Modern Communication Services

Modern Communication Services

Society is becoming more informationally and visually oriented every day. Personal computing facilitates easy access, manipulation, storage, and exchange of information. These processes require reliable transmission of data information. Communicating documents by images and the use of high resolution graphics terminals provide a more natural and informative mode of human interaction than just voice and data. Video teleconferencing enhances group interaction at a distance. High definition entertainment video improves the quality of picture at the expense of higher transmission bit-rates, which may require new transmission means other than the present overcrowded radio spectrum. A modern Telecommunications network (such as the broadband network) must provide all these different services (multi-services) to the user.
Differences between traditional (telephony) and modern communication services

Conventional telephony communicates using:

* the voice medium only
* connects only two telephones per call
* uses circuits of fixed bit rate

In contrast, modern communication services depart from the conventional telephony service in these three essential aspects. Modern communication services can be:

* Multimedia
* point to point, and
* multi-rate

These aspects are examined Individually in the following three sub-sections.

* Multi-media: A multi-media call may communicate audio, data, still images, or full-motion video, or any combination of these media. Each medium has different demands for communication qualities, such as:
o bandwidth requirement
o signal latency within the network, and
o signal fidelity upon delivery by the network

Moreover, the information content of each medium may affect the information generated by other media. For example, voice could be transcribed into data via voice recognition and data commands may control the way voice and video are presented. These interactions most often occur at the communication terminals, but may also occur within the network .

* Multi-point: A multi-point call involves the setup of connections among more than two people. These connections can be multi-media. They can be one way or two way communications. These connections may be reconfigured many times within the duration of a call. A few examples will be used to contrast point-to-point communications versus multi-point communications. Traditional voice calls are predominantly two party calls, requiring a point-to-point connection using only the voice medium. To access pictorial information in a remote database would require a point-to-point connection that sends low bit-rate queries to the database, and high bit-rate video from the database. Entertainment video applications are largely point-to-multi-point connections, requiring one way communication of full motion video and audio from the program source to the viewers. Video teleconferencing involves connections among many parties, communicating voice, video, as well as data. Thus offering future services requires flexible management of the connection and media requests of a multi-point, multi-media communication call .
* Multi-rate A multi-rate service network is one which allocates transmission capacity flexibly to connections. A multi-media network has to support a broad range of bit-rates demanded by connections, not only because there are many communication media, but also because a communication medium may be encoded by algorithms with different bit-rates. For example, audio signals can be encoded with bit-rates ranging from less than 1 kbit/s to hundreds of kbit/s, using different encoding algorithms with a wide range of complexity and quality of audio reproduction. Similarly, full motion video signals may be encoded with bit-rates ranging from less than 1 Mbit/s to hundreds of Mbit/s. Thus a network transporting both video and audio signals may have to integrate traffic with a very broad range of bit-rates.


Light tree

Light tree

Definition
The concept of light tree is introduced in a wavelength routed optical network, which employs wavelength -division multiplexing (WDM).
Depending on the underlying physical topology networks can be classified into three generations:

a).First Generation: these networks do not employ fiber optic technology; instead they employ copper-based or microwave technology. E.g. Ethernet.
b).Second Generation: these networks use optical fibers for data transmission but switching is performed in electronic domain. E.g. FDDI.
c).Third Generation: in these networks both data transmission and switching is performed in optical domain. E.g. WDM.

WDM wide area networks employ tunable lasers and filters at access nodes and optical/electronic switches at routing nodes. An access node may transmit signals on different wavelengths, which are coupled into the fiber using wavelength multiplexers. An optical signal passing through an optical wavelength-routing switch (WRS) may be routed from an output fiber without undergoing opto-electronic conversion.

A light path is an all-optical channel, which may be used to carry circuit switched traffic, and it may span multiple fiber links. Assigning a particular wavelength to it sets these up. In the absence of wavelength converters, a light path would occupy the same wavelength continuity constraint.

A light path can create logical (or virtual) neighbors out of nodes that may be geographically far apart from each other. A light path carries not only the direct traffic between the nodes it interconnects, but also the traffic from nodes upstream of the source to nodes upstream of the destination. A major objective of light path communication is to reduce the number of hops a packet has to traverse.

Under light path communication, the network employs an equal number of transmitters and receivers because each light path operates on a point-to-point basis. However this approach is not able to fully utilize all of the wavelengths on all of the fiber links in the network, also it is not able to fully exploit all the switching capability of each WRS.
A light tree is a point to point multipoint all optical channel, which may span multiple fiber links. Hence, a light tree enables single-hop communication between a source node and a set of destination nodes. Thus, a light tree based virtual topology can significantly reduce the hop distance, thereby increasing the network throughput.

Requirements:
1. Multicast -capable wavelength routing switches (MWRS) at every node in the netwok.
2. More optical amplifiers in the network.

LIDAR

LIDAR

LIDAR (Light Detection and Ranging) is an optical remote sensing technology that measures properties of scattered light to find range and/or other information of a distant target. The prevalent method to determine distance to an object or surface is to use laser pulses. Like the similar radar technology, which uses radio waves, which is light that is not in the visible spectrum, the range to an object is determined by measuring the time delay between transmission of a pulse and detection of the reflected signal. LIDAR technology has application in Geomatics, archaeology, geography, geology, geomorphology, seismology, remote sensing and atmospheric physics.[1] Other terms for LIDAR include ALSM (Airborne Laser Swath Mapping) and laser altimetry. The acronym LADAR (Laser Detection and Ranging) is often used in military contexts. The term laser radar is also in use but is misleading because it uses laser light and not the radiowaves that are the basis of conventional radar.


General description

The primary difference between lidar and radar is that with lidar, much shorter wavelengths of the electromagnetic spectrum are used, typically in the ultraviolet, visible, or near infrared. In general it is possible to image a feature or object only about the same size as the wavelength, or larger. Thus lidar is highly sensitive to aerosols and cloud particles and has many applications in atmospheric research and meteorology.

An object needs to produce a dielectric discontinuity in order to reflect the transmitted wave. At radar (microwave or radio) frequencies, a metallic object produces a significant reflection. However non-metallic objects, such as rain and rocks produce weaker reflections and some materials may produce no detectable reflection at all, meaning some objects or features are effectively invisible at radar frequencies. This is especially true for very small objects (such as single molecules and aerosols).

Lasers provide one solution to these problems. The beam densities and coherency are excellent. Moreover the wavelengths are much smaller than can be achieved with radio systems, and range from about 10 micrometers to the UV (ca. 250 nm). At such wavelengths, the waves are "reflected" very well from small objects. This type of reflection is called backscattering. Different types of scattering are used for different lidar applications, most common are Rayleigh scattering, Mie scattering and Raman scattering as well as fluorescence. The wavelengths are ideal for making measurements of smoke and other airborne particles (aerosols), clouds, and air molecules.

A laser typically has a very narrow beam which allows the mapping of physical features with very high resolution compared with radar. In addition, many chemical compounds interact more strongly at visible wavelengths than at microwaves, resulting in a stronger image of these materials. Suitable combinations of lasers can allow for remote mapping of atmospheric contents by looking for wavelength-dependent changes in the intensity of the returned signal.

