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Raman Optical Comb Generation in Hydrogen-filled Hollow Core Fiber

Chunbai Wu, Oregon Center for Optics, University of Oregon

(Session 5 : Friday from 5:00-7:00)

Abstract. Title: Raman Optical Comb Generation in Hydrogen-filled Hollow Core Fiber Chunbai Wu, Erin Mondloch, Cade Gledhill and M. G. Raymer Oregon Center for Optics, University of Oregon Abstract: Frequency comb generation has attracted many research efforts in recent years, with applications such as optical atomic clocks and attosecond pulse synthesis. In addition, super-continuum generation of light has been demonstrated in specially structured photonic crystal fibers. Recently, researchers at the University of Bath developed a large hollow-core (single-defect) fiber with Kagome- or square-lattice pattern cladding. [1] These fibers show high transmission spectra spanning from ultra-violet to infrared. High pressure hydrogen gas is filled in the fiber's hollow core throughout the length of the fiber, and up to 45 vibrational and rotational Raman transition lines of molecular hydrogen are observed following a high-power 10-ns IR laser pulse being coupled into the fiber. [2] The question outstanding is to what extent are the phases of these optical comb lines correlated. Perfect correlation would in principle allow deterministic phase tailoring to create attosecond pulses. A preliminary simplified model calculation indicated a high degree of correlation would exist. [2] To understand better this cascaded, coherent stimulated Raman scattering (SRS), we solve the quantum mechanical model of SRS [2,3] for the temporal evolution of first-order Stokes and anti-Stokes fields at the end of the fiber, as well as the spatial evolution of molecular polarization (collective vibrational state) stored in the hydrogen gas. (Higher order Raman lines are neglected from the equations because they are much weaker.) In the high-gain transient regime, the degree of anti-correlation between complex Stokes and anti-Stokes fields is calculated and found to equal unity throughout the duration of the pulses, even at large phase mismatch of wave vectors induced by the dispersion of the fiber. This result indicates that the generated first-order Stokes and anti-Stokes fields are nearly perfectly phase anti-correlated, although the absolute value of the phase is random due to the spontaneous initiation of the SRS process. Stokes and anti-Stokes fields are generated with opposite spectral phase. Further analysis on higher order Stokes and anti-Stokes fields is needed. In experiment, we collaborate with researchers at University of Bath (who produce the fiber). We designed a high-pressure gas chamber for filling the fiber at its ends, by using commercially-available "Swagelok" fittings. We have observed multiple Raman scattering lines from hydrogen and the team is now attempting to verify the phase coherence between them. Reference: [1] F. Couny, F. Benabid, P. S. Light, Opt. Lett. 31, 3574 (2006) [2] F. Couny, F. Benabid, P. J. Roberts, P. S. Light, M. G. Raymer, Science 318, 1118 (2007) [3] S. Ya. Kilin, Europhys. Lett. 5, 419 (1988)