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N R Heckenberg

Publications and source records attributed to N R Heckenberg.

8 recordsLinked to original sources
PubMed2005

Linewidth reduction in a large-smile laser diode array.

We present theory and simulations for a spectral narrowing scheme for laser diode arrays (LDAs) that employs optical feedback from a diffraction grating. We calculate the effect of the so-called smile of the LDA and show that it is possible to reduce the effect by using a cylindrical lens set at an angle to the beam. The scheme is implemented on a 19-element LDA with smile of 7.6 microm and yields frequency narrowing from a free-running width of 2 to 0.15 nm. The experimental results are in good agreement with the theory.

Journal Article↗
PubMed2004

Laser frequency locking by direct measurement of detuning.

We present a new method of laser frequency locking in which the feedback signal is directly proportional to the detuning from an atomic transition, even at detunings many times the natural linewidth of the transition. Our method is a form of sub-Doppler polarization spectroscopy, based on measuring two Stokes parameters (I2 and I3) of light transmitted through a vapor cell. It extends the linear capture range of the lock loop by as much as an order of magnitude and provides frequency discrimination equivalent to or better than those of other commonly used locking techniques.

Journal Article↗
PubMed2003

Controlling chaos in a Lorenz-like system using feedback.

We demonstrate that the dynamics of an autonomous chaotic laser can be controlled to a periodic or steady state under self-synchronization. In general, past the chaos threshold the dependence of the laser output on feedback applied to the pump is submerged in the Lorenz-like chaotic pulsation. However there exist specific feedback delays that stabilize the chaos to periodic behavior or even steady state. The range of control depends critically on the feedback delay time and amplitude. Our experimental results are compared with the complex Lorenz equations which show good agreement.

Journal Article↗
PubMed2002

Controlling the complex Lorenz equations by modulation.

We demonstrate that a system obeying the complex Lorenz equations in the deep chaotic regime can be controlled to periodic behavior by applying a modulation to the pump parameter. For arbitrary modulation frequency and amplitude there is no obvious simplification of the dynamics. However, we find that there are numerous windows where the chaotic system has been controlled to different periodic behaviors. The widths of these windows in parameter space are narrow, and the positions are related to the ratio of the modulation frequency of the pump to the average pulsation frequency of the output variable. These results are in good agreement with observations previously made in a far-infrared laser system.

Journal Article↗
PubMed2001

Transforming chaos to periodic oscillations.

We demonstrate that the dynamics of an autonomous chaotic class C laser can be controlled to a periodic state via external modulation of the pump. In the absence of modulation, above the chaos threshold, the laser exhibits Lorenz-like chaotic pulsations. The average amplitude and frequency of these pulsations depend on the pump power. We find that there exist parameter windows where modulation of the pump power extinguishes the chaos in favor of simpler periodic behavior. Moreover we find a number of locking ratios between the pump and laser output follow the Farey sequence.

Journal Article↗
PubMed2001

Dynamical tunnelling of ultracold atoms.

The divergence of quantum and classical descriptions of particle motion is clearly apparent in quantum tunnelling between two regions of classically stable motion. An archetype of such non-classical motion is tunnelling through an energy barrier. In the 1980s, a new process, 'dynamical' tunnelling, was predicted, involving no potential energy barrier; however, a constant of the motion (other than energy) still forbids classically the quantum-allowed motion. This process should occur, for example, in periodically driven, nonlinear hamiltonian systems with one degree of freedom. Such systems may be chaotic, consisting of regions in phase space of stable, regular motion embedded in a sea of chaos. Previous studies predicted dynamical tunnelling between these stable regions. Here we observe dynamical tunnelling of ultracold atoms from a Bose-Einstein condensate in an amplitude-modulated optical standing wave. Atoms coherently tunnel back and forth between their initial state of oscillatory motion (corresponding to an island of regular motion) and the state oscillating 180 degrees out of phase with the initial state.

Journal Article↗
PubMed2001

Micromanipulation of chloroplasts using optical tweezers.

This paper describes experiments using optical tweezers to probe chloroplast arrangement, shape and consistency in cells of living leaf tissue and in suspension. Dual optical tweezers provided two-point contact on a single chloroplast or two-point contact on two adhered chloroplasts for manipulation in suspension. Alternatively, a microstirrer consisting of a birefringent particle trapped in an elliptically polarized laser trap was used to induce motion and tumbling of a selected chloroplast suspended in a solution. We demonstrate that displacement of chloroplasts inside the cell is extremely difficult, presumably due to chloroplast adhesion to the cytoskeleton and connections between organelles. The study also confirms that the chloroplasts are very thin and extremely cup-shaped with a concave inner surface and a convex outer surface.

Chloroplasts↗