Abstract:
1064-nm and 1319-nm light respectively generated by two lasers is combined and injected into a doubly resonant sum-frequency generator. The optical path length of the sum-frequency generator is adjusted responsive to feedback of the 1319-nm light to maintain 1319-nm resonance. Feedback of 1064-nm light is concurrently used to adjust the 1064-nm laser responsive to the optical path length to maintain 1064-nm resonance. Light output from the sum-frequency generator is compared to the sodium D 2a  wavelength i.e., approximately 589 nm, and the 1319-nm laser is responsively adjusted to eliminate any differential. This abstract is provided to comply with the rules requiring an abstract, and is intended to allow a searcher or other reader to quickly ascertain the subject matter of the technical disclosure. It is submitted with the understanding that it will not be used to interpret or limit the scope or meaning of the claims.

Description:
STATEMENT OF GOVERNMENT INTEREST 
   The conditions under which this invention was made entitle the Government of the United States, as represented by the Secretary of the Air Force, to the entire right, title and interest therein, including foreign rights, pursuant to paragraph I(a) of Executive Order 10096. 

   BACKGROUND OF THE INVENTION 
   This invention relates to lasers and nonlinear optics and, more particularly, uses several laser beams in conjunction with sum-frequency generation to produce a high-power beam having a desired wavelength, e.g., the sodium D 2a  spectral line. 
   The earth&#39;s atmosphere distorts light traveling through it because of time and spatially varying fluctuations in its refractive index. Such distortion limits the resolution of terrestrial telescopes, and was the motivation for launching the successful, but very expensive, Hubble telescope into earth orbit. However, ground-based telescopes can achieve a resolution surpassing that of the Hubble space telescope by means of adaptive optics, a technique whereby the surface of a deformable telescope mirror is changed as a function of time to compensate for atmospheric distortion. 
   Measuring the distortion requires that there be a bright optical source in the sky, such as a bright star, located close to the object to be observed. Since natural stars of sufficient brightness are too rare to permit compensated imaging except over a tiny fraction of the sky, an alternative is to produce one or more artificial guide stars by means of back-scattered light from a laser beam or similar source of coherent radiation directed from the site of the telescope into the area of the sky proximate to the object to be observed. Such guide stars can be produced by exciting resonance fluorescence at a 589-nm wavelength, i.e., at the sodium D 2a  spectral line, from a layer of sodium atoms that circumscribes the earth in the mesosphere at an altitude of approximately 90 km. 
   Although high-power dye lasers can be operated at this wavelength to excite the sodium layer, they are inefficient, bulky, expensive, and use chemicals which are toxic and which degrade over time, concomitantly resulting in degradation of performance. The desirability of an all-solid-state sodium beacon excitation source has long been recognized. W. Happer, G. J. MacDonald, C. E. Max, and F. J. Dyson, “Atmospheric-turbulence compensation by resonant optical backscattering from the sodium layer in the upper atmosphere,”  J. Opt. Soc. Am. A , Vol. 11, No. 2, pp. 263–276 (January 1994). However, the goal of building a reliable and efficient device that generates sufficient power with good beam quality has been elusive, despite over a decade of endeavor. 
   By a natural coincidence, sodium resonance radiation at λ d =589 nm can be produced by sum-frequency generation of the Nd:YAG laser lines at λ p =1064 nm and λ s =1319 nm. This has motivated the development of pulsed sources using single-pass sum-frequency generation in lithium triborate (LiB 3 O 5 ). T. Jeys and V. Daneu, “Diode-pumped, Nd:YAG source of sodium-resonance radiation for atmospheric adaptive optics,”  ESO Workshop on Laser Technology for Laser Guide Star Adaptive Optics Astronomy , Garching, Germany (Jun. 23–26, 1997); and a low-power continuous-wave source using doubly resonant sum-frequency generation in lithium niobate (LiNbO 3 ). J. D. Vance, C.-Y. She, and H. Moosmuller, “Continuous-wave, all-solid-state, single frequency 400-mW source at 589 nm based on doubly resonant sum-frequency mixing in a monolithic lithium niobate resonator,”  Applied Optics , Vol. 37, No. 2, pp. 4891–4896 (Jul. 20, 1998). The pulsed sources have proved too unreliable for use in conjunction with adaptive optics, whereas absorption in lithium niobate precludes the generation of high power in this crystal. 
