In the long term, integrated optics could play a ubiquitous role in optical and electronic systems. To realize that role, an intensive period of basic research is needed that will expand the range of physical structures available for device design. With success in that effort, integrated optics could impact future computer system designs, display technologies, and high speed electronic systems.
A consistent, analytic technique is used to describe the performance of various wave-guide deflectors, in which a spatially varying phase shift is created electro-optically across the optical beam. We use Fraunhofer diffraction theory to obtain the far field out-put intensity distributions of single prism elements, arrays of identical or phase-staggered prism elements and arrays of channel waveguide elements. The deflection of the optical beam is continuous or discontinuous, depending on the phase distribution across the beam which is determined by the geometry of the device. T.Te calculate the capacitance of the different interdigital electrode structures, with constant and linearly varying gap-widths; the values are in good agreement with experimental measurements. The performance capabilities of different deflectors are assessed in terms of number of resolvable spots, power, bandwidth and crosstalk. Simple phase distributions are shown to be useful for determining most deflector characteristics, including the power per unit bandwidth.
The design and performance of some signal processing devcies using fiber and integrated optics are discussed. Analog-to-digital (A/D) conversion, logic, and computation are among the signal processing functions which can be performed using integrated optical devices. Optical fibers can be used for signal delay and storage and for implementing traversal filters and delay-matched filters. Some other interesting possibilities make use of both fiber delay line and integrated optical elements. These include variable-length delay lines and programmabe transversal filters.
A detailed analysis of a high-speed electrooptic guided wave analog-to-digital (A/D) converter has been performed. It is concluded that the development of converters in the 4-6 bit 1-2 GS/sec range should be feasible. Analyses are presented which detail the design considerations for the electrooptic modulator chip, the laser sampler, and the analog signal amplifier. A design plan is presented for a 6-bit 1-GS/sec converter which utilizes an array of 6 LiTaO3 interferometric waveguide modulators to perform the conversion and a frequency-doubled mode-locked Nd:YAG laser to perform the sampling of analog signals having 500 MHz bandwidths. Preliminary waveguide device results are also reported, including the successful demonstration of an interferometric modulator with a 17-dB (98%) extinction ratio.
Stepped switched optical directional couplers with sections of unequal lengths are studied. The switching diagrams for two- and three-section directional couplers were obtained. The concepts of "quasi-bar" or "quasi-cross" states are introduced for couplers with unequal sectional lengths. Loci tracing these quasi states and contours corresponding to different cross-talk levels are obtained. These curves are useful in estimating of the cross talk level due to fabrication errors.
We present the theory and experimental results of the performance of an integrated optics device that is composed of a prism-waveguide dual coupler. In the device, a circular and collimated beam of light is coupled into a LiNb03 Ti indiffused waveguide, with an efficiency of 94%, propagates 1 cm. in the waveguide, where electrodes can process the optical signal, and then the guided light is coupled out by the same prism with the same high efficiency. After presenting the theory, which is applicable to any type of waveguide, we show the experimental results that demonstrate the high efficiency throughput, and the high degree of collimation of the emerging beam.
An approach toward constructing an integrated waveguide-hologram memory is presented. The fundamental unit of the device, which is cascadable for integration, is a combination of a small waveguide hologram and an optical switch of deflection type which enables the selective read-out of the hologram. The waveguide hologram was recorded by using an Arion laser in an amorphous As2S3 (recording layer) / Ti-diffused LiNbO3 (slab waveguide) structure, which was designed for deriving high diffraction efficiency. The stored pictorial information was successfully read out by the it light of a He-Ne laser satisfying the propagation constant matching condition for the aberration-free imaging. The optical switch must be compact and capable of controlling a guided wave having a large width required for reconstructing a two-dimentional image with sufficient resolution. The design considerations are described of an electrooptic TIR switch in which an effective-index gap is formed by high-index cladding to obtain a large separation angle. It was found and experimentally confirmed that the switch has an appropriate performance, although the component size and the switching ratio must be traded off. The detailed imaging and switching characteristics are presented and the feasibility of the memory is discussed.
