Transformation of the multimode terahertz quantum cascade laser beam into a Gaussian, using a hollow dielectric waveguide

We demonstrate that a short hollow dielectric tube can act as a dielectric waveguide and transform the multimode, highly diverging terahertz quantum cascade laser beam into the lowest order dielectric waveguide hybrid mode, EH(11), which then couples efficiently to the free-space Gaussian mode, TEM(00). This simple approach should enable terahertz quantum cascade lasers to be employed in applications where a spatially coherent beam is required.     

Extinction paradox and actual power scattered in light beam scattering: a two-dimensional study

The extinction paradox is examined by applying partial-wave analysis to a two-dimensional light beam interacting with a long transverse cylinder without absorption, assuming always short wavelengths. We show that the (conventional) power scattered, Psca, except for a very narrow beam hitting a transparent cylinder on axis, is always double the power directly intercepted by the scatterer, Pitc, including a zero result for Psca when the incident beam is basically off the material surface. This contradicts the interpretation that attributes one half of Psca to edge diffraction by the scatterer. Furthermore, we identify the shadow-forming wave (SFW) from the partial-wave sum in the forward direction and show that the actual power scattered or, equivalently, the power depleted from the incident beam is equal to one unit of Pitc for a narrow beam, gets larger for a broader beam, and approaches 2Pitc for a very broad beam. The larger value in the latter cases is due to the extent of divergence of the SFW beam out of the incident beam at distances well beyond the Rayleigh range.     

Thermal effects in a hollow waveguide beam launch for CO(2) laser power delivery

Thermal effects caused by launching conditions in a CO(2) laser beam delivery that uses metallic hollow waveguides is investigated. It is found that front-end clipping is the main cause of thermal loading and generates a steep temperature gradient at the fiber front end while the continuous beam attenuation produces a temperature distribution declining slowly along the waveguide.     

Polarization properties of a four-layer dielectric waveguide

A new integrated-optics polarizer system is analyzed. Calculations are made and experiments carried out to demonstrate the performance of this system. Pis’ma Zh. Tekh. Fiz. 24, 35–39 (March 26, 1998)     

The energy of a photon flux in a dielectric waveguide

Propagation of a monochromatic electromagnetic field in a round dielectric waveguide is considered within the framework of a quantum field approach. A Hamiltonian of the photon flux-dielectric waveguide system is found and expressions for the system energy in the linear and nonlinear approximations are obtained. It is shown that the energy depends on the ratio of the numbers of photons with left-and right-handed helicity.     

Infrared multiphoton dissociation with a hollow fiber waveguide

A novel scheme for performing infrared multiphoton dissociation (IRMPD) is presented in which a hollow fiber waveguide (HFWG) is used to transmit IR radiation into the ion storage region of a mass spectrometer. Efficient dissociation of oligonucleotide and protein ions is demonstrated on an ESI-FTICR instrument in which IRMPD is performed in the external ion reservoir and on a quadrupole ion trap. Using a simple optical scheme consisting of a single focusing lens and an x, y translator, the 10.6-microm IR laser beam, initially 3.5 mm in diameter, is focused into the vacuum-sealed HFWG. The small internal diameter and the high transfer efficiency of the waveguide allow IR radiation of high power density to be employed for IRMPD. In studies performed on a quadrupole ion trap, a 500-microm-i.d. waveguide was used as a medium to transmit IR radiation directly through a 700-microm orifice in the ring electrode. Efficient IRMPD of both a 12-mer oligonucleotide and the protein melittin were performed at laser powers of 0.5 and 3.2 W, respectively.     

Wavelength-selective filter based on a hollow optical waveguide

In this paper, we describe a theoretical and experimental study of a wavelength-selective filter derived from hollow optical waveguides composed of Bragg reflectors with defect layers on a silicon substrate. The defect states of the transmission filter at wavelengths of 1519 and 1571 nm were realized using one-dimensional photonic crystals (1D PCs) formed from a-Si and SiO(2). The transmission spectra of the filter waveguides and the band structure of the defect 1D PCs were calculated using the two-dimensional finite-difference time-domain and transfer matrix methods, respectively. The device exhibited the narrow bandwidths of 0.5 and 1.1 nm for wavelengths of 1571 and 1519 nm, respectively.     

