Abstract:
By introducing magneto-optical garnets with high Faraday rotation and low optical loss in a ring resonator, a nonreciprocal phase shift is generated to split the resonance wavelengths of clockwise and counter-clockwise modes under magnetic field. There are three main applications based on this nonreciprocal effect, optical isolators, optical circulators, and tunable optical filters. The concept of the tunable filters and the design of optical isolators for TE and TM modes are described in the paper. With proper optical ring isolator configurations, optical circulators can be realized.

Description:
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
       [0001]    This invention was made with Government support under Grant No. HR0011-09-C-0123, awarded by DARPA. The Government has certain rights in this invention. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    The present invention relates generally to semiconductor devices, and, more specifically, to a ring-resonator based optical isolator and circulator. 
         [0004]    2. Description of the Related Art 
         [0005]    As electronic devices increase in speed, and are more commonly used in optical systems, integration of electronic and optical semiconductor devices has become more common. Such circuits, often called photonic integrated circuits, have found uses in several consumer and commercial applications. 
         [0006]    Proper integration of optical devices for inter-chip and intra-chip optical interconnections is important in the performance of the final integrated circuit. Some optical components, such as lasers, modulators, and photodetectors, can be monolithically integrated with electronic devices, however, other optical devices, such as optical isolators, are difficult to integrate with other electronic and optical devices. 
         [0007]    Optical isolators are important devices in optical systems in that optical isolators minimize light reflections into transmitting lasers, and thus reduce instabilities and system noise in optical and electro-optical systems. Typically, optical isolators are integrated using a magneto-optical effect, and the mechanisms used to integrate optical isolators through magneto-optical effects are classified into three main categories: non-reciprocal Transverse Electric (TE)-Transverse Magnetic (TM) (TE-TM) mode conversion, non-reciprocal loss (NRL), and non-reciprocal phase shift (NRPS) devices. 
         [0008]    Each type of device has associated performance and integration issues. A Nonreciprocal TE-TM mode converter in a waveguide creates an inherent phase mismatch between the TE and TM modes. In order to reach high conversion efficiency, phase matching is required between these two modes, however, zero birefringence is challenging to achieve in a waveguide. 
         [0009]    A device employing the NRL technique has been demonstrated by combining a Semiconductor Optical Amplifier (SOA) and magneto-optical film, achieving a reasonable amount of isolation. However, the SOA requires continuous power consumption to compensate for the extra insertion loss due to the lossy magneto-optic material. Another approach to realize NRL is to guide backward light to a radiation mode, which increases optical loss and thus lowers performance. 
         [0010]    NRPS devices has been embodied in a Mach-Zehnder Interferometer (MZI) for waveguide optical isolators, with a reasonable extinction ratio, however, these devices are typically sized on the order of millimeters, which makes monolithic integration difficult and expensive. NRPS combining microresonators for miniaturization of isolators has been proposed, which determines the size of the optical isolator by the size of the resonator, which can be on the order of microns, which remains difficult to integrate. 
         [0011]    Problems associated with using discrete optical isolators incorporating such conversions are typically associated with performance of the system using such discrete devices. By integrating optical isolators with other photonic and electronic devices, performance can be improved and cost and size of the final circuitry is much smaller, thus creating additional applications for such integrated systems. 
         [0012]    It can be seen, then, that there is a need in the art for optical isolators that are more readily integrated with other optical and electrical devices. 
       SUMMARY OF THE INVENTION 
       [0013]    To minimize the limitations in the prior art, and to minimize other limitations that will become apparent upon reading and understanding the present specification, the present invention provides methods for making optical isolators and optical isolators. An optical isolator having semiconductor-device dimensions in accordance with one or more embodiments of the present invention comprises a resonator structure carrying forward light in a first direction and backward light in a second direction, and a magneto-optical film, coupled to the resonator structure, the magneto-optical film creating a shift between the forward light and the backward light travelling in the resonator structure, wherein the resonator structure and the magneto-optical film are sized to reside on a semiconductor chip. 
         [0014]    Such an optical isolator further optionally comprises the resonator structure being a ring resonator structure, the magneto-optical film being coupled on a top of the ring resonator structure, a coil, coupled to the magneto-optical film, for controlling a magnetic field in the optical isolator, controlling a current in the coil filtering at least one of the forward light and the backward light by wavelength, a width and/or a height of the resonator structure being between 100 nanometers and 10 microns, the magneto-optical film being made from a material selected from a group consisting of yttrium iron garnet (YIG), bismuth iron garnet (BIG), bismuth-substituted gadolinium iron garnet (BiGa 2 Fe 5 O 12 ), and cerium-substituted yttrium iron garnet (Ce:YIG), the magneto-optical film being coupled on an inner diameter top of the resonator structure, and the resonator structure being a double ring resonator structure. 
