Scintillation properties of a rectangular dark hollow beam

Yangjian Cai, Yan Zhang

The scintillation properties of a rectangular dark hollow beam (DHB) in a weak turbulent atmosphere are investigated. Explicit expression for the on-axis scintillation index of a rectangular DHB is derived. It is found that the scintillation index value of a rectangular DHB can be smaller than that of Gaussian, elliptical Gaussian and rectangular flat-topped beams in a weak turbulent atmosphere under certain conditions. Our results will be useful in long-distance free-space optical communications. The scintillation properties of a rectangular DHB are closely controlled by its initial beam parameters.

Optimizing anti-Stokes Raman scattering in gas-filled hollow-core photonic crystal fibers

A. Nazarkin, A. Abdolvand, P. St. J. Russell

Anti-Stokes Raman scattering in gas-filled hollow-core photonic crystal fibers is discussed. It is shown that the efficient anti-Stokes generation observed under conditions of significant wave mismatch is caused by phase locking of the interacting fields. This leads to the establishment of a phase difference that is independent of the optical path. An optimization technique, based on the adjustment of the wave mismatch along a gas-filled hollow fiber using pressure control, is proposed. Anti-Stokes conversion efficiencies close to the theoretical maximum of 50% are predicted.

Reduction of Nonlinear Phase Noise in DPSK Transmission Using a NALM

C. Stephan, K. Sponsel, G. Onishchukov, B. Schmauss, G. Leuchs

Performance of a NALM as phase-preserving amplitude 2R-regenerator in a DPSK transmission system has been investigated experimentally and numerically. A 3dB improvement of eye opening or alternatively a 3dB increase of fibre-launched power are demonstrated.

A vibration-insensitive optical cavity and absolute determination of its ultrahigh stability

Y. N. Zhao, J. Zhang, A. Stejskal, T. Liu, V. Elman, Z. H. Lu, L. J. Wang

We use the three-cornered-hat method to evaluate the absolute frequency stabilities of three different ultrastable reference cavities, one of which has a vibration-insensitive design that does not even require vibration isolation. An Nd: YAG laser and a diode laser are implemented as light sources. We observe similar to 1 Hz beat note linewidths between all three cavities. The measurement demonstrates that the vibration-insensitive cavity has a good frequency stability over the entire measurement time from 100 mu s to 200 s. An absolute, correlation-removed Allan deviation of 1.4 x 10(-15) at 1 s of this cavity is obtained, giving a frequency uncertainty of only 0.44 Hz. (C) 2009 Optical Society of America

Continuous Variable Entanglement and Squeezing of Orbital Angular Momentum States

M. Lassen, G. Leuchs, U. L. Andersen

We report the first experimental characterization of the first-order continuous variable orbital angular momentum states. Using a spatially nondegenerate optical parametric oscillator (OPO) we produce quadrature entanglement between the two first-order Laguerre-Gauss modes. The family of orbital angular momentum modes is mapped on an orbital Poincare sphere, where the mode's position on the sphere is spanned by the three orbital parameters. Using a nondegenerate OPO we produce squeezing of these parameters, and as an illustration, we reconstruct the "cigar-shaped" uncertainty volume on the orbital Poincare sphere.

Solitary Pulse Generation by Backward Raman Scattering in H-2-Filled Photonic Crystal Fibers

A. Abdolvand, A. Nazarkin, A. V. Chugreev, C. F. Kaminski, P. St. J. Russell

Using a hydrogen-filled hollow-core photonic crystal fiber as a nonlinear optical gas cell, we study amplification of ns-laser pulses by backward rotational Raman scattering. We find that the amplification process has two characteristic stages. Initially, the pulse energy grows and its duration shortens due to gain saturation at the trailing edge of the pulse. This phase is followed by formation of a symmetric pulse with a duration significantly shorter than the phase relaxation time of the Raman transition. Stabilization of the Stokes pulse profile to a solitonlike hyperbolic secant shape occurs as a result of nonlinear amplification at its front edge and nonlinear absorption at its trailing edge (caused by energy conversion back to the pump field), leading to a reshaped pulse envelope that travels at superluminal velocity.

Correlation measurement of squeezed light

Leonid A. Krivitsky, Ulrik L. Andersen, Ruifang Dong, Alexander Huck, Christoffer Wittmann, Gerd Leuchs

We study the implementation of a correlation measurement technique for the characterization of squeezed light which is nearly free of electronic noise. With two different sources of squeezed light, we show that the sign of the covariance coefficient, revealed from the time-resolved correlation data, is witnessing the presence of squeezing in the system. Furthermore, we estimate the degree of squeezing using the correlation method and compare it to the standard homodyne measurement scheme. We show that the role of electronic detector noise is minimized using the correlation approach as opposed to homodyning where it often becomes a crucial issue.

A method to unmix multiple fluorophores in microscopy images with minimal a priori information

S. Schlachter, S. Schwedler, A. Esposito, G. S. Kaminski Schierle, G. D. Moggridge, C. F. Kaminski

The ability to quantify the fluorescence signals from multiply labeled biological samples is highly desirable in the life sciences but often difficult, because of spectral overlap between fluorescent species and the presence of autofluorescence. Several so called unmixing algorithms have been developed to address this problem. Here, we present a novel algorithm that combines measurements of lifetime and spectrum to achieve unmixing without a priori information on the spectral properties of the fluorophore labels. The only assumption made is that the lifetimes of the fluorophores differ. Our method combines global analysis for a measurement of lifetime distributions with singular value decomposition to recover individual fluorescence spectra. We demonstrate the technique on simulated datasets and subsequently by an experiment on a biological sample. The method is computationally efficient and straightforward to implement. Applications range from histopathology of complex and multiply labelled samples to functional imaging in live cells. (C) 2009 Optical Society of America

Demonstration of a universal one-way quantum quadratic phase gate

Yoshichika Miwa, Jun-ichi Yoshikawa, Peter van Loock, Akira Furusawa

We demonstrate a quadratic phase gate for one-way quantum computation in the continuous-variable regime. This canonical gate, together with phase-space displacements and Fourier rotations, completes the set of universal gates for realizing any single-mode Gaussian transformation such as arbitrary squeezing. As opposed to previous implementations of measurement-based squeezers, the current gate is fully controlled by the local oscillator phase of the homodyne detector. Verifying this controllability, we give an experimental demonstration of the principles of one-way quantum computation over continuous variables. Moreover, we can observe sub-shot-noise quadrature variances in the output states, confirming that nonclassical states are created through cluster computation.

Experimental observation of spectral Bloch oscillations

Christoph Bersch, Georgy Onishchukov, Ulf Peschel

We report on the first, to our knowledge, experimental observation of spectral Bloch oscillations in an optical fiber employing the interaction between a probe signal and a traveling-wave periodic potential. The spectrum of weak probe pulses is shown to oscillate on account of their group-velocity mismatch to the periodic field. The behavior of a cw probe spectrum reveals the actual discrete nature of the effect. Recurrences of the spectrum after one and two Bloch periods are demonstrated. (C) 2009 Optical Society of America