Showing papers on "Guided wave testing published in 1973"

Efficiency of optical-grating couplers

Abstract: The distributed coupling between one guided wave and a finite number M of freely propagating waves is considered as it applies to optical-grating couplers and prism couplers. Starting from a scattering-matrix representation, general amplitude and power relations among the various incident and outgoing waves are discussed. These relations are based on the reciprocity, symmetry, uniformity, or losslessness of the coupler. It is shown that the high efficiency (80%) of the prism coupler can also be obtained with a grating coupler, provided that the guided wave interacts with only one free wave (M = 1). Various methods of achieving this are proposed. Among them is a scheme that yields a wavelength-selective coupler. The optimization of couplers is discussed for a number of typical coupling situations. The effects of losses in couplers are also treated.

Optical film-fiber coupler

Abstract: There is disclosed an arrangement for coupling guided optical waves from thin-film waveguides to fiber waveguides, and vice versa. The thin film in the former type of guide is provided with an end region which is gradually tapered in thickness along the path of the wave to be guided therein to a cut-off value for the wave. This tapered region of the film causes the wave to propagate out of the film into the lower refractive index substrate of the guide along a small angle with respect to the film-substrate interface. Situated in the substrate slightly below the film-substrate interface is a cavity having a crosssectional shape and cross-sectional dimensions adapted for receiving an end of the fiber waveguide. By properly selecting the orientation and position of the cavity in the substrate, and of the fiber in the cavity, the fiber can be made to intercept the wave coupled out of the film. To insure efficient coupling of the wave into the fiber, the cavity is provided with a hemispherical end shape and is filled with a transparent material having a refractive index exceeding that of the substrate. A hemispherical lens is thereby formed which focuses and directs the wave into the fiber. Efficient coupling of the guided wave from the thin film to the fiber, or from the fiber to the thin film, is thus possible.

Light-guiding switch, modulator and deflector employing antisotropic substrate

Abstract: There is disclosed a light beam switching, modulating and deflecting device which is comprised of a thin dielectric film or strip waveguide deposited on an anisotropic (i.e., uniaxial) crystalline substrate. The guide and substrate parameters are selected so that one polarization of an optical wave is supported as a guided mode in the guide and the other, orthogonal polarization is not supported. Applied electric field components or acoustic strain components interact with the substrate to cause a coupling of the guided wave from the supported polarization to the unsupported polarization. The wave, upon being coupled to the unsupported polarization, is automatically and immediately radiated out of a guided mode into a freely propagating substrate mode. The unique principle of operation of the device, involving a combination of polarization discrimination in the guide and polarization coupling through an electro-optic or acousto-optic modulation of the substrate, eliminates the need for phase-matching. Noncritical, spatially uniform electric field or acoustic strain components applied to the substrate can thus provide the desired effect. The structure and operation of the device are thus generally simpler than those of similar prior art alternatives.

Measurement of thin films by optical waveguiding technique

Abstract: In order to measure the thickness and refractive index of a transparent thin film on a substrate of higher refractive index, such as a film of silicon oxide on a substrate of silicon, an optical wave is coupled into and guided through the film. The guided wave is coupled into the film by means of the phenomenon of the tunneling into the film of an evanescent wave from a beam of light incident on the surface of an adjacent coupling medium at an angle greater than the critical angle for total internal reflection in the medium. The intensity minima of the optical radiation which can be coupled out of the film back into the coupling medium (overall reflection), as a function of angle, yields data from which the desired thickness and refractive index can be determined.

Beam deflection and amplitude modulation of 10.6-μm guided waves by free-carrier injection in GaAs-AlGaAs heterostructures

Abstract: Beam deflection and amplitude modulation of 10.6‐μm guided waves by photoinjected free carriers are demonstrated in thin‐film GaAs–AlGaAs heterostructure waveguides. At high injection levels (N ≳ 5 × 1017 cm−3), amplitude modulation (up to 100%) of the 10.6‐μm guided wave dominates, but at lower injection levels beam deflection is dominant. External deflection angles of the order of 1° are obtained with nanosecond response times.

Second‐harmonic generation due to a guided wave structure consisting of quartz coated with a glass film

Abstract: Phase‐matched second‐harmonic generation (SHG) was obtained in the visible spectrum by using the nonlinear waveguide. The guide consisted of quartz as the cladding and glass film as the waveguiding section. y‐cut quartz was used for the nonlinear medium. A Q‐switched YAG laser was used as the pumping source. The guided modes of the pump were TE00 and TM00, and that of the second harmonic (SH) was TM02. The phase‐matched thickness 2b of the glass film was 2.55 μm with an allowable tolerance of about 30 A for an interaction length of 10 mm. The peak output power was 70 mW. The results were in agreement with theory.

Light‐wave coupling to optical waveguides by a tapered cladding medium

Abstract: A transition from a symmetric to an asymmetric waveguide can serve as a mode‐ and polarization‐dependent light‐wave coupler. Coupling experiments with an efficiency up to 70% are reported. Furthermore, total reflection occurs at the transition if the angle of incidence of the guided wave exceeds a critical value. Coupling and total reflection are explained, using the concept of the effective refractive index.

Modulation of the Optical Guided Wave by uv Light and Electron Beam Excitations

Abstract: An effective modulation of a He–Ne laser beam at a wavelength of 6328 A guided in a semiconductor ZnS thin film has been obtained when the guided wave travels transversely across the region irradiated by intense uv light. This seems to be caused by the absorption effect by means of the optically excited free carriers. The modulation ratio has shown monotonic increase with the excitation intensity. At maximum excitation, a modulation ratio of 35% was obtained. For normal transmission of a laser beam through the excited shallow region near the surface, measurements of reflectance change and phase retardation have been also carried out under uv light and pulsed electron excitations, are discussed in connection with the absorption effects by the free carriers generated by these excitations. In the case of excitation by an electron beam of 15 mA and 30 kV, the fractional increase of the absorption was 1×10−3.