Modelling the transverse-mode response of a two-port SAW resonator

An equation is derived for the transverse-mode frequencies of a two-port surface-acoustic-wave (SAW) resonator with uniform reflection gratings. It is postulated that only odd-order (symmetric) transverse modes should be excited in such a grating, due to its symmetrical placement with respect to the interdigital transducer (IDT). Frequency response modeling involves a summation of one-dimensional transmission-matrix computations for the fundamental mode and participating transverse ones. Excellent agreement is obtained between theory and experiment for a 360-MHz resonator with three close-in transverse modes, attributed to symmetric modes of odd order m=3, 5, 7.     

Passband widening of transversely coupled resonator filters using the fundamental symmetric and antisymmetric modes

This paper describes a method for widening the passband of transversely coupled resonator filters (TCF) using only the fundamental symmetric and antisymmetric modes. The coupling of modes analysis in the transverse direction is applied to the TCF design to investigate the passband width. As a result, it is found that the passband width can be increased by making the surface acoustic wave (SAW) velocity difference between the interdigital transducer (IDT) region and the resonator gap region smaller. It is proposed that a grating structure be applied to the common ground bar, instead of the uniform metal, to reduce the SAW velocity difference. Using the grating-type common ground bar, filters are fabricated on ST-quartz substrate. The passband of a single filter with a center frequency of 248 MHz is widened up to 410 kHz without any increase of the insertion loss. The effect of the impedance mismatch at the junction of two cascaded devices is investigated. It is shown that the filter performance is improved by reduction of the small parasitic capacitance existing at the cascade point. Experimentally, the capacitance formed between the bus bar of the IDT and the bottom surface of the ceramic package is reduced. The insertion loss is reduced by 0.0 dB, and 3 dB passband is widened by 8 kHz for a filter with a center frequency of 248 MHz. On the basis of these two improvements, cascaded TCFs are fabricated. For a filter with a center frequency of 248 MHz, an insertion loss of 5.5 dB and a 3-dB passband width of 270 kHz are obtained     

Microwave measurements of effective dielectric constant ofsemiconductor waveguides via periodic-structure photoexcitation

In this paper a new method for measurements of the effective dielectric constant of layered or full-substrate semiconductor waveguides at microwave frequencies is presented. The light-induced generation of a photoconductivity grating within the excited waveguide section is utilized. Incoming swept-frequency signals produce a stop-band reflection spectrum which exhibits a dominant major-lobe peak value at the grating center frequency. The effective dielectric constant is calculated from a simple analytical expression by substituting the measured center frequency. Theoretical background, error estimation, and an example are presented     

Second-harmonic generation in second-harmonic fiber Bragg gratings

We consider the production of second-harmonic light in gratings resonant with the generated field, through a Green's function approach. We recover some standard results and obtain new limits for the uniform grating case. With the extension to nonuniform gratings, we find the Green's function for the second harmonic in a grating with an arbitrary phase shift at some point. We then obtain closed form approximate expressions for the generated light for phase shifts close to π/2 and at the center of the grating. Finally, comparing the uniform and phase-shifted gratings with homogeneous materials, we discuss the enhancement in generated light and the bandwidth over which it occurs, and the consequences for second-harmonic generation in optical fiber Bragg gratings.     

Beam separator for high-order harmonic radiation in the 3-10 nm spectral region

We present the scheme of a beam separator for ultrashort high-order harmonic radiation below 10 nm. The system consists of a collimating mirror and two plane grazing-incidence gratings in compensated configuration. The first grating acts as the beam separator: it diffracts the extreme ultraviolet (XUV) light into the first order while reflecting the fundamental laser beam into the zero order. The diffracted light goes to a second grating that compensates both for the spectral dispersion and for the temporal broadening of the XUV ultrashort pulse caused by the diffraction at the first grating. The system can be designed for any wavelength in the 3-40 nm region. Since the gratings are operated at extreme grazing incidence, the area of the optical surface illuminated by the fundamental laser pulse is large, and therefore there is no risk of damage of the optical surfaces. The effects on the phase of the ultrashort pulse for narrowband applications are discussed.     

