Patent Publication Number: US-8983241-B2

Title: Optical waveguide switch

Description:
This application is a national stage application of PCT Application No. PCT/CN2011/084423, filed on Dec. 22, 2011, which claims the benefit of U.S. Provisional Application No. 61/426,400, entitled “High extinction ratio optical waveguide switch using asymmetric Mach-Zehnder interferometer,” by Bing Li, and filed on Dec. 22, 2010, the entire content of which is incorporated herein by reference. 
    
    
     TECHNICAL FIELD 
     The disclosure relates to optical waveguide switches in general and, more particularly, to waveguide switches utilizing Mach-Zehnder interferometers. 
     BACKGROUND 
     It is a common technology to use Mach-Zehnder interferometer (MZI) to construct an optical waveguide switch or modulator. In U.S. Pat. No. 7,817,881, the waveguide capacitor concept is introduced in which the free carriers can be stored inside the waveguide core in order to modulate the refractive index of the material. 
     In silicon photonics, the free-carrier dispersion effect is used to modulate the refractive index of the silicon, and then, produces a switch or amplitude modulation through the MZI. However, in the free-carrier dispersion effect, the injected free carrier affects not only the real part of the refractive index, but also the imaginary part of the index, i.e., it will cause the attenuation of the light while it modulates the phase. As a result, optical switches of the MZI type can not reach a very high extinction ratio due to the fact that the two light beams from two arms of the MZI will have the different power level when they interfere. 
     SUMMARY 
     In general, this disclosure describes an optical waveguide switch that could reach a very high extinction ratio. It should be noted that the optical waveguide switch disclosed here shall be constructed by semiconductor material. In particular, this disclosure describes an asymmetric MZI, in which different waveguide capacitor structures are used in two arms of the MZI: one arm with a waveguide capacitor in which the free carriers have small mobility and therefore magnitude modulation is much greater than the phase modulation, and the other arm with a waveguide capacitor in which the free carriers have large mobility and therefore the phase modulation is much greater than the magnitude modulation, respectively. Using the asymmetric MZI in accordance with this disclosure, one can design an algorithm to achieve almost unlimited extinction ratio during the switching operation. 
     In one embodiment, the optical waveguide switch comprises an asymmetric MZI, wherein one arm comprises an abrupt PN junction and the other arm has a PIN junction. One can intentionally increase the background ion implant density with the net implant density fixed, by applying implants with reversed tone (P or N) alternatively and making them overlap to each other. In this manner, a very high extinction ratio could be achieved. 
     In another embodiment, this disclosure is directed to an optical waveguide switch using an asymmetric MZI including a waveguide capacitor on each arm. The waveguide capacitor is a ridge-loaded waveguide capacitor structure that is constructed by loading a semiconductor film on top of a regular ridge waveguide. On one arm, which can be referred as the magnitude-modulation arm, the loaded semiconductor film and the silicon ridge of the waveguide capacitor will be heavily doped (background ion density is high but not the net doping density), while they are lightly doped on the other arm, the phase modulation arm. 
     In another embodiment, this disclosure is directed to an optical waveguide switch using an MZI including two waveguide capacitors on each arm. Each arm of the MZI has both phase section and magnitude section. The phase section comprises a waveguide capacitor with lightly doped loaded semiconductor film and silicon ridge. The magnitude section comprises a waveguide capacitor with heavily doped loaded semiconductor film and silicon ridge. 
     The details of one or more aspects of the disclosure are set forth in the accompanying drawings and the description below. Other features, objects, and advantages will be apparent from the description and drawings, and from the claims. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a simplified diagram of a top view depicting one example configuration of an optical waveguide switch in accordance with this disclosure. 
         FIG. 2  is a cross-sectional view taken along line  1 - 1  of the optical waveguide switch depicted in  FIG. 1 . 
         FIG. 3  is a simplified diagram depicting an optical waveguide switch with an asymmetric MZI using ridge-loaded waveguide capacitor in accordance with this disclosure. 
         FIG. 4  is a schematic diagram of an optical waveguide switch with phase and magnitude sections in each arm of the MZI in accordance with this disclosure. 
     
    
    
