Atomic line filter
Encyclopedia
Band-pass filter
A band-pass filter is a device that passes frequencies within a certain range and rejects frequencies outside that range.Optical band-pass filters are of common usage....
used in the physical science
Physical science
Physical science is an encompassing term for the branches of natural science and science that study non-living systems, in contrast to the life sciences...
s for filtering electromagnetic radiation
Electromagnetic radiation
Electromagnetic radiation is a form of energy that exhibits wave-like behavior as it travels through space...
with precision, accuracy, and minimal signal strength loss. Atomic line filters work via the absorption or resonance
Resonance
In physics, resonance is the tendency of a system to oscillate at a greater amplitude at some frequencies than at others. These are known as the system's resonant frequencies...
lines
Atomic spectral line
In physics, atomic spectral lines are of two types:* An emission line is formed when an electron makes a transition from a particular discrete energy level of an atom, to a lower energy state, emitting a photon of a particular energy and wavelength...
of atomic vapors and so may also be designated an atomic resonance filter (ARF).
The three major types of atomic line filters are absorption-re-emission ALFs, Faraday filters and Voigt filters. Absorption-re-emission filters were the first type developed, and so are commonly called simply “atomic line filters”; the other two types are usually referred to specifically as “Faraday filters” or “Voigt filters”. Atomic line filters use different mechanisms and designs for different applications, but the same basic strategy is always employed: by taking advantage of the narrow lines of absorption or resonance in a metallic vapor, a specific frequency
Frequency
Frequency is the number of occurrences of a repeating event per unit time. It is also referred to as temporal frequency.The period is the duration of one cycle in a repeating event, so the period is the reciprocal of the frequency...
of light bypasses a series of filters that block all other light.
Atomic line filters can be considered the optical equivalent of lock-in amplifier
Lock-in amplifier
A lock-in amplifier is a type of amplifier that can extract a signal with a known carrier wave from an extremely noisy environment . It is essentially a homodyne with an extremely low pass filter...
s; they are used in scientific applications requiring the effective detection of a narrowband signal (almost always laser light) that would otherwise be obscured by broadband sources, such as daylight
Daylight
Daylight or the light of day is the combination of all direct and indirect sunlight outdoors during the daytime. This includes direct sunlight, diffuse sky radiation, and both of these reflected from the Earth and terrestrial objects. Sunlight scattered or reflected from objects in outer space is...
. They are used regularly in Laser Imaging Detection and Ranging (LIDAR
LIDAR
LIDAR is an optical remote sensing technology that can measure the distance to, or other properties of a target by illuminating the target with light, often using pulses from a laser...
) and are being studied for their potential use in laser
Laser
A laser is a device that emits light through a process of optical amplification based on the stimulated emission of photons. The term "laser" originated as an acronym for Light Amplification by Stimulated Emission of Radiation...
communication systems. Atomic line filters are superior to conventional dielectric optical filters such as interference filter
Interference filter
An interference filter or dichroic filter is an optical filter that reflects one or more spectral bands or lines and transmits others, while maintaining a nearly zero coefficient of absorption for all wavelengths of interest...
s and Lyot filter
Lyot filter
A Lyot filter, named for its inventor Bernard Lyot, is a type of optical filter that uses birefringence to produce a narrow passband of transmitted wavelengths...
s, but their greater complexity makes them practical only in background-limited detection, where a weak signal is detected while suppressing a strong background. Compared to etalons, another high-end optical filter, Faraday filters are significantly sturdier and may be six times cheaper at around US$
United States dollar
The United States dollar , also referred to as the American dollar, is the official currency of the United States of America. It is divided into 100 smaller units called cents or pennies....
15,000 per unit.
History
The predecessor of the atomic line filter was the infrared quantum counter, designed in the 1950s by Nicolaas BloembergenNicolaas Bloembergen
Nicolaas Bloembergen is a Dutch-American physicist and Nobel laureate.He received his Ph.D. degree from University of Leiden in 1948; while pursuing his PhD at Harvard, Bloembergen also worked part-time as a graduate research assistant for Edward Mills Purcell at the MIT Radiation Laboratory...