         [0015]    A method for making an optical isolator having semiconductor-device dimensions in accordance with one or more embodiments of the present invention comprises sizing a ring resonator structure for integration on a semiconductor chip, forming the semiconductor-chip sized ring resonator structure to carry forward light in a first direction and backward light in a second direction, and coupling a magneto-optical film to the ring resonator structure, wherein the magneto-optical film creates a shift between the forward light and the backward light travelling in the ring resonator structure. 
         [0016]    Such a method further optionally comprises comprising coupling a coil to the magneto-optical film, controlling a current in the coil filtering at least one of the forward light and the backward light by wavelength, the magneto-optical film being made from a material selected from a group consisting of yttrium iron garnet (YIG), bismuth iron garnet (BIG), bismuth-substituted gadolinium iron garnet (BiGa 2 Fe 5 O 12 ), and cerium-substituted yttrium iron garnet (Ce:YIG), and the semiconductor-device dimensions being between 100 nanometers and 10 microns. 
         [0017]    A semiconductor optical isolator in accordance with one or more embodiments of the present invention comprises an input semiconductor waveguide on a semiconductor chip, an output semiconductor waveguide on the semiconductor chip, a ring resonator on the semiconductor chip, coupled between the input waveguide and the output waveguide, and a magneto-optical film, coupled to the ring resonator, wherein the magneto-optical film creates a shift between a forward light and a backward light travelling in the resonator structure. 
         [0018]    Such an optical isolator further optionally comprises the magneto-optical film being made from a material selected from a group consisting of yttrium iron garnet (YIG), bismuth iron garnet (BIG), bismuth-substituted gadolinium iron garnet (BiGa 2 Fe 5 O 12 ), and cerium-substituted yttrium iron garnet (Ce:YIG), and at least one of a height and a width of the ring resonator is between 100 nanometers and 10 microns. 
         [0019]    Other features and advantages are inherent in the system disclosed or will become apparent to those skilled in the art from the following detailed description and its accompanying drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0020]    Referring now to the drawings in which like reference numbers represent corresponding parts throughout: 
           [0021]      FIG. 1  illustrates a schematic diagram of an integrated semiconductor laser, isolator, and modulator on an SOI wafer in accordance with one or more embodiments of the present invention; 
           [0022]      FIGS. 2A and 2B  illustrate top and side views of configurations of the ring optical isolator of the present invention for the TM and TE modes in accordance with one or more embodiments of the present invention; 
           [0023]      FIG. 3A  illustrates the design metrics used to evaluate resonators in accordance with one or more embodiments of the present invention; 
           [0024]      FIGS. 3B and 3C  illustrate the influence of coupling between waveguides and the resonator with respect to isolation ratio and insertion loss in accordance with one or more embodiments of the present invention; 
           [0025]      FIG. 3D  illustrates the dependency of isolation ratio on the quality factor of the resonator in accordance with one or more embodiments of the present invention; 
           [0026]      FIG. 4  illustrates a transmission spectra of a single-ring resonator of the present invention and a series-cascaded double-ring resonator of the present invention; 
           [0027]      FIGS. 5A-5B  illustrate design plots for double-ring isolators in accordance with one or more embodiments of the present invention; 
           [0028]      FIG. 6  illustrates the influence of Q factor on isolation ratio for a double-ring isolator in accordance with one or more embodiments of the present invention; 
           [0029]      FIGS. 7A-7B  illustrate comparisons of a single-ring isolator of the present invention and a double-ring isolator of the present invention; 
           [0030]      FIG. 8  illustrates changes in isolation ratio with the confinement factor for the double-ring isolator of 50-μm radius in accordance with one or more embodiments of the present invention; 
           [0031]      FIG. 9  illustrates an optical circulator in accordance with one or more embodiments of the present invention; and 
           [0032]      FIG. 10  illustrates a process chart in accordance with the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0033]    In the following description, reference is made to the accompanying drawings which form a part hereof, and which is shown, by way of illustration, several embodiments of the present invention. It is understood that other embodiments may be utilized and structural changes may be made without departing from the scope of the present invention. 
       Overview 
       [0034]    Photonic integrated circuits have attracted attention for application to inter-chip and intra-chip optical interconnects. Most of the optical components, such as lasers, modulators, and photodetectors, can be monolithically integrated. However, optical isolators, which avoid the unwanted reflected light back to the laser and thus reduce instabilities and relative intensity noise, are still difficult for integration with other optoelectronic devices. 
