Component fabrication for integrated optical systems on LiNb03 is reviewed. The fabrication of an Integrated Optical RF Spectrum Analyzer  has demonstrated the manufacturability, evaluation, and assembly of components such as geodesic lenses, surfaces, tranducers, waveguides, photodiode array detectors and GaA1As laser diodes. These components will become increasingly important in the development of future integrated optical devices. The sequence and technique of fabrication, alignment, and assembly of these components in integrated optical devices are of major importance to successful achievement of device performance goals. Those sequences and fabrication techniques which were used in the development of the RF Integrated Optical Spectrum Analyzer components are discussed.
We report here three different techniques for making high-refractive-index transparent films on LiNbO3 in the form of either planar or two-dimensional structures. Such techniques should be very useful for making a number of integrated optical components on LiNbO3.
The laser-power-handling properties of planar out-diffused and in-diffused lithium niobate waveguides are considered in terms of fabrication process characteristics. The percentage of laser induced power-loss is found to increase as the square of both the input-coupled laser power and the beam's transmission distance in the guide, regardless of the fabrication details. Titanium in-diffused waveguides are found not to exhibit laser-induced power-loss for beams proloaaatina along their z-axis.
Low-loss planar and channel waveguides were fabricated in BK-7 substrates by ion exchange using dilute AgNO3/NaNO3 melts. waveguide refractive index profiles were inferred by comparison with effective refractive index measurements made on the guided modes. The diffusion constant of silver and its activation energy in BK-7 were calculated from the measured refractive index data. It was found that a post-exchange annealing of the waveguides resulted in a significant decrease in the waveguide losses. A Fabry-Perot waveguide resonator loss measurement technique was developed to measure the attenuation in these waveguides because of the unreliability of prism coupling methods at low-loss levels. Losses as low as 0.068 dB/cm were measured in channel waveguides.
Thin films of V2O5, Nb2O5, and Ta2O5 have been grown under a variety of conditions including reactive rf sputtering, reactive bias sputtering, and post deposition oxidation. The refractive index and guided-wave attenuation have been measured for both as-grown and annealed films. Except for V205, where low losses were not achieved, general guidelines were established for optimal growth conditions resulting in high refractive index low loss films. Films of Nb2O5 and Ta2O5 have been used in the fabrication of guided wave Bragg cells and waveauidp Lunphura lenses.
Planar graded index Si02 optical waveguides characterized by very low scattering have been fabricated on silicon substrates. The waveguides were thermally grown on silicon in two different ways. In one case, a Si02 thickness of 15 microns resulted in waveguiding with a total waveguide attenuation of 0.6 dB/cm. In another case, intentional doping of the Si02 layer allowed waveguiding to take place at a thickness of 6.6 microns with a total waveguide attenuation of 2.3 dB/cm. In each case, the relatively small magnitude of the refractive index change suggests that coupling to the silicon substrate is an important loss mechanism. Measurements imply that scattering loss from the waveguide itself is exceedingly low, and that continuing efforts to increase the field confinement may result in waveguides characterized by a substantially smaller value for the total waveguide attenuation.
Ion implantation of semiconductors is a very attractive technique for forming the guided wave components needed in integrated optical circuits. Implanted protons produce lattice defects which act as electron traps, thereby lowering the free carrier concentration and the subsequent negative plasma contribution to the index of refraction, producing waveguiding. Careful control of implant parameters, along with post-implant thermal annealing, allow optimization of defect annealing and diffusion sufficient to reduce the waveguiding loss and to provide the precise desired waveguided mode structure. Due to the statistical nature of the implantation, precise device practicality is achieved, along with planar waveguides exhibiting losses at 1.15 μm less than 2 cm-1. Characterization of the effects of thermal processing on the optical and electronic properties of proton implanted n-type GaAs is reported. The thermal processing includes variations in the substrate temperature during implantation and post-implantation annealing. Capacitance-voltage and infrared reflectivity measurements provide a measurement of the degree, depth, and uniformity of compensation, while free carrier absorption loss in the substrate mode tails and optical guided mode profiles are obtained using a scanning galvanometer mirror and a detector slit. A revised theoretical model of the compensation process based upon the classical Drude model is presented along with a variety of unique loss and mode profile determinations correlated with implant fluence and temperature processing.