This paper describes the integrated optic implementation of a Bragg spectrum analyzer which utilizes a surface-guided coherent optical wave and a surface acoustic wave. The Integrated Optic Spectrum Analyzer (IOSA) employs surface geodesic lenses for beam focusing, a surface acoustic wave input transducer, an optical surface waveguide, a solid state laser and a detector array coupled to the waveguide. Design principles are given to selecting component parameters such as optical beam diameter, detector cell size, lens apertures and focal lengths, acoustic transducer design and input power level so as to obtain specific RF resolution, spurious level and signal to noise ratio. Design parameters are.presented for a 400 to 800 MHz spectrum analyzer with a projected resolution of 4 MHz, and a 40 dB dynamic range.
Experimental observations of the in-plane scattered energy distribution in Ti-diffused LiNb03 waveguides have been performed with a view toward determining and subsequently eliminating sources of scattering in this material. Results suggest that scattering from refractive-index variations is more important than scattering from surface roughness in commercial samples. However, scattering from contaminants at or near the surface exists and can be reduced by using less Ti diffusant and/or by polishing the finished waveguide. Variations in scattering as a function of propagation direction and mode polarization suggest that the scattering centers are anisotropic with regard to either their refractive index, their shape or both. Diagnostic experiments to ascertain the nature of these scatterers, including electrooptic, photorefractive, and thermal studies, are described.
Scattered light which remains is an optical waveguide to be guided to the detector is an important limiting factor to the performance of integrated optical devices. In sputtered glass and niobium oxide waveguides the principal mechanisms for scattering are the scattering from refractive index variations in the waveguide and from surface roughness of the waveguide. Theories for scattering from surface roughness, in the regime expected for these waveguides, predict that the wavelength variation of the scattering should be proportional to the inverse square of the wavelength. Theory for scattering from refractive index variations predicts a wavelength dependence ranging from inverse fourth power of the wavelength to no dependence upon wavelength, when the range of possible scattering diameters is varied from small to large with respect to the optical wavelength. Experiments on a number of relatively lossy waveguides indicate that there is little relation between overall waveguide loss and the magnitude of the scattering in the near forward direction. Furthermore, overall loss and in-plane scattering have different variation with wavelength suggesting that the mechanisms involved in loss and in scattering are different. Waveguide surface preparation prior to sputtering the waveguide appears to be an important factor in determining overall quality but more as it influences the bulk properties of the waveguide film than as a direct scattering mechanism. Homogeneity of the waveguide film appears to be the controlling factor in production of low scatter waveguides.
The geometrical theory of axially symmetric geodesic lenses is discussed. An algorithm was developed and used to obtain the optimal shape of a lens that is corrected for spherical aberrations. The effects of lens surface distortions on the performance characteristics are determined. For a lens to be near diffraction limited it was found that its ideal shape must be maintained to within 10-4 X lens diameter.
The application of single point diamond turning to the fabrication of aberration-corrected, aspheric geodesic lenses in LiNb03 for use in integrated optics is reported. F/5 lenses with useful apertures of 4 mm and 5 mm have been produced. Lens profile accuracies ranging between 1.5 and 3 pm total indicator reading and depth accuracies of 0.25 to 2.0 μm have been achieved. The machined surfaces require only light polishing, which produces little effect on the figure accuracy, to remove residual machining marks. Waveguides grown by Ti-indiffusion are found to be characterized by very low scattering losses. Optical image spot sizes of 1.35 to 2.23 times diffraction-limited have been measured for input beam widths of 1.0 to 3.28 mm for a lens in which the edge rounding has not been incorporated into the aspheric correction. The edge rounding has been included in the correction for subsequent lenses.