   Various theoretical studies have been completed comparing the efficiency of different pulse formats, including continuous wave. J. M. Telle, P. W. Milonni, and P. D. Hillman, “Comparison of pump-laser characteristics for producing a mesospheric sodium guidestar for adaptive optical systems on large aperture telescopes,” in  High - Power Lasers , S. Basu, Editor,  Proceedings of SPIE , Vol. 3264, pp. 37–42 (January 1998). 
   The advantages of continuous wave are high duty cycle and narrow bandwidth. Both effects enhance the sodium fluorescence return, as long as saturation is avoided. Continuous wave sources have been used, but have only been able to generate low beam power, i.e., power too low for astronomical imaging using adaptive optics to correct atmospheric distortion. 
   It follows that there is a need in the art for a terrestrial apparatus capable of generating an artificial guide star by means of back-scattered light from a laser beam or similar source of coherent radiation and, more particularly, by creating such a guide star by exciting resonance fluorescence from a layer of sodium atoms that circumscribes the earth in the mesosphere. The present invention has fulfilled this need in the art. 
   SUMMARY OF THE INVENTION 
   Briefly, the invention is comprised of an injection-locked 1064-nm laser, an injection-locked 1319-nm laser, a doubly resonant sum-frequency generator (hereinafter referred to as “SFG”), and a heated sodium-vapor cell for measuring Doppler-free fluorescence. Each of the injection-locked lasers includes a single-frequency high-power ring oscillator containing a pair of Brewster-cut Nd:YAG rods. The ring oscillators are respectively injection locked by low power, single-frequency, linearly-polarized lasers using the Pound-Drever-Hall stabilization technique, and are unidirectional when so injection-locked. Resonance of the respective laser frequencies in the oscillators is maintained by electronic feedback servo circuitry. 
   The 1064-nm and 1319-nm light, respectively generated by the two injection-locked lasers, is combined by a dichroic mirror and, after being mode matched, is injected into the SFG comprised of a bowtie resonator that includes a lithium triborate (LiB 3 O 5 ) crystal for generating 589-nm light and a scraper mirror for extracting the 589-nm light. 
   The optical path length of the SFG is adjusted responsive to feedback of the 1319-nm light to maintain 1319-nm resonance. Feedback of 1064-nm light is concurrently used to adjust the 1064-nm low-power laser responsive to the foregoing changes in the SFG optical path length, to maintain 1064-nm resonance in the SFG. 
   The extracted 589-nm light is directed towards the desired celestial area. It has the wavelength and power required to generate sodium D 2a  fluorescence in the mesospheric sodium layer. A small portion of the 589-nm light is directed through the heated sodium-vapor cell, where Doppler-free spectral features of the sodium fluorescence are detected. The 1319-nm low-power laser is responsively adjusted, either manually or by an automatic mechanism, to maintain the output from the SFG at the desired sodium D 2a  wavelength, i.e., approximately 589.159 nm. The spectral linewidth of the extracted 589-nm light is of the order of 10 kHz, much narrower than the 1 GHz spectral linewidth of atmospheric D 2a  resonance and the 10 MHz spectral linewidth of Doppler-free D 2a  resonance. 
   Another embodiment employs an additional 1064-nm low-power laser. One of the two 1064-nm low-power lasers is tuned such that the light output from the SFG has a wavelength at the center of the sodium D 2a  spectral line, while the other low-power laser is detuned such that the output from the SFG has a wavelength slightly off of the spectral line. The two light beams are oriented with orthogonal polarization states and then co-axially combined. 