. This was a quantum mechanical amplifier
Amplifier
Generally, an amplifier or simply amp, is a device for increasing the power of a signal.In popular use, the term usually describes an electronic amplifier, in which the input "signal" is usually a voltage or a current. In audio applications, amplifiers drive the loudspeakers used in PA systems to...
theorized by Joseph Weber
Joseph Weber
Joseph Weber was an American physicist. He gave the earliest public lecture on the principles behind the laser and the maser and developed the first gravitational wave detectors .-Early education:...
to detect infrared
Infrared
Infrared light is electromagnetic radiation with a wavelength longer than that of visible light, measured from the nominal edge of visible red light at 0.74 micrometres , and extending conventionally to 300 µm...
radiation with very little noise. Zero spontaneous emission was already possible for x-ray
X-ray
X-radiation is a form of electromagnetic radiation. X-rays have a wavelength in the range of 0.01 to 10 nanometers, corresponding to frequencies in the range 30 petahertz to 30 exahertz and energies in the range 120 eV to 120 keV. They are shorter in wavelength than UV rays and longer than gamma...
and gamma ray
Gamma ray
Gamma radiation, also known as gamma rays or hyphenated as gamma-rays and denoted as γ, is electromagnetic radiation of high frequency . Gamma rays are usually naturally produced on Earth by decay of high energy states in atomic nuclei...
amplifiers and Weber thought to bring this technology to the infrared spectrum. Bloembergen described such a device in detail and dubbed it the “infrared quantum counter”.
The media of these devices were crystal
Crystal
A crystal or crystalline solid is a solid material whose constituent atoms, molecules, or ions are arranged in an orderly repeating pattern extending in all three spatial dimensions. The scientific study of crystals and crystal formation is known as crystallography...
s with transition metal
Transition metal
The term transition metal has two possible meanings:*The IUPAC definition states that a transition metal is "an element whose atom has an incomplete d sub-shell, or which can give rise to cations with an incomplete d sub-shell." Group 12 elements are not transition metals in this definition.*Some...
ion
Ion
An ion is an atom or molecule in which the total number of electrons is not equal to the total number of protons, giving it a net positive or negative electrical charge. The name was given by physicist Michael Faraday for the substances that allow a current to pass between electrodes in a...
impurities, absorbing low-energy light and re-emitting it in the visible range. By the 1970s, atomic vapors were used in atomic vapor quantum counters for detection of infrared electromagnetic radiation, as they were found to be superior to the metallic salt
Salt
In chemistry, salts are ionic compounds that result from the neutralization reaction of an acid and a base. They are composed of cations and anions so that the product is electrically neutral...
s and crystals that had been used.
The principles hitherto employed in infrared amplification were put together into a passive sodium ALF. This design and those that immediately followed it were primitive and suffered from low quantum efficiency
Quantum efficiency
Quantum efficiency is a quantity defined for a photosensitive device such as photographic film or a charge-coupled device as the percentage of photons hitting the photoreactive surface that will produce an electron–hole pair. It is an accurate measurement of the device's electrical sensitivity to...
and slow response time. As this was the original design for ALFs, many references use only the designation “atomic line filter” to describe specifically the absorption-re-emission construction. In 1977, Gelbwachs, Klein and Wessel created the first active atomic line filter.
Faraday filters, developed sometime before 1978, were “a substantial improvement” over absorption-re-emission atomic line filters of the time. The Voigt filter, patent
Patent
A patent is a form of intellectual property. It consists of a set of exclusive rights granted by a sovereign state to an inventor or their assignee for a limited period of time in exchange for the public disclosure of an invention....
ed by James H. Menders and Eric J. Korevaar on August 26, 1992, was more advanced. Voigt filters were more compact and “[could] be easily designed for use with a permanent magnet”. By 1996, Faraday filters were being used for LIDAR.
