Jump to content

Atomic line filter

From Wikipedia, the free encyclopedia

This is an old revision of this page, as edited by Zbxgscqf (talk | contribs) at 19:10, 18 June 2006 (→‎LIDAR: missing ref). The present address (URL) is a permanent link to this revision, which may differ significantly from the current revision.

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.

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]

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[1]. 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.[3]

History

The first type of atomic line filter worked by the absorption-re-emmission principal. This design was primitive and suffered from low quantum efficiency and slow response time. Atomic line filters were improved greatly when Faraday filters were developed, which happened sometime before 1972. The Faraday filter design greatly improved "the performance" of previous atomic line filters[1]. By 1996, ALFs were being used for LIDAR.

The Voigt filter, patented by James H. Menders and Eric J. Korevaar on August 26th, 1992,[4]was as an improvement to the Faraday filter design; advantages of this design include increased compactness and Voigt filters, "could be easily designed for use with a permanent magnet."[1]

Qualities

Positive

  • They may continue operation if jostled.[5]
  • They have relatively wide fields of view (around 180°).[6]
  • The exact parameters (such as temperature, magnetic field strength, length, etc.), may be tuned to a precise implementation. However, these values must be calculated by a computer due to the extreme complexity of the system.[7]
  • They are very efficient (>50%)[citation needed]. This is partially because the, "crossed polarizers ... serve to block out background light with a rejection ratio better than 10-5" [8]. The passband of a typical Faraday filter may be a few Ghz.[5]

Negative

  • The target frequency must be near absorption lines of the vapor cell.[5]

Common components

File:FaradayFilter.png
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.
File:ALFConcept.png
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[9].

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.

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[10]. Three common vapor cell materials are sodium, potassium and cesium. Note that non-metallic vapors may be used, though: neon has been used in Faraday filters[11].

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[12].

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.

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.

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."[8].

Types

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[13]. A low-pass filter blocks radiation above the frequency of the fluorescence light.

However, "for measurement of windspeed these filters have two drawbacks, slow response time (about 500 ns for the alkali atoms) and low quantum efficiency"[14].[1] This was improved by the "Fast ALF" design of Eric Korevaar in 1989 which detected emitted flourescence without photosensitive plates[1].

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[15]. This effect is generally related to and enhanced by the Zeeman Effect[5], or the splitting of atomic absorption lines in the presence of the magnetic field[16][17]. Light at the resonant frequency of the vapor is exits an 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 through which the light must pass and the Verdet constant of the vapor in the cell. This relationship is represented the following equation:

[18]

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[10]. 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)[8].

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[5], measuring the efficiencies of antibiotics[19] and general filtering applications.

Atomic line filters compete with etalons, another high-end optical filter. Faraday filters may be cheaper to implement at approximately $20,000 per unit[15] [20].

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"[1]. The total energy consumption of the latter system is "30 to 35 times less" than that of the former[21].[10]

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[22]. 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.

This is a valuable faculty for meteorologists and climatologists, for:

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.[3]

And, "The capability of making daytime measurements of the mesopause requires an ultra-narrow band spectral filter blocking out the solar background light."[3], making atomic line filters perfect for this application.

See also

Citations

  1. ^ a b c d e f g Korevaar & Menders 1998
  2. ^ Oehry, Schupita & Sumetsberger 1994
  3. ^ a b c Hedin 2002, p. 8
  4. ^ Webster's 2006
  5. ^ a b c d e Popescu & Walther 2004, p. 1
  6. ^ Weber 2003, p. 467
  7. ^ Hedin 2002, p. 26
  8. ^ a b c Bloom, Korevaar & Chan 1998
  9. ^ Hedin 2002, p. 24
  10. ^ a b c Zhao 2005
  11. ^ Endo, Yabuzaki & Kitano 1978
  12. ^ Menders, Searcy & Ross 1993
  13. ^ Molisch & Oehry 1998, p. 361
  14. ^ Kremer & Korevaar 1989
  15. ^ a b Friedman 2005
  16. ^ Hedin 2002, p. 25
  17. ^ Fitzpatrick 2002
  18. ^ Hedin 2002, p. 16
  19. ^ Nelson 1996
  20. ^ Billmers, Contarino & Allocca 1996
  21. ^ Snyder & Walther 1993, p. 4-8
  22. ^ Sherman 2005

References


Retrieved from "https://en.wikipedia.org/w/index.php?title=Atomic_line_filter&oldid=59312758"