   An electro-optic polarization rotator switches between the two beams and only the selected beam proceeds through the remainder of the apparatus, as previously described. Where the selected beam is the one at the sodium D 2a  spectral line, the mesospheric sodium layer becomes fluorescent, while no fluorescence is produced when the other beam is selected. This allows the Rayleigh background light from the lower atmosphere to be measured and subtracted from the fluorescence of the mesospheric guide star. 
   Other aspects and advantages of the present invention will become apparent from the following detailed description, taken in conjunction with the accompanying drawings, and illustrating by way of example the principles of the invention. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     In the accompanying drawings: 
       FIG. 1  is a schematic drawing of the present invention; and 
       FIG. 2  is a schematic drawing illustrating an alternative embodiment of the present invention wherein  FIG. 1  is modified by adding several elements, particularly a low-power 1064-nm laser. 
   

   DETAILED DESCRIPTION 
     FIG. 1  is a schematic diagram of continuous-wave sodium beacon excitation source  4 , an embodiment of the present invention. Solid lines denote optical beam paths, while dashed lines denote electrical connections. Faraday isolators consist of Faraday rotators sandwiched between polarization beam splitters. For simplicity, beam dumps associated with polarization beam splitters are not shown. 
   Excitation source  4  is comprised of four interconnected modules, modules  5 ,  6 ,  7  and  8 . Module  5  comprises a 1064-nm injection-locked laser. Module  5  includes laser  11 , which produces a low-power, single frequency, linearly polarized, continuous-wave 1064-nm beam. The wavelength of laser  11  can be adjusted over a limited range. Lightwave Electronics Corporation of Mountain View, Calif., manufactures a device of this type. As will be explained in conjunction with the subsequent discussion of module  7 , doubly resonant SFG  13  generates 589-nm light and servo amplifier  15  electronically adjusts the frequency of laser  11  to maintain locking of the 1064-nm light to SFG  13 . 
   It is also to be noted that the wavelengths mentioned herein, i.e., 1064, 589, and 1319 nm, are approximations accurate to 1 nm. As will become clear, the respective wavelengths output by the described optical elements are controlled to a resolution better than 5×10 −6  nm during the operation of the invention by virtue of feedback loops that enable the invention ultimately to generate a high-power laser beam having the precise wavelength and stability that is desired. 
   The 1064-nm light from laser  11  passes through Faraday isolator  17  to prevent light from counter-propagating back into laser  11 . The light then passes through half-wave plate  19 , which rotates the polarization direction; through mode-matching lens  21 ; and then through electro-optic radio frequency phase modulator  23 . 
   Local oscillator  25  produces an 18 MHz sine-wave voltage, which is applied to phase modulator  23  to produce phase sidebands on the 1064-nm light. These sidebands are used for Pound-Drever-Hall locking of 1064-nm high-power ring oscillator  27  to laser  11 , and also for locking the output of module  5  to SFG  13 . Input coupler  29  for ring oscillator  27  and input coupler  31  of SFG  13  reflect the phase sidebands because the sidebands are not resonant with either ring oscillator  27  or SFG  13  while the foregoing two components are locked onto the resonance of the wavelength of laser  11 . The voltage from the local oscillator  25  is also applied to mixer  33  in module  7  and mixer  35  in module  5 . 
   After traversing Faraday isolator  37 , half-wave plate  39 , mode-matching lens  41 , and turning mirrors  43 , the low-power 1064-nm beam from laser  11  passes through the partially transmitting input coupler  29  and is injected into ring oscillator  27 . The reflective surfaces of input coupler  29  and mirrors  45  and  47  of ring oscillator  27  are flat. Mirrors  45  and  47  are highly reflective at 1064 nm. Mirror  47  is mounted on piezoelectric transducer  49 . The latter element is capable of translating mirror  47  to adjust the length of the optical path of ring oscillator  27 . 
   Piezoelectric transducer  49  is driven by servo amplifier  51 , which in turn is controlled by the zero-crossing error voltage output of mixer  35 . Servo amplifier  51  includes proportional-integral analog circuitry to supply proper negative feedback to piezoelectric transducer  49 , and provides gain roll-off and phase margin sufficient to avoid exciting the first mechanical resonance of piezoelectric transducer  49 . 
