Contains reports on nine research projects.U.S. Air Force - Office of Scientific Research (Contract F49620-82-C-0091)U.S. Air Force - Rome Air Development CenterJoint Services Electronics Program (Contract DAAG29-83-K-0003)National Science Foundation Grant (Grant PHY 82-710369

Bernacki, Bruce

Ezekiel, Shaoul

Hemmer, Philip R.

Hergenroeder, Leo

Jain, Sudhanshu H.

Kierstead, John D.

Kuchar, J.

Lamela-Rivera, Horatio

Miller, S.

Morris, D.

Natoli, V. D.

Neils, C.

O'Connor, C. E., III

Pevtschin, Veronique

Prentiss, Mara G.

Rizk, R. S.

Robinson, I.

Shahriar, M. S.

Smith, Stephen P.

Zarinetchi, Farhad

DSpace@MIT

Quantum Optics and Photonics21.0 Quantum Optics and PhotonicsAcademic and Research StaffProf. S. Ezekiel, Dr. P.R. Hemmer, J. Kierstead, Dr. H. Lamela-Rivera, B. Bernacki, D.MorrisGraduate StudentsL. Hergenroeder, S.H. Jain, V.D. Natoli, C.E. O'Connor 111, V. Pevtschin, M.G. Prentiss,M.S. Shahriar, S.P. Smith, F. ZarinetchiUndergraduate StudentsS. Miller, I. Robinson, R.S. Rizk, C. Neils, J. KucharU.S. Air Force - Office of Scientific Research (Contract F49620-82-C-0091)U.S. Air Force - Rome Air Development CenterJoint Services Electronics Program (Contract DAAG29-83-K-0003)National Science Foundation Grant (Grant PHY 82- 710369)21.1 Investigation of Error Sources in a Laser RamanClockWe have continued our precision studies of 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 performanceshowed a stability of 1 x 10-11 for a 5000 second averaging time. This compares fa-vorably with commercial cesium clocks when difference in atom transit time and tran-sition frequency are taken into consideration.Currently we are studying potential sources of long term frequency error in theRaman clock. Some of the error sources are similar to those in microwave clocks, suchas the effects of path length phase shift, external magnetic fields, background slope,atomic beam misalignment and second order Doppler. The other error sources areunique to the Raman clock and include laser frequency detuning, laser intensitychanges, laser beam misalignment, optical atomic recoil, the presence of nearbyhyperfine levels, and other smaller effects.Our present investigations have centered around error sources that are unique to theRaman process. For example, we have found that laser detuning from resonance by1% of the atomic linewidth can cause a fractional error of 2.4 x 10- 11. Also a 1% changein laser intensity can generate an error of 2.5 x 10- 12. Efforts are underway to find waysof reducing both detuning and intensity errors. In addition, laser misalignment can alsobe a source of significant error. A laser beam translation of 0.1 mm can cause an error139Quantum Optics and Photonicsof 3 x 10-11. The use of fiberoptics is being investigated for the minimization of misa-lignment error.PublicationsHemmer, P.H., G.P. Ontai, and S. Ezekiel, "Precision Studies of Stimulated ResonanceRaman Interactions in an Atomic Beam," J. Opt. Soc. Am. B (1986).Hemmer, P.H., V.D. Natoli, M.S. Shahriar, B. Bernacki, H. Lamela-Rivera, S.P. Smithand S. Ezekiel, "Study of Several Error Sources in a Laser Raman Clock," Pro-ceedings of 41st Annual Frequency Control Symposium, Philadelphia,Pennsylvania, 1987.21.2 Physics of Stimulated Resonance Raman EffectIn conjunction with our experimental studies of the stimulated resonance Raman ef-fect for potential clock applications, we have also performed detailed theoretical studies.Among other things, these theoretical studies have resulted in the development of aclassical mechanical model for the quantum mechanical resonance Raman process,namely a set of three classical coupled pendulums.In this pendulum model, individual pendulum oscillations correspond to atom fieldcomposite states and the coupling springs serve to couple the pendulum oscillations inthe same way that the laser field couples atom field composite states. Using standardapproximations for atom field calculations, it is found that a one to one correspondenceexists between the (complex) pendulum oscillation amplitudes and the (complex)composite state amplitudes. Moreover, it is also found that the familiar atom field"dressed" states are analogous to normal modes of the coupled pendulum system.Thus, the pendulum system provides the means to actually "see" what happens inthe resonance Raman interactions. This allows us to give simple physical interpretationsto many of our more puzzling experimental observations. For example, in our two zoneRaman studies in an atomic beam, we find that the effects of laser detuning becomessmaller at higher laser intensities. Using pendulums, we find that this occurs becausethe Raman interaction generates a single atom field superposition dressed state at highlaser intensities, (single pendulum mode); but laser detuning effects result from the in-terference of two superposition states in the region between interaction zones.In addition to the pendulum analogy, we are also performing density matrix calcu-lations of the Raman lineshapes and comparing these to our experimental results.21.3 Observation of Raman-Ramsey Fringes in a CesiumAtomic Beam Using a Semiconductor LaserStimulated resonance Raman interactions have been observed in a cesium atomicbeam using a semiconductor laser at 852 nm. The semiconductor laser, which has alinewidth of 30 MHz, was amplitude modulated at about 4.6 GHz and the resultingsidebands were used to excite the Raman transition at 9.212 GHz. Using separated fieldexcitation, Ramsey fringes for a separation of 8 cm have been observed with a linewidth140 R.L.E. P.R. No. 129Quantum Optics