Abstract:
Various optical isolators are disclosed. One embodiment provides an optical isolator comprising a waveguide that includes polymer magneto-optical media. In a particular embodiment, the waveguide is dimensioned for single mode operation in the selected isolation range. A cross-section of the waveguide is inhomogeneous in terms of magneto-optical materials. Polymer magneto-optical material is a part of the optical waveguide structure. The inhomogeneity induces the propagation constant shift, which is propagation-direction-dependent. An embodiment is characterized by a cutoff frequency for forward propagating waves that is different than the cutoff frequency for reverse waves; the dimensions and direction of magnetization of the waveguide can be tailored so that, in a particular embodiment, the cutoff frequency for forward propagating waves is lower than the cutoff frequency for reverse waves.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims the benefit of U.S. Provisional Application No. 61/802,172, filed on Mar. 15, 2013, the disclosure of which is hereby incorporated by reference. The disclosures of the following applications are also hereby incorporated by reference: U.S. application Ser. No. 13/219,355 filed Aug. 26, 2011; U.S. application Ser. No. 12/496,630 filed Jul. 1, 2009 (now U.S. Pat. No. 8,009,942); and U.S. Provisional Application No. 61/133,609 filed Jul. 1, 2008. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    Optical isolators are optical components that transmit light in one direction but block it in the backward-propagating direction. They are used when the reversely-propagating light needs to be avoided. For example, lasers become instable when reflected light comes back to them. 
         [0003]    Conventional optical isolators consist of a Faraday rotator and two polarizers. The linear-polarization angle needs to be rotated by 45 degrees in a Faraday rotator, which is typically a few millimeters long. Thus, this type of optical isolator becomes relatively large. It is also a challenge to build Faraday rotators and polarizers in a guided-optics format, i.e. it is hard to integrate them on chip. In order to construct guided-wave optical isolators, an asymmetric Mach-Zehnder waveguide interferometer is commonly used. This optical isolator does not require polarizers, but requires high-precision interference for blocking backward propagating light waves; one branch of the interferometer is a nonreciprocal phase shifter, and the other is a reciprocal phase shifter. However, this optical isolator still requires two long waveguides. 
         [0004]    U.S. Pat. No. 8,009,942 (“Yoshie et al.”) describes an optical isolator implemented as a waveguide section utilizing materials that induce a propagation constant shift that is propagation-direction dependent. Yoshie et al. describes an isolator characterized by a cutoff frequency for forward propagating waves that is different than the cutoff frequency for reverse waves. Yoshie et al. describes use of magneto-optical materials in a wave guide to construct a wave guide with a cross section that is inhomogeneous in terms of magnetic properties. 
       SUMMARY OF THE INVENTION 
       [0005]    It is possible to construct a waveguide isolator characterized by cutoff frequencies that are propagation-direction dependent using crystalline magnetic materials such as bismuth-substituted iron garnets and GaAs:Mn. However, fabricating crystalline magneto-optical materials requires certain substrates such as, for example garnets and GaAs. Due to this limitation, it is a challenge to build optical isolators on some popular low-cost substrates such as silicon, silica and plastics. However, recently, some polymer materials have been found to show the Faraday Effect. Such polymers can provide magneto-optical media for constructing waveguides on substrates that are more readily compatible with typical materials used for mass-produced chips. 
         [0006]    One embodiment of the present invention provides an optical isolator comprising a waveguide that includes polymer magneto-optical media. In a particular embodiment, the waveguide is dimensioned for single mode operation in the selected isolation range. A cross-section of the waveguide is inhomogeneous in terms of magneto-optical materials. Polymer magneto-optical material is a part of the optical waveguide structure. The inhomogeneity induces the propagation constant shift, which is propagation-direction-dependent. In a typical embodiment, this device works as an optical isolator from a cut-off frequency of the lowest mode forward waves (lower frequency) to one for the lowest mode reverse waves (higher frequency). However, in some embodiments, a device might works as an optical isolator from a cut-off frequency of the lowest forward mode waves (lower frequency) to a cutoff frequency of a different reverse mode (e.g., the next highest reverse mode rather than the lowest reverse mode). 
         [0007]    An embodiment of the present invention includes a waveguide section utilizing materials that induce a propagation constant shift that is propagation-direction-dependent. An embodiment of the inventive isolator is characterized by a cutoff frequency for forward propagating waves that is different than the cutoff frequency for reverse waves; the dimensions and direction of magnetization of the waveguide can be tailored so that, in a particular embodiment, the cutoff frequency for forward propagating waves is lower than the cutoff frequency for reverse waves. 
