Abstract:
An optical switch is fabricated on a single substrate and switches planar waveguides into and out of alignment with each other. The switch is constructed by selective deposition and etching of layers that make up the desired waveguides and physical structures of the switch. The physical structures include a movable structure upon which one of the waveguides resides, the movable structure being freed from the substrate by deep etching underneath it. A heat source may be used to provide heat to the movable structure, which causes its thermal expansion and resulting movement of the waveguide on it into or out of alignment with another waveguide. A third waveguide may be located so as to align with the waveguide on the movable structure when it is at an ambient temperature.

Description:
FIELD OF THE INVENTION  
         [0001]    This invention relates generally to optical switching and, more specifically, to optical switching of a planar waveguide.  
         BACKGROUND OF THE INVENTION  
         [0002]    In the field of optical signal communication, it is often desirable to select between two or more optical signals to be transmitted along a single optical waveguide. The different optical signals may represent, for example, different communications signals on a telecommunications system. Similarly, it is often desirable to select between two possible waveguides along which a given optical signal will be transmitted. In such a case, the switching may be to direct a certain optical signal along a selected one of multiple output paths. For such uses, as well as others, it would be desirable to have a high-speed optical switch that was compact, efficient and easy to produce.  
           [0003]    In the past, optical switches have used a number of different techniques to perform the desired switching operation. A simple switch configuration, often referred to as a “1×2 switch” can be operated to connect one optical waveguide to either one of two other optical waveguides. One method of switching is to have a region through which an optical signal being switched must pass, the region having a refractive index or change in refractive index upon which the directing of the optical signal to a given waveguide depends. By supplying some means by which the refractive index of this region may be changed, the path of the optical signal is likewise changed, directing it to a different output, for example.  
           [0004]    Another type of optical switching relies on a physical movement of one or more switch components to select the position of an optical signal being switched. In some cases, these switches have made use of actuators that move an optical fiber waveguide from a first position to a second position, the two positions realizing coupling, respectively, between two different output fibers. In another actuator-type switch, a coupling lens positioned between a source fiber and multiple output fibers is moved to change the focus of the signal being coupled from one output fiber to another. In such devices, to initiate switching, force is applied to a movable component in one of several ways. For example, a piezoelectric module may be used that undergoes a physical change in the presence of an electrical switching signal. Such a device is shown in U.S. Pat. No. 4,303,302. In another switch type, materials with known coefficients of thermal expansion are arranged such that the heating of one or more of these materials causes a physical movement of one point relative to another. In U.S. Pat. No. 5,446,811, such a configuration is used to cause the movement of an optical fiber secured to the displaced material, thereby enabling the switching.  
         SUMMARY OF THE INVENTION  
         [0005]    In accordance with the present invention, an optical switch is provided that is fabricated on a single substrate. A first planar waveguide and a second planar waveguide are both fabricated on the substrate, and the second waveguide is provided with a degree of freedom that allows it to be moved between two positions. In a first position, the second waveguide is aligned with the first waveguide such that an optical signal can be coupled between them. In the second position, the second waveguide is not aligned with the first waveguide. However, in a preferred embodiment, the second position places the second waveguide in alignment with a third planar waveguide that is also fabricated on the substrate.  
           [0006]    The present invention provides for an optical switch that can be produced as a single fabrication process. The waveguides may be part of a layer of material that is deposited on the substrate and etched to form the desired core regions of planar waveguides. A cladding material is also deposited on the substrate, typically prior to and after the deposition of the waveguide material, so that it completely surrounds the planar waveguides. This provides the necessary refractive index boundary to allow a total internal reflection condition within the waveguides. Selective etching of the cladding material layer allows the formation of a movable structure to which the second waveguide is rigidly fixed, preferably by its residing within it. The movable structure may be fixed relative to a remainder of the cladding material layer, and a portion of the substrate below the movable structure is typically removed to allow it limited movement in a space above the substrate. A physical stop may also be used, and is particularly beneficial when the switch relies on movement caused by thermal expansion. The stop is located so as to limit the motion of the movable structure in a direction in which the structure moves when changing the alignment between the second waveguide and the first waveguide. That is, when the movable structure contacts the stop, the second waveguide is properly aligned with the first waveguide. Further increases in the force driving the movement of the movable structure therefore do not change the alignment between the waveguides.  
           [0007]    In a preferred embodiment, switching is implemented by using a heat source to heat the movable structure, which then moves between different positions in response to its own thermal expansion. Such a heat source may consist of one or more conductive pads that may be deposited on the substrate, typically on top of one or more previously deposited layers. A resistance heating material may be integrated into the movable structure, and connected to the heating pads that, if provided with an electrical potential, cause an electrical current to pass through the heating material. This, in turn, results in resistive heating of the movable structure. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0008]    The above and further advantages of the invention may be better understood by referring to the following description in conjunction with the accompanying drawings in which:  
         [0009]    [0009]FIG. 1 is a schematic top view of an optical switch according to the present invention; and  
         [0010]    FIGS.  2 A- 2 G depict a cross sectional schematic view of the fabrication stages of a switch like that shown in FIG. 1. 
