Thin-film Optical Waveguide Research Articles

Fluctuation and scattering properties of RF sputtered glass thin film optical waveguides

AbstractThe fluctuating scattering properties of a Corning 7059 glass thin‐film waveguide RF‐sputtered on pyrex glass are studied theoretically and experimentally. Light propagating in the thin film waveguide is radiated owing to imperfections of the thin film boundaries and refractive index irregularities in the film. It is thus possible to estimate the imperfection characteristics of the waveguide by comparing the measured scattering pattern with the computed data. An example of the measured results indicates that the correlation lengths of each irregularity are bx/λ ∼ 0.1 and bz/λ ∼ 1.0 and the ratio of the dispersions is G (= Vf/(Vλ)) ∼ 1.0. Here bx and bz are the correlation lengths in the film thickness direction and in the propagation direction, Vf and V are the rms values of the boundary and refractive irregularities, and λ is the wavelength. From these results it is found that both the refractive index irregularity in the thin film and the boundary irregularity contribute to the scattering loss in the glass thin‐film waveguide created by RF sputtering.

Thin film optical waveguides fabricated by ion- assisted techniques

R.f. sputtering and low temperature field-assisted diffusion are compared as techniques for manufacturing planar optical waveguides in glass. The emphasis of the paper is on how well these processes meet criteria derived from the operating requirements for waveguiding layers, e.g. index homogeneity, absorption and scattering loss, index profile control, configurability.

Light scattering from a glass thin-film optical waveguide

Measurements were made of the light scattered out of the Corning 7059 glass thin-film optical waveguide sputtered onto substrates of pyrex glass. The angular distributions of the scattered light can be explained by the waveguide imperfections not only of surface roughness of air-film-substrate interfaces but of refractive-index inhomogeneities. The measured scattering patterns are consistent with the theory taking account of both imperfections, assuming a correlation length of bx/λ ∼ 0.05 and bz/λ ∼1.0, where bx and bz are the correlation lengths in the directions perpendicular and parallel to the waveguide surface.

Spectroscopic properties and aging processes in thin-film optical waveguides made of organic compounds

Spectroscopic properties and aging processes in thin-film optical waveguides made of organic compounds

Method of successive approximations for calculating the eigenvalues of optical thin-film waveguides

Method of successive approximations for calculating the eigenvalues of optical thin-film waveguides

Extremely low-loss glass thin-film optical waveguides utilizing surface coating and laser annealing

Application of a surface coating to Corning 7059 glass thin-film optical waveguides has been shown to significantly reduce their scattering loss. Combining the technique of CO2 laser annealing of glass waveguides with surface coating, we measure waveguide loss as low as 0.01 dB/cm. By attributing the decrease in attenuation to elimination of the effects of surface irregularities, we were able to identify surface and bulk contributions to scattering losses in the original waveguide. By making such an identification before and after laser annealing, we confirm that CO2 laser annealing of glass waveguides reduces both surface and bulk contributions to scattering losses.

Characteristics of Multilayered Thin-Film Optical Tunable Separators Using Surface Acoustic Waves

New type optical tunable separators consisting of multilayered thin-film optical waveguides are studied. The coupling strength and coupling length can be controlled by surface acoustic waves. For this coupler, amorphous Nb2O5, Ta2O5 and ZnS film waveguides are fabricated on a substrate, Y-cut LiNbO3, by R.F. sputtering method. Tunable and wavelength-selective properties, that are important factors for active wave separators and tunable optical filters in multi-wavelength optical communication systems, are discussed with analysis of propagation properties of surface acoustic waves in film guides.

Fabrication of chirped gratings on GaAs optical waveguides

After a brief review of the applications and fabrication techniques of gratings in integrated and guided wave optics, a new method of fabricating chirped gratings is presented. The method is based on the interference of two spherical laser beams after spatial filtering by pinholes. The experimental conditions required for fabricating antisymmetric linearly chirped gratings, symmetric quadratically chirped gratings and uniform gratings are presented by way of example. By this method chirped gratings with small variations in the period can be fabricated on GaAs thin film optical waveguides. Profiles and patterns of chemically etched corrugations are quite uniform, mainly because of the direct interference of beams immediately after spatial filtering.