         [0008]    A particular embodiment is constructed as a single-mode waveguide on a substrate. The cross-section of the waveguide is inhomogeneous in terms of magneto-optic materials. At least one part of the cross-section is a non-reciprocal magneto-optical medium, which has nonzero off-diagonal permittivity tensor components. This inhomogeneity induces the propagation constant shift, which is propagation-direction-dependent so that the device works as an optical isolator from the cutoff frequency of forward waves (lower frequency) to one for reverse waves (higher frequency). Various configurations, i.e. structures with various distributions of the magneto-optical medium, may be used consistent with the principles of the invention. 
         [0009]    In some embodiments, the magneto-optical polymer material is magnetized prior to placement within the device that includes the optical isolator waveguide structure. In some embodiments, the magnetization of the magneto-optical polymer material is provided or enhanced by one or more magnets placed external to the optical isolator structure but within, on, or near the device that includes the optical isolator waveguide structure. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0010]    The novel features of the invention are set forth in the appended claims. However, for purpose of explanation, several aspects of a particular embodiment of the invention are described by reference to the following figures. 
           [0011]      FIG. 1  illustrates an optical isolator  100  in accordance with one embodiment of the invention. 
           [0012]      FIG. 2  illustrates an optical isolator  200  in accordance with another embodiment of the invention. 
           [0013]      FIG. 3  illustrates an optical isolator  300  in accordance with another embodiment of the invention. 
           [0014]      FIG. 4  illustrates an optical isolator  400  in accordance with another embodiment of the invention. 
           [0015]      FIG. 5  illustrates an optical isolator  500  in accordance with another embodiment of the invention. 
           [0016]      FIG. 6  illustrates an optical isolator  600  in accordance with another embodiment of the invention. 
           [0017]      FIG. 7  illustrates an optical isolator  700  in accordance with another embodiment of the invention. 
           [0018]      FIG. 8  illustrates an optical isolator  800  in accordance with another embodiment of the invention. 
           [0019]      FIG. 9  illustrates an optical isolator  900  in accordance with another embodiment of the invention. 
           [0020]      FIG. 10  illustrates an optical isolator  1000  in accordance with another embodiment of the invention. 
           [0021]      FIG. 11  illustrates an optical isolator  1100  in accordance with another embodiment of the invention. 
           [0022]      FIG. 12  illustrates an optical isolator  1200  in accordance with another embodiment of the invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0023]    The following description is presented to enable any person skilled in the art to make and use the invention, and is provided in the context of particular applications and their requirements. Various modifications to the exemplary embodiments will be readily apparent to those skilled in the art, and the generic principles defined herein may be applied to other embodiments and applications without departing from the spirit and scope of the invention. Thus, the present invention is not intended to be limited to the embodiments shown, but is to be accorded the widest scope consistent with the principles and features disclosed herein. 
         [0024]      FIG. 1  illustrates an optical isolator  100  in accordance with one embodiment of the invention. Optical isolator  100  includes substrate portions  101  and  102 , first waveguide portion  103 , and second waveguide portion  104 . In the illustrated embodiment, substrate portion  102  comprises silicon dioxide (SiO2) (which is a thin layer on top of silicon portion  101 ). However, in other embodiments, a substrate portion such as substrate portion  102  may comprise other materials, preferably a dielectric materials having a low refractive index. First waveguide portion  103  comprises silicon (Si). Second waveguide portion  104  comprises a magneto-optical polymer (“MO polymer”). 
         [0025]    The example of  FIG. 1  may be constructed by, for example, on a silicon-on-insulator (“SOI”) wafer, patterning a silicon layer with high resolution lithography and dry etching to form a silicon waveguide on silica. Then, a magneto-optical polymer, for example, an electronic-grade poly(3-dodecylthiophene-2,5-diyl), can be spun-coated to form portion  104 . The optical mode propagation is non-reciprocal for TM-like modes due to asymmetry of the magneto-optical media distribution in the vertical (z) direction. The aspect ratio for supporting the lowest TM-like mode is such that dimension a is larger than b. The waveguide is dimensioned to support a single mode for a TM-like mode. 
         [0026]      FIG. 2  illustrates an optical isolator  200  in accordance with another embodiment of the invention. The example of  FIG. 2  may be constructed, for example, on a thermally oxidized silicon substrate, coating and patterning a magneto-optical polymer by using spin coating and photolithography and dry etching to form a magneto-optical polymer waveguide portion  203  on top of silicon dioxide portion  202  (which is a thin layer on top of silicon portion  201  formed from thermally oxidizing a silicon substrate). It should be understood that in some embodiments of the invention, including the embodiment of  FIG. 2 , optical intensity may exist in a portion of the substrate. For example, in the embodiment of  FIG. 2 , some optical intensity resides in a portion of silica layer  202 , which is non-magnetic. Thus, the combination of magneto-optical polymer portion  203  and a portion of silica layer  202  provides a waveguide that is inhomogeneous in terms of magnetic properties. In the illustrate embodiment, the optical mode propagation is non-reciprocal for a single-mode TM-like mode. 
