Exit Face Research Articles

Spatial coherence in incoherently excited optical fibers

The degree of spatial coherence on the exit face of optical fibers, excited by a spatially incoherent source, has been evaluated assuming that the guided modes are represented by Hermite-Gauss polynomials. Experimental results, obtained with quasi-monomode and multimode parabolic fibers, are shown to agree very well with our theory.

Line spread function, crosstalk, and modulation transfer function for silicon IR detector arrays

The line spread function, crosstalk, and modulation transfer function (MTF) are computed for stripe geometries, assuming the absorption length is greater than the detector resolution element size. It is assumed that the radiation incident surface is antireflection coated and that the reflectivity of the exit face is a parameter. Numerical computations are carried out to find the optimal focal-plane depth (least crosstalk), and the crosstalk is calculated at this optimal focal-plane depth as a function of both absorption coefficient and detector width. The MTF is then calculated for various detector widths.

The reverse laser drilling of transparent materials

Within a limited range of incident laser-beam intensities, laser drilling of a sapphire wafer initiates on the surface of the wafer where the laser beam exits and proceeds upstream in the laser beam to the surface where the laser beam enters the wafer. This reverse laser drilling is the result of the constructive interference between the laser beam and its reflected component on the exit face of the wafer. Constructive interference occurs only at the exit face of the sapphire wafer because the internally reflected laser beam suffers no phase change there. A model describing reverse laser drilling predicts the ranges of incident laser-beam intensity where no drilling, reverse laser drilling, and forward laser drilling can be expected in various materials. The application of reverse laser drilling in fabricating feed-through conductors in silicon-on-sapphire wafers for a massively parallel processor is described.

Connecticut River Fishways: Model Studies

The fishways at the Vernon and Turners Falls Dams were optimized by use of hydraulic model studies. In both cases, slotted weirs were modified so that a better head loss distribution was obtained. In addition, the diffusers were modified to produce acceptable exit face velocity distributions. At Turners Falls, the fish entrance was modified to produce velocity conditions known to be acceptable to the two migrating species of fish. At Vernon Dam, flow conditions were optimized at the fish exit and attraction water intake. Modeling of these fishways has resulted in design changes that should improve fish passage and minimize water consumption.

Connection between spatial coherence and modal structure in optical fibers and semiconductor lasers

The evaluation of the relative amplitudes of the modes of a guiding structure is not easy when the relative amplitudes of the modes are very different among themselves. In this paper we show how a measurement of the modulus of the complex degree of spatial coherence on the exit face of a passive or active guiding structure, supporting a small number of guided modes, allows us to determine the model content of the propagating field. Experimental results obtained performing measurements on a low-mode optical fiber and a stripe DH laser are also presented.

Analysis and measurement of modal noise in an optical fibre

Modal noise in analogue and digital optical fibre systems has been demonstrated to be a significant effect. In this letter, we give some theoretical and experimental results concerning the behaviour of the signal-to-noise ratio in presence of modal noise considering the power from the exit face of a short sample of a graded-index multimode optical fibre.

Anti-Stokes stimulated Raman scattering of light by polaritons

A theoretical analysis is made of anti-Stokes stimulated Raman scattering (STRS) of light by polaritons in noncentrosymmetric crystals. This is due to the superposition of parametric four-photon and two-stage three-photon processes, in addition to three-photon processes associated with the possible existence of thermal polariton excitations. These processes are mutually coherent and their contributions interfere. The fluctuation-dissipation theory of polariton STRS is applied to derive and analyze general formulas for the frequency-angular distribution of the Stokes and anti-Stokes radiation intensity at the exit face of a plane-parallel crystal layer pumped by a given plane monochromatic wave. Unlike the phonon process, frequency-tunable polariton anti-Stokes STRS is found in scattering at any angles within the polariton range of scattering angles. The anti-Stokes processes also affect the Stokes scattering. In particular, a doublet fine is observed near the center of the gain line with a dip on one side and a on the other. If the is sufficiently large, it may determine the line observed in strong STRS. The anti-Stokes lines are generally considerably narrower than the Stokes lines but their peak intensities are of the same order of magnitude. They also have a doublet structure with a dip and whose width, in this case, is of the order of the total line width. Spontaneous anti-Stokes scatterng is attributable only to thermal polariton excitations in the medium.

