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Home , Atomic line filter, Polarizer

US007058110B2

(12) United States Patent (10) Patent N0.: US 7,058,110 B2 Zhao et a]. (45) Date of Patent: Jun. 6, 2006

(54) EXCITED STATE ATOMIC LINE FILTERS 6,009,111 A * 12/1999 CorWin et a1...... 372/32

(75) Inventors: Zhong-Quan Zhao, San Diego, CA * Cited by examiner (US); Michael Joseph Lefebvre, Del Mar; _CA (Us); Damel H‘ Leshe’ Primary ExamineriMinsun Oh Harvey Enclmtas’ CA (Us) Assistant ExamineriDung (Michael) T. Nguyen

( 73 ) A sslgnee' : CIZXGJSH)T E t erpnses' C Orp . ’ S an D'lego’ (74) Attorney, Agent, or Firmilohn R. Ross

( * ) Notice: Subject to any disclaimer, the term of this patent is extended or adjusted under 35 (57) ABSTRACT U.S.C. 154(b) by 364 days.

( 21 ) A PP 1, N0.: 10/682 , 567 An excited state atomic line ?lter. The P resent invention solves the problem of lack of ground state resonant lines in 22 Filed: Oct. 9, 2003 at Wavelen g ths substantiall y lon g er than those of visible . Atomic line ?lters of the Faraday or Voigt crossed (65) PI‘iOI‘ PllbliCatiOIl Data type are provided in Which alkali atomic Us 2005/0078729 A1 Apr 14 2005 vapor in a vapor cell is excited With a pump beam to an ' ’ intermediate excited state Where a resonant absorption line, (51) Int_ CL at a desired , is available. A magnetic ?eld is Hols 3/22 (200601) applied to the cell producing a rotation for (52) U 5 Cl 372/56, 372/32 polarized light at near the resonant absorption (58) Fi'el'd 0 """ " ’ 372/56 lines. Thus all light is blocked by the cross except See application ?le for comple light near one of the spaced apart resonant lines. However, ' the polarization of light at certain Wavelengths near the (56) References Cited resonant is rotated in the cell and therefore passes through the output polarizer. U.S. PATENT DOCUMENTS 5,731,585 A * 3/1998 Menders et a1...... 250/382 11 Claims, 14 Drawing Sheets

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U.S. Patent Jun. 6, 2006 Sheet 3 0f 14 US 7,058,110 B2

Pump Wavelength I Power: 852.112 nm (6 i!S1,.‘, - 6 2PM) I ~10 W/cm2 Cs Cell 1 Length: 120 C“! 1 cm Magnetic Held: 80 Gauss M <—— ~100%transmissio1|

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F/G. 3F? F/6.- 35 /D0 027.0 mwm\(QM W. WMWNMI m /\ (Wm) A \I/ F/6. 3 c l F/é. 3,0 U.S. Patent Jun. 6, 2006 Sheet 6 6f 14 US 7,058,110 B2

15/6, 35' U.S. Patent Jun. 6, 2006 Sheet 7 0f 14 US 7,058,110 B2

Rb at 1529.3 nm

VPiezo-Pump = 47.0 Volt ~

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Rb vapor temperature: 120 "C Rb cell length: 1 cm Strength: 140 Gauss Pump laser Wavelength: 780.24 nm (vaccum) Pump power: 10.8 mW Pump laser effective area: 0.778 mm2 /C/ 4 U.S. Patent Jun. 6, 2006 Sheet 8 0f 14 US 7,058,110 B2

30 OH Absorption Peak Flrst 2. Window Window 1‘39 m

2 8%:55E52 1

Rayleigh Scattering 0.5

0 1000 1 400 1600 wavelength (nm) FIG, 5' U.S. Patent Jun. 6, 2006 Sheet 9 0f 14 US 7,058,110 B2