         [0027]      FIG. 3  illustrates an optical isolator  300  in accordance with another embodiment of the invention. The example of  FIG. 3  may be constructed, for example, on a SOI substrate, coating a magneto-optical polymer and then coating and patterning the silicon and magneto-optical polymer after coating the magneto-optical polymer. The resulting isolator  300  comprises silicon portion  303  and magneto-optical polymer portion  304  arranged as shown on silicon dioxide portion  302  (which is a thin layer on top of silicon portion  301 ). The optical mode propagation is non-reciprocal for a single-mode TM-like mode. 
         [0028]      FIG. 4  illustrates an optical isolator  400  in accordance with another embodiment of the invention. The example of  FIG. 4  may be constructed, for example, on a plastic (or glass) substrate  401 , coating a magneto-optical polymer and a non-magnetic polymer and patterning the non-magnetic polymer by photolithography and dry etching to form magneto-optical polymer portion  402  and non-magnetic polymer portion  403 . The optical mode propagation is non-reciprocal for single-mode TM-like modes. 
         [0029]      FIG. 5  illustrates an optical isolator  500  in accordance with another embodiment of the invention. The example of  FIG. 5  may be constructed, for example, on a plastic (or glass) substrate  501 , coating a magneto-optical polymer and a non-magnetic polymer and patterning the non-magnetic polymer by photolithography and further patterning the non-magnetic polymer and a portion of the magneto-optical polymer by dry etching. As shown, this results in a first magneto-optical polymer portion  502 , a second magneto-optical polymer portion  503 , and a non-magneto-optical polymer portion  504 . The optical mode propagation is non-reciprocal for a single-mode TM-like mode. 
         [0030]      FIG. 6  illustrates an optical isolator  600  in accordance with another embodiment of the invention. The example of  FIG. 6  may be constructed, for example, on a SOI substrate, patterning a silicon layer by lithography and dry etching and coating and patterning a magneto-optical polymer by spin coating and lithography with alignment and dry etching to form adjacent portions  603  (silicon) and  604  (magneto-optical polymer) on silicon dioxide portion  602  (which is a thin layer on top of silicon portion  601 ). The optical mode propagation is non-reciprocal for a single-mode TE-like mode. To support a TE-like mode as the lowest mode, the waveguide is dimensioned to have an aspect ratio such that dimension a is less than dimension b. 
         [0031]      FIG. 7  illustrates an optical isolator  700  in accordance with another embodiment of the invention. The example of  FIG. 7  may be constructed, for example, on a thermally oxidized substrate (comprising silicon portion  701  and silicon dioxide portion  702 ), coating and patterning a plastic layer to form portion  703  and coating and patterning a magneto-optical polymer to form portion  704  by spin coating and lithography with alignment and dry etching. The optical mode propagation is non-reciprocal for a single-mode TE-like mode. 
         [0032]      FIG. 8  illustrates an optical isolator  800  in accordance with another embodiment of the invention. The example of  FIG. 8  may be constructed, for example, on a SOI substrate, patterning a silicon layer by lithography and dry etching and coating and patterning a magneto-optical polymer by spin coating and lithography with alignment and dry etching to form silicon portion  803  and magneto-optical polymer portion  804 , arranged as shown on silicon dioxide layer  802  (which is a thin layer on top of silicon layer  801 . The optical mode propagation is non-reciprocal for a single-mode TE-like mode. 
         [0033]      FIG. 9  illustrates an optical isolator  900  in accordance with another embodiment of the invention. The example of  FIG. 9  may be constructed, for example, on a plastic (or glass) substrate  901 , coating and patterning a magneto-optical polymer to form portion  902 , followed by coating and patterning of a non-magnetic polymer to form portions  903  and  904 . The optical mode propagation is non-reciprocal for a single-mode TE-like mode. In one particular embodiment, portion  903  may comprise SiO 2  and portion  904  may comprise amorphous silicon in the form of, respectively, spin-on glass and spin-on silicon. Although most silicon and silica materials used in industry are not polymers, spin-on-glass and spin-on-silicon are silicon-containing polymers dissolved in a solvent or formed in a sol-gel manner, and they can be spin coated on substrates. After annealing, they become like silica and amorphous silicon, respectively. 
