Abstract:
An optoelectronic apparatus for controlling a signal includes an optical waveguide having a variable refractive index; an active device formed within the waveguide, the device having three electrodes, a drain, a source and a gate; and wherein the device is located within the waveguide so that current flowing from the drain to the source changes the refractive index.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is a divisional of, and claims priority from, commonly-owned, U.S. patent application Ser. No. 11/859,665, filed on Sep. 21, 2007 now U.S. Pat. No. 7,711,212. 
    
    
     STATEMENT REGARDING FEDERALLY SPONSORED-RESEARCH OR DEVELOPMENT 
     Not Applicable. 
     INCORPORATION BY REFERENCE OF MATERIAL SUBMITTED ON A COMPACT DISC 
     Not Applicable. 
     FIELD OF THE INVENTION 
     The invention disclosed broadly relates to the field of electro-optic devices and more particularly relates to the field of electro-optic modulators. 
     BACKGROUND 
     Electro-optic modulators use an electric field to change the index of refraction of the substrate or medium where the beam of light (or laser) is found. Electro-optic modulators are optical devices which allow control of the power, phase, or polarization of a beam of light with an electrical control signal applied via a capacitor, p-i-n diode, or JFET, for example. The functionality is based on an electro-optic effect, that is the modification of the refractive index of a medium (e.g., crystal substrate) by an electric field in proportion to the field strength. Electro-optic devices can use free carrier dispersion to produce the refractive index change. In semiconductor materials, such as Silicon, the refractive index of light may be varied by varying the charge carrier concentration along the optical path. For example, when an electric potential is supplied to a Silicon P-N junction to forward bias a diode, the doped regions inject charge carriers into the Silicon depletion region. By free carrier dispersion, the change in the concentration of free charge carriers alters the refraction index. 
     Up to now the devices used to change the concentration of free charge carriers have been capacitors and p-i-n/p-n diodes. The majority of electro-optic modulators make use of the injection and/or depletion of free carriers, and the associated free carrier dispersion, within a region of the optical waveguide in which light is confined. Two types of two-terminal electrical devices have predominantly been used to modulate the density of free carriers, these being metal-oxide-semiconductor (MOS) capacitors, and p-i-n/p-n (diodes). The primary drawback of the MOS capacitor configuration is the limited overlap between the approximate 10 nm thick depletion/accumulation/inversion layer (where the concentration of free carriers is modulated by the gate voltage), and the optical mode, having a spatial extent of approximately 500 nm×500 nm, typically. While the p-i-n diode device geometry has been designed to dramatically improve upon the optical overlap issue, several other problems exist. 
     The p-i-n diode modulator can be used in two primary modes of operation. The first, in which the diode is first forward biased to inject minority carriers into the depletion region/waveguide core, and is then reverse biased to sweep out these carriers, is intrinsically slow. In forward bias, while forward current is flowing, the charge density within the waveguide takes a much longer time to reach steady state in comparison with when the diode is reverse biased. This results from the slow dynamics of minority carriers in silicon, and limits the fundamental modulation frequency at which an optical modulator/switch can be driven. 
     The second mode of operating a p-i-n diode modulator is in reverse bias depletion mode only. In this case, the diode is never forward biased to inject carriers, and only the concentration of the existing majority carriers (from dopants, thermal generation, etc.) is modulated within the diode depletion region/waveguide core. While the reverse bias only mode of operation enables intrinsically much faster modulation than permitted by the forward-reverse mode, the magnitude of modulation of the free carrier concentration is approximately ten times smaller, implying a ten times smaller change in the refractive index caused by free carrier dispersion. Therefore, p-i-n diode modulators operated in reverse bias only mode must be approximately ten times as long in order to attain the same modulation depth. The device footprint is thus adversely affected in order to obtain high speed modulation, often requiring the usage of traveling wave electrodes, further complicating the design. 
     Prior art two-terminal p-i-n/p-n diode modulators have the limitations that in forward-reverse bias operation, the forward biased diode turn-on is slow, restricting operation at higher speeds. 
     In reverse bias only operation, there is no introduction of excess carriers as in forward bias case, leading to the limitation that the net change in carrier concentration and waveguide refractive index is small. An additional solution to these problems is therefore required, in order to enable high frequency modulation/switching, within an ultra-compact device footprint. 
     SUMMARY OF THE INVENTION 
     The invention adds a control terminal to the control device to provide a finer adjustment to the injection of these free carriers into the modulating medium by controlling the depletion region of a JFET and thus obtain better response times. 
     Briefly, according to an embodiment of the invention an apparatus for controlling a light signal includes an optical waveguide having a variable refractive index; an active device formed within the waveguide, the device having three electrodes, a drain, a source and a gate; and wherein the device is located within the waveguide so that current flowing from the drain to the source changes the refractive index. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       To describe the foregoing and other exemplary purposes, aspects, and advantages, we use the following detailed description of an exemplary embodiment of the invention with reference to the drawings, in which: 
         FIG. 1A  is a cross-section of a junction field effect transistor (JFET) electrooptic waveguide, illustrating a state of operation with high drain-source current according to another embodiment of the invention. 
         FIG. 1B  is a cross-section of the JFET electrooptic waveguide of  FIG. 1A , illustrating a state of operation with low drain-source current according to another embodiment of the invention. 
         FIG. 2A  is a cross-section of a gated diode electrooptic waveguide control device operating at a high current state for an optical modulator according to another embodiment of the invention. 
         FIG. 2B  is a cross-section of a gated diode electrooptic waveguide control device operating at a low current state for an optical modulator according to another embodiment of the invention. 
         FIG. 3  is a Mach-Zehnder Interferometer with incorporated electrooptic phase shifter section, illustrating drain (D), source (S), and gate (G) terminals of the JFET according to another embodiment of the invention. 
         FIG. 4  is a flowchart of a modulator method according to another embodiment of the invention. 
     
