Method for suppressing solar background in a receiver of laser light

Two new passive atomic filters that operate at 422.67 nm and 460.73 nm respectively are disclosed. The filter wavelengths overlap Fraunhofer lines, thereby providing outstanding sunlight rejection. The new calcium filter utilizes collisional energy transfer with Xenon to wavelength shift the violet light to 657.28 nm. An internal photon conversion efficiency of 25% was recorded. The new strontium filter utilizes collisions with noble gases to produce emission at 689.26 nm. An internal photon conversion efficiency of 45% was recorded.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to atomic filters in general and to passive atomic 
filters in particular. 
2. Description of the Prior Art 
Atomic resonance filters are optical bandpass filters that employ sharp 
atomic transitions to perform ultranarrowband (.about.0.001 nm) filtering 
of light. Further, the bandwidth is independent of aperture size and field 
of view. The properties of atomic filters make them ideally suited for the 
detection of weak laser signals against continuum backgrounds. 
Atomic filters can be divided into two classes: active and passive. 
Promising active filters include sodium, thallium, rubidium, magnesium, 
and calcium. Active devices require power sources, usually in the form of 
optical radiation, to sustain filter operation. These sources add 
complexity and cost, and may reduce reliability. In several instances, the 
absorbed optical power has been shown to contribute filter noise. 
On contrast to the large number of active filters, only two passive filter 
schemes have been developed. These filters employ rubidium and cesium 
atoms and they operate near 420 and 456 nm, respectively. Passive filters 
are advantageous for applications that stress low level light detection, 
high reliability, low cost, and design simplicity. There are several 
important shortcomings with these passive atomic filters. First, these 
filters emit at wavelengths greater than 700 nm. Few photomultiplier tubes 
are available to receive these emissions. Second, there is no inherent 
sunlight rejection. Third, the optical bandwidths cannot be pressure 
broadened to accommodate Doppler shifted signals. 
Therefore, a principal object of the present invention is to provide a 
passive filter which maximizes solar background rejection. It is also an 
object to provide a filter that emits int he red spectral region (below 
700 nm) and has a variable optical bandwidth. 
SUMMARY OF THE INVENTION 
Two new passive atomic filters are disclosed that employ a novel method of 
wavelength-shifting. Two preferred embodiments of the passive atomic 
filter are based on calcium and strontium. The calcium filter operates at 
422.67 nm and overlaps the g Fraunhofer line, at which wavelength the 
solar background is reduced by a factor of 40. The strontium filter 
operates at 460.73 nm in a Fraunhofer dip at which the solar background is 
reduced by fifty percent. Additionally, both new filters possess a single 
noise passband compared to the four noise passbands of the rubidium and 
cesium filters.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
The passive atomic filters based on calcium and strontium exhibit improved 
performance over the cesium and the rubidium filters in three ways: 
superior sunlight rejection, an output emission wavelength in the red vs 
deep red, and a pressure-broadened linewidth. The Fraunhofer overlap 
combined with its single noise passband gives the passive calcium filter a 
hundredfold theoretical sunlight rejection advantage compared to the other 
passive devices while the advantage for strontium is nearly ten fold. The 
importance of red vs deep red emission lies in fact that well-developed, 
large-area, low-noise photo-multiplier tubes are available in the spectral 
region below 700 nm. The ability to broaden the linewidth is critical for 
receiving Doppler-shifted signals from satellite transmitters. 
The method to shift the wavelength of the signal light disclosed herein is 
unique. It involves rapid collisional energy transfer induced by a 
molecular curve crossing interaction. No other atomic filter, either 
active or passive utilizes this effect to achieve wavelength-shifting. A 
comparison of the features of prior art filters and present invention is 
summarized below in Table I. 
TABLE I 
__________________________________________________________________________ 
Signal Emission 
Relative 
Wavelength- 
Wavelength 
Wavelength 
Sunlight 
Shifting 
Atom (nm) (nm) Rejection 
Method 
__________________________________________________________________________ 
Cesium 
455,459 
852,894 
0 db 
Radiative cascade 
Rubidium 
420,421 
780,795 
0 db 
Radiative cascade 
Calcium 
423 657 +22 db 
Molecular curve-crossing 
Strontium 
461 689 +9 db 
Molecular curve-crossing 
__________________________________________________________________________ 
The energy level diagram for the passive calcium and strontium filters are 
shown in FIG. 1. The values for strontium are shown in parenthesis beside 
the values for calcium. It can be modeled by a simple three level atom. 
Signal photons at the 4p.sup.1 P.sub.1 -4s.sup.1 S.sub.o resonsance 
transition are absorbed by the calcium atoms. Collisions with xenon 
populate the 4p.sup.3 P.sub.J mestastable triplet level at a rate K.sub.T. 
Atoms return to the ground level by emission of red light at the 
intercombination wavelength of 657.3 nm. The corresponding energy levels 
for strontium ar 5p.sup.1 P.sub.1, 5s.sup.1 5.sub.o and 5p.sup.3 P.sub.J 
respectively. Emission occurs at 689.3 nm. 
The generic wavelength-shifting mechanism for this filter is the 
collisionally-induced cascade process. The internal photon conversion 
efficiency, .eta..sub..phi., is the ratio of emitted red photons to 
absorbed violet photons. For the process under consideration it is given 
by the expression 
##EQU1## 
where A.sub.i is the spontaneous emission rate of the singlet level. 
