Atomic line filter
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An atomic line filter is a device in a class of advanced optical filters. Atomic line filters are used in the physical sciences for filtering out background light with great precision, accuracy and efficiency. ALFs are thus narrow-band band-pass filters. Atomic line filters work with atomic vapors, depending upon specific absorption or resonance lines of the vapor cell in the filter. One may also be known as an ALF, atomic resonance filter or ARF.
The three major implementations of atomic line filters include absorption-re-emission ALFs, Faraday filters and Voigt filters. Note that the term "absorption-re-emission" was retroactive and only partially established, so the phrase "atomic line filter" might mean only that absorption type. In contrast, "Faraday filter" and "Voigt filter" are almost always used when one is describing those types. While all atomic line filters use different effects and designs for the specific implementation, the same basic strategy is always employed. This involves 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.
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). Because ALFs are very effective at doing this, they have filled a niche in the field of optical filtering: they are used regularily in Laser Imaging Detection and Ranging (LIDAR) and are being studied for their potential in the field of laser communication systems. Compared to more "conventional" dielectric optical filters such as interference filters and Lyot filters, ALFs are superior, but so much more complex that their use is only warranted in background-limited detection. Compared to etalons, another high-end optical filter, Faraday filters are significantly more sturdy and may be cheaper at around $20,000 per unit.
- 1 Types
- 2 Properties
- 2.1 Input/Output
- 2.2 Response time and transmission rate
- 2.3 Effectiveness
Types
Absorption-re-emission
An absorption-re-emission ALF absorbs the desired wavelength of light and undergoes fluorescence, emitting light which bypasses other, broad band filters. In passive absorption-re-emission ALFs, a high-pass filter blocks all low-energy incoming light. 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-emits the signal light by undergoing fluorescence at a lower frequency. In an active ALF, optical or electrical pumping is used for exciting these atoms so they may absorb or emit light of other wavelengths A low-pass filter blocks radiation above the frequency of the fluorescence light.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 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. This effect is generally 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 through which the light must pass and the Verdet constant (which is dependent on temperature, wavelength and sometimes field-intensity) 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 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°, instead of a Faraday rotator, by the Voigt Effect (magnetic birefringence).Properties
A technical definition of an atomic line filter is as an "ultra-narrow-band, large-acceptance-angle, isotropic optical filter". "Ultra-narrow-band", as described above, defines the very thin band or range that an ALF may accept; an ALF generally has a passband on the order of .001 nanometer. That atomic line filters also have wide acceptance angles (near 180°) is another important characteristic of the devices.These optical filters have important properties which define them and their place in the world of scientific research. Again, while each implementation of an atomic line filter is unique, some qualities of all of them are consistent and define the parent category discussed here.
The exact parameters (such as 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. In plain absorption-re-emission ALFs, the frequency of light must be shifted in order for the filter to operate, and in a passive device, this must always fall by the First Law of Thermodynamics. This means that passive filters are rarely able to work with infrared light, because the output frequency would be too low. Also, if photomultiplier tubes are used the, "output wavelength of the ARF should lie in a spectral region in which commercial, large-area, long-lived PMT's 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 photocathodes 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 excited so that it will absorb other frequencies of light.
Response time and transmission rate
Another important property of an absorption-re-emmission atomic line filter is the response time of the system, because this above all else dictates the transfer speed of information gathered from the light source. The response time of such an ALF, in turn, 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".Many methods of decreasing the response time of ALFs have been developed. Even in the late 1980s, certain gases could have been used to catalyze the decay of the electrons of the vapor cell. In 1989, too, Eric Korevaar had developed his Fast ALF design which detected emitted flourescence without photosensitive plates. With such methods employed, gigahertz frequencies are easily attainable.
Effectiveness
Efficiency
Atomic line fliters are inherently very efficient filters, generally classified as "ultra-high-Q" as their Q factor 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 by imperfect lenses, filters and windows.Band-pass
The width of the band-pass of an atomic line filter is usually equal to the Doppler profile of the vapor cell; this naturally the range of frequencies at which a vapor cell will be excited by any pure light source. The Doppler profile is the width of the spectrum of Doppler shifted radiation emitted by the vapor cell due to the thermal motion of its atoms. This value is less for larger atoms at lower temperatures, a system considered more ideal.There are, however, 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, for laser tracking of an object with a changing velocity that produces Doppler shifts of its own beyond the Doppler profile of the atomic vapor. For this application, the band-pass of the ALF would have to include frequencies greater and less than the maximum and minimum values for the light shifted off of the moving object. The method for this involves placing an inert gas in the vapor cell. This gas both widens the spectral line and increases the transmission rate of the filter as a whole.
