Abstract:
A doping profile for a modulator facilitates rapidly changing the carrier density in a waveguide. The carrier density change causes rapid changes in the index of refraction of the waveguide. Example modulators include a ring modulator and a Mach Zender modulator. A charge reciprocating section may be provided to control the amount of injected charge.

Description:
RELATED APPLICATION 
     This application claims priority to U.S. Provisional Application Ser. No. 61/099,859 (entitled: PINIP Electrooptic Modulator in Silicon Operating At the Optical Phonon Limited Speeds, filed Sep. 24, 2008) which is incorporated herein by reference. 
    
    
     GOVERNMENT FUNDING 
     This invention was made with Government support under Grant Number 0300387 awarded by the National Science Foundation and under Grant Number W911NF-06-1-0057 awarded by the U.S. Army Department of Defense. The United States Government has certain rights in the invention. 
    
    
     BACKGROUND 
     An all-silicon electro-optic modulator is a key component in electronic photonic integrated circuits. Carrier dispersion based electro-optic modulators on Silicon-On-Insulator substrates have been demonstrated based on a MOS capacitor, a PIN diode or a PN junction. However, in order to achieve high performance devices with high extinction ratio for high data rate on small silicon foot print, one needs to break the traditional tradeoff between speed and extinction ratio. MOS based devices can potentially scale in speed to many tens of Gbits/s; however the effective index change obtained is limited due to small overlap of the optical mode with carrier concentration change. On the other hand PIN based devices with laterally formed junctions provide high extinction ratio but are limited in speed due to the carrier injection dynamics. Hence a tradeoff exists between speed and extinction ratio due to the available electrooptic structures. The present invention is a device which achieves both high-speed and high-extinction ratio through a novel doping profile. 
     SUMMARY 
     An electro-optic modulator back-to-back diodes structure facilitates rapidly changing the carrier density in a photonic structure to rapidly change the index of refraction of the photonic structure. A ring modulator or a Mach-Zehnder modulator may be used as the photonic structure. A charge reciprocating section may also be used to allow for controlling amount of injected charge. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1A  is a cross section representation of an optical modulator according to an example embodiment. 
         FIG. 1B  is a top view of an optical modulator integrated with a ring resonator according to an example embodiment. 
         FIG. 2  is a top view of the optical modulator of  FIG. 1B  with contacts for providing a modulation signal according to an example embodiment. 
         FIG. 3  is a cross section of an alternative modulator according to an example embodiment. 
         FIG. 4  is a cross section representation of a further alternative modulator according to an example embodiment. 
         FIG. 5  is a cross section representation of yet a further alternative modulator according to an example embodiment. 
         FIG. 6  is a cross section representation of still a further alternative modulator according to an example embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     In the following description, reference is made to the accompanying drawings that form a part hereof, and in which is shown by way of illustration specific embodiments which may be practiced. These embodiments are described in detail to enable those skilled in the art to practice the invention, and it is to be understood that other embodiments may be utilized and that structural, logical and electrical changes may be made without departing from the scope of the present invention. The following description of example embodiments is, therefore, not to be taken in a limited sense, and the scope of the present invention is defined by the appended claims. 
     An optical modulator achieves both high speed and high extinction ratio through a novel doping profile. Various embodiments of the modulator include two back-to-back diodes formed by laterally doped layers of p-i-n-i-p. The electrical rise and fall times in one embodiment are 10 ps and 15 ps respectively which approach a fundamental limit imposed by carrier terminal velocity in silicon for a photonic structure such as waveguide geometries dictated by index contrast in silicon on insulator (SOI) substrates. Based on such a structure, an electro-optic modulator may be formed that operates at 40 Gbit/s non-return to zero (NRZ) with a high extinction ratio (&gt;10 dB) within the dimensions of ˜10 microns. 
       FIG. 1A  is a cross section representation an optical modulator  100  that includes a photonic structure such as a waveguide  105  embedded in a PINIP device  110  in the shape of a ridge. The PINIP device  110  in one embodiment is formed of a two P doped silicon regions  115 ,  120  that are separated from an N doped area  125  by intrinsic regions  130 ,  135 . P doped region  115  may be coupled to a contact region  140 , and P doped region  120  may be coupled to a contact region  145 . In some embodiments, the PINIP device  110  is formed as a relatively thin slab on SOI, whereas the waveguide  105  is substantially thicker. A charge reciprocating area or ridge  150  may be formed on intrinsic region  135 . The PINIP device  110  provides high speed transitions of carrier density in the waveguide. The refractive index of the waveguide is modulated due to the carrier dispersion effect in silicon. The doping levels and dimensions of one example device are outlined in Table 1. The concentrations shown in Table 1 are example concentrations only, and may be varied over a wide range of concentrations to optimize performance for various size and configuration modulators. 
     
