Design of Optical Nand Gate By Nonlinear Device SOA

Transcription

1 INTERNATIONAL JOURNAL OF ELECTRICAL SYSTEMS AND CONTROL (IJESC) Vol. 3, No. 1, Jan-June 2011, pp. 1 5 Design of Optical Nand Gate By Nonlinear Device SOA Yogesh Bhomia 1, Nikita Jain 2 and Devendra Soni 3 Abstract: All-optical digital devices are key components for advanced signal processing in next generation optical networks and optical computing. In most digital systems, photonic integrated circuits are required to carry out high speed energy efficient functionalities. Since we know that it s easy to construct optical combinational as well as optical sequential circuits with the help of optical gates. As we know from the digital logic design that we can construct any combinational circuit with the help of Universal gates like NOR and NAND. So here in this paper, design of Optical NOR gate have been focused. 1. INTRODUCTION In order to meet the ever-increasing demand of data communication for future optical networks, high-speed digital processing is required. Photonics signal elaboration at the optical layer is attractive to perform various computational functionalities, such as packet buffering, bit-length conversions, header processing, switching, retiming and reshaping, and overcoming all the speed electronics limitations. In recent years, a lot of effort has been spent in these fields and all-optical digital processing seems to be one of the most promising technologies to bring increased capacity, flexibility, and scalability to the next generation systems in the optical domain. Due to their great potential in optical computation, several all optical Digital devices have been proposed as building elements for more complex subsystems, including optical threshold functions, logic gates, buffers, flip-flops, shift registers, and binary counters, exploiting nonlinear effects of semiconductor optical amplifiers (SOAs). 2. IMPORTANT TERMINOLOGIES 2.1 Four Wave Mixing When a high-power optical signal is launched into a fiber, the linearity of the optical response is lost. One such nonlinear effect, which is due to the third-order electric 1 Vice Principal, ARYA Institute of Engineering & Technology, Jaipur, bhomia9yogesh@rediff mail. com 2 Astt. Prof., Bhagwant University, Ajmer, nikitaait@gmail.com 3 Arya College of Engg. & Information Technolgy, Jaipur, sonierdevendra@yahoo.co.in susceptibility is called the optical Kerr effect. 1), 2) Fourwave mixing (FWM) is a type of optical Kerr effect, and occurs when light of two or more different wavelengths is launched into a fiber. Fig. 1: Schematic of Four Waves Mixing in Frequency Domain Generally speaking FWM occurs when light of three different wavelengths is lauched into a fiber, giving rise to a new wave (know as an idler), the wavelength of which does not coincide with any of the others. FWM is a kind of optical parametric oscillation. In the transmission of dense wavelength division multiplexed (DWDM) signals, FWM is to be avoided, but for certain applications, it provides an effective technological basis for fiber-optic devices. FWM also provides the basic technology for measuring the nonlinearity and chromatic dispersion of optical fibers. The idler frequency fidler may be determined by f idler = fp 1 + fp 2 f probe Where: fp 1 and fp 2 are the pumping light frequencies, and fprobe is the frequency of the probe light.