Sources of noise
For all of their efficiency, atomic line filters are not perfect; there are many sources of error, of "noise", in a given system that 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 itself and may happen to be within the bandpass of the second broad band filter. This situation is worsened if the filter is designed for output in the IR range as most of the radiation would be in that spectrum. Also, this heat radiation may stimulate the vapor and cause it to create the radiation it is trying to detect in the first place. Other error 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 of the target absorption line may absorb incorrect frequencies of light and pass them through to the output sensor.Others
Radiation trapping
Radiation trapping is one of the most important phenomena occurring in an atomic line filter as it 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]" of the ALFs. Later, radiation trapping was studied, analyzed and ALFs were optimized to account for it.Stark shifts and Zeeman splitting
In all atomic line filters, the position and widths of the vapor cell resonance lines are among the most important properties. By two physics phenomena, the Stark effect and Zeeman splitting, the base absorption lines may be split into finer lines. Analysis of these phenomena allows for manipulation of electric and magnetic fields in order to alter other properties of the filter (ie. shifting the passband); "Stark and Zeeman tuning... can be used to tune the detector" .History
The predecessor of the atomic line filter was Nicolaas Bloembergen's infrared quantum counter of the 1950s. This in turn, was designed as a quantum mechanical amplifiers theorized by Joseph Weber to compete with masers. Masers have an "inherent limiting noise temperature of hv/k due to spontaneous emission". and an amplifier of microwaves with theoretically nonexistant spontaneous emissions (and therefore less noise) was desired. Zero spontaneous emission was already possible for x-ray and gamma ray amplifiers; Weber thought to bring this technology to microwaves. Bloembergen described such a device and dubbed it an "infrared quantum counter". These devices worked with transition metal ions embedded in crystals that absorbed light and reemitted it in the visible range. This up-conversion of frequencies was another powerful properties of the proto-atomic line filters, and by the 1970s, atomic vapors were used in atomic vapor quantum counters for detection of infrared electromagnetic radiation. For this and other applications, atomic vapors were far superior to the solid metallic salts previously used.The principles hitherto employed in microwave and infrared amplification were put together into a passive sodium ALF working at a wavelength of 2.34 micrometers. This design and those that immediately followed it were primitive and suffered from low quantum efficiency and slow response time. As this was the original design for ALFs, many sources 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 ALF.
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 26th, 1992, is more advanced. Voigt filters are more compact; and now ALFs, "could be easily designed for use with a permanent magnet." By 1996, atomic line filters were being used for LIDAR.
Common components
Collimator
Preceding an atomic line filter is a collimator, which straightens incident light rays for passing through the rest of the filter consistently.First conventional filter
In an absorption-re-emission atomic line filter, after the collimator comes a high-pass filter by which almost half of the incoming light (that of too long a wavelength), is either absorbed or deflected.In Faraday and Voigt filters, the first polarizing plate is used here.
Vapor cell
One thing common to all atomic line filters is the vapor cell, the component that follows the high-pass filter. While every implementation of each kind of ALF is different, the construction of the vapor cells in each is relatively similar. 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. 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 or ceramic is used as the vapor is usually kept at temperatures upwards of 100°C.Most ALF vapor cells use alkali metals because of their high vapor pressures; most also have absorption lines and resonance in the desired spectrums. Three common vapor cell materials are sodium, potassium and cesium. Note that non-metallic vapors may also be used: neon has been used in Faraday filters. As the early quantum counters used solid state transition and rare earth metal ions in crystals, it is concievable that such a medium could be used in the ALFs of today. This is presumably not done because of the vast superiority of atomic vapors in this capacity.
Second conventional filter
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 components
Other systems may be used in conjuction with the rest of an atomic line filter for practicality's sake. 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 ... 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 that it may be usable.Applications
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 weak, narrowband signals; however, they may be used for edge detection, measuring the efficiencies of antibiotics and general filtering applications.
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 simple dielectric optical filters (eg. interference 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 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". 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 have been proposed and developed.
LIDAR
With the ability to effectively track weak laser signals, comes the ability to do LIDAR. However, most LIDAR implementations cannot operate during the daytime due to sunlight which drowns out the laser signals "to study the thermal structure, diurnal/semi-diurnal tides, and seasonal variations in the mesopause region".Thus, an "ultra-narrow band spectral filter blocking out the solar background light" is needed. Atomic line filters are perfect for this application, so for the past decade, Faraday filters have been used to do this.
LIDAR entails firing lasers at desired portions of the atmosphere where some of the light is backscattered. 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 these properties may be, "significant... and cannot be studied by nighttime data only".
