Contains table of contents for Part II, table of contains for Section 1 and reports on three research projects.Charles Stark Draper LaboratoryJoint Services Electronics Program (Contract DAAL03-86-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

Donoghue, John J.

Ezekiel, Shaoul

Hemmer, Philip R.

Kierstead, John D.

Lamela-Rivera, Horatio

Prentice, Mara G.

Shahriar, M. Selion

Smith, Stephen P.

Snitzer, Elias

Zarinetchi, Farhad

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.4L : , ,Part 11 Applied 'yscsIC 4 ASection Atomic, Molecular and OpticalSection 2 Plasma PhysicsSection 3 ElectromageticsV4A VV:a;4 ,isr~ 4~' I ~ ~ dA tt4ni A F :4 4ftrAt tt'~: fti.444* 444 4444' 4'4 4 115__ _116 RLE Progress Report Number 131Section 1 Atomic,PhysicsChapter 1Chapter 2Chapter 3Chapter 4Molecular and OpticalQuantum Optics and Photonics.Basic Atomic Physics.Molecular Physics.Small Angle X-Ray and Neutron Scattering -Its Application to Supramolecular Solutions.117118 RLE Progress Report Number 131Chapter 1. Quantum Optics and PhotonicsChapter 1. Quantum Optics and PhotonicsAcademic and Research StaffProfessor Shaoul Ezekiel, Dr. Philip R. Hemmer, Dr. Mara G. Prentice, Dr. Horatio Lamela-Rivera, Bruce Bernacki, John Kierstead, Dr. Elias SnitzerGraduate StudentsRobert Barat, M. Selion Shahriar, Stephen P. Smith, Farhad Zarinetchi, John DonoghueSponsorsCharles Stark Draper LaboratoryJoint Services Electronics Program (Contract DAAL03-86-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 Center1.1 Laser Raman ClockStudiesWe are continuing our precision studies ofstimulated resonance Raman interaction in anatomic beam to determine its applicability toclocks and to demonstrate any advantagesover conventional (microwave excited)atomic clocks. Up to this point, we haveachieved a stability of 1 x 10- 11 for a 5000second averaging time using a sodium beamand a dye laser. This time compares favor-ably with commercial cesium clocks after dif-ferences in atom transit time and transitionfrequency are taken into consideration.The stimulated resonance Raman process isillustrated in figure la in which we show aRaman transition between two long-livedstates, 1 and 3, induced by two laser fields atfrequencies (w and w2. These two laser fieldsare generated from the same laser to reducethe effects of laser jitter. It is well knownthat the Raman transition linewidth for weak,copropagating laser fields is primarily deter-mined by the decay rates of the long-livedstates 1 and 3. Therefore, the linewidth isset by the transit time. To achieve an effec-tively long transit time and a very narrowlinewidth, Ramsey's technique of separatedfield excitation is used (figure ib). Figure icshows a typical Raman-Ramsey fringepattern obtained with a sodium beam and adye laser. The central fringe linewidth in thisdata is 2.3 kHz, which is consistent with thetransit time. The Raman clock is simply amicrowave oscillator stabilized to the centralRaman-Ramsey fringe.1.1.1 Ramsey Fringes in a CesiumBeam Using a Semiconductor LaserRecently, we observed an ultra-narrow, 1kHz (FWHM) wide Raman-Ramsey fringe ina cesium atomic beam with a 15 cm inter-action zone separation and semiconductorlaser with a free running linewidth of 20M Hz. Our observation showed us that itmight be possible to develop a smaller,simpler, and less expensive clock based onthe resonance Raman effect in cesium with asemiconductor laser.The signal to noise ratio of the observedRamsey fringes, shown in figure 2, is about400 : 1 for a 1 second time constant. This isabout ten times smaller than the shot noiselimit expected for the present setup. Thelarge fluorescence background and thefluorescence intensity fluctuations caused bythe large degree of laser frequency jitter arethe primary causes of the lower signal tonoise ratio. Semiconductor lasers, which useoptical feedback techniques and with fre-quency jitters on the order of 1 MHz, couldreduce intensity fluctuations. To minimizethe fluorescence background, we will shieldthe interaction region magnetically so thatthe m = 0, Am = 0 Ramsey fringe will be119Chapter 1. Quantum Optics and Photonics2 3(b) atomicbeam)1W2( o0-(C) N_____ i ' I ' I-41/T 0 4/TW 2 -W I ---(T=L/v)Figure 1. (a) Schematic of stimulated resonanceRaman transition, Y2 is the linewidth of the intermediatestate. (b) Schematic configuration of the separatedfields Raman interaction. (c) A typical Raman-Ramseyfringe pattern. T is the transit time in the separatedfields interaction.centered in a dip in the fluorescence. Thisdip is produced by several overlapping