       
         
               
             
               
               
               
             
           
               
                 TABLE 1 
               
               
                   
               
               
                 PINIP Device Parameters 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                   
                 Intrinsic region doping 
                 5 × 10 16 /cm 3   
               
               
                   
                 N region doping 
                 10 19 /cm 3   
               
               
                   
                 P region doping 
                 10 19 /cm 3   
               
               
                   
                 N region width 
                 600 nm 
               
               
                   
                 Waveguide dimensions (width × height) 
                 450 nm × 250 nm 
               
               
                   
                 Distance from waveguide edge to doped 
                 300 nm 
               
               
                   
                 regions 
               
               
                   
                   
               
             
          
         
       
     
     In one embodiment, the PINIP device may be formed as a ring resonator, as illustrated a top view in  FIG. 1B . The PINIP structure is integrated into a silicon ring cavity on an SOI platform in one embodiment. Note that only the outer ring, waveguide  105  supports an optical mode while the inner ring, ridge  150  is used only as part of the electrical PINIP structure. 
     The PINIP device  110  operates as a high field transport device where carriers are accelerated through the intrinsic region at the saturation velocity in silicon. This structure may be used for study of high field behavior of electrons (NIPIN) and holes (PINIP). The high field, near saturation velocity transport in PINIP may be used for electro-optic modulation in an SOI photonic device. As discussed above, waveguide  105 , which is in the shape of a ridge, is used for guiding light, while ridge  150  is used as a charge reciprocating structure. The double ridge structure creates symmetry in the electrical response of the PINIP device. The charge injecting regions are connected to a strip waveguide  170  through a 50 nm thick slab of intrinsic silicon  175 . The entire structure may be clad in SiO 2 . 
     The charge injecting regions in one embodiment have uniform doping concentrations of 10 19 /cm 3 . The wave guiding regions may be slightly p doped with a typical dopant concentration of 5×10 16 /cm 3  so that the carrier density changes are unipolar. This significantly decouples the performance of the device from the time response of recombination of electrons and holes. Such decoupling is useful to avoid pattern dependency and timing jitter associated with carrier dispersion devices due to recombination effects. 
     An example modulator formed in accordance with modulator  100  may exhibit a carrier injection time of 10 ps and extraction time of 15 ps enabling ˜100 GHz operation of a silicon electro-optic device. These times allow the device to operate at optical phonon limited speeds (carrier velocity of 10 7  cm/s) in a silicon electro-optic device. Such a device may be integrated with a microring resonator or a Mach-Zehnder interferometer to form an electro-optic modulator. Such modulators may operate at 40 Gbit/s with 12 dB extinction ratio and 2.25 fJ/bit/micron-length power dissipation. In some embodiments, the modulator may be used as a part of a wavelength division multiplexing scheme to increase the number of optical wavelengths in use to &gt;25. This would allow modulations of greater than one Tera bit /second optical data streams. Such modulators may overcome the hold time restriction of PIN devices. A data ON state can be held for hundreds of nanoseconds enabling usage of telecom quality data with long sequences of ones and zeros. Such modulators may provide an extra level of optical design flexibility where double ring optical response can be tuned to increase the dispersion tolerance for long distance communications. 
     The modulator  100  has a free carrier dispersion of silicon that may be modeled by the following equations for the refractive index and absorption coefficient for a wavelength of 1.55 μm in silicon.
 
Δ n=Δn   e   +Δn   h =−(8.8×10 −22   Δn+ 8.5×10 −18 (Δ p ) 0.8 )
 
