Engineering Seminar on Microphotonic Technology
Microphotonic is an emerging field encompassing photonics, electro-optics, integrated opto -VLSI chips, broad-band photo-receiver arrays and related systems to manipulate, transmit and detect photons at microscopic level. The 21st century is sometimes noted as the Age of Light and indeed, optical technology has been adopted into our daily lives, with fiber optics-based broadband, LCD TVs, optical disks, illuminations, laser therapy, solar cells and more. In fact, optics contributes greatly to making our lifestyles more convenient and comfortable. OMRONâ„¢s Microphotonics is a key driving force for the steady advancement of this optical technology. Microphotonics is a light-wave control technology designed to maximize the potential qualities of light such as brightness, speed and energy.
The photon is one of the elementary particles. Its interactions with electrons and atomic nuclei account for a great many of the features of matter, such as the existence and stability of atoms, molecules, and solids. These interactions are studied in quantum electrodynamics (QED), which is the oldest part of the Standard Model of particle physics. Photonics is the science and technology of generating and controlling photons, particularly in the visible and near infra-red light spectrum. Photonics as a science is closely related to quantum optics and optoelectronics with somewhat unclear boundaries. Quantum optics frequently implies fundamental research, while photonics often refers to more application-related research. The term optoelectronics, which by construction is a somewhat narrower field than photonics dealing only with active elements involving an electrical interaction, nonetheless frequently is used to include passive photonic elements as well.
2. Overview of Microphotonics
Microphotonics is a branch of technology that deals with directing light on microscopic scale. It is used in optical networking. Microphotonics employs at least two different materials with a large different materials index of refraction to squeeze the light down to a small size. Generally speaking virtually all of microphotonics relies on Fresnel reflection to guide the light. If the photons reside mainly in the higher index material, the confinement is due to total internal reflection. If the confinement is due many distributed Fresnel reflections, the device is termed a photonic crystal. There are many different types of geometries used in microphotonics including: optical waveguides, optical microcavities, Arrayed Waveguide Gratings. Light bounces off the small yellow square that it looks like a scrap of metal, something a child might pick up as a plaything. But it isnâ„¢t a toy, and it isnâ„¢t metal. Made of a few ultrathin layers of nonconducting material, this photonic crystal is the latest in a series of materials that reflect various wavelengths of light almost perfectly. Photonic crystals are on the cutting edge of microphotonics: technologies for directing light on a microscopic
scale that will make a major impact on telecommunications. In the short term, microphotonics could break up the logjam caused by the rocky union of fiber optics and electronic switching in the telecommunications backbone. Photons barreling through the networkâ„¢s optical core run into bottlenecks when they must be converted into the much slower streams of electrons that are handled by electronic switches and routers. To keep up with the Internetâ„¢s exploding need for bandwidth, technologists want to replace electronic switches with faster, miniature optical devices, a transition that is already under way Because of the large payoff-a much faster, all-optical Internet-many competitors are vying to create such devices. Large telecom equipment makers, including Lucent Technologies, Agilent Technologies and Nortel Networks, as well as a number of startup companies, are developing new optical switches and devices. Their innovations include tiny micro mirrors, silicon waveguides, even microscopic bubbles to better direct light. But none of these fixes has the technical elegance and widespread utility of photonic crystals. In lab, photonic crystals are providing the means to create optical circuits and other small, inexpensive, low-power devices that can carry, route and process data at the speed of light. The trend is to make light do as many things as possible, it may not replace electronics completely, but you want to make light do as much as you can. Conceived in the late 1980s, photonic crystals are to photons what semiconductors are to electrons, offering an excellent medium for controlling the flow of light. Like the doorman of an exclusive club, the crystals admit or reflect specific photons depending on their wavelength and the design of the crystal. In the 1990s, Joannopoulos MIT professor suggested that defects in the crystalsâ„¢ regular structure could bribe the doorman, providing an effective and efficient method to trap the light or route it through the crystal. Researchers in this field explained how crystal filters could pick out specific streams of light from the flood of beams in wavelength division multiplexing, or WDM, a technology used to increase the amount of data carried per fiber . The labâ„¢s work on two-dimensional photonic crystals set the stage for the worldâ„¢s smallest laser and electromagnetic cavity, key components in building integrated optical circuits. But even if the dream of an all-optical Internet comes to pass, another problem looms. So far, network designers have found ingenious ways to pack more and more information into fiber optics, both by improving the fibers and by using tricks like WDM. But within five to 10 years, some experts fear it wonâ„¢t be possible to squeeze any more data into existing fiber optics. The way around this may be a type of photonic crystal recently created by Joannopoulosâ„¢ group: a Ëœperfect mirrorâ„¢ that reflects specific wavelengths of light from every angle with extraordinary efficiency. Hollow fibers lined with this reflector could carry up to 1,000 times more data than current fiber optics-offering a solution when glass fibers reach their limits. And because it doesnâ„¢t absorb and scatter light like glass, the invention may also eliminate the expensive signal amplifiers needed every 60 to 80 kilometers in todayâ„¢s optical networks Joannopoulos is now exploring the theoretical limits of photonic crystals. How much smaller can devices be made, and how can they be integrated into optical chips for use in telecommunications and, perhaps, ultrafast optical computers? Says Joannopoulos: ËœOnce you start being able to play with light, a whole new world opens up. Conceived in the late 1980s, photonic crystals are to photons what semiconductors are to electrons, offering an excellent medium for controlling the flow of light.
