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Self-mixing Laser Interferometry
Self-mixing or back-injection laser interferometry is an Interferometry, interferometric technique in which a part of the light reflected by a vibrating target is reflected into the Optical cavity, laser cavity, causing a modulation both in ''amplitude'' and in ''frequency'' of the emitted optical beam. In this way, the laser becomes sensitive to the distance traveled by the reflected beam thus becoming a distance, speed or vibration sensor. The advantage compared to a traditional measurement system is a lower cost thanks to the absence of collimation optics and external photodiodes. Background After the development of the classic external interferometric configurations (Michelson interferometer, Michelson and Mach–Zehnder interferometer, Mach-Zehnder interferometers) which consisted of lenses, beam splitter, mirrors, and Corner reflector, corner cube, the possibility of creating a much simpler and more compact system was investigated. Starting in the 1980s, this new configurati ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Interferometric Fringe
Interferometry is a technique which uses the ''interference'' of superimposed waves to extract information. Interferometry typically uses electromagnetic waves and is an important investigative technique in the fields of astronomy, fiber optics, engineering metrology, optical metrology, oceanography, seismology, spectroscopy (and its applications to chemistry), quantum mechanics, nuclear and particle physics, plasma physics, remote sensing, biomolecular interactions, surface profiling, microfluidics, mechanical stress/strain measurement, velocimetry, optometry, and making holograms. Interferometers are devices that extract information from interference. They are widely used in science and industry for the measurement of microscopic displacements, refractive index changes and surface irregularities. In the case with most interferometers, light from a single source is split into two beams that travel in different optical paths, which are then combined again to produce interf ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Shot Noise
Shot noise or Poisson noise is a type of noise which can be modeled by a Poisson process. In electronics shot noise originates from the discrete nature of electric charge. Shot noise also occurs in photon counting in optical devices, where shot noise is associated with the particle nature of light. Origin In a statistical experiment such as tossing a fair coin and counting the occurrences of heads and tails, the numbers of heads and tails after many throws will differ by only a tiny percentage, while after only a few throws outcomes with a significant excess of heads over tails or vice versa are common; if an experiment with a few throws is repeated over and over, the outcomes will fluctuate a lot. From the law of large numbers, one can show that the relative fluctuations reduce as the reciprocal square root of the number of throws, a result valid for all statistical fluctuations, including shot noise. Shot noise exists because phenomena such as light and electric current co ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Dark Current (physics)
In physics and in electronic engineering, dark current is the relatively small electric current that flows through photosensitive devices such as a photomultiplier tube, photodiode, or charge-coupled device even when no photons enter the device; it consists of the charges generated in the detector when no outside radiation is entering the detector. It is referred to as reverse bias leakage current in non-optical devices and is present in all diodes. Physically, dark current is due to the random generation of electrons and holes within the depletion region of the device. The charge generation rate is related to specific crystallographic defects within the depletion region. Dark-current spectroscopy can be used to determine the defects present by monitoring the peaks in the dark current histogram's evolution with temperature. Dark current is one of the main sources for noise in image sensors such as charge-coupled devices. The pattern of different dark currents can result in a ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Free Spectral Range
Free spectral range (FSR) is the spacing in optical frequency or wavelength between two successive reflected or transmitted optical intensity maxima or minima of an interferometer or diffractive optical element. The FSR is not always represented by \Delta\nu or \Delta\lambda, but instead is sometimes represented by just the letters FSR. The reason is that these different terms often refer to the bandwidth or linewidth of an emitted source respectively. In general The free spectral range (FSR) of a cavity in general is given by :\left, \Delta\lambda_\text\ = \frac\left, \left(\frac\right)^ \ or, equivalently, :\left, \Delta\nu_\text\ = \frac\left, \left(\frac\right)^\ These expressions can be derived from the resonance condition \Delta \beta L = 2\pi by expanding \Delta \beta in Taylor series. Here, \beta = k_0 n(\lambda) = \fracn(\lambda) is the wavevector of the light inside the cavity, k_0 and \lambda are the wavevector and wavelength in vacuum, n is the refractive index of ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Optical Path Length