Properties
A technical definition of an atomic line filter is as an “ultra-narrow-band, large-acceptance-angle, isotropic optical filter”. “Ultra-narrow-band” defines the thin range of frequencies that an ALF may accept; an ALF generally has a passbandPassband
A passband is the range of frequencies or wavelengths that can pass through a filter without being attenuated.A bandpass filtered signal , is known as a bandpass signal, as opposed to a baseband signal....
on the order of 0.001 nanometer. That atomic line filters also have wide acceptance angles (near 180°) is another important characteristic of the devices; conventional dielectric filters based on the spacing of reflective or refractive layers change their effective spacing when light enters at an angle.
The exact parameters (temperature, magnetic field strength, length, etc.) of any filter may be tuned to a specific application. These values are calculated by computers due to the extreme complexity of the systems.
Input/output
Atomic line filters may operate in the ultravioletUltraviolet
Ultraviolet light is electromagnetic radiation with a wavelength shorter than that of visible light, but longer than X-rays, in the range 10 nm to 400 nm, and energies from 3 eV to 124 eV...
, visible
Visible spectrum
The visible spectrum is the portion of the electromagnetic spectrum that is visible to the human eye. Electromagnetic radiation in this range of wavelengths is called visible light or simply light. A typical human eye will respond to wavelengths from about 390 to 750 nm. In terms of...
and infrared regions of the electromagnetic spectrum
Electromagnetic spectrum
The electromagnetic spectrum is the range of all possible frequencies of electromagnetic radiation. The "electromagnetic spectrum" of an object is the characteristic distribution of electromagnetic radiation emitted or absorbed by that particular object....
. In absorption-re-emission ALFs, the frequency of light must be shifted in order for the filter to operate, and in a passive device, this shift must be to a lower frequency (i.e. red shifted) simply because of energy conservation. This means that passive filters are rarely able to work with infrared light, because the output frequency would be impractically low. If photomultiplier tubes
Photomultiplier
Photomultiplier tubes , members of the class of vacuum tubes, and more specifically phototubes, are extremely sensitive detectors of light in the ultraviolet, visible, and near-infrared ranges of the electromagnetic spectrum...
(PMTs) are used then the “output wavelength of the ARF should lie in a spectral region in which commercial, large-area, long-lived PMT’s [sic] possess maximum sensitivity”. In such a case, active ALFs would have the advantage over passive ALFs as they would more readily, “generate output wavelengths in the near UV, the spectral region in which well-developed photocathode
Photocathode
A photocathode is a negatively charged electrode in a light detection device such as a photomultiplier or phototube that is coated with a photosensitive compound...
s possess their highest sensitivity”.
In a passive ALF, the input frequency must correspond almost exactly to the natural absorption lines of the vapor cell. Active ARFs are much more flexible, however, as the vapor may be stimulated so that it will absorb other frequencies of light.
Faraday and Voigt filters do not shift the frequency or wavelength of the signal light.
Response time and transmission rate
The response time of an absorption-re-emission atomic line filter directly affects the rate information is transmitted from the light source to the receiver. Therefore, a minimal response time is an important property of these ALFs. The response time of such an ALF, is largely dependent on the spontaneous decay of the excited atoms in the vapor cell. In 1988, Jerry Gelbwachs cited, “typical rapid spontaneous emission times are ~ 30 nsSecond
The second is a unit of measurement of time, and is the International System of Units base unit of time. It may be measured using a clock....
, which suggests that the upper limit on the information rate is approximately 30 MHz
Hertz
The hertz is the SI unit of frequency defined as the number of cycles per second of a periodic phenomenon. One of its most common uses is the description of the sine wave, particularly those used in radio and audio applications....
”.