Reduction of scattering from a glass thin-film optical waveguide by CO2 laser annealing

Laser annealing with a CO2 laser is utilized to improve the quality of Corning 7059 glass thin-film optical waveguides. The 1.0-μm-thick 7059 glass film is deposited by sputtering onto a thermally oxidized silicon substrate. Measurement of the absorption coefficient of 7059 glass at λ = 10.6 μm yields a value of 1.12 x 104 cm−1. Dark-field photomicrographs taken before and after laser annealing imply that both surface defects and bulk inhomogeneities are removed by laser annealing. Waveguide attenuation is measured to be 17.4 dB/cm befor laser annealing and 0.6 dB/cm afterwards.

Determining the absorption coefficient of absorbing thin films with optical waveguides

A new simple method of determining the absorption coefficient of absorbing thin films is proposed as an application of the complex propagation constant of a guided wave in a thin film optical waveguide. This method was applied to both cases as examples: low index (polyurethane film top layer containing rhodamine 6G dye) and high index (evaporated ZnSe film top layer) absorbing top layers on the main glass guide. The absorption coefficients so determined were compared with the values given by Lambert's method.

Optical waveguide filters for the visible spectrum

A study has been made of wavelength selective devices which use a grating structure superimposed on a thin film optical waveguide. Good agreement was found between the theoretical and experimental wavelength response of filters in three forms of optical waveguide: with the grating ion-beem etched into a single homogeneous guiding layer, with the grating in a photoresist film on a homogeneous layer and with the grating in a photoresist film on a silver ion-exchanged waveguide. Details of the method of fabrication of the gratings and waveguides are given. Filters made with gratings ion-beanm etched into waveguide had bandwidths of 0.2 to 0.5 nm at 600 nm while the filters with superimposed gratings in photoresist had somewhat wider bandwidths.

Integrated-optic spectrum analyzer(Invited Paper)

This paper describes the integrated-optic implementation of a Bragg spectrum analyzer that employs the interaction between a coherent optical guided wave and a surface acoustic wave to determine the power spectral density of the input. The integrated-optic spectrum analyzer (IOSA) consists of an injection laser diode, a thin-film optical waveguide, waveguide lenses, a surface-acoustic-wave (SAW) transducer, and a linear detector array with CCD readout. Design principles are given for selecting component parameters such as optical beam width, detector cell size, lens aperture and focal length, and acoustic transducer design so as to obtain specific RF resolution, spurious level, and signal-to-noise ratio. Design parameters are presented for a 750-1250-MHz spectrum analyzer with a resolution of 4 MHz and a 40-dB dynamic range. Also described in the paper is the development of the state-of-the-art component technology for the spectrum analyzer.

Field analysis of harmonic generation in thin-film integrated optics

Harmonic generation of light in thin-film optical waveguides is analyzed theoretically by evaluating the total electromagnetic fields that satisfy the polarization driven wave equation and the electromagnetic boundary conditions at both interfaces. Analytical expressions are given for the amplitude of the growing wave component of the total fields. The results agree with those of normal mode analysis only in the limit of phase-matched harmonic generation. This analysis is believed to be applicable to a large range of wave interaction phenomena in integrated optics.

Scattering of guided optical beams by surface acoustic waves in thin films

A new approach to the problem of light scattering by surface acoustic waves in thin-film optical waveguides is presented. In this analysis electromagnetic fields are found to rigorously satisfy both the polarization driven wave equation and the electromagnetic boundary conditions at the frequencies of interest. A component of the solution field is found to evolve with propagation distance into a linearly growing guided wave mode that dominates the solutions under conditions that can be defined in terms of waveguide mode parameters. Analytical formulas for the growth coefficient are obtained for both the elasto–optic and corrugation scattering mechanisms for the geometries TEm → TEm, TEm → TMm, TMm → TMm and TMm → TEm The present results are found to be consistent with previous normal mode analyses of this phenomenon for the cases in which comparison is possible. Numerical calculations for thin films of As2S3 and Corning 7059 glass on fused silica substrates indicate that the elasto-optic effect does not always dominate the scattering cross section and that the corrugation mechanism must often be taken into account.