Micromachined thin film plate acoustic resonators utilizing the lowest order symmetric lamb wave mode

Thin film integrated circuits compatible resonant structures using the lowest order symmetric Lamb wave propagating in thin aluminum nitride (AlN) film membranes have been studied. The 2-mum thick, highly c-oriented AlN piezoelectric films have been grown on silicon by pulsed, direct-current magnetron reactive sputter deposition. The films were deposited at room temperature and had typical full-width, half-maximum value of the rocking curve of about 2 degrees. Thin film plate acoustic resonators were designed and micromachined using low resolution photolithography and deep silicon etching. Plate waves, having a 12-mum wavelength, were excited by means of both interdigital (IDT) and longitudinal wave transducers using lateral field excitation (LW-LFE), and reflected by periodical aluminum-strip gratings deposited on top of the membrane. The existence of a frequency stopband and strong grating reflectivity have been theoretically predicted and experimentally observed. One-port resonator designs having varying cavity lengths and transducer topology were fabricated and characterized. A quality factor exceeding 3000 has been demonstrated at frequencies of about 885 MHz. The IDT based film plate acoustic resonators (FPAR) technology proved to be preferable when lower costs and higher Qs are pursued. The LW-LFE-based FPAR technology offers higher excitation efficiency at costs comparable to that of the thin film bulk acoustic wave resonator (FBAR) technology     

Study on the quality of interference fringes from a pulsed UV source for application in a biprism based fiber Bragg grating writing

This paper presents a study on the quality of interference fringes formed from a pulsed UV (255 nm, 5.6 kHz, and 40 ns) source for an application in writing fiber Bragg gratings (FBGs). The interference fringes of separation of about 8 microm, formed by a biprism of apex angle 2 degrees , were studied for their contrast, evolution of contrast, and positional and intensity stability over a period of 5 min (over 10(6) pulses). Second harmonic UV (255 nm) sources of different spatial coherence and pointing stability characteristics were employed as the inputs. It is established that the UV fringes contrast and interference pattern stability with time is largely controlled by the optical resonator geometry of the fundamental wavelength (510 nm) copper vapor laser (CVL) oscillator. In particular, the generalized diffraction filtered resonator (GDFR) CVL produced the highest quality second harmonic beam with the highest fringes contrast and stability. The implications of these results were studied by employing these UV sources for the fabrication of the C-band FBGs by a 24 degrees apex angle biprism.     

Transmission and reflection coefficients for surface transverse waves propagating under shorted metal strip gratings

Coupled-mode theory is used for the analysis of surface transverse waves (STWs) propagating under metal-strip gratings in the YZ plane of quartz. Analytical expressions are obtained for the reflection and transmission coefficients of waves propagating on rotated Y-cut substrates of weakly piezoelectric trigonal, tetragonal, or hexagonal crystals. A quadratic dependence of the stopband width on the height-to-period ratio of the electrodes is found. This is an agreement with stopband width measurements. The results can be used in the analysis of STW grating structures, particularly the two-port resonator filter.     

Embossed Bragg Gratings Based on Organically Modified Silane Waveguides in InP

Considering the large variety of applications for optical glass waveguide gratings, the effective production method of embossing for micropatterning, and the unique advantages of InP-based materials, we expect that hybridization of embossed optical glass waveguide gratings and InP substrates will inevitably lead to new applications in integrated optics. We present our preliminary results of research toward the development of solgel-derived glass waveguide gratings made by embossing on InP. Theoretically, the dependence of the stop-band FWHM and transmission contrast of the grating filter on the grating length, and the relationship between the Bragg grating's reflective wavelength and the dopant concentration in the solgel waveguide, are obtained. Experimentally, using organically modified silane, we solve the problem of mismatching of SiO(2) and InP, and successfully fabricate an embossed glass grating with a second-order Bragg reflection wavelength of 1580 nm and a FWHM of 0.7 nm fabricated upon a solgel waveguide on an InP substrate.     