7-76.61!) U.S. Patent Jun. 6, 2006 Sheet 10 0f 14 US 7,058,110 B2

90

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SIGNAL l A’ SIGNAL FREOU ENCY FREOUE NCY Hg. 78 Fig. 7A Fig. 7C

RECEIVER FAHADAY : E FILTER TRANSMISSION ; L = I cm (FOR FILTERING I : B = so a Fig 8A BEACON BEAM AT . : T=121 C - RECEIVING : : TRANSCEIVER) i 5 i I VOIGT EFFECT E i E 1 ‘ cm STABILIZATION FILTER I ; g = _ TRANSMISSION : I : B = em Go FIg. as (FoR LOCKING TRANSMIT : _- . T = 102.5 c BEAM) i I i l - t

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I TRANSMISSIONREFERENCE CELL 92-16“?' ‘ (Cs ABSORPTION 0.22 nm Fig. 8C PEAKS No MAGNETIC + FIELD} OPTICAL L (INCREASING TO LEFT) U.S. Patent Jun. 6, 2006 Sheet 11 0f 14 US 7,058,110 B2

9

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1 10 Wavelength (pm) U.S. Patent Jun. 6, 2006 Sheet 12 0f 14 US 7,058,110 B2 U.S. Patent Jun. 6, 2006 Sheet 13 0f 14 US 7,058,110 B2