Many methods of decreasing the response time of ALFs have been developed. Even in the late 1980s, certain gases were used to catalyze the decay of the electrons of the vapor cell. In 1989, Eric Korevaar had developed his Fast ALF design which detected emitted fluorescence without photosensitive plates. With such methods employed, gigahertz frequencies are easily attainable.
Efficiency
Q factor
In physics and engineering the quality factor or Q factor is a dimensionless parameter that describes how under-damped an oscillator or resonator is, or equivalently, characterizes a resonator's bandwidth relative to its center frequency....
is in the 105 to 106 range. This is partially because the, “crossed polarizers … serve to block out background light with a rejection ratio better than 10-5”. The passband of a typical Faraday filter may be a few GHz. The total output of a Faraday filter may be around 50% of the total input light intensity. The light lost is reflected or absorbed by imperfect lenses, filters and windows.
Band-pass
The band-pass of an atomic line filter is usually equal to the Doppler profile of the vapor cell, the natural range of frequencies at which a vapor cell will be excited by a pure light source. The Doppler profile is the width of the spectrum of Doppler shifted radiation emitted by the vapor cell due to its thermal motion. This value is less for larger atoms at lower temperatures, a system considered more ideal.There are some circumstances where this is not the case, and it is desirable to make the width of the transition line larger than the Doppler profile. For instance, when tracking a quickly accelerating object, the band-pass of the ALF must include within it the maximum and minimum values for the reflected light. The accepted method for increasing the band-pass involves placing an inert gas in the vapor cell. This gas both widens the spectral line and increases the transmission rate of the filter.
Sources of noise
For all of their efficiency, atomic line filters are not perfect; there are many sources of error, or “noise”, in a given system. These are manifest as electromagnetic radiation independent of the working processes of the filter and the intensity of the signal light. One source of error is the thermal radiationThermal radiation
Thermal radiation is electromagnetic radiation generated by the thermal motion of charged particles in matter. All matter with a temperature greater than absolute zero emits thermal radiation....
of and within the ALF itself. Some thermal radiation comes directly from the filter and happens to be within the bandpass of the second broad band filter. More noise is created if the filter is designed for output in the infrared range, as most of the thermal radiation would be in that spectrum. These emissions may stimulate the vapor and create the radiation it is trying to detect in the first place.
Active atomic line filters are more likely to produce noise than passive ones because actives have no “state selectivity”; the pumping source may accidentally excite atoms hit by the wrong light up to the critical energy level, emitting radiation spontaneously.
Other errors may be caused by atomic absorption/resonance lines not targeted but still active. Though most “near” transitions are over 10 nanometers away (far enough to be blocked by the broad-band filters), the fine
Fine structure
In atomic physics, the fine structure describes the splitting of the spectral lines of atoms due to first order relativistic corrections.The gross structure of line spectra is the line spectra predicted by non-relativistic electrons with no spin. For a hydrogenic atom, the gross structure energy...
and hyperfine structure
Hyperfine structure
The term hyperfine structure refers to a collection of different effects leading to small shifts and splittings in the energy levels of atoms, molecules and ions. The name is a reference to the fine structure which results from the interaction between the magnetic moments associated with electron...
of the target absorption line may absorb incorrect frequencies of light and pass them through to the output sensor.
Relevant phenomena
Radiation trapping
Radiation trapping, imprisonment of resonance radiation, radiative transfer of spectral lines, line transfer or radiation diffusion is a phenomenon in physics whereby radiation may be "trapped" in a system as it is emitted by one atom and absorbed by another....
in an atomic line filter may seriously affect the performance and therefore tuning of an ALF. In the original studies of atomic line filters in the 1970s and early 1980s, there was a “large overestimation of the [signal bandwidth]”. Later, radiation trapping was studied, analyzed and ALFs were optimized to account for it.
