Abstract:
An all optical logic circuit includes a micro-ring resonator ( 110 ) optically coupled to a waveguide ( 115 ) The waveguide ( 115 ) provides multiple optical input signals (INPUT A, INPUT B) and an optical probe signal (PROBE) at a different frequency (lambda s) than the optical input signals (INPUT A, INPUT B) to the micro-ring resonator ( 110 ) such that the probe signal (PROBE) exhibits logical amplitude transitions as a function of the multiple input signals (INPUT A, INPUT B) The logical amplitude transitions of the optical probe signal (PROBE) correlate to an ANDing or NANDing of the optical input signals (INPUT A, INPUT B) In one embodiment, the all optical logic circuit is an integrated silicon device.

Description:
RELATED APPLICATIONS 
     This application is a nationalization under 35 U.S.C. 371 of PCT/US2007/018845, filed Aug. 24, 2007 and published as WO 2008/024512 A2 on Feb. 28, 2008, which claimed priority under 35 U.S.C. 119(e) to U.S. Provisional Patent Application Ser. No. 60/839,970, filed Aug. 24, 2006; which applications and publication are incorporated herein by reference and made a part hereof. 
    
    
     GOVERNMENT FUNDING 
     This invention was made with Government support under Grant Number F49620-1-0424 awarded by Air Force Office of Scientific Research and NSF career award. The United States Government has certain rights in the invention. 
    
    
     BACKGROUND 
     Logical devices are well known in the semiconductor arts. Various electrical circuits have been used to take one or more electrical input signals and provide an output signal that has a level representative of a logical combination of the one or more input signals. An AND logical device may be used to take two or more input signals and provide an output signal that is a logical “1” only if all of the input signals are also representative of a logical “1”. A logical “1” may correspond to a high voltage, and a logical “0” may correspond to a low voltage. 
     There is a desire to provide such logic functions using optical signals as opposed to electrical signals. It is desired to provide such functions using compact optical devices. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a top view of a silicon ring resonator that provides logic functions according to an example embodiment. 
         FIG. 2  illustrates resonant wavelengths for the ring resonator of  FIG. 1 . 
         FIG. 3  illustrates a transfer function for all-optical pulse modulation of the ring resonator of  FIG. 1 . 
         FIG. 4  is a block schematic diagram of an experimental setup for all optical logic utilizing the ring resonator of  FIG. 1 . 
         FIG. 5  illustrates waveforms of control signals and output as a logic function of a control signal according to an example embodiment. 
         FIG. 6  is a ring resonator with an integrated PIN junction according to an example embodiment. 
         FIG. 7  is a logical block diagram of an optical logic AND device according to an example embodiment. 
         FIG. 8  is a logical block diagram illustrating multiple NAND devices for implementing various logic devices 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 sufficient 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 all-optical logic device with a micron-size silicon ring resonator utilizes a free-carrier dispersion effect in silicon. Logical AND and NAND operations may be provided at 310 Mbps with higher than 10 dB extinction ratio. A free-carrier-lifetime-limited bit-rate can be significantly improved by active carrier extraction in some embodiments. 
     Highly developed micro-fabrication technology enables ultra-compact optical devices integrated on silicon. All-optical logic operation is obtained using an integrated silicon device. 
     In one embodiment shown in a top view generally at  100  in  FIG. 1 , a silicon micro-ring resonator  110  is coupled to a straight waveguide  115 . The resonator and waveguide may be fabricated on a SOI substrate using E-beam lithography, plasma dry etching, and plasma enhanced chemical vapor deposition (PECVD) for SiO2 cladding deposition. The silicon waveguides forming the resonator  110  and waveguide  115  in one embodiment have a width of approximately 450 nm and height of approximately 250 nm. The radius of the ring  110  in one embodiment is approximately R=5 μm, and the spacing between the ring and the straight waveguide is approximately 150 nm. A fiber-to-fiber insertion loss for a quasi-TE mode (electric field parallel to the substrate) is measured to be 10.4 dB in one embodiment. The view of device  100  is prior to the SiO 2  cladding deposition. 
     A transmission spectrum for the quasi-TE mode is shown in  FIG. 2 . Two resonances exist at the wavelengths λ1=1550.7 nm and λ2=1568.7 nm respectively. The transmission of the waveguide drops by about 16 dB at both resonances. The insets  210  and  220  of  FIG. 2  show a zoom-in spectra around both resonant wavelengths. A full-width-half-maximal (FWHM) bandwidths of the resonances are approximately Δλ1=0.14 nm and Δλ2=0.16 nm, corresponding to Q1=11,076 and Q2=9,804, respectively. The wavelengths of the pump (λ C ) and the probes for AND (λ P1 ) and NAND (λ P2 ) gates are marked in the insets. 