F/ 6, m A U.S. Patent Jun. 6, 2006 Sheet 14 0f 14 US 7,058,110 B2 US 7,058,110 B2 1 2 EXCITED STATE ATOMIC LINE FILTERS ?lters described in the above referenced patents utiliZe alkali such as cesium and rubidium to produce these metal The present invention related to optical ?lters and espe vapors. These metals are preferred because their vapors may cially to atomic line ?lters (ALF). be produced at relatively loW . HoWever, good absorption lines from these vapors are generally BACKGROUND OF THE INVENTION in the visible and the near visible spectral region such as 780 nm and 852 nm. Atomic line ?lters are a class of optical ?lters Which have Many optical symptoms operate at Wavelengths substan acceptance bandwidths on the order of 0.001 nm. In one tially longer than the visible and near visible. A good prior art type of ALF, broadband light containing narroW example is light With Wavelengths in the range of 1.5 band signal light is passed through a ?rst color glass ?lter micron. For example, ?ber optic communication is typically Which cuts off light at Wavelengths beloW a threshold value. at Wavelengths in the range of about 1.2 micron to about The signal and remaining noise light enter an atomic vapor 1.65 microns (see FIG. 5). Typical long haul ?lter that only absorbs the signal light Within the atom’s 0.001 nm operate Within a C or L band. (C band is 1520 nm to 1570 acceptance bandWidth thereby exciting those absorbing nm, and L band is 1570 nm to 1620 nm). Shorter range ?ber atoms to an intermediate energy level. A pump beam further optics requiring higher quality ?ber optics may operate excites those atoms to a second, higher energy level that then Within an s band (13141.48 microns). Also, there is a need decays through various processes including ?uorescence, to for ?lters at even longer Wavelength such as about 1.5 to 5 the ground state of the atom. The emitted ?uorescence microns for laser tracking and for free space laser commu occurs at Wavelengths beloW the threshold value of the ?rst 20 nications. Applicants have searched for good lines color glass ?lter. A second color glass ?lter then cuts off any in the alkali metals at these Wavelengths Without success. Wavelengths above the threshold which effectively permits passage of only the emitted narroWband ?uorescence. In SUMMARY OF THE INVENTION effect, the incoming signal has been internally shifted in Wavelength by the atomic vapor, Which then alloWs the use 25 The present invention solves the problem of lack of of tWo overlapping color glass ?lters to block any back ground state resonant lines in at Wavelengths substantially ground radiation. longer than those of visible light. Atomic line ?lters of the Another type of prior art atomic line ?lter takes advantage Faraday or Voigt crossed polariZer type are provided in of either the or the Voigt effect where an Which alkali metal atomic vapor in a vapor cell is excited atomic vapor in a magnetic ?eld produces polarization 30 With a pump beam to an intermediate excited state Where a rotation in order to pass a narroW spectral band of light resonant absorption line, at a desired Wavelength, is avail through tWo crossed polariZers. These ?lters are known able. A magnetic ?eld is applied to the cell producing a respectively as Faraday ?lters or Voigt ?lters. An important polariZation rotation for polariZed light at Wavelengths near use for these ?lters is to block background light so that a the resonant absorption lines. Thus all light is blocked by the beacon laser beam can be detected by a Wide ?eld-of-vieW 35 cross polariZers except light near one of the spaced apart detector. resonant lines. HoWever, the polarization of light at certain Operational principles of Faraday ?lters can be under Wavelengths near the resonant is rotated in the cell and stood by reference to FIGS. 7AiC. Crossed polariZers 90 therefore passes through the output polariZer. and 91 serve to block out background light With a rejection ratio better than 10's. Because these polariZers only Work 40 BRIEF DESCRIPTION OF THE DRAWINGS over a limited Wavelength region in the , a broad band interference ?lter may be used in conjunction With the FIG. 1A ShoWs a laboratory demonstration of the present Faraday ?lter. BetWeen the polariZers, an atomic vapor invention With a cesium cell. (Which in many of these ?lters is cesium or rubidium) in a FIG. 1B ShoWs a laboratory demonstration of the present magnetic ?eld axially aligned With the path of the beam, 45 invention With a rubidium cell. rotates the polariZation of the laser signal by 90°, While FIG. 2 Is a transmission curve at approximately 1469.49 leaving background light at other Wavelengths