In this paper we discuss three integrated devices using GaAs/GaAlAs DH injection lasers. The first of these achieves angular scanning by adjusting the current levels between two closely spaced stripe contacts, thereby creating an asymmetric gain/refractive index through which the laser beam propagates. A deflection of the peak laser beam intensity of ±14' along the p-n junction is observed. Another structure involves operation of a branching waveguide stripe-geometry laser. This structure produces interference fringes in the far field which are spatially scanned by electrically varying the optical phase in each leg of the branching waveguide in a manner analogous to microwave phased array radar. The last structure consisted of 10 conducting 3 om stripe contacts on centers separated by D and coupled by curved waveguide sections. This device produces a high-power, phase-locked beam of small angular divergence parallel to the p-n junction Data from four stripe geometries with D ranging from 10 Um to 27.4 im are compared and overall optimal characteristics are shown to occur for D = 101.1. In that case, threshold current densities are comparable to those of solid area laser, differential quantum efficiency is approximately 60%, and maximum observed pulsed power per facet is on the order of 1 watt.
A review is given of recent progress in the development of integrated optical circuits involving the Al Ga1-x As/GaAs double heterostructure system. Various devices utilizing x. periodic corrugations or gratings are described briefly, whereas alternate attempts to fabricate optical circuits by wet chemical etching are discussed in more detail. The current trend to explore other III-V compounds is considered, with emphasis on the quaternary system GaxIni-xPyAs1-y Lattic matching of this quaternary to InP results in long wave-length emission, suitable for use with present optical fibers. A number of reasons are also given for increased investigation of this quaternary lattice-matched to GaAs.
A silicon photodiode array with 50 dB dynamic range has been developed for integrated optical spectrum analyzer systems. The device consists of 140 pixels on a 12 micrometer pitch. Each pixel can be addressed within a period of 2μseconds. The device is butt coupled to an integrated optical waveauide spectrum analyzer circuit fabricated from lithium niobate.
The successful evolution of III-V optical and electronic devices from laboratory curiosities to production components has opened exciting new possibilities for vastly improved electronic and optical electronic systems. GaAs FETs and semiconductor lasers can now be commercially obtained from a number of companies in the Uniteld States and Japan. Researchers have recently demonstrated GaAs digital integrated circuits and CCDs2 that can reach speeds and low-powered dissipations that cannot be achieved with similar geometry silicon devices. The complexity (i.e., number of active components) of these circuits is rapidly growing. Laboratory GaAs circuits with ~ 100 gates have been demonstrated.
This paper describes the various parameters which must be specified or measured for the cw GaA/As injection laser diode when it is to be used as a source in integrated optical applications. These include width of the current stripe, effective waveguide thickness and location, diode thermal response, and effect of modulation and drive current on the optical power and spectrum. In each case, examples are given citing the expected or measured behavior and comparing this with results obtainable in available cw laser diodes.
Adjustable access couplers for multimode fiber-optic networks have been constructed. The 4-port fiber-coupled devices make use of voltage-controlled total internal reflection by a thin film of nematic liquid crystal. The devices can serve as tee-couplers in unidirec-tional single-strand data buses or in duplexed two-strand buses. This active-coupler approach gives low loss between any pair of addressed terminals on the bus. In the devices, multimode fibers are coupled to rod lenses, and those lenses are, in turn, coupled to flint glass prisms that confine a 6-μm liquid-crystal layer. The achromatic switches have a 1.6 dB optical insertion loss, a tap-ratio that is controllable from -48 dB to -4.6 dB, a directionality of 44 dB, and an operating voltage of 5 to 20 V rms at 1 kHz. The structure is extendable to matrix switching arrays and to higher-order switching functions.
This paper describes the Multiwavelength Monolithic Integrated Fiber Optic Terminal (MMIFOT) being developed by MDAC-STL under the sponsorship of NASA/JSC. The first phase of this program has addressed the development of a monolithic integrated optical receiver/demultiplexer chip for an ei qht channel wavelength multiplexed fiber optic link. Under the second phase of this program Ã¢â‚¬â€? a transmitter/multiplexer chip will be developed.