Lidar has been used extensively for atmospheric research and meteorology. With the deployment of the GPS in the 1980's precision positioning of aircraft became possible. GPS based surveying technology has made airborne surveying and mapping applications possible and practical. Many have been developed, using downward-looking lidar instruments mounted in aircraft or satellites. A recent example is the NASA Experimental Advanced Research Lidar.

LIDAR is an acronym for LIght Detection And Ranging.
What can you do with LIDAR?

* Measure distance
* Measure speed
* Measure rotation
* Measure chemical composition and concentration


of a remote target where the target can be a clearly defined object, such as a vehicle, or a diffuse object such as a smoke plume or clouds.

Applications

Other than those applications mentioned above, there are a wide variety of applications of LIDAR.

Archaeology

LiDAR has many applications in the field of archaeology including aiding in the planning of field campaigns, mapping features beneath forest canopy, and providing an overview of broad, continuous features that may be indistinguishable on the ground. LiDAR can also provide archaeologists with the ability to create high-resolution digital elevation models (DEMs) of archaeological sites that can reveal micro-topography that are otherwise hidden by vegetation. LiDAR-derived products can be easily integrated into a Geographic Information System (GIS) for analysis and interpretation. For example at Fort Beausejour - Fort Cumberland National Historic Site, Canada, previously undiscovered archaeological features have been mapped that are related to the siege of the Fort in 1755. Features that could not be distinguished on the ground or through aerial photography were identified by overlaying hillshades of the DEM created with artificial illumination from various angles. With LiDAR the ability to produce high-resolution datasets quickly and relatively cheaply can be an advantage. Beyond efficiency, its ability to penetrate forest canopy has led to the discovery of features that were not distinguishable through traditional geo-spatial methods and are difficult to reach through field surveys.

Meteorology

The first LIDARs were used for studies of atmospheric composition, structure, clouds, and aerosols. Initially based on ruby lasers, LIDARs for meteorological applications were constructed shortly after the invention of the laser and represent one of the first applications of laser technology.

Elastic backscatter LIDAR is the simplest type of lidar and is typically used for studies of aerosols and clouds. The backscattered wavelength is identical to the transmitted wavelength, and the magnitude of the received signal at a given range depends on the backscatter coefficient of scatterers at that range and the extinction coefficients of the scatterers along the path to that range. The extinction coefficient is typically the quantity of interest.

Differential Absorption LIDAR (DIAL) is used for range-resolved measurements of a particular gas in the atmosphere, such as ozone, carbon dioxide, or water vapor. The LIDAR transmits two wavelengths: an "on-line" wavelength that is absorbed by the gas of interest and an off-line wavelength that is not absorbed. The differential absorption between the two wavelengths is a measure of the concentration of the gas as a function of range. DIAL LIDARs are essentially dual-wavelength elastic backscatter LIDARS.

Raman LIDAR is also used for measuring the concentration of atmospheric gases, but can also be used to retrieve aerosol parameters as well. Raman LIDAR exploits inelastic scattering to single out the gas of interest from all other atmospheric constituents. A small portion of the energy of the transmitted light is deposited in the gas during the scattering process, which shifts the scattered light to a longer wavelength by an amount that is unique to the species of interest. The higher the concentration of the gas, the stronger the magnitude of the backscattered signal.

Doppler LIDAR is used to measure wind speed along the beam by measuring the frequency shift of the backscattered light. Scanning LIDARs, such as NASA's HARLIE LIDAR, have been used to measure atmospheric wind velocity in a large three dimensional cone. ESA's wind mission ADM-Aeolus will be equipped with a Doppler LIDAR system in order to provide global measurements of vertical wind profiles. A doppler LIDAR system was used in the 2008 Summer Olympics to measure wind fields during the yacht competition. Doppler LIDAR systems are also now beginning to be successfully applied in the renewable energy sector to acquire wind speed, turbulence, wind veer and wind shear data. Both pulsed and continuous wave systems are being used. Pulsed systems using signal timing to obtain vertical distance resolution, whereas continuous wave systems rely on detector focusing.

Geology

In geology and seismology a combination of aircraft-based LIDAR and GPS have evolved into an important tool for detecting faults and measuring uplift. The output of the two technologies can produce extremely accurate elevation models for terrain that can even measure ground elevation through trees. This combination was used most famously to find the location of the Seattle Fault in Washington, USA. This combination is also being used to measure uplift at Mt. St. Helens by using data from before and after the 2004 uplift. Airborne LIDAR systems monitor glaciers and have the ability to detect subtle amounts of growth or decline. A satellite based system is NASA's ICESat which includes a LIDAR system for this purpose. NASA's Airborne Topographic Mapper is also used extensively to monitor glaciers and perform coastal change analysis.

Physics and Astronomy

A world-wide network of observatories uses lidars to measure the distance to reflectors placed on the moon, allowing the moon's position to be measured with mm precision and tests of general relativity to be done. MOLA, the Mars Orbiting Laser Altimeter, used a LIDAR instrument in a Mars-orbiting satellite (the NASA Mars Global Surveyor) to produce a spectacularly precise global topographic survey of the red planet.

In September, 2008, NASA's Phoenix Lander used LIDAR to detect snow in the atmosphere of Mars.

In atmospheric physics, LIDAR is used as a remote detection instrument to measure densities of certain constituents of the middle and upper atmosphere, such as potassium, sodium, or molecular nitrogen and oxygen. These measurements can be used to calculate temperatures. LIDAR can also be used to measure wind speed and to provide information about vertical distribution of the aerosol particles.

At the JET nuclear fusion research facility, in the UK near Abingdon, Oxfordshire, LIDAR Thomson Scattering is used to determine Electron Density and Temperature profiles of the plasma.

Biology and conservation

LIDAR has also found many applications in forestry. Canopy heights, biomass measurements, and leaf area can all be studied using airborne LIDAR systems. Similarly, LIDAR is also used by many industries, including Energy and Railroad, and the Department of Transportation as a faster way of surveying. Topographic maps can also be generated readily from LIDAR, including for recreational use such as in the production of orienteering maps.

In oceanography, LiDAR is used for estimation of phytoplankton fluorescence and generally biomass in the surface layers of the ocean. Another application is airborne lidar bathymetry of sea areas too shallow for hydrographic vessels.

Redwood ecology

The Save-the-Redwoods League is undertaking a project to map the tall redwoods on California's northern coast. LIDAR allows research scientists to not only measure the height of previously unmapped trees but to determine the biodiversity of the redwood forest. Dr. Stephen Sillett who is working with the League on the North Coast LIDAR project claims this technology will be useful in directing future efforts to preserve and protect ancient redwood trees.

