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

Experimental evaluation of an atomic line filter for spaceborne application

B. P. Oehry, Walter Schupita, Brigitte Sumetsberger, Andreas F. Molisch, Gottfried Magerl

Published Dec 9, 1994• 0 citations
Publisher

Abstract

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.

DOI
10.1117/12.195873

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•Nov 26, 1998•0 citations
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.
When optical signals are embedded in background noise, narrowband optical filters are needed for receivers to achieve quantum limited performance. Such is typically the case with daytime optical communications and lidar. Narrowband filtered detectors. with acceptance bands of just a few picometers (pm) and high out of band rejection are well suited for these applications. The main thrust of this program has been the development of two means for narrowband detection: the Fast Atomic Line Filter/ Field Ion Detector (FALF/FID) and the Faraday magneto-optic filter. The FALF/FIDs transduce optical radiation within a narrow band directly into electrical signals. They are laser pumped filters based on the spectroscopy of atomic vapors. We designed, built and tested an infrared FALF/FID with an rise time of under 10 ns and a quantum efficiency of >25%. Faraday filters are imaging magneto-optic filters with passbands of a few pm. Faraday filters with passbands in the visable and the near infra-red were constructed and tested. Transmission spectra were measured, and found to agree closely with theoretical predictions. Both of these narrowband detection schemes must be used in conjunction with a tuned narrowband laser source. We developed a frequency locked alexandrite laser designed to match the acceptance band of a rubidium based filter. Finally, a preliminary demonstration of a laser interrogator using a tuned infrared diode laser in conjunction with a filtered receiver was demonstrated. Atomic line filter, Alkali metal vapor, Faraday filter, Active optical filter, Laser pumped filter, Magneto-optic filter, Laser communications.
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.
A narrow-band, gate-able matched optical filter with large extinction ratio and high transmission is required for ocean lidar applications at wavelength of 532 nm. Two types of filters, the induced dichroism excited-state atomic line (IDEAL) and excited state anomalous Faraday dispersion optical filter (ESFADOF), are studied. They are operating on the 4P1/2 to 8S1/2 excited states transition line of potassium vapor and a narrow band at 532 nm. The measured critical features of the IDEAL filter are: 1.3 GHz (i.e..01A) narrow pass band, approximately 45% peak transmission, high extinction coefficient in the order of 10-5, very fast rise time (<3ns) and a 15 ns decay time determined by resonant enhanced two photon transition. The physical mechanism of the IDEAL filter operation is explained in terms of resonant enhanced two photon transition. Good agreement between experiment and the derived theoretical model is achieved. The pertinent features of the excited (ESFADOF) filter were measured to be: approximately 15% peak transmission, high extinction coefficient in the order of 10-5, very fast rise time (<3ns), and a decay time in the tens of microsecond range governed by resonant photon trapping. The physical processes contributing to the ESFADOF filter action are resonant photon trapping and Faraday anomalous dispersion. The experimental results confirmed theoretical predictions. The applications of these filters are discussed in terms of improving the lidar system performance. Specifically, the narrow bandwidth of these filters eliminates the background noise caused by stray solar radiation and thereby enhances the dynamic range of the lidar system. Furthermore the fast efficient gating of the filters minimizes reflection from the ocean surface, which is several orders of magnitude higher than the return signal from underwater targets.
Abstract In free-space optical information transmission and in LIDAR systems, very narrow band filtering is often needed to eliminate broad-band solar background noise. Ideally, such filters should have a bandwidth on the order of a few hundred MHz to a few GHz. It is very difficult to produce such narrow-band filters by conventional means, but we know that atomic absorption lines have just that bandwidth. An atomic vapour cell actually is an excellent band-block filter. In order to convert it to a bandpass filter, we just have to find a way to determine whether absorption has taken place.