Design of a compact dual-mode ring resonator bandpass filter with harmonic suppression

Abstract

An approach to suppress the harmonic responses in a dual-mode circular ring resonator bandpass filter is presented. Feed lines with one-sided coupling arms are suitably relocated around the ring resonator to introduce phase interference between two degenerate modes for simultaneously suppressing the second and third harmonics. Three open loop resonators at the rim of the ring are adopted to further suppress the second harmonic. By fabricating on a FR4 glass-epoxy base, this approach is demonstrated to have effectiveness in broadening the filter's rejection band in both simulations and measurements without affecting the fundamental response or increasing the device size.     

Off-axis pivot mounting for aberration-corrected concave gratings at normal incidence

The off-axis pivot-point mounting for toroidal uniform line-space (TULS) gratings and spherical varied line-space (SVLS) is presented: One scans the spectrum by rotating the grating about the pivot point. The best choice for the location of the point is discussed: For a monochromator the location is chosen such that the grating is moved approximately along the bisector of the subtended angle; for a spectrometer, it is chosen such that the grating is moved approximately along the direction of the incident ray. Minimizing the spectral defocusing caused by the rotation of the grating determines the optimum length of the pivot arm. The pivot points for TULS and SVLS gratings are found to be located on opposite sides with respect to the normal to the grating. In a comparison of the optical performance, the spectral focusing is similar, but the spatial aberrations can be fewer for a SVLS grating.     

变截面闭管中非线性驻波场的实验研究

当变截面闭管（CTVCS）的固有一阶反共振频率设计在接近共振基频的倍频处,共振基频激励下的二次谐波将受到极大抑制,以致在闭管末端可获得高声压、低谐波畸变的波形。相比等截面闭管,CTVCS内的二次谐波特性较为复杂,且在研究范围内,谐波随基波声压级的增长未出现明显的饱和趋势。     

Doubly rotated contoured quartz resonators

Doubly rotated contoured quartz resonators are used in the design of temperature-compensated stable clocks and dual-mode sensors for simultaneous measurements of pressure and temperature. The design of these devices is facilitated by models that can predict frequency spectra associated with the three thickness modes and temperature and stress-induced frequency changes as a function of crystalline orientation. The Stevens-Tiersten technique for the analysis of the C-mode of a doubly rotated contoured quartz resonator is extended to include the other two thickness modes. Computational results for harmonic and anharmonic overtones of all three thickness modes of such resonators help in optimizing the radius of curvature of the contour and electrode shape for suppression of unwanted modes and prevention of activity dips. The temperature and stress-induced changes in thickness-mode resonator frequencies are calculated from a perturbation technique for small dynamic fields superposed on a static bias. The static bias refers to either a temperature or stress-induced static deformation of the resonator plate. Phenomenological models are also used for calculating the temperature and stress-induced changes in resonant frequencies as a function of crystalline orientation. Results for the SBTC-cut quartz plate with a spherical convex contour of 260 mm indicate that normal trapping occurs for the third (n=3) and fifth (n=5) harmonic of the A-mode, the fundamental (n=1) and third (n=3) harmonic of the B-mode, and the fundamental (n=1) and fifth (n=5) harmonic of the C-mode     

Flat broadband wavelength conversion based on cascaded second-harmonic generation and difference frequency generation in segmented quasi-phase matched gratings

Bandwidth enhancement and response flattening of wavelength conversion based on single-pass and double-pass cascaded second harmonic generation and difference frequency generation were investigated in segmented quasi-phase matched (QPM) gratings. It is shown that the signal and pump bandwidths are both efficiently widened by increasing the segment number of the QPM grating and optimising the poling period of each segment. The ripple on the matching response is also very small. The conversion bandwidth in a 3-cm-long three-segment waveguide reaches 150–160?nm, which is over the whole conventional band and long-wavelength band. Larger signal bandwidth can be obtained with a little response flatness penalty and conversion efficiency penalty, which can be compensated by increasing the input pump power. Compared with a sinusoidally chirped optical superlattice device, a wavelength converter based on the segmented gratings has higher conversion efficiency, broader bandwidth and better pump-wavelength tolerance, and is easier to fabricate in practice.     