In all atomic line filters, the position and widths of the vapor cell resonance lines are among the most important properties. By the Stark effect
Stark effect
The Stark effect is the shifting and splitting of spectral lines of atoms and molecules due to presence of an external static electric field. The amount of splitting and or shifting is called the Stark splitting or Stark shift. In general one distinguishes first- and second-order Stark effects...
and Zeeman splitting, the base absorption lines may be split into finer lines. “Stark and Zeeman tuning… can be used to tune the detector.” Consequently, manipulation of electric and magnetic field
Magnetic field
A magnetic field is a mathematical description of the magnetic influence of electric currents and magnetic materials. The magnetic field at any given point is specified by both a direction and a magnitude ; as such it is a vector field.Technically, a magnetic field is a pseudo vector;...
s may alter other properties of the filter (i.e. shifting the passband).
Types
Absorption-re-emission
An absorption-re-emission atomic line filter absorbs the desired wavelength of light and emits light that bypasses broadband filters. In passive absorption-re-emission ALFs, a high-pass filterHigh-pass filter
A high-pass filter is a device that passes high frequencies and attenuates frequencies lower than its cutoff frequency. A high-pass filter is usually modeled as a linear time-invariant system...
blocks all low-energy incoming light. The vapor cell absorbs the signal, which coincides with the vapor’s thin absorption line, and the cell’s atoms become excited. The vapor cell then re-emits the signal light by undergoing fluorescence
Fluorescence
Fluorescence is the emission of light by a substance that has absorbed light or other electromagnetic radiation of a different wavelength. It is a form of luminescence. In most cases, emitted light has a longer wavelength, and therefore lower energy, than the absorbed radiation...
at a lower frequency. A low-pass filter
Low-pass filter
A low-pass filter is an electronic filter that passes low-frequency signals but attenuates signals with frequencies higher than the cutoff frequency. The actual amount of attenuation for each frequency varies from filter to filter. It is sometimes called a high-cut filter, or treble cut filter...
blocks radiation above the frequency of the fluorescent light. In an active ALF, optical
Optical pumping
Optical pumping is a process in which light is used to raise electrons from a lower energy level in an atom or molecule to a higher one. It is commonly used in laser construction, to pump the active laser medium so as to achieve population inversion...
or electrical pumping is used for exciting these atoms so they absorb or emit light of different wavelengths. For active ALFs, other systems of conventional filters may be needed.
Faraday filter
A Faraday filter, magneto-optical filter, FADOF or EFADOF (Excited Faraday Dispersive Optical Filter) works by rotating the polarization of the light passing through the vapor cell. This rotation occurs near its atomic absorption linesSpectral line
A spectral line is a dark or bright line in an otherwise uniform and continuous spectrum, resulting from a deficiency or excess of photons in a narrow frequency range, compared with the nearby frequencies.- Types of line spectra :...
by the Faraday effect
Faraday effect
In physics, the Faraday effect or Faraday rotation is a Magneto-optical phenomenon, that is, an interaction between light and a magnetic field in a medium...
and anomalous dispersion. Only light at the resonant frequency of the vapor is rotated and the polarized plates block other electromagnetic radiation. This effect is related to and enhanced by the Zeeman Effect
Zeeman effect
The Zeeman effect is the splitting of a spectral line into several components in the presence of a static magnetic field. It is analogous to the Stark effect, the splitting of a spectral line into several components in the presence of an electric field...
, or the splitting of atomic absorption lines in the presence of the magnetic field. Light at the resonant frequency of the vapor exits a FADOF near its original strength but with an orthogonal polarization.
Following the laws which govern the Faraday effect, the rotation of the targeted radiation is directly proportional to the strength of the magnetic field, the width of the vapor cell and the Verdet constant
Verdet constant
The Verdet constant is an optical "constant" that describes the strength of the Faraday effect for a particular material.The Verdet constant for most materials is extremely small and is wavelength dependent. It is strongest in substances containing paramagnetic ions such as terbium...