     
    
     DETAILED DESCRIPTION  
       [0011]    Shown in FIG. 1 is a 1×2 optical switch that uses a thermally actuated switching element to change the position of a planar waveguide and thereby effectuate the desired switching action. The switch is fabricated from a single substrate  10  that may be, for example, a semiconductor material such as silicon. An input optical waveguide  12  is fabricated on the substrate  10  and passes through a portion of the switch that is free to move relative to the substrate. The movable portion includes two “arms”  14 ,  16  which together form a “wishbone” shape. Each of the arms is fixed to the substrate at one end, and together form a single laterally moving segment  18  at the other end. Preferably, each arm  14 ,  16 , at an ambient operating temperature, has an arcuate shape, as shown in the figure. As shown, the arms reside in an open area  17  of the material layer from which they are fabricated.  
         [0012]    The input waveguide  12  is a planar waveguide that follows along arm  14  to segment  18 , where it ends and is transmitted across a small gap to one of two output waveguides  20 ,  22 . Preferably, the respective waveguides are angled so as to achieve Brewster&#39;s angle at the coupling point, as is known in the art. Each of the two output waveguides  20 ,  22  is arranged to convey the optical signal from the input waveguide  12  to a different destination. Which of the waveguides receives the output from the input waveguide depends on the relative positioning of the movable portion formed by the arms  14 ,  16 . The position of the arms shown in FIG. 1 is at an ambient operating temperature, and results in an optical signal transmitted along input waveguide  12  to be coupled to output waveguide  22 . Those skilled in the art will understand that the drawing in FIG. 1 is not to scale, but rather is for illustrative purposes. In order to switch to the other output, i.e., output waveguide  20 , heat is input to the arms  14 ,  16 .  
         [0013]    The heating of arms  14 ,  16  is accomplished using a heater consisting of heater pads  24 ,  26  and a thermal element  28  that runs on top of or through the arms  14 ,  16 . The heater element may be a simple resistance heater, such that an electrical source could be applied to pads  24 ,  26  to provide the desired heating of the material. As the temperature of the arms  14 ,  16  increases, the material from which the arms are fabricated expands. The wishbone shape of the arms results in them having a relatively high degree of stiffness in a first direction, while having significantly more flexibility in a perpendicular direction, which, in the figure, is the direction shown by arrow  30 . Thus, as the arms heat up, the segment  18  moves quickly to a second position in which the input waveguide  12  is aligned with output waveguide  20 . A mechanical stop  32  is used to control the movement of the segment  18  in the direction of the arrow  30 , so that it is not necessary to precisely control the temperature of the arms  14 ,  16 . If the heater is overdriven, additional flexing of the arms results, but the input waveguide  12  remains aligned with output waveguide  20 . It is recognized that the deflection of the arms  14 ,  16  may result in the introduction of a certain degree of birefringence into the input waveguide. It may be desirable to design the waveguide with a predetermined amount of birefringence that is reduced when activation of the switch puts the waveguide under stress. The birefringence may also be designed into the waveguide in such a way that it is minimized when the switch is at a position halfway between the two output waveguides, so that the birefringence in each of the switch positions is approximately equal.  
         [0014]    As mentioned above, the optical switch is formed on a single substrate through a series of deposition and etching steps. In particular, the process must include a step of etching that undercuts the arms  14 ,  16 , so that they are free to move in response to the thermal changes. The dashed line  34  shown in FIG. 1 surrounds a region that is reduced by undercut etching following implementation of a fabrication method described herein. An example of such a fabrication process is described below in conjunction with FIGS.  2 A- 2 G. Those skilled in the art will understand that this series of steps describes just one specific way to fabricate the optical switch, and that variations on this method may exist.  
         [0015]    In FIGS.  2 A- 2 G, the view is of a cross section of a switching apparatus during fabrication, the section being taken through a plane perpendicular to the plane of the page in FIG. 1, in a location and direction indicated by the section line II-II. In a first step of the method, shown in FIG. 2A, a substrate  36  has deposited on it a layer of a material  38  having a relatively low index of refractive index. A good candidate for this material is silicon dioxide (SiO 2 ), which may be deposited by flame hydrolysis deposition (FHD), a known vacuum deposition method. This layer will serve to provide a refractive index boundary for the core of the input waveguide  12  of the switch. Following deposition of the layer  38 , another layer is deposited, also preferably by FHD. This layer, shown in FIG. 2B, is a material  40  with a refractive index that is significantly higher than that of the material  38 . When the layer  38  is SiO 2 , a good choice for the material  40  is SiO 2  with a dopant added to raise the refractive index. For example, by adding a dopant such as germanium to SiO 2  during deposition, a layer is produced that has a significantly higher refractive index than the SiO 2  alone.  
