An atomic line filter (ALF) is a more effective
optical band-pass filter used in the
physical science
Physical science is a branch of natural science that studies non-living systems, in contrast to life science. It in turn has many branches, each referred to as a "physical science", together called the "physical sciences".
Definition
Physi ...
s for filtering
electromagnetic radiation with precision, accuracy, and minimal signal strength loss. Atomic line filters work via the
absorption
Absorption may refer to:
Chemistry and biology
*Absorption (biology), digestion
**Absorption (small intestine)
*Absorption (chemistry), diffusion of particles of gas or liquid into liquid or solid materials
*Absorption (skin), a route by which s ...
or
resonance lines 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 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
A lock-in amplifier is a type of amplifier that can extract a signal with a known carrier wave from an extremely noisy environment. Depending on the dynamic reserve of the instrument, signals up to a million times smaller than noise components, p ...
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.
They are used regularly in ''Laser Imaging Detection and Ranging'' (
LIDAR
Lidar (, also LIDAR, or LiDAR; sometimes LADAR) is a method for determining ranges (variable distance) by targeting an object or a surface with a laser and measuring the time for the reflected light to return to the receiver. It can also be ...
) and are being studied for their potential use in
laser
A laser is a device that emits light through a process of optical amplification based on the stimulated emission of electromagnetic radiation. The word "laser" is an acronym for "light amplification by stimulated emission of radiation". The firs ...
communication systems.
Atomic line filters are superior to conventional dielectric optical filters such as
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. An interference filter ...
s and
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. Lyot filters are often used in astronomy, particularly for solar astronomy.
A Lyot fi ...
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$
The United States dollar (symbol: $; code: USD; also abbreviated US$ or U.S. Dollar, to distinguish it from other dollar-denominated currencies; referred to as the dollar, U.S. dollar, American dollar, or colloquially buck) is the official ...
15,000 per unit.
History
The predecessor of the atomic line filter was the
infrared quantum counter
Infrared (IR), sometimes called infrared light, is electromagnetic radiation (EMR) with wavelengths longer than those of visible light. It is therefore invisible to the human eye. IR is generally understood to encompass wavelengths from around ...
, designed in the 1950s by
Nicolaas Bloembergen
Nicolaas Bloembergen (March 11, 1920 – September 5, 2017) was a Dutch- American physicist and Nobel laureate, recognized for his work in developing driving principles behind nonlinear optics for laser spectroscopy. During his career, he was a p ...
. This was a quantum mechanical
amplifier
An amplifier, electronic amplifier or (informally) amp is an electronic device that can increase the magnitude of a signal (a time-varying voltage or current). It may increase the power significantly, or its main effect may be to boost the v ...
theorized by
Joseph Weber to detect
infrared radiation with very little noise.
Zero spontaneous emission was already possible for
x-ray
An X-ray, or, much less commonly, X-radiation, is a penetrating form of high-energy electromagnetic radiation. Most X-rays have a wavelength ranging from 10 picometers to 10 nanometers, corresponding to frequencies in the range 30  ...
and
gamma ray 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
crystals with
transition metal
In chemistry, a transition metal (or transition element) is a chemical element in the d-block of the periodic table (groups 3 to 12), though the elements of group 12 (and less often group 3) are sometimes excluded. They are the elements that can ...
ion
An ion () is an atom or molecule with a net electrical charge.
The charge of an electron is considered to be negative by convention and this charge is equal and opposite to the charge of a proton, which is considered to be positive by conv ...
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
salts 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
The term quantum efficiency (QE) may apply to incident photon to converted electron (IPCE) ratio of a photosensitive device, or it may refer to the TMR effect of a Magnetic Tunnel Junction.
This article deals with the term as a measurement of ...
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,
patented by James H. Menders and Eric J. Korevaar on August 26, 1992,
was more advanced. Voigt filters were more compact and "
ouldbe 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
Isotropy is uniformity in all orientations; it is derived . Precise definitions depend on the subject area. Exceptions, or inequalities, are frequently indicated by the prefix ' or ', hence ''anisotropy''. ''Anisotropy'' is also used to describe ...
optical filter".