Variation of radiation force values with differing gas content of propagating media

The experiments discussed here involve investigations of variations in radiation force values for ultrasound incident on an inverted cone of a sensitive radiation pressure balance. The variations were observed to coincide with changes in the oxygen content of a constant volume of water between the transducer and the inverted cone balance pan (Fig. 1; Materials and Methods). The relationship between intensity per unit area of ultrasound entering unit volume of water (I0 and the intensity at the exit face (/2) is expressed by the equation 12 = l ~ e x p ( 1 0 -3~) where /~ represents the intensity absorption coefficient for water. The absorption of water increases when water is progressively aerated, compared to degassed water and can be accounted for by considering gas bubbles as inhomogeneities in

The effect of porous barriers on the molecular composition and total flux of a reactive gas mixture

When a gas mixture that is governed by a pressure dependent equilibrium is passed through a porous barrier, the exit molecular fluxes are functions of the equilibrium constant and the atomic concentrations at the exit face of the barrier rather than of the molecular weights and impingement pressures as has been tacitly assumed for Knudsen flow conditions. It is shown that whether or not molecules of the gas react with each other, the ratio of the total atomic escape flux of each elemental constituent to the total atomic impingement flux of that element for Knudsen flow should be a/l, where l is the barrier thickness and a is the same constant, of the order of the average diameter of barrier pores for each element in the gas mixture. The equilibrium prediction is confirmed for the sodium chloride monomer/dimer equilibrium mixture passed through alumina barriers with pores of the order of 1–10 μ diameter and various thicknesses. Measured dimer fluxes are reduced by the barriers to less than 10−3 times the dimer fluxes in conventional effusion; yet the measured fluxes agree to within about 30% with predictions from the measured monomer fluxes and the monomer/dimer equilibrium constant. The prediction that total atomic flux reductions for different vapors have the same dependence on a/l is confirmed by measurement with sodium chloride vapor and zinc vapor. The influence of porous barriers on the apparent heats of vaporization of monomers and dimers and the possibility of significant influence of surface diffusion on the experimental measurements are discussed.

Laser damage in germanium

We present a photographic study of areas of damage which occurred on a number of germanium etalons used as output mirrors of a pulsed CO 2 TEA laser. A search of the damage revealed certain characteristics which appeared at many locations on any particular etalon face, and on each of the several etalons used. The damage on the inside cavity face was always greater, both in extent and in depth, than that on the outside or exit face of the etalon. However, the patterns observed showed similar characteristics on both faces. A detailed description of the topographical features of the damage is given followed by an account of the proposed damage mechanisms.

Evaporation and damage of entry and exit faces of a transparent solid dielectric by a giant laser pulse

An analysis is made of the evaporation of the entry face and of the spallation of the exit face of a transparent solid dielectric by a giant laser pulse. It is pointed out that the nature of the damage process is largely governed by the absorption of laser radiation in the surface region as a result of avalanche multiplication of electrons. The main damage mechanism at the entry face is the evaporation. The rate of evaporation and its threshold are calculated. It is shown that the damage asymmetry of the entry and exit faces is due to the attenuation of the radiation passing through the absorbing hot surface layer (due to the damage to the exit face) which shifts the absorption radiation maximum toward the surface region. Overheating of matter at the absorption maximum causes an explosion under the surface and this results in spallation of the exit face and ejection of the heated material. Quantitative estimates for sapphire illuminated with a giant ruby laser pulse are in good agreement with the experimental results.

Raman scattering by polaritons under nonstationary conditions. II: Local and wave nonstationary cases

AbstractThe theoretical investigation of stimulated Raman scattering (RS) by polaritons under nonstationary conditions, when local or wave nonstationary properties are separately important has been carried out on the basis of a fluctuation‐dissipation method. General experssions are found which describe the scattered radiation (Stokes and polariton) nonstationary angular distribution at the piezo‐electric crystal scattering layer exit face. In particular, for the local nonstationary case, the detailed analysis of the cases of small stationary gain and of very short and moderately short pump pulses at large stationary gain is carried through. For small stationary gain and also in the case of very short pulses with arbitrary gain, the spontaneous RS regime is realized, the Stokes wave intensity time dependence being on the whole a purely parametric one. The delay effects connected with the finite nature of the scattering polaritons' relaxation time become significant when the coherent component of the polaritons is important. Typical numerical calculations of the scattered radiation pulse shape and peak intensity are carried out for pump pulses of the rectangular and domed form. The most important general manifestations of nonstationary properties of the polariton RS are the apperance of deformation, asymmetry and delay of the scattered radiation pulses relative to the pump pulses; the angular tuning of the pump duration and the scattered radiation frequency tuning; the different character of the influence of local and wave nonstationary properties and so on. These effects are determined by shape and duration of the pump pulses and also by the stationary gain parameter in the material and by the scattering polaritons' relaxation time (for the local nonstationary case) or by the group delay time of the Stokes wave relatively to the pump wave (for the wave nonstationary case).

Two-photon absorption in cadmium selenide

The nonlinear absorption coefficient β of a CdSe crystal was deduced from the attenuation of the radiation of a single-frequency neodymium laser at liquid-nitrogen and room temperatures. This was done for different orientations of the crystal. The coefficient β was several times higher than the values obtained for multimode laser radiation. Allowance was made for the space-time structure of the laser pulses and for the wave reflected from the exit face of the crystal. A study was made of the nonlinear absorption associated with the conversion of the fundamental-frequency radiation into the second harmonic whose frequency was located within the absorption band of cadmium selenide.

