Acousto-optic tunable filters (AOTFs) optimised for operation in the 2-4μm region

Acousto-Optic Tunable Filters (AOTFs) are electronically-controlled bandpass optical filters. They are often preferred in applications in spectroscopy where their agility and rapid random-access tuning can be deployed to advantage. When used for spectral imaging a large aperture (typically 10mm or more) is desired in order to permit sufficient optical throughput. However, in the mid IR the λ2 dependence on RF drive power combined with the large aperture can prove to be a hurdle, often making them impractical for many applications beyond about 2μm. We describe and compare a series of specialised free-space configurations of AOTF made from single crystal tellurium dioxide, that require relatively low RF drive power. We report on AOTFs specifically optimised for operation with a new generation of Supercontinuum source operating in the 2-4μm window and show how these may be used in a spectral imaging system. Finally, we describe an AOTF with an (acoustic) Fabry-Perot cavity operating at acoustic resonance rather than the conventional travelling-wave mode; the acoustic power requirement therefore being reduced. We present an analysis of the predicted performance. In addition, we address the practical issues in deploying such a scheme and outline the design of a prototype "resonant AOTF" operating in the 1-2μm region

Design and fabrication of high quality-factor 1-d photonic crystal/photonic wire extended microcavities

We have successfully demonstrated experimentally the fabrication and measurement of high quality-factor one-dimensional photonic crystal/photonic wire extended cavities based on silicon-on-insulator. The cavities that we have investigated ranged from 3 to 8 m in length. A quality-factor of nearly 74 000 was measured at a cavity length of 5 m through the use of tapering both within and outside the cavity, showing good agreement with the finite-difference time-domain simulation approach use

Optical properties of split ring resonator metamaterial structures on semiconductor substrates

Metamaterials based on single-layer metallic Split Ring Resonators (SRR) and Wires have been demonstrated to have a resonant response in the near infra-red wavelength range. The use of semiconductor substrates gives the potential for control of the resonant properties of split-ring resonator (SRR) structures by means of active changes in the carrier concentration obtained using either electrical injection or photo-excitation. We examine the influence of extended wires that are either parallel or perpendicular to the gap of the SRRs and report on an equivalent circuit model that provides an accurate method of determining the polarisation dependent resonant response for incident light perpendicular to the surface. Good agreement is obtained for the substantial shift observed in the position of the resonances when the planar metalisation is changed from gold to aluminium

Repeatability of fractional flow reserve despite variations in systemic and coronary hemodynamics

Objectives This study classified and quantified the variation in fractional flow reserve (FFR) due to fluctuations in systemic and coronary hemodynamics during intravenous adenosine infusion. Background Although FFR has become a key invasive tool to guide treatment, questions remain regarding its repeatability and stability during intravenous adenosine infusion because of systemic effects that can alter driving pressure and heart rate. Methods We reanalyzed data from the VERIFY (VERification of Instantaneous Wave-Free Ratio and Fractional Flow Reserve for the Assessment of Coronary Artery Stenosis Severity in EverydaY Practice) study, which enrolled consecutive patients who were infused with intravenous adenosine at 140 μg/kg/min and measured FFR twice. Raw phasic pressure tracings from the aorta (Pa) and distal coronary artery (Pd) were transformed into moving averages of Pd/Pa. Visual analysis grouped Pd/Pa curves into patterns of similar response. Quantitative analysis of the Pd/Pa curves identified the “smart minimum” FFR using a novel algorithm, which was compared with human core laboratory analysis. Results A total of 190 complete pairs came from 206 patients after exclusions. Visual analysis revealed 3 Pd/Pa patterns: “classic” (sigmoid) in 57%, “humped” (sigmoid with superimposed bumps of varying height) in 39%, and “unusual” (no pattern) in 4%. The Pd/Pa pattern repeated itself in 67% of patient pairs. Despite variability of Pd/Pa during the hyperemic period, the “smart minimum” FFR demonstrated excellent repeatability (bias −0.001, SD 0.018, paired p = 0.93, r2 = 98.2%, coefficient of variation = 2.5%). Our algorithm produced FFR values not significantly different from human core laboratory analysis (paired p = 0.43 vs. VERIFY; p = 0.34 vs. RESOLVE). Conclusions Intravenous adenosine produced 3 general patterns of Pd/Pa response, with associated variability in aortic and coronary pressure and heart rate during the hyperemic period. Nevertheless, FFR – when chosen appropriately – proved to be a highly reproducible value. Therefore, operators can confidently select the “smart minimum” FFR for patient care. Our results suggest that this selection process can be automated, yet comparable to human core laboratory analysis

Characterisation at infrared wavelengths of metamaterials formed by thin-film metallic split-ring resonator arrays on silicon

The infrared reflectance spectra at normal incidence for split-ring resonator arrays fabricated in thin films of three different metals on a silicon substrate are reported. The results are compared with a finite difference time domain simulation of the structures and a simple and novel equivalent-circuit method for the calculation of the first and second resonant wavelengths

Coupling strength control in photonic crystal/photonic wire multiple cavity devices

Resonance splitting has been demonstrated for two coupled micro-cavities with control of the free spectral range between the resonance peaks, together with a normalised transmission level of approximately 60%. Coupled micro-cavity-based structures that were separated by two closely spaced in-line coupler sections between the two micro-cavities have also been successfully fabricated and measured. The coupling strength of the two cavities was controlled via the use of hole tapering in the middle section between the two cavities. 2D finite-difference time-domain simulation shows close agreement with the results of measurements

Increasing optical metamaterials functionality

Gold Split Ring Resonators (SRRs) were fabricated on silicon substrates by electron beam lithography and lift-off, with overall dimensions of approximately 200 nm. Reflectance spectra from the SRRs are similar to those published elsewhere. New devices are proposed based on the additional functionality afforded by the use of a silicon substrate

Transmission of PhC coupled-resonator waveguide (PhCCRW) structure enhanced via mode matching

A method for increasing the coupling efficiency between ridge optical waveguides and PhCCRWs is described. This increase is achieved via W1 channel waveguide sections, formed within a two-dimensional triangular lattice photonic crystal using mode-matching. The mode-matching is achieved by low quality-factor modified cavities added to both the input and output ports of the PhCCRW. A three dimensional finite-difference time-domain method has been used to simulate light propagation through the modified PhCCRW. We have fabricated PhCCRWs working at 1.5µm in silicon-on-insulator material. Measurements and simulations show that the overall transmission is improved by a factor of two

Acousto-optic tunable filters for imaging applications in the 2-4~μm with low RF drive power

The λ2 dependence on acoustic field intensity (and hence RF drive power) can render large aperture acousto-optic tunable filters impractical for many applications beyond about 2 μm. One potential technique for reducing the RF drive-power requirement is to configure an acousto-optic tunable filter such that the interaction region is at acoustic resonance. We describe an acousto-optic tunable filter that operates at resonance and present an analysis of the predicted performance. In addition, we address the practical issues in deploying such a scheme. Finally, we present results of a prototype "resonant acousto-optic tunable filter" operating in the 1-2 μm region