BACKGROUND OF THE INVENTION 0001 1. Field of the Invention 0002 The present invention relates to a method of embodying all-optical XOR logic gate by using semiconductor optical amplifiers. More particularly, the present invention relates to a technique to embody all-optical XOR logic gate by utilizing the inverter characteristic of semiconductor optical amplifiers controllable with input currents, illumination signals, and pumping signals. 0003 2. Description of the Related Art 0004 In recent years, needs for high speed and large capacity of calculating systems have been increased. However, almost all calculation systems based on silicon material and electric signals have barriers in speed and capacity. 0005 On the other hand, calculation systems using optical elements based on InP are coping with such barriers of speed and information processing capacity. 0006 Generally, single optical logic elements such as AND, OR, XOR, NAND, NOR, and NXOR are integrated to constitute calculating systems. The integration technique is indispensable to calculation systems using optical signals. 0007 Recently, researches have been focused on the XOR logic element development since XOR logic has various applications to communication purposes such as decision-making and packet switching (Refer to IEEE Photonics Technology Letters, Vol. 13, No. 7, pp. 750-752 (2001) by T. Fjelde, A. Kloch, D. Wolfson, B. Dagens, A. Coquelin, I. Guillemot, F. Gaborit, F. Poingt, and M. Renaud.). Examples include: XOR using Ultrafast Nonlinear Interferometer (UNI) presented by C. Bintjas, M. Kalyvas, G. Theophilopoulos, T. Stathopoulos, H. Avramopoulos, L. Occhi, L. Schares, G. Guekos, S. Hansmann, and R. Dall'Ara in IEEE Photonics Technology Letters, Vol. 12, No. 7, pp. 834-836 (2000); XOR using Tetrahertz Optical Asymmetric Demultiplexer (TOAD) presented by Pousite, Blow, Kelly, and Manning in Opt. Commun. 156, pp. 22-26 (1998); XOR using Sagnac gate presented by T. Houbavlis, K. Zoiros, A. Hatziefremidis, H. Avramopoulos, L. Occhi, G. Guekos, S. Hansmann, H. Burkhard, and R. Dall'Ara in Electronics Letters, Vol. 35, No. 19, pp. 1650-1652 (1999); and XOR using Interferometric Wavelength Converter (IWC) presented by T. Fjelde, D. Wolfson, A. Kloch, B. Dagens, A. Coquelin, I. Guillemot, F. Gaborit, F. Poingt, and M. Renaud in Elecronic Letters, Vol. 36, No. 22, pp. 1863-1864 (2000). 0008 Aforementioned elements using UNI, TOAD, and Sagnac gate have the advantage of high operation speed but are difficult to apply to optical calculation systems requiring high density integration because their core element is optical fiber that is difficult to integrate with other elements. 0009 On the other hand, optical logic elements using single Semiconductor Optical Amplifier (SOA) are not only stable, small-sized, and easy to combine with other optical elements but also have the merit of having independence on polarization and wavelength. (Refer to Elecronic Letters, Vol. 36, No. 22, pp. 1863-1864 (2000) by T. Fjelde, D. Wolfson, A. Kloch, B. Dagens, A. Coquelin, I. Guillemot, F. Gaborit, F. Poingt, and M. Renaud.) A forementioned IWC is an element made by integrating these semiconductor optical amplifiers, but not suitable for large-scale fabrication due to its complicated fabrication process. 0010 Moreover, since XOR logic elements disclosed so far depend on clock signal or continuous wave (CW) light, they need additional beam other than input signals A and B.

SUMMARY OF THE INVENTION 0011 The present invention is to resolve the aforementioned problems in the prior art. It is an object of the present invention to provide a method embodying all-optical XOR logic elements by using cross-gain modulation (XGM) of semiconductor optical amplifiers that are more stable and easier to combine with other elements than optical fiber-based elements and that can reduce the size and speed limitation of logic elements by eliminating the need of clock signal or CW light. 0012 To achieve the objects of the present invention, the present invention embodies all-optical XOR logic elements by impinging pump-signals and illumination signals together onto two semiconductor optical amplifiers and then summing output signals with the XGM characteristic arising from gain saturation and wavelength conversion of said semiconductor optical amplifiers. 0013 The present invention does not have clock signal or CW light dependence, since it embodies XOR logic element by using the XGM characteristic of two single semiconductor optical amplifiers. 0014 Therefore, if signal A and signal B that are input to the two single semiconductor optical amplifiers have the same speed, element realization is possible without clock signal. 0015 In the XOR logic element in accordance with the present invention, since Boolean A{overscore (B)} and {overscore (A)}B are obtained first and then summed, signals A{overscore (B)} and {overscore (A)}B are separable. Therefore, various forms of logic are realizable with using only one all-optical XOR logic element. 0016 The above and other features and advantages of the present invention will be more clearly understood for those skilled in the art from the following detailed description in conjunction with the accompanying drawings, which form part of this disclosure.

BRIEF DESCRIPTION OF THE DRAWINGS 0017 FIG. 1 shows the basic logic concept of XOR logic element in accordance with the present invention. 0018 FIG. 2 illustrates an output screen of Boolean logic signal A{overscore (B)} by XOR logic element in accordance with the present invention. 0019 FIG. 3 illustrates an output screen of Boolean logic signal {overscore (A)}B by XOR logic element in accordance with the present invention. 0020 FIG. 4 shows an experimental configuration for embodying XOR logic element in accordance with the present invention. 0021 FIG. 5 illustrates an output screen of logic signal of XOR logic element operating at 5 Gb/s in accordance with the present invention.