Military and law enforcement

One situation where LIDAR has notable non-scientific application is in traffic speed law enforcement, for vehicle speed measurement, as a technology alternative to radar guns. The technology for this application is small enough to be mounted in a hand held camera "gun" and permits a particular vehicle's speed to be determined from a stream of traffic. Unlike RADAR which relies on doppler shifts to directly measure speed, police lidar relies on the principle of time-of-flight to calculate speed. The equivalent radar based systems are often not able to isolate particular vehicles from the traffic stream and are generally too large to be hand held. LIDAR has the distinct advantage of being able to pick out one vehicle in a cluttered traffic situation as long as the operator is aware of the limitations imposed by the range and beam divergence. Contrary to popular belief LIDAR does not suffer from “sweep” error when the operator uses the equipment correctly and when the LIDAR unit is equipped with algorithms that are able to detect when this has occurred. A combination of signal strength monitoring, receive gate timing, target position prediction and pre-filtering of the received signal wavelength prevents this from occurring. Should the beam illuminate sections of the vehicle with different reflectivity or the aspect of the vehicle changes during measurement that causes the received signal strength to be changed then the LIDAR unit will reject the measurement thereby producing speed readings of high integrity. For LIDAR units to be used in law enforcement applications a rigorous approval procedure is usually completed before deployment. Jelly-bean shaped vehicles are usually equipped with a vertical registration plate that, when illuminated causes a high integrity reflection to be returned to the LIDAR, many reflections and an averaging technique in the speed measurement process increase the integrity of the speed reading. In locations that do not require that a front or rear registration plate is fitted headlamps and rear-reflectors provide an almost ideal retro-reflective surface overcoming the reflections from uneven or non-compliant reflective surfaces thereby eliminating “sweep” error. It is these mechanisms which cause concern that LIDAR is somehow unreliable. Most traffic LIDAR systems send out a stream of approximately 100 pulses over the span of three-tenths of a second. A "black box," proprietary statistical algorithm picks and chooses which progressively shorter reflections to retain from the pulses over the short fraction of a second.

Military applications are not yet known to be in place and are possibly classified, but a considerable amount of research is underway in their use for imaging. Their higher resolution makes them particularly good for collecting enough detail to identify targets, such as tanks. Here the name LADAR is more common.

Five LIDAR units produced by the German company Sick AG were used for short range detection on Stanley, the autonomous car that won the 2005 DARPA Grand Challenge.

Vehicles

Lidar has been used to create Adaptive Cruise Control (ACC) systems for automobiles. Systems such as those by Siemens and Hella use a lidar device mounted in the front of the vehicle to monitor the distance between the vehicle and any vehicle in front of it. Often, the lasers are placed onto the bumper. In the event the vehicle in front slows down or is too close, the ACC applies the brakes to slow the vehicle. When the road ahead is clear, the ACC allows the vehicle to speed up to speed preset by the driver.

Imaging

3-D imaging is done with both scanning and non-scanning systems. "3-D gated viewing laser radar" is a non-scanning laser radar system that applies the so-called gated viewing technique. The gated viewing technique applies a pulsed laser and a fast gated camera. There are ongoing military research programmes in Sweden, Denmark, the USA and the UK with 3-D gated viewing imaging at several kilometers range with a range resolution and accuracy better than ten centimeters.

Coherent Imaging Lidar is possible using Synthetic Array Heterodyne Detection which is a form of Optical heterodyne detection that enables a staring single element receiver to act as though it were an imaging array.

Imaging LIDAR can also be performed using arrays of high speed detectors and modulation sensitive detectors arrays typically built on single chips using CMOS and hybrid CMOS / CCD fabrication techniques. In these devices each pixel performs some local processing such as demodulation or gating at high speed down converting the signals to video rate so that the array may be read like a camera. Using this technique many thousands of pixels / channels may be acquired simultaneously. In practical systems the limitation is light budget rather than parallel acquisition.

LIDAR has been used in the recording of a music video without cameras. The video for the song "House of Cards" by Radiohead is believed to be the first use of real-time 3D laser scanning to record a music video.

3D Mapping

Airborne LIDAR sensors are used by companies in the Remote Sensing area to create point clouds of the earth ground for further processing (e.g. used in forestry).

RADAR

RADAR
Radar is an object detection system that uses electromagnetic waves to identify the range, altitude, direction, or speed of both moving and fixed objects such as aircraft, ships, motor vehicles, weather formations, and terrain. The term RADAR was coined in 1941 as an acronym for radio detection and ranging. The term has since entered the English language as a standard word, radar, losing the capitalization. Radar was originally called RDF (Radio Direction Finder, now used as a totally different device) in the United Kingdom.

A radar system has a transmitter that emits microwaves or radio waves. These waves are in phase when emitted, and when they come into contact with an object are scattered in all directions. The signal is thus partly reflected back and it has a slight change of wavelength (and thus frequency) if the target is moving. The receiver is usually, but not always, in the same location as the transmitter. Although the signal returned is usually very weak, the signal can be amplified through use of electronic techniques in the receiver and in the antenna configuration. This enables radar to detect objects at ranges where other emissions, such as sound or visible light, would be too weak to detect. Radar uses include meteorological detection of precipitation, measuring ocean surface waves, air traffic control, police detection of speeding traffic, determining the speed of basesballs and by the military.

RAdio Detection And Ranging ,in short RADAR relies on sending and receiving electromagnetic radiation, usually in the form of radio waves (see Radio) or microwaves. Electromagnetic radiation is energy that moves in waves at or near the speed of light. The characteristics of electromagnetic waves depend on their wavelength. Gamma rays and X rays have very short wavelengths. Visible light is a tiny slice of the electromagnetic spectrum with wavelengths longer than X rays, but shorter than microwaves. Radar systems use long-wavelength electromagnetic radiation in the microwave and radio ranges. Because of their long wavelengths, radio waves and microwaves tend to reflect better than shorter wavelength radiation, which tends to scatter or be absorbed before it gets to the target. Radio waves at the long-wavelength end of the spectrum will even reflect off of the atmospheres ionosphere, a layer of electrically-charged particles in the earths atmosphere. The challenges for radar are stealth technology,clutter,jamming. It has certain applications like the traffic control,maritime navigation,millitary safety,air traffic control,meteorology etc.

History

Several inventors, scientists, and engineers contributed to the development of radar. The first to use radio waves to detect "the presence of distant metallic objects" was Christian Hülsmeyer, who in 1904 demonstrated the feasibility of detecting the presence of a ship in dense fog, but not its distance.He received Reichspatent Nr. 165546 for his pre-radar device in April 1904, and later patent 169154 for a related amendment for ranging. He also received a patent[9] in England for his telemobiloscope on September 22, 1904.

In August 1917 Nikola Tesla first established principles regarding frequency and power level for the first primitive radar units. He stated, " by their [standing electromagnetic waves] use we may produce at will, from a sending station, an electrical effect in any particular region of the globe; [with which] we may determine the relative position or course of a moving object, such as a vessel at sea, the distance traversed by the same, or its speed."

Before the Second World War developments by the Americans, the Germans, the French, the Soviets, and the British led to the modern version of radar. In 1934 the French Émile Girardeau stated he was building a radar system "conceived according to the principles stated by Tesla" and obtained a patent (French Patent n° 788795 in 1934) for a working dual radar system, a part of which was installed on the Normandie liner in 1935. The same year, American Dr. Robert M. Page tested the first monopulse radar and the Soviet military engineer P.K.Oschepkov, in collaboration with Leningrad Electrophysical Institute, produced an experimental apparatus RAPID capable of detecting an aircraft within 3 km of a receiver.[16] Hungarian Zoltán Bay produced a working model by 1936 at the Tungsram laboratory in the same vein.

