An atomic line filter, or ALF, (sometimes atomic resonance filter or ARF) is a class of optical band-pass filters used in the physical sciences for filtering light with great precision, accuracy and efficiency. Atomic line filters are supposedly named thus because they work on the atomic scale, depending upon specific absorption or resonance lines of the atoms in the filter. Optical redirects here. ... The frequency axis of this symbolic diagram would be logarithmically scaled. ... Prism splitting light Light is electromagnetic radiation with a wavelength that is visible to the eye (visible light) or, in a technical or scientific context, electromagnetic radiation of wavelengths that are studied in the field of optics. ... Look up absorption in Wiktionary, the free dictionary Absorption may refer to: In physics: absorption (chemistry) - absorption of particles of gas or liquid in liquid or solid material as studied in physical chemistry absorption (optics) - absorption of photons by a material Absorption (acoustics) - absorption of sound waves by a material... The Tacoma Narrows Bridge (shown twisting) in Washington collapsed spectacularly, under moderate wind, in part because of resonance. ...

The three major implementations of atomic line filters include Faraday filters, Voigt filters and absorption-re-emission ALFs^[1]. While all atomic line filters use different effects and designs for the specific implementation, the same basic strategy is always employed, taking advantage of a narrow line of absorption or resonance in a metallic vapor so that a very specific frequency of light may be manipulated (through polarization or absorption and re-emission) to bypass a series of filters that block all other light.

Specifically, an absorption-re-emission ALF absorbs the desired wavelength of light and emits fluorescence which bypasses other, broad band filters. A Faraday filter always uses the Faraday Effect to rotate the polarization of a specific frequency of light to pass through two crossed polarizers, and a Voigt filter works similarly, but uses the Voigt Effect to rotate the light, instead.^[2] In physics, the Faraday effect or Faraday rotation is an interaction between light and a magnetic field. ...

As an ALF generally has a passband on the order of .001 nanometer, atomic line filters are often used in scientific applications requiring the detection of laser light that would otherwise be drowned out by other, more abundant light sources, such as daylight^[3]. Because ALFs are very effective at doing this, they have filled a niche in the field of optical filtering: they are used regularily in LIDAR and are being studied for their potential in the field of laser communication systems.^[4] In telecommunications, passband is the portion of spectrum, between limiting frequencies (or, in the optical regime, limiting wavelengths), that is transmitted with minimum relative loss or maximum relative gain. ... A nanometre (American spelling: nanometer) is 1. ... Look up daylight in Wiktionary, the free dictionary. ... LIDAR (Light Detection and Ranging; or Laser Imaging Detection and Ranging) is a technology that determines distance to an object or surface using laser pulses. ...

History

The first type of atomic line filter was the basic absorption-re-emmission one. This design was primitive, however, and suffered from low quantum efficiency and slow response time. It did evolve and improve, but a revolution was found in the Faraday filter. A graph showing variation of quantum efficiency with wavelength of the CCD chips in the Hubble Space Telescopes Wide Field and Planetary Camera 2. ... In telecommunication, response time is the time a system or functional unit takes to react to a given input. ...

Faraday filters have been used since at least 1972, improving "the performance" of previous atomic line filters^[5]. By 1996, they were being used for LIDAR. Faraday filters compete mostly with etalons, another high end optical filter. Faraday filters may be cheaper to implement at approximately $20,000 per unit^{[6] [7]}, and may be more reliable as they can continue operation if jostled. They also have relatively wide fields of view (around 180°)^[8].^[9] LIDAR (Light Detection and Ranging; or Laser Imaging Detection and Ranging) is a technology that determines distance to an object or surface using laser pulses. ... In optics, a Fabry-Perot interferometer or etalon is typically made of a transparent plate with two reflecting surfaces, or two parallel highly-reflecting mirrors. ... The field of view is the part of the observable world that is seen at any given moment. ...

The Voigt filter was patented by James H. Menders and Eric J. Korevaar on August 26th, 1992.^[10] The Voigt filter was seen as an improvement to the already present Faraday filter design; advantages of it include increased compactness and Voigt filters, "could be easily designed for use with a permanent magnet."^[11] A patent is a set of exclusive rights granted by a state to a person for a fixed period of time in exchange for the regulated, public disclosure of certain details of a device, method, process or composition of matter (substance) (known as an invention) which is new, inventive, and...

Common components

A diagram of the parts of a Faraday filter. In a Voigt filter, the magnetic field would be rotated 90 degrees. Note that the two polarizer plates are perpendicular in direction of polarization.

The method of filtration behind an Absorption-re-emission ALF.

Collimator

Preceding an atomic line filter is a collimator, which straightens incident light rays for passing through the rest of the filter consistently^[12]. How a lead collimator filters a stream of rays. ...

