Abstract:
An electro-optically coupled switch includes first and second waveguides which are aligned in parallel to each other, with a thin, flat layer of cross-coupling material sandwiched therebetween. A voltage source is provided to establish a strong uniform electric field that is oriented perpendicular across the entire layer of cross-coupling material between the waveguides. Incorporated with the voltage source is a switch for changing the electric field, to thereby alter the refractive index of the cross-coupling material for transferring the transmission of an optical signal from one waveguide to the other.

Description:
[0001]    This application is a divisional of application Ser. No. 14/687,726, filed Apr. 15, 2015, which is currently pending. The contents of application Ser. No. 14/687,726 are incorporated herein by reference. 
     
    
     FIELD OF THE INVENTION 
       [0002]    The present invention pertains generally to systems and methods that employ switches and modulators during the transmission of optical signals through optical waveguides. More specifically, the present invention pertains to optical switches and modulators that employ a cross-coupling material which is sandwiched between two waveguides, wherein the waveguides are aligned parallel to each other, and an electric field, E, is used to change the refractive index, tic, of the cross-coupling material to transfer an optical signal from one waveguide to the other. The present invention is particularly, but not exclusively, useful as an electro-optically coupled switch wherein the cross coupling material is structured as a thin, flat layer, and the electrical field, E, is strong and uniform, with flux lines oriented substantially perpendicular to the entire layer of cross-coupling material and confined between the waveguides. 
       BACKGROUND OF THE INVENTION 
       [0003]    It is well known that an optical waveguide is a physical structure which guides electromagnetic waves (e.g. light) through the structure. The guidance, or confinement, of light by the waveguide is the result of internal reflections within the waveguide. As a physical event, these internal reflections result when the difference between the refractive index, n wg , of the waveguide material, and that of the surrounding environment, n e , has a certain value. Otherwise, there may be no confinement, or inefficient confinement, of light within the waveguide. 
         [0004]    It is also well known that an applied electric field can change the refractive index of a material through a linear or nonlinear electro-optic effect such as the well-known Pockets&#39; effect (linear) or the Kerr effect (nonlinear). In particular, the Pockets&#39; electro-optic effect is a case wherein the influence of a voltage that is applied across a material will change the index of refraction, n, of the material by an amount, Δn, which can be mathematically expressed as: 
         [0000]      Δ n=−rn   3   E/ 2
 
         [0000]    where r is the Pockets&#39; constant, and E is the strength of the electric field. 
         [0005]    In the context of a planar, waveguide coupler switch, an electric field E is applied between two cross-coupled optical waveguides which are separated by an electro-optic material having a refractive index, n eo . When applied, the electric field, E, changes the refractive index, n eo , of the cross-coupling material to modify the cross-coupling characteristics between the two optical waveguides. As a result, light traveling along one waveguide is moved to the other waveguide. 
         [0006]    With the above in mind, the design of a vertical, waveguide optical switch as envisioned for the present invention involves several interactive factors of particular importance. These include: the separation distance, d, between the waveguides (i.e. the thickness of the cross-coupling material); the refractive index of the cross-coupling material, n c , (also sometimes referred to herein as n eo ); and the design (i.e. configuration) of the electric field E. 
         [0007]    In particular, insofar as the design of the electric field is concerned, the ability of the device (i.e. electro-optic switch) to configure and confine the electric field, E, relative to the cross-coupling material is of paramount importance. Specifically, the concern here for a design of the electric field, E, is three-fold. First: the electric field, E, passing through the cross-coupling material should be uniform (i.e. the electric field flux lines are parallel to each other). Second: flux lines of the electric field, E, should be confined to the cross-coupling material. And third: the flux lines of the electric field, E, should be aligned with the polarization direction of the cross-coupling material (i.e. perpendicular to the light beam pathway in the waveguides). The purpose for harmonizing these factors is to optimize the electro-optic modulation efficiency of the device. 