         [0035]    Integrated optical resonators, typically made at least partially of magneto-optical material. Such materials are typically transparent to the wavelength(s) of interest. Several compounds, such as yttrium iron garnet (YIG), bismuth iron garnet (BIG), bismuth-substituted gadolinium iron garnet (BiGa 2 Fe 5 O 12 ), cerium-substituted yttrium iron garnet (Ce:YIG) have been developed, and such materials have low (less than 0.03 cm −1 ) optical absorption in the range from 1.5 microns to 5 microns, while at 1.3 microns, the absorption coefficient of Ce:YIG is approximately 1 cm −1 . 
         [0036]    Although all of the materials listed above, as well as other magneto-optical materials, are useful within the scope of the present invention, Ce:YIG is a preferred candidate for a magneto-optic material for use in the present invention, because Ce:YIG has a high Faraday rotation, low optical loss, and, for integration purposes, is compatible with silicon and silica. For example, and not by way of limitation, a waveguide made of epitaxially-sputtered Ce:YIG exhibited 5.8 dB/cm and 9.7 dB/cm of optical loss for TM mode and TE mode, respectively. 
         [0037]    The present invention provides approaches and devices that integrate optical isolators with other optical devices, e.g., lasers, modulators, etc., and electronic devices, by incorporating a high Faraday rotation and low optical loss magneto-optical material into a microring resonator structure, such that the optical isolator is sized to be integrated on a semiconductor chip. Such a compact isolator can be integrated with semiconductor lasers and modulators and electronic devices. 
       Integrated Optical Isolator Structures 
       [0038]      FIG. 1  illustrates a schematic diagram of an integrated semiconductor laser, isolator, and modulator on an SOI wafer in accordance with one or more embodiments of the present invention. 
         [0039]    Structure  100  is shown, with optical isolator  102  resident on a Semiconductor-On-Insulator (SOI) wafer  104 . SOI wafer  104  typically comprises a substrate  106 , an insulator layer  108 , and a semiconductor layer  110 , but can comprise additional layers as desired. Typically, substrate  106  is silicon, but can be other materials, such as silicon carbide, without departing from the scope of the present invention. Further, insulator layer  110  is typically silicon oxide, but can be other insulator materials, and semiconductor layer  110  is typically silicon, but can be other semiconductor materials, including compound semiconductors, and semiconductor layer  110  can be doped, undoped, or selectively or gradient doped, without departing from the scope of the present invention. 
         [0040]    Optical isolator  102  is co-resident with at least one other device on SOI wafer  104 , and two such devices, laser  112  and modulator  114 , are shown. Additional devices, including electronic devices which may be resident in semiconductor layer  110 , grown on semiconductor layer  110 , or bonded to semiconductor layer  110 , are also possible within the scope of the present invention, which would electrically, optically, or otherwise integrates or couple the optical isolator  102  with such devices. 
         [0000]    Nonreciprocal Ring Resonator with Magnetic Garnets 
         [0041]      FIGS. 2A and 2B  illustrate top and side views of configurations of the ring optical isolator of the present invention for the TM and TE modes in accordance with one or more embodiments of the present invention. 
         [0042]      FIG. 2A  illustrates a TM mode optical isolator  200  in a top view  202  and a side cutaway view  204 . Resonator  206  (also called a ring resonator or micro-ring resonator) is shown, with film  208  (also called a magneto-optical film, typically Ce:YIG but can be other materials) coupled to a top of the resonator  206 . Input waveguide  210  and output waveguide  212  couple light into and out of the resonator  206 , and optional multi-turn coil  214 , when desired, is coupled either within or to film  208 . 
         [0043]    Optical isolator  200  is a ring resonator, and thus has a non-reciprocal phase shift (NRPS), where the degeneracy of the clockwise mode and the counterclockwise mode no longer exists. Therefore, the resonance wavelengths for the forward light (the light travelling clockwise in the resonator  206 ) and the backward light (the light travelling counter-clockwise in the resonator  206 ) are different, which results in isolation between the forward light and the backward light. The isolation (e.g., the phase and/or frequency shift between forward and backward light) is determined by the Faraday rotation of the film  208  and the quality factor (Q) of the ring resonator  206 . 
         [0044]    To achieve this nonreciprocal quality of resonator  206  in the TM mode, the boundary between the magneto-optic layer (film  208 ) and the resonator  206  should be horizontal (along the x-axis), and thus film  208  is typically attached on the top of the resonator  206 . Thus, film  208  is coupled “on top of” or “over” resonator  206 . Typically, film  208  is coupled to resonator  206  through bonding or deposition, but can also be deposited in other ways without departing from the scope of the present invention. 
         [0045]    For resonator  206 , where the case of the magneto-optic film  208  is coupled on top of resonator  206  as shown in  FIG. 2A , only TM-like light will experience NRPS. Using perturbation theory, the NRPS is found to be 
