Contains reports on five research projects.Joint Services Electronics Program (Contract DAALO3-86-K-0002)National Science Foundation (Grant PHY 82-10369)U.S. Air Force - Office of Scientific Research (Contract F49620-82-C-0091)U.S. Air Force - Rome Air Development Cente

Barat, Robert

Bernacki, Bruce

Bevilaqua, J.

Ezekiel, Shaoul

Hawkeye, T.

Hemmer, Philip R.

Kierstead, John D.

Kuchar, J.

Lamela-Rivera, Horatio

Morris, D.

Neils, M. C.

Prentice, Mara G.

Shahriar, M. S.

Smith, Stephen P.

Zarinetchi, Farhad

DSpace@MIT

Quantum Optics and Photonics22.0 Quantum Optics and PhotonicsAcademic and Research StaffProf. S. Ezekiel, Dr. P.R. Hemmer, Dr. M.G. Prentice, Dr. H. Lamela-Rivera, B. Bernacki,D. Morris, J. KiersteadGraduate StudentsR. Barat, M.S. Shahriar, S.P. Smith, F. ZarinetchiUndergraduate StudentsJ. Bevilaqua, T. Hawkeye, J. Kuchar, M.C. NeilsSponsorsJoint Services Electronics Program (Contract DAALO3-86-K-0002)National Science Foundation (Grant PHY 82-10369)U.S. Air Force - Office of Scientific Research (Contract F49620-82-C-0091)U.S. Air Force - Rome Air Development Center22.1 Measurement of Fresnel-Drag in Moving MediaUsing Aring Resonator TechniqueIt has been known for many years that the observed velocity of light in a movingmedium differs from that in a stationary medium. This effect, namely the Fresnel-drag,was explained by the special theory of relativity. While special relativity has been veryaccurately tested, no tests of comparable precision can be claimed regarding thetheory's predictions for light propagation in moving media.We have performed careful measurements of the Fresnel-drag in various movingglass media and tested the dependence of the drag coefficient on refractive index anddispersion. A solid medium was used in our experiments to provide a precisely knownvelocity and index. To further ensure the accuracy of these measurements we havestudied the drag dependence on other key parameters such as the thickness and velocityof the glass samples, as well as the angle between the light beam and glass normal.Our technique is based on moving a glass plate of known index and dispersion backand forth inside a ring resonator as shown in figure 22.1. In this way, the Fresnel-draggenerated by the motion of the glass induces a nonreciprocal phase shift which mani-fests itself as a difference in the resonance frequencies of the cavity for oppositelypropagating field directions. The precision measurement of small resonance frequencydifference in a ring cavity is similar to the measurement of nonreciprocal phase shift dueto inertial rotation, i.e., Sagnac effect.The effective drag coefficient was measured for four glass samples namely; BK-7,SF-1, SF-57 and fused silica which have different indices of refraction and dispersion.Averaging fifteen separate measurements resulted in an overall agreement with theory153Quantum Optics and Photonicsthat was well within our + 2.8x10 - 4 measurement uncertainty, thus verifying the dragdependence on both the refractive index as well as the dispersion of the moving me-dium. These measurements provided the most accurate verification of Fresnel-drag thusfar.The use of a solid medium together with ac detection techniques contributed to theimproved experimental verification of the Fresnel-drag reported here. Aside from vary-ing sample index and dispersion we also varied sample thickness, velocity and angleof incidence. Further improvement in the accuracy of the measurements can be madeby placing the cavity within an evacuated chamber to exclude air currents, and by usinga smoother bearing for guiding the motion of the glass sample.PublicationSanders, G.A., and S. Ezekiel, "Measurement of Fresnel-Drag in Moving Media Usinga Ring Resonator Technique," J. Opt. Soc Am. B (1988), to be published).NEDIUNr- -- ---- - -.cAU II' A.i -~CcFigure 22.1 Experimental setup.22.2 Observation of Ultra-Narrow Raman RamseyFringes in a Cesium Atomic Beam Using aSemiconductor LaserWe have observed an ultra narrow, 4 kHz wide Raman Ramsey fringe in a cesiumatomic beam using a semiconductor laser with a free running linewidth of 45 MHz. To154 RLE Progress Report Number 130It - -- _- -Quantum Optics and Photonicsour knowledge, this is the narrowest atomic resonance recorded using a semiconductorlaser and has applications in the development of new time and frequency standards.A clock based on the resonance Raman effect in cesium could be smaller, simpler andless expensive