Diffractive Quality Research Articles

Study of a Coaxial Gyrotron Cavity With Improved Mode Selection

The resonant frequency, the diffractive quality factor, and the mode competition in the coaxial gyrotron cavity with a tapered inner rod have been investigated. The resonant frequency and the diffractive quality factor can be adjusted by changing the tilt angle and the radius of the tapered inner rod. When the inner rod radius is far smaller than the caustic radius of the given mode, the resonant frequency and the diffractive quality factor are hardly affected by the tilt angle of the tapered inner rod; the resonant frequency increases slightly but the diffractive quality factor increases noticeably with the increase of the tilt angle of the tapered inner rod when the inner rod radius is a little smaller than the caustic one; the resonant frequency and the diffractive quality factor decrease with the increase of the tilt angle of the tapered inner rod when the inner rod radius is larger than the caustic one. The mode competition can be improved with an appropriate design of the tilt angle and the radius of the tapered inner rod in a coaxial gyrotron cavity with a tapered inner rod.

Eigenmode on a Coaxial Resonator With Longitudinal Inner-Outer Corrugation

In this paper, eigenequations of TE and TM mode about coaxial resonator with longitudinal inner-outer corrugation are derived by the surface impedance theory and the first-order transmission line equations with mode coupling coefficients are obtained by means of the transmission line and coupling wave theory. According to these equations, the resonant frequency, diffractive quality factor Q, and field profile geometry of the eigenmode of the coaxial resonator can be calculated, and the effect of the inner (outer) slot depth and tooth width on the resonant frequency and Q factor can be discussed. Numerical results display as follows: 1) the resonator can effectively suppress the mode competition and 2) the inner slot depth and tooth width of the resonator does not almost affect the resonant frequency, and the outer slot depth and tooth of the resonator can affect the resonant frequency, whereas the Q factor is not affected by them.

Design and analysis of metallic photonic band gap cavity for a Gyrotron

Design and analysis of a 35 GHz metal photonic band gap (PBG) resonant cavity operating in the TE041-like mode has been presented. The dispersion characteristics of a 2D metal PBG structure comprising of triangular array of rods has been obtained using FDTD method. Global band gap regions have been obtained to be used for the PBG cavity design. A mode map has been also generated to examine the occurrence of the possible modes in the PBG cavity. Electromagnetic simulation of the designed PBG cavity has been performed to study the operating modes and quality factors. Role of the number of metal rods layers around the defect in PBG structure has also been illustrated for confining the desired mode and deciding the diffractive quality factor. PBG cavity results have been compared with the analogous cylindrical cavity for the designed mode along with the possible nearby modes to examine the mode competition. It has been found that the mode competition has been successfully reduced in designed PBG cavity and nearly a single mode operation is achieved. It is hoped that present study would be useful for the application of the metal PBG structures in the Gyrotron devices to alleviate the mode competition problem while operating in the higher order modes.

Effects of the spatial frequency composition of the target pattern and the number of quantization levels in diffractive beam shaper design

A mathematical model is derived, and numerical simulation is analyzed for laser beam shaping by using multilevel phase-only diffractive optical elements (DOEs). We used the simulated annealing algorithm to design the beam shapers. The result has an essential effect on the diffractive pattern quality caused by the spatial frequency composition of target patterns for the same incident gaussian beam size and target pattern area. The root mean square error between the diffractive and target patterns is smaller for the target patterns with lower spatial frequencies. Moreover, the effect of spatial frequency composition can be relaxed for the cases of larger incident gaussian beam size. In addition, finer quality control of a diffraction pattern can be obtained by increasing the number of quantization levels at the DOEs.

Analysis of the Characteristics of a Coaxial Gyrotron Cavity With a Tilted Inner Rod

The effects of the tilted inner rod of a coaxial gyrotron cavity on the resonant frequency, the diffractive quality factor, and the longitudinal field distribution for transverse electric (TE) modes are investigated. As a practical application, a numerical analysis is given to TE62 mode in the 330-GHz wideband continuous frequency tunable coaxial gyrotron proposed by O. Dumbrajs' group. Results show that a small tilt angle of the inner rod will cause a noticeable change in the resonant frequency, the diffractive quality factor, and the longitudinal field distribution for TE modes in a coaxial gyrotron cavity due to the mode coupling. The resonant frequency and the diffractive quality factor increase with increasing tilt angle of the inner rod; if the tilt angle θ is 0.4°, the resonant frequency shift will be about 1.5 GHz.

