguide coupler of this application, therefore, may be implemented as part of the I/O port of such a photonic IC to connect the external waveguide. In an application where a photonic IC may be coupled to an external fiber link, the cross section of the fiber core (e.g., around 8-9 microns) may be greater than the cross section of the on-chip waveguide core which may be a fraction of one micron in a high-index-contrast design such as a silicon core in a silicon oxide cladding. In addition, such a waveguide coupler may be implemented in an optical path on the chip to change the cross section of light and to allow for efficient optical coupling between different parts of an optical path, e.g., different optical elements or devices.  
      Waveguide couplers of this application generally use a transitional structure between two waveguides with a spatially varying cross section profile along the direction of optical propagation to gradually transform the mode of guided light from one waveguide to the other waveguide. This gradual transformation is “gradual” in the sense that the mode of the guided light adiabatically changes as it passes through the transitional structure. This requirement of adiabatic change reduces or minimizes the optical loss caused by the change of the guided mode. One way to meet this adiabatic requirement is that the transitional structure has an extended length so that the cross section changes gradually over this extended length.  
      The use of this extended length of the transitional structure, however, is undesirable in integrated photonic ICs because the extended length of the transitional structure becomes a barrier to miniaturizing the circuits. In photonic ICs, it is desirable that the length of this transitional structure be as small as possible to make the waveguide coupler compact and small because, like electronic IC counterparts, each component on photonic ICs should be minimized in order to integrate a large number of functionalities on a given real estate of a chip. Examples of waveguide couplers described in this application are specially structured to provide a strong lateral spatial confinement in the waveguide couplers and thus to reduce the length of transitional structure while still maintaining the optical adiabatic condition.  
      In one implementation, such an optical coupler may include a substrate to support a mesa, a first waveguide formed on the mesa and having one tapered end section which adiabatically transforms an optical mode guided in the first waveguide, and a second waveguide formed on the substrate and having a cross section larger than the first waveguide and a refractive index less than the first waveguide. The second waveguide has one waveguide section in which the first waveguide and said mesa are conformingly embedded to place the first waveguide near a center of the second waveguide.  
      In another implementation, an optical coupler may include a cladding layer having a mesa, a first waveguide core, and a second waveguide core. The index of the first waveguide core is greater than the cladding layer. The first waveguide core is formed on the mesa and has a tapered end section to adiabatically transform a mode of guided light. The second waveguide core has a cross section greater than a cross section of and an index less than an index of the first waveguide core. The second waveguide core is formed over the cladding layer and the first waveguide core to have a solid section and a hollow section. The hollow section has an opening to conformingly enclose the tapered end section and the mesa to surround the tapered end section by the mesa and the second waveguide core.  
       FIG. 1  shows an exemplary input waveguide coupler  100  integrated on a substrate  110  to interconnect a large waveguide  120  and a small waveguide  130 . The substrate  110  may be a suitable semiconductor material such as Si, GaAs, and InP, or a non-semiconductor material such as quartz, glass, and polymer (e.g., polyimide, polycarbonate, and polymethyl methacrylate (PMMA)) materials. The large waveguide  120  has a solid section to receive and guide an input optical beam  101  and a hollow section in which at least a part of the small waveguide  130  is embedded. The large waveguide  120  is the waveguide core and its cladding is formed by air or a low-index dielectric medium above the substrate  110 . The embedded portion of the small waveguide  130  are in contact with and conforms to contacted inner surfaces of the hollow section of the large waveguide  120 . The refractive index of the large waveguide  120  is designed to be less than that of the small waveguide  130  so that the hollow section of the large waveguide  120  in effect becomes the waveguide cladding of a waveguide core formed by the embedded portion of the small waveguide  130 . Both waveguides  120  and  130  may be singe-mode waveguides with different cross sections. The low index waveguide  120  may coexist with high index waveguide  130  as the cladding of the waveguide  130 . Alternatively, the waveguide  120  may partially cover the waveguide  130  and terminate at a location where the light is transformed from the fundamental mode of the large waveguide  120  to the fundamental mode of the small waveguide  130 .  
      The large waveguide  120  may be implemented with different materials, including fluorinated polyimide, acrylate, PMMA, PolySiloxane, silicon oxynitride, titanium oxide, glass and others. The refractive index of the large waveguide  120  may be typically set between about 1.4 and about 1.6. The small waveguide  130  has an index higher than that of the large waveguide  120 . Exemplary materials for the small waveguide  130  include Si, amorphous Si, silicon nitride, titanium oxide, silicon carbide and others.  
      The substrate  110  is a dielectric material with a refractive index less than the index of the waveguide  130  and operates as a part of the cladding for the waveguides  130  and  120 . The index of the substrate  110  is preferably less than that of the waveguide  120  and may be close or equal to the index of the waveguide  120 . In some implementations, the substrate  110  may include a support substrate and a low index cladding layer on the top of the support substrate. In other implementations, the substrate  110  is used both as a support substrate and a low-index waveguide cladding layer. In one implementation, for example, the substrate  110  may include a silicon oxide cladding layer on a silicon substrate, the high-index waveguide  130  may be silicon, and the low-index waveguide  130  may be a polymer. The index contrast for the waveguide  130  may be higher than that for the waveguide  120 .  