Photonics is the science and technology of generating and controlling photons, particularly in the visible and near infra-red light spectrum. Photonics as a science is closely related to quantum optics and optoelectronics with somewhat unclear boundaries. Quantum optics frequently implies fundamental research, while photonics often refers to more application-related research. The term optoelectronics, which by construction is a somewhat narrower field than photonics dealing only with active elements involving an electrical interaction, nonetheless frequently is used to include passive photonic elements as well. The term photonics may, but doesnâ„¢t always, imply a goal of establishing an electronics of photons instead of electrons. Polaritonics differs with photonics in that the fundamental information carrier is a phonon-polariton, which is an admixture of photons and phonons, and operates in the range of frequencies from 300 terahertz to approximately 10 terahertz. Photonics typically operates at frequencies on the order of hundreds of terahertz.
Bundle of photons dispersed by a prism The field of photonics has a strong interest in optical communication. The science and applications of photonics are usually based on laser light.
4. CONCEPT INVOLVED IN MICROPHOTONIC TECHNOLOGY
The refractive index (or index of refraction) of a material is the factor by which the phase velocity of electromagnetic radiation is slowed in that material, relative to its velocity in a vacuum. It is usually given the symbol n, and defined for a material by:where er is the materialâ„¢s relative permittivity, and Ã‚Âµr is its relative permeability. For a non-magnetic material, Ã‚Âµr is very close to 1, therefore n is approximately . The phase velocity is defined as the rate at which the crests of the waveform propagate; that is, the rate at which the phase of the waveform is moving. The group velocity is the rate that the envelope of the waveform is propagating; that is, the rate of variation of the amplitude of the waveform. It is the group velocity that (almost always) represents the rate that information (and energy) may be transmitted by the wave, for example the velocity at which a pulse of light travels down an optical fiber.
4.1 TOTAL INTERNAL REFLECTION
The larger the angle to the normal, the smaller is the fraction of light transmitted, until the angle when total internal reflection occurs. (The colour of the rays is to help distinguish the rays, and is not meant to indicate any colour dependence.)
Total internal reflection Total internal reflection is an optical phenomenon. It occurs when light is refracted (bent) at a medium boundary enough to send it backwards, effectively reflecting all of the light. When light crosses materials with different refractive indices, the light beam will be partially refracted at the boundary surface, and partially reflected. However, if the angle of incidence is shallower (closer to the boundary) than the critical angle, the angle of incidence where light is refracted so that it travels along the boundary, then the light will stop crossing the boundary altogether and instead totally reflect back internally. This can only occur where light travels from a medium with a higher refractive index to one with a lower refractive index. For example, it will occur when passing from glass to air, but not when passing from air to glass. Total internal reflection can be demonstrated using a semi-circular glass block. A ray box shines a narrow beam of light (a ray) onto the glass. The semi-circular shape ensures that a ray pointing towards the center of the flat face will hit the surface at right angles. This prevents refraction at the air/glass boundary. At the glass/air boundary what happens will depend on the angle. Where ?c is the critical angle: If ? < ?c, as with the red ray, the ray will split. Some of the ray will reflect off the boundary, and some will refract as it passes through. If ? > ?c, as with the blue ray, all of the ray reflects from the boundary. None passes through. The second situation is total internal reflection. This physical property makes optical fibres useful, and rainbows and prismatic binoculars possible. It is also what gives diamonds their distinctive sparkle, as diamond has an extremely high refractive index. An important side effect of total internal reflection is the propagation of an evanescent wave across the boundary surface. This wave may lead to a phenomenon known as frustrated total internal reflection.
4.1.1 Frustrated Total Internal
While it is true that the creation of an evanescent wave does not affect the conservation of energy under ordinary conditions, i.e. the evanescent wave transmits zero net energy, if a medium with a higher refractive index is placed less than several wavelengths distance from the boundary of the first medium, the strength of the evanescent wave will be large enough to effect a change in the field of the second material. Electrons driven by the field allow energy to flow across the gap and into the second higher refractive index medium. A common example in everyday use is a beam splitter. A transparent, low refractive index material is sandwiched between two prisms of another material. This allows the beam to tunnel through from one prism to the next in a process very similar to quantum tunneling while at the same time altering the direction of the incoming ray.
