Asymptotics of evanescence

Abstract

The wave from an isotropic point source in two and three dimensions is separated into its homogeneous (H) and evanescent (E) parts, with respect to a distinguished direction z, and the far field r θ evaluated asymptotically as a function of polar angle θ. Only in the ‘forward needle’ θ = 0 (three dimensions) and the transverse directions θ = π/2 (two and three dimensions) is the E wave of comparable strength to the H wave. Uniform asymptotic approximations (in terms of Bessel functions and Fresnel integrals) accurately interpolate between these directions of significant evanescence and all other angles (where the amplitude of E relative to H decays as 1/r¹/²). The forward needle in three dimensions is analogous to glory scattering.     

Pseudocolour encoding by lensless one-step rainbow holography with a synthesized slit

Abstract

Pseudocolour encoding of holographic images by lensless one-step rainbow holography with a synthesized slit is proposed. When a different portion of the object is recorded the reference wave is incident at a different angle θ and during each exposure the incident angle changes continuously around this angle θ. The pseudocolour holographic image is obtained by white-light reconstruction of the lensless one-step rainbow hologram with synthesized slit. The basic advantage of this technique is the elimination of a narrow slit and a lens in the encoding process. The theoretical analysis and experimental result are presented.     

Polarization Holographic Gratings

Abstract

The photoinduced anisotropy (dichroism and birefringence) in AgCl emulsions is studied as well as the possibility for chemical fixing of this anistropy. A theoretical relation between the diffraction efficiency of amplitude—phase polarization holographic gratings and the photoanisotropy of the recording medium is derived. Based on this relationship an estimation of the properties of the gratings is presented. Polarization holographic gratings with spatial frequencies of 200 to 2000 mm^?1 and diffraction efficiency up to 1·8% are recorded. The gratings are stable, and do not change in time for more than a year.     

Experimental study of a bow-tie-shaped optical fiber ring resonator with two 2 × 2 fiber couplers

An experimental study of a bow-tie-shaped double-coupler fiber ring resonator is presented. Multiple resonances of the transmitted output intensity and the splitting of the main resonance dip or peak have been observed. The experimental results are discussed and compared with theoretical results. The observed output property suggests the possible applications of the resonator as periodic Butterworth-like, narrow-band passing, and blocking filters.     

Difference in Finesse Between Output and Circulating Intensities in Optical Fibre Ring and Loop Resonators

Abstract

When the geometry of an optical fibre ring or loop resonator with two or more fibre couplers is complex, the shapes and linewidths of the resonance curves of the output intensities and the circulating (resonant) intensities, and consequently their finesses, may become different. Applying degenerate two-wave mixing to some configurations of these fibre ring or loop resonators can also bring about a noticeable difference between these finesses. A theoretical study of this aspect is presented. Results obtained from this study can improve our understanding of the basic characteristics of these fibre resonators.     

Growth kinetics of (θ′– and θ-phases in Al–3Cu alloy

Abstract

The precipitation of θ′ and θ in Al–3Cu alloy has been investigated, using a resistivity and a transmission electron microscopy (TEM) method. Both stages of the transformation obey the Avrami equation, ξ = 1? exp (?ktⁿ), where ξ is the precipitated fraction of excess solute. The growth parameters are n = 1·68±0·10 for θ′ precipitation and n = 0·63 ± 0·04 for θ precipitation, and the corresponding activation energies are 85·3 ± 10·9 and 111·1±5·2 kJ, respectively. The n-value for the θ′ precipitation stage corresponds to the model proposed by Ham for diffusion-limited growth of disc-like particles, and is in agreement with the TEM results of this investigation. The activation energy for θ′ precipitation is in the range for θ′ boundary migration reported by Aaronson and Laird. The n-value and activation energy for θ precipitation suggests a growth model in which concurrent dissolution of θ′ supplies solute to θ particles via dislocation paths.

MST/480     

Polarization Conversion Using Prism-coupled Surface Plasmon-polaritons

Abstract

Evanescent prism coupling through a thin silver film provides a convenient mechanism for excitation of surface plasmon-polaritons (SPPs). However, the SPP is observed as a dip in p-polarized reflectivity, which may be inconvenient in sensor application. Here we observe the effect of rotation of the plane of incident polarization. This leads to the observation of polarization mixing at SPP excitation, and a peak reflectivity signal is obtained between crossed polarizers. This provides a powerful mechanism for SPP observation as a peak, which may be more easily monitored.     