unrotated, nm With the cesium cell. and thus blocked by the polariZers. FIG. 3 Is a draWing of a preferred embodiment of the In the case of the Faraday ?lter the magnetic ?eld is present invention. applied in the direction of the signal beam, and in the case 50 FIG. 3A ShoW Wavelengths locking features using dual of the Voigt ?lter the magnetic ?eld is applied perpendicular etalons. to the signal beam direction and at 45 degrees to the FIG. 3B ShoW Wavelengths locking features using dual direction of each of the tWo cross polariZers. etalons. Prior art atomic line ?lters patents issued to co-Workers of FIG. 3C ShoW Wavelengths locking features using dual applicant includes US. Pat. Nos. 4,983,844; 5,267,010; 55 etalons. 5,502,558; 5,731,585 and 6,151,340 each of Which are FIG. 3D ShoW Wavelengths locking features using dual incorporated herein by reference. The ’844 patent discloses etalons. a fast atomic line ?lter Which utiliZes a pump laser and a high FIG. 3E ShoWs expected transmission curve for the FIG. voltage potential to produce pairs from atoms excited by 3 embodiment. photons With Wavelengths corresponding to a resonant fre 60 FIG. 4 ShoWs actual transmission curve at 1529.3 nm for quency. The other patents describe applications of Faraday the rubidium cell laboratory demonstration. and/or Voigt ?lters. FIG. 5 ShoWs Wavelength bands in Wide use for ?ber One problem With atomic line ?lters such as those optic communication. referred to above is that their operation depends on the FIG. 6A ShoW some atomic transmission for cesium and existence of a good sharp resonant absorption line near the 65 rubidium. spectral range to be ?ltered. Many of these sharp resonant FIG. 6B ShoW some atomic transmission for cesium and absorption lines are characteristic of atomic vapors and the rubidium. US 7,058,110 B2 3 4 FIG. 7A ShoW features of a prior a Faraday ?lter. Wavelength Locking FIG. 7B ShoW features of a prior art Faraday ?lter. To fully utilize the excited atomic line ?lters of the present FIG. 7C ShoW features of a prior art Faraday ?lter. invention, the signal laser frequency must be stabilized FIG. 8 Shows background spectral data. Within the transmission bandWidth of the ?lters during FIG. SA Show prior art Faraday and Voigt results. operation Which may encounter temperature drift, vibration and mechanical shock. Efforts have been put forWards to FIG. SB Show prior at Faraday and Voigt results. stabilize the laser frequency, the success, hoWever, is limited FIG. SC Show prior art Faraday and Voigt results. up to date. Earlier Wavelength locker design described in FIG. 9 Shows an application of the present invention. prior art patents cited in the Background section utilized an FIG. 10A ShoW etalon data. extended cavity laser With an ALF inside the cavity. Appli FIG. 10B ShoW etalon data. cants have discovered that this design is sensitive to vibra FIG. 10C ShoW etalon data. tion and mechanical shock. The laser is scanned in fre FIG. 10D ShoW a laboratory demonstration of a signal quency to locate the peak ALF transmission position. The laser locking technique. laser is then periodically scanned and tuned to keep the frequency at the peak transmission position. Additional DETAILED DESCRIPTION OF PREFERRED complications results from the fact that the laser frequency EMBODIMENTS is very sensitive to the laser diode temperature Which is affected not only by the environmental temperature, but also Prototype Results by the tuning of the laser current. 20 Preferred embodiments of the present invention employ a pair of micro-etalons in order to provide frequency locking Cesium Vapor Cell of the signal laser. Preferably the transmission peaks of the FIG. 1 shoWs a prototype layout used by Applicants to etalons differ by the full Width at the half maximum demonstrate the present invention. For this demonstration, a (FWHM) of the transmission band of the atomic line ?ler 10 mm long cesium cell, 4 containing cesium vapor at 157° 25 and the peak frequency of one of the etalon is located at one C. is placed in a magnetic ?eld of 150 Gauss produced by side of the ?lter transmission band and the peak frequency tWo ring magnets, 6 (Dexter Permag PP96-1213-1). The of the other etalon is located at the other side of the ?lter cesium vapor Was excited With a 852 nm pump beam to the transmission band. The laser Wavelength is locked at the Cs 62P3/2 level as shoWn in FIG. 3 by pump laser 8 Which middle of the pair of the transmission bands. FIG. 10A is a diode laser supplied by SDL (noW part of JDS Uniphase) 30 shoWs the simulated etalon transmission spectra of a pair of (SDL 5722-H1). For this demonstration a tunable