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         [0000]    where E y  and E z  are components of the electrical field in they and directions, H x  and are components of the magnetic field in the x and z directions, y 0  and y 1  are the top and bottom boundary of the magneto-optic film  208  (shown as reference numerals  216  and  218 , respectively), and ∈ yz  represents the off-diagonal terms of the dielectric tensor of the film  208 , which is a function of the Faraday coefficient of the material. The thickness of film  208  is the difference between y 0    216  and y 1    218 , and the interface  220  between film  208  and resonator  206  is essentially at y 0    216 . 
         [0046]    In order to maximize the NRPS in resonator  206 , the peak of the field intensity profile should be positioned close to or at the interface  220  of the magneto-optic film  208  and the resonator  206 . Maximization or other field intensities are achieved through resonator  206  design parameters. 
         [0047]    In resonator  206 , the external magnetic field should be in the radial direction. However, it is difficult to make a permanent magnet with a magnetic field in a uni-radial direction and then align this field to the resonator  206  ring structure. Thus, an optional addition to structure  200  employs multi-turn coil  214 , which is shown attached to film  208 , to generate the magnetic field B  222  (pointing inward toward the center of the ring resonator  206 ) shown in top view  202  of  FIG. 2A . By controlling the direction of the current in coil  214 , either clockwise or counterclockwise (shown as counter-clockwise in side view  204 , with the head of the arrow pointing out at point  224  and the tail of the current arrow pointing in at point  226 ), the direction of the generated magnetic field can be selectively directed toward or away from the center of the ring resonator  206 . Such an approach eliminates external magnets from structure  200  and enables the integrity of the optical isolator, at a small expense of additional power consumption. Further, by adjusting the current in coil  214 , structure  200  becomes a tunable filter as well as an optical isolator. 
         [0048]      FIG. 2B  illustrates a TE mode isolator  228  in top view  230  and side view  232 . For TE polarized light, the magneto-optic layer boundary  234 , at film  204  edge x 0    236 , should be in a vertical direction (normal to the plane of the resonator  206  ring structure), and thus the film  204 , with thickness shown between x 0    236  and x 1    238 , is attached on the sidewall of the resonator  206  ring as shown. 
         [0049]    For this case, where TE light is employed and the magneto-optic film  204  is attached on the sidewall of the resonator  204  ring, the external magnetic field direction B  240  is vertical, and only TE-like light will experience the nonreciprocal phase shift, which can be written as 
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         [0050]    Similar to the case of the TM mode optical isolator  200 , the magnetic field intensity in the interface of the magnet-optic film  204  and the silicon waveguide should be as large as possible to increase the NRPS and thus the performance of the isolator  228 . Isolator  228  can use an external permanent magnet (not shown) to provide additional magnetic field strength if needed. 
       Resonator Design 
       [0051]    The ring isolator  200 / 228  can be realized in many platforms where optical waveguides and resonators  206  can be integrated. Although described with respect to SOI wafer  104  platforms herein, because of certain advantages of silicon photonics, other materials or platforms may be used without departing from the scope of the present invention. 
         [0052]    Design of isolators  200 / 228 , and, as described hereinbelow, a TE mode isolator  228 , uses design metrics in order to produce figures-of-merit to determine the suitability of a given isolator  200 / 228  for a desired application. 
         [0053]      FIG. 3A  illustrates the design metrics used to evaluate resonators. Typically, the metrics used to generate the figures-of-merit are the isolation ratio and the insertion loss of the isolator  200 / 228 , which provide evaluation points for isolators  200 / 228 . Graph  300  illustrates a typical isolator  200 / 228 , with forward light  302  and backward light  304  travelling in isolator  200 / 228 , with insertion loss  306  and isolation ratio  308  shown. 
         [0054]    Initially, design of the isolator  200 / 228  determines the dimensions of ring resonator  206 . As shown in  FIG. 2B , with a TE-mode isolator  228 , material for the film  208  is chosen, which is typically Ce:YIG and a thickness for the film  208  is chosen, which was chosen for the following example as 200 nm of film  208  thickness. The refractive index and Faraday coefficient of Ce:YIG are 2.22 and 4500°/cm, respectively. Film  208  is then attached onto the sidewall of resonator  206 . 
         [0055]    Table 1 summarizes the resonance wavelength splitting between forward and backward light for different widths  250  of the resonator  206  ring while the height  252  was fixed at 700 nm. A width of 300 nm is chosen as typical for the following example isolator  228 , as such width provides reasonable NRPS and does not impose an excessive burden on fabrication techniques used to construct isolator  228 . Other widths  250  and heights  252  are possible within the scope of the present invention. Width  250  and height  252  are dimensioned or sized such that the resonator  206  can reside on a semiconductor chip, e.g., width  250  and height  252  are typically in the range of 10 nanometers to 100 microns, preferably between 100 nanometers and 10 microns, and more preferably between 100 nanometers and 1 micron. 
         [0000]    
       