However, it was the British who were the first to fully exploit it as a defence against aircraft attack. This was spurred on by fears that the Germans were developing death rays. Following a study of the possibility of propagating electromagnetic energy and the likely effect, the British scientists asked by the Air Ministry to investigate concluded that a death ray was impractical but detection of aircraft appeared feasible. Robert Watson-Watt demonstrated to his superiors the capabilities of a working prototype and patented the device in 1935 (British Patent GB593017) It served as the basis for the Chain Home network of radars to defend Great Britain.

The war precipitated research to find better resolution, more portability and more features for radar. The post-war years have seen the use of radar in fields as diverse as air traffic control, weather monitoring, astrometry and road speed control.

Principles

The radar dish, or antenna, transmits pulses of radio waves or microwaves which bounce off any object in their path. The object returns a tiny part of the wave's energy to a dish or antenna which is usually located at the same site as the transmitter. The time it takes for the reflected waves to return to the dish enables a computer to calculate how far away the object is, its radial velocity and other characteristics.

Reflection

Electromagnetic waves reflect (scatter) from any large change in the dielectric or diamagnetic constants. This means that a solid object in air or a vacuum, or other significant change in atomic density between the object and what is surrounding it, will usually scatter radar (radio) waves. This is particularly true for electrically conductive materials, such as metal and carbon fiber, making radar particularly well suited to the detection of aircraft and ships. Radar absorbing material, containing resistive and sometimes magnetic substances, is used on military vehicles to reduce radar reflection. This is the radio equivalent of painting something a dark color.

Radar waves scatter in a variety of ways depending on the size (wavelength) of the radio wave and the shape of the target. If the wavelength is much shorter than the target's size, the wave will bounce off in a way similar to the way light is reflected by a mirror. If the wavelength is much longer than the size of the target, the target is polarized (positive and negative charges are separated), like a dipole antenna. This is described by Rayleigh scattering, an effect that creates the Earth's blue sky and red sunsets. When the two length scales are comparable, there may be resonances. Early radars used very long wavelengths that were larger than the targets and received a vague signal, whereas some modern systems use shorter wavelengths (a few centimeters or shorter) that can image objects as small as a loaf of bread.

Short radio waves reflect from curves and corners, in a way similar to glint from a rounded piece of glass. The most reflective targets for short wavelengths have 90° angles between the reflective surfaces. A structure consisting of three flat surfaces meeting at a single corner, like the corner on a box, will always reflect waves entering its opening directly back at the source. These so-called corner reflectors are commonly used as radar reflectors to make otherwise difficult-to-detect objects easier to detect, and are often found on boats in order to improve their detection in a rescue situation and to reduce collisions.

For similar reasons, objects attempting to avoid detection will angle their surfaces in a way to eliminate inside corners and avoid surfaces and edges perpendicular to likely detection directions, which leads to "odd" looking stealth aircraft. These precautions do not completely eliminate reflection because of diffraction, especially at longer wavelengths. Half wavelength long wires or strips of conducting material, such as chaff, are very reflective but do not direct the scattered energy back toward the source. The extent to which an object reflects or scatters radio waves is called its radar cross section.

Polarization

In the transmitted radar signal, the electric field is perpendicular to the direction of propagation, and this direction of the electric field is the polarization of the wave. Radars use horizontal, vertical, linear and circular polarization to detect different types of reflections. For example, circular polarization is used to minimize the interference caused by rain. Linear polarization returns usually indicate metal surfaces. Random polarization returns usually indicate a fractal surface, such as rocks or soil, and are used by navigation radars.

Interference

Radar systems must overcome unwanted signals in order to focus only on the actual targets of interest. These unwanted signals may originate from internal and external sources, both passive and active. The ability of the radar system to overcome these unwanted signals defines its signal-to-noise ratio (SNR). SNR is defined as the ratio of a signal power to the noise power within the desired signal.

In less technical terms, SNR compares the level of a desired signal (such as targets) to the level of background noise. The higher a system's SNR, the better it is in isolating actual targets from the surrounding noise signals.

Noise

Signal noise is an internal source of random variations in the signal, which is generated by all electronic components. Noise typically appears as random variations superimposed on the desired echo signal received in the radar receiver. The lower the power of the desired signal, the more difficult it is to discern it from the noise (similar to trying to hear a whisper while standing near a busy road). Noise figure is a measure of the noise produced by a receiver compared to an ideal receiver, and this needs to be minimized.

Noise is also generated by external sources, most importantly the natural thermal radiation of the background scene surrounding the target of interest. In modern radar systems, due to the high performance of their receivers, the internal noise is typically about equal to or lower than the external scene noise. An exception is if the radar is aimed upwards at clear sky, where the scene is so "cold" that it generates very little thermal noise.

There will be also flicker noise due to electrons transit, but depending on 1/f, will be much lower than thermal noise when the frequency is high. Hence, in pulse radar, the system will be always heterodyne. See intermediate frequency.

Clutter

Clutter refers to radio frequency (RF) echoes returned from targets which are uninteresting to the radar operators. Such targets include natural objects such as ground, sea, precipitation (such as rain, snow or hail), sand storms, animals (especially birds), atmospheric turbulence, and other atmospheric effects, such as ionosphere reflections and meteor trails. Clutter may also be returned from man-made objects such as buildings and, intentionally, by radar countermeasures such as chaff.

Some clutter may also be caused by a long radar waveguide between the radar transceiver and the antenna. In a typical plan position indicator (PPI) radar with a rotating antenna, this will usually be seen as a "sun" or "sunburst" in the centre of the display as the receiver responds to echoes from dust particles and misguided RF in the waveguide. Adjusting the timing between when the transmitter sends a pulse and when the receiver stage is enabled will generally reduce the sunburst without affecting the accuracy of the range, since most sunburst is caused by a diffused transmit pulse reflected before it leaves the antenna.

While some clutter sources may be undesirable for some radar applications (such as storm clouds for air-defence radars), they may be desirable for others (meteorological radars in this example). Clutter is considered a passive interference source, since it only appears in response to radar signals sent by the radar.

There are several methods of detecting and neutralizing clutter. Many of these methods rely on the fact that clutter tends to appear static between radar scans. Therefore, when comparing subsequent scans echoes, desirable targets will appear to move and all stationary echoes can be eliminated. Sea clutter can be reduced by using horizontal polarization, while rain is reduced with circular polarization (note that meteorological radars wish for the opposite effect, therefore using linear polarization the better to detect precipitation). Other methods attempt to increase the signal-to-clutter ratio.

Constant False Alarm Rate (CFAR, a form of Automatic Gain Control, or AGC) is a method relying on the fact that clutter returns far outnumber echoes from targets of interest. The receiver's gain is automatically adjusted to maintain a constant level of overall visible clutter. While this does not help detect targets masked by stronger surrounding clutter, it does help to distinguish strong target sources. In the past, radar AGC was electronically controlled and affected the gain of the entire radar receiver. As radars evolved, AGC became computer-software controlled, and affected the gain with greater granularity, in specific detection cells.