than a conventional cesium clock.The stimulated resonance Raman process is illustrated in figure 22.2 where we showa Raman transition between two long-lived states, 1 and 3, induced by two laser fieldsat frequencies wc, and 02. As is well-known, the Raman transition linewidth for weakcopropagating laser fields is primarily determined by the decay rates of the long livedstates 1 and 3. Thus, the linewidth is set by the transit time. To achieve an effectivelylong transit time, and thus a very narrow linewidth, Ramsey's technique of separatedfield excitation is used.2852 nm1Figure 22.2 Schematic of Raman transition in cesium.In our present experiment, the long-lived states 1 and 3 in figure 22.2 are respectivelythe 62S1/ 2(F = 3) and (F=4) ground state hyperfine levels in cesium separated by 9.2GHz, and state 2 is the 62P3/2 (F = 4) level. The optical transitions are components ofthe cesium D2 line at 852 nm.The experimental setup is shown schematically in figure 22.3. The laser diode is asingle frequency, double heterostructure, AIGaAs laser (Hitachi HLP-1400) operatingat 852 nm and mounted on a thermoelectric device. The linewidth of the laser was 45MHz as measured by a Fabry Perot interferometer.Generation of the two optical fields, w, and w2 is accomplished by modulating thelaser current at 4.6 GHz with a microwave VCO, shown in figure 22.3, thus yielding twoamplitude modulation sidebands separated by 9.2 GHz. By generating wc and w2 in thismanner, the effect of laser jitter can be eliminated. In our previous work using a sodiumbeam and a dye laser, the second optical field was generated by frequency shifting inan external Bragg cell.Figure 22.4(a) shows the modulation sidebands of the modulated laser as measuredby a short, plane-parallel, scanning Fabry-Perot cavity with a free spectral range of 25GHz. As can be seen, the separation between sidebands is 4.6 GHz and that there isno evidence of any laser instabilities. In contrast, figure 22.4(b) shows the spectrumof the unmodulated single mode laser.155Quantum Optics and PhotonicsCesiumBeam9.2 GHz -~ 4+-LaserDiode7.5 cm -- 'BS.PSD 2Figure 22.3. Experimental setup.As indicated in figure 22.3, the output from the modulated laser interacts with thecesium atomic beam in zones "A" and "B," and the fluorescence from zone "B" is col-lected onto a photomultiplier tube. Figure 22.5 shows the demodulated fluorescencefrom zone "B" which exhibits the Ramsey fringes as the microwave oscillator is slowlyswept through the center of one of the cesium Zeeman transitions. As shown in thefigure, the central fringe has a width of 4 kHz, which is in good agreement with thepredicted width for a 7.5 cm zone separation and a 200'C oven temperature. To ob-serve the Raman transition, the laser and sidebands were held by means of a servo nearthe maximum of the fluorescence lineshape in zone "B." It should be noted that theseRamsey fringes are still preliminary and the details of the fringe shapes have yet to becarefully investigated.As in the sodium Raman clock studies, the central Ramsey fringe associated with themagnetic field insensitive m = 0, 6 m = 0 transition can be used as a reference for thestabilization of the microwave oscillator. The Raman clock would have an advantageover the conventional cesium clock, in that it does not require state selection magnets,nor a microwave cavity.156 RLE Progress Report Number 130IQuantum Optics and Photonicslo-25 GHz- -25 GHz- 1(b,Figure 22.4 Spectrum of: (a) Modulated laser at 4.6 GHz (b) Unmodulated laser.4 kHz- - - - ~ft f-- - = -Figure 22.5 Ramsey fringes for a 7.5 cm zone separation.Although optical pumping has recently been considered as a replacement for the stateselection magnets in the conventional cesium clock, the microwave cavity, however, isstill needed.Future work will be focused on the study of error sources in semiconductor excitedcesium Raman clocks. In addition, several improvements to our present setup are beingconsidered. For example, we propose to use a laser with a spectral width equal to orless than the cesium natural width of 5 MHz instead of the present 45 MHz. Also, wewish to consider simple methods of laser cooling the cesium beam, to increase thetransit time for a given zone separation and oven temperature.Since semiconductor lasers are also available at 780 nm near the resonanceexcitation of rubidium, it would be worthwhile to consider a rubidium Raman clock inaddition to cesium. Finally, the