Guide/antiguide optical intensity modulator

An electrorefractive index-controllable guide/antiguide optical intensity modulator has been created and characterized. The operating mechanism, beam propagation method simulation, device design and fabrication, and calculated and experimental results of this guide/antiguide modulator are given. The operational principle of the modulator is based on the field-induced waveguide (FIG) concept; that is, electric-field-induced refractive index changes cause the waveguide to be turned on and off electrically. The modulator is formed with a central narrow FIG electrode sandwiched between two antiguide electrodes. The electrooptic effects, along with carrier effects, have been exploited to adjust the refractive index under the guide and adjacent antiguide electrodes by changing reverse biases applied to them. The modulation is determined mainly by changing the lateral refractive index profiles. In the on state, a waveguide is formed under the central electrode, and the input light propagates along this waveguide. To realize the best off state, an antiguide situation is created that causes the light to diffract out of the central guide rapidly. An on/off ratio larger than 23 dB, a propagation loss 1 dB, and a record optical bandwidth from 1.0 to 1.55 m have been obtained for this modulator.     

A design for a nonreciprocal phase shifter

A design is described for a nonreciprocal phase shifter, which is the key element in the constitution of a Mach-Zehnder interferometric isolator consisting of a magneto-optical channel waveguide. A four-layered structure waveguide is dealt with in which a cover layer having a larger refractive index than that of the film is placed on the magneto-optic thin film; it is revealed that the phase shifter length can be shortened by up to 6 mm at the 1.15 m wavelength, which is half the length of a nonreciprocal phase shifter constructed of a three-layered waveguide without a large refractive index cover layer. Two types of channel waveguides are investigated: a rib waveguide and a strip-loaded waveguide; they are found to have the same phase shifter length for the optimum condition.     

“Anomalous” refractive index dispersion curves—a relativistic interpretation

The Einstein field equations (R =0) are seen as possible candidates for a set of unified field equations. Three solutions of these field equations are used for a new interpretation and reformulation of the refractive index of an isotropic material medium. The new formulation explains the basic features of anomalous refractive index dispersion curves. It also predicts that the refractive index is a function of the angle of incidence when the plane in which the measurement is made is not tangential to the surface of the spherical gravitating mass, thereby providing a suitable test for the theory and hence of general relativity.     

Visualization of the photorefractive grating in BSO

A technique based on self-imaging at finite conjugates has been experimentally demonstrated for visualization of laser-induced photorefractive gratings in BSO. A simple model theory has been used to provide the correct rules of geometrical optics of self-imaging. A novel technique has been used for photoinducing the periodic structure. An application has been made to determine the harmonic components of the nonsinusoidal refractive index modulation in the BSO for a grating period of 10 m and for external DC voltages of 6.0 and 3.0 kV. The technique is directly applicable to the visualization and study of laser-induced gratings (LIG) in thin nonlinear optical media.     

Asymmetry of the angular distribution of radiation of channeled relativistic electrons in optically transparent crystals

It has been shown that optical and ultraviolet radiation from relativistic electrons at planar channeling in optically transparent crystals is characterized by an unusual dependence on the polar and azimuth angles. A fraction of radiation with the frequency ω near which the derivative of the refractive index is nonzero, n'(ω) = dn(ω)/dω ≠ 0, should be observed at an angle close to π/2 with respect to the electron beam. For normal dispersion (n'(ω) > 0), this angle is smaller than π/2, whereas for anomalous dispersion (n'(ω) < 0), it is larger than π/2 (“backward” radiation). A pronounced dependence of the radiation intensity on the azimuth angle φ, i.e., azimuthal asymmetry, appears beyond the region of normal and anomalous dispersion at a fixed polar angle θ. In particular, the ratio of radiation intensities at angles φ = 0 and π/2 at θ = π/2 reaches a maximum value of about the square of the refractive index.     

