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2007


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Frequency-domain displacement sensing with a fiber ring-resonator containing a variable gap

Vollmer, F., Fischer, P.

SENSORS AND ACTUATORS A-PHYSICAL, 134(2):410-413, 2007 (article)

Abstract
Ring-resonators are in general not amenable to strain-free (non-contact) displacement measurements. We show that this limitation may be overcome if the ring-resonator, here a fiber-loop, is designed to contain a gap, such that the light traverses a free-space part between two aligned waveguide ends. Displacements are determined with nanometer sensitivity by measuring the associated changes in the resonance frequencies. Miniaturization should increase the sensitivity of the ring-resonator interferometer. Ring geometries that contain an optical circulator can be used to profile reflective samples. (c) 2006 Elsevier B.V. All rights reserved.

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DOI [BibTex]

2007


DOI [BibTex]


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An easy to use bluetooth scatternet protocol for fast data exchange in wireless sensor networks and autonomous robots

Mockel, R., Spröwitz, A., Maye, J., Ijspeert, A. J.

In Proceedings of the 2007 IEEE/RSJ International Conference on Intelligent Robots and Systems, pages: 2801-2806, IEEE, San Diego, CA, 2007 (inproceedings)

Abstract
We present a Bluetooth scatternet protocol (SNP) that provides the user with a serial link to all connected members in a transparent wireless Bluetooth network. By using only local decision making we can reduce the overhead of our scatternet protocol dramatically. We show how our SNP software layer simplifies a variety of tasks like the synchronization of central pattern generator controllers for actuators, collecting sensory data and building modular robot structures. The whole Bluetooth software stack including our new scatternet layer is implemented on a single Bluetooth and memory chip. To verify and characterize the SNP we provide data from experiments using real hardware instead of software simulation. This gives a realistic overview of the scatternet performance showing higher order effects that are difficult to be simulated correctly and guaranties the correct function of the SNP in real world applications.

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DOI [BibTex]

DOI [BibTex]


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Observation of the Faraday effect via beam deflection in a longitudinal magnetic field

Ghosh, A., Hill, W., Fischer, P.

PHYSICAL REVIEW A, 76(5), 2007 (article)

Abstract
We show that magnetic-field-induced circular differential deflection of light can be observed in reflection or refraction at a single interface. The difference in the reflection or refraction angles between the two circular polarization components is a function of the magnetic-field strength and the Verdet constant, and permits the observation of the Faraday effect not via polarization rotation in transmission, but via changes in the propagation direction. Deflection measurements do not suffer from n-pi ambiguities and are shown to be another means to map magnetic fields with high axial resolution, or to determine the sign and magnitude of magnetic-field pulses in a single measurement.

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DOI [BibTex]


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Circular differential double diffraction in chiral media

Ghosh, A., Fazal, F. M., Fischer, P.

OPTICS LETTERS, 32(13):1836-1838, 2007 (article)

Abstract
In an optically active liquid the diffraction angle depends on the circular polarization state of the incident light beam. We report the observation of circular differential diffraction in an isotropic chiral medium, and we demonstrate that double diffraction is an alternate means to determine the handedness (enantiomeric excess) of a solution. (c) 2007 Optical Society of America.

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DOI [BibTex]

DOI [BibTex]

2004


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Simple and low-cost compliant leg-foot system

Meyer, F., Spröwitz, A., Lungarella, M., Berthouze, L.

In Proceedings of the IEEE/RSJ International Conference on Intelligent Robots and Systems (IROS 2004), 1, pages: 515-520, IEEE, Sendai, Japan, 2004 (inproceedings)

Abstract
We present the design of a simple and low- cost humanoid leg-foot system featuring compliant joints and springy feet. The mechanical compliance of the individual joints can be adjusted by means of visco-elastic material, or metal. To explore some of the relevant characteristics of the proposed system, we performed a series of experiments in which the leg was dropped from a fixed height. Combinations of different materials in the joints (silicone rubber, latex, and brass) as well as a rigid or a compliant foot were used. Additional data were obtained through of a Lagrangian analysis of the leg-foot system. Our analyses show that compliant joints not only reduce impactive forces, but also induce smoother joint trajectories. Further, by employing a compliant foot, a higher energy efficiency for the movement is achieved.

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DOI [BibTex]

2004


DOI [BibTex]

2001


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Isotropic second-order nonlinear optical susceptibilities

Fischer, P., Buckingham, A., Albrecht, A.

PHYSICAL REVIEW A, 64(5), 2001 (article)

Abstract
The second-order nonlinear optical susceptibility, in the electric dipole approximation, is only nonvanishing for materials that are noncentrosymmetric. Should the medium be isotropic, then only a chiral system. such as an optically active liquid, satisfies this symmetry requirement. We derive the quantum-mechanical form of the isotropic component of the sum- and difference-frequency susceptibility and discuss its unusual spectral properties. We show that any coherent second-order nonlinear optical process in a system of randomly oriented molecules requires the medium to be chiral. and the incident frequencies to be different and nonzero. Furthermore, a minimum of two nondegenerate excited molecular states are needed for the isotropic part of the susceptibility to be nonvanishing. The rotationally invariant susceptibility is zero in the static field limit and shows exceptionally sensitive resonance and dephasing effects that are particular to chiral centers.

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DOI [BibTex]

2001


DOI [BibTex]


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Reply to “Comment on ‘Phenomenological damping in optical response tensors’”

Buckingham, A., Fischer, P.

PHYSICAL REVIEW A, 63(4), 2001 (article)

Abstract
We show that damping factors must not be incorporated in the perturbation of the ground state by a static electric field. If they are included, as in the theory of Stedman et al. {[}preceding Comment. Phys. Rev. A 63, 047801 (2001)], then there would be an electric dipole in the y direction induced in a hydrogen atom in the M-s = + 1/2 state by a static electric field in the x direction. Such a dipole is excluded by symmetry.

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DOI [BibTex]