possibility of a millimeter wave Raman clock is alsobeing explored.157(a)Quantum Optics and PhotonicsPublicationHemmer, P.R., H. Lamela-Rivera, S.P. Smith, B.E. Bernacki, and S. Ezekiel, "Observa-tion of Ultra-Narrow Ramsey Raman Fringes in a Cesium Atomic Beam Using aSemiconductor Laser," submitted to Optics Lett.22.3 Coupled Pendulum Model of the StimulatedResonance Raman EffectThe stimulated resonance Raman effect has found numerous potential applicationsin such diverse areas as spectroscopy, collisional studies and Raman lasers. In addition,we are exploring the possibility of using the stimulated resonance Raman interaction forportable clock development (see section 22.2 above). To date, there has not been asimple physical model to describe this interaction. Theoretical treatments using eitherperturbation theory or dressed states have been devised. The dressed state approachhas been more successful because it describes the existence of a "trapped state" whichis transparent to resonant excitation fields. This accounts for the non-absorption reso-nances which have been observed experimentally by a number of researchers. How-ever, these models do not offer a simple physical explanation of either the formation ofthe trapped state or the influence of experimental conditions on observables.In our research we have shown that the analogy between the stimulated resonanceRaman interaction and the well-known system of three classical coupled pendulumscan provide considerable physical insight. A set of three classical coupled pendulumsis used to model the stimulated resonance Raman interaction. This model provides asimple, intuitive, physical description of the resonance Raman process and can also beused to interpret experimental observations, including the dynamics of Raman inducedtransparency, the physical nature of Ramsey fringes in separated field excitation, andthe effects of off-resonant laser excitation. The model has also been extended to sug-gest what might be observed for very strong laser fields.PublicationHemmer, P.R., and M.G. Prentiss, "Coupled Pendulum Model of the Stimulated Reso-nance Raman Effect," submitted to J. Opt. Soc. Am. B.22.4 Laser Raman Clock Using SodiumWe have continued our precision studies of dye laser induced stimulated resonanceRaman interactions in a sodium atomic beam with emphasis on Ramsey's method ofseparated oscillatory fields. We observed Raman-Ramsey fringes for a field separationof up to 30 cm, and the data were consistent with theoretical predictions. We have alsobeen investigating the performance of a clock based on this interaction in a sodiumatomic beam to determine the feasibility of such a scheme and to demonstrate anypossible advantages over conventional microwave excited clocks. Recent performanceof our sodium Raman clock showed a stability of 1x10-11 for a 5000 second averagingtime. This compares favorably with commercial cesium clocks when difference in atomtransit time and transition frequency are taken into consideration.158 RLE Progress Report Number 130Quantum Optics and PhotonicsCurrently we are studying potential sources of long term frequency error in a Ramanclock. Some of the error sources are similar to those in microwave clocks, such as theeffects of path length shift, external magnetic fields, background slope, atomic beammisalignment and second order Doppler. The other error sources are unique to theRaman clock and include laser frequency detuning, laser intensity changes, laser beammisalignment, optical atomic recoil, the presence of nearby hyperfine levels, and othersmall effects.Recently we have performed theoretical and experimental studies of an importanterror source related to Ramsey fringe phaseshift as a function of laser detuning in a twozone Raman excitation scheme. In our set up, laser detuning arises from many reasonssuch as laser drift away from the optical resonance (see section 22.2 above), misalign-ment of beams, and so on. Experimentally, we found that the Ramsey fringe phase shiftdepends strongly on initial state preparation, as well as, laser intensity in the interactionregions. We have used the Bloch vector approach to model the Raman interaction inthe separated field excitation and have found that the observed phase shift is simply thephase of the 1-3 off-diagonal density matrix element at the end of the first interactionzone. This phase shift is therefore of fundamental interest and, in fact, is directly relatedto the phase