Abstract:
An optical integration circuit includes a semiconductor optical amplifier (SOA), a readout mechanism coupled to the SOA, and an optical filter coupled to an output of the SOA. The SOA has a decaying response function and an input for receiving an optical input signal having a first wavelength. The SOA is configured to output an optical signal representing a temporal integration of the optical input signal. The readout mechanism provides an optical readout signal having a second wavelength to the SOA for measuring a state of the SOA. The optical filter is configured to receive the signal representing the temporal integration of the optical input signal and block optical signals having the first wavelength.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a national stage application under 35 U.S.C. §371 of International Application No. PCT/US2010/026830 filed Mar. 10, 2010, which claims priority benefit to U.S. Provisional Patent Application No. 61/158,986, filed Mar. 10, 2009, which is incorporated herein by reference herein in its entirety. 
    
    
     FIELD OF DISCLOSURE 
     The disclosed system and method relate to optical computational systems. More specifically, the disclosed system and method relate to an optical system for performing an integration. 
     BACKGROUND 
     Optical signals have a high bandwidth which has led to them being incorporated in many signal processing applications. Accordingly, various optical circuits have been developed for performing various computations such as adding or subtracting. However, devices for performing complex signal processing computations, such as integration, have not been developed. 
     Accordingly, a device for performing optical integration is desirable. 
     SUMMARY 
     An optical integration circuit is disclosed including a semiconductor optical amplifier (SOA), a readout mechanism coupled to the SOA, and an optical filter coupled to an output of the SOA. The SOA has a decaying response function and an input for receiving an optical input signal having a first wavelength. The SOA is configured to output an optical signal representing a temporal integration of the optical input signal. The readout mechanism provides an optical readout signal having a second wavelength to the SOA for measuring a state of the SOA. The optical filter is configured to receive the signal representing the temporal integration of the optical input signal and block optical signals having the first wavelength. 
     A method is also disclosed in which an optical input signal and an optical signal of a pulse train are received at a semiconductor optical amplifier (SOA), an optical signal having an amplitude that is an integral of the optical input signal is output, and the integrated optical signal is filtered to remove the optical input signal. The optical input signal has a first amplitude and a first wavelength, and the optical signal of the pulse train has a second amplitude and a second wavelength. 
     An optical integration circuit including an optical coupler, a semiconductor optical amplifier (SOA), and an optical filter is also disclosed. The optical coupler is configured to receive a first plurality of optical input signals each having a first wavelength and a second plurality of optical readout signals each having a second wavelength. The SOA is configured to receive an optical signal having the first and second wavelengths from the optical coupler and to output an optical signal representing a temporal integration of the optical input signal. The optical filter is coupled to an output of the SOA and is configured to receive the optical signal representing to temporal integration from the SOA and remove optical signals having the first wavelength. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a block diagram of one example of an optical integration circuit. 
         FIG. 2  shows a cross section of an exemplary semiconductor optical amplifier (SOA) useful in the circuit of  FIG. 1 . 
         FIG. 3A  illustrates one example of input pulses received at an input of the optical integration circuit. 
         FIG. 3B  illustrates one example of output pulses from the optical integration circuit in response to the receiving the input pulses illustrated in  FIG. 3A . 
         FIG. 4A  is an energy versus time graph showing the response of a semiconductor optical amplifier that receives a series of optical pulses. 
         FIG. 4B  is an example oscilloscope trace of a plurality of optical signals of a pulse train. 
     
    
    