2 2 International Journal of Electrical Systems and Control (IJESC) This condition is called the frequency phase-matching condition. When the frequencies of the two pumping Waves are identical, the more specific term degenerated four-wave mixing (DFWM) is used, and the equation for this case may be written f idler = 2f p f probe where: fp is the frequency of the degenerated pumping wave. 2.2 Intrinsic Gain The analog fiber optic link has a conversion loss given by equation1. Due to the parallel combination of Rload and the output impedance only half of the detected photodiode current is available and hence the factor ½ is used in equation 1, 2 and 4. The link has a conversion loss of db and with an optical loss of 1 db in the link the total RF loss will be db. The conversion loss is also referred as intrinsic gain of a link. With better slope efficiency and reactive impedance matching at the laser and photodiode the loss can be reduced. VCSEL laser has higher slope efficiency compared to FP and DFB lasers and help in improving the conversion loss of the link. The intrinsic gain increases with an increase in optical power of an externally modulated link. A directly modulated link does not shown any improvement in gain with increase in optical power. Gi = 10.Log10(ηl. R/2)2...(1) Where ηl = Slope efficiency of the laser diode in W/A R = Responsivity of the photodiode in A/W 2.3 Noise Performance of The Link The total noise performance of an analog fiber optic link depends on the individual noise contribution of various components in the link. Three major noise sources in an analog link are Laser noise (RIN noise) generated at the laser, shot noise at the photodiode detection and thermal noise at the receiver circuit. 2.4 Laser Noise The laser noise arises from the random fluctuations in the intensity of the optical signal generated at the laser diode. The laser noise is measured directly at the transmitter and is referred to as relative intensity noise (RIN) in the laser diode specification. The RIN is the ratio of mean square amplitude of the noise fluctuations per unit bandwidth (<P2>) to the square of the DC optical power (Po2). The laser noise power Plaser in an analog fiber optic link for direct detection is given by Equation 2. Plaser = RINlaser.(Iph/2)2.Rl...(2) Where Photodiode current Iph = (Ibias Ith). ηl.r.α...(3) hl = Slope efficiency of the laser in W/A α = Optical loss in ratio [10 ( optical loss in db/10)] R = Responsivity of the photodiode A/W Rl = Load resistance at the photodiode receiver (Ibias Ith) = Bias current above threshold current ma 2.5 Shot Noise The average photocurrent generated at the photodiode is associated with a shot noise current generated due to the random process by which the current is generated at the photo detector. The shot noise power Pshot is given by Equation 4. Where Pshot = 2.e.[(Iph + Id)/2].Rl...(4) e = Charge of the current carrier ( C) Id = dark current of the photodiode in A. 2.6 Thermal Noise Thermal noise is associated with the receiver circuit and the thermal noise power P Thermal_noise is given by Equation 5. P Thermal_noise = k. To...(5) Where k = Boltzman constant ( J/K) To = Temperature in Kelvin (290 K) 2.7 Total Noise The total noise is the summation of the above three major noise sources in a fiber optic link. The total noise power of the link Ptotal noise is given by Equation 6. Ptotal noise = PnLaser + Pshot + PThermal_noise...(6) 2.8 Equivalent Input Noise (Ein In Dbm/ Hz) of The Link The EINlink is the total noise referred to the link input by the following Equation 7. EINlink = Ptotalnoise/Glink...(7) Where the link gain Glink = ( ηl.α.r/2)2 2.9 Noise Figure of The Link The noise figure (NF in db) of the link is given by the Equation 8. NFlink = (EINlink dbm/hz) + (174 dbm/hz)...(8)

3 Design of Optical Nand Gate By Nonlinear Device SOA Noise Equivalent Bandwidth Einbw The equivalent input noise for a given noise bandwidth is given by Equation 9. EINbw = EINlink + 10.log10(BW)...(9) 3. COMPONENTS USED 1. Optical transmitter 2. Polarization control 3. Variable optical attenuator 4. Bidirectional coupler 5. Semiconductor optical amplifier 6. Band pass filter 7. EDFA 8. Circulator 3.1 Semiconductor Optical Amplifier A schematic diagram of an SOA is shown in Fig. The device is driven by an electrical current. The active region in the device imparts gain, via stimulated emission, to an input signal. The output signal is accompanied by noise. This additive noise, amplified spontaneous emission (ASE), is produced by the amplification process. SOAs are polarisation sensitive. This is due to a number of factors including the waveguide structure and the gain material. Polarisation sensitivity can be improved by the use of square-cross section waveguides and strained quantum-well material. Fig. 3: Gain Power Charecteristics This gain saturation can cause significant signal distortion. It can also limit the gain achievable when SOAs are used as multi-channel amplifiers in wavelength division (WDM) multiplexed systems. SOAs are normally used to amplify modulated light signals. If the signal power is high then gain saturation will occur. This would not be a serious problem if the amplifier gain dynamics were a slow process. However in SOAs the gain dynamics are determined by the carrier recombination lifetime (few hundred picoseconds). This means that the amplifier gain will react relatively quickly to changes in the input signal power. This dynamic gain can cause signal distortion, which becomes more severe as the modulated signal bandwidth increases. These effects are even more important in multichannel systems where the dynamic gain leads to inter-channel crosstalk. This is in contrast to optical fibre amplifiers, which have recombination lifetimes of the order of milliseconds leading to negligible signal distortion. SOAs also exhibit nonlinear behaviour. These nonlinearities can cause problems such as frequency chirping and generation of inter-modulation products. However, nonlinearities can also be of use in using SOAs as functional devices such as wavelength converters. As its name suggests, an SOA is made from semiconductor optoelectronic material (indium phosphide -based) and is pumped electrically to create a population inversion that generates optical gain via stimulated emission. 3.2 Optical Circulator Fig. 2: SOA The gain of an SOA is influenced by the input signal power and internal noise generated by the amplification process. As the input signal power increases the gain decreases as shown in Fig Isolators The principle of operation of an isolator is as shown in the figure. Assume that the light signal has the v ertical SOP as shown. It is passed through a polarizer, which passes only light energy in the vertical SOP and blocks light energy in the horizontal SOP. The Faraday rotator is used to rotate the SOP, say, clockwise by 45 degrees, regardless of the direction of propagation. This is again followed by polarizer that passes only SOPs with 45