         [0008]    In light of the above, it is an object of the present invention to provide an electro-optically coupled switch having a cross-coupling material with a refractive index, n c , that ensures good optical confinement between two waveguides. Another object of the present invention is to provide an electro-optically coupled switch with a cross-coupling material having a refractive index, n c , that establishes a strong electro-optic modulation coefficient. Yet another object of the present invention is to design the structure for an electro-optic switch having the proper waveguide separation to achieve strong waveguide cross-coupling; while maximizing the electro-optic efficiency of the device by providing good optical confinement in the cross-coupling material that facilitates the transfer of light into or out of the waveguide. Another object of the present invention is to provide an electro-optically coupled switch wherein a uniform electric field, E, is confined and directed through a layer of cross-coupling material that is sandwiched between two optical waveguides, and wherein the electric field intensity is normal to the layer of cross-coupling material. Still another object of the present invention is to provide an electro-optically coupled switch that is simple to manufacture, is easy to use and is comparatively cost effective. 
       SUMMARY OF THE INVENTION 
       [0009]    In accordance with the present invention, a vertical electro-optically coupled switch includes first and second waveguides, with a layer of cross-coupling material positioned between the waveguides. In combination, the first and second waveguides, together with the cross-coupling material located therebetween, create what is sometimes hereinafter referred to as a waveguide stack. In any event, an electric field, E, is established through the cross-coupling material. Variations in E can then be made (i.e. a switching voltage, V π ) to change the refractive index of the cross-coupling material, n c  (i.e. n c ≡n eo ). The intended result here is to transfer the transmission of an optical signal, λ, from one waveguide to the other. Several structural aspects of the cross-coupling material, as well as functional aspects, of the electric field, E, are particularly important. 
         [0010]    For purposes of the present invention, the layer of cross-coupling material should have a depth, d, and it should be coextensive with the length, L, of the waveguides. As envisioned for the present invention, the refractive index of a first waveguide, n wg1 , will be equal to, or nearly equal to, the refractive index of a second waveguide, n wg2  (i.e. n wg1 ≈n wg2 ). Importantly, however, the refractive index of the cross-coupling material, n c , needs to be much greater than the respective indexes n wg1  and n wg2  of the first and second waveguides (i.e. n wg1 &lt;&lt;n c &gt;&gt;n wg2 ). Specifically, this selection of refractive indexes is made, along with consideration of the distance, d, to achieve strong waveguide cross-coupling, good optical confinement, and an optimum electro-optic modulation efficiency. Typically, the distance, d, between waveguides will be smaller than the value of L/n wg  (i.e. d&lt;L/n wg ). Further, the waveguide width, W, is optimized to improve optical confinement and to reduce optical loss. 
         [0011]    With regard to the electric field, E, as noted above it must be strong and uniform. Further, flux lines of the electric field, E, are to be oriented substantially perpendicular to the layer of cross-coupling material that is positioned between the waveguides. Furthermore, the electric field, E, is to be confined between the waveguides across the entire layer of the cross coupling material. To do this a filler material having a refractive index, n f , is positioned against the cross-coupling material between the waveguides. 
         [0012]    For a construction of the present invention, the depth, d, of the cross-coupling material, the length, L, of the waveguides, and the refractive indexes n wg1 , n wg2 , and n c , as well as the field strength for E, all need to be selected ,and based upon the wavelength, λ, of the optical signal that is being transmitted. As envisioned for the present invention, the cross-coupling material may be a polymer, when the first and second waveguides are also polymers. The cross-coupling material may also be a polymer when the waveguides are a SiON/silica material. On the other hand, if the waveguides are doped materials then, depending on the doping used, the cross-coupling material can either be a polymer, a PIN planar-diode-structure semiconductor, or a PIN multiple-quantum-well semiconductor, 
         [0013]    A voltage source is connected to the waveguide stack for selectively establishing a uniform electric field, E, through the cross-coupling material. Preferably, the electric field, E, is confined in the cross-coupling material by a filler material which encloses the cross-coupling material between the first waveguide and the second waveguide. Furthermore, and most importantly, the electric field, E, is oriented everywhere across the cross-coupling material, perpendicular to the layer of cross-coupling material. 
         [0014]    Incorporated with the voltage source is an electric switch. Specifically, this switch is a means for imposing a switching voltage, V π , to the waveguide stack. In particular, the switching voltage, V π , is used to selectively change the refractive index, n c , of the cross-coupling material. 
