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Proceedings of SPIE, the International Society for Optical Engineering/Proceedings of SPIE

A 535 nm Active Atomic Line Filter Employing The Thallium Metastable State As An Absorbing Medium

C. S. Liu, P. J. Chantry, C. L. Chen

Published Jan 30, 1987• 7 citations
Publisher

Abstract

A new kind of active atomic line filter (ALF) using the thallium metastable state as an absorption medium is discussed. The basic principle underlying the operatiqn of a Tℓ-ALF is based on photodissociative excitation of TℓCℓ, or transfer excitation of Tℓ(2P3/2) in Tℓ-Cs mixtures. The operating wavelength of a Tℓ-ALF is at 535 nm which matches the wavelength of a frequency-doubled Nd-Bel laser. The results of modeling calculations and related experimental studies are presented.

DOI
10.1117/12.937287

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Stimulated emission of the 535 nm line of thallium was observed when a Tl-He mixture was pumped optically in the violet wing of a pressure-broadened resonance line of thallium at 377.6 nm by a recombination He–Ca laser emitting at λ = 373.7 nm. Pumping resulted from the absorption transitions between repulsive terms of the Tl-He quasimolecule. The average stimulated radiation power reached 5 mW when the pulse repetition frequency was 6 kHz. The efficiency of conversion of the useful part of the pump pulse was 45%.

Soviet Journal of Quantum Electronics•Feb 28, 1984•5 citations
A new kind of active atomic line filter (ALF) using the thallium metastable state as an absorption medium is discussed. The basic principle underlying the operatiqn of a Tℓ-ALF is based on photodissociative excitation of TℓCℓ, or transfer excitation of Tℓ(2P3/2) in Tℓ-Cs mixtures. The operating wavelength of a Tℓ-ALF is at 535 nm which matches the wavelength of a frequency-doubled Nd-Bel laser. The results of modeling calculations and related experimental studies are presented.
The major efforts of this project were: (1) to reassess the technical merits of a Tantalum atomic line filter (ALF) employing a low vapor density T1C1 and (2) to measure the excited state cesium trapping lifetimes and to derive the self quenching cross sections of excited state cesium atoms. These two separate efforts address alternative approaches to the implementation of an ALF based on metastable T1 atoms, T1 sub m, whose absorption wavelength (535 nm) matches the frequency doubled Nd:Bel laser. The first approach uses photodissociation to produce T1 sub m directly from T1C1. The second approach involves resonantly pumping the Cs in a T1-Cs mixture and collisionally transferring energy to the T1 to sustain the required density of T1 sub m.
Atomic line filters take advantage of the sharp spectral features offered by electronic transitions in free atoms. The basic idea is to convert a narrow atomic absorption profile into an equally narrow optical transmission filter by detecting the fluorescence radiation that is emitted after a photon has been absorbed. We developed the metastable thallium atomic line filter from the basic physical idea to a laboratory prototype in order to evaluate the usefulness of the technology for satellite borne backscatter LIDARs. Atomic thallium provides for an unsurpassed combination of features for this purpose. The input wavelength of the filter is 535 nm, which matches the frequency doubled Nd:BEL laser. The filter is an active device, it upconverts the 535 nm input into 378 nm output that is detected by a PMT. The experimental results were as expected concerning the characteristics directly related to atomic properties, i.e. the filter has only 10 ns response time, its optical bandwidth is 0.004 nm, and its acceptance angle is only limited by the device geometry. An optimized setup, with the size constraints of a space-borne system, displays a total quantum efficiency of 2%, i.e. from input photons to detected output photons. This constitutes a remarkable and unsurpassed value for atomic line filters but is too low for the application in mind. (A ground-based, scaled-up version of our prototype would reach about 10% quantum efficiency). In addition, the 500 degree(s)C operating temperature of the vapor cell requires sophisticated thermal layout and thermal shielding, which means a lot of added mass and volume. In summary, we found that atomic line filters can be built that offer characteristics not found with other technologies, but their applicability for space borne systems is questionable.
We present a new atomic line filter concept for the detection of frequency-doubled Nd:BEL laser radiation. Advantage is taken of the thallium 6(2)P(3/2) metastable energy level whose large lifetime can be preserved even in small vapor cells. The filter is pumped intermittently by a low-power spectral lamp and thereby combines the benefits of active and passive operation. Analysis of a proposed final design shows that simple engineering solutions exist, and excellent filter properties can be deduced. We have experimentally demonstrated the new operating principle in a preliminary setup.
A new atomic filter is proposed. It operates at 534.9 nm. The new filter offers two important advantages for openchannel laser communications. First, its wavelength matches the output of the frequency-doubled Nd:BEL laser. Second, the filter overlays a solar Fraunhofer line, thereby offering intrinsic sunlight suppression. Ultranarrowband optical filtering takes place within the singlet manifold of neutral-calcium vapor. Excellent filter properties are deduced from spectroscopic examination of the atomic system. A novel feature of the new filter is the potential for greater-than-unity internal photon conversion.
Stimulated emission of the 535 nm line of thallium was observed when a Tl-He mixture was pumped optically in the violet wing of a pressure-broadened resonance line of thallium at 377.6 nm by a recombination He–Ca laser emitting at λ = 373.7 nm. Pumping resulted from the absorption transitions between repulsive terms of the Tl-He quasimolecule. The average stimulated radiation power reached 5 mW when the pulse repetition frequency was 6 kHz. The efficiency of conversion of the useful part of the pump pulse was 45%.