and Photonicsof approximately 2 kHz. This fringe width is consistent with the theoretically predictedvalue. Work is under way to use the Raman/Ramsey fringe to stabilize a microwaveoscillator in a way similar to our sodium Raman clock employing a dye laser.The interest in the semiconductor laser excited Raman cesium clock is based on itspotential small size, light weight, and low cost, as well as high performance.21.4 Influence of Atomic Recoil on the Spectrum ofResonance Fluorescence From a Two-Level AtomThe fluorescence spectrum of a two-level atom, which is of fundamental importanceto the understanding of atom-field interaction, has received considerable attentionduring the past few years. Calculations show that for a stationary two-level atom in amonochromatic excitation field, the fluorescence spectrum is composed of three peakswhich are symmetric with respect to the excitation-field frequency. A number of ex-periments have been conducted to measure this spectrum, and such symmetric spectrahave indeed been observed. However, under certain conditions, asymmetric spectrawere also observed. In particular, we observed a symmetric spectrum for atoms in auniform field and an asymmetric spectrum in a field gradient.We have recently investigated the cause of this asymmetry which has been hithertounexplained and we now feel confident that it is caused by atomic recoil. We haveperformed a detailed calculation that takes into account atomic recoil, the field gradientand the direction of the observation of the fluorescence and showed that the spectrumof resonance fluorescence becomes asymmetric consistent with observations. More-over, we also showed that by including the forces on the atoms due to the laser field,asymmetry can also occur in a uniform field.PublicationsPrentiss, M.G. and S. Ezekiel, "Influence of Atomic Recoil on the Spectrum of Reso-nance Fluorescence From a Two-Level Atom," Phys. Rev. A 35, 922 (1987).21.5 Studies in a Passive Resonator GyroscopeA passive resonator rotation sensor or "gyroscope" is a ring resonator in whichcounter-propagating light beams within the resonator experience a non-reciprocalphase shift due to an applied rotation rate. Because of this phase shift the resonancefrequency of the resonator for the counter-propagating directions are not identical. Thisdifference in resonance frequency, Af, is measured by means external to the resonator,e.g., by locking the frequencies of the external laser beams to corresponding resonancesof the cavity.Our research demonstrated that because of backscattering within the cavity, the dif-ference in resonance frequency Af cannot be measured as long as the applied rotationis below a certain value determined by the degree of backscattering. This behaviorwhich is called "lock-in" has long been observed in ring laser gyros where a laser am-plifier is placed within the resonator. In our passive approach, lock-in occurs if all theQuantum Optics and Photonicsinformation about backscattering is fed faithfully to the servo loops which hold the ex-ternal laser frequencies to the cavity resonance.Lock-in was observed using mechanical rotation and also non-mechanical rotation,e.g., by sweeping the frequency of one of the external lasers. The theory of lock-inrelative to our passive set-up was developed and used to understand the observed be-havior in the neighborhood of lock-in. Finally, we demonstrated several non-mechanical schemes of eliminating lock-in in passive resonator gyroscope.The operation of servo loops which lock the external laser frequencies to the cavityresonance requires the generation of discriminants from the resonances of the cavity.These discriminants are produced by modulating the perimeter of the cavity sinusoidallyat 30 kHz and demodulating the output intensity of the cavity using a phase sensitivedetector (lock-in amplifier). In normal operation of the gyroscope the phase of thelock-in amplifier is adjusted to yield the largest demodulated output. In this case thelock-in amplifier is in phase with respect to the modulation on the perimeter of thecavity. When we set the phase of the lock-in amplifier to be 90 degrees with respectto the modulation on the cavity (i.e., detecting the quadrature signal) we did not getzero across the resonance lineshape. This indicated that the cavity lifetime influencedthe discriminant signal. This is a plausible argument since our cavity linewidth of 50kHz is comparable to the 30 kHz modulation frequency.The distortions observed in quadrature were in good agreement with our computergenerated models for this process, and furthermore, our model predicts a decrease in theamplitude of this distortion as we either increase the cavity linewidth or decrease themodulation frequency. We have experimentally verified this by first modulating at 1kHz, and then using a cavity with linewidth of 500 kHz. These distortions in thediscriminant are important because they can generate errors in the measurement of Af.The short term rms noise of the gyroscope with an integration time of 3 sec is 0.05deg/hr which is close to the shot noise on the light. Currently we are investigating bothshort term and long term error sources.PublicationsZarinetchi, F., and S. Ezekiel, "Observation of Lock-In Behavior in Passive ResonatorGyroscope," Optics Lett. 11, 401, (1986).21.6 Studies of Nonreciprocal Phase Shift in a FiberopticRing Resonator Gyroscope.A nonpolarization maintaining fiber optic ring resonator with a finesse of 80 has beenused to investigate error sources in a fiber ring resonator rotation sensor. Detailed ob-servations of the