A physiological study to determine the mechanism of carbon dioxide clearance during apnoea when using transnasal humidified rapid insufflation ventilatory exchange (THRIVE).

Clinical observations suggest that compared with standard apnoeic oxygenation, transnasal humidified rapid-insufflation ventilatory exchange using high-flow nasal oxygenation reduces the rate of carbon dioxide accumulation in patients who are anaesthetised and apnoeic. This suggests that active gas exchange takes place, but the mechanisms by which it may occur have not been described. We used three laboratory airway models to investigate mechanisms of carbon dioxide clearance in apnoeic patients. We determined flow patterns using particle image velocimetry in a two-dimensional model using particle-seeded fluorescent solution; visualised gas clearance in a three-dimensional printed trachea model in air; and measured intra-tracheal turbulence levels and carbon dioxide clearance rates using a three-dimensional printed model in air mounted on a lung simulator. Cardiogenic oscillations were simulated in all experiments. The visualisation experiments indicated that gaseous mixing was occurring in the trachea. With no cardiogenic oscillations applied, mean (SD) carbon dioxide clearance increased from 0.29 (0.04) ml.min-1 to 1.34 (0.14) ml.min-1 as the transnasal humidified rapid-insufflation ventilatory exchange flow rate was increased from 20 l.min-1 to 70 l.min-1 (p = 0.0001). With a cardiogenic oscillation of 20 ml.beat-1 applied, carbon dioxide clearance increased from 11.9 (0.50) ml.min-1 to 17.4 (1.2) ml.min-1 as the transnasal humidified rapid-insufflation ventilatory exchange flow rate was increased from 20 l.min-1 to 70 l.min-1 (p = 0.0014). These findings suggest that enhanced carbon dioxide clearance observed under apnoeic conditions with transnasal humidified rapid-insufflation ventilatory exchange, as compared with classical apnoeic oxygenation, may be explained by an interaction between entrained and highly turbulent supraglottic flow vortices created by high-flow nasal oxygen and cardiogenic oscillations.

Heterodyne Fizeau interferometer for testing flat surfaces.

A heterodyne Fizeau interferometer, which uses a rotating radial grating to achieve the required optical frequency shift, is described. An analysis of the effects of grating ruling errors shows that they may be nearly eliminated by averaging the interferometer phase readings over integral numbers of grating revolutions. Experimental tests indicate that the interferometer is capable of measuring with a reproducibility of lambda/200 (lambda = 632.8 nm), limited by temperature effects.

Reduction of false correlations with binary phase-only filters.

Binary phase-only filters have considerable potential for use in optical correlation systems. However, simple correlation systems using these filters can give false correlation peaks and suffer from inherent noise. We describe a method by which these problems can be overcome by combining the results of sine and cosine binary phase-only correlations and demonstrate it by numerical simulation. Some problems of realizing the method in practice are also discussed.The authors are with Mechanical Engineering Laboratory, Namiki 1-2, Tsukuba-shi, Ibaraki 305, Japan.

Phase-difference amplification in near-real-time phase-stepping interferometers.

We present results from two interferometer systems incorporating phase amplification and phase stepping that operate in near real time. Each system contains two interferometers. The first interferometer projects an interferogram of the test object onto the write side of an optically addressed phase-only liquid-crystal spatial light modulator (LCSLM). The read side of the LCSLM is illuminated by two beams from the second interferometer that are adjusted so that their +n- and -n-order beams are diffracted back along the optic axis. These produce an output interferogram that is phase amplified by a factor 2n. This phase distribution is retrieved by phase stepping.

Phase-only modulation using a twisted nematic liquid crystal television.

We used a commercially available liquid crystal television display unit as a spatial optical phase-only modulator. To do this, we removed the integral polarizers from the unit and double passed the light under modulation through it. We found that it was possible to obtain continuous phase modulation from 0 to PI with essentially no change in the state of polarization of the output light and absorption changes of <2.5%. We wrote computer-generated phase-only holograms on the LCTV and reconstructed them optically.

Adaptive wave-front correction by means of all-optical feedback interferometry.