In optics, optical path length (OPL, denoted ''Λ'' in equations), also known as optical length or optical distance, is the product of the geometric length of the optical path followed by light and the refractive index of homogeneous medium through which a light ray propagates; for inhomogeneous optical media, the product above is generalized as a path integral as part of the ray tracing procedure. A difference in OPL between two paths is often called the optical path difference (OPD). OPL and OPD are important because they determine the phase of the light and governs interference and diffraction of light as it propagates. Formulation In a medium of constant refractive index, ''n'', the OPL for a path of geometrical length ''s'' is just :\mathrm = n s .\, If the refractive index varies along the path, the OPL is given by a line integral :\mathrm = \int_C n \mathrm d s,\quad where ''n'' is the local refractive index as a function of distance along the path ''C''. An e ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Refractive Index
In optics, the refractive index (or refraction index) of an optical medium is a dimensionless number that gives the indication of the light bending ability of that medium. The refractive index determines how much the path of light is bent, or refracted, when entering a material. This is described by Snell's law of refraction, , where ''θ''1 and ''θ''2 are the angle of incidence and angle of refraction, respectively, of a ray crossing the interface between two media with refractive indices ''n''1 and ''n''2. The refractive indices also determine the amount of light that is reflected when reaching the interface, as well as the critical angle for total internal reflection, their intensity (Fresnel's equations) and Brewster's angle. The refractive index can be seen as the factor by which the speed and the wavelength of the radiation are reduced with respect to their vacuum values: the speed of light in a medium is , and similarly the wavelength in that medium is , where ''� ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Signal-to-noise Ratio
Signal-to-noise ratio (SNR or S/N) is a measure used in science and engineering that compares the level of a desired signal to the level of background noise. SNR is defined as the ratio of signal power to the noise power, often expressed in decibels. A ratio higher than 1:1 (greater than 0 dB) indicates more signal than noise. SNR, bandwidth, and channel capacity of a communication channel are connected by the Shannon–Hartley theorem. Definition Signal-to-noise ratio is defined as the ratio of the power of a signal (meaningful input) to the power of background noise (meaningless or unwanted input): : \mathrm = \frac, where is average power. Both signal and noise power must be measured at the same or equivalent points in a system, and within the same system bandwidth. Depending on whether the signal is a constant () or a random variable (), the signal-to-noise ratio for random noise becomes: : \mathrm = \frac where E refers to the expected value, i.e. in thi ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Superheterodyne Receiver
A superheterodyne receiver, often shortened to superhet, is a type of radio receiver that uses frequency mixing to convert a received signal to a fixed intermediate frequency (IF) which can be more conveniently processed than the original carrier frequency. It was long believed to have been invented by US engineer Edwin Armstrong, but after some controversy the earliest patent for the invention is now credited to French radio engineer and radio manufacturer Lucien Lévy. Virtually all modern radio receivers use the superheterodyne principle; except those software-defined radios using ''direct sampling''. History Heterodyne Early Morse code radio broadcasts were produced using an alternator connected to a spark gap. The output signal was at a carrier frequency defined by the physical construction of the gap, modulated by the alternating current signal from the alternator. Since the output frequency of the alternator was generally in the audible range, this produces an aud ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Free Spectral Range
Free spectral range (FSR) is the spacing in optical frequency or wavelength between two successive reflected or transmitted optical intensity maxima or minima of an interferometer or diffractive optical element. The FSR is not always represented by \Delta\nu or \Delta\lambda, but instead is sometimes represented by just the letters FSR. The reason is that these different terms often refer to the bandwidth or linewidth of an emitted source respectively. In general The free spectral range (FSR) of a cavity in general is given by :\left, \Delta\lambda_\text\ = \frac\left, \left(\frac\right)^ \ or, equivalently, :\left, \Delta\nu_\text\ = \frac\left, \left(\frac\right)^\ These expressions can be derived from the resonance condition \Delta \beta L = 2\pi by expanding \Delta \beta in Taylor series. Here, \beta = k_0 n(\lambda) = \fracn(\lambda) is the wavevector of the light inside the cavity, k_0 and \lambda are the wavevector and wavelength in vacuum, n is the refractive index of ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Signal-to-noise Ratio
Signal-to-noise ratio (SNR or S/N) is a measure used in science and engineering that compares the level of a desired signal to the level of background noise. SNR is defined as the ratio of signal power to the noise power, often expressed in decibels. A ratio higher than 1:1 (greater than 0 dB) indicates more signal than noise. SNR, bandwidth, and channel capacity of a communication channel are connected by the Shannon–Hartley theorem. Definition Signal-to-noise ratio is defined as the ratio of the power of a signal (meaningful input) to the power of background noise (meaningless or unwanted input): : \mathrm = \frac, where is average power. Both signal and noise power must be measured at the same or equivalent points in a system, and within the same system bandwidth. Depending on whether the signal is a constant () or a random variable (), the signal-to-noise ratio for random noise becomes: : \mathrm = \frac where E refers to the expected value, i.e. in thi ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