Spatial and Temperature Profile Effects on the Fundamental and the SH Waves

The IDRI effect in a crystal of LiIO3 is well pronounced when an intense laser beam is used (E ­ 105 V/cm). The effect causes sharp bending of the fundamental ray due to the refraction at the exit face of the crystal whereas the SH does not show such a behaviour, due to the weak field associated with it. This effect is shown experimentally using a laser beam of energy 1·79 eV which is focused near the exit face of the crystal (0·191 cm) of thickness 0·364 cm. The focused laser beam creates opaque sharp edges of size 6 w which produce the fine structure of the ruby radiations due to diffraction. Simultaneously, SHG yields a Fraunhoffer diffraction pattern due to the focusing. The effect of focusing increases the rate of various processes considerably (degradation of SH power, SH structure formation, radiation-induced damaging of the crystal, broadening of the central maximum and the ratio of the first maximum to the last maximum of SHG). The previous results are explained and some new results are verified in the light of the theoretical and experimental approaches. It has been shown independently that the focusing parameter is always associated in the experiments even with the unfocused laser beams, due to the self-focusing effect which is also expected from theory.

Laser induced damage and the water content of vitreous quartz

Using the OH absorption band at 2.73 μm to grade the bulk water content of several samples of vitreous quartz, it was found that an increase in water content: (a) Transferred the site for ruby laser induced damage from the entrance face to the exit face. (b) Increased the threshold for entrance face damage from 210 MW/cm 2 to over 1330 MW/cm 2 and, (c) increased the degree of self focussing of the laser beam in the medium.

Scientific and practical problems related to the calculation of hydrodynamic parameters of seepage through earth dams

"Scientific and practical problems related to the calculation of hydrodynamic parameters of seepage through earth dams." La Houille Blanche, 59(5-6), pp. 413–414

Correction to "Bandwidth and threshold calculations for angle-turned parametric oscillators"

In the above paper, Basu and Steier have derived an expression [equation (18)] for the threshold of a single resonant optical parametric oscillator in which they take account of the effects of noncolinearity and walk-off of the interacting beams. In deriving (18), an erroneous assumption has been made, viz., that under low gain condition all three waves - pump, signal, and idler - may be taken to be constant over the length of the crystal. While this is a good approximation for the pump wave and the resonated wave (idler in their notation), it is not so for the non-resonant wave since this has zero amplitude at the entrance face to the nonlinear medium and grows to a level comparable to the idler amplitude by the time it reaches the exit face.

Shielding to fast neutrons provided by iron barriers

A study was made of the dose distributions and of the differential neutron energy spectra across the exit face of iron slabs of 6, 9 and 12 inch thickness exposed to an isotropic broad spectrum neutron source (Am-Be). A comparison was presented of the dose values observed with Hurst-type proportional counters and those calculated with the Monte Carlo codes 05R and Morse. The 05R program utilized the DNA 4124 cross-section set as point cross-sections while Morse utilized a set of troup-transfer cross-sections generated from the DNA 4124 set with the code Supertog. The results show a marked dependence on the representation of the cross-section sets. Detailed comparisons of calculated and measured emergent neutron spectra were also made. The spectra were measured with an NE213 scintillation spectrometer which was calibrated for use with the Ferdor unfolding code. In addition to the Monte Carlo calculations, the Anisn one-dimensional discrete ordinate code was used for a S16-P8 calculation with the same group transfer cross-section set used with Morse.

Importance of Fresnel reflections in laser surface damage of transparent dielectrics

It is shown that interference between incident and reflected light waves at the surfaces of a transparent dielectric material significantly influences surface damage thresholds. For normal incidence the light that is reflected at the entrance face of a surface is 180° out of phase with the incident light, and the resulting interference causes a sizable reduction in the electric field at the entrance face. The light reflected at the exit face suffers no phase shift, and as a result its interference with the incident light slightly increases the electric fields experienced at the exit surface. The analysis predicts a definite ratio for the damage threshold of the entrance and exit surfaces. This and one other prediction are experimentally investigated for Owens-Illinois ED-2 laser glass.

Determination of the angular divergence of laser radiation by a transforming system of prisms

A simple method is suggested for measuring the angular divergence of laser radiation. In this method a transforming system of prisms increases a transverse dimension of a laser beam in the plane of the prism bases. An analysis is made of the refraction (by this system of prisms) of a laser beam incident on the exit faces of the prisms at an angle close to the total-internal-reflection angle. The shape of the caustic surface, forming the boundary of the laser beam at the exit from the prism system, is calculated. It is shown that this system is a quasipoint source of light, for which the wider side of the transverse cross section of the beam is practically proportional to the angular divergence. The proposed method is used in compact units for measuring the angular divergence of the radiation emitted by pulsed lasers.