Control of the refractive index change in fused silica glasses induced by a loosely focused femtosecond laser

We demonstrate waveguide fabrication using of a loosely focused femtosecond laser that induces both nonlinear ionization and nonlinear propagation in silica glasses without any scanning process. The numerical aperture is chosen to be 0.007 to avoid spatial splitting of the laser pulses during the nonlinear propagation of the femtosecond laser pulses inside the fused silica glass. It also enables a uniform cylindrical refractive index change, which acts as an optical waveguide, to be induced. We found that the induction of irregular refractive index changes is related to the free electron density of the focused area and is controlled by decreasing the NA. The length, position, and core diameter of the fabricated waveguide can be controlled by the pulse-width, energy, and irradiation time of the incident laser. By using this technique, we varied the length of the fabricated waveguides between 20 m and 6 mm, while keeping the core diameter at around 5 m. PACS 42.70.Ce; 42.82.Et; 61.80.Ba     

Buried channel waveguides in plasma-enhanced chemical vapour deposited silicon oxynitride layers on silicon substrates,formed by electron-beam irradiation

It is shown that buried channel waveguides may be formed from a bilayer planar guide (made by plasma-enhanced chemical vapour deposition of silicon oxynitride) by bombardment with low-energy electrons. This results in an expansion of the material, and hence a decrease in the refractive index. Consequently, irradiation around a narrow stripe will induce lateral confinement. Data are presented for the electron-induced refractive index and volume changes, together with preliminary results for single-mode channel guides operating at 1.52m wavelength.     

The effect of interdiffusion on the change of refractive index of an AlGaAs/GaAs quantum well structure

The change of the refractive index due to quantum well (QW) disordering is calculated for light propagating normal to the Al_0.3Ga_0.7As/GaAs QW layers (i.e. along the QW growth direction). A hyperbolic function is used to model the above QW confinement profile after disordering, i.e. thermal interdiffusion of trivalent atoms across the well-barrier interfaces. The refractive index difference (n) is evaluated for two cases, where case I refers to the difference between a partially disordered QW and a more extensively disordered QW, while case II refers to the difference between an as-grown QW and a partially disordered QW. The results demonstrate that good photon confinement (large n > 0) can be achieved for both cases, where n increases with increasing QW width and decreases with annealing time for case I while for case II it increases with annealing time. In comparing the two cases, a shorter annealing time is required to achieve the same value of n if the case II structures are used. The change of refractive index obtained here demonstrates a larger value of n than that produced by the variation of the concentration of free carriers in the bulk material.     

Z-scan measurements of nonlinear refraction and Kerr-lens mode-locking with Yb3+:KY(WO4)2

The nonlinear refractive index n₂ of Yb³⁺:KY(WO₄)₂ crystal has been measured using picosecond Z-scan technique. The magnitude of n₂ was found to be 8.7 × 10^–16 cm²/W at wavelength of 1.08 m. The numerical modeling based on fluctuation model showed a great potential of this crystal as active medium for Kerr-lens mode-locking.     

Broadband near-infrared fibers dispersion measurement using white-light spectral interferometry

A new technique for an experimental determination of the effective refractive index, group refractive index and dispersion of fibers in a broad near-infrared spectral range is presented. The method is based on a white-light spectral interference which utilizes an unbalanced Michelson interferometer. The effective refractive index is obtained by a direct fitting the cosine function to the spectral interference pattern recorded by a low resolution spectrometer. The method has been tested in the spectral range of 1000-1700 nm both with standard telecommunication fibers and a sample of a photonic fiber. The accuracy of dispersion measurement () exceeds those from the previously reported near-infrared white-light spectral interference methods.     

Self-focussing of CO2 laser radiation in SF6

Self-focussing of high-power TEA CO₂ laser pulses for a number of 10 m P-band lines is reported in SF₆ molecular gas. Application of this effect to estimating the intensity-dependent refractive index of the gas is demonstrated.     