     DETAILED DESCRIPTION 
     This disclosure describes an optical waveguide switch using asymmetric MZI to achieve a high extinction ratio, in which different waveguide capacitor structures are used in two arms of the MZI: one arm with a waveguide capacitor in which the background ion density is high to achieve the mainly magnitude modulation and the other arm with a waveguide capacitor in which the background ion density is low to achieve mainly the phase modulation, respectively. It should be noted that the optical waveguide switch disclosed here shall be constructed by semiconductor material. Two particular waveguide capacitors having these properties are the PN junction built in a waveguide and PIN junction built in a waveguide. The PN junction has higher background-ion density and the PIN junction has lower background-ion density. Here, the said “higher” or “lower” is relative to each other, i.e., the one is higher than the other and then the other is lower. 
       FIG. 1  is a simplified diagram of a top view depicting one example configuration of an optical waveguide switch in accordance with this disclosure. As shown in  FIG. 1 , in one embodiment, from the input end to the output end, an optical waveguide switch  40  comprises two input ports  51 ,  52 , a pair of input silicon waveguides  17 ,  18 , input 3-dB coupler  3  coupling the waveguides  17 ,  18 , two arms  19 ,  20 , a pair of output silicon waveguides  24 ,  25 , output 3-dB coupler  4  coupling the waveguides  24 ,  25  and output ports  53 ,  54 . Arm  19  comprises an abrupt PN junction  190 . Arm  20  comprises a PIN junction  200 . Magnitude modulation electrode  22  is located over the P doping area  191  of the abrupt PN junction  190  of arm  19 , while phase modulation electrode  23  is built over the P doping area  201  of the PIN junction  200  of arm  20 . GND electrode  21  covers a portion of the N doping areas  192 ,  202  of both the abrupt PN junction  190  and the PIN junction  200 . 
     The difference between the abrupt PN junction  190  and the PIN junction  200  is the background ion implant density. According to the free-carrier dispersion theory, 
     
       
         
           
             
               
                 
                   
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     Where, Δα represents the imaginary part of index modulation (corresponding to the attenuation), and Δn is the real part of the index modulation. One will notice that the imaginary part of the index modulation is related to the mobility of the carriers (μ) while the real part of the index modulation (produce the phase shift) is irrelevant to the mobility. And the mobility of the free carriers inside silicon could be derived by the expression (3). 
     
       
         
           
             
               
                 
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     As seen in the expression (3), μ varies with the background ion implant N s . In the abrupt PN junction, the volume where the carrier injection takes place has either P-type or N-type doping, versus that, in PIN junction, the volume where the carrier is injected is intrinsic originally (i.e., there is no background ion density). Therefore, the PIN junction will have more Δn and less Δα, comparing to the PN junction, in which, when the background implant density is very high, the Δα (the magnitude modulation) will be much more significant than Δn (the phase modulation). 
     In particular, one can intentionally increase the background ion implant density with the net implant density fixed, by applying implants with reversed tone (P or N) alternatively and making them overlap to each other. So, the PN junction designed and fabricated in such way will be very efficient on introducing the attenuation to the light, while the other arm (arm  20 ) of the MZI with PIN junction will be efficient on generating the phase modulation with small parasitic attenuation. 
     Once this asymmetric MZI in  FIG. 1  is constructed, one can design an algorithm to achieve almost unlimited extinction ratio during the switching operation. Assuming the input and output 3-dB couplers  3 , 4  are perfectly balanced (in real application, an un-perfect balanced 3-dB coupler can also be compensated with a modified algorithm), as well as the static loss and optical length of the two arms  19 , 20  of the MZI, the output of the device in  FIG. 1  can be derived as the follows, 
     
       
         
           
             
               
                 
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     are the output of the input 3-dB coupler  3 , where B1 is corresponding to input amplitude of the arm  19  and B2 is corresponding to input amplitude of the arm  20 . And, 
     
       
         
           
             
               
                 
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     are the light-wave fields at the end of the two arms of the MZI. C1 is for PN junction arm  19 , and C2 is for the PIN junction arm  20 . And then, the output of the device in  FIG. 1  at the output ports  53 ,  54  of the output 3-dB coupler  4 , can be described by the following expression (6), 
     
       
         
           
             
               
                 
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     Where, D1 is the output of the port  53  and D2 is the output of the port  54 . In the expression (5), the item −Δα(N epn ,N hpn ,N s )·L pn  is the attenuation introduced by the arm  19 , in which the L pn  is the length of the PN junction and Δα is the attenuation constant determined by expression (1), where the N epn , N hpn , and N s  are the injected electron density, hole density, and background ion density respectively. The item −j·Δβ(N epin ,N hpin )·L pin  is the phase modulation introduced by the arm  20 , and the Δβ is the phase propagation constant modulation caused by the index modulation expressed by the expression (2). And N epin  and N hpin  are the injected carrier densities and L pin  is the length of the PIN junction  200  respectively. 
     Without any carrier injection to either arm, the device in  FIG. 1  is at the CROSS state, where the output of the output ports  53 ,  54  could be described as expression (7). 
     
       
         
           
             
               
                 