    
    
     While the invention as claimed can be modified into alternative forms, specific embodiments thereof are shown by way of example in the drawings and will herein be described in detail. It should be understood, however, that the drawings and detailed description thereto are not intended to limit the invention to the particular form disclosed, but on the contrary, the intention is to cover all modifications, equivalents and alternatives falling within the scope of the present invention. 
     DETAILED DESCRIPTION 
     An embodiment of the invention makes use of the extra performance and functionality introduced by incorporating a third control terminal into the electrical device used to modulate free carrier concentration within the optical modulator. Several embodiments of the disclosed device are possible, however, the primary geometry makes use of a junction field effect transistor (JFET), incorporated within an optical waveguide. 
     Referring to  FIG. 1A , there is shown an illustration of an optical waveguide  100  whose characteristics (e.g., refractive index) are modified by the JFET  102 . The waveguide  100  includes the JFET  102  formed within the waveguide  100 . The light travels perpendicular to the plane of the cross-section of the JFET  102 . The JFET has a drain  104 , a source  106 , and a gate  108 . The drain and source are formed in a p-type semiconductor material (e.g., silicon). The parts of the p-type region where the drain and source are located are more heavily doped than its middle. The part of the n-type region where the gate is located is more heavily doped than the part which connects to the drain-source channel. A positive voltage is applied at the drain  104  and a voltage, V gate , greater than the drain voltage (V drain ) is applied at the gate. The drain to source path forms a channel where the current, I ds , which flows perpendicular to the light passing through the optical waveguide  100 . A depletion region  110  is formed at the junction between the n-type region and the p-type region. Under the conditions of  FIG. 1A , this depletion region  110  is small and the current I ds  is positive while the refractive index (Δη) is also positive. 
     A background concentration of majority carriers is introduced within the optical waveguide, by means of applying a bias current through the conductive channel (between the two terminals, source  108  and drain  104 ) within the waveguide  100  itself. The gate terminal  108  is used to modulate the concentration and/or spatial distribution of the free carriers within the optical waveguide  100 , thereby modulating the refractive index affecting the light traveling within the waveguide. Such a JFET geometry permits the relative refractive index change between the ON and OFF states to be as large as in the case of forward-reverse operation of a p-i-n diode, on account of providing the bias current. In addition, the intrinsic modulation speed of the JFET configuration can be as large as that of the reverse bias only mode of the p-i-n diode, because only majority carriers are modulated by the gate terminal. 
     Referring now to  FIG. 1B , there is illustrated the optoelectronic device of  FIG. 1A , operating at a low current state. In this case, the drain voltage (V drain ) is positive and the gate voltage (Vg or V gate ) is much greater than the drain voltage (V drain ). This produces a bigger depletion region  110 , causing the drain to source current I ds  to be substantially smaller such that the change in the refractive index of the waveguide is approximately zero. 
       FIG. 2A  is a cross-section of a gated diode  202  electrooptic waveguide  200  operating at a high current state where Vd&gt;0, Vd&lt;&lt;Vg, according to another embodiment of the invention. The gated diode  202  includes a drain made from a p+ (heavily doped)) semiconductor material and the source is made from an n+ (highly doped) semiconductor. 
     Referring to  FIG. 2B  there is shown a cross-section of the gated diode of  FIG. 2   a  operating in a low current state according to another embodiment of the invention. In this state of operation, the depletion region  210  is longer than in the state of  FIG. 2A . This results in a condition where Ids and Δn are approximately zero. 