Design and optimisation of high-brightness 980 nm laser diodes with distributed phase correction

Designs for 980 nm Al _x Ga_1–x As/In_0.2Ga_0.8As/GaAs high-power, high-brightness semiconductor lasers/amplifiers with distributed phase correction and short-cavity lengths (i.e. cavity lengths 560–1040 m) are presented. The proposed lasers/amplifiers employ a single mode feed waveguide coupled to a power amplifier with a laterally graded effective refractive index (GRIN) profile to control the lateral mode shape and phase. The lateral index of the power amplifier has a hyperbolic secant (HYSEC) profile, which can be approximately realised by tailoring the effective refractive index of the amplifier using a series of discrete etches. The epitaxial and structure designs of the laterally discretised GRIN lasers/amplifiers are presented. Finally, a method for improving the effective refractive index discretisation is described.     

Optical properties of cuprous telluride films

The structure of vacuum-deposited cuprous telluride Cu₂Te films deposited on carbon substrates was observed by means of an electron microscope. Films on glass substrates were also investigated by X-ray diffraction. The optical constants (the refractive index n and the extinction coefficient k) of Cu₂Te thin films were determined from the measured transmittance and reflectance at normal incidence of light in the wavelength range 0.4 m to 2.5 m. The variation of the optical constants with thickness for different evaporated films have been determined.     

Near Field Observation of a Refractive Index Grating and a Topographical Grating by an Optically-Trapped Gold Particle

We observed the near field for a refractive index grating fabricated on a planar light waveguide circuit (PLC) by scanning an optically-trapped 100 nm diameter gold particle. We demonstrate that stable trapping and scanning occur with a Gaussian laser beam at the scan velocity of 1.6 m/s and Nd:YAG laser power of 25mW. The scattered Ar laser light from the gold particle is strong at high refractive indexes of the grating with periods of 1.06 m and 0.53 m both by s and p polarized illumination. In addition, we observed the surface profile of the optical disk tracking groove with and without the gold particle. © 2004 The Optical Society of Japan     

Microstructural parameters and optical constants of ZnTe thin films with various thicknesses

Different thickness of polycrystalline ZnTe films have been deposited onto glass substrates at room temperature by vacuum evaporation technique. The structural characteristics studied by X-ray diffraction (XRD) showed that the films are polycrystalline and have a zinc blende (cubic) structure. The calculated microstructure parameters revealed that the crystallite size increases and microstrain decreases with increasing film thickness. The transmittance and reflectance have been measured at normal and near normal incidence, respectively, in the spectral range 400-2500 nm. For ZnTe films of different thicknesses, the dependence of absorption coefficient, α on the photon energy showed the occurrence of a direct transition with band gap energy (For ZnTe films of different thicknesses) confirming the independency of deduced energy gap on film thickness. The refractive indices have been evaluated in terms of envelope method, which has been suggested by Swanepoul in the transparent region. The refractive index could be extrapolated by Cauchy dispersion relationship over the whole spectra range, which extended from 400 to 2500 nm. It was observed that the refractive index, n increased upon increasing the film thickness up to 508 nm, lying within the experimental error for further increases in film thickness.     

Application of the Z-scan technique for determining non-linear anisotropic change of refractive index in Cr4+ : YAG at powerful continuous-wave 1.06-μm probing

An experimental study and a theoretical modeling of non-linear refraction in a continuous-wave-pumped Cr⁴⁺:YAG crystal at wavelength 1.06 m are reported. Analysis of the processes that result in the non-linear change of Cr⁴⁺:YAG refractive index allows us to conclude that the physical mechanisms contributing to the change are the ground-state absorption saturation (minor part) and the sample inhomogeneous heating (dominant part). The role of the refractive index change anisotropy, appearing when the crystal is bleached, is addressed for the first time to our knowledge. PACS 42.70.Hj; 42.65.Hw; 42.60.Gd; 78.20.Nv     