         
               
             
               
               
               
             
               
               
               
             
           
               
                 TABLE 1 
               
             
             
               
                   
               
               
                 Resonance wavelength splitting of the ring resonator and the confinement 
               
               
                 factor of the magneto-optic film. The height of the ring is 700 nm. 
               
             
          
           
               
                   
                 Resonance wavelength 
                 Confinement factor 
               
               
                 Width of the ring 
                 splitting 
                 of MO 
               
               
                   
               
             
          
           
               
                 200 nm 
                  173 pm 
                 65.42% 
               
               
                 300 nm 
                   78 pm 
                 29.34% 
               
               
                 400 nm 
                 30.2 pm 
                 12.35% 
               
               
                 500 nm 
                 14.7 pm 
                 6.21% 
               
               
                   
               
             
          
         
       
     
         [0056]      FIGS. 3B and 3C  illustrate the influence of coupling between waveguides  210 / 212  and the resonator  206  with respect to isolation ratio  308  and insertion loss  306 . Coupling parameters between resonator  206  and waveguides  210  and  212  are then determined. The power coupling ratio between the waveguides  210 / 212  and the resonator  206  is represented by            WG  shown on graph  310  in  FIG. 3B  for various values of Q, while the round-trip loss in the resonator  206  is represented by γ ring  and are shown in  FIG. 3C  on graph  314 . The resonator  206  loss can be dominated by scattering loss or material absorption, depending on fabrication techniques used in fabricating resonator  206 . The isolation ratio  308  decreases with an increase in coupling because the transmission spectrum becomes broader. However, the insertion loss  306  can benefit from the strong coupling because a larger portion of power is coupled to the output waveguide  212  instead of lost in the resonator  206 . 
         [0057]      FIG. 3D  illustrates the dependency of isolation ratio  308  on the quality factor of the resonator  206 . 
         [0058]    Resonators with higher Quality (Q) factors (where Q is the resonant frequency divided by the bandwidth) have narrower transmission spectra, and thus higher isolation ratios  308 . Although resonators  206  with lower Q factors are lossier, the insertion loss  306  can be compensated or overcome by stronger coupling. Therefore, the insertion loss versus coupling-loss ratio (           WG /γ ring ) is independent of Q factors as shown in  FIG. 3C . However, the power coupling ratio cannot exceed one since there is no gain in a passive device. Graph  316  illustrates the loss versus coupling-loss ratio where            WG =0.5γ ring , graph  318  illustrates the loss versus coupling-loss ratio where            WG =γ ring , and graph  320  illustrates the loss versus coupling-loss ratio where            WG =2γ ring . 
       Series-Cascaded Double Ring Isolator 
       [0059]    In order to further increase the isolation ratio and bandwidth, especially for ring resonators  206  with lower Q factors, higher order rings can be fabricated within the scope of the present invention. 
         [0060]      FIG. 4  illustrates a transmission spectra of a single-ring resonator of the present invention and a series-cascaded double-ring resonator of the present invention. 
         [0061]      FIG. 4 , in graph  400 , shows the comparison of transmission spectra between a single-ring resonator and a series-cascaded double-ring resonator having the same Q factor. The transmission of forward light  402  and backward light  404  in the single-ring resonator, and the transmission of forward light  406  and backward light  408  in the double-ring isolator, are shown. The double-ring isolator exhibits larger isolation ratio and bandwidth at a small expense of insertion loss, which can be compensated by stronger coupling between the waveguides and the resonator  406  ring. 
         [0062]    Similar to the design of a single-ring isolator, the coupling is also an important factor that affects the isolation ratio and insertion loss for a double-ring isolator. In addition to the waveguide-ring power coupling ratio (           WG ), the ring-ring power coupling ratio (           ring ) also affects the performance of a double-ring isolator in accordance with the present invention. 
         [0063]      FIGS. 5A-5B  illustrate the dependence of isolation ratio and insertion for double-ring isolators in accordance with one or more embodiments of the present invention. 
         [0064]    Graphs  500 - 510  illustrate various ratios of            WG  and γ ring  for            ring /γ ring  where the Q factor of both ring resonators is given as equal to 10 5 . Graph  500  illustrates the condition where            WG =γ ring , graph  502  illustrates the condition where            WG =2γ ring , graph  504  illustrates the condition where            WG =3γ ring , and  FIG. 5A  illustrates the effect of the change of these parameters on the isolation ratio  308 . Graph  506  illustrates the condition where            WG =γ ring , graph  508  illustrates the condition where            WG =2γ ring , graph  510  illustrates the condition where            WG =3γ ring , and  FIG. 5B  illustrates the effect of the change of these parameters on the insertion loss  306 . 