     The weak split of resonances, represented by the double-notch feature of the resonant spectrum, may be caused by a weak reflection inside the ring resonator  110 . The photon lifetime of the ring resonator at λ1-resonance can be obtained from Q as τcav1=Q·λ/(2πc)=9.1 ps. This lifetime gives what may be a fundamental limit to the operation speed of the device. In practice, the operating speed of a fabricated device is limited by the longer carrier lifetime. 
     The logic operation is based on an all-optical modulation mechanism in silicon micro-ring resonator. When a strong optical control pulse or input signal and a weak cw probe light are coupled into the ring resonator through two different resonances, the control pulse generates free carriers in the ring resonator due to the two-photon absorption (TPA) effect. The generated free carriers reduce the refractive index of silicon through a plasma dispersion effect, and blue shifts the ring resonances. The probe light is therefore modulated by the resonance shift. After the control pulse leaves, the resonant wavelength and the transmission of the probe light relax back due to the fast surface recombination of the free carriers. The relaxation time is determined by the carrier lifetime of approximately 0.5 ns in the ring resonator  110 . 
     To show the relationship between the modulation depth and the energy of the control pulse, the transfer functions of the all-optical modulation process is illustrated in  FIG. 3  for both the positive modulation, line  310  and negative modulation, line  320 , cases, using a 14-ps long Gaussian pulse as the control pulse. A positive modulation is obtained if the wavelength of the probe light is at λ P1  illustrated in  FIG. 2 , so that the device has low transmission without the control pulse, as line  330  in the inset  335  of  FIG. 3  shows. A negative modulation, shown as line  340  in the inset  335 , is obtained at the probe wavelength of λ P2  with high transmission before the control pulse comes. 
     Due to the nonlinear nature of this TPA-based process, there is a clear threshold energy the control pulse has to pass to obtain large modulations. A logic-gate-like behavior is obtained on both transfer functions with a sharp transition region sandwiched between flat regions. Specifically, there is a 10-dB increase of modulation depth on the positive-modulation transfer function when the control pulse energy increased from 2.6 pJ to 5.2 pJ. Similarly, there is a 10-dB increase of modulation depth on the negative-modulation transfer function when the control pulse energy increased from 4.1 pJ to 8.2 pJ. Therefore, with properly chosen control pulse energy, dramatically different modulations may be obtained with two control pulses together compared to with only one control pulse, enabling logic operations of AND and NAND gates. 
     An experimental setup to demonstrate all-optical logic is shown in  FIG. 4  at  400 . A cw light from tunable laser  410  is modulated at  415  with a PRBS 27-1 return-to-zero (RZ) signal, which is amplified such as by an erbium-doped fiber amplifier (EDFA)  418  and split into two at splitter  420  and sent into a resonator device  425  from two opposite directions via polarization controllers (PC)  430  and  432  to avoid interference between them. One of the control signals may also be attenuated at  434 . The wavelength of the control light in one embodiment is fixed at the shorter-wavelength edge of a resonance of the ring resonator, as shown by an arrow  230  in  FIG. 2 . A cw probe light from a laser  440  tuned at another resonance of the ring resonator, indicated at  240  in  FIG. 2 , is polarized at  442  and coupled together with one of the control signals at  445  and sent into the device  425 . The output of the probe light is separated from the control light using an optical circulator  450  and an optical filter  455 . The waveforms of the probe light are then detected at  460  and observed on an oscilloscope  465 . The polarization of both control and probe light are set to be TE-like by the polarization controllers  430 ,  432  and  442 . A polarizer  470  may also be used to provide the combined control signal and probe to the device  425 . 
     In one embodiment, the two control signals have bit-rates of 310 Mbit/s and pulse widths of 200 ps. The average optical power of each control signal is about 2 mW. The waveforms of these two control signal synchronized at the device are show at  510  and  520  in  FIG. 5 . When both control signal are in logic ‘1’, which means two control pulses are coupled into the ring resonator simultaneously, the total optical power is higher than the threshold to obtain large modulation, and a positive or negative modulation is imposed onto the probe light depending on the wavelength of the probe light. When one or both of the control signals are ‘0’, the total optical power is less than the threshold, and very little modulation is observed on the probe output. This results in the AND (λProbe=λP1) and NAND (λProbe=λP2) operations with extinction ration ˜10 dB, as evident from  530  and  540  respectively. 
