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Optics Letters

4227-nm atomic filter with superior solar background rejection

Jerry A. Gelbwachs

Published Feb 15, 1990• 11 citations
Publisher

Abstract

A novel atomic resonance filter with powerful solar background rejection is proposed. The filter wavelength matches the g Fraunhofer line at 422.7 nm. Atomic filtering occurs within the singlet manifold of neutral calcium. Superior sunlight discrimination arises mainly from two factors: overlay with an intense Fraunhofer line and signal levels devoid of fine and hyperfine structures. The new filter should exhibit high throughput and low noise.

DOI
10.1364/ol.15.000236

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Optics Letters•Jul 15, 1989•19 citations
A novel atomic resonance filter with powerful solar background rejection is proposed. The filter wavelength matches the g Fraunhofer line at 422.7 nm. Atomic filtering occurs within the singlet manifold of neutral calcium. Superior sunlight discrimination arises mainly from two factors: overlay with an intense Fraunhofer line and signal levels devoid of fine and hyperfine structures. The new filter should exhibit high throughput and low noise.
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 demonstration of the first magnetooptic atomic filter that overlays a strong solar Fraunhofer line is reported. Compared with alkali magnetooptic filters, this filter enjoys a large reduction in solar interference and a significant decrease in the number of noise passbands. The filter utilizes the strong Ca(4p/sup 1/P/sub 1/-4s/sup 1/S/sub 0/) transition at 422.7 nm. Under the weak magnetic field experimental conditions, a maximum transmission efficiency of 55% and a symmetrical double-peaked transmission spectrum with 1.5 GHz wide passbands were observed. The filter's frequency response was measured with a laser intensity modulation technique. No falloff was observed at 176 MHz, the highest frequency available with the apparatus. Calculations indicate that further improvements in filter performance can be achieved by optimizing the magnetic field and the cell temperature.>
The atomic resonance filter (ARF) is art ultra- high-Q (105–106) optical filter that is isotropic and insensitive to environmental factors and possesses a 180° acceptance angle.1 We are developing a novel ARF for satellite applications. Our filter operates in a Fraunhofer dip in the solar spectrum, thereby offering the key advantage of reduced solar background noise. Other important operational characteristics of the filter are: a projected conversion efficiency of blue-green light into the wavelength-shifted output light of ~90%, a 10-ns response time, and an output in the near UV, a spectral region that is efficiently detected by commercial large-area long-lived photomultiplier tubes.
An optically pumped active Ca atomic resonance filter is demonstrated, applicable to background-limited optical communications through scatter channels. In pump saturation, the filter should detect 50% of the incident 423-nm signal power with an internal photon gain of 6 and a response time of 10 micros, 2 orders of magnitude faster than a passive Ca filter. Response time of 100 micros has been demonstrated, limited by available pump power. The filter maintains the wide field of view and reduced solar background associated with atomic absorption at the Ca Fraunhofer line while permitting higher data rate communications.