LPE growth of Li(Nb, Ta)O3 solid-solution thin film waveguides on LiTaO3 substrates

Li(Nb, Ta)O3 solid-solution thin film optical waveguides were fabricated by liquid phase epitaxial growth from a solution containing V2O5 flux. Phase relations of the LiVO3-Li(Nb1-xTax)O3 systems were studied by differential thermal analysis, and LPE growth of the films was performed on Y-plates of LiTaO3 substrates. Appropriate growth conditions for obtaining high crystallinity films were investigated based on X-ray examinations. Optical waveguiding characteristics such as refractive indices and optical propagation are described.

Effects of deposition parameters on optical loss for rf-sputtered Ta2O5 and Si3N4 waveguides

Thin-film optical waveguides of tantalum oxide and silicon nitride were prepared by rf diode sputtering. The optical loss α was found to be a sensitive function of the following deposition parameters: reactive gas composition, total gas pressure, gas flow rate, substrate temperature, substrate bias, and postdeposition annealing. Increasing the percentage oxygen in argon from 5% to 30% raised the loss from 2 to 45 dB/cm and changed the film structure from amorphous to crystalline. Optical losses below 1 dB/cm were produced in amorphous Ta2O5 films at 0.5 Pa, 10% O2 in Ar, low gas flow, and low substrate temperatures. The log α was a linear function of the percentage of reactive gas for both Ta2O5 and Si3N4. Optical losses of 15 dB/cm in amorphous Si3N4 films were obtained at 1.5 Pa with 10% N2 in Ar, independent of substrate temperature and gas flow.

Volume diffraction gratings with inclined layers in thinfilm optical waveguides

A study was made of the diffraction coupling (in and out) of radiation by means of volume phase diffraction gratings with inclined layers in thin-film and stripe waveguides. Under optimal conditions, the efficiency of coupling of radiation into a thin-film waveguide could approach ~100%.

Single mode grating coupling between thin-film and fiber optical waveguides

Optical directional coupling by means of a periodic perturbation is described between a clad, single mode, cylindrical fiber, and a thin-film planar waveguide. Codirectional coupling was achieved to a clad fiber, with 0.4 percent efficiency; allowing for the geometric mismatch, this is effectively 30 percent coupling. The fiber guided mode is accessed by reducing the fiber diameter by heating and pulling. Mode phase matching is achieved with a periodic grating sputter etched into the film. Unwanted grating induced interactions are minimized by restricting the guide parameters or by modifying the coupling mechanism. A perturbation analysis is used to calculate coupling coefficients and the design procedure for an optimum structure is explained. Measurements on grating couplers are described and the film/fiber codirectional coupling is shown to be effected by the grating.

Holographic interference lithography for integrated optics

In this paper, some of our recent work in the use of holographic interference lithography and various material removal techniques to corrugate thin-film optical waveguides are reviewed. The specific applications of these periodic thin-film devices in the distributed feedback and distributed Bragg reflector semiconductor laser, and as output grating couplers, are described. Recent results in the use and fabrication of chirped and curved-line gratings are also summarized.

Attenuation in thin film optical waveguides due to roughness-induced mode coupling

Marcuse's mode-coupling theory for symmetrical planar optical waveguides was extended to cover the asymmetrical case. Computer programs were written to study the particular case of coupling between the zeroth order guided transverse electric (TE) mode and the radiation modes as a function of interface roughness. It is shown that a simple expression proposed by Tien gives results which correspond to the most pessimistic results obtained by the mode-coupling theory. R.f. sputtered thin film planar optical waveguides were prepared. The zeroth order TE guided mode having a vacuum wavelength of 0.6328 μm was excited in the thin film by means of a prism coupler. The attenuation of this mode was inferred from the reduction in intensity of the radiation modes which was measured with a photomultiplier. A Talystep was used to obtain the film thickness and also profiles of the roughness of the film-substrate and film-superstrate interfaces. The roughness profiles were computer-processed to determine the r.m.s. amplitude and the correlation length of the interface roughness. The refractive index was determined from the film thickness and the synchronous coupling angle using the reduced SYNCANG method. These data were used to compute the attenuation of the zeroth order guided TE mode from the mode-coupling theory. The correlation coefficient between the theoretical and experimental results was 0.96.