         [0065]    Strong coupling (both            WG  and γ ring ) typically results in a smaller isolation ratio  308 . However, there is an optimal ring-ring coupling value to minimize the insertion loss  306 , because the coupling between the rings splits each transmission peak for forward and backward light. By properly choosing the coupling, the insertion loss can be minimized with larger isolation bandwidth. 
       Influence of Q Factor on Isolation Ratio 
       [0066]      FIG. 6  illustrates the influence of Q factor on isolation ratio for a double-ring isolator in accordance with one or more embodiments of the present invention. 
         [0067]    Similar to the single-ring isolator  200 / 228 , the isolation ratio increases with the Q factor. The waveguide-ring coupling and the ring-ring coupling (           WG =           ring =         ) is assumed as equal for the calculation, and graphs  600 - 608  illustrate the dependence of the ratio of            WG  to γ ring  on isolation ratio  308 . Graph  600  illustrates the design where            WG =γ ring , graph  602  illustrates the design where            WG =2γ ring , graph  604  illustrates the design where            WG =3γ ring , graph  606  illustrates the design where            WG =4γ ring , and graph  608  illustrates the design where            WG =5γ ring . 
         [0068]    From these design graphs  600 - 608 , isolators in accordance with the present invention, with specific isolation ratios and insertion losses, can be obtained by choosing proper waveguide-ring coupling, ring-ring coupling, and the Q factor. 
         [0069]      FIGS. 7A-7B  illustrate comparisons of a single-ring isolator of the present invention and a double-ring isolator of the present invention. 
         [0070]    Graphs  700 - 706  are shown, which indicate that the isolation ratio of the double-ring isolator has a stronger dependence on Q factor than the single-ring isolator, and thus it increases faster in the double-ring isolator than in the single-ring isolator as shown by graphs  700  and  702  in  FIG. 7A . Graphs  704  and  706  show the relationship between the isolation ratio and Faraday coefficient of the magneto-optic material used in the isolator. Two common materials used for isolators are Ce:YIG and BIG, and their Faraday coefficient are approximately 4500°/cm and 3000°/cm, respectively, in the 1550 nm spectral regime. Graphs  704  and  706  suggest that advantages of a double-ring isolator structure become more prevalent when larger Q factors and larger Faraday coefficient materials are used in the design and construction of the isolators of the present invention. 
       Confinement Factor of Magneto-Optic Film 
       [0071]    Although the above discussion obtains NRPS of the isolator of the present invention first through geometry and dimensioning of the ring isolator, and then using the calculated NRPS to calculate the isolation ratio  308  and insertion loss  306 , there is another relationship between the NRPS and Q factor, which are both dependent on the confinement factor of the magneto-optic film  208 . 
         [0072]    According to the equations provided herein, the maximum resonance wavelength splitting occurs when the peak of the optical profile is positioned at the interface of the resonator  206  and the magneto-optic film  208 . However, because the refractive index of the material used in the resonator, which is typically silicon (where n=3.45) is much larger than that of Ce:YIG (n=2.22), the peak of the optical profile when silicon and Ce:YIG are used in device  200  would be located inside the resonator  206  silicon core. Thus, the peak of the optical profile can be positioned close to or at the interface only when the width of the silicon core of the resonator  206  is very small. It is possible to use another material with a lower refractive index, e.g., silicon nitride which has n=1.98 at 1550 nm, which may change the dimensions required for resonator  206 . The optimal dimensioning of lower-index material waveguides will be larger than the dimensions of higher-index material waveguides, and thus the optical mode of the waveguide will also be larger. Thus, the peak intensity will be lower in the lower-index material waveguides, and since NPRS is proportional to field intensity differences between the two interfaes of the film  208 , the NPRS of lower-index material waveguides used in the present invention will be smaller as well, but are still useable in the present invention. 
         [0073]    When magneto-optic film  208  is used a waveguide cladding, the NRPS is approximately linearly proportional to the optical confinement factor in the film  208 . Larger confinement factors will allow for more resonance wavelength splitting. However, sidewall roughness of the resonator  206  ring, and film  208  absorption, will create additional waveguide scattering loss, which will degrade the Q factor of the ring resonator  206 . Such losses will tend to cancel any benefits obtained from larger resonance wavelength splitting, and thus the confinement factor has relatively little influence on the isolation ratio  308 . 
         [0074]      FIG. 8  shows changes in isolation ratio with the confinement factor for the double-ring isolator of 50-μm radius in accordance with one or more embodiments of the present invention. 
         [0075]    Graphs  800 - 808  show changes in percentages of optical confinement factor in the film  208  from graph  800  at 10%, graph  802  at 20%, graph  804  at 30%, graph  806  at 40%, and graph  808  at 50%, showing that the isolation ratio for each is relatively constant regardless of the percentage of the confinement factor. 
       Optical Circulator 
       [0076]      FIG. 9  illustrates an optical circulator in accordance with one or more embodiments of the present invention. 