(which is dependent on the temperature
Temperature
Temperature is a physical property of matter that quantitatively expresses the common notions of hot and cold. Objects of low temperature are cold, while various degrees of higher temperatures are referred to as warm or hot...
of the cell, wavelength
Wavelength
In physics, the wavelength of a sinusoidal wave is the spatial period of the wave—the distance over which the wave's shape repeats.It is usually determined by considering the distance between consecutive corresponding points of the same phase, such as crests, troughs, or zero crossings, and is a...
of the light and sometimes intensity of the field) of the vapor in the cell. This relationship is represented the following equation:
Voigt filter
A Voigt filter is a Faraday filter with its magnetic field shifted to be perpendicular to the direction of the light and at 45° to the polarization of the polarized plates. In a Voigt filter, the vapor cell acts as a half wave plateWave plate
A wave plate or retarder is an optical device that alters the polarization state of a light wave travelling through it.- Operation :A wave plate works by shifting the phase between two perpendicular polarization components of the light wave. A typical wave plate is simply a birefringent crystal...
, retarding one polarization by 180° per the Voigt effect
Voigt Effect
The Voigt Effect, is one of a class of effects, resulting in what is called magnetic birefringence, or magnetic double refraction. It is a magneto-optical phenomenon with a similar origin to the Faraday effect. In the Faraday effect, the polarization of light can be rotated when passed through a...
.
Common components
Collimator
A collimator is a device that narrows a beam of particles or waves. To "narrow" can mean either to cause the directions of motion to become more aligned in a specific direction or to cause the spatial cross section of the beam to become smaller.- Optical collimators :In optics, a collimator may...
, which straightens incident light rays for passing through the rest of the filter consistently; however, collimated light is not always necessary. After the collimator, a high-pass filter blocks almost half of the incoming light (that of too long a wavelength). In Faraday and Voigt filters, the first polarizing plate
Polarizer
A polarizer is an optical filter that passes light of a specific polarization and blocks waves of other polarizations. It can convert a beam of light of undefined or mixed polarization into a beam with well-defined polarization. The common types of polarizers are linear polarizers and circular...
is used here to block light.
The next component in an atomic line filter is the vapor cell; this is common to all atomic line filters. It either absorbs and re-emits the incident light, or rotates its polarization by the Faraday or Voigt effect. Following the vapor cell is a low-pass filter, designed to block all of the light that the first filter did not, except the designated frequency of light which came from the fluorescence. In Faraday and Voigt filters, a second polarizing plate is used here.
Other systems may be used in conjunction with the rest of an atomic line filter for practicality. For instance, the polarizers used in the actual Faraday filter don’t block most radiation, “because these polarizers only work over a limited wavelength region … a broad band interference filter is used in conjunction with the Faraday filter”. The passband of the interference filter may be 200 times that of the actual filter. Photomultiplier tubes, too, are often used for increasing the intensity of the output signal to a usable level. Avalanche
Avalanche breakdown
Avalanche breakdown is a phenomenon that can occur in both insulating and semiconducting materials. It is a form of electric current multiplication that can allow very large currents within materials which are otherwise good insulators. It is a type of electron avalanche...
photomultiplier
Photomultiplier
Photomultiplier tubes , members of the class of vacuum tubes, and more specifically phototubes, are extremely sensitive detectors of light in the ultraviolet, visible, and near-infrared ranges of the electromagnetic spectrum...
s, which are more efficient, may be used instead of a PMT.
Vapor cell
While every implementation of each kind of ALF is different, the vapor cell in each is relatively similar. The thermodynamic properties of vapor cells in filters are carefully controlled because they determine important qualities of the filter, for instance the necessary strength of the magnetic field. Light is let into and out of this vapor chamber by way of two low-reflection windows made of a material such as magnesium fluorideMagnesium fluoride
Magnesium fluoride is an inorganic compound with the formula MgF2. The compound is a white crystalline salt and is transparent over a wide range of wavelengths, with commercial uses in optics.-Production and structure:...