First polarizer

After the collimator, in the direction of propogation, comes a first polarizing plate by which almost half of the incoming light, (that of the wrong polarization state), is either absorbed or deflected.

In an absorption-re-emission ALF, a high-pass filter is used here. A high-pass filter is a filter that passes high frequencies well, but attenuates (or reduces) frequencies lower than the cutoff frequency. ...

Vapor cell

One thing common to all atomic line filters is the vapor cell, which is the device that follows the first polarizing plate. While every implementation of each kind of ALF is different, the construction of the vapor cells in each is relatively similar. Most ALF vapor cells use alkali metals. These metals are often used because they may be in the vapor state at a low temperature; they also often have absorption lines and resonance in the desired visible spectrum^[13] Three common vapor cell materials are sodium, potassium and cesium^[14]. Note that non-metallic vapors may be used, though: neon has been used in Faraday filters^[15]. The alkali metals are the series of elements in Group 1 (IUPAC style) of the periodic table (excluding hydrogen in all but one rare circumstance): lithium (Li), sodium (Na), potassium (K), rubidium (Rb), caesium (Cs), and francium (Fr). ... ... The visible spectrum (or sometimes optical spectrum) is the portion of the electromagnetic spectrum that is visible to the human eye. ... General Name, Symbol, Number sodium, Na, 11 Chemical series alkali metals Group, Period, Block 1, 3, s Appearance silvery white Atomic mass 22. ... General Name, Symbol, Number potassium, K, 19 Chemical series alkali metals Group, Period, Block 1, 4, s Appearance silvery white Atomic mass 39. ... General Name, Symbol, Number Caesium, Cs, 55 Series Alkali metals Group, Period, Block 1(IA), 6, s Density, Hardness 1879 kg/m3, 0. ... Together with the metals and metalloids, a nonmetal is one of three categories of chemical elements as distinguished by ionization and bonding properties. ... General Name, Symbol, Number neon, Ne, 10 Chemical series noble gases Group, Period, Block 18, 2, p Appearance colorless Atomic mass 20. ...

The thermodynamic properties of vapor cells in a filter are carefully controlled as they determine many important qualities of the filter, for instance the necessary strength of the magnetic field^[16].

Light is let into and out of this vapor chamber by way of two non-reflective windows, possibly made of magnesium fluoride. The other sides of the cell may be of any opaque material, though generally a heat resistant metal or ceramic is used as the vapor is usually kept at temperatures upwards of 100°C. Magnesium fluoride (MgF2) is a white crystalline salt composed of one magnesium ion and two fluoride ions. ... Hot metal work from a blacksmith Look up Metal in Wiktionary, the free dictionary. ... Fixed Partial Denture, or Bridge The word ceramic is derived from the Greek word κεραμικος (keramikos, having to do with pottery). The term covers inorganic non-metallic materials whose formation is due to the action of heat. ...

Second Polarizer

Following the vapor cell is the second polarizing plate, designed to block all of the light that the first polarizer did not, except a designated frequency of light which is rotated or fluorescent.

In an absorption-re-emission ALF, a low-pass filter is used here. A low-pass filter is a filter that passes low frequencies well, but attenuates (or reduces) frequencies higher than the cutoff frequency. ...

Interference filter

Other systems may be used in conjuction with the rest of an atomic line filter for practicality. For instance, the polarizers used in the actual Faraday filter don't actually block most radiation, "because these polarizers only work over a limited wavelength region in the infrared, a broad band interference filter is used in conjunction with the Faraday filter."^[17]. In telecommunication, an interference 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. ...

Types of atomic line filters

Absorption-re-emission

In absorption-re-emission ALFs, the first polarized filter is replaced with a high-pass filter. The vapor cell absorbs the signal itself, which coincides with the vapor's thin absorption line, and the cell's atoms become excited. The vapor cell then re-emit the signal light by undergoing fluorescence at a low wavelength. In an active ALF, a pump beam is used for further exciting these atoms first. In a passive ALF, no pump beam is used^[18]. A low-pass filter blocks radiation above the frequency of the fluorescence light. A high-pass filter is a filter that passes high frequencies well, but attenuates (or reduces) frequencies lower than the cutoff frequency. ... Fluorescence induced by exposure to ultraviolet light in vials containing various sized Cadmium selenide (CdSe) quantum dots. ... A low-pass filter is a filter that passes low frequencies well, but attenuates (or reduces) frequencies higher than the cutoff frequency. ...

However, "for measurement of windspeed these filters have two drawbacks, slow response time (about 500 ns for the alkali atoms) and low quantum efficiency"^[19].^[20] This was improved by the "Fast ALF" design of Eric Korevaar in 1989 which detected emitted flourescence without photosensitive plates^[21]. A graph showing variation of quantum efficiency with wavelength of the CCD chips in the Hubble Space Telescopes Wide Field and Planetary Camera 2. ...