On the Theory of Stimulated Raman Radiation in an Optical Resonator

Stimulated Raman radiation involving an arbitrary number of Stokes components in an optical resonator is considered for the case when a pumping wave is directly incident on a Raman-active condensed substance at an angle θ to the resonator axis. The expressions are derived for the steady-state radiation intensities of all the Stokes components generated, and the number of the components versus the pumping wave intensity is discussed. The conditions of the effective conversion of the pumping wave into radiation of Stokes components are elucidated. It is shown that in a frequency region close to the point ?² n/? ω² = 0 (n is the effective refractive index of the substance inside the resonator) locking Stokes components is feasible due to a ‘tunnel’ traversal of femtosecond pulses through a dispersive medium, provided the latter occupies a characteristic tunnel length along the resonator axis. For the case of the three components generated, the wave band over which locking occurs is derived and a frequency modulation of the first and the third Stokes components beyond this band is predicted. The possibility of an experimental indication of the band discussed, due to a discontinuous disappearance of the frequency modulation in the band, is noted. For locking four Stokes components the possibility is shown of the change of their stable phase relation on variation of the pumping wave intensity.     

Polarimetric investigation of materials with both linear and circular anisotropy

Abstract

We investigate light propagation through materials with both linear and circular anisotropy and find the relation of the amplitude and polarization transfer functions to the four anisotropic characteristics: linear circular birefringence, and linear and circular dichroism. We determine these four characteristics of anisotropic samples by measuring the output intensity and polarization corresponding to different input polarization azimuths and fitting the theoretical and experimental results. In our experiments we have used films of side-chain azobenzene polyesters in which optical anisotropy had been previously induced on illumination with elliptically polarized light.     

Polarization separation of light in holographic grating reflection in conical mounting

Abstract

We experimentally investigate the interaction of linearly polarized light with a holographic grating in a conical mounting. Due to the periodic structure, the polarization properties of the reflected zeroth-order beam are highly sensitive to the conical angle. When a focused Gaussian beam with linear polarization impinges on an air–grating interface at an exceptional conical angle, a spatial splitting of the reflected beam is observed behind a polarizer. We find that it can be interpreted using the anisotropy of the polarization distribution in holographic grating reflection.     

Bilayer Coatings for Light Refraction Without Change of Polarization

Abstract

Wave refraction without change of polarization at a dielectric—dielectric interface is achieved with a bilayer coating of two transparent thin films. For given refractive indices of all media, the thicknesses of the two films are determined as functions of the angle of incidence. The polarization-independent reflectance and the differential reflection phase shift of the coated substrate are also calculated. Examples are presented of MgF₂—ZnS and Ge—MgF₂ bilayer coatings on a Ge substrate that refract infrared radiation without change of polarization. Such coatings are useful in the construction of polarization-preserving beam deflectors and beam splitters.     

Angle-dependent light emission from aligned multiwalled carbon nanotubes under CO(2) laser irradiation

This paper reports the light emission from aligned multiwalled carbon nanotubes (MWNTs) under continuous wave CO(2) laser (λ = 10.6?μm) irradiation. Results indicate that the light emission is dependent on the angle θ between the laser incident direction and the nanotube axis. The relative intensity of the light emission at certain wavelengths shows a Lorentzian feature when θ varies from 0° to 90°. The Lorentzian fitting curve displays a distinct tendency between shorter (λ<600?nm) and longer wavelength (λ>700?nm). A minimum intensity was observed at θ(m) close to 67° under shorter wavelength, whereas a maximum intensity was shown at θ(m) of about 60° at longer wavelength. These results show the anisotropic property of aligned MWNTs.     

Lamellar Gratings as Polarization Components for Specularly Reflected Beams

Abstract

High spatial frequency lamellar gratings are shown to function as phase compensators, quarter-wave and half-wave retarders, and polarization rotators that operate on specularly reflected (zeroth-order) beams. These gratings are designed using rigorous coupled-wave and modal grating diffraction theories. Controlling the geometrical parameters of these gratings allows for engineering the phase retardation and polarization conversion introduced to a reflected beam. Fabrication and operational tolerances for these elements are discussed. Wavelength and polar angle of incidence variation affect the performance of these elements more strongly than variations in other geometrical and operational parameters.     