Clutter may also originate from multipath echoes from valid targets due to ground reflection, atmospheric ducting or ionospheric reflection/refraction. This clutter type is especially bothersome, since it appears to move and behave like other normal (point) targets of interest, thereby creating a ghost. In a typical scenario, an aircraft echo is multipath-reflected from the ground below, appearing to the receiver as an identical target below the correct one. The radar may try to unify the targets, reporting the target at an incorrect height, or - worse - eliminating it on the basis of jitter or a physical impossibility. These problems can be overcome by incorporating a ground map of the radar's surroundings and eliminating all echoes which appear to originate below ground or above a certain height. In newer Air Traffic Control (ATC) radar equipment, algorithms are used to identify the false targets by comparing the current pulse returns, to those adjacent, as well as calculating return improbabilities due to calculated height, distance, and radar timing.

Jamming

Radar jamming refers to radio frequency signals originating from sources outside the radar, transmitting in the radar's frequency and thereby masking targets of interest. Jamming may be intentional, as with an electronic warfare (EW) tactic, or unintentional, as with friendly forces operating equipment that transmits using the same frequency range. Jamming is considered an active interference source, since it is initiated by elements outside the radar and in general unrelated to the radar signals.

Jamming is problematic to radar since the jamming signal only needs to travel one-way (from the jammer to the radar receiver) whereas the radar echoes travel two-ways (radar-target-radar) and are therefore significantly reduced in power by the time they return to the radar receiver. Jammers therefore can be much less powerful than their jammed radars and still effectively mask targets along the line of sight from the jammer to the radar (Mainlobe Jamming). Jammers have an added effect of affecting radars along other lines of sight, due to the radar receiver's sidelobes (Sidelobe Jamming).

Mainlobe jamming can generally only be reduced by narrowing the mainlobe solid angle, and can never fully be eliminated when directly facing a jammer which uses the same frequency and polarization as the radar. Sidelobe jamming can be overcome by reducing receiving sidelobes in the radar antenna design and by using an omnidirectional antenna to detect and disregard non-mainlobe signals. Other anti-jamming techniques are frequency hopping and polarization. See Electronic counter-counter-measures for details.

Interference has recently become a problem for C-band (5.66 GHz) meteorological radars with the proliferation of 5.4 GHz band WiFi equipment.

Radar engineering

A radars components are:

* A transmitter that generates the radio signal with an oscillator such as a klystron or a magnetron and controls its duration by a modulator.
* A waveguide that links the transmitter and the antenna.
* A duplexer that serves as a switch between the antenna and the transmitter or the receiver for the signal when the antenna is used in both situations.
* A receiver. Knowing the shape of the desired received signal (a pulse), an optimal receiver can be designed using a matched filter.
* An electronic section that controls all those devices and the antenna to perform the radar scan ordered by a software.
* A link to end users.

Antenna design

Radio signals broadcast from a single antenna will spread out in all directions, and likewise a single antenna will receive signals equally from all directions. This leaves the radar with the problem of deciding where the target object is located.

Early systems tended to use omni-directional broadcast antennas, with directional receiver antennas which were pointed in various directions. For instance the first system to be deployed, Chain Home, used two straight antennas at right angles for reception, each on a different display. The maximum return would be detected with an antenna at right angles to the target, and a minimum with the antenna pointed directly at it (end on). The operator could determine the direction to a target by rotating the antenna so one display showed a maximum while the other shows a minimum.

One serious limitation with this type of solution is that the broadcast is sent out in all directions, so the amount of energy in the direction being examined is a small part of that transmitted. To get a reasonable amount of power on the "target", the transmitting aerial should also be directional.

Parabolic reflector

More modern systems use a steerable parabolic "dish" to create a tight broadcast beam, typically using the same dish as the receiver. Such systems often combine two radar frequencies in the same antenna in order to allow automatic steering, or radar lock.

Parabolic reflectors can be either symmetric parabolas or spoiled parabolas:

* Symmetric parabolic antennas produce a narrow "pencil" beam in both the X and Y dimensions and consequently have a higher gain. The NEXRAD Pulse-Doppler weather radar uses a symmetric antenna to perform detailed volumetric scans of the atmosphere.
* Spoiled parabolic antennas produce a narrow beam in one dimension and a relatively wide beam in the other. This feature is useful if target detection over a wide range of angles is more important than target location in three dimensions. Most 2D surveillance radars use a spoiled parabolic antenna with a narrow azimuthal beamwidth and wide vertical beamwidth. This beam configuration allows the radar operator to detect an aircraft at a specific azimuth but at an indeterminate height. Conversely, so-called "nodder" height finding radars use a dish with a narrow vertical beamwidth and wide azimuthal beamwidth to detect an aircraft at a specific height but with low azimuthal precision.

Types of scan

* Primary Scan: A scanning technique where the main antenna aerial is moved to produce a scanning beam, examples include circular scan, sector scan etc
* Secondary Scan: A scanning technique where the antenna feed is moved to produce a scanning beam, examples include conical scan, unidirectional sector scan, lobe switching etc.
* Palmer Scan: A scanning technique that produces a scanning beam by moving the main antenna and its feed. A Palmer Scan is a combination of a Primary Scan and a Secondary Scan.

Synergetics

Synergetics

Inpired by the Laser theory and founded by Hermann Haken and Arne Wunderlin, Synergetics is an interdisciplinary science explaining the formation and self-organization of patterns and structures in 'open' systems far from thermodynamic equilibrium. Self-organization requires a 'macroscopic' system, consisting of many nonlinearly interacting subsystems. Depending on the external control parameters (environment, energy-fluxes) self-organization takes place. Essential in Synergetics is the order-parameter concept which was originally introduced in the Ginzburg-Landau theory in order to describe phase-transitions in thermodynamics.

The order parameter concept is generalized by Haken to the 'enslaving-principle' saying that the dynamics of fast-relaxing (stable) modes is completely determined by the 'slow' dynamics of as a rule only a few 'order-parameters' (unstable modes). The order parameters can be interpreted as the amplitudes of the unstable modes determining the macroscopic pattern. As a consequence, self-organization means an enormous reduction of degrees of freedom (entropy) of the system which macroscopically reveals as increase of 'order' (pattern-formation). This far-reaching macroscopic order is independent on the details of the microscopic interactions of the subsystems. This is why Synergetics explains the self-organization of patterns in so many different systems in physics, chemistry, biology and even social systems.

Spintronics

Spintronics

Spintronics (a neologism meaning "spin transport electronics"), also known as magnetoelectronics, is an emerging technology that exploits the intrinsic spin of electrons and its associated magnetic moment, in addition to its fundamental electronic charge, in solid-state devices.

History

The research field of Spintronics emerged from experiments on spin-dependent electron transport phenomena in solid-state devices done in the 1980s, including the observation of spin-polarized electron injection from a ferromagnetic metal to a normal metal by Er. Jiveshwar Sharma (Jove) and Johnson and Silsbee (1985), and the discovery of giant magnetoresistance independently by Albert Fert et al. and Peter Grünberg et al. (1988). The origins can be traced back further to the ferromagnet/superconductor tunneling experiments pioneered by Meservey and Tedrow, and initial experiments on magnetic tunnel junctions by Julliere in the 1970s. The use of semiconductors for spintronics can be traced back at least as far as the theoretical proposal of a spin field-effect-transistor by Datta and Das in 1990.