         [0077]    Circulator  900  comprises a plurality of isolators  200 / 228 , shown as isolator  228  in  FIG. 9 . Port  1   902 , port  2   904 , and port  3   906  are also shown, where light propagation is allowed from port  1   902  to port  2   904 , and from port  2   904  to port  3   906 . Typically, other propagation directions are prohibited in such a circulator  900 , however, other circulator designs can allow other propagation paths. Direction  908  is shown to show the forward direction of light through the isolators  228 , and magnetic field B  240  is shown, with the field lines pointing into the page as indicated by the tail of the arrow indicator. 
         [0078]    One advantage of this configuration compared to circulators with cascaded isolators is to eliminate the need for a coupler, typically a 3 dB coupler, in port  2   904 , thus, the present invention&#39;s circulator  900  eliminates at least 3 dB of loss in any system using the circulator  900  of the present invention. Each ring isolator  200  can also be replaced by a double-ring isolator as discussed herein if increased extinction ratios are desired in an application of circulator  900 . 
       Tunable Optical Filter 
       [0079]    The present invention, as shown herein in at least  FIGS. 2A and 2B , can also be used as a tunable optical filter by controlling the external field, typically with multi-turn coil  214 , but also with external magnets or electromagnets as desired. Typically, for an optical isolator, the magnetization of magneto-optic film  208  is in the saturation region to reach maximum NRPS. However, for a tunable optical filter, the magnetization of the film  208  is in the linear region, i.e., the magnetization of film  208  is proportional to the external magnetic field. Therefore, the NRPS is tunable by adjusting the current in the coil  214 . 
       Bidirectional Wavelength-Selective Isolator 
       [0080]    Another application of the isolator  200 / 228  of the present invention is a bidirectional wavelength-selective isolator for two interleaved sets of optical wavelengths, where isolator  200 / 228  adjusts the splitting of the resonance wavelength to be approximately half of the free spectral range. This is useful in wavelength-interleaved bidirectional transmission, as such a configuration minimizes back reflection for both Wavelength Division Multiplexed (WDM) channel sets in opposite directions, and therefore reduce the number of the fiber links required in an optical transmission system. 
       Process Chart 
       [0081]      FIG. 10  illustrates a process chart in accordance with the present invention. 
         [0082]    Box  1000  illustrates sizing a ring resonator structure for integration on a semiconductor chip. 
         [0083]    Box  1002  illustrates forming the semiconductor-chip sized ring resonator structure to carry forward light in a first direction and backward light in a second direction. 
         [0084]    Box  1004  illustrates coupling a magneto-optical film to the ring resonator structure, wherein the magneto-optical film creates a shift between the forward light and the backward light travelling in the ring resonator structure. 
       REFERENCES 
       [0085]    The following references are incorporated herein by reference:
   1. T. R. Zaman, X. Guo, and R. J. Ram, “Faraday rotation in an InP waveguide,” Applied Physics Letters 90, 23514-23511-23514-23514-23513 (2007).   2. H. Shimizu, and Y. Nakano, “Fabrication and characterization of an InGaAsP/InP active waveguide optical isolator with 14.7 dB/mm TE mode nonreciprocal attenuation,” J. Lightwave Technol. 24, 38-43 (2006).   3. Y. Shoji, H. Yokoi, and T. Mizumoto, “Enhancement of magneto optic effect in optical isolator with GaInAsP guiding layer by selective oxidation of AlInAs,” Jpn. J. Appl. Phys. 1, Regul. Pap. Short Notes Rev. Pap. 43, 590-593 (2004).   4. J. Fujita, M. Levy, J. R. M. Osgood, L. Wilkens, and H. Dotsch, “Waveguide optical isolator based on Mach—Zehnder interferometer,” Applied Physics Letters 76, 2158-2160 (2000).   5. N. Kono, K. Kakihara, K. Saitoh, and M. Koshiba, “Nonreciprocal microresonators for the miniaturization of optical waveguide isolators,” Opt. Express 15, 7737-7751 (2007).   6. S. Sang-Yeob, Q. Xiaoyuan, and B. J. H. Stadler, “Integrating yttrium iron garnet onto nongarnet substrates with faster deposition rates and high reliability,” Applied Physics Letters 87, 121111-121111-121111-121111-121113 (2005).   7. T. Shintaku, A. Tate, and S. Mino, “Ce-substituted yttrium iron garnet films prepared on Gd 3 Sc 2 Ga 3 O 12  garnet substrates by sputter epitaxy,” Applied Physics Letters 71, 1640-1642 (1997).   8. M. Wallenhorst, M. Niemoller, H. Dotsch, P. Hertel, R. Gerhardt, and B. Gather, “Enhancement of the nonreciprocal magneto-optic effect of TM modes using iron garnet double layers with opposite Faraday rotation,” J. Appl. Phys. 77, 2902-2905 (1995).   9. H. Yokoi, T. Mizumoto, N. Shinjo, N. Futakuchi, and Y. Nakano, “Demonstration of an optical isolator with a semiconductor guiding layer that was obtained by use of a nonreciprocal phase shift,” Appl. Opt. 39, 6158-6164 (2000).   10. S. Mino, M. Matsuoka, A. Tate, A. Shibukawa, and K. Ono, “Completely Bi-substituted iron garnet (BIG) films prepared by electron cyclotron resonance (ECR) sputtering,” Jpn. J. Appl. Phys. 1, Regul. Pap. Short Notes 31, 1786-1792 (1992).   11. Z. Haifeng, J. Xiaoqing, Y. Jianyi, Z. Qiang, Y. Tianbao, W. Minghua, T. Yu, and T. Yu, “Wavelength-selective optical waveguide isolator based on nonreciprocal ring-coupled Mach-Zehnder interferometer,” J. Lightwave Technol. 26, 3166-3172 (2008).   12. L. Myoung Soo, H. In Kag, and K. Byoung Yoon, “Bidirectional wavelength-selective optical isolator,” Electron. Lett. 37, 910-912 (2001).   
 