         [0015]    In a preferred embodiment of a waveguide stack for the present invention, the first waveguide and the second waveguide are made of a SiON/silica material, and the cross-coupling material is a polymer. For this embodiment, the means for imposing V π  on the waveguide stack includes a first transparent electrical contact that is connected with the voltage source and is positioned between the first waveguide and the cross-coupling material. A second transparent electrical contact which is connected with the voltage source and positioned between the second waveguide and the cross-coupling material is also included. In a variation of the preferred embodiment, the first waveguide, the second waveguide and the cross-coupling material can all be made of a polymer. 
         [0016]    In a first alternate embodiment of the present invention, the first waveguide and the second waveguide are each made of a same, lightly-doped, electrically-conductive material, and the waveguides are individually positioned in contact with the voltage source. Specifically, both the first waveguide and the second waveguide are N doped. The means for imposing the switching voltage, V π , to the waveguide stack will then include a first N +  doped layer that is positioned in electrical contact between the first N doped waveguide and the voltage source. Similarly, a second N +  doped layer is positioned in electrical contact between the second N doped waveguide and the voltage source. For this embodiment of the present invention the cross coupling material is preferably a polymer. 
         [0017]    In a second alternate embodiment of the present invention, the first waveguide is P doped and the second waveguide is N doped. In this case, the means for imposing V π  to the waveguide stack includes a first doped layer positioned in electrical contact between the first P doped waveguide and the voltage source. Also, a second N +  doped layer is positioned in electrical contact between the second N doped waveguide and the voltage source. For this second alternate embodiment the cross-coupling material can be either a PIN planar-diode-structure semiconductor, or a PIN multiple-quantum-well semiconductor. 
         [0018]    For an operation of the present invention, the switch can include a first input port at the upstream end of the first waveguide, and a first output port at the downstream end of the first waveguide. Also, the switch can include a second output port at the downstream end of the second waveguide. With this arrangement, when an incoming optical signal, λ, is received at the first input port it can be selectively routed to the second output port by the switching voltage, V π . As an additional feature of the present invention, a second input port can be used at the upstream end of the second waveguide. In this case, when an incoming optical signal, λ′, is received at the second input port, it can be selectively routed to the first output port by the switching voltage, V π . 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0019]    The novel features of this invention, as well as the invention itself, both as to its structure and its operation, will be best understood from the accompanying drawings, taken in conjunction with the accompanying description, in which similar reference characters refer to similar parts, and in which: 
           [0020]      FIG. 1  is a perspective-schematic view of a system for transmitting optical signals, which includes an electro-optically coupled switch in accordance with the present invention; 
           [0021]      FIG. 2  is a cross-section view of an embodiment of the electro-optically coupled switch for the present invention as seen along the line  2 - 2  in  FIG. 1 ; 
           [0022]      FIG. 3  is a cross-section view of an exemplary switch in accordance with the present invention, as seen along the line  3 - 3  in  FIG. 1 , showing the switch/modulation functionality of the present invention; 
           [0023]      FIG. 4  is a cross-section view of another embodiment of the electro optically coupled switch for the present invention as seen along the line  4 - 4  in  FIG. 1 ; and 
           [0024]      FIG. 5  is a cross-section view of still another embodiment of the electro-optically coupled switch for the present invention as seen along the line  5 - 5  in  FIG. 1 . 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0025]    Referring initially to  FIG. 1 , an electro-optically coupled switch in accordance with the present invention is shown and is generally designated  10 . As shown, the switch  10  includes an enclosure  12  for holding and protecting the electro-optic components of the switch  10 . Also, an access connector  14  is provided for connecting the electro-optic components (not shown in  FIG. 1 ) with an external voltage source  16 . A queue control  18  and a routing control  20  are incorporated with the voltage source  16  to respectively provide for the sequencing, routing and modulation of optical signals, λ, as they pass through the electro-optically coupled switch  10 . 