Conventional electronic devices rely on the transport of electrical charge carriers - electrons - in a semiconductor such as silicon. Now, however, physicists are trying to exploit the 'spin' of the electron rather than its charge to create a remarkable new generation of 'spintronic' devices which will be smaller, more versatile and more robust than those currently making up silicon chips and circuit elements. The potential market is worth hundreds of billions of dollars a year. See Spintronics

All spintronic devices act according to the simple scheme: (1) information is stored (written) into spins as a particular spin orientation (up or down), (2) the spins, being attached to mobile electrons, carry the information along a wire, and (3) the information is read at a terminal. Spin orientation of conduction electrons survives for a relatively long time (nanoseconds, compared to tens of femtoseconds during which electron momentum decays), which makes spintronic devices particularly attractive for memory storage and magnetic sensors applications, and, potentially for quantum computing where electron spin would represent a bit (called qubit) of information. See Spintronics

Magnetoelectronics, Spin Electronics, and Spintronics are different names for the same thing: the use of electrons' spins (not just their electrical charge) in information circuits. See Magnetoelectronics, Spin Electronics, and Spintronics

Theory

Electrons are spin-1/2 fermions and therefore constitute a two-state system with spin "up" and spin "down". To make a spintronic device, the primary requirements are to have a system that can generate a current of spin polarized electrons comprising more of one spin species—up or down—than the other (called a spin injector), and a separate system that is sensitive to the spin polarization of the electrons (spin detector). Manipulation of the electron spin during transport between injector and detector (especially in semiconductors) via spin precession can be accomplished using real external magnetic fields or effective fields caused by spin-orbit interaction.

Spin polarization in non-magnetic materials can be achieved either through the Zeeman effect in large magnetic fields and low temperatures, or by non-equilibrium methods. In the latter case, the non-equilibrium polarization will decay over a timescale called the "spin lifetime". Spin lifetimes of conduction electrons in metals are relatively short (typically less than 1 nanosecond) but in semiconductors the lifetimes can be very long (microseconds at low temperatures), especially when the electrons are isolated in local trapping potentials (for instance, at impurities, where lifetimes can be milliseconds).

Metals-based spintronic devices

The simplest method of generating a spin-polarised current in a metal is to pass the current through a ferromagnetic material. The most common application of this effect is a giant magnetoresistance (GMR) device. A typical GMR device consists of at least two layers of ferromagnetic materials separated by a spacer layer. When the two magnetization vectors of the ferromagnetic layers are aligned, the electrical resistance will be lower (so a higher current flows at constant voltage) than if the ferromagnetic layers are anti-aligned. This constitutes a magnetic field sensor.

Two variants of GMR have been applied in devices: (1) current-in-plane (CIP), where the electric current flows parallel to the layers and (2) current-perpendicular-to-plane (CPP), where the electric current flows in a direction perpendicular to the layers.

Other metals-based spintronics devices:

* Tunnel Magnetoresistance (TMR), where CPP transport is achieved by using quantum-mechanical tunneling of electrons through a thin insulator separating ferromagnetic layers.
* Spin Torque Transfer, where a current of spin-polarized electrons is used to control the magnetization direction of ferromagnetic electrodes in the device.

Applications

The storage density of hard drives is rapidly increasing along an exponential growth curve, in part because spintronics-enabled devices like GMR and TMR sensors have increased the sensitivity of the read head which measures the magnetic state of small magnetic domains (bits) on the spinning platter. The doubling period for the areal density of information storage is twelve months, much shorter than Moore's Law, which observes that the number of transistors that can cheaply be incorporated in an integrated circuit doubles every two years.

MRAM, or magnetic random access memory, uses a grid of magnetic storage elements called magnetic tunnel junctions (MTJ's). MRAM is nonvolatile (unlike charge-based DRAM in today's computers) so information is stored even when power is turned off, potentially providing instant-on computing. Motorola has developed a 1st generation 256 kb MRAM based on a single magnetic tunnel junction and a single transistor and which has a read/write cycle of under 50 nanoseconds (Everspin, Motorola's spin-off, has since developeda 4 Mbit version). There are two 2nd generation MRAM techniques currently in development: Thermal Assisted Switching (TAS) which is being developed by Crocus Technology, and Spin Torque Transfer (STT) on which Crocus, Hynix, IBM, and several other companies are working.

Another design in development, called Racetrack memory, encodes information in the direction of magnetization between domain walls of a ferromagnetic metal wire.

Semiconductor-based spintronic devices

In early efforts, spin-polarized electrons are generated via optical orientation using circularly-polarized photons at the bandgap energy incident on semiconductors with appreciable spin-orbit interaction (like GaAs and ZnSe). Although electrical spin injection can be achieved in metallic systems by simply passing a current through a ferromagnet, the large impedance mismatch between ferromagnetic metals and semiconductors prevented efficient injection across metal-semiconductor interfaces. A solution to this problem is to use ferromagnetic semiconductor sources (like manganese-doped gallium arsenide GaMnAs), increasing the interface resistance with a tunnel barrier, or using hot-electron injection.

Spin detection in semiconductors is another challenge, which has been met with the following techniques:

* Faraday/Kerr rotation of transmitted/reflected photons
* Circular polarization analysis of electroluminescence
* Nonlocal spin valve (adapted from Johnson and Silsbee's work with metals)
* Ballistic spin filtering

The latter technique was used to overcome the lack of spin-orbit interaction and materials issues to achieve spin transport in silicon, the most important semiconductor for electronics.

Because external magnetic fields (and stray fields from magnetic contacts) can cause large Hall effects and magnetoresistance in semiconductors (which mimic spin-valve effects), the only conclusive evidence of spin transport in semiconductors is demonstration of spin precession and dephasing in a magnetic field non-colinear to the injected spin orientation. This is called the Hanle effect.

Applications

Advantages of semiconductor-based spintronics applications are potentially lower power use and a smaller footprint than electrical devices used for information processing. Also, applications such as semiconductor lasers using spin-polarized electrical injection have shown threshold current reduction and controllable circularly polarized coherent light output. Future applications may include a spin-based transistor having advantages over MOSFET devices such as steeper sub-threshold slope.

Quantum mirage

Quantum mirage

In physics, a quantum mirage is a peculiar result in quantum chaos. Every system of quantum dynamical billiards will exhibit an effect called scarring, where the quantum probability density shows traces of the paths a classical billiard ball would take. For an elliptical arena, the scarring is particularly pronounced at the foci, as this is the region where many classical trajectories converge. The scars at the foci are colloquially referred to as the "quantum mirage".

The quantum mirage was first experimentally observed by Hari Manoharan, Christopher Lutz and Donald Eigler at the IBM Almaden Research Center in San Jose, California in 2000. The effect is quite remarkable but in general agreement with prior work on the quantum mechanics of dynamical billiards in elliptical arenas.