         [0016]    After deposition of the layer  40 , the material in that layer is selectively etched to remove all but several channels of higher refractive index material that will correspond to the respective waveguide cores for the switch. In the preferred embodiment, reactive ion etching is used, although any of a variety of known methods of selective etching may be used as well. The technique of reactive ion etching is known in the art, and the specific steps involved are not repeated herein, nor depicted in the figures. Those skilled in the art will be also familiar with other existing methods of selectively removing portions of the material  40 . Following the etching, the several channels of higher refractive index material are left, and are shown in FIG. 2C. Although different configurations are possible, the channels in FIG. 2C correspond to the input and output waveguides of FIG. 1. In particular, the channel  42  of FIG. 2C corresponds to input waveguide  12  of FIG. 1, and channels  44  and  46  of FIG. 2C correspond, respectively, to output waveguides  22  and  20  of FIG. 1. Those skilled in the art viewing FIG. 2C will understand that the channels extend significantly in a direction perpendicular the drawing page, as well as parallel to it.  
         [0017]    Once the channels  42 ,  44  and  46  are formed, further deposition of material  38  takes place. As shown in FIG. 2D, the additional deposition, also preferably by FHD, raises the level of the material  38  so that it completely covers the channels  42 ,  44  and  46 . Indeed, the material  38  surrounds the channels on all sides and, having a lower index of refraction than that of the core material  40 , serves as a cladding for the respective waveguides. Because of the cross sectional nature of FIG. 2D, only one surface of channel  42  is visible and, because of the location at which the section is taken, channels  44  and  46  are entirely hidden behind the cladding material  38 .  
         [0018]    With the core and cladding portions of the waveguides in place, the heater pads for coupling heat to the arms of the switch are deposited on the material  38  layer. The pads may be of a metal material, and a preferred method for forming the pads is by sputtering deposition. One of the heater pads is shown in FIG. 2E. Again, although not necessary, the location of the pad  48  is kept consistent with its location in FIG. 1 (in which it corresponds to pad  24 ), to assist in describing the invention. Due to the location at which the cross section of FIG. 2E is taken, the second heating pad shown in FIG. 1 is not visible in FIG. 2E.  
         [0019]    [0019]FIG. 2F is taken along the same section line as the rest of FIGS.  2 A- 2 G, and depicts a step in which deep reactive ion etching is used to remove large sections of the material  38 . As is known in such an etching process, a photoresist material is developed on the material  38  layer at the locations where the arms  14 ,  16  are to be located, and all of the other area outside of the open region  17  within which the arms reside in FIG. 1. When the material  38  is etched, the portions of the material  38  that correspond to arms  14  and  16  are isolated from the rest of the material except at the contact points at either side of the open region  17 . In FIG. 2F, the segment  50  of material  38  will correspond to arm  14  of FIG. 1. The segment corresponding to the arm  16  is not visible in the view of FIG. 2F. At this stage of the fabrication process, the arms  14 ,  16  are still attached to the material layer beneath them.  
         [0020]    In order to free the arms from the underlying material, an “undercut etch” is performed. An etching solution that is applied to the open region  17  removes more of the material  38 , including portions residing under the segments corresponding to arms  14 ,  16 . In fact, in the preferred embodiment, a portion of the underlying substrate  36  is also removed beneath the open region. The etching solution not only eats downward into the silicon base, making it deeper, and also eats laterally into the side walls of the open region. This etching process removes the material that lies below the sections corresponding to arms  14 ,  16 , thereby freeing them from any restriction except at the side contact points. Using the same cross sectional view as before, the resulting configuration is depicted in FIG. 2G. As shown, the segment  50  is disconnected from the underlying structure, and a portion of the substrate  36  is removed, creating a shallow pocket that corresponds to the area identified by dashed line  34  of FIG. 1.  
         [0021]    While the invention has been shown and described with reference to a preferred embodiment thereof, it will be recognized by those skilled in the art that various changes in form and detail may be made herein without departing from the spirit and scope of the invention as defined by the appended claims. For example, although the preferred embodiment specifies certain materials for the planar waveguide and the surrounding structure, those skilled in the art will quickly identify analogous materials that may be substituted. The waveguides may be glass or silicon, or other materials known to be useful for such purposes. The configuration of the switch may also be modified without departing from the nature of the invention. The preferred embodiment suggests the use of a wishbone-shaped movable element, but other shapes are anticipated, and would operate in an analogous manner. Likewise, those skilled in the art will understand that, while the preferred embodiment is shown as a 1×2 optical switch, the principles disclosed herein apply to numerous different types of switching arrangements. In addition, known techniques for optimizing performance should be incorporated herein, such as compensation for any birefringence on the waveguide due to stress during its displacement. Naturally, known coupling strategies for optimizing the coupling between the input and output waveguides should also be implemented. Finally, means of changing the position of a movable component in the switch other than thermal expansion may also be used.