Δα=Δα e +Δα h =8.5×10 −18   Δn+ 6.0×10 −18   Δp  
 
     where Δn is the change in refractive index, Δα is the change in absorption coefficient of intensity, ΔN is the injected electron density per cm 3 , and ΔP is the injected hole density per cm 3  The deviation from the classical Drude model is included in the 0.8 power dependency on the hole concentration which arises due to the non-parabolic shape of the band structure of silicon. 
     In one embodiment, PINIP device  110  consists of two adjacent diodes arranged in opposite directions and sharing the N doped region. It is symmetrical about the N doped region in one embodiment. Charge transport takes place during the turn-on and turn-off times of the diodes resulting in fast carrier density changes. The turn-on and turn-off times of the diodes may be determined by the time taken for the carriers to form the depletion region as they are swept under high electric fields. The carriers are accelerated to the carrier terminal velocity in silicon (10 7  cm/s) under electric fields exceeding 10 4  V/cm. By using a symmetric electrical structure for the diodes, fast transients are produced during the build up of and depletion of carriers. 
     Simulated electrical transient characteristics show that the PINIP device conducts only during the transition time of the applied voltage thus creating fast electrical transitions. Asymmetry in the rise and fall times is believed due to a non-uniform distribution of the electric field in the intrinsic region. The rise time is determined by the transit time of carriers from the thin slab region to the center of the waveguide region. The electric field in the slab region is higher than the electric field in the waveguide region, leading to a faster rise time (10 ps) as compared to the fall time. The rise time of this device is 2 orders of magnitude smaller than the rise time in PIN carrier injection devices which is on the order of 1 ns (determined by the free carrier lifetime). The device also shows reduced dependence of peak carrier density on the applied voltage. This reduces the effect of noise in the applied voltage on the output waveforms. 
     The dual-diode PINIP device provides a way to control injected charge while enabling high speed transitions. When a positive voltage is applied, the injection of carriers is believed to stop as soon as the second diode goes into reverse bias. As a consequence, the injected charge is limited to the charge required to reverse-bias the second diode. The injected charge is clamped to the intrinsic hole concentration (which is identical in both diodes). Similarly, when a negative voltage is applied, the first PIN region will be reverse-biased and the charge injected into the second PIN region is controlled by the first PIN region. The peak charge concentration may be controlled by the amount of charge that can be exchanged between the forward and reverse diodes. 
       FIG. 2  illustrates a top view of connections for providing electrical signals to a PINIP optical ring modulator  100 . A signal is provided by contact  145  to region  120 , while region  115  has contacts  140 . This arrangement of contacts may be used to test the device, while other contact arrangements may also be used to provide suitable electrical connections to the PINIP diodes for modulating light. Such connections may be formed using common processing steps used to form silicon based integrated circuitry. 
     The transient optical response of the PINIP double-ridge ring modulator may be compared with the optical response of a PIN embedded ring modulator. Both the devices were assumed to be fabricated using waveguides with a 1 ns carrier life time (total surface recombination velocity of 16,000 cm/s due to surface recombination and interface recombination). The ring resonator may be modeled as a ring cavity with a lossless coupler as an input. The ring cavity may be simulated by iteratively calculating the fields in the ring and the coupler. In response to a symmetric square pulse of ±2.5 V for modulators with a quality factor of 20,000 in a single PIN modulator one can observe the effect of storage time due to accumulated carriers. In a PINIP modulator this effect is not present. A chirp-like transient at the rising edge of optical transmission may occur and improves the eye opening and is caused by the interference between optical energy being released from the cavity and the input optical energy. The optical fall time of the PINIP modulator is given by the photon lifetime of the cavity. The turn-off time is determined by the optical ring-down time of the cavity given by the photon lifetime (λ 2 /2πcΔλ=16 ps) for a cavity with a quality factor of 20,000). 
     Since the device is unipolar, the effect of the carrier recombination on the device performance is small in the absence of the oppositely charged carriers. Note that even though the surface states at the oxide/silicon interface act as traps for the carriers they do not lead to recombination. This is in strong contrast to PIN-based device performance which strongly depends on the recombination lifetime. The transmission of both PIN and PINIP devices for surface recombination velocities (SRVs) of 100 cm/s to 20,000 cm/s reveals a strong performance dependence of the PIN device on the SRV in strong contrast to the performance of PINIP devices which shows no SRV dependence. 
     The PINIP devices can modulate data with extremely long identical bit sequences since the state hold time is &gt;1 μs in the absence of carrier recombination processes. The state hold time is limited by the leakage current of the structure which mainly arises from thermal generation in the depletion region. The PINIP devices also do not suffer from the timing jitter that is characteristic of PIN-based carrier injection modulators. 
