Atomic line filter

Optical band pass filter used in the physical sciences

An atomic line filter (ALF) 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 (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. 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 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. 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. was more advanced. Voigt filters were more compact and "[could] be easily designed for use with a permanent magnet".

Atomic line filters may operate in the ultraviolet, visible and infrared regions of the electromagnetic spectrum. 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.

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".

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. 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."

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.

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. 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 (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:

A Voigt filter is a Faraday filter with its magnetic field shifted to be 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° per the Voigt effect.

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". 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; many alkali metals also have absorption lines and resonance in the desired spectra. 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. measuring the efficiencies of antibiotics and general filtering applications.

Without an atomic line filter, laser tracking and communication may be difficult. Usually, intensified charge coupled device cameras must be used in conjunction with simple dielectric optical filters (e. g. interference filters) to detect laser emissions at a distance. Intensified CCDs are inefficient and necessitate 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 more efficiently. "[Atomic line filters] with passbands of about 0.001 nm have been developed to improve the background rejection of conventionally filtered laser receivers". so space based, underwater and agile laser communications with ALFs have been proposed and developed.