Faraday filter

Polarization mechanism of a Faraday filter by the Faraday effect

A Faraday filter, Magneto-optical filter or FADOF (Faraday Dispersive Optical Filter) works by rotating the polarization of the light passing through the vapor cell near its atomic absorption lines by the Faraday Effect due to anomalous dispersion. Only light at the resonant frequency of the vapor is rotated and the polarized plates block other electromagnetic radiation^[6]. This effect is generally related to and enhanced by the Zeeman Effect^[22], or the splitting of atomic absorption lines in the presence of the magnetic field^[23][24]. Light at the resonant frequency of the vapor is exits an FADOF near its original strength but with an orthogonal polarization. Image File history File links Faraday-effect. ... Image File history File links Faraday-effect. ... In physics, the Faraday effect or Faraday rotation is an interaction between light and a magnetic field. ... ... In physics, the Faraday effect or Faraday rotation is an interaction between light and a magnetic field. ... The Tacoma Narrows Bridge (shown twisting) in Washington collapsed spectacularly, under moderate wind, in part because of resonance. ... wave f[o[Image:Light-wave. ... The Zeeman effect (IPA ) is the splitting of a spectral line into several components in the presence of a magnetic field. ...

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 through which the light must pass and the Verdet constant of the vapor in the cell. This relationship is represented the following equation: The Verdet constant is an optical constant that describes the strength of the Faraday effect for a particular material. ...

^[25]

Voigt filter

A Voigt filter is a Faraday filter with its magnetic field directed perpendicular to the direction of the light and at 45° to the polarization of the polarized plates^[26]. In a Voigt filter, the vapor cell acts as a half wave plate, retarding one polarization by 180°, instead of a Faraday rotator, by the Voigt Effect (magnetic birefringence)^[27]. A wave plate is an optical device that alters the polarization state of a light wave travelling through it. ...

Applications

Drawing of the receiver end of a laser tracking system from a United States patent, #05202741

Atomic line filters are most often used in LIDAR and other excercises in laser tracking and detection, for their ability to filter out daylight and effectively discern laser signals; however, they may be used for edge detection^[28], measuring the efficiencies of antibiotics^[29] and general filtering applications. A patent is a set of exclusive rights granted by a state to a person for a fixed period of time in exchange for the regulated, public disclosure of certain details of a device, method, process or composition of matter (substance) (known as an invention) which is new, inventive, and... LIDAR (Light Detection and Ranging; or Laser Imaging Detection and Ranging) is a technology that determines distance to an object or surface using laser pulses. ... Lasers range in size from microscopic diode lasers (top) with numerous applications, to football field sized neodymium glass lasers (bottom) used for inertial confinement fusion, nuclear weapons research and other high energy density physics experiments. ... Look up daylight in Wiktionary, the free dictionary. ... The goal of edge detection is to mark the points in a digital image at which the luminous intensity changes sharply. ... An antibiotic is a drug that kills or slows the growth of bacteria. ...

Note that for actual applications of these filters, the exact parameters (such as temperature, magnetic field strength, length, etc.) of ALF must first be calculated by a computer because of the extreme complexity of the system^[30]

Laser tracking and communication

Without an atomic line filter, laser tracking and communication may be difficult. Usually, intesified CCD cameras must be used in conjunction with dielectric optical filters to detect a laser emissions at a distance. Intensified CCDs are very inefficient and neccesitate the use of a pulsed laser transmission within the visible light spectrum. With the superior filtering system of an ALF, a non-intensified CCD may be used with a continuous wave laser much more efficiently. "Atomic line filters (ALFs) with passbands of about 0.001 nm have been developed to improve the background rejection of conventionally filtered laser receivers"^[31]. The total energy consumption of the latter system is "30 to 35 times less" than that of the former^[32].^[33] CCD can stand for: Charge-Coupled Device Confraternity of Christian Doctrine Carbonate Compensation Depth Council for a Community of Democracies MiniCD This page concerning a three-letter acronym or abbreviation is a disambiguation page — a navigational aid which lists other pages that might otherwise share the same title. ... A continuous wave (CW) is an electromagnetic wave of constant amplitude and frequency. ... A nanometre (American spelling: nanometer) is 1. ...