The topology of “vacuum” in a dielectric waveguide

The energy states of a nonlinear system of the electromagnetic field-dielectric waveguide type is considered. It is shown that this system possesses a manifold of “vacuum” states with a zero “internal” energy. The presence of such states is evidence that the system can feature field states in the form of three-dimensional solitary waves.     

Ultrasonic guided wave scattering in a plate overlap

Guided wave scattering in a plate overlap is investigated by numerical calculations and experimental measurements of transmission and reflection factors from the overlap region. In the numerical study, a hybrid boundary element-finite element method is used to calculate the guided wave scattered field from the overlap region. Transmission and reflection factors are calculated for incident A0 and S0 Lamb and n0 shear horizontal waves, including higher modes generated through mode conversion phenomena. In addition, parametric studies of transmission and reflection factors in this problem are performed numerically over various incident modes, frequencies, and overlap lengths. For verification and comparison with numerical results, experiments were conducted to measure the transmission and reflection factors for incident Lamb and shear horizontal waves in steel plates with two different overlap areas. The experimental results agree well with the numerical calculations. The numerical and experimental results show that it is highly feasible to carry out efficient Lamb wave nondestructive evaluation (NDE) in overlapped plates and in multilayer structures with various lap joints by selecting various modes and tuning frequency.     

Light propagation in a planar dielectric waveguide with a gyrotropic layer

The nature of the magneto-optic Kerr effect in a planar dielectric waveguide geometry has been investigated by calculation of the Jones matrix for a planar waveguide structure with a gyrotropic magnetic material as one wall. The intensity of the component of the field that is in the polarization state orthogonal to the input was calculated as a function of length of the gyrotropic material and input polarization state. The degree of polarization rotation depends on the relative orientation of the magnetization in the magnetic material and the direction of propagation. It is found that there exists an optimal waveguide length and input polarization at which the output signal is maximized and that a significant enhancement in polarization rotation is available with respect to free-space reflection. These results indicate that a magnetic-film-bounded planar waveguide can be used for device applications such as magnetic field sensors or magneto-optic modulators.     

Analysis of mode scattering from an abruptly ended dielectric slab waveguide by an accelerated iteration technique

A new modification of the integral equation method using an iteration technique with "accelerating" parameters is presented to solve the problem of guided-mode scattering from an abruptly ended asymmetrical slab waveguide. The optimal choice of the parameters is shown to be closely connected with the variational principle. The electric-field distribution at the terminal plane, the reflection coefficient of the guided mode, and the far-field radiation pattern are computed. Numerical results are presented for several cases of abruptly ended waveguides, including the systems with constant and variable profiles of the refractive indices. The phenomenon of the radiation pattern rotation is examined in detail.     

Infinite guided modes in a planar waveguide with a biaxially anisotropic metamaterial

We investigate in detail the guided modes in a two-layered planar waveguide where one layer is filled with an ordinary right-handed material (RHM) and the other is filled with a biaxially anisotropic metamaterial. We show that the mode properties are closely dependent on the spatial dispersion relation of the anisotropic medium. When the dispersion equation for the anisotropic medium becomes a two-sheet or a one-sheet hyperbola type, an infinite number of guided modes can be supported simultaneously in the waveguide, which is completely different from the cases of RHM and isotropic metamaterial. We also investigate the mode distributions of the planar waveguide in the lossy case, where we discover that the dominant mode in the waveguide is a forward wave while the higher-order modes are backward waves under the two-sheet hyperbolic dispersion. Numerical results validate our theoretical analysis.     