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     Here, we ignore the static loss of the 3-dB couplers and the MZI&#39;s arms. To bring the device into the BAR state, one injects carriers into the arm  20  to introduce π phase shift. If no carrier injection into the PN junction  190 , the C1 and C2 in expression (5) will have un-balanced power level due to the item of −Δα(N epin ,N hpin ,0)·L pin . For instance, if one introduce 5×10 17  cm^-3 carrier density modulation for both electrons and holes in PIN junction  200 , the extinction ratio D1/D2 will be less than 30 dB. 
     The compensation injection into the PN junction  190  is calculated by the following iteration process: 1) introduce an attenuation along the PN junction arm  19  and make it equal to the attenuation caused by −Δα(N epin ,N hpin ,0)·L pin ; 2) then increase the injection into the PIN junction  200  slightly to balance the phase item of the magnitude arm −j·Δβ(N epn ,N hpn )·L pn ; 3) then modify the PN junction injection to make the attenuation balanced again; 4) repeat the step 1)-3) until both required carrier injection converged. With 2×10 18  cm^-3 background ion density in the PN junction  190  area, only extra 23.5% percent carrier injection into the PIN junction  200  is required, to improve the extinction ratio of the BAR state up to almost infinity, if no other un-perfections need to be considered. 
       FIG. 2  is a cross-sectional view taken along line  1 - 1  of the optical waveguide switch depicted in  FIG. 1 . In  FIG. 2 , PN junction  190  is a waveguide capacitor formed by a ridge waveguide, which comprises the P doping area  191  and the N doping area  192 . Also, PIN junction  200  is a waveguide capacitor formed by a ridge waveguide, which comprises the N doping area  202 , intrinsic area  203  and the P doping area  201 . The capacitance is the junction capacitance of PN or PIN. Please note that the background ion density of the PN junction  190  is higher than that of the PIN junction  200 . 
     The methodology of the optical waveguide switch using the asymmetric MZI invented here can have more varieties of implementation (or embodiments).  FIG. 3  is a simplified diagram depicting an optical waveguide switch asymmetric MZI using ridge-loaded waveguide capacitor in accordance with this disclosure. 
     Waveguide capacitor is a terminology to describe the optical waveguide inside which there is a capacitor to store the charges (the electrons or holes in semiconductor). It is a basic building block for all electro-optic devices using free carrier dispersion effect, as described in U.S. Pat. No. 7,817,881. Actually, the previous described PN junction and PIN junction built in optical waveguides are also two types of waveguide capacitors where the capacitance itself is the junction capacitance of PN or PIN. 
     As seen in  FIG. 3 , in this embodiment, the optical waveguide switch  40  comprises an asymmetric MZI using waveguide capacitors. In contrast to the structure in  FIG. 1 , the abrupt PN junction and the PIN junction are replaced by waveguide capacitors. In  FIG. 3 , the waveguide capacitor  26  is a ridge-loaded waveguide capacitor structure that is constructed by loading a semiconductor film  260  on top of a regular ridge waveguide  261  (assuming it is a silicon-on-insulator waveguide). The ridge waveguide  261  comprises silicon slab  262  and the silicon ridge  263 . In the same way, the waveguide capacitor  27  is formed by loading a semiconductor film  270  on top of a regular ridge waveguide  271 . The ridge waveguide  271  comprises silicon slab  272  and the silicon ridge  273 . If the loaded semiconductor is a poly-silicon, and the thin dielectric film sandwiched in-between (not depicted) is the gate-oxide layer, the capacitance that this waveguide capacitor utilized is actually MOS capacitor in a regular CMOS process. On the arm  19  (the magnitude modulation arm), both loaded semiconductor film  260  and the silicon ridge  263  will be heavily doped with background ion density. Please note that it is done by alternatively applying P- and N-type doping and the volume must have low net implant density to avoid heavy free-carrier attenuation when there is no electrical voltage applied. On the arm  20  (the phase modulation arm), the loaded semiconductor film  270  and the silicon ridge  273  will be lightly doped to ensure little background ion density. 
     The net doping density on both arms needs also to be great enough to ensure the conductivity of the material so that the waveguide capacitor  26  and  27  can be charged or discharged in a time short enough. The net doping density on the loaded semiconductor and the silicon underneath can be non-uniform, e.g., the center portion (corresponding to the waveguide ridge) can have less net density, while the side portion (on the side of the waveguide ridge) can have more net density, in order to satisfy the requirement of the high conductivity and low optical attenuation of the waveguide. 
     The invented method can also have other different types of configurations.  FIG. 4  is a schematic diagram of an optical waveguide switch with the phase and the magnitude sections in both arms of the MZI in accordance with this disclosure. 
     In this embodiment as shown in  FIG. 4 , the physical structures of two arms of the MZI are actually the same. Each arm of the MZI has both phase section and magnitude section. Arm  19  has the phase section  15  and the magnitude section  16 , while arm  20  comprises the phase section  28  and the magnitude section  29 . As described above referring to the waveguide capacitors in  FIG. 3 , the phase section  15  has the waveguide capacitor structured the same as the waveguide capacitor  27  in  FIG. 3 , and the magnitude section  16  structured the same with the waveguide capacitor  26  in  FIG. 3 . Please note that on the arm  20  in  FIG. 4 , the phase section  28  has the same structure with the phase section  15  and the magnitude section  29  is of the same structure with the magnitude section  16 . 
     However, the device in  FIG. 4  will be operated in an asymmetric manner: the phase sections  15  and  28  of the two arms (also referred to as north and south arm) will be operated differentially, i.e., when the phase section  15  of the north arm  19  has a carrier injection, the phase section  28  of the south arm  20  will have a carrier extraction, and vice versa. The magnitude section  16  and  29  afterwards will be used to compensate the parasitic attenuation caused by the operation in the phase section  15  and  28 . The detailed algorithm of the operation can be derived using a similar procedure described earlier in this disclosure. 
     Various aspects of the disclosure have been described. These and other aspects are within the scope of the following claims.