   Ring oscillator  27  also includes intracavity concave lenses  53  and Nd:YAG rods  55 . A combination of focusing by lenses  53  and thermal lensing in rods  55  obtains a stable cavity. Rods  55  are cut at the Brewster angle, helping to ensure linearly polarized operation of ring oscillator  27 . Diode arrays (not shown) pump rods  55  transversely. 
   The high-power output of ring oscillator  27  is transmitted through input coupler  29 , whereupon it joins the phase sidebands reflected from input coupler  29 . The output and sidebands then pass through polarizing beam splitter  57  to enhance the degree of linear polarization over that obtainable from ring oscillator  27  alone. A small fraction of the beam power is reflected from beam splitter  59  and impinges on light detector  61 . The radio frequency output of light detector  61  is applied to mixer  35 , where it is mixed in quadrature with the signal from local oscillator  25 , i.e., a 90° phase difference between the radio frequency input from  61  and the signal from local oscillator  25 , to produce the input to servo amplifier  51 . 
   The output of mixer  35  is generated using the Pound-Drever-Hall stabilization technique, a laser frequency stabilization technique well known to those skilled in optics. The output of servo amplifier  51  is applied to piezoelectric transducer  49 , which adjusts the optical path length of ring oscillator  27  to maintain resonance in the cavity, i.e., the cavity optical path length must be an integral number of light wavelengths. The resonance of ring oscillator  27  with respect to the output of laser  11  maintained by means of the aforementioned feedback loop causes the former to lase unidirectionally at the central frequency of the 1064-nm beam emanating from laser  11 . In this regime of operation, ring oscillator  27  is said to be injection locked to the output of laser  11 . 
   Module  6  comprises a 1319-nm injection-locked laser. Module  6  includes laser  63 , which produces a low-power, single-frequency, linearly polarized, continuous-wave 1319-nm laser beam. Lightwave Electronics Corporation of Mountain View, Calif., manufactures such a device. The wavelength of laser  63  can be adjusted over a limited range. The low-power light from  63  passes through Faraday isolator  65  to prevent light from counter-propagating back into laser  63 . The 1319-nm light then passes through half-wave plate  67 , which rotates the polarization direction, through mode-matching lens  69 , and then through an electro-optic radio frequency phase modulator  71 . 
   Local oscillator  73  generates a 21 MHz sine-wave voltage, which is applied to phase modulator  71  to produce phase sidebands on the 1319-nm light. These sidebands are used for Pound-Drever-Hall locking of 1319 nm high-power ring oscillator  75  to laser  63 , and also for locking SFG  13  to the output of module  6 . Input coupler  77  for oscillator  75  and input coupler  31  of SFG  13  reflect the sidebands because the sidebands are not resonant with either ring oscillator  75  or SFG  13  while the foregoing two components are locked onto the resonance of the wavelength of laser  63 . The voltage from local oscillator  73  also is applied to mixer  79  of module  6  and mixer  81  in module  7 . After traversing Faraday isolator  83 , half-wave plate  85 , mode-matching lens  87 , and turning mirrors  89 , the low power 1319-nm beam from laser  63  passes through partially transmitting input coupler  77  and is injected into ring oscillator  75 . 
   Mirrors  91  and  93  of the cavity for ring oscillator  75  are highly reflective at 1319 nm. The reflective surfaces of input coupler  77  and mirrors  91  and  93  are flat. Input coupler  77  and mirrors  91  and  93  are specially coated to reflect light at 1319 nm, and to be highly transmissive at 1064 nm to prevent lasing at the latter wavelength. Mirror  93  is mounted on piezoelectric transducer  95 . The latter element is capable of translating mirror  93  to adjust the cavity optical path length. 
