Page 159 - Tunable Lasers Handbook
P. 159

4  CO,  Isotope lasers and Their Applications   139

                     kHz/Torr of the laser's gas mixture. Because the typical CO,  fill pressures in the
                     saturable absorber cells used to line-center-stabilize the lasers in the two-channel
                     calibration system were about 40 mTorr, a first-order guess-estimate indicated an
                     approximately  1.2-kHz systematic error in the beat measurements. The magni-
                     tude of  such an error was too small to worry about too much during the first few
                     years of calibrating the CO, laser transitions. When the uncertainties in the mea-
                     sured results  diminished  from  about  20 to  25  kHz  to  about 5 kHz  or less.  it
                     seemed prudent to initiate a more precise theoretical and experimental endeavor
                     for evaluating the  effect of pressure shift on the frequency calibration of  CO,  -
                     laser  transitions.  Thus  "Pressure  Shifts  in  Carbon  Dioxide  and  Its  Isotopes"
                     became  the topic  of  the  PhD  dissertation  of  SooHoo who  then  proceeded  to
                     compile a vast amount of experimental data and all available theoretical interpre-
                     tations that took years of  assiduous work  [112]. The in many ways  surprising
                     outcome of  this research was summarized in two publications by  SooHoo et a/.




                             l   ~   l  '  l   ~   l  '   l
                                                             co,   lLP(20)
                                                         1   63  kHz/Torr
                                                        4                            1
                                        BLUE  SHIFT          BLUE  SHIFT/      /
                                                                                     !


                                                                                L 1



                             I I  ~ ~  I I  ~ ~  I I  ' '  I I  ' '  I I
                                                        ,t        'r\



                            la
                         13 la
                         13
                           co,
                           co,   I-R(20)                     BLUE  SHIFT  >
                                I-R(20)
                           47
                              kHz/Torr
                           47  kHz/Torr
                            BLUE  SHIFT                 6l                       ,  ,  ,
                       8                                8   ,   ,   ,  ,   ,  ,   ,  ,
                        0    20   40   60   80   100      0   20   40   60   80   100
                                               PRESSURE  (rn  Torr)
                     FIGURE  19  Typical pressure shift data sequences, all "blue"  shifts, one for each C02 isotope
                     and rotational-vibrational  branch transition. Note that a "blue shift" sequence may have either a posi-
                     tive or a negative slope depending on whether  the fixed reference  line was above or below the fre-
                     quency of  the transition  that  was pressure shifted. (Reprinted with permission  from SooHoo er  al.
                     [76]. 0 1985 IEEE.)
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