J. Nelson

Local light-ray rotation

We present a sheet structure that rotates the local ray direction through an arbitrary angle around the sheet normal. The sheet structure consists of two parallel Dove-prism sheets, each of which flips one component of the local direction of transmitted light rays. Together, the two sheets rotate transmitted light rays around the sheet normal. We show that the direction under which a point light source is seen is given by a Mobius transform. We illustrate some of the properties with movies calculated by ray-tracing software.

Ray-optical negative refraction and pseudoscopic imaging with Dove-prism arrays

A sheet consisting of an array of small, aligned Dove prisms can locally (on the scale of the width of the prisms) invert one component of the ray direction. A sandwich of two such Dove-prism sheets that inverts both transverse components of the ray direction is a ray-optical approximation to the interface between two media with refractive indices +n and –n. We demonstrate the simulated imaging properties of such a Dove-prism-sheet sandwich, including a demonstration of pseudoscopic imaging.

Waveguide grating mirror in a fully suspended 10 meter Fabry-Perot cavity

We report on the first demonstration of a fully suspended 10 m Fabry-Perot cavity incorporating a waveguide grating as the coupling mirror. The cavity was kept on resonance by reading out the length fluctuations via the Pound-Drever-Hall method and employing feedback to the laser frequency. From the achieved finesse of 790 the grating reflectivity was determined to exceed 99.2% at the laser wavelength of 1064 nm, which is in good agreement with rigorous simulations. Our waveguide grating design was based on tantala and fused silica and included a ≈ 20 nm thin etch stop layer made of Al2O3 that allowed us to define the grating depth accurately and preserve the waveguide thickness during the fabrication process. Demonstrating stable operation of a waveguide grating featuring high reflectivity in a suspended low-noise cavity, our work paves the way for the potential application of waveguide gratings as mirrors in high-precision interferometry, for instance in future gravitational wave observatories.

Single-Ended Differential Protection in MTDC Networks Using Optical Sensors

This paper presents a method for rapid detection of faults on voltage source converter multiterminal HVdc transmission networks using multipoint optical current sensing. The proposed method uses differential protection as a guiding principle, and is implemented using current measurements obtained from the optical current sensors distributed along the transmission line. Performance is assessed through detailed transient simulation using MATLAB/Simulink models, integrating inductive dc-line terminations, detailed dc circuit-breaker models, and a network of fiber-optic current sensors. Moreover, the feasibility and required performance of optical-based measurements is validated through laboratory testing. Simulation results demonstrate that the proposed protection algorithm can effectively, and within very short period of time, discriminate between faults on the protected line (internal faults), and those occurring on adjacent lines or busbars (external faults). Hardware tests prove that the scheme can be achieved with the existing, available sensing technology.

Experimental demonstration of a suspended diffractively coupled optical cavity

All-reflective optical systems are under consideration for future gravitational wave detector topologies. One approach in proposed designs is to use diffraction gratings as input couplers for Fabry-Perot cavities. We present an experimental demonstration of a fully suspended diffractively coupled cavity and investigate the use of conventional Pound-Drever-Hall length sensing and control techniques to maintain the required operating condition.

Translational, rotational, and vibrational coupling into phase in diffractively coupled optical cavities

All-reflective optical systems are under consideration for future gravitational wave detector topologies. A key feature of these all-reflective systems is the use of Fabry-Perot cavities with diffraction gratings as input couplers; however, theory predicts and experiment has shown that translation of the grating surface across the incident laser light will introduce additional phase into the system. This translation can be induced through simple side-to-side motion of the coupler, yaw motion of the coupler around a central point (i.e., rotation about a vertical axis), and even via internal resonances (i.e., vibration) of the optical element. In this Letter we demonstrate on a prototype-scale suspended cavity that conventional cavity length-sensing techniques used to detect longitudinal changes along the cavity axis will also be sensitive to translational, rotational, and vibrational motion of the diffractive input coupler. We also experimentally verify the amplitude response and frequency dependency of the noise coupling as given by theory.

Photorealistic visualization of imaging in canonical optical resonators

We investigate the capability of open source freeware ray tracing rendering software to demonstrate the imaging properties of canonical optical resonators. This software can directly visualize the imaging properties of canonical resonators. We can also demonstrate effects such as the trapping of light rays in geometrically stable resonators and the formation of fractal intensity patterns in the eigenmodes of geometrically unstable resonators.

Frequency characterisation of an optically-interrogated Rogowski coil for smart grid protection applications

A preliminary frequency characterization of an optically-interrogated Rogowski coil designed for smart grid protection applications is presented in this paper. The investigation into the sensor performance carried out according to the requirements of the 5P protection class specified by the IEC 60044-8 standard revealed that the maximum permissible error is breached at the 4th harmonic for the prototype device. It is demonstrated through numerical simulations that by amending a number of parameters, such as the piezoelectric stack capacitance or the Rogowski coil resistance, the sensor performance can be improved and the class requirements readily satisfied. A refined design of the device is proposed accordingly.

Development and testing of optically-interrogated current sensors

We report on the design and construction of a low cost, accurate optically-interrogated low voltage transducer (LVT) which, when combined with a Rogowski coil or conventional current transformer, forms an optically-interrogated current sensor (OCS). The LVT has been designed to be capable of being mass produced. The OCS requires no power supply and is interrogated remotely. Furthermore, it can be multiplexed in series or in parallel, allowing for multiple current measurements at different sites to be made by the same interrogator. The LVT and OCS have been tested against the accuracy requirements of the IEC 60044-8 standards for electronic current transformers (ECTs) both by direct signal injection, and by primary current injection when interfaced with a conventional PX class CT. The LVT and OCS are shown to meet the stringent 5P protection class requirements and also the additional requirements of the Transient Protection Electronic (TPE) class.

Three-dimensional self-similar fractal light in canonical resonators

Unstable canonical resonators can possess eigenmodes with a fractal intensity structure [Karman et al., Nature 402, 138(1999)]. In one particular transverse plane, the intensity is not merely statistically fractal, but self-similar [Courtial and Padgett, PRL 85, 5320 (2000)]. This can be explained using a combination of diffraction and imaging with magnification greater than one.nnHere we show that the same mechanism also shapes the intensity cross-section in the longitudinal direction into a self-similar fractal, but with a different magnification. This results in three-dimensional, self-similar, fractal intensity structure in the eigenmodes.