       FIG. 2A  is a top view of the waveguide coupler  100  to show additional structural details. As illustrated in  FIGS. 1 and 2 A, the embedded portion of the small waveguide  130  has a tapered section  134  with a tip  135  and a straight section  133 . The tapered section  134  begins at the tip  135  and gradually increases its cross section. The end of the tapered section  134  conforms to the cross section of the straight section  133 . This embedded portion of the waveguide is in contact with and conforms to the a part of the inner surfaces of the hollow section of the waveguide  120 . Accordingly, the inner part of the hollow section of the large waveguide  120  includes a corresponding tapered hollow section conformingly in contact with the tapered section  134  and a straight hollow section conformingly in contact with the embedded straight section  133 .  FIGS. 2B and 2C  show two cross sectional views taken along the lines BB and CC as marked in  FIG. 2A , respectively, to show the solid section and the hollow section of the large waveguide  120 .  
      Notably,  FIGS. 1 and 2 C show that the hollow section of the large waveguide  120  is deeper than the height of the small waveguide to include a low-index mesa  112  with a height, H, underneath the small waveguide  133  and protruded above the substrate  110 . The shape of the mesa  112  conforms to the shape of the small waveguide  130  by having a straight mesa section  113  corresponding to the straight section  133  and a tapered mesa section  114  corresponding to the tapered section  134 . Hence, the shape of the small waveguide  130  show in  FIG. 2A  is the shape of the mesa  112 . The refractive index of the mesa  112  is designed to be less than that of the small waveguide  130  so that the mesa  112  forms a part of the waveguide cladding for the embedded portion of the small waveguide  130 . The index of the small waveguide  130  is much higher than the indices of the large waveguide  120  and the mesa  112 . Hence, this structure forms a high-index-contrast waveguide in the embedded section. In particular, the presence of the mesa  112  allows the embedded portion of the small waveguide  130  to deeply “bury” within the large waveguide  120 . Hence, the high-index core formed by the embedded part of the small waveguide  130  is in close proximity to the center of the low index mode distribution. This structure strongly confines the guided light in the embedded small waveguide  130  and provides highly efficient coupling from large waveguide  120  to the small waveguide  130 . Simulations based on the coupled mode equations verified this enhancement. This structure can achieve the desired optically adiabatic condition with a small length of the tapered section  134 . In addition, this efficient coupling allows for reduction of the power requirements for the off-chip light source.  
      In operation, the above waveguide coupler  100  may operate to couple light from the large waveguide  120  to the small waveguide  130 . Light is coupled between two waveguides  120  and  130  by both evanescent coupling and “butt coupling.” The relative amount of each type of coupling is controlled the amount of tapering and the shape of the tapering of the high index contrast waveguide  130 .  FIG. 3  shows that an input beam  310  is coupled into the large waveguide  320  with a low-index contrast in a fundamental mode  320 . As the light encounters the tapered waveguide  130 , the high-index contrast and the taper  134  cause the mode  320  to change and to shrink in the adiabatic manner without significant optical loss. At the straight section  133  of the high-index waveguide  130 , the mode  320  is transformed into a fundamental mode  330  of the waveguide  130 . Light in the mode  330  continues to propagate in the waveguide  130 .  
      The coupler can certainly operate in an inverse direction to couple light from the waveguide  130  to the waveguide  120 . In this mode of operation, the light initially guided by the waveguide  130  hits the tapered section  134  and the mode expands as the cross section of the tapered section  134  reduces along the direction of light propagation. At the end the tip  135  of the high index guide  130 , the optical mode of the light is transformed and is substantially matched to the mode of the low index guide  120 .  
      In the fundamental mode, the optical energy of the waveguide mode concentrates at the center of the waveguide. Hence, it is desirable to place the small waveguide  130  at or near the center of the large waveguide  120  to effectuate an efficient coupling between the modes of the waveguides  120  and  130 . As the position of the waveguide  130  moves away from the center of the waveguide  130 , the coupling efficiency decreases and a longer interaction length is needed to achieve a complete mode transform between the modes of the waveguides  120  and  130 .  
      For example, consider a waveguide coupler where the large waveguide  120  has a 3-micron square cross section and is made of a polymer with a refractive index of 1.6 and the small waveguide  120  has a 0.3-micron square cross section and is made of Si with a refractive index of 3.5. Assume that both waveguides are single-mode waveguides. When the waveguide  130  is at the center of the large waveguide, the tapered section with a length of less than 20 microns is sufficient to completely transform the fundamental modes between the waveguides with a coupling loss less than 1 dB. In comparison, if the waveguide  130  is placed near the edge of the large waveguide  120 , the tapered section with a length of more than 200 microns may be needed to completely transform the fundamental modes between the waveguides with a coupling loss less than 1 dB. Hence, the position of the waveguide  130  within the waveguide  120  may cause the length of the tapered region to change as much as 10 times in this particular example. Similar dependence of the optical coupling in mode transform and the position of the waveguide  130  in the waveguide  120  can be found in waveguides with other cross section profiles. Accordingly, the mesa structure  112  is designed to place the waveguide  130  near or at the center of the waveguide  120  to reduce the length of the tapered region for the adiabatic mode transformation.  
