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Fiber Bragg Grating
A fiber Bragg grating (FBG) is a type of distributed Bragg reflector constructed in a short segment of optical fiber that reflects particular wavelengths of light and transmits all others. This is achieved by creating a periodic variation in the refractive index of the fiber core, which generates a wavelength-specific dielectric mirror. Hence a fiber Bragg grating can be used as an inline optical filter to block certain wavelengths, can be used for sensing applications, or it can be used as wavelength-specific reflector. History The first in-fiber Bragg grating was demonstrated by Kenneth O. Hill, Ken Hill in 1978. Initially, the gratings were fabricated using a visible laser propagating along the fiber core. In 1989, Gerald Meltz and colleagues demonstrated the much more flexible transverse holographic inscription technique where the laser illumination came from the side of the fiber. This technique uses the #Interference, interference pattern of ultraviolet laser light to cr ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Theory
A theory is a systematic and rational form of abstract thinking about a phenomenon, or the conclusions derived from such thinking. It involves contemplative and logical reasoning, often supported by processes such as observation, experimentation, and research. Theories can be scientific, falling within the realm of empirical and testable knowledge, or they may belong to non-scientific disciplines, such as philosophy, art, or sociology. In some cases, theories may exist independently of any formal discipline. In modern science, the term "theory" refers to Scientific theory, scientific theories, a well-confirmed type of explanation of nature, made in a way Consistency, consistent with the scientific method, and fulfilling the Scientific theory#Characteristics of theories, criteria required by modern science. Such theories are described in such a way that scientific tests should be able to provide Empirical evidence, empirical support for it, or Empirical evidence, empirical contradi ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Ultraviolet
Ultraviolet radiation, also known as simply UV, is electromagnetic radiation of wavelengths of 10–400 nanometers, shorter than that of visible light, but longer than X-rays. UV radiation is present in sunlight and constitutes about 10% of the total electromagnetic radiation output from the Sun. It is also produced by electric arcs, Cherenkov radiation, and specialized lights, such as mercury-vapor lamps, tanning lamps, and black lights. The photons of ultraviolet have greater energy than those of visible light, from about 3.1 to 12 electron volts, around the minimum energy required to ionize atoms. Although long-wavelength ultraviolet is not considered an ionizing radiation because its photons lack sufficient energy, it can induce chemical reactions and cause many substances to glow or fluoresce. Many practical applications, including chemical and biological effects, are derived from the way that UV radiation can interact with organic molecules. The ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Radio Frequency
Radio frequency (RF) is the oscillation rate of an alternating electric current or voltage or of a magnetic, electric or electromagnetic field or mechanical system in the frequency range from around to around . This is roughly between the upper limit of audio frequencies that humans can hear (though these are not electromagnetic) and the lower limit of infrared frequencies, and also encompasses the microwave range. These are the frequencies at which energy from an oscillating current can radiate off a conductor into space as radio waves, so they are used in radio technology, among other uses. Different sources specify different upper and lower bounds for the frequency range. Electric current Electric currents that oscillate at radio frequencies (RF currents) have special properties not shared by direct current or lower audio frequency alternating current, such as the 50 or 60 Hz current used in electrical power distribution. * Energy from RF currents in conduct ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Phased-array
In antenna theory, a phased array usually means an electronically scanned array, a computer-controlled array of antennas which creates a beam of radio waves that can be electronically steered to point in different directions without moving the antennas. In a phased array, the power from the transmitter is fed to the radiating elements through devices called ''phase shifters'', controlled by a computer system, which can alter the phase or signal delay electronically, thus steering the beam of radio waves to a different direction. Since the size of an antenna array must extend many wavelengths to achieve the high gain needed for narrow beamwidth, phased arrays are mainly practical at the high frequency end of the radio spectrum, in the UHF and microwave bands, in which the operating wavelengths are conveniently small. Phased arrays were originally invented for use in military radar systems, to detect fast moving planes and missiles, but are now widely used and have spread to c ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Dispersion (optics)
Dispersion is the phenomenon in which the phase velocity of a wave depends on its frequency. Sometimes the term chromatic dispersion is used to refer to optics specifically, as opposed to wave propagation in general. A medium having this common property may be termed a dispersive medium. Although the term is used in the field of optics to describe light and other electromagnetic waves, dispersion in the same sense can apply to any sort of wave motion such as acoustic dispersion in the case of sound and seismic waves, and in gravity waves (ocean waves). Within optics, dispersion is a property of telecommunication signals along transmission lines (such as microwaves in coaxial cable) or the Pulse (signal processing), pulses of light in optical fiber. In optics, one important and familiar consequence of dispersion is the change in the angle of refraction of different colors of light, as seen in the spectrum produced by a dispersive Prism (optics), prism and in chromatic aberration ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Chirp
A chirp is a signal in which the frequency increases (''up-chirp'') or decreases (''down-chirp'') with time. In some sources, the term ''chirp'' is used interchangeably with sweep signal. It is commonly applied to sonar, radar, and laser systems, and to other applications, such as in spread-spectrum communications (see chirp spread spectrum). This signal type is biologically inspired and occurs as a phenomenon due to dispersion (a non-linear dependence between frequency and the propagation speed of the wave components). It is usually compensated for by using a matched filter, which can be part of the propagation channel. Depending on the specific performance measure, however, there are better techniques both for radar and communication. Since it was used in radar and space, it has been adopted also for communication standards. For automotive radar applications, it is usually called linear frequency modulated waveform (LFMW). In spread-spectrum usage, surface acoustic wave (S ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Fiber Laser
A fiber laser (or fibre laser in Commonwealth English) is a laser in which the active gain medium is an optical fiber doped with rare-earth elements such as erbium, ytterbium, neodymium, dysprosium, praseodymium, thulium and holmium. They are related to doped fiber amplifiers, which provide light amplification without lasing. Fiber nonlinearities, such as stimulated Raman scattering or four-wave mixing, can also provide gain and thus serve as gain media for a fiber laser. Characteristics An advantage of fiber lasers over other types of lasers is that the laser light is both generated and delivered by an inherently flexible medium, which allows easier delivery to the focusing location and target. This can be important for laser cutting, welding, and folding of metals and polymers. Another advantage is high output power compared to other types of laser. Fiber lasers can have active regions several kilometers long, and so can provide very high optical gain. They can sup ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Raised Cosine
The raised-cosine filter is a filter frequently used for pulse-shaping in digital modulation due to its ability to minimise intersymbol interference (ISI). Its name stems from the fact that the non-zero portion of the frequency spectrum of its simplest form (\beta = 1) is a cosine function, 'raised' up to sit above the f (horizontal) axis. Mathematical description The raised-cosine filter is an implementation of a low-pass Nyquist filter, i.e., one that has the property of vestigial symmetry. This means that its spectrum exhibits odd symmetry about \frac, where T is the symbol-period of the communications system. Its frequency-domain description is a piecewise-defined function, given by: :H(f) = \begin 1, & , f, \leq \frac \\ \frac\left [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