singlezone Raman transitions between different mlevels in a manner similar to the way it isproduced in the sodium system. If weassume shot-noise limited detection, thefractional frequency stability would be 3 x10-11/T1/2. This would compare well withcommercially available cesium clocks. Thesignal to noise ratio could be improvedfurther by increasing the atomic beamthroughtput, a practice which is currentlybeing studied in optically pumped cesiumclocks.1 kHzFigure 2. Raman-Ramsey fringes in a cesium atomicbeam, obtained using a semiconductor laser with a freerunning linewidth of 20 MHz. The linewidth (FWHM)is 1 kHz, consistent with the transit time for a 15 cmzone separation and a 200 degree C oven temperature.The signal to noise ratio is 400.1.1.2 Error Sources in a SodiumRaman ClockSome of the error sources in a Raman clockare similar to those found in microwaveclocks and include the effects of shiftingpath length, external magnetic fields, back-ground slope, atomic beam misalignment,and second-order Doppler. Other sources oferrors are unique to the Raman clock andinclude laser frequency detuning, laser inten-sity changes, optical pumping variations,laser beam misalignment, optical atomicrecoil, and the presence of nearby hyperfinelevels.1.1.3 AC-Stark Shifts in a Two-ZoneRaman InteractionOur expanded study of laser detuning effects(common mode) include other effects inaddition to those from optical resonance thatmight occur, for example, because of misa-lignment of beams. This detuning causes thefringe minimum to shift in frequency as amanifestation of the AC-Stark effect. Previ-ously, we performed preliminary theoreticaland experimental investigations into theresults from this effect. We found that theshift depends on the amount of detuning, thelaser intensity, and the population differencebetween the ground levels before the inter-action began.We extended our investigations of theAC-Stark shift to consider the effects of laserintensity in greater detail and also to deter-120 RLE Progress Report Number 131mine the role of Zeeman sublevels. Forexample, we found experimentally that theAC-Stark shift is almost insensitive to thevariation of the ratio of the intensities of thelaser fields at the two frequencies as long asthe sum of the intensities remainsunchanged. When the theoretical expressionfor the AC-Stark shift was extended toinclude the case of unequal intensities, thedegree of insensitivity was predicted correctly(figure 3). This observation is unexpectedwhen the three level system is considered tobe two independent two-level systems.We also predicted theoretically that theAC-Stark shift does not depend strongly onthe exact laser intensity profile as long as thetime integrated intensity experienced by themoving atom remains unchanged. In a twolevel system, this would be analogous to theequivalent, for example, of two 7i pulses ofdifferent shapes, where 7r is the time intergralof the square root of the laser intensity (inunits of Rabi frequency) as experienced bythe atom. This predicted insensitivity to laserprofile has been verified experimentally(figure 4).Finally, we have augmented the theory of theRaman interaction in an ideal three levelsystem to account for the effects of the addi-tional Zeeman sublevels. We have accountedfor these effects to first order by introducingeffective within-system decay rates to theclosed three level system equations. Such anaugmented anomalous has successfully pre-dicted an experimentally observed anomalousslope reversal in the AC-Stark shift as a func-tion of detuning.The slope reversal mentioned above can beachieved by varying laser intensity or theinitial population difference between theground states. This implies that for a set ofvalues of these parameters, the slopebecomes small. Figure 5a shows a highresolution experimental plot of the AC-Starkshift versus laser detuning obtained underconditions that are experimentally determinedto minimize the slope of the AC-Stark shift.This data is shot noise limited and wasobtained with an averaging time of 1 sec. Ascan be seen, the AC-Stark shift is less thanthe noise over a laser detuning of +0.2y2where Y2 is the linewidth of the intermediatelevel. In comparison, figure 5b shows theChapter 1. Quantum Optics and Photonics(n /) 222/2 --r ~/2- /2 - 5 ---- +_____Figure 3. Illustration of insensitivity of AC-Stark shiftto differences in Rabi frequencies. (Top trace) Datashowing AC-Stark shift for unequal Rabi frequencies,I 1 2/ 2 12 = 2. (Middle trace) Theoretical AC-Starkshift for I 0 2/ 2 12 = 2. (Bottom trace) TheoreticalAC-Stark shift for 21 = II 2 I but the same(I 22 + 10 2 12). Here, f1 and f1 2 are the Rabi frequen-cies corresponding to the 1-2 and 3-2 transitions,respectively. 