Atomic line filter

An atomic line filter is a more effective 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.
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 that would otherwise be obscured by broadband sources, such as daylight. They are used regularly in Laser Imaging Detection and Ranging 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.

History

The predecessor of the atomic line filter was the infrared quantum counter, designed in the 1950s by Nicolaas Bloembergen. This was a quantum mechanical amplifier theorized by Joseph Weber to detect infrared radiation with very little noise. Zero spontaneous emission was already possible for x-ray 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 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 " be 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 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 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 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 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 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 are used then 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 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".
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 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.

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. 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.

Relevant phenomena

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 ". 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.

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.
Image:Faraday-effect.svg|thumb|240px|right|Polarization of light by a Faraday filter.

Faraday filter

A Faraday filter, magneto-optical filter, FADOF or EFADOF works by rotating the polarization of the light passing through the vapor cell. This rotation occurs near its atomic absorption lines by the Faraday effect and anomalous dispersion. 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 of the vapor in the cell. This relationship is represented the following equation: