Patent Abstract:
An optical modulator structure includes at least two waveguide structures for inputting and outputting an optical signal. At least one ring resonator structure provides coupling between the at least two waveguide structures. The at least one ring resonator structure includes Ge or SiGe.

Full Description:
PRIORITY INFORMATION 
     This application claims priority from provisional application Ser. No. 60/759,877 filed Jan. 18, 2006, which is incorporated herein by reference in its entirety. 
    
    
     SPONSORSHIP INFORMATION 
     This application was made with government support awarded by DARPA under Grant No. HR0011-05-C-0027. The government has certain rights in the invention. 
    
    
     BACKGROUND OF THE INVENTION 
     The invention is related to the field of ring resonators, and in particular to Ge/Si resonator-based modulators for optical data communications in silicon photonics. 
     It is highly desired to have a field effect based modulator by using materials compatible with Si-CMOS platform. Ring resonators are gaining more and more interest due to its very small footprint (&lt;a few tens μm), extremely high sensitivity to refractive index change, large extinction ratio and small power consumption. There are several reports on Si based ring modulators, where the refractive index change is induced by free carrier absorption. 
     It is well known that field effect devices are theoretically able to operate at the highest speed. Epitaxial SiGe on Si has been proposed for modulator devices by using Franz-Keldysh effects. However, there are several challenges needed to be solved in order to achieve workable ring modulator. First, Ge on Si is a high refractive index contrast system and its single mode dimension size is very small. Next, the index difference between Si and Ge is very large and it results in a very small coupling efficiency between Si waveguide and Ge (or SiGe) ring. Furthermore, depending on the operating composition of Si in SiGe, the Q-factor of Ge (or SiGe) ring could be low due to intrinsic absorption, which could result a low extinct ratio. 
     SUMMARY OF THE INVENTION 
     According to one aspect of the invention, there is provided an optical modulator structure. The optical modulator structure includes at least two waveguide structures for inputting and outputting an optical signal. At least one ring resonator structure provides coupling between the at least two waveguide structures. The at least one ring resonator structure includes Ge or SiGe. 
     According to another aspect of the invention, there is provided a method of performing optical modulation. The method includes using at least two waveguide structures for inputting and outputting an optical signal. Also, the method includes providing coupling between the at least two waveguide structures using at least one ring resonator structure, the at least one ring resonator structure comprising Ge or SiGe. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIGS. 1A-1B  are schematic diagrams illustrating the inventive ring resonator modulator structure; 
         FIGS. 2A-2B  are schematic diagrams illustrating another embodiment of the inventive ring resonator modulator structure; 
         FIG. 3  is a schematic diagram illustrating an embodiment of a ring resonator modulator structure having multiple ring resonators; and 
         FIGS. 4A-4B  are graphs demonstrating the overall performance of the invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The invention provides a ring resonator modulator structures that utilizes either Ge or SiGe ring resonators. This allows for more compact ring resonator structures that can be used in modulator structures requiring less space and higher performance. 
       FIGS. 1A-1B  show the inventive ring resonator modulator structure  2 . In particular,  FIG. 1A  shows a top view of the ring resonator modulator structure  2  and  FIG. 1B  shows a cross-section view of the ring resonator modulator structure  2 . The ring resonator modulator structure  2  includes a Ge or SiGe ring resonator structure  4 . A doped poly-Si layer  10  is formed on top of the ring resonator structure  4  and acts as the top contact. A Si substrate  12  with an opposite doping type acts as the bottom contact to provide the vertical field and RF signal. 
     Input and output waveguides  6  and  14  are located laterally next to the ring resonator structure  4  to provide lateral coupling between the input and output waveguides  6  and  14  and the ring resonator structure  4 . The waveguides  6  and  14  can include Si or SiON waveguide. Short channel waveguides  8  can be inserted into the input and output waveguides  6  and  14  at the area close to ring resonator structure  4  to enhance side coupling. The short channel waveguides  8  can comprise Ge or SiGe. The ring resonator structure  4  and the short channel waveguides  8  can be fabricated by selective growth of Ge or SiGe in a trench. Both TE and TM can be used to couple into and out of the waveguides  6  and  14 . A resonator with a Q on the order of 100 that permits ultrafast modulation speeds (resonant photon lifetime on the order of 160 fs) with adequate extinction ratio on the order of 3 to 4 dB. 
       FIGS. 2A-2B  shows another embodiment of the inventive ring resonator modulator structure  20 . In particular,  FIG. 2A  shows a top view of the ring resonator modulator structure  20  and  FIG. 2B  shows a cross-section view of the ring resonator modulator structure  20 . The ring resonator modulator structure  20  includes a Ge or SiGe ring resonator structure  22 . The ring resonator structure  22  is formed on a dopant layer comprising two n-type regions and a p-type region formed between the n-type regions. The dopant layer  28  is formed on a Si substrate  26 . 