         [0026]    Still referring to  FIG. 1 , it will be seen that the enclosure  12  includes an input port  22  for optically connecting an optical waveguide  24  with the switch  10 . Similarly, an input port  26  is provided by the enclosure  12  for optically connecting an optical waveguide  28  with the switch  10 . It is to be appreciated that the optical waveguides  30  and  32  will have similar connections with the enclosure  12 . 
         [0027]    In  FIG. 2  the internal, electro-optic components for a preferred embodiment of the switch  10  are shown. There it will be seen that the switch  10  includes a waveguide  34  and a waveguide  36  that are respectively protected by a cladding  38  and a cladding  40 . In more detail, each waveguide  34  and  36  has a width, W, and a length, L, and they are vertically aligned in parallel with each other. Further, as shown, the switch  10  includes a metal connector  42  (e.g. +V) and a metal connector  44  (e.g. −V) which are respectively connected with a transparent electrical contact  46  and a transparent electrical contact  48 . Further, a cross-coupling material  50  is positioned between the transparent electrical contacts  46  and  48 . In accordance with the present invention, the transparent electrical contacts  46  and  48  are in direct contact with the cross-coupling material  50 , and are everywhere separated from each other by a distance, d. Further, the transparent electrical contacts  46  and  48  are positioned opposite each other from the cross-coupling material  50 . And, they are each positioned between the cross-coupling material  50  and a respective waveguide  34  and  36 . Additionally, a filler material  52  is provided to electrically confine the cross-coupling material  50  between the transparent electrical contacts  46  and  48 . 
         [0028]    Within the combination of components for the switch  10  shown in  FIG. 2 , the differences in the refractive index of the various materials used are important. In detail, the refractive index of waveguide  34  (a first waveguide), n wg1 , will be equal to, or nearly equal to, the refractive index of waveguide  36  (a second waveguide), n wg2 . For purposes of the present invention, the refractive indexes of the waveguides  34  and  36  will be the same, or nearly the same, n wg1 ≈n wg2 . Importantly, however, the refractive index of the cross-coupling material  50 , n c , (also sometimes noted herein as n eo ) needs to be much greater than the respective indexes n wg1  and n wg2  of the first and second waveguides  34  and  36  (i.e. n wg1 &lt;&lt;n c &gt;&gt;n wg2 ). As noted above, this arrangement is made to achieve strong waveguide cross-coupling, good optical confinement in the cross-coupling material, and efficient electro-optic modulation, with a proper waveguide separation distance, d. For example, n c =1.7, n wg =1.57, and d=0.5 μm. Also, the refractive index of the filler material  52 , n f , needs to be smaller than all of the others (i.e. n c &gt;&gt;n wg(1 and 2) &gt;n f , and n wg1 ≈n wg2 ). As shown, the metal connector  42  and the metal connector  44  are separately connected with the voltage source  16 . Thus, a +V can be provided to the metal connector  42  by the voltage source  16 , and a −V can be provided to the metal connector  44 . The result is that a switching voltage, ΔV π , can be applied through the cross-coupling material  50  that will change its refractive index, n c . As envisioned for the present invention, the cross-coupling material  50  may be a polymer, when the waveguides  34  and  36  are also polymers, or when the waveguides  34  and  36  are made of a SiON/silica material. 
         [0029]    An operation of the switch  10  will be best appreciated with reference to  FIG. 3 . There it will be seen that, depending on the influence of the switching voltage, V π , an optical signal, λ, can be directed either onto a pathway  54  (solid arrows) or a pathway  56  (dashed arrows). The consequence of this is that, the switching voltage, V π , can be used to guide an optical signal, λ, which enters the switch  10  through the input port  22  to exit the switch  10  from either the output port  58  of waveguide  36  or the output port  60  of waveguide  34 . 
         [0030]    With the above in mind, and by returning to  FIG. 1 , it will be appreciated that the routing control  20  can influence the voltage source  16  to selectively establish the switching voltage, V π , and thereby generate the electrical field, E. Importantly, the electrical field, E, when generated, is uniform with the flux lines of the field oriented substantially perpendicular to the length, L, of the waveguides  34  and  36 . As mentioned above, the purpose here is to influence the transit of an optical signal, λ, through the switch  10 . 