Performance criteria for components required for an electrooptic A/D converter currently under development are discussed. These components include pulsed diode lasers, guided-wave modulator arrays on LiNbO3, high-speed avalanche photodiodes and high-speed digital circuitry. Efforts will be made to relate the effects of component performance to converter performance.
Recent developments in the design of integrated optical circuits for performing optical numerical computations are discussed. The use of systolic architectures for these IOC's is described and the natural marriage of IOC's with the systolic concept is discussed. Examples include optical binary correlation, polynomial evaluation, and matrix multiplication.
Electro-optic modulators have been constructed which provide high efficiency modulation of channel guided waves in LiNb03 with a capability of large dynamic range, good linearity, and high packing density for use in large modulator arrays. Modulation of an input guided wave has been observed by creating a variable index gradient with an E-field generated by surface electrodes. The dominant modulation mechanism is determined by the degree of light energy confinement in the interaction region. Both an imaging or gradient effect and a guided mode effect are possible. Modulation efficiencies of greater than 0.95 have been achieved with a 1 mm interaction length and an E-field strength of 4 x 10-6 v/m. Linearity of about 1% has been demonstrated. Design considerations and experimental performance data are presented.
Measurements are described at 0.83 µm and 1.3 µm to investigate the directional coupling between two identical, single mode, Ti :diffused channel waveguides for the ordinary (TE) and extraordinary (TM) polarizations in Z-cut, X-propagating LiNb03. The coupling parameters for each polarization are a function of the waveguide fabrication parameters and hence of the waveguide mode dispersion. Also the coupling characteristics for the TE and TM modes vary somewhat differently with changes in mode dispersion. Hence, for different waveguide fabrication conditions, the TE and TM mode coupling coefficients may be equal at one coupler gap or may be unequal for all gaps or, most significantly, may be equal for all gaps. Experimentally determined coupling variables are compared with theoretical ones, calculated using channel mode dispersion parameters and diffusion parameters determined from mode measurements on planar waveguides. Experimental/theoretical agreement is generally good and the calculations correctly predict the effect on the coupling characteristics of changes in waveguide fabrication parameters.
Many methods have been presented and discussed for the solution of modes and propagation constants of optical waveguides [1,2]. Waveguides normally obtained and used for integrated optics (e.g., titanium indiffused lithium niobate) have index profiles which are anisotropic, of arbitrary cross-section, and of arbitrary index profile. It would be beneficial to require our method to be capable of treating waveguides of this type.
Chirp gratings lens is the key technology for the next generation of integrated optical signal processing devices. These lenses have been made on LiNbO3 substrate by various fabrication processes. This paper will report the current status of this technology development.
We have designed and fabricated an integrated-optics Mach-Zehnder interferometric modulator. The electrodes are 3 μm thick asymmetric coplanar striplines, formed by ion-beam etching techniques. Complete modulation is achieved with 6.5 volts for the 6 mm long device at 0.83 μm wavelength and with 18 volts at 1.3 μm wavelength. The modulator's optical characteristics have been measured up to 3 GHz and its rf characteristics up to 18 GHz. The rf characteristic of the stripline electrodes are highly sensitive to fabrication and packaging. In fact, by electroplating instead of sputter-depositing the gold stripline electrodes and by modifying the device package, the 3 dB bandwidth of our device increased from 3.5 GHz to 16.5 GHz. In addition, resonances, commonly observed in previous devices, were eliminated after the modifications, yielding for the first time, smooth frequency response up to 17 GHz. Since this particular modulator retains a dc electrical bias, it performs either as an intensity modulator by applying a π/2 dc phase bias to achieve maximum linearity or as a frequency shifter by changing the dc bias point to π. In addition, we analyzed the principle of operation of the Y-junction by observing both the in-phase and the out-of-phase modes of a multimode waveguide modulator.