A novel adaptive wave-front correction system based on an all-optical feedback interferometer is described. In this system the two-dimensional output fringe intensity from a Mach-Zehnder interferometer with large radial shear is optically fed back to an optically addressed phase-only liquid-crystal spatial light modulator. Consequently, without a separate aberration-free reference wave, the modulator phase approximates the conjugate of the interferometer phase that is directly related to the phase of the input aberrated wave front, so this system is applicable in adaptive optics. We successfully achieved real-time correction of aberrated wave fronts: A diffraction pattern that was seriously distorted because of aberrations was transformed into a diffraction-limited spot immediately after the feedback loop was closed.

Surface-profile measurement by means of a polarization Sagnac interferometer with parallel optical feedback.

We describe a novel feedback interferometer for real-time, unambiguous measurement of surface profiles that consists of a polarization Sagnac interferometer and an optically addressed phase-only spatial light modulator. In this system the output intensity from the Sagnac interferometer is optically fed back to the phase modulator placed in one arm of the interferometer, which produces a sawtooth fringe intensity profile (instead of the conventional cosinusoidal one) that is directly and unambiguously related to the surface profile. Preliminary experimental results demonstrate the feasibility of applying this system to surface-profile measurement.

Determination of interferometer phase distributions by use of wavelets.

A new technique for directly extracting phase gradients from two-dimensional (2-D) interferometer fringe data is presented. One finds the gradients by tracking the maximum modulus of the continuous wavelet transform of the fringe data and the phase distribution that is obtained, with a small error, by integration. Problems associated with phase unwrapping are thereby avoided. The technique is compared with standard methods, and excellent agreement is found. In common with Fourier-transform methods, the technique is capable of extracting the full 2-D phase distribution from a single image.

Reconfigurable free-space optical interconnections with a phase-only liquid-crystal spatial light modulator.

A range of space-invariant and space-variant holographic optical interconnections are demonstrated with the aid of a nematic liquid-crystal panel that is capable of nearly continuous phase modulation. Results on fan-out, nearest-neighbor/next-nearest-neighbor interconnects, and 8-bit perfect shuffle are presented. The diffraction efficiencies are close to 15% of incident light in all cases.

Grating interferometer with extremely high stability, suitable for measuring small refractive index changes.

We describe here a grating interferometer with extremely high stability and which can be used as a differential refractometer. The instrument uses heterodyne techniques to achieve high sensitivity. We present an analysis of the operation of the system and results which show that it has a long term stability of the order of 1/1500 wavelength over 2 h.

Programmable optical logic systems using free-space optical interconnections.

A free-space optical logic technique is presented that utilizes a two-dimensional array of diffractive optical elements. Each optical element focuses light to multiple, separate positions in the output focal plane. The focal spots from different optical elements are allowed to overlap spatially, resulting in interference. By changing the phase shift between the optical elements, one can create different optical logic operations in the focal plane. The technique is demonstrated by the use of two input beams incident onto a multiplexed optical element written onto a programmable spatial light modulator. The optical element simultaneously creates both AND and XOR logic functions in the output plane.

Multiple-beam lateral shear interferometry for optical testing.

Lateral shear interferometry is used to obtain the lateral aberrations of a lens. The zeroth-order fringe in an interferogram obtained from a wedge-plate lateral shear interferometer, however, directly displays the lateral aberration curve of a test lens. Nevertheless, the intensity distribution, is cosinusoidal. Multiple-beam interferometry results in sharpened fringes; hence the multiple-beam wedge-plate shear interferometer displays the lateral aberration curve of a lens sharply, provided the shear is small. For large shear, some new artifacts appear in the interferogram, which are also explained.

Real-time phase-difference amplification with a liquid-crystal spatial light modulator.

We describe a simple system for achieving real-time phase-difference amplification of interferograms. We arrange the interferogram such that it contains high-spatial-frequency carrier fringes and project it onto the write side of an optically addressed phase-only spatial light modulator. The resultant phase pattern on the modulator is read out by two readout beams, and diffraction by the carrier fringes provides the spatial heterodyning that is necessary for achieving phase-difference amplification. We present results that demonstrate real-time phase-difference amplification by as much as a factor of 10.

Focal-length measurement by multiple-beam shearing interferometry.

The application of multiple-beam shearing interferometry to lens focal-length measurement is described. A coated shearing plate interferometer was used in transmission to produce sharp multiple-beam fringes that rotate as the collimation of the incoming wave front from the lens under test changes. The test lens was used to collimate light from a point source that was translated longitudinally, and the focal length was determined from the rate of rotation of the fringes as the source moved. This method is simple, accurate, and lends itself to automatic determination of focal length.