         [0031]    For an exemplary operation of the switch  10 , refer back to  FIG. 1 . In this example, consider an optical signal, λ in-a , as input from optical waveguide  24 , into the waveguide  36  via input port  22 . Also consider an optical signal, λ′ in-b , as input from optical waveguide  28 , into the waveguide  34  via input port  26 . For purposes of this example, subscript “a” pertains to waveguide  36 , while subscript “b” pertains to waveguide  34 . 
         [0032]    With cross-reference between  FIG. 1  and  FIG. 3 , and first considering only the optical signal, λ, it is to be appreciated that with no switching voltage, V π , there is no electric field, E, through the cross-coupling material  50 . Accordingly, optical signal, λ, in optical waveguide,  24  will enter switch  10  via input port  22 , transit switch  10  on pathway  54 , and exit from switch  10  via the output port  58  ( FIG. 3 ) and into the optical waveguide  30  as optical signal, λ out-a . On the other hand, with a switching voltage, V π , imposed on the cross coupling material  50 , an electric field, E, is generated through the cross coupling material  50  to change the refractive index, n c  (n eo ), of the cross coupling material  50 . In this case, the optical signal, λ in-a , will transit switch  10  on pathway  56 , and exit from switch  10  via the output port  60  ( FIG. 3 ), and into the optical waveguide  32  as optical signal, λ out-b . 
         [0033]    Similarly, when considering the optical signal, λ′, it is to be appreciated that with no switching voltage, V π , optical signal, λ′ in-b , will enter switch  10  from optical waveguide  28  via input port  26 . Optical signal, will then transit switch  10  and exit via the output port  60  ( FIG. 3 ) and into the optical waveguide  32  as optical signal, λ′ out-b . With a switching voltage, V π , imposed on the cross-coupling material  50 , however, the optical signal, λ′ in-b , will transit switch  10  to exit from switch  10  via the output port  58  ( FIG. 3 ), and into the optical waveguide  30  as optical signal λ′ out-a . 
         [0034]    Still referring to  FIG. 1  it is to be appreciated that the switch  10  can be used either as a switch or as a modulator. Further, it will be appreciated that the queue control  18  can be used as a gate to provide for alternating or sequential access of the optical signals, λ and λ′, to the switch  10 . As will be appreciated by the skilled artisan, when switch  10  is used as a modulator, only one continuous wave (CW) light input port  22  and one optical output port (e.g. output port  58 ,  FIG. 3 ) are required, 
         [0035]      FIG. 4  shows an alternate embodiment for the present invention wherein the waveguide  34  and the waveguide  36  are each made of a same, lightly-doped, electrically-conductive material. As shown, the waveguides  34  and  36  are individually positioned in contact with the voltage source  16 . For one alternate embodiment of the present invention, both the waveguide  34  and the waveguide  36  are N doped. Accordingly, the means for imposing the switching voltage, V π , includes an N +  doped layer  62  that is positioned in electrical contact between the N doped waveguide  34  and the metal connector  44 . Similarly, an N +  doped layer  64  is positioned in electrical contact between the N doped waveguide  36  and the metal connector  42 . Preferably, for this alternate embodiment of the present invention, the cross coupling material  50  is a polymer. 
         [0036]      FIG. 5  shows another alternate embodiment of the present invention wherein the waveguide  34  is P doped and the waveguide  36  is N doped. In this case, the means for imposing V π  includes a P +  doped layer  66  positioned in electrical contact between the P doped waveguide  34  and the metal connector  44 . Also included is an N +  doped layer  68  which is positioned in electrical contact between the N doped waveguide  36  and the metal connector  42 . In this case, the cross-coupling material  50  can be either a PIN planar-diode-structure semiconductor, or a PIN multiple-quantum-well semiconductor. 
         [0037]    While the particular Vertical Electro-Optically Coupled Switch as herein shown and disclosed in detail is fully capable of obtaining the objects and providing the advantages herein before stated, it is to be understood that it is merely illustrative of the presently preferred embodiments of the invention and that no limitations are intended to the details of construction or design herein shown other than as described in the appended claims.