Quantum corral

The mirage occurs at the foci of a quantum corral, a ring of atoms arranged in an arbitrary shape on a substrate. The quantum corral was demonstrated in 1993 by Lutz, Eigler, and Michael Crommie, now a professor at the University of California, using an ellipitical ring of cobalt atoms on a copper surface. The ferromagnetic cobalt atoms reflected the surface electrons of the copper inside the ring into a wave pattern, as predicted by the theory of quantum mechanics.

The size and shape of the corral determine its quantum states, including the energy and distribution of the electrons. To make conditions suitable for the mirage the team at Almaden chose a configuration of the corral which concentrated the electrons at the foci of the ellipse.

When scientists placed a magnetic cobalt atom at one focus of the corral, a mirage of the atom appeared at the other focus. Specifically the same electronic properties were present in the electrons surrounding both foci, even though the cobalt atom was only present at one focus.

Applications

IBM scientists are hoping to use quantum mirages to construct atomic scale processors in the future.

The term quantum mirage refers to a phenomenon that may make it possible to transfer data without conventional electrical wiring. Instead of forcing charge carriers through solid conductors, a process impractical on a microscopic scale, electron wave phenomena are made to produce effective currents. Leading the research are physicists Donald Eigler, Hari Manoharan, and Christopher Lutz of the IBM facility in San Jose, California.

All moving particles have a wavelike nature. This is rarely significant on an everyday scale. But in atomic dimensions, where distances are measured in nanometer s (nm), moving particles behave like waves. This phenomenon is what makes the electron microscope workable. It is of interest to researchers in nanotechnology , who are looking for ways to deliver electric currents through circuits too small for conventional wiring.

A quantum mirage is a spot where electron waves are focused so they reinforce each other. The result is an energy hot zone, similar to the acoustical hot zones observed in concrete enclosures, or the electromagnetic wave focus of a dish antenna . In the case of electron waves, the enclosure is called a quantum corral. An elliptical corral produces mirages at the foci of the ellipse. A typical quantum corral measures approximately 20 nm long by 10 nm wide. By comparison, the range of visible wavelengths is approximately 390 nm (violet light) to 750 nm (red light). One nanometer is 10 -9 meter, or a millionth of a millimeter.

Terahertz radiation

Terahertz radiation

In physics, terahertz radiation refers to electromagnetic waves sent at frequencies in the terahertz range. It is also referred to as submillimeter radiation, terahertz waves, terahertz light, T-rays, T-light, T-lux and THz. The term is normally used for the region of the electromagnetic spectrum between 300 gigahertz (3x1011 Hz) and 3 terahertz (3x1012 Hz), corresponding to the submillimeter wavelength range between 1 millimeter (high-frequency edge of the microwave band) and 100 micrometer (long-wavelength edge of far-infrared light).

Introduction

Like infrared radiation or microwaves, these waves usually travel in line of sight. Terahertz radiation is non-ionizing submillimeter microwave radiation and shares with microwaves the capability to penetrate a wide variety of non-conducting materials. Terahertz radiation can pass through clothing, paper, cardboard, wood, masonry, plastic and ceramics. It can also penetrate fog and clouds, but cannot penetrate metal or water.

The Earth's atmosphere is a strong absorber of terahertz radiation, so the range of terahertz radiation is quite short, limiting its usefulness for communications. In addition, producing and detecting coherent terahertz radiation was technically challenging until the 1990s.

Sources

Terahertz radiation is emitted as part of the black body radiation from anything with temperatures greater than about 10 kelvin. While this thermal emission is very weak, observations at these frequencies are important for characterizing the cold 10-20K dust in the interstellar medium in the Milky Way galaxy, and in distant starburst galaxies. Telescopes operating in this band include the James Clerk Maxwell Telescope, the Caltech Submillimeter Observatory and the Submillimeter Array at the Mauna Kea Observatory in Hawaii, the BLAST balloon borne telescope, and the Heinrich Hertz Submillimeter Telescope at the Mount Graham International Observatory in Arizona. Planned telescopes operating in the submillimeter include the Atacama Large Millimeter Array and the Herschel Space Observatory. The opacity of the Earth's atmosphere to submillimeter radiation restricts these observatories to very high altitude sites, or to space.

As of 2004[update] the only viable sources of terahertz radiation were the gyrotron, the backward wave oscillator ("BWO"), the far infrared laser ("FIR laser"), quantum cascade laser, the free electron laser (FEL), synchrotron light sources, photomixing sources, and single-cycle sources used in Terahertz time domain spectroscopy such as photoconductive, surface field, Photo-dember and optical rectification emitters. The first images generated using terahertz radiation date from the 1960s; however, in 1995, images generated using terahertz time-domain spectroscopy generated a great deal of interest, and sparked a rapid growth in the field of terahertz science and technology. This excitement, along with the associated coining of the term "T-rays", even showed up in a contemporary novel by Tom Clancy.

There have also been solid-state sources of millimeter and submillimeter waves for many years. AB Millimeter in Paris, for instance, produces a system that covers the entire range from 8 GHz to 1000 GHz with solid state sources and detectors. Nowadays, most time-domain work is done via ultrafast lasers.

In mid-2007, scientists at the U.S. Department of Energy's Argonne National Laboratory, along with collaborators in Turkey and Japan, announced the creation of a compact device that can lead to a portable, battery-operated sources of T-rays, or terahertz radiation. The group was led by Ulrich Welp of Argonne's Materials Science Division. This new T-ray source uses high-temperature superconducting crystals grown at the University of Tsukuba, Japan. These crystals comprise stacks of Josephson junctions that exhibit a unique electrical property: when an external voltage is applied, an alternating current will flow back and forth across the junctions at a frequency proportional to the strength of the voltage; this phenomenon is known as the Josephson effect. These alternating currents then produce electromagnetic fields whose frequency is tuned by the applied voltage. Even a small voltage – around two millivolts per junction – can induce frequencies in the terahertz range, according to Welp.

In 2008 engineers at Harvard University announced they had built a room temperature semiconductor source of coherent Terahertz radiation. Until then sources had required cryogenic cooling, greatly limiting their use in everyday applications.

In 2009 it was shown that in addition to X-rays, T-waves are produced when unpeeling adhesive tape. The observed spectrum of this terahertz radiation exhibits a peak at 2 THz and a broader peak at 18 THz. The radiation is not polarized. The mechanism of terahertz radiation is tribocharging of the adhesive tape and subsequent discharge.

