We report an experimental investigation of the 5S 1/2 -7S 1/2 one-color two-photon transition in rubidium vapor with a single continuous-wave laser operating at 760 nm. By utilizing a robust intensity modulation method, high-resolution spectra with a signal-to-noise ratio of ~120 are obtained. A perfect match between experimental results and the theoretical analysis is obtained by characterizing the probability of a two-photon transition with vapor temperature and laser power. Moreover, the influence of electric dipole selection rules on the two-photon transition is also investigated by changing the polarization of the laser beam. Since the 5S 1/2 -7S 1/2 twophoton transition is insensitive to stray magnetic fields and has an extremely narrow linewidth, we confidently believe that it can provide a concrete frequency standard for quantum telecommunication in the S-band window (1480-1530 nm). Meanwhile, the atomic system at 760 nm will serve as a good candidate for a quantum repeater.

We report the measurement of the Kerr nonlinear refractive index of the rubidium vapor via the high sensitivity z-scan method by using an optical frequency comb. The novel self-focusing and self-defocusing effects of the vapor are presented with red and blue detunings of the laser frequency. The optical nonlinear characteristics of the rubidium vapor are clearly interpreted under different experimental parameters. Furthermore, the Kerr nonlinear refractive index n2 is obtained from the measured dispersion curve, and it basically occurs on the order of 10−6 cm2/W. The evolutions of the Kerr nonlinear coefficient n2 with the laser power and frequency detuning, respectively, are studied. To the best of our knowledge, the use of pulsed lasers to measure the Kerr nonlinear refractive index n2 of atomic vapor has not been reported yet. The direct measurement of the Kerr nonlinear coefficient will greatly help us understand and optimize nonlinear optical processes and find its more potential applications in quantum optics.

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We experimentally demonstrate the cesium electric quadrupole transition from the 6S 1/2 ground state to the 7D 3/2,5/2 excited state through a virtual level by using a single laser at 767 nm. The excited state energy level population is characterized by varying the laser power, the temperature of the vapor, and the polarization combinations of the laser beams. The optimized experimental parameters are obtained for a high resolution transition interval identification. The magnetic dipole coupling constant A and electric quadrupole coupling constant B for the 7D 3/2,5/2 states are precisely determined by using the hyperfine levels intervals. The results, A = 7.39 (0.06) MHz, B = -0.19 (0.18) MHz for the 7D 3/2 state, and A = -1.79 (0.05) MHz, B =1.05 (0.29) MHz for the 7D 5/2 state, are in good agreement with the previous reported results. This work is beneficial for the determination of atomic structure information and parity non-conservation, which paves the way for the field of precision measurements and atomic physics.

We investigate the input and propagation characteristics and geometric parametric instability of the partial Gaussian beam limited by the fiber face area in a graded-index multimode fiber. The theoretical simulation shows that the energy of the partial Gaussian beam and the coupling efficiency of the fiber face are restricted by the fiber face area for the different powers and spot sizes of the input Gaussian beam. The spot intensity pattern of the partial Gaussian beam exhibits a standard oscillating distribution in space as the beam undergoes periodic oscillations with propagation. Also, the dynamic evolution process from parametric sidebands to a supercontinuum is affected by the peak power, the spot size of the partial Gaussian beam, and the fiber length. Finally, the experimental output spectra with different powers of the partial Gaussian beam and fiber lengths in a graded-index multimode fiber confirm the prediction of theoretical simulations. This work provides practical guidance for optimizing supercontinuum source expansion and spectral power density.