resonant backscatter revealed that Rayleigh backscatter was the pri-mary source of offset drift when a cavity modulation technique was used. Using thiscommon modulation technique a long-term variation of offset of 42o/hr was observed.Short-term noise three times the short noise limit was measured.By using external acousto-optic modulators to frequency modulate the twocounterpropagating beams at different frequencies prior to entering the resonator, the142 R.L.E. P.R. No. 129Quantum Optics and Photonicseffect of the backscatter was eliminated. Long-term variation of the offset on the orderof 80 /hr was observed. One possible source of this variation is the change in thebirefringence of the fiber. The use of polarization, or better, a single polarizationresonator would significantly reduce the effect of this birefringence. The short-termnoise was approximately 16 times higher than shot noise. This was attributed to thenoise added by the acousto-optic frequency modulators.Preliminary observations of the optical Kerr effect showed an offset of 20/hr/mwattinput power difference. This is in approximate agreement with the calculated value ofthe optical Kerr effect and may be eliminated by a simple servo technique.PublicationsEzekiel, S., "A Brief History of Fiber Gyros and M.I.T.'s Contributions to Fiber GyroDevelopment," S.P.I.E. 719, 9, (1987).21.7 Investigation of Noise in a Fiber InterferometerGyroscopeIn a multiturn fiber interferometer gyroscope a broadband light source, e.g., asuperluminescent diode (SLD), is used to reduce any undesirable backscatter and alsothe optical Kerr effect. However, SLDs do not produce a lot of power and their lifetimeis limited because they operate at a much higher current density than semiconductorlasers.We have investigated the use of a chirped frequency semiconductor laser to producean effectively broad width so as to replace the SLD. The frequency of a semiconductorlaser may be chirped by varying the injection current. A number of techniques forchirping the laser have been studied with emphasis on the effect of mode hopping andon any broadband noise created by the chirping.Using a chirped laser with a width of 28 GHz, we were able to reduce the opticalKerr effect from 500/hr to a negligible value. At the same time, we achieved a short termnoise close to the short noise with the intensity on the detector being one hundred timeslarger than with a superluminescent diode.21.8 Sensitive Techniques for Detection of ResidualHigher Order Modes in a Quasi-Single-Mode FiberPure single mode propagation in an optical fiber is important in many applications,such as wideband fiberoptic communication and precision interferometric fiberopticsensors.Various methods exist for evaluating single mode fibers that are based on cutoffwavelength measurements. For instance, the cutoff wavelength may be determined byobserving the near field pattern or by the rise in bending loss. Other methods are basedon mode interference. The cutoff wavelength methods are not able to detect a smallcontribution of higher order modes and the interferometric schemes either are elaborate,143Quantum Optics and Photonicsi.e., require an external interferometer setup, or they need special equipment such astunable sources. Our scheme is also based on mode interference within the fiber butrequires neither a special light source nor an elaborate setup.In our experiment light from a single frequency He-Ne source is focused into anoptical fiber, part of which is wrapped around a 2-cm diameter piezo-electric transducer(PZT). By driving the PZT with a sinusoidal signal at fm, we can generate a modulationof the difference between the propagation constants for the fundamental and a higherorder mode. The output of the fiber is detected on a movable small aperture photo di-ode and passed through an amplifier before phase sensitive demodulation by a lock-inamplifier. Using a multi-mode fiber with core diameter of 5.2 microns we were able todemonstrate the sensitivity of this technique. We measured residual higher order modeamplitudes as low as 10- 4 of the primary mode.PublicationsNakajima, Y., and S. Ezekiel, "Sensitive Determination of the Existence of Higher-OrderModes in a Quasi-Single-Mode Fiber," Opt. Lett. 11, 815, (1986).21.9 Investigation of Absolute Stability ofWater-Vapor-Stabilized Semiconductor LaserSingle-frequency semiconductor lasers are useful in a number of applications, suchas fiber optic communication, interferometry, sensors, and high-resolutionspectroscopy. In most of these applications the frequency of the laser must be long-term stable. A simple and convenient method of stabilizing the laser frequency is to lockit to a transition in an atomic or molecular vapor, e.g., cesium, rubidium, or water(H2 0) vapor. We have investigated the long term stability of the frequency of adouble-heterostructure AIGaAs laser locked to a transition in a simple water vapor cell.We demonstrated that the stability depended on the vapor temperature which influ-enced the pressure shift in water vapor. Pressure-induced frequency shifts resultingfrom temperature variations were found to range from 3 MHz/oC at room temperatureto 1 MHz/°C at O'C. We determined that an absolute stability of 1 part in 1010 with anaveraging time constant of 10 sec could be achieved by controlling the temperature ofthe water vapor to 0.01 C.PublicationsPevtschin, V., and S. Ezekiel, "Investigation of Absolute Stability of Water-Vapor-Stabilized Semiconductor Laser," Opt. Lett. 12, 172, (1987).144 R.L.E. P.R. No. 129

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