       FIG. 3  is a Mach-Zehnder Interferometer with incorporated electrooptic phase shifter section, illustrating drain (D), source (S), and gate (G) terminals of the JFET according to another embodiment of the invention. In practical application, JFET or gated diode electrooptic waveguides may be used as optical phase shifters, and can be combined with passive waveguides to form a variety of electrically controlled tunable/modulated optical devices. Examples may include but are not limited to phase shifters, directional couplers, power splitters, interferometers, resonators, switches, filters, and modulators.  FIG. 3  illustrates one such possible device application in the form of a Mach-Zehnder interferometer  300 . At the input  302 , light is divided into two waveguide branches by an optical power splitter  304 . In the one branch, light travels through a passive waveguide, while in the other branch, light passes through an active phase shifter section  306 , which incorporates a JFET or gated diode electrooptic waveguide. The drain, source, and gate terminals of the JFET are illustrated schematically in the figure. The relative phase, or optical path length, between the active and passive branches is controlled by the gate voltage at the electrooptic waveguide, as discussed above. At the output  308  of the Mach-Zehnder interferometer, light in both waveguide branches is recombined with a second power splitter/combiner. The relative phase of the light in the two branches at the output results in either constructive or destructive interference, leading to amplitude modulation of the transmitted output power. In this manner, the Mach-Zehnder interferometer with an incorporated JFET phase shifter section can be used as an optical amplitude modulator. 
     Referring to  FIG. 4  there is shown a flow chart illustrating a method  400  of modulating a light beam carrier signal passing though a waveguide. In step  402  there is provided a majority carrier bias current through a conductive (low resistance) channel to improve the total ΔP (or ΔN) (carrier concentration) swing, compared to reverse bias only operation. In step  404  the conductivity of the channel using a gate electrode is modulated. Step  406  depletes the channel width using a reverse biased diode. Step  408  uses spatial modulation of current across waveguide, and current extinction by moving depletion region, modulates Δn (refractive index) across waveguide. This mode of operation results in a similar Δn as forward-reverse bias operation of p-i-n, with the speed of minority carrier dynamics as in reverse bias only operation. 
     The result of the method  400  is similar to a JFET device, but uses a forward biased p-i-n diode to provide the bias current. However, current will be mostly minority in this case, as opposed to majority current through the JFET resistive channel. 
     Biasing Scheme for Operation: 
     The JFET electrooptic waveguide is biased into the high current state by application of a drain-source current Ids flowing transversely through the rib waveguide core, as shown in the top portion of  FIG. 1A . This is a bias current consisting of majority carrier holes, on account of the being p+-p−-p+ doping profile between the drain and source terminals. This bias condition requires a positive drain voltage Vdrain&gt;0, where the source terminal is assumed to be connected to ground, Vsource=0. In order to prevent the drain-gate junction from becoming forward biased, a small reverse bias must be placed on the gate, such that Vdrain&lt;Vgate. However, in the high current state, this reverse bias must not be excessively large, so as to produce a depletion region which pinches off the conductive channel between the drain and source terminals. In the high current state, the drain-source current produces a large increase in the charge carrier concentration within the waveguide core (as compared to the case with Ids is approximately zero), resulting in a significant change in the refractive index of the waveguide. 
     In contrast, the low current state is achieved by increasing the reverse bias voltage on the gate, such that Vdrain&lt;&lt;Vgate. This causes the high resistivity depletion region to expand further into the waveguide core, dramatically decreasing the conductivity of the path between drain and source terminals, and effectively turning off the drain-source current, Ids is approximately zero. In the low current state, the charge carrier concentration within the waveguide core is much lower than in the high current state, returning the waveguide refractive index back to near its unperturbed value |Δn|˜0, with no voltage or current applied at the terminals. 
     Biasing and gate operation for the gated diode configuration is similar, with the exception that the drain-source bias current will begin to flow when V exceeds a minimum voltage, in order to reach the forward bias condition of the drain-source p-n/p-i-n junction. The operating bias current Ids can be set as desired by adjusting Vdrain as needed. 
     Details of Doping Profile for JFET and Gated Diode Configurations: 
     The exact doping profile required for device operation ultimately depends upon multiple design factors, including but not limited to tolerable free-carrier induced optical losses, restrictions on the available external bias current/voltage control circuitry, and minimization of deleterious ohmic heating effects within the device. Nevertheless, the table below presents a general description of the doping profile required to achieve the desired mode of operation. 
     