external etalons by CVI Technical Optics. The peak transmission of cavity diode laser 10 (Near Focus Velocity 6326) is used for each of the etalons can be adjusted by changing the etalons the signal beam so that a Wide spectral range could be spacing and/or charging the angular position of the etalon examined. The laser is tunable Within the range of relative to the sample beam as shoWn in FIGS. 10B and 10C. 1446*1557 nm. The output beam is linearly polarized. A 35 Etalon performance is also affected by temperature and long pass ?lter 12 blocks virtually all Wavelengths beloW pressure changes. about 1.0 pm including light at the pump beam Wavelength Several parameters need to be considered in designing an of 852 nm and Wallaston polarizing prism 14 is arranged etalon: With its polarization direction perpendicular to the polariza a) Transmission Wavelength and Free Spectra Range tion direction of the signal beam from tunable laser 10. 40 An etalon is compose of tWo highly re?ective parallel Neutral density ?lter 16 reduces the intensity of light across surfaces With the medium betWeen the tWo surfaces a broad spectrum to match the intensity of the signal light either air or solid. Etalon transmissions are results of operating range of the test instruments. Thus, substantially multiple beam interference. The transmitted all light is blocked except light from laser 10 Which is length is determined by polarization rotated Within cesium cell 4. 45 FIG. 2 shoWs the results obtained by scanning the laser through the range of about 0.05 nm centered at 1469 nm. As Where, n' is the index of of the medium in the is evident from the ?gure almost 100% of the light With a etalon gap; h is the thickness of the etalon gap; 6' is the Wavelength band of less than 0.01 nm is transmitted With refraction angle inside the medium; and m is the substantially zero transmission outside a band of about 0.05 50 interference order. The transmission peaks are equally nm. The tWo side lobes shoWn in FIG. 2 Would produce spaced in frequency With frequency spacing, the so some noise but in many application this Would be insigni? called free spectra range (FSR), determined by cant. C/(2.n'.h. cos 6'), Where C is the . The transmission Wavelength for a given interference order Rubidium Vapor 55 can be tuned by varying the angle of refraction, or the A second bench top experiment Was conducted using a spacing of the etalon, or index of refraction of the rubidium cell. This experiment Was very similar to the material in the space betWeen the surfaces that material cesium cell experiment and is shoWn in FIG. 1B. The only is typically a gas such as air and n is affected by the difference is that the vapor cell 4 is a rubidium vapor cell and pressure and temperature of the gas. FIGS. 10B and the pump laser is a 780 nm pump laser model Vortex 6013 60 10C plot the frequency shift of the transmission band supplied by NeW Focus With of?ces in San Jose, Calif. and With angle of refraction and etalon spacing, respec the half Wave plate 9 is chosen to correspond to the 780 nm tively, for an air-spaced etalon With 7.5 mm spacing or Wavelength. The results Were equivalent to those shoWn for 20 GHz FSR at 852 nm central transmission Wave the co-experiment. This experiment shoWs that an atomic length. To achieve sub 100 MHz resolution angle line ?lter can be provided in accordance With the present 65 tuning of the transmission band, the required angular invention to provide an excellent ?lter for the very important control accuracy is on the order of hundreds of micro C band of ?ber optics communication. See FIG. 5. radian. US 7,058,110 B2 5 6 b) FWHM and Finesse shoWing ground-to-air tracking. The technique used is to The ?nesse (f) of the etalon de?ned as the FSR divided by provide a beacon laser operating at or Within a precise the FWHM of the transmission bands is only deter Wavelength range With a Wide enough angle to be relatively mined by the re?ectance (R) of the etalon certain of including the tracking system. This provides the tracking system With a narroWband ?lter Which Will be as e?icient as possible at the beacon laser Wavelength and as narroW enough so as to discriminate JEN; su?iciently against background light so that the signal to noise ratio is large enough to permit tracking. optical bandpass ?lters typically achieve band The larger the re?ectance, the larger the ?nesse and Widths of 0.5% to 1% of the center Wavelength, and suffer therefore narroWer transmission bandwidth for a given from increased transmission loss With decreasing band spacing or FSR. To produce a FWHM of 0.5 GHZ at Width, limited acceptance angle, and Wavelength shift With 780 