4 4 International Journal of Electrical Systems and Control (IJESC) degrees orientation. Thus the light signal from left to right is passed through the device without any loss. On the other hand, light entering the device from the right due to the reflections, with the same 45 degree orientation, is rotated another 45 degrees by the Faraday rotator, and thus blocked by the first polarize Circulator Fig. 4: Operation of Isolator The isolator constructed using Spacial Walk-off Polarizer (SWP) and Half- Waveplate is shown in the figure below. This forms the basic principle of circulators. The SWP is used to split the light into two components i.e. horizontal and vertical. The Half Wave plate is similar to the Faraday rotator except that it rotates 45 degree clockwise the light coming from left to right and 45 degrees counterclockwise, the light coming from right to left. Fig. 5: Isolator Constructed using Spacial Walk-off Polarizer (SWP) and Half-Waveplate Fig. 6: Types of Circulator Connections (a) Strict Sense Circulator with Four Ports. (b) Non Strict Sense Circulator in Ladder Topology. (c) Non Strict Sense three Port Circulator. An optical circulator is a generalized isolator having three or more ports. While an isolator causes loss in the isolation direction, a circulator collects the light and directs it to a nonreciprocal output port. The figure drawn above shows the several possible circulator configurations. Figure 6(a) illustrates the port mapping for a four port circulator. The ports cyclically map This is called a strict sense circulator because every input port has a specific non-reciprocal output port. Construction of a strict-sense circulator with more ports becomes inelegant but ones with three ports becomes simple. Figure 6(b) illustrates a non-strict-sense circulator having any number of ports greater than two. In this case each input port has a specific non-reciprocal output port except for the last port; the light input to the last port is lost. The ladder diagram reflects the optical path within the component and indicates the disconnect between the first and the last ports. Figure 6(c) illustrates the three port non-strict-sense circulator. This circulator has significance in telecommunication applications because return of light from port 3 to port 1 is not often required. For instance the reflected light from a fiber Bragg grating need only be separated from the input light without loss, but as optical links are not typically operated in reverse there is no need for strict-sense behaviour. 4. THEORETICAL ANALYSIS For each logic function, only one semiconductor device is used, reducing costs, complexity, and energy consumption of the schemes. In our applications, A is used as the input clock pulse, while B and C are external control signals. The experimental setup of the A B gate is shown in Fig. 1(a). The logic function is performed

5 Design of Optical Nand Gate By Nonlinear Device SOA 5 by filtering out the FWM signal ( λfwm = 2λB λa) between A and B. When a clock pulse A comes in and the control signal B = 1, a pulse at λfwm is generated. If B = 0, there is no FWM generation, and no pulse is present at the gate output. Variable optical attenuators (VOA) and polarization controllers (PC) are used in order to adjust the input optical power and the signals polarization state. This way it is possible to maximize the FWM efficiency in the SOA. However, the polarization dependence of the gate can be eliminated by polarization diversity technique. The filtered ASE noise emission given out by the SOA can be a limiting factor in cascaded configurations. A counter-propagating CW light can be introduced into the device in order to keep slightly saturated the SOA, thus reducing the noise emission. 5. PRACTICAL DESIGN Fig. 7: Logic Function for Input A and B Output of Fig.1 is given as input to the port 1 of the circulator, so an invertedoutput of the port 1 will appear at the output port 3. The logic function is obtained between the input control signal A, B, and clock pulse C, is a NAND gate. The proposed gate can be considered as a combination of the nonlinear effects. Fig 2 shows the logic function of NAND gate. Fig. 8: Logic Function for NAND Gate 6. CONCLUSION Design of Optical NAND gate is proposed exploiting FWM and XGM nonlinear effects in SOAs. The optical gate can be used for design of optical flip flop, which is the basic of optical memory. As we already discussed that most of the power is wasted in optical to electronic and electronic to optical conversion in high speed data transmission. So with the help of optical memory we can save that power as well as the time and hence improved speed can be formed. REFERENCE [1] Machzender Interferometer and its Applications Rekha Mehra, Jitendra Tripathi, International Journal of Computer Applications ( ), 1- no. 9. [2] All-Optical Clocked Flip-Flops and Binary Counting Operation using SOA Based SR Latch and Logic Gates, Jing wang, Gianluka Meloni, Gianluka Berrettini, Luca Poti, and Antonella Bogoni. [3] All Optical Clocked D Type Flip Flop Exploiting SOA Based Optical SR Latch and Logi Gates, J.Wang, G.Berrettini, G.Meloni, A.Bogoni. [4] Four Wave Mixing in Optical Fibers and its Applications, Osamu Aso, Masateru Tadakuma, and Shu Namiki. [5] A Bidirectional Link using Optical Circulator, Mukrsh K Bhongade, S.Suresh Kumar. [6] Semiconductor Optical Amplifiers and Their Applications, Michael connelly.