Resonance ring interferometry with incoherent light

A new method of measuring the phase nonreciprocity in a passive ring resonator using a light source of low coherence is described. The method provides effective suppression of noise from the backscattering of light in the ring resonator and insensitivity of the interferometer to excursions of the resonance frequencies of the resonator due to reciprocal effects (e.g., thermal expansion) and also permits modulation and compensation of the phase nonreciprocity by using a device outside the resonator to shift the frequency of the light. One version of a low-coherent resonance ring interferometer is examined, viz., a two-transit asymmetric interferometer with a rotating mirror. Pis’ma Zh. Tekh. Fiz. 24, 24–29 (September 12, 1998)     

Fiber Optics: Research and Development

Abstract

With twisted stacks of N polarizers P or retarders R, the polarization of a light beam can be cycled around the Poincaré sphere on N similar arcs of great P or small R circles. We calculate the phase changes around these cycles (geometric for P; geometric + dynamical for R). In the continuum limit N → ∞ of a smoothly twisting medium, a P stack forces the light to follow its changing polarization, and the phase is the solid angle of the associated loop on the sphere; for an R stack, on the other hand, it is only in the adiabatic limit of slow twist (where the dynamical phase is large) that the geometric phase corresponds to that of the loop specified by the changing eigenpolarization of the medium. The predicted phase shifts are observed as fringe shifts in an interferometer for N=2, 3 and 4.     

Polarization switching time for semiconductor laser radiation

Expressions are derived for the polarization switching time for semiconductor laser radiation. These expressions can be used to assess the influence of the laser diode parameters on the polarization switching time of the output radiation. The results of an analysis of these expressions show that the main parameters influencing the polarization switching of the output radiation from a semiconductor laser are the nonlinear gains. Pis’ma Zh. Tekh. Fiz. 23, 87–95 (May 12, 1997)     

Diffractive light attenuators with variable transmission

Abstract

A new application of diffractive optical elements (DOEs) for continuous or multistage adjustment of optical radiation intensity is described. The diffractive attenuators are linear or circular gratings (amplitude or phase) with constant period and diffraction efficiency that varies across the grating. The zero order of diffraction is used as the output and transmitted through the grating without angular deviation. The diffractive attenuators, in distinction to conventional analogues, allow one to change the intensity of the light beam according to predetermined function and have no limitations for power of the regulated light beam. These elements can be used in optical systems as a beam splitter with adjusted splitting coefficient. The experimental results on a circular diffractive attenuator fabricated by direct laser writing on a chromium film are presented. The range of transmission variation was 20 times within a 340° angle of attenuator turn. The possibility to use a phase diffractive attenuator as a light radiation modulator for a powerful technological laser is discussed.     

Conversion of 1/f fluctuations in crystal resonator within an inter resonance gap

This paper presents general relationships for transformation coefficients of BAW or SAW crystal resonator amplitude and phase fluctuations through the 1/f flicker noises of its motional and static equivalent parameters within the resonator inter resonance gap. Approximate functions of phase and amplitude power spectral densities are found based on Leeson's oscillator open loop model and are given with detailed consideration of Butler and Colpitts modes of operation with the assumption of full and zero inter noise correlation. It is also substantiated that a low-noise frequency region of crystal resonator operation exists in which the fluctuation influence of its motional inductance and capacity tend to zero in oscillators. Five examples are given as an illustration of a good agreement of the measured data with the prediction curves, giving a possibility of resonator power phase and amplitude spectral densities valuation at an arbitrary offset frequency from the carrier through the 1/f flicker noises of resonator parameters. Emphasis is laid in conclusion on the possible way of parameter spectral densities definition.     

Al-Cu合金析出相在等径角挤压中的演变

为了研究Al-Cu合金中两种不同析出相(θ′和θ相)在ECAP变形过程中的变化.采用透射电镜(TEM)和硬度测试方法研究了析出相形貌变化以及对合金性能的影响.结果表明:在本实验中,θ′和θ相其破碎、回溶速度明显不同,两者的破碎方式也不同.θ′相先是与基体失去取向关系,随后从其内部产生位错使其破碎,而θ相是被外部基体位错所切割、破碎.θ′相与位错的相互作用方式类似于绕过机制,θ相与位错的作用方式类似于切割机制.两种状态样品的硬度在变形过程中的变化趋势相同,但在第1道次后θ相状态样品的硬度增加值高于θ′相状态.     

Polarization Aberration Effects Propagated in Optical Systems

Abstract

The propagation and imaging of polarized light through optical systems described by a polarization aberration expansion is treated by combining a scalar operator calculus with the Jones calculus. The martrix-operator framework provides a means for handling diffraction and propagation in optical systems containing polarization aberrations. An expansion for the polarization aberration function of an optical system, similar to the expansion of the wavefront aberration function into defocus, tilt, piston, and higher-order terms, is analysed. These polarization aberration terms introduce phase changes in the diffraction image proportional to the first and second derivatives of the non-polarization aberrated image structure.