     The bit-rate may be limited by the free-carrier lifetime in the resonator. In order to avoid inter-symbol interference, a second control pulse may come in after all the carriers generated by the first control pulse have recombined. To increase the speed, one can be actively extracting the carriers from the ring resonator, instead of waiting for the carriers in the ring to recombine at the Si/SiO 2  interfaces. Carrier lifetime can be reduced to ˜30 ps by reversely biasing a p-i-n junction built across or integrated with the ring resonator, enabling logic operation at ˜5 Gbit/s as shown in  FIG. 6 . 
     In further embodiments, the control pulses may be generated from other logic optical devices as shown in  FIG. 7  generally at an optical AND or NAND device  700 . A first optical input A control signal at  710  and a second optical input B control signal at  720  may be provided to a ring resonator  730 . The input signal control pulses may be provided from independent sources as opposed to a single split laser pulse in the experimental setup. A control probe  740  is also provided to the resonator  730 . The control signals and probe may be provided on the same waveguide coupling to the resonator, or may be provided on separate waveguides. Similarly, an output signal  750  may be provided on a separate or same waveguide. 
     The power levels of the control pulses may be adjusted, and more than two control pulses or input signals may be combined to pass a threshold for logically modeling the probe signal. The term micro-ring resonator is used to describe micron or smaller ring type resonators. The size and other parameters of the ring resonators, such as refractive index may be modified to obtain different desired resonant characteristics. In further embodiments, the probe signal obtained from the resonator may be amplified and fed into further logical optical circuitry, either tuned to treat the probe signal as an input signal to be combined with other probe signals and provide yet a further probe signal output responsive to the input probe signals, or to use the probe signal as another probe signal to be modulated by further input signals. In still further embodiments, the frequency and power level of the probe output signal may be modified to that of an input signal. 
     A NAND gate, such as one incorporating a ring resonator as described herein, has the property of functional completeness, Any other logic function, such as a AND, OR, NOR, etc, can be implemented using NAND gates. An entire processor may be created using NAND gates. In one embodiment shown in  FIG. 8  at  800 , several logical devices  805 ,  810  and  815  may be coupled together to provide further logical functions. Each of the devices may be a NAND gate in one embodiment. Outputs from gates  805  and  810  are provided to respective optical switches  820 ,  825  which operate to control a pair of inputs  830 ,  835  to gate  815 . In one embodiment, the outputs of gates  805  and  810  are amplified such as by EDFAs, and control the optical switch to modulate the input signals  830  and  835  to gate  815  at desired times. Inputs  830  and  835  may also be amplified to proper levels to function as control signals for logic gate  815 . In one embodiment, the all-optical switch has the same two resonant frequencies as the NAND gate, and the output probe has a power level that is over the threshold intensity for full modulation of the input signal. If the output is high, the all-optical switch lets a new input signal pass into the next NAND gate, and if it was low, no input signal would pass (or vice versa). Several more NAND gates may be cascaded in this manner to provide a multitude of logical functions. 
     In various embodiments of all optical logic devices, the output of one all-optical logic device may be used as an input for a second all-optical device. The input to both all-optical logic devices has the same wavelength in one embodiment. A third all-optical logic device may be used as a converter, and the two inputs to such converter have the same wavelength as the probes and outputs of the first and second all-optical logic devices, and the wavelength of the probe and hence the output of the converter is the same as that of the input to the first and second all-optical logic devices. 
     In one embodiment, the coupling between the probe and the resonator of the converter is such that when the inputs to the converter are zero, the output from the converter is low or zero. In this embodiment, the output from the first all-optical logic devices provides a first input to the converter. A second input, which can be constant, at the same frequency is provided to the converter from another source such that the power of the second input alone is sufficiently below the threshold to achieve deep modulation of the converter&#39;s probe signal that little or none of the probe passes through the resonator and there is hence no output. However, when the first input is high, the sum of the first and second inputs to the converter exceeds the modulation threshold, and the probe signal passes through the resonator into the output waveguide, thereby providing an input to the second all-optical logic device at the proper input wavelength. 
     In another embodiment, the coupling between the probe and resonator of the converter is such that the probe signal passes through the resonator unmodulated when the inputs are zero, and the converter operates to convert a low output from the first all-optical logic devices into a high input to the second all-optical logic devices, or to convert a high output from the first all-optical logic devices into a low input to the second all-optical logic devices. 
     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 interpret or limit the scope or meaning of the claims.