An atomic line filter (ALF) is an advanced optical band-pass filter used in the physical sciences for filtering electromagnetic radiation with precision, accuracy, and minimal signal strength loss. Atomic line filters work via the absorption 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 amplifiers; 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) and are being studied for their potential use in laser communication systems. Atomic line filters are superior to conventional dielectric optical filters such as interference filters and Lyot filters, 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$15,000 per unit.
The media of these devices were crystals with transition metal ion 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 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 “[could] be easily designed for use with a permanent magnet”. By 1996, Faraday filters were being used for LIDAR.
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.
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.
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.
Atomic line filters 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 or absorbed by imperfect lenses, filters and windows.
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.
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 of the target absorption line may absorb incorrect frequencies of light and pass them through to the output sensor.
Radiation trapping 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 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 fields may alter other properties of the filter (i.e. shifting the passband).
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 (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:
Preceding an atomic line filter may be a collimator, 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 photomultipliers, which are more efficient, may be used instead of a PMT.
Most ALF vapor cells use alkali metals 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. Note that non-metallic vapors such as neon 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.
[Atomic line filters] are ideally suited for applications in which weak laser signals are detected against a continuum backgroundAtomic 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 antibiotics and general filtering applications.
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.
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