   Piezoelectric transducer  95  is driven by servo amplifier  97 , which is in turn controlled by the zero-crossing error voltage output of mixer  79 . Servo amplifier  97  includes proportional-integral analog circuitry to supply proper negative feedback to piezoelectric transducer  95 , and provides gain roll-off and phase margin sufficient to avoid exciting the first mechanical resonance of piezoelectric transducer  95 . Ring oscillator  75  also includes intracavity lenses  99  and Nd:YAG rods  101 . A combination of focusing by lenses  99  and thermal lensing in rods  101  is used to obtain a stable cavity. Diode arrays (not shown) pump rods  101  transversely. 
   Ring oscillators  27  and  75  are each an example of a power oscillator to be frequency-locked to lasers  11  and  63 , respectively. Each of ring oscillators  27  and  75  could be replaced by a ring oscillator having a number of rods other than two, or by a power oscillator having a completely different configuration, e.g., a Nd:YAG slab or fiber. 
   The high-power output of ring oscillator  75  is transmitted through input coupler  77 , whereupon it joins the phase sidebands reflected from input coupler  77 . The output and sidebands then pass through polarizing beam splitter  103  to enhance the degree of linear polarization over that obtainable from ring oscillator  75  alone. 
   A small fraction of the beam power is reflected from beam splitter  105  and impinges on light detector  107 . The radio-frequency output of detector  107  is applied to mixer  79 , where it is mixed in quadrature with the signal from local oscillator  73  to produce the input to servo amplifier  97 . The output of mixer  79  is obtained by using the Pound-Drever-Hall stabilization technique. 
   The output of servo amplifier  97  is applied to piezoelectric transducer  95 , which adjusts the optical path length of ring oscillator  75  to maintain resonance in the cavity. The resonance of ring oscillator  75  with respect to the output of laser  63  maintained by means of the aforementioned feedback loop causes the former to lase unidirectionally at the central frequency of the 1319-nm beam emanating from laser  63 . In this regime of operation, ring oscillator  75  is said to be injection locked to the output of laser  63 . 
   Alternatively, high-power amplifiers could be used to increase the low power of lasers  11  and  63 , instead of high-power ring oscillators  27  and  75 , respectively. Such an alternative embodiment would not require internal feedback loops using beam splitter  59 , light detector  61 , mixer  35  and servo amplifier  51 , with respect to ring oscillator  27 ; or beam splitter  105 , light detector  107 , mixer  79  and servo amplifier  97 , with respect to ring oscillator  75 . Another embodiment of the invention would involve respectively replacing modules  5  and  6  with high-power single-frequency lasers. Each of the high-power lasers would have to be frequency controllable. 
   Turning to module  7 , each of the infrared beams from modules  5  and  6  pass through convex mode-matching lenses  111 , half-wave plates  113  and polarization beam splitters  114 , to allow independent adjustment of the power of the infrared beams ultimately impinging on SFG  13 . The 1064-nm beam emanating from module  5  is reflected onto this path by turning mirrors  115 , and after traversing the aforementioned optical elements it impinges on dichroic mirror  117 . The 1319-nm beam emanating from module  6  is reflected by turning mirror  116  to impinge on dichroic mirror  117  after traversing a separate set of the aforementioned optical elements having optical properties appropriate to the 1319-nm wavelength, i.e., appropriate anti-reflection coatings and an appropriate design for half-wave plate  113 . The two beams respectively impinge on opposite surfaces of dichroic mirror  117 , with the two surfaces being respectively coated to reflect the 1064-nm beam and transmit the 1319-nm beam, thus combining them into a single co-axial beam. 
   Doubly resonant SFG  13  consists of two highly reflective concave mirrors  119 , lithium triborate (LiB 3 O 5 ) crystal  121 , flat input coupler  31  which partially transmits both 1064-nm and 1319-nm light, and flat highly reflective mirror  123  mounted on piezoelectric transducer  125 . As an example, crystal  121  is 2 cm long and uses noncritical type-I phase matching. As previously noted, input coupler  31  reflects the phase sidebands from both the 1064-nm and 1319-nm light because the sidebands are not resonant with SFG  13  while it is locked onto resonance with the wavelengths (center frequencies) of lasers  11  and  63 . 