Device configurations of optical channel waveguide arrays coupled to integrated charge-coupled devices (CCDs) will be presented. Optical and electrical performance of these devices will be discussed. A channel waveguide array formed in a fan-out pattern is then introduced as a means of enhancing optical waveguide focal plane resolution in integrated optical devices utilizing optical waveguide lenses. High spatial resolution can thus be obtained without making detector spacings too small, thus avoiding detector problems with regard to fabrication, cross talk, linearity and charge transfer efficiency. To fabricate the fan-out channel waveguide array, a new differential heating and photoresist lift-off process is discussed which allows high resolution patterns to be reproducibly formed in polyurethane. Propagation of light from a HeNe laser in these fan-out channel waveguide arrays has been demonstrated with only a small amount of scattering. Low scattering is consistent with the smooth channel waveguide surfaces apparent in scanning electron micro-scope pictures presented. Applications of optical channel waveguide arrays coupled to integrated CCDs to fiber multiplexing and transversal filtering will also be discussed.
Thick film optical waveguides have been integrated with mechanical fiber optics connectors on a single substrate. Micron alignment accuracies are possible by photolitho-graphically fabricating the mechanical fiber guides in the same medium as the optical wave-guides. This allows efficient coupling of light from optical fibers to the thick film light guides. Coupling losses of less than 1 dB have been experimentally verified. Mass producibility, low cost and flexibility of design should make these integrated optics very useful as fiber optics connectors and splitters as well as fiber optics to integrated optics interfaces.
Light guide condensers with dimensions on the order of centimeters and numerical apertures of about 0.7 have been optimized with the aid of a computer to provide a uniform illuminance and emittance distribution within 6% and a controlled polar luminance distribution across the object plane.
This paper discusses the behavior of guided modes in the optical-range multilayer strip waveguide with different index cross sections which are possible in this structure including the S-shaped refractive index distribution. The strip has the largest index of refraction. Special attention is paid to the case for which there is a "potential barrier" nearby the strip inside the multilayer structure. The layer nearest to the strip has a smaller refractive index which separates the strip from the outside layer. The dispersion and attenuation characteristics of modes are investigated. The S-type strip waveguide may result from increases in the dissipation factor in the outer (from the strip) layer or quasi-single-mode operation with heating losses of higher order unwanted modes.
The confined propagation of guided optical modes in fiber guides with rectangular (square) cores is proposed and analyzed. An analysis is made of the waveguide characteristics of different types of a singly and doubly clad optical fiber guides with rectangular cores, including the W-shaped refractive index distribution. A detailed investigation is made of the characteristic equation and dispersion characteristics of guided modes in such fiber guides are found. Prospective applications are presented. It is shown that it is possible to make single-mode fiber guides with rectangular core with good dispersion characteristics, and relatively large transverse dimensions of the rectangular core as well as a self-filtering fiber guide can be constructed. 17ibex guides with rectangular (square) core are one kind of the large family of fibers for optical communications systems. In some practical cases it is necessary to provide the fiber guides with rectangular core. It is important for compatibility with terminal devices. Some fibers are now available with losses as low as a few decibels per kilometer. However, some basic problems must be solved.(1,2) A practical method of splicing fibers with injection semiconductor lasers is one of them, because in order to increase the efficiency of the coupling between laser and fiber it seems better to have the geometry of the fiber's core cross section the same as the injection region of the semiconductor laser.( 3) In the first part of this paper we shall investigate a fiber guide with rectangular core that is surrounded by single infinite (in comparison with wavelength) cladding. In the second part we shall investigate optical fiberguide which is composed of three portions: the rectangular core, inner cladding, and outer cladding. The main stress is on fiber guide W-type when the rectangular core has the largest refractive index of the three, and the outer cladding index is the next largest.