. The other sides of the cell may be of any opaque material, though generally a heat-resistant metal
Metal
A metal , is an element, compound, or alloy that is a good conductor of both electricity and heat. Metals are usually malleable and shiny, that is they reflect most of incident light...
or ceramic
Ceramic
A ceramic is an inorganic, nonmetallic solid prepared by the action of heat and subsequent cooling. Ceramic materials may have a crystalline or partly crystalline structure, or may be amorphous...
is used as the vapor is usually kept at temperatures upwards of 100 °C.
Most ALF vapor cells use alkali metal
Alkali metal
The alkali metals are a series of chemical elements in the periodic table. In the modern IUPAC nomenclature, the alkali metals comprise the group 1 elements, along with hydrogen. The alkali metals are lithium , sodium , potassium , rubidium , caesium , and francium...
s because of their high vapor pressures; many alkali metals also have absorption lines and resonance in the desired spectra. Common vapor cell materials are sodium
Sodium
Sodium is a chemical element with the symbol Na and atomic number 11. It is a soft, silvery-white, highly reactive metal and is a member of the alkali metals; its only stable isotope is 23Na. It is an abundant element that exists in numerous minerals, most commonly as sodium chloride...
, potassium and caesium
Caesium
Caesium or cesium is the chemical element with the symbol Cs and atomic number 55. It is a soft, silvery-gold alkali metal with a melting point of 28 °C , which makes it one of only five elemental metals that are liquid at room temperature...
. Note that non-metallic
Nonmetal
Nonmetal, or non-metal, is a term used in chemistry when classifying the chemical elements. On the basis of their general physical and chemical properties, every element in the periodic table can be termed either a metal or a nonmetal...
vapors such as neon
Neon
Neon is the chemical element that has the symbol Ne and an atomic number of 10. Although a very common element in the universe, it is rare on Earth. A colorless, inert noble gas under standard conditions, neon gives a distinct reddish-orange glow when used in either low-voltage neon glow lamps or...
may be used. As the early quantum counters used solid state
Solid-state laser
A solid-state laser is a laser that uses a gain medium that is a solid, rather than a liquid such as in dye lasers or a gas as in gas lasers. Semiconductor-based lasers are also in the solid state, but are generally considered as a separate class from solid-state lasers .-Solid-state...
metal ions in crystals, it is conceivable that such a medium could be used in the ALFs of today. This is presumably not done because of the superiority of atomic vapors in this capacity.
Applications
Atomic line filters are most often used in LIDAR and other exercises in laser tracking and detection, for their ability to filter daylight and effectively discern weak, narrowband signals; however, they may be used for filtering out the earth’s thermalThermal radiation
Thermal radiation is electromagnetic radiation generated by the thermal motion of charged particles in matter. All matter with a temperature greater than absolute zero emits thermal radiation....
background, measuring the efficiencies of antibiotic
Antibiotic
An antibacterial is a compound or substance that kills or slows down the growth of bacteria.The term is often used synonymously with the term antibiotic; today, however, with increased knowledge of the causative agents of various infectious diseases, antibiotic has come to denote a broader range of...
s and general filtering applications.
Laser tracking and communication
Without an atomic line filter, laser tracking and communication may be difficult. Usually, intensified charge-coupled device cameras must be used in conjunction with simple dielectric optical filters (e.g. interference filters) to detect laser emissions at a distance. Intensified CCDs are inefficient and necessitate the use of a pulsed laser transmission within the visible spectrum. With the superior filtering system of an ALF, a non-intensified CCDCharge-coupled device
A charge-coupled device is a device for the movement of electrical charge, usually from within the device to an area where the charge can be manipulated, for example conversion into a digital value. This is achieved by "shifting" the signals between stages within the device one at a time...
may be used with a continuous wave
Continuous wave
A continuous wave or continuous waveform is an electromagnetic wave of constant amplitude and frequency; and in mathematical analysis, of infinite duration. Continuous wave is also the name given to an early method of radio transmission, in which a carrier wave is switched on and off...
laser more efficiently. “[Atomic line filters] with passbands of about 0.001 nm have been developed to improve the background rejection of conventionally filtered laser receivers”. The total energy consumption of the latter system is “30 to 35 times less” than that of the former, so space-based, underwater and agile laser communications with ALFs have been proposed and developed.