4.1.2 FRESNAL EQUATION
The Fresnel equations, deduced by Augustin-Jean Fresnel, describe the behaviour of light when moving between media of differing refractive indices. The reflection of light that the equations predict is known as Fresnel reflection. When light moves from a medium of a given refractive index n1 into a second medium with refractive index n2, both reflection and refraction of the light may occur.
In the diagram on the right, an incident light ray PO strikes at point O the interface between two media of refractive indexes n1 and n2. Part of the ray is reflected as ray OQ and part refracted as ray OS. The angles that the incident, reflected and refracted rays make to the normal of the interface are given as ?i, ?r and ?t, respectively. The relationship between these angles is given by the law of reflection and Snellâ„¢s law. The fraction of the intensity of incident light that is reflected from the interface is given by the reflection coefficient R, and the fraction refracted by the transmission coefficient T. The Fresnel equations may be used to calculate R and T in a given situation. The calculations of R and T depend on polarisation of the incident ray. If the light is polarised with the electric field of the light perpendicular to the plane of the diagram above (s-polarised), the reflection coefficient is given by:
where ?t can be derived from ?i by Snellâ„¢s law. If the incident light is polarised in the plane of the diagram (p-polarised), the R is given by:
The transmission coefficient in each case is given by Ts = 1 - Rs and Tp = 1 - Rp. If the incident light is unpolarised (containing an equal mix of s- and p-polarisations), the reflection coefficient is R = (Rs + Rp)/2. The reflection and transmission coefficients correspond to the ratio of the intensity of the incident ray to that of the reflected and transmitted rays. Equations for coefficients corresponding to ratios of the electric field amplitudes of the waves can also be derived, and these are also called Fresnel equations. At one particular angle for a given n1 and n2, the value of Rp goes to zero and a p-polarised incident ray is purely refracted. This angle is known as Brewsterâ„¢s angle, and is around 56Ã‚Â° for a glass medium in air or vacuum. When moving from a more dense medium into a less dense one (i.e. n1 > n2), above an incidence angle known as the critical angle, all light is reflected and Rs = Rp = 1. This phenomenon is known as total internal reflection. The critical angle is approximately 41Ã‚Â° for glass in air.
When the light is at near-normal incidence to the interface (?i Ã‹ ?t Ã‹ 0), the reflection and transmission coefficient are given by:
For common glass, the reflection coefficient is about 4%. Note that reflection by a window is from the front side as well as the back side, and that some of the light bounces back and forth a number of times between the two sides. The combined reflection coefficient for this case is 2R/(1 + R). Repeated reflection and refraction on thin, parallel layers is also known as Fabry-Perot interference, this effect is responsible for the colours seen in oil films on water, used in optics to make reflection free lenses and perfect mirrors, etc. It should be noted that the discussion given here is only valid when the permeability Ã‚Âµ is equal to the vacuum permeability Ã‚Âµ0 in both media. This is true for most dielectric materials, but the completely general Fresnel equations are more complex.
5. MICROPHOTONIC DEVICES
5.1 PHOTONIC CRYSTAL
The opal in this bracelet contains a natural periodic microstructure responsible for its iridescent color. It is essentially a natural photonic crystal, although it does not have a complete photonic band gap. Photonic crystals are periodic dielectric or metallo-dielectric (nano)structures that are designed to affect the propagation of electromagnetic waves (EM) in the same way as the periodic potential in a semiconductor crystal affects the electron motion by defining allowed and forbidden electronic energy bands. The absence of allowed propagating EM modes inside the structures, in a range of wavelengths called a photonic band gap, gives rise to distinct optical phenomena such as inhibition of spontaneous emission, highreflecting omnidirectional mirrors and lowloss- waveguiding among others. Since the basic physical phenomenon is based on diffraction, the periodicity of the photonic crystal structure has to be in the same lengthscale as half the wavelength of the EM waves i.e. ~300 nm for photonic crystals operating in the visible part of the spectrum. This makes the synthesis cumbersome and complex. To circumvent nanotechnological methods with their big and complex machinery, different approaches have been followed to grow photonic crystals as self-assembled structures from colloidal crystals. A prominent example of a photonic crystal is the naturally occurring gemstone opal. Its opalescence is essentially a photonic crystal phenomenon based on Bragg diffraction of light on the crystalâ„¢s lattice planes. Another well-known photonic crystal is found on the wings of some butterflies such as the blue Morpho (Morpho granadensis). Photonic crystals are attractive optical materials for controlling and manipulating the flow of light. They are of great interest for both fundamental and applied research, and are expected to find commercial applications soon. Two-dimensionally periodic photonic crystals already have reached a level where integrated-device applications are in sight, whereas their three-dimensional counterparts are still far from commercialization but will offer additional advantages possibly leading to new device