In this paper we review the properties of two integrated optics devices which we have developed for the purpose of making physical measurements. The first is an integrated optics ring resonator fabricated by ion exchange in glass, and the second is quasi-waveguides, using their m-lines to measure refractive index and thickness of films of refractive index lower than the substrate.
As data rates and pin counts increase interconnection of chips, boards and modules will become increasingly difficult by conventional means. Optical interconnection through the use of high speed serial optical links will be an attractive and viable alternative technique. GaAs optical and electronic device technology has advanced to the stage where the monolithic integration of both components is possible. While initial implementations will be rather simple, it is now possible to project that in the next ten years VLSI complexity circuits incorporating both optical and electronics functions will become a reality.
The optoelectronic properties of In0.53Ca0.47As, grown lattice matched to InP, suggest that a high performance integrated photoreceiver for use in fiber communications systems operating in the 1.3-1.5 μm spectral band is possible. Research directed at demonstrating such a system is reviewed in this article.
The materials and processing required for semiconductor laser diodes and GaAs integrated circuits are different and often incompatible. This has been primarily responsible for the relatively slow development of integrated optoelectronic/electronic structures. Therefore, it is important to develop a method for combining the two dissimilar technologies using materials and processes which do not place prohibitive restrictions on the performance or geometry of the optical and electronic devices. The laser-in-a-well technique has been developed to overcome many of the problems of monolithically integrating an AlGaAs laser diode with complex GaAs circuitry. Optoelectronic transmitters containing a transverse junction stripe laser, a driver FET and a 36 gate 4:1 multiplexer are in process. The ability of the laser-in-a-well technique to combine the TJS processing with the GaAs electronics has already been demonstrated with the on-chip testing of working multiplexers on wafers containing lasers.
Q-switched semiconductor diode lasers with an integrated modulator have been operated with full on/off modulation at rates of 3 GHz. In addition, modulation of the lasers has been shown up to a detector-limited frequency of 6 GHz. A new model of these devices, which includes amplified spontaneous emission and high gain, predicts the possibility of a new mode of Q-switched operation with the capacity for repetition rates of tens of gigahertz and binary pulse position modulation at rates of the order of 10 Gbit/sec.
The electro-optical effect (Pockels Effect) induced in zincblende structures can be utilized to modify the phase and polarization characteristics of the transmitted light. The III-V group of semiconductors and their ternary and quaternary mixtures represent a particularly important group of zincblende structures because these also provide the most important semiconductor laser materials. The characteristics of different crystal orientations and their possible uses in optical communication systems are reviewed. Polarization modulators are practically interesting devices which can be fabricated from these materials in waveguide form. Possible applications for these - in addition to polarization control - are intensity modulation and optical isolation. Polarization characteristics are analyzed with the help of Jones matrices. An example of a simple polarization modulator is presented and experimental results discussed. A good agreement with theoretical predictions was attained.
We report the first experimental results on a new (100) waveguide geometry for use as a electro-optic polarization modulator in III-V semiconductors. Theoretical investigations have predicted large electro-optic modal conversion efficiencies (up to 100%) at low voltages and with short guide lengths. Our preliminary experiments on A1GaAs/GaAs demonstrated 8% conversion, and appear to follow theory closely. With minor improvements, higher conversion efficiencies are expected.
Optoelectronic integration of an InGaAsP/InP laser is discussed. Low threshold buried heterostructure lasers on semi-insulating InP substrate and junction FETs compatible with integration are developed.