   The end surfaces of crystal  121  are anti-reflection coated for wavelengths of 1064 nm, 1319 nm, and 589 nm; thus, the ends transmit light of all three foregoing wavelengths with minimal loss. The fold angle ⊖ of SFG  13  should be kept as small as possible to minimize aberrations. Moreover, the optimal choice for the reflectivities of input coupler  31  depends on the available power of the beams produced by the lasers of modules  5  and  6 . 
   Scraper mirror  127  is inserted at the Brewster angle in the resonator between curved mirrors  119  such that the light in SFG  13  passes through crystal  121  before impinging on scraper mirror  127 . Scraper mirror  127  has a coating on its front surface onto which light is incident, to highly transmit the p-polarized infrared light, i.e., the 1064-nm and 1319-nm light, and highly reflect the s-polarized 589-nm light. This extracts the s-polarized 589-nm light from SFG  13 . 
   Input coupler  31  transmits a portion of the infrared light circulating within SFG  13  and incident on input coupler  31  from within SFG  13 . This transmitted light, together with the phase sidebands and the portion of the externally incident infrared light at the central frequencies reflected by input coupler  31 , is directed by turning mirror  129  onto dichroic beam splitter  131 , which separates it into 1064-nm and 1319-nm beams. The 1319-nm beam passes through dichroic beam splitter  131 , reflects off of turning mirror  132 , and passes through half-wave plate  133  and polarization beam splitter  134  before impinging on light detector  135 . The angular rotation of half-wave plate  133  is set so as to reduce the infrared power impinging on detector  135  to a level that avoids saturating this detector. Polarization beam splitter  134  shunts the superfluous power to a beam dump (not shown). 
   Detector  135  produces a voltage signal based on the Pound-Drever-Hall stabilization technique. Mixer  81  mixes in quadrature the signal from light detector  135  with the signal from local oscillator  73  to produce a zero-crossing error voltage input to servo amplifier  136 . Responsive to the signal from servo amplifier  136 , piezoelectric transducer  125  translates mirror  123  to adjust the length of the optical path of SFG  13  to keep it locked onto resonance with the 1319-nm output of module  6 . Servo amplifier  136  includes proportional-integral analog circuitry to supply proper negative feedback to piezoelectric transducer  125 , and provides gain roll-off and phase margin sufficient to avoid exciting the first mechanical resonance of piezoelectric transducer  125 . 
   Dichroic beam splitter  131  reflects the 1064-nm beam through half-wave plate  137  and polarization beam splitter  139 , and onto light detector  140 . The angular orientation of half-wave plate  137  is set so as to reduce the infrared power impinging on detector  140  to a level that avoids saturating this detector. Polarization beam splitter  139  shunts the superfluous power to a beam dump (not shown). 
   Detector  140  produces a voltage signal based on the Pound-Drever-Hall stabilization technique. Mixer  33  mixes in quadrature the signal from light detector  140  with the signal from local oscillator  25  to produce a zero-crossing error voltage input to servo amplifier  15 . 
   The wavelength of the 1064-nm light produced by laser  11  is adjusted responsive to the signal from servo amplifier  15 , to maintain the locked resonance of the output of module  5  to SFG  13 . Servo amplifier  15  includes proportional-integral analog circuitry to supply proper negative feedback to a frequency actuating transducer (a piezoelectric element) included within laser  11 , and provides gain roll-off and phase margin sufficient to avoid exciting the first mechanical resonance of the frequency actuating transducer of laser  11 . 
   An alternative configuration (not shown) eliminates dichroic beam splitter  131  and instead sends both infrared wavelengths reflected from input coupler  31  to a single light detector. The signal produced by the single detector is then split and applied to mixers  33  and  81 , respectively. The optical path length of SFG  13  and the wavelength of the light emitted by laser  11  are then adjusted as previously described. 