nm With a FSR of 25 GHZ, the required re?ectance temperature. Atomic line ?lters have demonstrated superior is about 94%. In practice, defects in Optics Will dete performance With bandWidths less than 0.02 nm, greater riorate the ?nesse of the etalons producing a someWhat than 30 degree acceptance angle, and little or no dependence Wider transmission band. upon temperature. HoWever, these ?lters are limited to c) Thermal Stability Wavelengths in the visible and near visible (<853 nm). A Etalons can be air-spaced or solid. For better thermal neW excited state Faraday resonance ?lter of the present performance, air-spaced etalons are the choice because 20 invention extends conventional proven atomic line ?lter the spacers can be made of temperature insensitive techniques to the 1550 nm region. This ?lter provides materials such as Zerodur. The cavity may also be increased background reduction of >100 over the best opti sealed to reduce index of refraction ?uctuation With cal ?lters Without the angular ?eld of vieW and temperature temperature. The quoted thermal stability form Wave shift limitations of dielectric optical ?lters. Precision Inc. is —0.007 GHz/0 C. for sealed air-spaced 25 For negligible electronic noise, an FPA used in beacon etalons made of Zerodur. acquisition Will have an SNR given by Preferred etalons are air-spaced With 7.690 mm spacing Which results in a free spectra range (FSR) of 19.5 GHZ. The re?ectivity of the coating is 90% at 780 nm, Which leads to 1/2 SNR = "PS ] a ?nesse of 31. The corresponding FWHM of the transmis 30 sion bands is about 0.7 GHZ. A schematic diagram of a Wavelength locker for locking a diode laser is shoWn in FIG. 10D. Mini etalons and micro Where PS is the per pixel detected signal poWer, P b is the per optics are used. The output from laser diode 70 is ?rst pixel background poWer and Af is the sensor integration collimated. A feW percent of the collimated beam Will be 35 bandWidth. For background limited performance it can be then picked off by 72 for Wavelength locking. shoWn that the IFOV The sampled beam Will be split into tWo beams, one of Which Will be sent to a small absorption cell 74 of atomic vapor for absolute Wavelength tuning of the laser Wave length, another beam Will be sent to the etalon pair assembly 40 76A and 76B for Wavelength locking signals from detection D1, D2 and D3 are used for tuning the etalon on both sides of the absorption peak. The Whole package Will be tempera When PSIPb, A?» is the bandpass, A is the pixel area, and LA ture-controlled With a TE cooler. The reader should not that is the background radiance (W/cmZ/SR/u). This important once etalons have been tuned and locked in place, the 45 result shoWs that decreasing the optical ?lter bandWidth by absorption cell may no longer be needed. Also, for locking a factor of 100 Will alloW the received beacon poWer density the beacon laser, the atomic line ?lter is preferably used for to be decreased by 100 for the same sensor ?eld of vieW and absolute tuning. background poWer density. Or, alternatively, Will alloW 100>< more solid angle to be illuminated thereby decreasing search Applications 50 time. Dielectric bandpass ?lters having narroW bandWidths Will Laser Tracking have center transmission Wavelength shift With angle of incidence (0) proportional to sin 20. For a SOTA ?lter of A preferred application of the present invention is for 0.1% bandWidth, this shift Would exceed the bandWidth for laser tracking of a moving target. In this case the moving 55 an angle of incidence of 10 degrees (FIG. 2). Thus the neW target may utiliZe a relatively Wide divergence beacon laser ultra-narroW ?lters being made for ?ber DWDM systems (ie Which is tracked by a tracking detector Which includes one 0.1% BW) Will not meet the vieWing angle requirements or more ?lters such as one of those described above. The needed for an acquisition sensor. The 1.55 micron atomic beacon laser system should be stabiliZe the beacon laser at line ?lter described above can readily achieve 1/100 of this the speci?c Wavelength band of the very narroW band ?lter. 60 bandWidth (0.02 nm @ 1550 nm) With greater than 30 In this case, a very narroW band ?lter is needed so that degree acceptance angle and 80% transmission. re?ected and scattered sunlight can be ?ltered out. FIG. 8 shoWs examples of spectral background looking directly at PREFERRED EMBODIMENT the sun 80, looking at a sunlit cloud 82, scattering from a clear sky and emission from a clear sky. A tracking system 65 Details of the principal components and features of a must compete With this light and must produce a su?icient preferred embodiment of the present invention