LIDAR
LIDAR comprises firing lasers at relevant portions of the atmosphere where light is backscatterBackscatter
In physics, backscatter is the reflection of waves, particles, or signals back to the direction they came from. It is a diffuse reflection due to scattering, as opposed to specular reflection like a mirror...
ed. By analyzing the reflected laser beam for Doppler shifts, wind speeds and wind directions in the target region may be calculated. The thermal structure, diurnal/semi-diurnal tide
Tide
Tides are the rise and fall of sea levels caused by the combined effects of the gravitational forces exerted by the moon and the sun and the rotation of the Earth....
s, and seasonal variations in the mesopause
Mesosphere
The mesosphere is the layer of the Earth's atmosphere that is directly above the stratosphere and directly below the thermosphere. In the mesosphere temperature decreases with increasing height. The upper boundary of the mesosphere is the mesopause, which can be the coldest naturally occurring...
region may thus be studied. This is a valuable faculty for meteorologists
Meteorology
Meteorology is the interdisciplinary scientific study of the atmosphere. Studies in the field stretch back millennia, though significant progress in meteorology did not occur until the 18th century. The 19th century saw breakthroughs occur after observing networks developed across several countries...
and climatologists
Climatology
Climatology is the study of climate, scientifically defined as weather conditions averaged over a period of time, and is a branch of the atmospheric sciences...
, as these properties can be significant.
However, without the ability to effectively track weak laser signals, collection of atmospheric data would be relegated to times of day where the sun's electromagnetic emissions did not drown out the laser's signal. The addition of an atomic line filter to the LIDAR equipment effectively filters interference to the laser's signal to the point where LIDAR data can be collected at any time of the day. For the past decade, Faraday filters have been used to do this. Consequently, scientists know significantly more today about the Earth’s middle atmosphere than they did before the advent of the FADOF.
See also
- Stimulated emissionStimulated emissionIn optics, stimulated emission is the process by which an atomic electron interacting with an electromagnetic wave of a certain frequency may drop to a lower energy level, transferring its energy to that field. A photon created in this manner has the same phase, frequency, polarization, and...
- Arecibo ObservatoryArecibo ObservatoryThe Arecibo Observatory is a radio telescope near the city of Arecibo in Puerto Rico. It is operated by SRI International under cooperative agreement with the National Science Foundation...
- Ferromagnetic resonanceFerromagnetic resonanceFerromagnetic resonance, or FMR, is a spectroscopic technique to probe the magnetization of ferromagnetic materials. It is a standard tool for probing spin waves and spin dynamics...
- Fraunhofer linesFraunhofer linesIn physics and optics, the Fraunhofer lines are a set of spectral lines named for the German physicist Joseph von Fraunhofer . The lines were originally observed as dark features in the optical spectrum of the Sun....
- Rayleigh scatteringRayleigh scatteringRayleigh scattering, named after the British physicist Lord Rayleigh, is the elastic scattering of light or other electromagnetic radiation by particles much smaller than the wavelength of the light. The particles may be individual atoms or molecules. It can occur when light travels through...
Further reading
- H. Chen, M. A. White, D. A. Krueger, and C. Y. She. Daytime mesopause temperature measurements with a sodium-vapor dispersive Faraday filter in a lidar receiver. Opt. Letters, 21(15):1093–1095, 1996.
- H. Chen, C. Y. She, P. Searcy, and E. Korevaar. Sodium-vapor dispersive Faraday filter. Optics Letters, 18:1019–1021, June 1993.