Grating Configuration Research Articles

Analysis of the bidirectional optical three-stage prism grating system for red blood cell velocity measurements in microvessels.

A bidirectional optical (BDO) three-stage prism grating system has been developed to measure online direction and magnitude of red blood cell (RBC) velocities in the microcirculation. The paper describes a mathematical evaluation of the BDO system. The optical grating configuration was simulated in terms of a spatial filter. The input signal was the simulated spatial light distribution of a group of moving RBCs Three methods for estimating the mean frequency (fmean) of the signal corresponding to RBC velocity were evaluated on the criteria of accuracy and noise sensitivity. All underestimated the RBC velocity. The reference method calculates fmean from the power density spectrum. A zero-crossing counting technique and a method based on time differentiation underestimated the mean frequency by 14·1±11·3 per cent (mean ±SD) and 0·9±0·1 per cent, respectively. The zero-crossing counting technique was more sensitive to noise than both other methods. The reference method, operating in the frequency domain, is preferred, because the power density spectrum obtained can be corrected for filtering errors induced by higher-order passbands of the optical filter.

Planar optical demultiplexer with chirped grating for WDM fibre systems

The theory presented allows the analysis of multimode planar multiplexers and demultiplexers using plane gratings of arbitrary grating constant and blaze angles. In a configuration with a plane chirped grating, the wavelength-selective device has to provide the correct phase matching of the divergent phase front of the input fibre. This refocuses the radiation onto the wavelength-dependent output fibre locations. Computations show an excellent device property for ideal chirp and blaze angles. Taking practical deviations into account, the diffraction behaviour of five different grating configurations have been calculated and optimised with regard to the blaze angle. A grating configuration consisting of four regions with constant angles leads to an intensity loss of only 0.72 dB compared to the ideal case.

Waveguide laser using a curved grating

A curved grating was introduced into the cavity of a glass tube waveguide laser. This replaces the commonly used lens–flat grating configuration and facilitates the operation of the laser system. Several positions of the grating are possible and allow the design of a more compact cavity. The performance is comparable to that of conventional cavities.

Theory of a distributed feedback laser

This paper treats distributed feedback laser operation in which a classical electromagnetic field obeying Maxwell's equations interacts with an active medium according to the laws of quantum mechanics. The theory describes arbitrarily intense two-mode operation of fundamental and higher modes in both index and gain grating configurations. Spacial hole burning and finite atomic linewidth are included in the analysis and are shown to give marked changes in some laser intensity profiles.

Characteristics of a hollow-core distributed feedback CO2laser

Using perturbation theory and a plane wave analysis, the scattering amplitudes and coupling coefficients for a planar, hollow-core, double grating distributed feedback (DFB) waveguide laser are derived. Waveguide reflectivity has been compared for a single and double grating waveguide configuration. For a waveguide dimension of 100 μm over a 10 cm length, reflectivity is enhanced from 75 percent for a single grating to 98 percent using a double grating configuration. Included in this analysis is the effect of phase relationship between the two gratings. Using this analysis a means has been devised whereby reflectivity of the DFB waveguide may be optimized during operation of the laser. With the double grating configuration, coupling is enhanced such that a greater plate separation is possible in nearly all cases. Use of a double grating allows the plate separation of the waveguide to be increased from 80 to 100 μm, reducing total waveguide loss from 15.9 to 8.1 dB/m. These results have led to a method of solving heretofore prohibitive characteristics of single grating hollow-core DFB waveguides. Formal relationships for the waveguide parameters where a double grating configuration is used have been presented for designing hollow-core DFB waveguide lasers operating at 10.6 μm.

Acoustic Surface Waves in Split Strip Periodic Metal Gratings on a Piezoelectric Surface

Ahrruct-The propagation of acoustic surface waves in configurations of split periodic metal gratings on the surfaces of piezoelectric materials is analyzed. The analysis is based on analytic solutions for the electric field distribution. Particular attention is paid to the details of the dis- persion relations near the resonances, where the stopband frequencies for various combinations of externally interconnected or disconnected strips are calculated as functions of strip geometry for stopbands of various order. several wavelengths is frequently used either to create multiple passbands or to minimize phase errors and bulk wave conver- sion which may be a problem, especially at high frequencies. So far the models used to analyze these transducers have been equivalent circuits with adjustable empirical parameters for the effective coupling coefficient, as well as for the electrostatic coupling between distantly spaced neighboring taps. The subject of the present paper is a field analysis of split strip periodic gratings with two metal strips per period. The analysis leads to analytic expressions for the dispersion rela- tions. Particular emphasis is paid to the behavior around the stopbands where the period of the structure is close to an in- tegral number of half wavelengths. From the dispersion rela- tion one can easily derive the decay of waves in the stopbands, a figure which is of central importance in the design of surface wave resonators. Numerical results are given for the promi- nent cases with one half wavelength between neighboring strips to show the effect of thinning as well as for a regular array with four strips per wavelength corresponding to the split finger transducer. models can be derived from the dispersion relations and the field solutions without the necessity of empirically adjustable parameters. In an accompanying paper, it is shown that equivalent circuit

Rapid Acoustooptic Tuning of a Dye Laser

Rapid tuning of a dye laser over a large wavelength range has been achieved by inserting an acoustooptic deflector between the dye cell and a diffraction grating. The deflector and grating combination serve as the tuning element. TeO(2), which has an anomalously low sound velocity, has been used to demonstrate acoustooptical tuning over a broad spectral range. Theoretically, tuning rates greater than 100 nm/microsec can be achieved over a wavelength range of up to 35 nm. The experimental data have been used to verify the analytical aspects of the beam deflector and diffraction grating configuration.

Optical Experiments with a Magnetically Rotatable Diffraction Grating

A striped-domain thin magnetic film, when covered by a Bitter colloid, forms a diffraction grating. Because of its possible application in display devices this phenomenon is attracting current interest. The theory of the grating formation is developed, and it is shown that external fields can and do greatly improve the grating configuration and efficiency. Improvements in diffracted light flux efficiency of as much as four to one have been observed.