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All optical circuits and for digital logic
 

All optical circuits and for digital logic

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Final Report for Laser Class, 2007

Final Report for Laser Class, 2007

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    All optical circuits and for digital logic All optical circuits and for digital logic Document Transcript

    • Bistable Digital Logic Devices Halim Bistable Digital Logic Devices for All Optical Circuits Mohammad Faisal Halim (Faissal) Laser Course Professor Dosinville 1
    • Bistable Digital Logic Devices Halim Bistable Digital Logic Devices for All Optical Circuits Contents Page Topic Number Prologue: Moore’s Law 3 Background: The Optical Architecture 5 Introduction: The Optical Logic Gate 6 Utilizing a Photonic Resonant-Tunneling Device Based on Photonic-Crystal Nanocavity 8 Single Wavelength Bistability 8 Conclusion 13 Appendix 14 2
    • Bistable Digital Logic Devices Halim Bistable Digital Logic Devices for All Optical CircuitsPrologue: Moore’s Law Figure 1: Moore’s LawThe relentless push for Moore’s Law has resulted in microchip circuit components thatare becoming so small that any further push towards miniaturization (in order to attainmore speed) will result in quantum mechanical effects becoming more pronounced in the 3
    • Bistable Digital Logic Devices Halimdevices. These effects will not just result in power losses, but will also result in thedevices having less reliable behavior: their behavior will acquire a statistical nature.Further, with the current state of electrical technology copper (which is used in theinterconnects, in chips) is approaching its information carrying capacity.The Harry Truman Approach: “If you can’t stand the heat, get out of the kitchen.” Thatquote is one way to justify moving out of the current paradigm to chip design, entirely,and migrate to an inherently faster one: using light to carry information. The dream ofoptical computing has existed for a long time, but the technology has been a long time incoming. The basic concept is this: use light, instead of electricity, to transmit informationacross a chip, and use logic gates that switch light, instead of currents and voltages, toperform logic operations. The primary advantage is that light travels much faster than acurrent, so chips that use light will be a lot faster, and optical switching could be donefaster than current and voltage switching (which is done in modern transistortechnology), thus speeding up processor speeds. 4
    • Bistable Digital Logic Devices HalimBackground: The Optical ArchitectureA computer microprocessor utilizing all optical circuitry (which would not be “circuitry”as taught in introductory physics) would have dielectric waveguides as to channel thelight inside the processor, and optical transistors and other optical bistable devices to dothe logic operations. Figure 2: Source: MODELING OF PHOTONIC BAND GAP STRUCTURES AND PROPOSED SYNTHESIS SCHEMES By Srivatsan Balasubramanian, RPI, 2002 Figure 3: Source: Hwang, Cho, Kang, Lee, Park, Rho, “Two-Dimensional Optical Interconnection Based on Two-Layered Optical Printed Circuit Board,” IEEE Photon. Technol. Lett., vol. 19, no. 6, pp 411-413, Mar. 15, 2007. 5
    • Bistable Digital Logic Devices HalimIntroduction: The Optical Logic GateThe following schematic shows a digital logic circuit, where the inputs and outputs are alllight, rather than voltages or currents. Figure 4: Schematic of a digital logic deviceThe way all optical technology is being developed, all inputs and outputs to such a devicewill have to be lasers. This is because while technologies like MEMS can be used toswitch light, these systems will be cumbersome, and will not have the desired speeds thatmotivated all optical computing, in the first place. Laser is used, instead, for theoreticaland experimental work in this area for the following reasons: 1. Laser light being monochromatic, can be controlled using structures that have a photonic band gap (PBG). 2. Laser light can be confined into tight spaces, thus allowing miniaturization of devices (if small enough wavelengths are used). 3. With technologies like opal and inverse opal based photonic crystals (PCs) that have a PGB at the wavelength of the laser, the laser light can be confined in microcavities that have a high Q, thus allowing for high intensities of lasers being generated within devices. 4. It is practical to generate high enough intensities of laser light in microdevices to utilize non-linear optical (NLO) effects, for use in optical switching. 6