Theoretical and technological uses under development

* Medical imaging:
o Terahertz radiation is non-ionizing, and thus is not expected to damage tissues and DNA, unlike X-rays. Some frequencies of terahertz radiation can penetrate several millimeters of tissue with low water content (e.g. fatty tissue) and reflect back. Terahertz radiation can also detect differences in water content and density of a tissue. Such methods could allow effective detection of epithelial cancer with a safer and less invasive or painful system using imaging.
o Some frequencies of terahertz radiation can be used for 3D imaging of teeth and may be more accurate and safer than conventional X-ray imaging in dentistry.
* Security:
o Terahertz radiation can penetrate fabrics and plastics, so it can be used in surveillance, such as security screening, to uncover concealed weapons on a person, remotely. This is of particular interest because many materials of interest have unique spectral "fingerprints" in the terahertz range. This offers the possibility to combine spectral identification with imaging. Passive detection of Terahertz signatures avoid the bodily privacy concerns of other detection by being targeted to a very specific range of materials and objects.
* Scientific use and imaging:
o Spectroscopy in terahertz radiation could provide novel information in chemistry and biochemistry.
o Recently developed methods of THz time-domain spectroscopy (THz TDS) and THz tomography have been shown to be able to perform measurements on, and obtain images of, samples which are opaque in the visible and near-infrared regions of the spectrum. The utility of THz-TDS is limited when the sample is very thin, or has a low absorbance, since it is very difficult to distinguish changes in the THz pulse caused by the sample from those caused by long term fluctuations in the driving laser source or experiment. However, THz-TDS produces radiation that is both coherent and broadband, so such images can contain far more information than a conventional image formed with a single-frequency source.
o A primary use of submillimeter waves in physics is the study of condensed matter in high magnetic fields, since at high fields (over about 15 teslas), the Larmor frequencies are in the submillimeter band. This work is performed at many high-magnetic field laboratories around the world.
o Submillimetre astronomy.
o Terahertz radiation could let art historians see murals hidden beneath coats of plaster or paint in centuries-old building, without harming the artwork.
* Communication:
o Potential uses exist in high-altitude telecommunications, above altitudes where water vapor causes signal absorption: aircraft to satellite, or satellite to satellite.
* Manufacturing:
o Many possible uses of terahertz sensing and imaging are proposed in manufacturing, quality control, and process monitoring. These generally exploit the traits of plastics and cardboard being transparent to terahertz radiation, making it possible to inspect packaged goods.

Terahertz versus submillimeter waves

The terahertz band, covering the wavelength range between 0.1 and 1 mm, is identical to the submillimeter wavelength band. However, typically, the term "terahertz" is used more often in marketing in relation to generation and detection with pulsed lasers, as in terahertz time domain spectroscopy, while the term "submillimeter" is used for generation and detection with microwave technology, such as harmonic multiplication.

Teleportation

Teleportation

Teleportation is the transfer of matter from one point to another, more or less instantaneously, either by paranormal means or through technological artifice. Teleportation has been widely utilized in works of science fiction.

Similar is apport, an earlier word used to describe what today might be called teleportation; and bilocation, in which an individual or object is said to be, or appears to be, located in two distinct places at the same instant in time. The word "teletransportation" (which simply expands Charles Fort's abbreviated term) was first employed by Derek Parfit as part of a thought exercise on identity.

The word was coined in 1931[1][2] by American writer Charles Fort to describe the strange disappearances and appearances of anomalies, which he suggested may be connected. He joined the Greek prefix tele- (meaning "distant") to the Latin verb portare (meaning "to carry"). Fort's first formal use of the word was in the second chapter of his 1931 book, Lo! "Mostly in this book I shall specialize upon indications that there exists a transportory force that I shall call Teleportation." Though, with his typical half-serious jokiness, Fort added, "I shall be accused of having assembled lies, yarns, hoaxes, and superstitions. To some degree I think so myself. To some degree, I do not. I offer the data."[3] Fort suggested that teleportation might explain various allegedly paranormal phenomena, though, typically, it's sometimes difficult to tell if Fort took his own "theory" seriously, or instead used it to point out what he saw as the inadequacy of mainstream science to account for strange phenomena.

Scenarios

One proposed means of teleportation is the transmission of data which is used to precisely reconstruct an object or organism at its destination. However, it would be impossible to travel from one point to another instantaneously; faster than light travel, as of today, is believed to be impossible. The use of this form of teleportation as a means of transport for humans still has considerable unresolved technical issues, such as recording the human body with sufficient accuracy to allow reproduction elsewhere (i.e., because of the uncertainty principle), and whether destroying a human in one place and recreating a copy elsewhere would provide a sufficient experience of continuity of existence. The reassembled human might be considered a different sentience with the same memories as the original, as could be easily proved by constructing not just one, but several copies of the original and interrogating each as to the perceived uniqueness of each. Each copy constructed using merely descriptive data, but not matter, transmitted from the origin and new matter already at the destination point would consider itself to be the true continuation of the original and yet this could not logically be true; moreover, because each copy constructed via this data-only method would be made of new matter that already existed at the destination, there would be no way, even in principle, of distinguishing the original from the copies. Many of the relevant questions are shared with the concept of mind transfer.

Dimensional teleportation is another proposed means of teleportation. Often shown in fictional works, particularly in fantasy and comic books, it involves the subject exiting one physical universe or plane of existence, then re-entering it at a different location. This method is rarely seriously considered by the scientific community, as the currently predominant theories about parallel universes assume that physical travel is not possible between them.

A third proposed means of teleportation common in science fiction (and seen in The Culture novels and The Terminator series of films) sends the subject through a wormhole or similar phenomenon, allowing transit faster than light while avoiding the problems posed by the uncertainty principle and potential signal interference. In both of the examples above, this form of teleportation is known as "Displacement" or "Topological shortcut" (Scientific American)[citation needed] which implies that this kind of teleportation may be similar in mechanism to time travel[citation needed]. Displacement teleporters would eliminate many probable objections to teleportation on religious or philosophical grounds, as they preserve the original subject intact — and thus continuity of existence.

Teleportation by means of the mind or innate personal abilities are sometimes referred to as p-Teleportation, "psychoportation", or "jaunting"; named after the fictional scientist (Jaunte) who discovered it in The Stars My Destination (originally titled Tiger! Tiger!), a science fiction novel by Alfred Bester. This method could hypothetically work through any of the mechanisms proposed above, but is usually portrayed in fiction as displacement-type or dimensional teleportation to simplify its use in the story.

Teleportation is the name given by science fiction writers to the feat of making an object or person disintegrate in one place while a perfect replica appears somewhere else. How this is accomplished is usually not explained in detail, but the general idea seems to be that the original object is scanned in such a way as to extract all the information from it, then this information is transmitted to the receiving location and used to construct the replica, not necessarily from the actual material of the original, but perhaps from atoms of the same kinds, arranged in exactly the same pattern as the original. A teleportation machine would be like a fax machine, except that it would work on 3-dimensional objects as well as documents, it would produce an exact copy rather than an approximate facsimile, and it would destroy the original in the process of scanning it. A few science fiction writers consider teleporters that preserve the original, and the plot gets complicated when the original and teleported versions of the same person meet; but the more common kind of teleporter destroys the original, functioning as a super transportation device, not as a perfect replicator of souls and bodies.

In 1993 an international group of six scientists, including IBM Fellow Charles H. Bennett, confirmed the intuitions of the majority of science fiction writers by showing that perfect teleportation is indeed possible in principle, but only if the original is destroyed. In subsequent years, other scientists have demonstrated teleportation experimentally in a variety of systems, including single photons, coherent light fields, nuclear spins, and trapped ions. Teleportation promises to be quite useful as an information processing primitive, facilitating long range quantum communication (perhaps unltimately leading to a "quantum internet"), and making it much easier to build a working quantum computer. But science fiction fans will be disappointed to learn that no one expects to be able to teleport people or other macroscopic objects in the foreseeable future, for a variety of engineering reasons, even though it would not violate any fundamental law to do so.