Scintillation properties of a rectangular dark hollow beam

The scintillation properties of a rectangular dark hollow beam (DHB) in a weak turbulent atmosphere are investigated. Explicit expression for the on-axis scintillation index of a rectangular DHB is derived. It is found that the scintillation index value of a rectangular DHB can be smaller than that of Gaussian, elliptical Gaussian and rectangular flat-topped beams in a weak turbulent atmosphere under certain conditions. Our results will be useful in long-distance free-space optical communications. The scintillation properties of a rectangular DHB are closely controlled by its initial beam parameters.     

Elastic light scattering with a LiNbO3 waveguide

We present the measurements of hemispherical elastic scattering of light guided by different modes of a planar LiNbO3 waveguide. It is shown that the fundamental and the lowest-order modes are more sensitive to the scattering properties of the air-core interface, whereas higher-order modes are more sensitive to the optical inhomogeneities of the core-substrate interface. We also demonstrate that because of static polarization of LiNbO3 crystal, the air-core interface is sensitive to the presence of dust particles in air, which causes a change in the scattered light based on time of observation. This sensitivity could be used to elaborate compact sensors of air contamination.     

The surface mode of a dielectric waveguide with metal substrate

The conditions of excitation and the waveguide characteristics are determined in an analytical form for the TM₀ mode of a planar waveguide with metal substrate. This mode has a surface character, in contrast to other (bulk) modes excited in the system. The propagation and damping constants and the energy flux density distributions are compared for the TM₀ and TE₀ modes.     

Radiation of a charge moving in a waveguide with a resonantly dispersive dielectric layer

The radiation of an ultrarelativistic point charged particle moving along the axis of a cylindrical waveguide containing a layer of resonantly dispersive dielectric medium is studied theoretically. It is shown that, in the case of a sufficiently thin waveguide, the dispersion leads to a significant contraction of the radiation, a decrease in the energy of harmonics, and an increase in the relative contribution due to the first mode as compared to the higher harmonics. These effects are less pronounced in waveguides with relatively large radii; still, they have to be taken into account, especially in determining the frequencies of harmonics. The role of the resonance dispersion is most significant in cases where the dielectric layer thickness is small compared to the waveguide radius.     

Improving near-field throughput in a hollow metallic tapered waveguide via wavefront modulation

Throughput enhancement through a 100?nm-aperture hollow tapered aluminum waveguide via wavefront modulation was investigated numerically. The propagating modes in the waveguide are controlled and efficiently transformed to the fundamental propagating mode with optical wavefront modulations via a centro-symmetrical 9-bit binary optical element. Combined with the adjustment effect of the focal length of the coupling lens, an optimal throughput enhancement of about 11 times is obtained.     

Circular-harmonic vector analysis of a dielectric waveguide with a cross-cut-circle cross section

Vector-wave solutions of dielectric waveguides with cross-cut-circle cross sections, which are commonly produced by reflow of the waveguide core on a planar substrate at high temperature, are rigorously analyzed by the use of the point-matching method. As a numerical example, the propagation constant, electromagnetic-field distribution, and convergence of mode solutions for a semicircular waveguide are presented, and the properties of these modes are discussed.     

Role of multiple scattering in cross-correlated light scattering with a single laser beam

Previous systems for measuring cross-correlated light scattering by small particles suspended in a liquid with multiple-scattering suppression have illuminated the particles with two laser beams. It is shown that multiple-scattering suppression should also occur in cross correlation for a system that employs a single laser beam and two closely spaced detectors with wide fields of view. The single-scattering, double-scattering, and single-double-scattering cross-term contributions to the intensity cross-correlation function are calculated. It is found that the two cross terms, when added together, are unimportant for both autocorrelation and cross correlation. The amplitude of the double-scattering term can be greatly diminished by judicious detector spacing because the spatial coherence area in the detector plane for double scattering is much smaller than that for single scattering.     

Design and development of a thin film dielectric beam combiner

A multilayer thin film device called a beam combiner for a specific photodissociation experiment was designed and developed. The device when operated at 45° angle of incidence transmits auv laser beam and reflects a visible laser beam at the same time with minimum energy loss. The efficiency of the device was found to be better than 90%.