     A simulation of the structure showed electro-optic modulation at 40 Gbit/s in an non return to zero (NRZ) modulation scheme with a resonator of quality factor 5,000. A relatively low quality factor resonator is used since in the absence of electrical fall time limitations, the speed of modulation is now given only by the cavity ring-down time. Some embodiments extend the speed of carrier injection modulators from a few Gb/s to as high as 40 Gb/s. In some embodiments, an applied voltage and corresponding optical transmission profile for an arbitrary bit sequence modulated with an extinction ratio (defined as 10 log 10 (P high /P low )) of 12 dB at 40 Gbit/s, and an assumed a loss of 8 dB/cm in the ring under critical coupling conditions result in an insertion loss of 3 dB at 40 Gbit/s with a peak injection of 5×10 16  cm −3 . The insertion loss and extinction ratio can be improved by optimizing the doping profiles or by designing a filter shape using multiple rings or a single add-drop ring filter. 
     In one embodiment, a maximum sequence of ones (logic high bits) that a modulator such as modulator  100  can modulate is greater than 1000 bits. The length of identical bit sequence is limited only by the storage time of carriers determined by the leakage current of the device making this an ideal component for on-chip modulation for intra chip communication. The estimated power dissipation of the device is 2.25 fJ/bit/micron length. The energy per bit is estimated from the total charge injected per bit per micron length of the waveguide (0.9 fC/bit/micron) multiplied by the switching voltage (5 V) and the bit transition probability (0.5). The modulator does not draw current while the state is being held except for the parasitic leakage current. This is in contrast to PIN devices where the recombination of carriers has to be compensated with a steady state current inversely proportional to the carrier lifetime. The compact size also avoids the need for traveling wave electrodes and reduces the drive current requirements. The modulator can be driven by an analog CMOS driving circuit made on the same SOI substrate. 
     In various embodiments, a high speed silicon electro-optic device may increase the modulation rate beyond 40 Gbit/s, and may be limited only by the photon lifetime of the cavity. The device shows electrical transitions of 10 ps which is close to the fundamental limit imposed by carrier saturation velocity in silicon for the dimensions dictated by the index contrast in an SOI system. In some embodiments, a 40 Gbit/s operation has a 12 dB extinction ratio and 2.25 fJ/micron energy dissipation per bit in a 10 micron-sized device limited only by the photon lifetime of the structure. 
     A further embodiment of the modulator is illustrated in  FIG. 3  at  300 . A Mach-Zehnder interferometer which uses a PINIP structure  310  in one of the arms  315 ,  320  to create an electro-optic modulator. Structure  310  has P doped regions  325  and  330 , and an N doped region  335 . Arm  315  extends through the modulator as a waveguide  340 , separated from a charge reciprocating structure  345  by N doped region  335 . In  FIG. 4  at  400 , the PINIP structure  410 ,  415  is shown in both arms. PINIP structure  410  and  415  may have the same structure as structure  310 , each including waveguide  425  and charge reciprocating structure  430 . The disclosed device uses a charge reciprocating structure (by incorporating a complimentary waveguide-like structure) to allow for control on the amount of injected charge. In general, this feature can be extended to a wide variety of doping profiles/ waveguide geometries. In some embodiment, the charge reciprocating structure or region is a part of the device that has an excess of holes or electrons that can be easily, rapidly, withdrawn for injection into the waveguide. The region also rapidly accepts the holes or electrons that are withdrawn from the waveguide. The close physical proximity of the region provides a way to rapidly inject and extract the charge within short time intervals. 
     In further embodiments, alternate doping profiles include NIPIN, NPIN, PNIP or any other multilayered doping profile which allows for incorporation of a charge reciprocating structure. The charge reciprocating structure could in general be a rectangular waveguide like feature or any other possible geometry that can be fabricated in a given semiconductor process sequence. 
       FIG. 5  is a cross section representation of a ring assisted Mach-Zehnder configuration at  500 . One arm includes a PINIP structure  510 . Another arm includes a ring resonator  520 , forming a ring assisted Mach-Zehnder interferometer. As in  FIG. 3 , the structure  510  may have a waveguide  525  and charge reciprocating structure  530 . 
       FIG. 6  is a cross section representation of an alternative ring assisted Mach-Zehnder configuration at  600 . In this embodiment, one arm includes a ring based PINIP modulator  610 . The PINIP modulator  610  include a waveguide  615 , P doped region  620  on the outside of the ring, and N doped region  625  and a charge reciprocating ring  630 . It may be similar in design to the modulator shown in  FIG. 1B  in some embodiments. 
     In various embodiments, the PINIP electro-optic modulator is a micrometer scale silicon device that can switch light at speeds exceeding 100 billion times per second. It can be manufactured in an industrial silicon CMOS fabrication facility. Carrier injections times of 10 ps and extraction times of 15 ps enable 100 GHz operation. When integrated into a resonator, the micron-size device may operate at 40 Gbits/s with 12 dB extinction ratio and 2.25 fJ/bit/micron-length power dissipation Due to its compact size it can be integrated into silicon microchips enabling data rates exceeding 10 trillion bits per second per interconnect with 100 channels. For telecommunications with 100&#39;s of channels, bandwidths approaching 10 trillion bits per second may be achieved on a purely silicon based platform. 
     The Abstract is provided to comply with 37 C.F.R. §1.72(b) to allow the reader to quickly ascertain the nature and gist of the technical disclosure. The Abstract is submitted with the understanding that it will not be used to limit the scope or meaning of the claims.