       
         
               
             
               
               
               
               
             
           
               
                 TABLE 1 
               
             
             
               
                   
               
               
                 General doping profile and polarity for 
               
               
                 p-channel JFET electrooptic waveguide. 
               
             
          
           
               
                   
                 Region 
                 Dopant density 
                 Polarity 
               
               
                   
                   
               
               
                   
                 Channel 
                 Low/Medium 
                 p-type 
               
               
                   
                 Drain 
                 High 
                 p-type 
               
               
                   
                 Source 
                 High 
                 p-type 
               
               
                   
                 Gate 
                 Medium/High 
                 n-type 
               
               
                   
                   
               
             
          
         
       
     
     
       
         
               
             
               
               
               
               
             
           
               
                 TABLE 2 
               
             
             
               
                   
               
               
                 General doping profile and polarity for p-channel 
               
               
                 gated diode electrooptic waveguide. 
               
             
          
           
               
                   
                 Region 
                 Dopant density 
                 Polarity 
               
               
                   
                   
               
               
                   
                 Channel 
                 Low/Medium 
                 p-type 
               
               
                   
                 Drain 
                 High 
                 p-type 
               
               
                   
                 Source 
                 High 
                 n-type 
               
               
                   
                 Gate 
                 Medium/High 
                 n-type 
               
               
                   
                   
               
             
          
         
       
     
     Those skilled in the art will understand that the device operation described here can also be extended to similar devices in which the conductive channel within the waveguide core is doped to be n-type rather than p-type (with the necessary inverted polarity of the drain, source, and gate regions). In addition, variations of the doping profile described here while preserving the overall operation of the devices are understood to be broadly included within the scope of this patent. 
     Therefore, while there has been described what is presently considered to be the preferred embodiment, it will understood by those skilled in the art that other modifications can be made within the spirit of the invention.