Computer modeling studies on planar dielectric optical waveguides clad with gallium arsenide indicate that the attenuation and mode index behave as exponentially damped sinusoids as the semiconductor thickness in increased. This effect is due to a periodic coupling between the TE0 mode of the dielectric and the lossy modes supported by the high refractive index gallium aresenide. For guided wave propagation near the semiconductor band edge, the complex permittivity of the gallium arsenide may be altered through electron-hole pair generation via second photon excitation. This pair generation process is investigated as a means of controlling guided wave propagation, and amplitude and phase modulators are examined.
Nonlinear optical phenomena in optical fibers are finding a variety of applications. This paper reviews some of these applications which can be either useful or undesirable. Useful applications are the amplification and generation of different optical frequencies or the production and shaping of ultrashort optical pulses. Undesirable applications would include the signal degradation which nonlinear phenomena could introduce in long single-mode optical-fiber transmission systems.
Nonlinear optical effects as second harmonic and difference frequency generation, parametric amplification and oscillation are studied to achieve efficient frequency conversion in optical waveguide structures. Our experiments with low loss Ti:LiNbO3 optical wave-guides and waveguide resonators are shortly reviewed. The results demonstrate that small, (tunable) integrated optical frequency converters can be fabricated using medium and low power (semiconductor) lasers as pump sources.
Parallel-processing concepts for solar energy conversion are being explored at NASA and through university grants as part of a high-risk effort to design a 50% efficient solar cell for the 1990's. The aim is to overcome the 56% loss associated with mismatch between the broad solar spectrum and the monoenergetic conduction electrons used to transport energy in conventional silicon solar cells (Fig. 1). The parallel-processing strategy would use surface plasmons or other guided electromagnetic waves for broadband energy transport, and an array of tunable diodes for energy extraction (Fig. 2). Tunable diode research has focused on inelastic tunneling in metal-oxide-semiconductor-metal films, with the idea of designing energy conversion devices which can be tuned to different frequencies by varying the load. This paper describes the technical barriers which must be overcome in order to harness surface plasmons for solar energy conversion, and describes current research on four key problems: practical techniques to phase-match sunlight to surface plasmons, structures to minimize ohmic loss and reradiation, mode conversion techniques to couple energy into the tunnel diodes, and junction designs to maximize the probability of plasmon capture by tunneling electrons.
The discovery of surface-enhanced Raman scattering (SERS) has generated widespread activity on two fronts: 1) investigations of the physical mechanisms responsible for SERS, and 2) applications of those mechanisms to intensify other optical processes. The term SERS refers to the phenomenon in which the normally weak Stokes-shifted Raman signal from a molecular monolayer is increased by factors of 106 or more when the molecular layer is placed on a suitably roughened metal substrate. It is now rather widely believed that SERS is caused by an interaction between the molecule and localized plasma resonances (LPR) in the roughened substrate. The LPR are oscillations of the conduction electrons within the "bumps on the roughened substrate, and are excited by an incident optical beam. The LPR develop very strong surface electric fields; a molecule placed in close proximity to the "bumps" interacts with the strong field of the LPR giving rise to the enhanced Raman scattering.
Components that utilize the overcoated surface plasmon mode are promising candidates for systems in the far-infrared and submillimeter because of the low attenuation and ease of fabrication. Grating couplers (antennas) with efficiencies in excess of 70% have been experimentally demonstrated in the far-infrared. Theoretical calculations show that these components can be scaled to lower frequencies while maintaining similar performance.
We present here an analysis of plane-wave absorption by a slightly lossy two-layer dielectric structure backed by a metal plate. It is shown first that there exists a close relationship between the guiding characteristics of the two-layer dielectric structure and the total absorption of a plane wave coupled by a prism into the lossy dielectric waveguide. On the basis of known surface-wave characteristics, a parametric study on the plane-wave absorption by the waveguide is carried out and the results show that such a waveguide structure can be used for the design of angularly selective polarization-independent optical absorbers or TM-polarized optical absorbers with a relatively large range of incident angle.