The effect of exit beam phase aberrations on parallel beam coherent x-ray reconstructions

Diffraction artifacts from imperfect x-ray windows near the sample are an important consideration in the design of coherent x-ray diffraction measurements. In this study, we used simulated and experimental diffraction patterns in two and three dimensions to explore the effect of phase imperfections in a beryllium window (such as a void or inclusion) on the convergence behavior of phasing algorithms and on the ultimate reconstruction. A predictive relationship between beam wavelength, sample size, and window position was derived to explain the dependence of reconstruction quality on beryllium defect size. Defects corresponding to this prediction cause the most damage to the sample exit wave and induce signature error oscillations during phasing that can be used as a fingerprint of experimental x-ray window artifacts. The relationship between x-ray window imperfection size and coherent x-ray diffractive imaging reconstruction quality explored in this work can play an important role in designing high-resolution in situ coherent imaging instrumentation and will help interpret the phasing behavior of coherent diffraction measured in these in situ environments.

TM Modes in Coaxial Cavities With Inner Surface Corrugations

We present the mathematical formulation and numerical results for the resonance characteristics of the TM <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">mp</sub> modes in a coaxial cavity with a longitudinally corrugated insert. Based on the spatial harmonics method, the eigenvalues of the transverse plane are calculated, and then, the Vlasov approximation is used to calculate the resonance frequency, diffractive quality factor, and longitudinal field profile of every mode. The numerical results are presented and compared with those found in the literature.

Optimization of a SCALPEL ® exposure tool using a diffractive image quality technique

Determining and maintaining the best exposure parameters, such as dose, focus, or illumination uniformity, is a primary concern in operating a modern day stepper or scanner. A commonly adopted means of obtaining lithographic image quality information relies on accurately measuring CD values across a wafer, die, or image field. Continually shrinking Linewidth have pushed this type of characterization into the realm of the CD-SEM; though greatly improved and automated in recent years, the accuracy required to reliably identify trends makes this a tedious and time consuming practice. We present here an alternative, flexible, and potentially rapid method for optimizing lithography conditions using a diffractive image quality (DIQ) technique, emphasizing its utility in a SCALPEL-based exposure system. A DIQ technique uses the fact that the maximum intensity of higher-order diffraction dispersions is a function of the (image) quality of the grating being illuminated so interrogation of gratings that have been exposed in resist can provide knowledge of how the aerial image quality can be varied for a given set of exposure conditions. We have evaluated grating diffraction efficiency using a probe beam (HeNe laser) introduced at a glancing angle. This arrangement has the advantage of looking at higher order intensity distributions while avoiding the zero order, as well as being fast since one is only collecting signal (eg. No feature recognition searches). Further, it enables the imaging optimization of a new system, such as SCALPEL, without tying up quantitative tools such as a CD-SEM. We have found that this metrology technique provides a quite sensitive and rapid identification of the best exposure conditions. Cross-correlation with a CD-SEM metrology tool has also been performed, in order to verify that the optimum imaging conditions identified through this DIQ technique correspond to the best CD conditions.

Mode selective properties of coaxial gyrotron resonators

In this paper, the mode-selective properties of coaxial gyrotron resonators are discussed for different cavity configurations. Expressions are derived for the ohmic loading of a coaxial conductor with longitudinal corrugations where the RF field inside the corrugations is fully taken into account, and for the diffractive quality factor in coaxial cavities with tapered inner conductor relative to that of an empty cylindrical cavity. The competing action of a kind of "surface mode" that appears in corrugated structures is investigated by considering its diffractive and ohmic quality factors. Additional mode competition problems that can arise in coaxial cavities with corrugated inner conductor due to second cyclotron harmonic interaction are investigated and solutions to the problem are proposed.

Coaxial cavities with corrugated inner conductor for gyrotrons

This paper investigates coaxial gyrotron cavities with longitudinal slots on the inner conductor as a means to reduce the number of possible competing modes. In the analytic theory the corrugated surface is treated as a homogeneous impedance surface ("impedance corrugation") to obtain simple formulas for the characteristic equation of the eigenmodes, for the electromagnetic fields and the wall losses. The developed model applies if the number of slots is sufficiently high (cutoff wavelength much larger than the corruption period). The characteristic equation in terms of the ratio C of the outer wall radius to the inner conductor radius is solved numerically to determine a range of eigenvalues and C where the eigenvalue curves are monotonically decreasing. In such a region a cavity having its inner conductor downtapered (radius decreasing toward the cavity output) can be used to reduce the diffractive quality factors of several modes, leaving the working mode undisturbed and without favoring other modes. In addition the electromagnetic field profiles are investigated, and in particular it is shown that for certain cavity parameters a mode could have its energy concentrated close to the inner conductor. As a check on the validity of the theoretical approximations, simulations with the MAFIA code are carried out. These give good agreement with the results of the analytic equations.

Diffractive multifocal intraocular lens image quality

The diffractive multifocal intraocular lens is implanted in the eye to replace a cataractous crystalline lens. Axially separated images from two diffraction orders provide near and distance vision. Overall image quality is affected by both the quality of the in-focus component and the energy distribution between images. Modulation transfer function and energy distribution data are presented. Measurement accuracy is limited by the diameter of the point spread function and by the difficulty in separating the focused and defocused components.