are shoWn in signal to noise ratio to stay on track. FIG. 9 is a draWing FIGS. 3, 3A, 3B, and 3C. This embodiment includes a US 7,058,110 B2 7 8 beacon laser unit 30 that might be located on a moving scanned to obtain an ALF transmission spectrum and the platform such as an unmanned aerial vehicle such as those pump laser is scanned to obtain an absorption spectrum. For shoWn in FIG. 9. It also includes excited state atomic line the beacon laser, the tWo etalons Will be angularly-tuned so ?lter unit 32 comprising rubidium ALF 34 and pump laser that their transmission spectra overlaps at the center of the unit 36. ALF spectrum. For the pump laser, the transmission spec The beacon laser unit comprises diode signal laser 38, trum of the tWo etalon overlap at the center of the absorption frequency locking etalons 40A and 40B, and detectors 42A spectrum. The etalons Will be ?xed to positions With UV and 42B providing frequency locking signals to processor 44 epoxy. Which utiliZes those signals to control the current to signal Control Electronics and Operation Procedure laser 38 through current control 46. Processor 44 also A microprocessor Will be used to control the laser current maintains control of the diode temperature and the chamber and process the outputs from the tWo etalon-photodiodes. temperature through thermoelectric coolers (not shoWn) and The diode laser TE cooler, the atomic vapor cell heater, and diode temperature control unit 46 and chamber temperature the TE cooler for the Whole package may also be controlled control unit 48. With the microprocessor. Pump laser unit 36 provides a laser beam locked precisely 5 To operate the Wavelength locker, at ?rst all temperature to a Wavelength 7t:780.027 nm to excite rubidium vapor in controls should be stabiliZed With the laser current ramped vapor cell 70 to the 52P3/2 excited state as shoWn in FIG. 6B. up to the operation value. The laser Wavelength Will then be Controls for this laser are similar to those for beacon laser locked to the position by balancing the light output from the 30. The unit includes diode pump laser 50, frequency tWo etalons through the tuning of the laser current With the locking etalons 52A and 52B and detectors 54A and 54B 20 microprocessor. providing frequency locking signals to processor 56 Which utiliZe these signals to control the current to pump laser 50 Packaging through current control 58. Processor 56 also maintains The etalons Will be made of temperature-insensitive mate control of the diode temperature and the chamber tempera rials and the Whole package may be temperature-controlled ture through thermoelectric coolers (not shoWn) and diode 25 as Well. So thermal stability should not be a critical issue. temperature control unit 57 and chamber temperature con HoWever, as shoWn in FIG. 10B, the peak transmission trol unit 59. The output beam 60 of pump laser unit 36 is frequency of the etalons is very sensitive to angle of inci co-aligned With beacon beam 62 from beacon laser 30 using dent. To lock the laser frequency better than a feW hundreds dichroic mirror 64. of MHZ, the incident beam should be stabiliZed better than Rubidium atomic line ?lter 34 comprises rubidium cell 30 1 mradian. To reduce spatial jitter of the incident beam from 70, tWo ring magnets 72 producing a 150 Gauss co-axial the beacon laser to the Wavelength locker, the Whole package magnetic ?eld through cell 70, cross polariZers 74 and 76, should be made of small siZe components With dimensions long pass ?lter 78 and detector 80. Temperature controller on the order of a feW centimeters. Furthermore, the beacon 82 controls the temperature of cell 70 via rubidium heater laser and the line locker should be integrated into one small 84. Cold ?nger 86 provides a condensation location Within 35 package. the cell to maintain a rubidium vapor-liquid equilibrium While the above description describes speci?c preferred Within cell 70. The signal from detector as shoWn at 88 may embodiment of the present invention, person skilled in this be utiliZed by pointing equipment (not shoWn) to maintain a art should understand that many changes and modi?cations telescope such as those shoWn in FIG. 9 pointed at beacon could be made Within the scope of the present invention. For laser 30. 