     DETAILED DESCRIPTION 
     Semiconductor optical amplifiers (SOAs) have been widely used in optical systems, SOAs experience cross-gain modulation (XGM) in which the gain of an SOA is depleted immediately after an optical pulse passes through the SOA and it then gradually increases over time. For many applications, the XGM of an SOA is undesirable and thus other optical amplifiers such as doped fiber amplifiers and Raman amplifiers are implemented. However, the system and method disclosed herein utilize the XGM of an SOA to advantageously provide an optical circuit for performing temporal integration of an optical input signal. 
       FIG. 1  is a block diagram of one example of an optical integration circuit or system  100 . As shown in  FIG. 1 , the optical integration circuit  100  includes a readout mechanism  102  coupled to an input of optical coupler  104  having a plurality of inputs. An SOA  106  is coupled to an output of the optical coupler  104 , and an opticai filter  108  is coupled to an output of the SOA  106 . 
     Readout mechanism  102  may be any device configured to provide one or more signals to SOA  106  for reading out a current state of SOA  106 . For example, readout mechanism  102  may be a pulse train generator configured to provide an optical pulse train in which the optical signals have uniform wavelengths and amplitudes. A mode-locked ring fiber laser (MLL) configured to provide pulses on the order of picoseconds is one example of such an optical pulse train generator. 
     Optical coupler  104  may be any optical coupler configured to couple optical signals of different wavelengths and amplitudes in separate fibers into a single fiber. In one example, the optical coupler  104  has two inputs with one input receiving optical signals from readout mechanism  102  and the other input receiving an optical input signal. An example of a suitable fiber coupler  104  is a thermally tapered and fused pair of single-mode fibers, with the cores of the fiber pair coming into contact such that optical energy may be exchanged. If optical coupler  106  is a multiport coupler, it may be implemented as a tree of 2:1 couplers as will be understood by one skilled in the art. The optical signals of the pulse train may have a wavelength λ 0 , and the optical input signals may have one or more wavelengths, λ 1 , λ 2 , etc., which are different from wavelength λ 0 . Additionally, the optical input signals have an amplitude that is greater than the amplitude of the optical signals provided by the readout mechanism  102  such that the readout signals do not have a significant effect on the XGM of the SOA  106  as described below. 
     SOA  106  is coupled to an output of the opticai coupler  104  and is configured to receive a combined optical input signal, which is a combination of the optical input signal and the readout signals from readout mechanism  102 . One example of an SOA  106  is illustrated in  FIG. 2 . As shown in  FIG. 2 , the SOA  106  includes a semiconductor substrate  200 , which may be a Group III-V compound substrate as will be understood by one skilled in the art. Substrate  200  may be an n-type substrate having an n-doped region  202  and a p-doped region  204 . Metal layers  206  and  208  may be formed on a top and a bottom surface of the substrate  200 . As shown in  FIGS. 1 and 2 , the charge pumping circuit  110  is coupled to SOA  106  for restoring the gain of SOA  106  through population inversion once the gain of the SOA  106  has been depleted. Charge pumping circuit  110  may be implemented as an electrical circuit in which a current is supplied to the substrate of the SOA  106 , or charge pumping circuit  110  may be implemented as an optical circuit in which light is used to perform population inversion of the SOA  106 . 
     Optical filter  108  is coupled to an output of the SOA  106  and is configured to pass the wavelengths of the readout signals and block the wavelengths of the optical input signals. For example, the optical filter  108  may be a short-pass, long-pass, or band-pass filter such as a thin film multi-layer dielectric filter, a fiber Bragg grating, or an arrayed waveguide grating, to name a few. 
     The operation of the optical integration circuit  100  is described with reference to  FIG. 1 . The optical input signals having a wavelength λ 1  are combined with readout signals provided by readout mechanism  102  having a wavelength λ 0  at optical coupler  104 . The combined optical signal is output to SOA  106 . 
     SOA  106  is pumped with electrons from the charge pumping circuit  110 , which contributes to the gain of the SOA  106 . When a pulse from one of the optical input signals having a wavelength λ 1  is received at the SOA  106 , the gain of the SOA  106  is depleted due to the depletion of electrons, which are used to increase the amplitude of the optical input signals. The external pumping of the SOA  106  by the charge pumping circuit  110  causes the gain of the SOA  106  to gradually increase, but if another pulse is received from an optical input signal, then the gain of the SOA  106  will again be depleted. The recovery time of the gain of the SOA  106  is based on its carrier lifetime, T e , which functions as the integration time constant. Thus, the gains of SOAs having smaller carrier lifetimes will increase at faster rates than the gains of SOAs having larger carrier lifetimes. Consequently, the faster gain recovery results in less temporal integration as will be understood by one skilled in the art. 
     The SOA  106  outputs a signal representing a temporal integration of the optical input signal to optical filter  108 . Optical filter  108  may be tuned such that the optical input signals having one or more wavelength λ 1 , λ 2 , etc., which are different from the wavelength, λ 0 , provided by readout mechanism  102 , are removed or otherwise filtered out. As described above, the optical filter  108  may be a long-pass, short-pass, or band-pass filter configured to pass the wavelengths of the readout signals while blocking the wavelengths of the optical input signals. 
     An optical integration circuit in accordance with  FIG. 1  was designed and tested. The optical signals of the pulse train were generated using a supercontinuum generator with spectral slicing to generate optical signals having pulse widths of approximately 3 picoseconds full-width at half-maximum (FWHM). 
     SOA  106  was an Alcatel A1901SOA available from Alcatel-Lucent of Murray Hill, N.J. The resting potential of SOA  106 , i.e., the maximum gain of the SOA when a control signal was not present, was equal to 43 fJ. A master pulse source having a 1.25 GHz mode-locked ring fiber laser (“MLL”) was used to generate the optical input signals having a digital value of ‘01210’ as illustrated in  FIG. 3A . The digital output of the optical integration circuit  100  is shown in  FIG. 3B . 
       FIG. 4A  is an energy versus time graph illustrating the response of the SOA  106  to excitation by multiple pulses of optical input signals, which are shown in  FIG. 4B . As shown in  FIG. 4A , each input optical input pulse decreases the gain of the SOA  106  due to XGM. The gain of the SOA  106  gradually increases over time due to the charge pumping circuit  110  providing electrons to the SOA  106 . The SOA carrier lifetime, T e , was approximately equal to 180 ps, but was adjustable between 100 to 300 ps by altering the pump current received from the charge pumping circuit  110 . 
     Although the optical integration circuit and method have been described in terms of exemplary embodiments, they are not limited thereto. Rather, the appended claims should be construed broadly, to include other variants and embodiments of the circuit and method, which may be made by those skilled in the art without departing from the scope and range of equivalents.