Photonic Computers

The benefits of having computers encompassed in every day life is clearly benefical. However, with this transition a greater demand will be placed on the computers ability to process the data. The cost to increase a computer’s processing power is expensive because the physical ability of the electrical system is reaching a peak in their preformance. The solution to this problem is a photonic computer. Photonic computers use photonic energy instead of electricity to store, manipulate, and transmit data. The speed of a photon is 3108 m/s. Since the speed of light is significantly faster then electricity it would make logical sence to harness this property. Great effort has already been taken to create a computer that opporates on photonic energy with better preformance then conventional computers. The only success that has been seen is in the laboratory and there has yet to be a photonic computer released to the public. The purpose of this report is to explain the componets of a photonic computer and to analyze how photonic energy is manipulated to preform a desired computaional function.

The majority of computers today operate on electricity, but with technological advancements, computers will soon operate on photonic energy. The main component of a photonic computer is light. The term light is usually misinterpreted because the definition is too broad. The production of light occurs by exciting electrons in an atom to a higher energy level. An electron is bumped to a higher energy level by using heat, light or electricity. Then the electron falls back down to a more stable lower energy level. As the electron returns to the lower energy level it releases a photon. The photon that is released is the product of the energy that excited the electron to the higher energy level. Each photon emitted from a specific energy level will have a wavelength that correlates. Therefore, it would be desirable to be able to control a specific wavelength in order to control a desired function. This can be accomplished by using a laser as one of the main components of a photonic computer.

A laser is a mechanism that can control the release of photons. The term laser is an acronym for light amplification by stimulated emission of radiation. In order to ensure the proper function and efficiency of a laser, the lasing medium needs be intense to ensure that the atoms are excited to a level two or three times that of their ground state. This will ensure that the population inversion will be relatively high. The population inversion describes the amount of atoms that are in the excited energy levels versus the amount of atoms in the ground state. When there is a high population inversion, this ensures that the laser will function properly and be able to consistently supply photons in a large collection. To understand the power of laser light one must analyzes the properties of the emitting photons.

Laser light has three main components; it is monochromatic, coherent, and directional. Monochromatic means that there is only one specific wavelength of light, i.e. one specific color. The wavelength of light is determined by the amount of energy released when the electron returns to the ground state. Coherent describes the organization of light, for example the photons that are emitted in a laser are all moving in unison. Directional illustrates that the laser light is a very tight beam that is very strong and concentrated. When monochromatic, coherent and directional components are present this describes stimulated emission. It is from these components that makes laser light and ordinary light different. Another crucial component of a laser are the mirrors that are placed at each end of the lasing medium. This allows the photons to reflect off one mirror and travel back to the other and with a high population inversion the photons traveling this path will eventually collide with other atoms in the lasing medium. It is known that a photon has a specific wavelength and phase; if it were to collide with another atom that shared the same excited state then stimulated emission can occur. Essentially, the mirrors allow for a cascading effect to occur, thus producing a very consistent and dense population inversion. The manner in which the laser light is emitted from this mechanism is through the one of the mirrors. The mirror has the ability to reflect the majority of photons present in the lasing medium but allows some photons through, thus emitting laser light.

With the properties of a laser explained, the next component to a photonic computer is the ability to transmit the laser light in order to compute a desired function. This is accomplished with the use of fiber optics. Optical fibers are widely used in fiber-optic communication, which permits transmission over longer distances and at higher data rates than other forms of communications. Fibers are used instead of metal wires because signals travel along them with less loss, and they are immune to electromagnetic interference. Optical fibers are also used to form sensors, and in a variety of other applications.Light is kept in the “core” of the optical fiber by total internal reflection. This causes the fiber to act as a waveguide.

Fibers which support many propagation paths or transverse modes are called multimode fibers (MMF). A multimode fiber has photons (rays) travling through it. The multimode fiber has relativley large core diameter, usually greater than 10 m. Fibers which support only a single mode are called singlemode fibers (SMF). Multimode fibers generally have a large-diameter core, and are used for short-distance communication links or for applications where high power must be transmitted. Singlemode fibers are used for most communication links longer than 200 meters (Wikipedia). Total internal reflection discribes the path photons travel inside of a fiber optic cable. To accomplish total internal reflection the incident angle must be greater then the critical angle.The photon that is reflecting in the core strikes the cladding which has a lower index of refraction thus total internal reflection can be occur. To calculate the critical angle in order to determine if total internal reflection will occur, snell’s law is applied. If the incident angle must be greater then the critical angle in order to determine if total internal reflection will occur, Snell’s law states ni sin c = nt sin i. It is known that for any incident angle that’s greater than the critical angle total internal reflection occurs, therefore an incident angle can not be greater than the 90. Solving for the critical angle Snell’s law states, c= sin-1(nt/ ni). This is the equation that is used to engineer fiber optic cable. The next component needed in order for a photonic computer to function is a photonic switch. This is needed because the photonic switch will replace the transistors in conventional computers. The method that is used to manipulate the photonic energy is by using total internal reflection to change to property of the photons.

