Atomic line filter

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.

[4] 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.

[7][8] 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.

[9][10] Zero spontaneous emission was already possible for x-ray and gamma ray amplifiers and Weber thought to bring this technology to the infrared spectrum.

[11] The media of these devices were crystals with transition metal ion impurities, absorbing low-energy light and re-emitting it in the visible range.

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

This means that passive filters are rarely able to work with infrared light, because the output frequency would be impractically low.

If photomultiplier tubes (PMTs) are used then the "output wavelength of the ARF should lie in a spectral region in which commercial, large-area, long-lived PMT's [sic] possess maximum sensitivity".

[16] In a passive ALF, the input frequency must correspond almost exactly to the natural absorption lines of the vapor cell.

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.

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".

In 1989, Eric Korevaar had developed his Fast ALF design which detected emitted fluorescence without photosensitive plates.

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.

These are manifest as electromagnetic radiation independent of the working processes of the filter and the intensity of the signal light.

More noise is created if the filter is designed for output in the infrared range, as most of the thermal radiation would be in that spectrum.

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.

In the original studies of atomic line filters in the 1970s and early 1980s, there was a "large overestimation of the [signal bandwidth]".

In an active ALF, optical or electrical pumping is used for exciting these atoms so they absorb or emit light of different wavelengths.

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.

[18][32] 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.

With the superior filtering system of an ALF, a non-intensified CCD may be used with a continuous wave laser more efficiently.

[3] The total energy consumption of the latter system is "30 to 35 times less" than that of the former,[35] so space-based, underwater and agile laser communications with ALFs have been proposed and developed.

A potassium Faraday filter designed, built and photographed by Jonas Hedin for making daytime LIDAR measurements at Arecibo Observatory. [ 1 ]
A graph of transmission to relative wavelength in a potassium FADOF centered at the D1 transition of 770.1093 nm . The graph is for a single polarization, so the maximum transmission is 0.5. The highlighted area is generally used as the transmission spectrum of the FADOF. No optical losses are shown.
Stark splitting in hydrogen . Energy eigenvalues of Stark shifts are shown here as a function of electric field strength.
This vector graphic depicts an abstraction of the methodology of an absorption re-emission ALF: how only a narrowband may bypass two broadband filters and create a very precise and accurate filter. Here, a careful manipulation of the frequency of incoming light may be translated into a spatial translation. A similar strategy is employed in both Faraday and Voigt filters, though in these filters, the polarization of the light is shifted and not the frequency.
Polarization of light by a Faraday filter.
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.
Drawing of the receiver end of a laser tracking system from US 5202741
Starfire Optical Range LIDAR laser.