With respect to the relation between .eta..sub..phi. and pressure, k.sub.T 
is proportional to pressure. As the pressure increases, .eta..sub..phi. 
rises linearly and approaches unity at high pressures. Also, the 
homogeneous component of the absorption linewidth is pressure dependent. 
Thus, we have a passive atomic filter in which .eta..sub..phi. and the 
optical bandwidth can be adjusted, though not independently, by varying 
gas pressure. 
The complete passive calcium atomic filter is composed of a calcium vapor 
cell buffered with xenon gas sandwiched between two conventional filters. 
The filter on the input side passes violet light while rejecting red 
light. The exit filter performs the reverse function. The passbands of the 
filters are mutually exclusive. A 15 cm atomic cell would need to be 
heated to between 300 and 350.degree. C. in order to be optically thick to 
signal light. The corresponding temperatures for the passive strontium 
filter are 50.degree. C. less. 
FIG. 2 illustrates a preferred embodiment of the invention. The atomic 
resonance filter 2 comprises a first optical filter 4 and a second optical 
filter 6 separated by heated cell 8. The atomic vapor 10 (calcium or 
strontium) is located in cell 8 along with buffer gas 12. Incoming light 
of wavelength .lambda..sub.s passes through the first filter 4, a short 
wavelength pass filter that has a cutoff wavelength .lambda..sub.c1. After 
absorption by the atomic vapor 10 and collisions with the buffer gas 12, 
light is emitted at wavelength .lambda..sub.o and strikes second filter 6, 
a long wavelength pass filter that has a cutoff wavelength 
.lambda..sub.c2. Light passing through the second filter at wavelength 
.lambda..sub.o strikes photomultiplier tube(s) 14. Thus, .lambda..sub.s 
&lt;.lambda..sub.c1 &lt;.lambda..sub.c2 &lt;.lambda..sub.o. 
EXAMPLE 
An experimental confirmation using calcium is outlined in J. A. Gelbwachs 
and Y. C. Chan, Optics Letters 16, 5, "Passive Fraunhofer-wavelength 
atomic filter at 422.7 nm" (1991) and is summarized below. The 
corresponding experiments have been performed on the strontium filter but 
have not yet been published. Summarizing that experiment, a cross-shaped 
stainless steel cell with sapphire viewports was used to contain the 
calcium and later the strontium vapor. The cell was electrically heated by 
surrounding heating tapes. 
A cw single-mode ring dye laser with Stilbene 3 dye pumped by the UV lines 
of an Ar ion laser excited the calcium atoms. The laser beam was expanded 
and collimated into a 1.5 cm diameter beam before entering the cell. 
Typical laser poser used was 30 mW. The optical path through the cell was 
15 cm. A GaAs photomultiplier tube was used to detect the spectrally 
resolution resolved signals, and the resulting photcurrents were amplified 
by a picoameter. 
During the conversion efficiency measurement, the laser was first tuned to 
the 4p.sup.1 P.sub.1 -4s.sup.1 S.sub.0 transition. Then photocurrents were 
recorded at the 422.7 nm and 657.28 nm settings of the spectrometer. The 
laser was then detuned from the resonance line and photocurrents at the 
two spectrometer settings were again recorded. The latter readings 
measured the background contributions due to the laser scattering and the 
dark current of the photomultiplier tube. The difference between the 
signals with the laser on-resonance and off-resonance represented the net 
emission signal. 
For the pressure-broadened absorption linewidth measurements, the laser 
frequency was scanned .+-.10 GHz across the resonance frequency. Emission 
profiles of the 422.67 nm fluorescence signal were recorded and were 
signal averaged with a digital oscilloscope. A small portion of the laser 
beam was sent into a Fabry-Perot etalon. The free spectral range of the 
etalon was 1.5 GHz and the transmission signals of the etalon were used as 
frequency markers. 
FIG. 3 shows the experimental data as expressed in terms of the internal 
photon conversion efficiency. The curve shows a monotonic increase of the 
conversion efficiency up to 25% at 650 Torr. The solid line in the figure 
is the calculated values from the least squares analysis. Experimental 
errors for the conversion efficiency as inferred from the standard 
deviations of the individual set of measurements are less than 10% of the 
conversion efficiency values. 
Also shown in FIG. 3 is the full-width half-maximum (FWHM) of the 
collisional broadened 4p.sup.1 P.sub.1 -4s.sup.1 S.sub.0 resonance 
emission as a function of xenon gas concentration. The measured pressure 
broadening coefficient for the resonance line is 13.9.+-.0.2 MHz/Torr. The 
ability to broaden the filter bandwidth to 10 GHz is useful for the 
reception of Doppler-shifted laser signals from rapidly moving 
transmitters such as those encountered onboard spacecraft. 
The corresponding results obtained for the strontium filter appear in FIG. 
4. Not that the conversion efficiency is maximized when the buffer gas is 
argon. At 700 Torr, 45% of 460.7 nm light is converted to 689.3 nm light. 
While argon is the preferred buffer gas, the filter can operate with 
reduced conversion efficiency with any of the other four noble gases, 
namely, helium, neon, krypton and xenon. This feature of the strontium 
filter operation is different for the calcium filter. For the calcium 
filter, xenon gas should be used to produce useful amounts of red light. 
Although the invention has been described in terms of two preferred 
embodiments, it will be obvious to those skilled in the art that 
alterations and modifications may be made without departing from the 
invention. Accordingly, it is intended that all such alterations and 
modifications be included within the spirit and scope of the invention as 
defined by the appended claims.