    • Bistable Digital Logic Devices Halim 5. With PCs the spontaneous emission of light of a chosen frequency can be prevented in a microcavity, thus allowing for high efficiency, low threshold lasers, for use in all optical devices.Currently, a number of hybrid technologies are being developed that use electro-opticeffects, but they are limited (in speed) in the long run, since they are still dependent onthe speed of electronics. All optical devices, on the other hand, will only be limited by thespeed of light in the dielectrics used, and the speeds with which the materials used insuch devices will react to the light. 7
    • Bistable Digital Logic Devices HalimUtilizing a Photonic Resonant-Tunneling DeviceBased on Photonic-Crystal NanocavityAccording to the paper “Optical bistable switching action of Si high-Q photonic-crystalnanocavities,” (Notomi, Shinya, Mitsugi, Kira, Kuramochi, Tanabe, OPTICS EXPRESSVol. 13, No. 7 Apr. 4, 2005) single wavelength and two wavelength bistable devices anbe made for al optical circuits, to implement digital logic. The device the group used isshown in Figure 5. Figure 5: Photonic resonant-tunneling device based on PC nanocavitySingle Wavelength BistabilityFigure 6 shows that the light transmission characteristics of the material under test varieswith the intensity of the incident light: as the intensity of the light (at any givenwavelength) increases, at a certain intensity the transmitted intensity goes up, and whenthe incident light is being lowered in intensity then at a certain intensity the transmittedintensity goes down. 8
    • Bistable Digital Logic Devices Halim Figure 6: Utilizing intensity dependent transmission curvesFigure 7 shows a schematic of what happens in Figure 6, for any chosen wavelength. Figure 7: A schematic of Figure 6, for a chosen wavelength. 9
    • Bistable Digital Logic Devices HalimWhat Figure 7 shows is this (the numbers match the numbers on the curves): 1) As the incident intensity is swept down then at some intensity the transmitted power drops down precipitously. 2) The point marked ‘2’ in the figure corresponds to the intensity at which the device is in the OFF state. 3) Then, as the incident intensity is increased then at a certain intensity that is greater than the intensity at which the device turned OFF the device turns ON, that is the transmitted intensity increases dramatically.The effects shown in Figure 7 are due to an effect called “Saturable Absorption” (LaserFundamentals, Silfvast). Figure 8, perhaps, gives a better depiction. Figure 8: Output signal versus input signal, showing bistability 10
    • Bistable Digital Logic Devices HalimFor a device, as depicted in Figure 4, an input (called “Signal” in Figure 4) could come inat an input intensity slightly less than I1(in) (from Figure 8), and the control signal(“Control,” from Figure 4) could have an intensity such that the total intensity of the twobeams is slightly greater than I2(in) (from Figure 8), and the resulting output signal(“Output”, from Figure 4) could be the output from an AND gate, as used in digital logic.In principle, this sort of bistability is achieved by a saturable absorber. As the incidentintensity is increased, the beam is absorbed by the absorber, until at a certain intensity[I1(in) (from Figure 8)] the absorber is bleached, ad so it suddenly lets the light through,with very little loss in intensity. When the incident laser is being reduced in intensity thenthe absorber will still have some stored photons, causing it to stay bleached until theincident beam is reduced in intensity to I2(in) (from Figure 8) – this is lower than I1,when the absorber looses its stored photons and the transmitted intensity goes down.Since I2(in) is less than I1(in) (both, from Figure 8) we see the hysteresis. 11
    • Bistable Digital Logic Devices HalimThe group also performed two wavelength bistable switching, and obtained the followingresult (which gets conceptually more complicated), as shown in Figure 9 (for the deviceshown in Figure 5). Figure 9: Digital LogicFigure 10 shows the two wavelength switching times and energies. Figure 10: Switching speeds and energies 12
    • Bistable Digital Logic Devices HalimConclusionAlthough all optical bistable switching devices are still not as fast as all electronicdevices, they have the potential to be made faster, and more energy efficient, particularlyin light of emerging technologies in fabrication of PCs that have 3D bandgaps.Some PBGs are shown in Figure 11. Figure 11: Photonic Bandgaps and material construction of the PCs 13
    • Bistable Digital Logic Devices Halim Appendix “Optical bistable switching action of Si high-Q photonic-crystal nanocavities,” (Notomi, Shinya, Mitsugi, Kira, Kuramochi, Tanabe, OPTICS EXPRESS Vol. 13, No. 7 Apr. 4, 2005) MODELING OF PHOTONIC BAND GAP STRUCTURES AND PROPOSED SYNTHESIS SCHEMES, By Srivatsan Balasubramanian, RPI, 2002 Hwang, Cho, Kang, Lee, Park, Rho, “Two-Dimensional Optical Interconnection Based on Two-Layered Optical Printed Circuit Board,” IEEE Photon. Technol. Lett., vol. 19, no. 6, pp 411-413, Mar. 15, 2007. Intel Corporation Laser Fundamentals, Silfvast 14