6 is the common mode laser detuning,and 0 is the AC-Stark shift, where 0 = -- correspondsto a shift of half the linewidth (FWHM) of the Ramseyfringe.theoretical AC-Stark shift under experimentalconditions in which the effective within-system decay rates have been used as a freeparameter to produce the flattest trace. Goodagreement between theory and experiment isclearly achieved when the effect of theZeeman sublevels is taken into account bythis one parameter fit.It should be noted that the suppressedAC-Stark shift of figure 6 (clock stability ofbetter than 2 x 10- 11 for a laser detuning of0.01 72 is still preliminary and does not repre-sent the ultimate achievable clock perform-121Chapter 1. Quantum Optics and PhotonicsFigure 4. Illustration of insensitivity of the AC-Starkshift to the exact laser beam profile. First interactionzone transit times, z, are as indicated (f22 remains con-stant, where Q2 is the average (squared) Rabi fre-quency). Top traces are experimental, bottom tracesare theory.ance. In the future, we plan to verify that theAC-Stark shift could be further suppressedby increasing the degree of Raman satu-ration. This could be achieved by eitherincreasing the laser intensity, or by slowingdown the atoms (by cooling, for example).In addition, numerical calculations are inprogress to identify the exact role played bythe Zeeman sublevels.1.1.4 Reduction of BackgroundSlope EffectsAnother important source of error is thebackground slope of the Raman-Ramseyfringe pattern. A variation in this slope,which might occur due to a change in thelaser intensity, corresponds to a shift in theclock frequency. One way to reduce thiseffect is to use the third, instead of first, har-monic of the modulation frequency in thephase sensitive demodulation. In general,third harmonic demodulation is accompaniedFigure 5. High resolution Ac-Stark shift vs. laserdetuning: (a) Experimental trace for 1 = 0.1 3 2,(Phl - p33)/N = 0.7 (normalized initial population differ-ence), and (b) Corresponding theoretical best fit, withr'= 0.27, where r' is the normalized difference of theeffective within-system decay rates.by a reduction in the signal to noise ratio ofat least 3, thus reducing the short term sta-bility. However, we have shown, exper-imentally as well as through numericalsimulation, that for the case of Raman-Ramsey fringes, the reduction in the signal tonoise ratio is less than a factor of 2 becauseof the unique shape of the Ramsey fringepattern. Therefore, the resulting reduction inthe short term stability using the third har-monic is much less than expected, and theincrease in the long term stability is pre-served. Our experimental determination ofthe amount of increase in the long term sta-bility is in progress.PublicationsHemmer, P.R., M.S. Shahriar, V.D. Natoli,and S. Ezekiel "AC-Stark Shifts in a TwoZone Raman Interaction," accepted forpublication in J. Opt. Soc. Am. B.Hemmer, P.R., H. Lamela-Rivera, S.P. Smith,B.E. Bernacki, M.S. Shahriar, and S.Ezekiel, "Observation of Ultra-NarrowRamsey Raman fringes in a CesiumAtomic Beam Using a SemiconductorLaser," submitted to Optics Lett.Shahriar, M.S., Raman Clock Studies, S.M.thesis, Dept. of Electr. Eng. and Comp.Sci., MIT, 1989.122 RLE Progress Report Number 1310.20 .1140/¥2 /0.1442270/W036Z 0 3 62 2-' 1Sfl_r,Chapter 1. Quantum Optics and Photonics1.2 Fiberoptic GyroscopeStudiesWe are studying a variety of issues con-cerning the performance of a fiberopticresonator gyroscope. In particular, weobserved stimulated Brillouin scattering andare examining its effect on gyro behavior andperformance. Furthermore, we have beenstudying the feasibility of a fiber lasergyroscope based on Brillouin scattering.1.3 Laser LinewidthNarrowingSingle frequency semiconductor lasersexhibit a rather broad spectral width in com-parison with other lasers. Many techniqueshave been developed for reducing thelinewidth of semiconductor lasers. Thesetechniques include use of optical feedbackand placement of the laser in an externalcavity. Our linewidth narrowing technique isbased on stimulated Brillouin scattering in afiber. Our preliminary experiments haveshown that it is possible to achieve verynarrow linewidths of less than a few Hertswith a good conversion efficiency.Professor Shaoul Ezekiel123Professor Daniel Kleppner124 RLE Progress Report Number 131

Quantum Optics and Photonics

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