   The 589-nm light reflected from scraper mirror  127  reflects off turning mirrors  141  and traverses collimating lens  142  before impinging on beam-splitting mirror  143 . Mirror  143  reflects most of the light as output light beam  144  to be directed towards the desired celestial area. Output beam  144  creates the desired artificial guide star by impinging on the mesospheric sodium layer and exciting the sodium atoms to cause fluorescence. Photon return from the mesosphere is enhanced by converting output beam  144  to circular polarization before propagating the light to the mesosphere. This conversion can be accomplished by passing output beam  144  through a quarter-wave plate (not shown). 
   Using a fold angle ⊖ of 7°, reflectivities for input coupler  31  of 0.954 at 1064 nm and 0.925 at 1319 nm, and with the lasers of modules  5  and  6  generating power of 24 watts and 15 watts, respectively, the power of output beam  144  was 22.5 watts of 589-nm light. The beam quality was diffraction limited. Lower reflectivities of input coupler  31  would be preferable if the power of the infrared beams generated by the lasers of modules  5  and  6  were higher. If, in this circumstance, the input coupler reflectivities are lowered this would broaden the infrared resonance bandwidth of SFG  13  and, consequently, necessitate that the radio frequencies applied to phase modulators  23  and  71  be increased so that the phase sidebands would lie well outside the resonance of SFG  13 . Also, the frequencies should be chosen to have a high common multiple to avoid any cross-talk noise in the event that a single light detector for SFG  13  were to be used. 
   Beam splitting mirror  143  transmits the remaining, unreflected portion of the 589-nm light onto turning mirror  145 , whereupon the light beam passes through telescoping and collimating lenses  147  and into heated sodium-vapor cell  149 . Highly reflective mirror  151  reflects the light back through cell  149  in the direction opposite to the direction from which it entered. The counter-propagating beams in cell  149  produce Doppler-free spectral features in the sodium fluorescence, which are sensed by side-port fluorescence detector  153 . The signal from detector  153  is used to form a zero-crossing error signal to apply negative feedback to laser  63 . The wavelength of laser  63  is adjusted responsive to the signal from detector  153  to stabilize and maintain the 589-nm light of output beam  144  at the center of the sodium D 2a  spectral line. 
     FIG. 2  shows continuous wave sodium beacon excitation source  155 , another embodiment of the present invention wherein module  5  is replaced by module  156 . Module  5  is augmented to form module  156  by adding an additional low-power 1064-nm laser  157 , Faraday isolator  159 , half-wave plate  161 , directional mirror  163 , polarization beam combiner  165 , electro-optic polarization rotator  167 , and polarization beam splitter  169 . The remaining components of excitation source  155 , i.e., modules  6 ,  7  and  8 , are identical to those of excitation source  4 , and operate as previously described. 
   Lasers  11  and  157  are tuned to slightly different wavelengths, such that output beam  144  generated using one of them is tuned to the center of the sodium D 2a  spectral line, while output beam  144  generated using the other low-power laser is tuned off of this resonance wavelength so as not to produce an artificial guide star. Half-wave plates  19  and  161  are rotated such that the beams from lasers  11  and  157  both impinge on polarization beam combiner  165  with orthogonal linear polarizations. 
   The combined beam then passes through electro-optic polarization rotator  167 , which, depending on the voltage applied to it, either leaves the polarizations at the two wavelengths unchanged or interchanges them. Electro-optic polarization rotator  167  switches between the beams from lasers  11  and  157  such that only the beam with the selected polarization, i.e., wavelength, proceeds through polarization beam splitter  169 . One of the two emerging beams next passes through lens  21  to phase modulator  23 , and then on to the remainder of the elements of excitation source  155 , as previously described in conjunction with excitation source  4 . 
   Switching between these two wavelengths thus switches the fluorescence from the mesospheric guide star on and off, while leaving the Rayleigh scattering from the lower atmosphere essentially unchanged. This allows the Rayleigh background to be subtracted from the fluorescence of the mesospheric guide star. Replacing electro-optic polarization rotator  167  by a half-wave plate and manually rotating the plate by 45° would produce the same switching effect, but the electro-optic switching is much faster. 