40 example, vapors other than rubidium and cesium could be FIG. 3A provides a qualitative representation of the used Speci?cally, other alkali metals are good choices, output of detection D1 and D2 of beacon signal laser 30 as Where a desirable resonant frequency exists betWeen tWo a function of the output Wavelength of beacon signal laser excited states and Where the vapor can be pumped to the ?rst 38. FIG. 3B shoWs qualitatively the relationship betWeen the excited state. Good application of the present invention ratios of the detector signals to the Wavelength Within the 45 includes tracking a transceiver in a laser communication narroW spectral range of about 1529.29 nm to about 1529.31 system. Also, the system may be used to track hot objects nm. Current to diode laser 38 is adjusted to maintain’the such as bullets or other missiles emitting radiation at infra output of beacon laser 38 precisely Within the transmission red Wavelengths. band of ?lter unit 32 as shoWn in FIG. 3F. Another application is for laser radar systems Where eye FIGS. 3C and 3D are similar graphs demonstrating the 50 safety is a concern. For these reasons the reader should control of the Wavelength of the pump laser beam from determine the scope of the invention from the appended pump laser unit 36. This Wavelengths can also be controlled claims and their legal equivalence. to a precession of about 10.0005 nm. Speci?cation for pump What is claimed is: 36 laser provide a response of 0.0034 nm per milliamps permitting Wavelength precision to about 10.0005 nm. 55 1. An infrared atomic line ?lter comprising (A) a metal vapor cell having an optical entrance port and Alignment and Packaging Procedure an optical exit port and containing a metal vapor The alignment and packaging process for both the pump de?ning a ?rst excited energy state, de?ning a ?rst laser and the beacon laser should be performed in a Well resonant frequency, and a second excited energy state controlled environment so that the laser Wavelength Will be 60 and having at least one absorption line, at or near a stable to about 100 MHZ even Without Wavelength locking desired ?lter Wavelength Within the infrared spectral in a period of one hour or so. At ?rst all components other range, said absorption line being equal, in energy, to a than the tWo etalons Will be aligned and ?xed to positions difference betWeen the second excited state and the ?rst either mechanically or using thermal/UV curing epoxy. The excited state; tWo etalons Will be attached to tWo precession (100 p. radian 65 (B) at least one magnet for imposing on said metal vapor or better resolution) angular alignment stages, respectively, a magnetic ?eld to produce polariZation rotation near by mechanical or vacuum means. The beacon laser is said at least one absorption line; US 7,058,110 B2 10 (C) a ?rst polariZer positioned to block polarized light at 5. The ?lter as in claim 4 Wherein said tWo etalons are a ?rst polarization from entering said Vapor cell tuned to produce transmission peaks on both sides of said through said entrance port; ?rst resonant frequency and feedback control to maintain (D) a second polariZer oriented perpendicular to said ?rst said diode laser operation at said resonant frequency based polariZation and located across the path of light exiting on input signals from said tWo frequency locking detectors. said exit port so as to block light exiting said Vapor cell that is polariZed perpendicular to said ?rst polariZation; 6. The ?lter as in claim 1 Wherein said metal Vapor is (E) a pump light source for producing light at said ?rst rubidium. resonant frequency for pumping said metal Vapor from 7. The ?lter as in claim 1 Wherein at least one magnet is said ground state to said ?rst excited state; tWo permanent magnets. Wherein said metal Vapor under the in?uence of said mag 8. The ?lter as in claim 7 Wherein said tWo permanent netic ?eld produces a polariZation rotation of light Within a narroW spectral band near said absorption line permitting magnets are tWo ring magnets. light Within this spectral band to pass through said second 9. The ?lter as in claim 1 Wherein said at least one magnet polariZer Whereas all light in a much Wider spectral range is is an electromagnet. not rotated in polariZation and is blocked by either the ?rst 10. The ?lter as in claim 1 Wherein said Vapor cell de?nes polariZer or the second polariZer. beam direction and at least one magnet is oriented to 2. The ?lter as in claim 1 Wherein said pump light source produce a magnetic ?eld parallel to said beam direction. is a pump laser system. 11. The ?lter as in claim 1 Wherein said Vapor cell de?nes 3. The ?lter as in claim 2 Wherein said pump laser system 20 is a diode laser system producing a pump beam. a beam direction and said at least one magnetic is oriented 4. The ?lter as in claim 3 Wherein said diode laser further to produce a magnetic ?eld perpendicular to said beam comprises tWo frequency locking etalons and tWo frequency direction. detectors for monitoring sample portions of said pump beam.