         [0034]    As those skilled in the art will appreciate, the exact relative sizes of the various waveguide portions (e.g., the relative size of the MO polymer portion relative to a non-magnetic polymer portion or silicon portion) will depend on the properties of the particular materials used. For example, depending on the permittivity values and related refractive indices of the materials used, the desired relative sizes of the MO polymer portion relative to other portion or portions may vary for different implementations. 
         [0035]    The following equations can be used to optimize the isolation for a particular implementation. In general, position-dependent permittivity tensor is given by the addition of two permittivity tensors: 
         [0000]    
       
         
           
             
               
                 
                   
                     
                       
                         
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         [0036]    The tensors {tilde over (∈)} Hermitian, and Δ{tilde over (∈)} is considered as a perturbation term. The propagation constant shift is written, using equation (1), as: 
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         [0000]    where ω is the angular frequency, E(y,z) is the normalized electric field, and x is the propagation direction and 
         [0000]        I   yx   =∫∫u ( y,z ) Im[E   y *( y,z ) E   x ( y,z )] dydz    
         [0000]        I   zy   =∫∫v ( y,z ) Im[E   z *( y,z ) E   y ( y,z )] dydz    
         [0000]        I   xz   =∫∫w ( y,z ) Im[E   x *( y,z ) E   z ( y,z )] dydz.   (3)
 
         [0037]    Given the electric field E(y,z) and dispersion  ω ( β ) of an un-perturbed mode and small perturbation Δ{tilde over (∈)}(y,z), we can obtain the dispersion relation  ω ( β +Δ β ) of forward (+) and backward (+) propagating waves from equation (2). The isolation frequency range can be maximized by maximizing Δβ. In a relatively narrow frequency range, Δβ is proportional to the isolation frequency range. 
         [0038]    In some embodiments, the MO polymer material is magnetized prior to being placed within the device that includes the optical isolating waveguide. However, in other embodiments, magnetization of the MO polymer waveguide portion may be achieved or enhanced by magnets placed external to the wave guide structure. 
         [0039]      FIGS. 10-12  illustrate various examples in which magnets are placed external to the wave guide structure to effect and/or enhance the magnetization of the relevant MO polymer waveguide portion. Although the examples are illustrated in the context of a waveguide isolator structure similar to that shown in  FIG. 2 , magnets may be used in conjunction with other illustrated embodiments. 
         [0040]      FIG. 10  illustrates optical isolator  1000  in accordance with another embodiment of the invention. In the  FIG. 10  embodiment, a magnet  1000 -M is placed in or just below the substrate comprising silicon portion  10001 . Silicon portion  1001  and silicon dioxide portion  1002  form a substrate under magneto-optical polymer portion  1003 . 
         [0041]      FIG. 11  illustrates optical isolator  1100 , in accordance with another embodiment of the invention. In the  FIG. 11  embodiment, a magnet  1100 -M is placed above the waveguide structure. Dielectric spacer  1104  is between magnet  1100 -M and magneto-optical polymer portion  1103 , which rest on silicon dioxide portion  1102  (which is a thin layer on top of silicon portion  1101 ). 
         [0042]      FIG. 12  illustrates optical isolator  1200  in accordance with another embodiment of the invention. In the  FIG. 12  embodiment, magnets are placed on both sides of the waveguide structure. Specifically, magnet  1200 -M 1  is place on the left and magnet  1200 -M 2  is place on the right. Dielectric spacer  1204  is placed between magnet  1200 -M 1  and magneto-optical polymer portion  1203 . Dielectric spacer  1205  is placed between magnet  1200 -M 2  and magneto-optical polymer portion  1203 . Magnet  1200 -M 1 , dielectric spacer  1204 , magneto-optical polymer portion  1203 , dielectric spacer  1205 , and magnet  1200 -M 2  reside on top of silicon dioxide portion  1202  (which is a thin layer on top of silicon portion  1201 ). 
         [0043]    Magnets are absorptive and, therefore, in the illustrated embodiments, the magnets are placed at a distance from the waveguide structure. In one embodiment, the magnets are placed one wavelength away (as measured in the dielectric material placed between the magnet and the waveguide). In some embodiments, magnet(s) are placed to optimize the magnetic flux through the relevant waveguide portion. 
         [0044]    While the present invention has been particularly described with respect to the illustrated embodiments, it will be appreciated that various alterations, modifications and adaptations may be made based on the present disclosure and are intended to be within the scope of the present invention. While the invention has been described in connection with what are presently considered to be the most practical and preferred embodiments, it is to be understood that the present invention is not limited to the disclosed embodiment but, on the contrary, is intended to cover various modifications and equivalent arrangements included within the scope of the inventive principles described herein.