concepts, when some technological aspects such as manufacturability and principal difficulties such as disorder are under control. The first commercial products involving twodimensionally periodic photonic crystals are already available in the form of photoniccrystal fibers, which use a nanoscale structure to confine light with radically different characteristics compared to conventional optical fiber for applications in nonlinear devices, guiding exotic wavelengths, and so on. The simplest form of a photonic crystal is a one-dimensionally periodic structure, such as a multilayer film (a Bragg mirror); electromagnetic wave propagation in such systems was first studied by Lord Rayleigh in 1887, who showed that any such onedimensional system has a band gap. 1dperiodic systems continued to be studied extensively, and appeared in applications from reflective coatings to distributed feedback (DFB) lasers. 2d-periodic optical structures, without band gaps, received limited study in the 1970s and 1980s. The possibility of two- and three-dimensionally periodic crystals with corresponding two- and three-dimensional band gaps was not suggested until 100 years after Rayleigh, by Eli Yablonovitch and Sajeev John in 1987, and such structures have since seen growing interest by a number of research groups around the world. With applications including LEDs, optical fiber, nanoscopic lasers, ultrawhite pigment, radio frequency antennas and reflectors, and photonic integrated circuits. Many research groups have recently succeeded in controlling the pace of light emission, varying from a light drizzle to a rainstorm, using photonic crystals. In the process, they have verified a 17-year old prediction of American physicist Eli Yablonovitch that ignited a world-wide rush to build tiny chips that control light beams. Researchers say it has many potential uses, not only as a tool for controlling quantum optical systems, but also in efficient miniature lasers for displays and telecom, in solar cells, and even in future quantum computers.
5.2 PHOTONIC SWITCH
Ã‚Â· It as an ultra-fast, automated, patch panel
Ã‚Â· Glimmerglass System 300 Fiber Connection Server
Ã‚Â· Built up around Micro Electro Mechanical Systems (MEMS)
Ã‚Â· Miniaturized, moving components
Ã‚Â· Built with IC manufacturing technology
Ã‚Â· Cost effective
5.3 Coherent control of matter in photonic crystal fibers
An exciting field known as Coherent Control of Quantum Systems emerged from previously unsuccessful attempts to employ lasers to steer chemical reactions. One of the main ideas is to use the phase coherences within a spectrally broad bandwidth laser pulse to drive quantum systems to a desired target state. In other words, we specify the initial and the final state of a quantum system and design the coherences within a laser pulse such that this goal is achieved with a maximum efficiency.
Solid large core, endlessly single mode fiber produced at the IAP and calculated mode profile.
The photonic crystal fiber (PCF) is one subgroup of photonic structures especially well suited for guiding light and alike. In contrast to fiber communications where most efforts concentrate on common standards, the many diverse scientific applications require a variety of different photonic structures. As the geometry of the PCFsâ„¢ cross section next to the materialsâ„¢ properties determines the fiber characteristics, these needs can be served with unprecedented flexibility.The fiber drawing facility at the University of Bern has recently been modified to allow for PCF fabrication. Currently, a novel production process is being implemented, which will greatly enhance our capabilities for PCF fabrication. Figure 2 shows one of the first microstructure fibers guiding light at 1064nm wavelength together with the simulated mode profile. Design and simulations are done via adapted public domain and commercially available software.
VCSELs (Vertical Cavity Surface Emitting Lasers) have become an established product as key light source in computer mice, fiber-optic data links, and laser printers. Their superior performance in terms of size, power consumption, spectral purity, and temperature stability give them advantageous characteristics for these applications. An important future emerging field of application for VCSELs is optical interconnect technology in computers (board to board or rack to rack). It is currently under development by major computer companies worldwide, and a large market is expected. A major bottleneck in this application is the limited bandwidth of the VCSEL under direct modulation. The data rate required in this technology is beyond 10 Gbit/s, ideally 40 Gbit/s. Current VCSELs are limited to bandwidths below 12.5 Gbit/s in research and industry. The workaround for existing optical interconnect prototypes is to use multichannel, parallel links. They require costly, bulky and energy inefficient VCSEL arrays, and should be ideally replaced by single, highspeed sources.
Electro-opto-thermal simulation of a VCSEL. The left face shows the temperature distribution isolines)_ and the electron current (flowlines), and the right face shows the optical intensity. The
diameter of the top, light-emitting opening is approx. 6mm.
Microphotonics is considered as the successor to electronics which breaks the logjam between fiber optics and electronic switching.As this technology becomes more readily available, the high speed pipes will extend further and further into the network.. Proposed technology demonstrator incorporates new advanced-technology concepts for miniaturization of optical devices on SOI platform.
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