Phase-sensitive Processing Research Articles

Small angle double quantum spectroscopy (SAQS NMR)

A new experiment for recording double quantum spectra is introduced. The 2D DQ NMR experiment yields phase sensitive spectra with double quantum frequencies in F1. The appearance of remote peaks is vastly suppressed by using a small flip angle double quantum excitation and reconversion. Pulse sequences and phase sensitive processing are discussed. The complexity of the SQ antiphase magnetization given in larger proton spin networks could be reduced by using the option of band selective decoupling during the preparation period. In addition, an ACCORDION element is applied by incrementing the J evolution delay in concert with the t1 period. With this the excitation of double quantum coherence over a wider range of J values is achieved. A broadband homodecoupled version of the DQ experiment is proposed, where correlation peaks with singlet response at F2 chemical shifts and double quantum frequencies in F1 are obtained. We call this experiment Small Angle double Quantum Spectroscopy SAQS NMR.

Triply resonant coherent four-wave mixing in silicon nitride microresonators.

Generation of multiple tones using four-wave mixing (FWM) has been exploited for many applications, ranging from wavelength conversion to frequency comb generation. FWM is a coherent process, meaning that its dynamics strongly depend on the relative phase among the waves involved. The coherent nature of FWM has been exploited for phase-sensitive processing in different waveguide structures, but it has never been studied in integrated microresonators. Waveguides arranged in a resonant way allow for an effective increase in the wavelength conversion efficiency (at the expense of a reduction in the operational bandwidth). In this Letter, we show that phase shaping of a three-wave pump provides an extra degree of freedom for controlling the FWM dynamics in microresonators. We present experimental results in single-mode, normal-dispersion high-Q silicon nitride resonators, and numerical calculations of systems operating in the anomalous dispersion regime. Our results indicate that the wavelength conversion efficiency and modulation instability gain in microcavities pumped by multiple waves can be significantly modified with the aid of simple lossless coherent control techniques.

Stability in computed optical interferometric tomography (Part II): in vivo stability assessment.

Stability is of utmost importance to a wide range of phase-sensitive processing techniques. In Doppler optical coherence tomography and optical coherence elastography, in addition to defocus and aberration correction techniques such as interferometric synthetic aperture microscopy and computational/digital adaptive optics, a precise understanding of the system and sample stability helps to guide the system design and choice of imaging parameters. This article focuses on methods to accurately and quantitatively measure the stability of an imaging configuration in vivo. These methods are capable of partially decoupling axial from transverse motion and are compared against the stability requirements for computed optical interferometric tomography laid out in the first part of this article.

Phase discrimination and simultaneous frequency conversion of the orthogonal components of an optical signal by four-wave mixing in an SOA

Simultaneous conversion of the two orthogonal phase components of an optical input to different output frequencies has been demonstrated by simulation and experiment. A single stage of four-wave mixing between the input signal and four pumps derived from a frequency comb was employed. The nonlinear device was a semiconductor optical amplifier, which provided overall signal gain and sufficient contrast for phase sensitive signal processing. The decomposition of a quadrature phase-shift keyed signal into a pair of binary phase-shift keyed outputs at different frequencies was also demonstrated by simulation.

Strain processing of intraoperative ultrasound images of brain tumours: Initial results

The purpose of the study was to investigate a method for strain calculation and its ability to discriminate between brain tumour and normal brain. During surgery of a low-grade astrocytoma and a metastasis, we acquired ultrasound (US) radiofrequency (RF) data with a hand-held probe at the dura mater. Using cross-correlation and phase-sensitive processing, we quantified the tissue displacements between consecutive US images and, subsequently, the local strain. In the elastograms, the tumour lesions were associated with lower strain levels than those found in the surrounding normal tissue. For both investigated cases, the strain images showed good agreement with the B-mode images. However, the results also indicated that the tumour interpretation might be different in the two modalities. An important finding was that the tissue motion caused by arterial pulsation is sufficient for generating elastograms. Requiring no specialised equipment or changes to acquisition procedures, strain data can be obtained as easily as conventional US imaging. (E-mail: Tormod.Selbekk@sintef.no)

Speckle reduction in pulse echo imaging using phase insensitive and phase sensitive signal processing techniques

In conventional B-scan imaging, speckle often distorts the true representation of small structures and lesions and hence reduces the diagnostic potential of the technique. Considerable speckle reduction has been achieved with an f 2.4 , cone hybrid system by cutting the large aperture transducers into sectors, and using either phase insensitive sector addition (PISA), phase insensitive sector multiplication (PISM), or an rf multiplicative processing technique. We have compared two modes of operation with the hybrid system. In the first, the cone acts as transmitter, and each sector of the f 2.4 spherical aperture is a reciver. In the second mode, the f 2.4 sphere acts as transmitter, and each sector of the cone is a receiver. Quantitative assessment using the contrast to speckle ratio developed by Patterson et al, indicates that processing under both modes results in reduced speckle noise and improved image quality, with the cone sector receive (rx) geometry offering the greatest improvement.

Speckle Reduction in Pulse Echo Imaging Using Phase Insensitive and Phase Sensitive Signal Processing Techniques

In conventional B-scan imaging, speckle often distorts the true representation of small structures and lesions and hence reduces the diagnostic potential of the technique. Considerable speckle reduction has been achieved with an f/2.4, cone hybrid system by cutting the large aperture transducers into sectors, and using either phase insensitive sector addition (PISA), phase insensitive sector multiplication (PISM), or an rf multiplicative processing technique. We have compared two modes of operation with the hybrid system. In the first, the cone acts as transmitter, and each sector of the f/2.4 spherical aperture is a receiver. In the second mode, the f/2.4 sphere acts as transmitter, and each sector of the cone is a receiver. Quantitative assessment using the contrast to speckle ratio developed by Patterson et al, indicates that processing under both modes results in reduced speckle noise and improved image quality, with the cone sector receive (rx) geometry offering the greatest improvement.