Propagation characteristics of novel single-mode slab waveguides, ARROW (antiresonant reflecting optical waveguides), ARROW-B, and their modified configurations, are investigated. The dispersion and radiation loss characteristics, field profile, confinement factor, and spot size of these ARROW-type waveguides are analyzed in detail by a mode analysis method using the system interference matrix. Approximate expressions for these characteristics are obtained in simple forms, which provide useful suggestions for the design of these waveguides. Possible combinations of materials and design conditions of ARROW-type waveguides are discussed on the basis of these results. >

Dispersion and radiation loss characteristics of antiresonant reflecting optical waveguides-numerical results and analytical expressions

We report on both polar and nonpolar light-induced orientation of a polyimide of high glass transition temperature (Tg ≈ 210 °C) with nonlinear optical azo dye molecules in the side chain. This photoinduced orientation is shown to occur at room temperature, i.e., at least 190 °C below the Tg value of the polymer, which indicates that the photoisomerization-induced movement of the nonlinear optical chromophores induces, in turn, a movement of the polyimide main chain. This allows for an efficient orientation of the dye molecules in spite of the stiffness of the polyimide main chain. A nonpolar orientation is induced by polarized light irradiation alone, whereas polar orientation is achieved by application of a dc field during the photoisomerization process. Further light irradiation in the absence of a dc field destroys the previously induced stable polar order. A detailed theoretical study of this light-induced depoling process is also presented.

Light-induced orientation in a high glass transition temperature polyimide with polar azo dyes in the side chain

We report on light-induced nonpolar orientation of sidechain nonlinear-optical polyimides [glass transition temperatures (Tg) up to 350 °C] containing no flexible connectors or tethers. The nonlinear-optical azo-dye chromophore is incorporated through the donor substituent as a part of the polymer backbone. This impressive photoinduced orientation occurs at room temperature, i.e., at least 325 °C below the Tg of one of the polymers that we investigated. Furthermore, it is shown that, after photoinduced ordering, this polymer must be heated to 350 °C to induce main-chain movement and to erase the orientation, whereas the direction of orientation of the chromophores can easily be controlled at room temperature simply by choice of the appropriate polarization of the irradiating light. This light-induced orientation process is shown to be useful for storing images in waveguides.

Light-induced orientation in azo-polyimide polymers 325 degrees C below the glass transition temperature

The reduction of the insertion loss of a spot-size transformer based on dual tapered waveguides (DTW-SST) by controlling the shape of the tapered portion is discussed. The transformer is easily fabricated using a shadow mask sputtering method. A method is described for controlling independently the parameters of the tapered portion, such as the maximum taper angle, and length, and the position of tapered portion with respect to the edge of the shadow mask. A large transformation ratio (5 and 1/5) and a low insertion loss (0.7 dB) have been obtained. >

Low-loss spot-size transformer by dual tapered waveguides (DTW-SST)

We report evidence of photoisomerization-induced changes in the optical thickness of molecularly thin (9 A) photochromic layers which consist of azosilane molecules immobilized on silicon oxide substrates. Further, this light-induced change in the optical thickness of these films can be reversed upon irradiation with light of an appropriate wavelength. This has been demonstrated by means of surface plasmon spectroscopy that allows the observation of very small changes in the optical thickness of ultrathin layers. The study of the thermal back isomerization reaction of the azobenzene molecules in these films reveals steric hindrance at the molecular level, as shown by using UV−vis spectroscopy.

Surface plasmon investigations of light-induced modulation in the optical thickness of molecularly thin photochromic layers

A new method for the measurement of mode field radius in axially symmetrical and nonsymmetrical single-mode fibers with non-Gaussian power distribution is proposed. It is based on monitoring the power passed through a variable-width slit in the far field. A few results for different types of single-mode optical fiber are presented.

F. Calvo

Papers

Measurement of mode field radius in axially nonsymmetrical single-mode fibers with arbitrary power distribution