       FIG. 4  shows another example of a waveguide coupler  400  of this application. Similar to the coupler  100 , the coupler  400  provides coupling between a large low-index waveguide  420  and a small, high-index waveguide  430  with a tapered section  434  and a straight section  433  in an embedded configuration. Hence, the embedded portion of the waveguide  430  is raised above the substrate  110  to be at or near the center of the large waveguide  420  by a mesa. Different from the device  100 , the tapered section  434  in the coupler  400  is designed to expand in its cross section from the end of the straight section  433  to a large end facet  435  that is close to the cross section of the large waveguide  430 . Accordingly, the hollow section of the large waveguide  420  is shaped to conform to the embedded part of the waveguide  430  and the underlying mesa above substrate  110 .  
       FIG. 4  further illustrates the mode transformation of light initially guided by the waveguide  430 . Light is initially guided in the waveguide  430  in a mode  401 . As the light in the mode  401  enters the tapered section  434  towards the waveguide  420 , it begins to expand after entering the tapered section  434  adiabatically. At the end facet  435 , the mode defined by the tapered section  434  substantially matches a mode  402  of the solid section of the waveguide  420 . Hence, the light in the tapered section  430  is coupled into the mode  420  and continues to propagate in the waveguide  420 . The reverse operation is possible to couple light initially guided in the waveguide  420  into the waveguide  430 .  
       FIGS. 5A, 5B ,  5 C,  5 D, and  5 E show an exemplary fabrication flow for fabricating the waveguide couplers  100  and  400  in  FIGS. 1 and 4 . First, a substrate  510  such as a semiconductor, a glass, or other suitable material is provided. A low-index cladding layer  110  and a high-index waveguide layer  130  are sequentially deposited over the substrate  501  ( FIG. 5A ).  FIG. 5B  shows that the layers  110  and  130  are patterned to form the desired tapered shape shown in either  FIG. 2A  or  FIG. 4  to form the small high-index waveguide  130 . Notably, the layer  110  is patterned below its top surface in contact with the bottom of the layer  130  to form the mesa  112 . Alternatively, the substrate  510  may be directly patterned to form the mesa  112  to operate as the low-index cladding layer without using the separately formed cladding layer  110 .  FIG. 5C  shows that a layer of a low-index waveguide layer  120  is next deposited over the patterned layers  130  and  110 . The low-index cladding layer  110  should have a refractive index less than indices of the layers  120  and  120 . Next in  FIG. 5D , the low-index waveguide layer  120  is patterned as a stripe to either completely cover the waveguide  130  or partially cover part of the waveguide  130  that is near the tapered region and expose the rest of the waveguide  130 . Up completion of this step, the large low-index waveguide  120  is formed to have a solid section and a hollow section conformingly wrapping around the waveguide  130  and the mesa  112 . Optionally, a low index cladding overlay  510  may be formed over the entire structure as the cladding material for the low-index waveguide  120 .  
      The waveguide couplers  100  and  400  in  FIGS. 1 and 4  may be generally used to interconnect a low-index large waveguide and a high-index small waveguide.  FIG. 6  shows a photonic chip where various photonic components, such as optical modulators, photodetectors, and transmitter circuits, are integrated on the same substrate. Fibers are used to send optical input signals into the chip and to transmit optical output signals of the chip off the chip. High-index waveguides such as Si waveguides integrated on the chip are used to direct on-chip optical signal. Waveguide optical couplers based on the designs in  FIGS. 1 and 4  may be used as the input or output couplers to couple the fibers to the on-chip waveguides whose core cross sections are smaller than the fiber cores. For example, when the coupler  100  is used as an input coupler, an input fiber may be directly coupled to the large waveguide  120  for efficient coupling from the fiber to the large waveguide  120 . The tapered section  134  then efficiently couples the light into the small on-chip waveguide  130  for further on-chip processing.  
       FIG. 6  illustrates one exemplary photonic chip  600  formed on a substrate  601 . The chip  600  may include one or more input optical couplers  620  to receive various input optical signals and one or more output optical couplers  640  to output optical signals. Each input coupler  620  or output coupler  640  may be implemented by a waveguide coupler described in this application to provide coupling between an off-chip waveguide  612  or  660  (e.g., a fiber) and an on-chip waveguide  632  (e.g., a small Si waveguide). The chip  601  may include various photonic and electronic components or devices. As illustrated, optical modulators  630  and transmitter circuits  631  may be implemented to form an on-chip optical transmitter module (TX) to send out optical signals to off-chip waveguides  650 . Optical detectors  634  and receiver circuits  633  may be implemented to form an on-chip receiver (RX) to receive optical signals from off-chip waveguides  660 . A light source such as a laser  610  may be used to supply an input light beam via an off-chip waveguide  612  to supply optical power to the chip  600 . Alternatively, a diode laser or LED may be integrated on the chip  600  to supply the light.  
      Only a few implementations are described. However, it is understood that variations and enhancements may be made.