"Ultra-narrow-band" defines the thin range of frequencies that an ALF may accept; an ALF generally has a
passband 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
ultraviolet,
visible and infrared regions of the
electromagnetic spectrum
The electromagnetic spectrum is the range of frequencies (the spectrum) of electromagnetic radiation and their respective wavelengths and photon energies.
The electromagnetic spectrum covers electromagnetic waves with frequencies ranging from b ...
.
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 tubes (photomultipliers or PMTs for short) are extremely sensitive detectors of light in the ultraviolet, visible, and near-infrared ranges of the electromagnetic spectrum. They are members of the class of vacuum tubes, more spe ...
(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
icpossess 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
A photocathode is a surface engineered to convert light (photons) into electrons using the photoelectric effect. Photocathodes are important in accelerator physics where they are utilised in a photoinjector to generate high brightness electron be ...
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
ns, which suggests that the upper limit on the information rate is approximately 30
MHz
The hertz (symbol: Hz) is the unit of frequency in the International System of Units (SI), equivalent to one event (or cycle) per second. The hertz is an SI derived unit whose expression in terms of SI base units is s−1, meaning that one he ...
".
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.
Effectiveness
Efficiency
Atomic line filters are inherently very efficient filters, generally classified as "ultra-high-Q" as their
''Q'' factor is in the 10
5 to 10
6 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
A thermal column (or thermal) is a rising mass of buoyant air, a convective current in the atmosphere, that transfers heat energy vertically. Thermals are created by the uneven heating of Earth's surface from solar radiation, and are an example ...
. 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 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 and
hyperfine structure
In atomic physics, hyperfine structure is defined by small shifts in otherwise degenerate energy levels and the resulting splittings in those energy levels of atoms, molecules, and ions, due to electromagnetic multipole interaction between the nucl ...
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 ...
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
ignal 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
The Stark effect is the shifting and splitting of spectral lines of atoms and molecules due to the presence of an external electric field. It is the electric-field analogue of the Zeeman effect, where a spectral line is split into several compo ...
and
Zeeman splitting
The Zeeman effect (; ) is the effect of splitting of a spectral line into several components in the presence of a static magnetic field. It is named after the Dutch physicist Pieter Zeeman, who discovered it in 1896 and received a Nobel prize ...
, 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 fields 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 filter 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 at a lower frequency. A
low-pass filter blocks radiation above the frequency of the fluorescent light. In an active ALF,
optical 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 lines
A spectral line is a dark or bright line in an otherwise uniform and continuous spectrum, resulting from emission or absorption of light in a narrow frequency range, compared with the nearby frequencies. Spectral lines are often used to identi ...
by the
Faraday effect and
anomalous dispersion
In optics, and by analogy other branches of physics dealing with wave propagation, dispersion is the phenomenon in which the phase velocity of a wave depends on its frequency; sometimes the term chromatic dispersion is used for specificity to o ...
. 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, 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
The Verdet constant is an optical property named after the French physicist Émile Verdet. It describes the strength of the Faraday effect for a particular material. For a constant magnetic field parallel to the path of the light, it can be calcul ...
(which is dependent on the
temperature of the cell,
wavelength 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 plate, retarding one polarization by 180° per the
Voigt effect.
Common components
Preceding an atomic line filter may be a
collimator
A collimator is a device which 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 (i.e., make collimated light or parallel rays), or to cause the spat ...
, 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 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
An avalanche is a rapid flow of snow down a slope, such as a hill or mountain.
Avalanches can be set off spontaneously, by such factors as increased precipitation or snowpack weakening, or by external means such as humans, animals, and eart ...
photomultiplier A photomultiplier is a device that converts incident photons into an electrical signal.
Kinds of photomultiplier include:
* Photomultiplier tube, a vacuum tube converting incident photons into an electric signal. Photomultiplier tubes (PMTs for sho ...