       CONCLUSION 
       [0098]    In summary, embodiments of the invention provide methods for making optical isolators and optical isolators. An optical isolator having semiconductor-device dimensions in accordance with one or more embodiments of the present invention comprises a resonator structure carrying forward light in a first direction and backward light in a second direction, and a magneto-optical film, coupled to the resonator structure, the magneto-optical film creating a shift between the forward light and the backward light travelling in the resonator structure, wherein the resonator structure and the magneto-optical film are sized to reside on a semiconductor chip. 
         [0099]    Such an optical isolator further optionally comprises the resonator structure being a ring resonator structure, the magneto-optical film being coupled on a top of the ring resonator structure, a coil, coupled to the magneto-optical film, for controlling a magnetic field in the optical isolator, controlling a current in the coil filtering at least one of the forward light and the backward light by wavelength, a width and/or a height of the resonator structure being between 100 nanometers and 10 microns, the magneto-optical film being made from a material selected from a group consisting of yttrium iron garnet (YIG), bismuth iron garnet (BIG), bismuth-substituted gadolinium iron garnet (BiGa 2 Fe 5 O 12 ), and cerium-substituted yttrium iron garnet (Ce:YIG), the magneto-optical film being coupled on an inner diameter top of the resonator structure, and the resonator structure being a double ring resonator structure. 
         [0100]    A method for making an optical isolator having semiconductor-device dimensions in accordance with one or more embodiments of the present invention comprises sizing a ring resonator structure for integration on a semiconductor chip, forming the semiconductor-chip sized ring resonator structure to carry forward light in a first direction and backward light in a second direction, and coupling a magneto-optical film to the ring resonator structure, wherein the magneto-optical film creates a shift between the forward light and the backward light travelling in the ring resonator structure. 
         [0101]    Such a method further optionally comprises comprising coupling a coil to the magneto-optical film, controlling a current in the coil filtering at least one of the forward light and the backward light by wavelength, the magneto-optical film being made from a material selected from a group consisting of yttrium iron garnet (YIG), bismuth iron garnet (BIG), bismuth-substituted gadolinium iron garnet (BiGa 2 Fe 5 O 12 ), and cerium-substituted yttrium iron garnet (Ce:YIG), and the semiconductor-device dimensions being between 100 nanometers and 10 microns. 
         [0102]    A semiconductor optical isolator in accordance with one or more embodiments of the present invention comprises an input semiconductor waveguide on a semiconductor chip, an output semiconductor waveguide on the semiconductor chip, a ring resonator on the semiconductor chip, coupled between the input waveguide and the output waveguide, and a magneto-optical film, coupled to the ring resonator, wherein the magneto-optical film creates a shift between a forward light and a backward light travelling in the resonator structure. 
         [0103]    Such an optical isolator further optionally comprises the magneto-optical film being made from a material selected from a group consisting of yttrium iron garnet (YIG), bismuth iron garnet (BIG), bismuth-substituted gadolinium iron garnet (BiGa 2 Fe 5 O 12 ), and cerium-substituted yttrium iron garnet (Ce:YIG), and at least one of a height and a width of the ring resonator is between 100 nanometers and 10 microns. 
         [0104]    The foregoing description of the preferred embodiment of the invention has been presented for the purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise form disclosed. Many modifications and variations are possible in light of the above teaching. It is intended that the scope of the invention be limited not by this detailed description, but by the claims attached hereto and the full breadth of equivalents to the claims.