   The intervals when the frequency of output beam  144  is switched off of the sodium resonance frequency would typically be too brief for the wavelength of laser  63  to be adjusted responsive to the signal from detector  153 , to maintain the 589-nm light of output beam  144  at the sodium D 2a  spectral line, i.e., the off interval would be over before laser  63  could respond by changing its output wavelength. Alternatively, the signal from detector  153  could be switched off, e.g., by opening or disconnecting the connection from detector  153  to laser  63 , concurrent with switching output beam  144  off of the sodium resonance frequency. 
   To facilitate rapid frequency switching of output beam  144 , rapid switching between lasers  11  and  157  is required while maintaining locking to both ring oscillator  27  and SFG  13 . To ensure such locking, the free spectral ranges of these elements (the speed of light divided by the optical path length) should be commensurate, i.e., the optical path lengths of ring oscillator  27  and SFG  13 , as well as the difference in frequency between lasers  11  and  157 , are chosen such that this difference in frequency is an integer multiple of the free spectral range of the cavity of ring oscillator  27  and an integer multiple of the free spectral range of the cavity of SFG  13 . 
   Alternatively, a second low-power 1319-nm laser juxtaposed with laser  63 , along with a Faraday isolator, half-wave plate, directional mirror, polarization beam combiner, electro-optic polarization rotator, and polarization beam splitter, could be used in conjunction with laser  63  to switch output beam  144  on and off of the sodium resonance frequency. These elements would cooperate with laser  63  as previously described with respect to the switching function of 1064-nm laser  157  used in conjunction with laser  11 , to the same effect. 
   To minimize the absorption of infrared light, all optical elements transmitting high-power 1319-nm light are made of INFRASIL® 301 optical quartz glass (fused silica). (INFRASIL is a registered trademark owned by Hereaus Corporation.) If too much light were to be absorbed, thermal lensing might occur. 
   As noted with respect to excitation source  4 , the optical path length of SFG  13  is changed to maintain locking with the output of module  6  and the output of module  5  is adjusted to keep it locked to the optical path length of SFG  13 . It is to be understood that this locking relationship could be reversed. In such a reversed configuration, the optical path length of SFG  13  would be adjusted to maintain locking with the output of module  5 . The output of module  6  would then be adjusted to keep it locked with the optical path length of SFG  13 . 
   Furthermore, another technique could be used as an alternative to the Pound-Drever-Hall stabilization technique disclosed herein, e.g., the Hansch-Couillaud technique. Moreover, piezoelectric transducers  49 ,  95  and  125  used for respectively adjusting optical path lengths are but one example of a number of apparatus that could be used for this purpose, e.g., voice coil actuators or stepper motors. 
   It should also be noted that the wavelength of output beam  144  might be varied from 589 nm to suit purposes other than creating a mesospheric guide star. For example, a different output wavelength could be obtained by substituting low-power lasers having wavelengths different than the 1064-nm and 1319-nm wavelengths of lasers  11  and  63 , respectively. Such a substitution would necessitate modifying ring oscillators  27  and  75  to enable modules  5  and  6  to generate high-power single-frequency laser beams. Sodium-vapor cell  149  would also have to be replaced with an element for providing an appropriate wavelength reference standard for stabilizing output  144  at the desired reference wavelength. 
   Depending on the desired output wavelength, the composition of crystal  121  might also have to be changed. The low-power wavelengths necessary to obtain a high-power output beam  144  having a specific wavelength, together with the aforementioned requisite modifications to various elements of excitation source  4 , could be calculated by or would otherwise be known to one skilled in optics. 
   It is to be understood that the preceding is merely a detailed description of several embodiments of this invention and that numerous changes to the disclosed embodiments can be made in accordance with the disclosure herein without departing from the spirit or scope of the invention. The preceding description, therefore, is not meant to limit the scope of the invention. Rather, the scope of the invention is to be determined only by the appended claims and their equivalents.