LIDAR

With the ability to effectively track weak laser signals, comes the ability to do LIDAR or Laser Imaging Detection and Ranging. So, for the past decade, Faraday filters have been used to measure sodium density, zonal and meridonal winds and indirectly temperature in the mesosphere and upper atmosphere during the daytime^[34]. In such an implementation, lasers are fired at desired portions of the atmosphere and some of the light is backscattered by parts of the atmosphere. By analyzing the reflected laser beam for Doppler shifts, wind speeds and directions in the target region may be calculated. LIDAR (Light Detection and Ranging; or Laser Imaging Detection and Ranging) is a technology that determines distance to an object or surface using laser pulses. ... General Name, Symbol, Number sodium, Na, 11 Chemical series alkali metals Group, Period, Block 1, 3, s Appearance silvery white Atomic mass 22. ... Temperature is also the name of a song by Sean Paul. ... The mesosphere (from the Greek words mesos = middle and sphaira = ball) is the layer of the Earths atmosphere that is directly above the stratosphere and directly below the thermosphere. ... Saturns atmosphere is made up of hydorgen, helium and methane ... Lasers range in size from microscopic diode lasers (top) with numerous applications, to football field sized neodymium glass lasers (bottom) used for inertial confinement fusion, nuclear weapons research and other high energy density physics experiments. ... In telecommunication, the term backscattering has the following meanings: 1. ... The Doppler effect is the apparent change in frequency or wavelength of a wave that is perceived by an observer moving relative to the source of the waves. ...

This is a valuable faculty for meteorologists and climatologists, for: Satellite image of Hurricane Hugo with a polar low visible at the top of the image. ... Climatology is the study of climate, and is a branch of the atmospheric sciences. ...

To study the thermal structure, diurnal/semi-diurnal tides, and seasonal variations in the mesopause region, continuous observations covering the whole diurnal cycle is required. It has been shown that the differences in the features bettween diurnal and nighttime mean temperature profiles, mesopause temperatures and altitudes are significant and are due to the effects of the diurnal tides and cannot be studied by nighttime data only.[35]

And, "The capability of making daytime measurements of the mesopause requires an ultra-narrow band spectral filter blocking out the solar background light."^[36], making atomic line filters perfect for this application. Image File history File links Cquote1. ... Image File history File links Cquote2. ...

See also

• Arecibo Observatory
• Fabry-Pérot interferometer
• LIDAR
• Ferromagnetic resonance
• Lyot filter
• Radiation trapping

The Arecibo Observatory is located in Arecibo, Puerto Rico on the north coast of the island. ... LIDAR (Light Detection and Ranging; or Laser Imaging Detection and Ranging) is a technology that determines distance to an object or surface using laser pulses. ... Ferromagnetic resonance, or FMR, is a spectroscopic technique to probe the magnetization of ferromagnetic materials. ... 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. ...

References

• Bloom, Scott H.; Korevaar, Eric; Chan, Victor; Chen, Irene; Rivers, Michael D.; Low, Amy. Laser communication transceiver and system. U.S. Patent Number: 5710652. Publication Date: 01/20/1998. Retrieved March 26, 2006.

• Endo, T.; Yabuzaki, T.; Kitano, M.; Sato, T.; Ogawa, T. Frequency-locking of a CW dye laser to absorption lines of neon by a Faraday filter. Retrieved April 8, 2006.

• Hedin, Jonas. LULEÅ UNIVERSITY LIBRARY. Design and Construction of a Potassium Faraday Filter for Potassium Lidar System Daytime Operation at Arecibo Observatory. Retrieved March 25, 2006.

• Korevaar, Eric J.; Menders, James H. (San Diego, CA). Voigt filter. U.S. Patent #: 5731585. Retrieved April 30, 2006.

• Kremer, Richard M., Korevaar; Eric I. Laser radar device. U.S. Patent #: 5029999. Retrieved May 8, 2006.

• Menders, J.; Searcy, Paul; Ross, David; Korevaar, Eric; Scharpf, William J.; Allocca, David. Gated blue cesium Faraday atomic line filter. Retrieved April 30, 2006.

• Molisch, Andreas F.; Oehry, Bernhard P. (1998). Radiation Trapping in Atomic Vapours. Oxford University Press: Oxford. ISBN 0198538669.

• Nelson, Wilfred H. Antibiotic susceptibility test. U.S. Patent #: 5573927. Retrieved May 8, 2006.

• Oehry, Bernard P.; Schupita, Walter; Sumetsberger, Brigitte; Molisch, Andreas F.; Magerl, Gottfried. Experimental evaluation of an atomic line filter for spaceborne application. Retrieved April 27, 2006.

• Popescu, Alexandru; Walther, Thomas.On the Potential of Faraday Anomalous Dispersion Optical Filter as High-Resolution Edge Filters. Retrieved April 27, 2006.

• Snyder, James J. DOE Scientific and Technical Information. http://www.osti.gov/bridge/servlets/purl/10166916-zTOYJR/native/10166916.pdf. U.S. Patent Number: 05202741. Publication Date: 04/13/1993. Retrieved March 26, 2006.

• Weber, Marvin J. (2003). Handbook of Optical Materials. CRC Press LLC: Boca Raton. ISBN 0849335124

• Webster's Online Dictionary. Invention: Voigt filter. Retrieved April 5, 2006.

• Zhao, Zhong-Quan. Excited state atomic line filters. Retrieved March 26, 2006.

Notes