     Input and output waveguides  24  are located laterally on the edge to the ring resonator structure  22  to provide lateral coupling between the input and output waveguides  24  and the ring resonator structure  22 . The waveguides  24  can include Si or SiON waveguide. Both TE and TM can be used to couple into and out of the waveguides  24 . A resonator with a Q on the order of 100 that permits ultrafast modulation speeds (resonant photon lifetime on the order of 160 fs) with adequate extinction ratio on the order of 3 to 4 dB. 
       FIG. 3  show an embodiment of a ring resonator modulator structure  30  having multiple ring resonators  32 . The ring resonator modulator structure  30  is similar to the ring resonator modulator structure  2  discussed for  FIG. 1 , except the ring resonator modulator structure  30  includes multiple ring resonator structures  32 . Input and output waveguides  34  are located laterally next to the ring resonator structures  32  to provide lateral coupling between the input and output waveguides  34  and the ring resonator structures  32 . 
     The waveguides  34  can include Si or SiON waveguides. A short channel waveguides  36  can be inserted into the input and output waveguides  34  at the area close to ring resonator structures  32  to enhance side coupling. The short channel waveguides  36  can comprise Ge or SiGe. The ring resonator structures  32  and the short channel waveguides  36  can be fabricated by selective growth of Ge or SiGe in a trench. Both TE and TM can be used to couple into and out of the waveguides  34 . A resonator with a Q on the order of 100 that permits ultrafast modulation speeds (resonant photon lifetime on the order of 160 fs) with adequate extinction ratio on the order of 3 to 4 dB. 
       FIGS. 4A-4B  are graphs demonstrating the overall performance of the invention.  FIG. 4A  shows actual and theoretical results associated with the relationship between the extinction ratio, Q value, and insertion loss of a single ring resonator modulator structure. Graph  40  illustrates the theoretical results of Si ring resonator structure and graph  42  show actual results of Ge ring resonator structure used in accordance with the invention. There is significant lower insertion loss in graph  42  as compared to graph  40 .  FIG. 4B  shows actual and theoretical results associated with the relationship between the extinction ratio, Q value, and insertion loss of a multiple ring resonator modulator structure having multiple Si ring resonator structures, shown in graph  44 , and multiple Ge ring resonator structures, shown in graph  46 . The multiple Ge ring resonator structures show lower insertion loss as compared to the multiple Si ring resonator structures for the same range of extinction ratios. Thus, this proves the performance is increased using the structures described herein. 
     Inclusion of a larger ring radius modulator increases the extinction ratio, although the insertion loss also increases (though not linearly proportional). Insertion loss includes of two components: loss from non-unity (less than 100%) coupling into the microring, and loss from material absorption. A larger ring proportionally increases the extinction ratio given the increased interaction length; a larger ring increases the material absorption insertion loss but does not affect the non-unity coupling insertion loss. An improvement of the extinction ratio divided by insertion loss can thus be expected with a larger ring radius high-speed intensity modulator. 
     Inclusion of higher-order filters will also increase the extinction ratio with a likewise expense of increasing insertion loss. Racetrack resonators further improve the ring-waveguide coupling, resulting in a larger extinction ratio for a given insertion loss in the modulator. 
     Absorption and refractive index change of Ge or SiGe material under electric field has been modeled based on Franz-Keldysh effect, and has been experimentally confirmed. Full first-principles numerical simulations of a Ge/Si ring intensity modulator have confirmed the physical operation of the device and have been used to design the device for optimum performance. These full 3D Finite Difference Time Domain (FDTD) simulations have been used to identify the waveguide design parameters that will make the best modulator, based on realistic achievable Ge/Si material absorption values. Numerical simulations have also been used to determine the best polarization that works for several device geometries. 
     Various modulator geometries have been simulated to extract the insertion loss and the extinction ratio. The current design results illustrate an insertion loss of 4.31 dB and an extinction ratio of 3.31 dB. These values are for a geometry that consists of a microring modulator with a 5 μm diameter and a 0.15 μm ring-waveguide gap. 
     A resonator having a Q in the order of 100 corresponds to photon lifetimes of 160 fs (1.5 THz). The RC limited bandwidth of the device is well above 40GHz. The device operates at a low voltage of &lt;3.3V and the power consumption is in the order of a few mW, compared to several W of LiNbO 3  or BaTiO 3  modulators currently used in telecommunications. 
     Also, the designed intensity modulator discussed here is directly compatible with the process flow of a germanium detector, permitting full optical channelizers in a monolithic silicon CMOS compatible chipset. The process flow to create this high-speed modulator is immediately compatible with CMOS process line foundries. The modulator is compact (in the order of 10× wavelengths or less in physical size), permitting high-density integration of photonic and electronic drivers and circuits on an identical materials platform. 
     Although the present invention has been shown and described with respect to several preferred embodiments thereof, various changes, omissions and additions to the form and detail thereof, may be made therein, without departing from the spirit and scope of the invention.

Technology Classification (CPC): 6