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 fluoride. The other sides of the cell may be of any opaque material, though generally a heat-resistant
metal
A metal (from Greek μέταλλον ''métallon'', "mine, quarry, metal") is a material that, when freshly prepared, polished, or fractured, shows a lustrous appearance, and conducts electricity and heat relatively well. Metals are typica ...
or
ceramic is used as the vapor is usually kept at temperatures upwards of 100 °C.
Most ALF vapor cells use
alkali metal
The alkali metals consist of the chemical elements lithium (Li), sodium (Na), potassium (K),The symbols Na and K for sodium and potassium are derived from their Latin names, ''natrium'' and ''kalium''; these are still the origins of the names ...
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, potassium and
caesium
Caesium ( IUPAC spelling) (or cesium in American English) is a chemical element with the symbol Cs and atomic number 55. It is a soft, silvery-golden alkali metal with a melting point of , which makes it one of only five elemental metals that a ...
. Note that
non-metallic vapors such as
neon
Neon is a chemical element with the symbol Ne and atomic number 10. It is a noble gas. Neon is a colorless, odorless, inert monatomic gas under standard conditions, with about two-thirds the density of air. It was discovered (along with krypton ...
may be used.
As the early quantum counters used
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
thermal background,
measuring the efficiencies of
antibiotic
An antibiotic is a type of antimicrobial substance active against bacteria. It is the most important type of antibacterial agent for fighting bacterial infections, and antibiotic medications are widely used in the treatment and prevention ...
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
A charge-coupled device (CCD) is an integrated circuit containing an array of linked, or coupled, capacitors. Under the control of an external circuit, each capacitor can transfer its electric charge to a neighboring capacitor. CCD sensors are a ...
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
CCD may be used with a
continuous wave
A continuous wave or continuous waveform (CW) is an electromagnetic wave of constant amplitude and frequency, typically a sine wave, that for mathematical analysis is considered to be of infinite duration. It may refer to e.g. a laser or particl ...
laser more efficiently. "
tomic line filterswith 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
backscatter
In physics, backscatter (or backscattering) is the reflection of waves, particles, or signals back to the direction from which they came. It is usually a diffuse reflection due to scattering, as opposed to specular reflection as from a mirror, ...
ed. By analyzing the reflected laser beam for
Doppler shift
The Doppler effect or Doppler shift (or simply Doppler, when in context) is the change in frequency of a wave in relation to an observer who is moving relative to the wave source. It is named after the Austrian physicist Christian Doppler, who d ...
s, wind speeds and wind directions in the target region may be calculated. The thermal structure, diurnal/semi-diurnal
tides, and seasonal variations in the
mesopause
The mesopause is the point of minimum temperature at the boundary between the mesosphere and the thermosphere atmospheric regions. Due to the lack of solar heating and very strong radiative cooling from carbon dioxide, the mesosphere is the cold ...
region may thus be studied. This is a valuable faculty for
meteorologists and
climatologists
Climatology (from Greek , ''klima'', "place, zone"; and , ''-logia'') or climate science is the scientific study of Earth's climate, typically defined as weather conditions averaged over a period of at least 30 years. This modern field of stud ...
, 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 emission
*
Arecibo Observatory
The Arecibo Observatory, also known as the National Astronomy and Ionosphere Center (NAIC) and formerly known as the Arecibo Ionosphere Observatory, is an observatory in Barrio Esperanza, Arecibo, Puerto Rico owned by the US National Science Fo ...
*
Ferromagnetic resonance
Ferromagnetic resonance, or FMR, is coupling between an electromagnetic wave and the magnetization of a medium through which it passes. This coupling induces a significant loss of power of the wave. The power is absorbed by the precessing magneti ...
*
Fraunhofer lines
In physics and optics, the Fraunhofer lines are a set of spectral absorption lines named after the German physicist Joseph von Fraunhofer (1787–1826). The lines were originally observed as dark features (absorption lines) in the optical spectru ...
*
Rayleigh scattering
References
Bibliography
*
* .
* .
* .
* .
* .
* .
* .
* .
* .
* .
* .
* .
* .
* .
* .
* .
* .
* .
* .
Patents
*
*
*
*
*
*
*
{{refend
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.
Optical filters
Meteorological instrumentation and equipment