difference between the atom-field dressed states.The model we developed renders the Raman system almost as simple as the two levelmicrowave system, and yet is complete enough to predict experimental observables withquantitative accuracy. Figures 22.6 (a) and 22.6 (b) show experimentally observedshift in the clock frequency as a function of laser detuning under two different sets ofconditions. Figure 22.6 (c) and 22.6 (d) show theoretical predictions, according to ourmodel, of the frequency shift we should expect corresponding to the same conditions.As can be seen, the results agree quite well both qualitatively and quantitatively.Moreover, we have verified this agreement over a wide range of parameters.Specifically, we have investigated the effects of varying, independently, the initialpopulation difference between levels 1 and 3 and the laser intensity in the interactionzones. Figures 22.6 (b) and 22.6 (d) correspond to a large initial population differenceand low intensity. As can be seen, the clock frequency is extremely sensitive to laserdetuning under this condition. Theory predicts, as shown in figure 22.6 (c), that thissensitivity becomes very small for small detuning if we use high intensity along with asmall difference in population. Figure 22.6 (a) confirms this experimentally.In summary, we are now able to choose operating conditions under which the clockis least sensitive to laser detuning. Theory also shows that under such conditions, theclock is also least sensitive to changes in laser intensity and population difference.Our current atomic beam design is such that we were not able to run the clock undersuch an optimal condition. We recently finished construction of a new beam that willrun under the desired conditions. Moreover, the new beam is designed to have in-creased signal-to-noise ratio, be more stable, and be less sensitive to external magneticfields.159Quantum Optics and Photonics-I . 0 - _l_(0) (b)16000wooo (0) ()1600Figure 22.6 Comparison of experimental results with theoretical predictions of Raman clock frequency shiftas a function of laser detuning.22.5 Alignment Insensitive Technique for Wideband andTuning of an Unmodified Semiconductor LaserMany applications such as laser spectroscopy, optical pumping and isotope sepa-ration require a tunable narrow-linewidth source in the near infrared. Semiconductorlasers are an attractive alternative for these applications due to their relatively low cost,small size, and simplicity of operation. However, to date, semiconductor lasers havefound limited spectroscopic applications because of their inability to be tuned to arbi-trary frequencies of interest within the laser gain curve. To alleviate this problem, op-tical feedback schemes have been devised using diffraction gratings or etalons toprovide frequency selective feedback. However, all these techniques require the useof antireflection-coated lasers and, in addition, are very sensitive to alignment of theexternal optics. These factors detract from the simplicity and low cost aspects inherentin semiconductor lasers.We have developed a simple optical feedback technique that permits an ordinarysemiconductor diode laser with no antireflection-coated facets to be tuned to arbitraryfrequencies within the laser gain curve. In addition, this method is not highly sensitiveto optical misalignments. Briefly, a one piece cat's eye retro-reflector serves as the160 RLE Progress Report Number 130Quantum Optics and Photonicsfeedback mirror and a tilted intra-cavity solid etalon provides frequency selectivity. Theuse of a one piece retro-reflector contributes greatly to the simplicity of the techniquebecause it eliminates the need for angular adjustment of the feedback mirror. Theetalon, having approximately one-fifth the optical thickness of the semiconductor laser,forces the laser to lase in the longitudinal mode of interest.Using our set up we were able to excite the D2 line in atomic Cesium very repeatablyusing a nominally 852 nm laser. If the laser were used without feedback, it would nothave been possible to excite the cesium transition reliably using either injection currentor temperature tuning.PublicationBernacki, B.E., P.R. Hemmer, S.P. Smith and S. Ezekiel, "Alignment Insensitive Tech-nique for Wideband Tuning of an Unmodified Semiconductor Laser," submitted toOptics Lett.Professor Daniel Kleppner162 RLE Progress Report Number 130

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