A photonic switch is used to preform the basic task of turning a circuit on or off. This is the most current form of opitcal processing. Photonic switches are used in mass communciation systems that use fiber optic cable to transmitte the information. The opitcal switch can process an expontial amount of information compared to old telephone cables system. The conjunction of the fiber optic cable and optical switches allows the world to communicate at speeds that use to be incomprehensiable. The physics behind the optical switch is interesting. There is a configuration of fiber optic cable and a photonic switch that is sending a signal. The photonic switch is responsible for preventing total internal reflection from occuring. Thus a photonic switch has the ability to change relationship between the incident angles and the critical angles. Hench, enabling for a particular singal being turned on or off. This is extremely important because it is from these photonic switches that commands are transmitted and enables for a functioning system. Another type of photonic switch utilizes Fourier optics. Fourier optics methods can be visualized by considering the Fraunhofer diffraction pattern of a single slit. The diffraction process transforms the slit in the object plane to a diffraction pattern in the distant image plane. This diffraction pattern contains information about the slit in a form in which smaller spatial details (narrower slits) are transformed into larger spatial displacement in the image plane (broader diffraction patterns). Smaller spatial detail can be referred to as a higher “spatial frequency”, and the diffraction pattern produces a plot in which greater distance from the optic axis implies greater spatial frequency. This kind of transformation, where a plot of light distribution is transformed into plot of spatial frequency is an example of a Fourier transformation and is a conceptual starting point for Fourier optics (hyper physics). The function of the optical switch is achieved by detecting the Fraunhofer diffraction pattern. The photons traveling have generated a specific Fraunhofer diffraction pattern and this specific pattern affects other photons traveling with it that will activate a sensor that will perform a desired function when the specific Fraunhofer diffraction pattern is present. Photonic switches are being used in conjunction with electrical processing power.

A photonic switch is used in conjunction with electrical processing. The electrical processing performs basic binary code and converts that information into photonic energy. It is from this photonic energy that the photonic switch operates. Then optical switches are connected to fiber optic cables that send the information to an electrical processor which then converts the photonic energy back into binary code. They are used in conjunction with electrical processing power because this is the aspect of the photonic computers that has yet to be discovered.

The final component of a photonic computer is holography processor. The creation of a holography processor has yet to be calculated. There reason for this is because this mechanical device would have to have the ability to generate a hologram that was similar to the binary code for a conventional computer. This is extremely difficult because holography is essentially the absorption of the Fraunhofer diffraction pattern. Laser light is emitted, travels through a beam splitter. The laser light that is emitted through the beam splitter travels to a mirror, which reflects the laser light through a convergent lens and hits a film sheet that is not exposed to ambient light. The remaining laser light that went through the beam splitter travels through a convergent lens which then travels to a mirror which reflects the laser light. This laser light hits the objects in the image plane. The laser light reflects off of the objects in the plane. The laser light that is reflected off of the objects gives a specific Fraunhofer diffraction pattern. The Fraunhofer diffraction pattern is then recorded on the on the film. Once the film is developed, laser light is passed through the film and the objects that are no longer there can now be seen. The laser light that is passing through the film generates the image stored because the laser light is being manipulated through the small diffraction patterns which produces the recorded object(s) image. Because the process to make a hologram is some what difficult and timely, it is hard to comprehend how one could create a holography processor. A holography processor would need to be able to change between a series of holograms in order to send data with photonic energy. Conventional computers use a series of binary code in order to transmit information and or commands to other components with in the system. The issue with photonic energy is that it has a specific wavelength and phase therefore a photonic switch would only be able to direct specific wavelengths of light. Because conventional computers work on a binary code, a photonic computer could not because of the properties of laser light. In addition, if it was feasible to transmit photonic energy in the red and a blue wavelength in a form of binary code, a processor is needed to be able to alter the sequence in which red and blue laser light is emitted.

The benefits of obtaining such technology would be great. If information can be stored as photonic matter, the fear of hacking would be completely eliminated. The reason for this is that photonic energy is completely unique. If a file was opened or altered then the properties of the original photonic energy would be altered or destroyed. Therefore, it would be physical impossible to enter a system with out being detected. The key in obtaining this future technology is developing a photonic processor.

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