DETAILED DESCRIPTION OF THE EMBODIMENT 0022 Hereinafter, the constitution and operation of the present invention is described in detail through preferred embodiments by referring to the accompanying drawings. 0023 FIG. 1 shows the basic logic concept of XOR logic element in accordance with the present invention. 0024 The XGM characteristic of semiconductor optical amplifiers (SOA1, SOA2) is adjustable by controlling input current, illumination signal, and pump-signal to the semiconductor optical amplifiers (SOA1, SOA2). The XGM characteristic is obtained by varying light intensity of pump-signal while maintaining increased state of input current at a fixed value and providing illumination signal at a constant light intensity. 0025 Utilizing the above characteristic, in the first semiconductor optical amplifier (SOA1) signal A is modulated using signal B as pump-signal and signal A as illumination signal, while in the second semiconductor optical amplifier (SOA2) signal B is modulated using signal A as pump-signal and signal B as illumination signal. 0026 Upon summing two output signals from the semiconductor optical amplifiers (SOA1, SOA2), embodied is an XOR logic element in which logic value is 1 when only one out of signals A and B has logic value of 1 and logic value is 0 when signals A and B are at the same logic level. 0027 Describing in more detail, if pump-signal is provided to semiconductor optical amplifiers (SOA1, SOA2) with high light intensity, carriers are exhausted in the semiconductor optical amplifiers. 0028 If illumination signals are applied concurrently to the semiconductor optical amplifiers (SOA1, SOA2), gain saturation occurs in the semiconductor optical amplifiers because carriers are exhausted by pump-signals, and therefore output of illumination signal diminishes gradually in its light intensity. 0029 As a result, output signal has reverse value of that of incident signal representing the data. 0030 Such XGM characteristic has the outputs as shown below by tables 1 and 2 for given incident signals A and B. 1TABLE 1ABOUT000010101110

0031

2TABLE 2ABOUT000011100110

0032 If the first semiconductor optical amplifier (SOA1) is impinged with signal A as illumination signal and signal B as pump-signal, the Boolean A{overscore (B)} such as in Table 1 is resulted. On the other hand, if the second semiconductor optical amplifier (SOA2) is impinged with signal A as pump-signal and signal Bas illumination signal, the Boolean {overscore (A)}B such as in Table 2 is resulted. 0033 Therefore, on summing output signals A{overscore (B)} and {overscore (A)}B, an XOR logic element having output value of A{overscore (B)}{overscore (A)}B as shown in Table 3 is realizable. 3TABLE 3ABXOR000011101110

0034 FIG. 4 shows an experimental configuration for embodying the all-optic XOR logic element in accordance with the present invention. 0035 First, pulse signals with spacing of 800 ps are generated using pulses (10) from fiber ring laser and pulse generator (14). 0036 The above pulses are input to 50:50 coupler (12a) and then branched into two parts. One of the branched signals is put through optical delay (20a) and delayed by 200 ps, and the other signal is put through attenuator (16) and polarization controller (18a). And the two signals are then summed to make signal A. 0037 Signal A having pattern of 1100 (One-One-Zero-Zero) is again put through polarization controller (18b) and optical delay (20b) and delayed by 600 ps to make signal B having pattern of 1001. 0038 For the first semiconductor optical amplifier (SOA1), signal A as illumination signal is input through Erbium-doped fiber amplifier (EDFA1) and optical isolator (22b), and signal B as pump-signal is input through optical isolator (22c). 0039 For the second semiconductor optical amplifier (SOA2), signal A is input through optical isolator (22a) as pump-signal, signal B is input through Erbium-doped fiber amplifier (EDFA2) and optical isolator (22d) as illumination signal. 0040 The first semiconductor optical amplifier (SOA1) generates Boolean A{overscore (B)} as shown in FIG. 2 and the second semiconductor optical amplifier (SOA2) generates Boolean {overscore (A)}B as shown in FIG. 3. Now the two generated signals are summed by 50:50 coupler (12b) to generate the output signal shown in FIG. 5 that is visualized by an oscilloscope connected to the signal analyzer (24). 0041 The detected output is found to be identical to Boolean A{overscore (B)}{overscore (A)}B that is shown in the logic table of TABLE 3 for all-optical XOR logic element. 0042 FIG. 5 represents the XOR logic element's operating characteristic resulting from summing outputs of the first semiconductor optical amplifier (SOA1) and the second semiconductor optical amplifier (SOA2). The figure ascertains signal A having pattern of 1100and signal B having pattern of 1001 are summed to make the output signal having pattern of 0101. 0043 Observing the output signals, when signal A is logic 0 and signal B is logic 1 or when signal A is logic 1 and signal B is logic 0, the output signal becomes logic 1. 0044 When two signals have the same logical value, in other words when both of signals A and B are either 0 or 1, the output signal has logic 0. 0045 Since this result is identical to that of TABLE 3, the characteristic of XOR logical element is found experimentally verifiable. 0046 By aforementioned logic experiment, it has now verified that all-optical XOR logic element operable at 5 Gb/s could be developed. 0047 The above all-optical XOR logic element, along with other all-optical logic elements, is indispensable in constituting a large-scale computing system because it is core elements for half-adders or full-adders that are the basic elements for logic calculations. 0048 Therefore, due to the efficient integration technique for logic elements as in the present invention, it is possible to control all the computation systems with optical signals without depending on electric signals. 0049 As shown in the above, since all-optical XOR logic element is embodied using the XGM characteristic of semiconductor optical amplifiers, the XOR logic element in accordance with the present invention is more stable and more easily connectable to other logical elements compared with other elements based on optical fiber. Also, since there is no need to provide clock signal or CW light, it is much favorable in the size and the speed limitation of logic elements. 0050 Although the present invention has been described along with the accompanying drawings, this only illustrates preferred embodiments but not limits the scope of the present invention. It will be apparent for those skilled in the art that various modifications may be made without departing from the spirit and scope of the present invention as defined by the appended claims.