Intravascular ultrasound tissue harmonic imaging in vivo

Tissue harmonic imaging (THI) has been shown to increase image quality of medical ultrasound in the frequency range from 2 to 10 MHz and might, therefore, also be used to improve image quality in intravascular ultrasound (IVUS). In this study we constructed a prototype IVUS system that could operate in both fundamental frequency and second harmonic imaging modes. This system uses a conventional, continuously rotating, single-element IVUS catheter and was operated in fundamental 20 MHz, fundamental 40 MHz, and harmonic 40 MHz modes (transmit 20 MHz, receive 40 MHz). Hydrophone beam characterization measurements demonstrated the build-up of a second harmonic signal as a function of increasing pressure. Imaging experiments were conducted in both a tissue-mimicking phantom and in an atherosclerotic animal model in vivo. Acquisitions of fundamental 20 and 40 MHz and second harmonic acquisitions resulted in cross sections of the phantom and a rabbit aorta. The harmonic results of the imaging experiments showed the feasibility of intravascular THI with a conventional IVUS catheter both in a phantom and in vivo. The harmonic acquisitions also showed the potential of THI to reduce image artifacts compared to fundamental imaging.     

Laser-induced Grating Phenomena

The interference pattern of two laser beams induces periodical variations of the optical properties of matter (refractive index and absorption coefficient). Induced grating structures have been observed in absorbing liquids and solids, plasmas, laser materials, dye solutions and semiconductors. These gratings have been applied to thermal diffusion measurements, excitation of high frequency normal and second sound waves, determination of electronic lifetimes, distributed feedback lasers and real-time holography. The literature on transient grating materials and applications is surveyed.     

Analysis of fiber bragg gratings by a side-diffraction interference technique

A new nondestructive, noncontact, and sensitive technique for fiber Bragg grating geometry and index-fault location measurements is presented. Two plane-wave probe laser beams are incident upon the grating from the side at angles that satisfy the Bragg-reflection condition. An interference pattern is formed behind the fiber between the first-order diffracted beam (from one probe beam) and the zero-order transmitted beam (from the second probe beam). The axial grating index modulation and the grating period are functions of the fringe visibility and the fringe period, respectively. The method is sensitive and is applicable even in the case of relatively weak gratings. Unchirped and chirped Bragg gratings have been studied with the proposed technique. We demonstrate accuracies of 1 x 10(-4) for measurement of the index modulation and 0.01 nm for measurement of the period. As well as for the analysis of most already-fabricated gratings, this technique is useful for in situ analysis of a long fiber Bragg grating as such a grating is translated along its axis during the fabrication process.     

Photopolarimeter based on two gratings recorded in thin organic films

A photopolarimeter based on two different kinds of diffraction gratings (a two-grating photopolarimeter) has been developed for real-time measurements of the four elements of the Stokes vector. The main elements of the device are a pure polarization grating and an ordinary transmission grating, both recorded by means of holographic techniques in thin films of organic materials. The first one consists of a diffraction grating recorded by two interfering opposite circularly polarized beams in a Langmuir-Blodgett film of an azo-compound material. The second component is a grating recorded by two interfering parallel circularly polarized beams in a thin film of a photosensitive polymer. Both gratings offer long time stability and good diffraction efficiency. Four photodiodes collect the first-order diffracted beams from these gratings, the output signals of which are read through an analog-to-digital converter by a PC. The optical alignment of the device is easy and the calibration is realized in a one-step procedure.     

Broadening the angular tolerance in two-dimensional grating resonance structures at oblique incidence

Broadening of the angular response of two-dimensional (2D) guided mode resonant spectral filters at oblique incidence is investigated. Coupling into multiple fundamental guided resonant modes having the same propagation constant but propagating in different planes (inherent multiple-plane diffraction by 2D gratings) is shown to significantly broaden the angular tolerance while maintaining narrow linewidth. Resonances have symmetric and broad angular responses when the incident wave is coupled to four resonant modes in a structure with a hexagonal grating pattern. Further broadening is implemented by enhancing the second Bragg diffraction of the 2D grating structure. Resonance with a narrow spectral linewidth (Dlambda(FWHM) approximately 1.6 x 10(-4)lambda(0)) and angularly tolerant to an ~6 mum beam diameter is obtained. A second approach utilizing a dual 2D grating configuration with a second grating on the substrate side is shown to increase the lateral confinement, causing the merging of two successive resonant bands. This results in further improvement of the angular/spectral linewidth ratio by ~80%.