Non-linear refractive indices of CS2-CCI4 mixtures

We have determined the two independent fast non-linear refractive indices for liquid mixtures of CS₂ and CCI₄ to within 5% relatively and 8% absolutely over their range of more than a decade. These indices describe non-linear propagation of optical pulses of arbitrary polarisation, which are of a too short duration to be affected by electrostriction. We report measurements, having relative accuracy better than 1 %, of the static Kerr constants of these mixtures. We combine these results with recent data on Rayleigh wing depolarisation ratios for the same mixtures to deduce the desired non-linear indices with the aid of molecular theory. The non-linear refractive index for linearly polarised light is shown to be two-thirds (±5%) of the index difference measured by the Kerr effect, at least for molar concentrations of CS₂ greater than 10%. Our measured relative values of the mixture Kerr constants are predicted to within experimental error if one simply replaces the Lorentz local field factors in the classical theory by their 0.6 powers.     

Injection lasers based on AlGaAsSb/GaSb and InGaAsSb/GaSb heterostructures

The isoperiodic heterostructures AIGaAsSb/GaSb and InGaAsSb/GaSb and injection lasers based on them are investigated. It is shown that in the heterostructures of the first type it is possible to obtain very low threshold currents at room temperature at wavelengths 1.72–1.78 m. Analysis of the experimental data and interpolation characteristics show that considerable optical confinement in the investigated heterostructures of the first type. Estimates show that the minimum threshold current in heterolasers for the spectral range 1.75–1.78 m is approximately 1 kA/cm². An investigation of the InGaAsSb/GaSb heterostructures shows that a case unusual for heterostructures of III-V compounds is realized in them when the material indicated has a smaller refractive index than the wideband layers. The antiwaveguide anomaly may be useful for the development of laser structures of new types.Translated from Trudy Ordena Lenina Fizicheskogo Instituta im. P. N. Lebedeva, Vol. 141, pp. 46–61, 1983.     

Die Anwendung des normierten Vašiček-Verfahrens bei nichtabsorbierenden Einfachaufdampfschichten auf Glasträgern

The author has modifiedVa?i?ek's polarimetric theory to a graphic method for determination of optical constants of thick homogeneous films on glass carriers. The method is verified for single layers deposited by heating salts. The kind of salt determines the gradient of the refractive index. This gradient and the state of the surface of the glass are responsible for deviations from the theoretical curves. A suitable method has been found for determining the refractive index and the thickness of the deposited film. In a modified form the method can also be used for inhomogeneous films. Sometimes it is possible to determine even a change in thickness less than 10 Å during the deposition of the film. The thickness can be calculated accurately only by measuring at three different light-frequencies, because the curves are periodical. The values for the different light-frequencies agree well with each other differing only by about 5%. The agreement is much better for films thicker than 10000 Å. The thickness of the films as obtained by applying this method is almost identical with the one given by an interference microscope.     

Optoelectronic properties of zinc blende ZnSSe and ZnBeTe alloys

The compositional dependence of the electronic band structure has been computed for zinc blende ZnSxSe1-x and Zn1-xBexTe alloys with composition x ranging from 0 to 1. The empirical pseudo potential method with the virtual crystal approximation have been used. A particlar attention has been paid to the effect of alloy disorder on the electronic properties of the II-VI studied compounds. For this purpose, the compositional disorder is added to the virtual crystal approximation as an effective potential. Such correction approximates significantly our calculated values of the band gap bowing parameters to experimental ones. The ZnSxSe1-x gap energy shows a nonlinear behavior with strong bowing for low compositions of sulfur. The Zn1-xBexTe compound, as it is known, can be direct or indirect semiconductor depending on its beryllium composition x. The electron effective mass and the refractive index have been investigated as well. Polynomial approximations are obtained for both the energy gap and the effective mass as functions of alloy composition at and X valleys.