We review the design and fabrication considerations of the antiresonant reflecting optical waveguide (AR-ROW), including materials selection for IC and MEMS compatibility, and present a variety of IO devices and sensors based on the ARROW which we have developed, along with pertinent CAD tools. Examples presented include: directional couplers, thermo- optic modulators, and Mach-Zehnder interferometers (MZI), including micromachined MZIs for detection of mechanical signals.
We present the first large-area, all-silicon rib waveguides and fiber guiding U-grooves fabricated on (110) silicon substrates. Waveguide rib structures with a height of 6.7 micrometers and widths of 5, 10, and 15 micrometer are RIE etched into lightly doped epitaxial silicon layers deposited on heavily doped silicon substrates. Fiber-guiding U-grooves with unique corner compensation structures are anisotropically etched by a 35 wt% KOH solution at 70 degrees C, with and without ultrasonic agitation of the etching solution. Devices fabricated without ultrasonic agitation have a U-groove bottom surface roughness (Ra) of 5606 angstrom, while devices with ultrasonic agitation have a surface roughness (Ra) of 341 angstrom. Ultrasonic etching improves the surface roughness of the U-groove bottoms by a factor of 16.4. We found that the variation of U-groove etch depth is grater than 1 micrometer across the wafer, even with ultrasonic agitation of the etchant. Examination of the waveguide modal structure indicates that devices with 6.7 micrometer tall and 10 micrometer wide rib structures propagate only one waveguide mode at 1550 nm. The total loss of the straight waveguides was as low as 4.88 dB, and the propagation loss was estimated to be 1.68 dB. These loss values indicate waveguide end-faces fabricated by anisotropic etching of (110) silicon do not need to be polished.
We discuss pressure sensors that utilize a combination of geometric and elasto-optic effects in a deformable slab waveguide forming a geodesic lens. Simulation of their performance and preliminary experimental results are presented.
Starting with a piece of PMMA (2 mm by 2 cm by 5 cm) a pattern for an integrated version of an optical gas sensor is recorded using computer controlled carbon dioxide laser beam etching. The sensor principle is the ATR-leaky mode spectroscopy using the shift of the plasmon resonance. Divergent light is created by optical fiber coupling. The resonance shift is observed with an array detector.
An integrated optic refractometer device was developed to perform a rapid one-step, homogeneous immunoassay. The device measures refractive index changes at the surface of a planar, singlemode, ion-exchange waveguide using difference interferometry. Anti-aflatoxin- B1 antibodies were attached to the waveguide surface to provide a bioselective coating for detecting and quantifying the aflatoxin-B1 antigen level in a sample. The detection limit of this small antigen must be determined using a competitive assay format. To determine feasibility of the competitive assay, we determined the biosensor response to a larger molecular weight competing antigen, namely HRP-labeled aflatoxin-B1. This labeled antigen will compete with unlabeled aflatoxin for binding sites on the sensor surface. Increased sample aflatoxin levels will result in a decreased time-dependent phase change of the helium-neon laser light beam. Phase change data were determined for various concentration levels of HRP-labeled aflatoxin- B1 antigen. The assay measurements were made over a 5-minute time period. Results indicated that a competitive assay is feasible. Future assay efforts should be able to demonstrate measurement of aflatoxin-B levels found in contaminated corn samples.
Metal membranes fabricated using a surface micromachining process can be integrated with passive optical waveguides on silicon substrates to form active devices. Electrostatic actuation of a membrane into contact with a waveguide alters both the real and imaginary parts of the propagation constant of the guided mode. This effect has been used to demonstrate both on-off and routing type integrated optic switches, and has potential application to other devices such as variable attenuators and tunable Bragg filters.
Near field scanning optical microscopy (NSOM) has been used to investigate the guided mode intensity distribution in channel waveguides, directional couplers, and Y-junctions. The intensity profile above the sample surface and transverse to the waveguide propagation direction has been measured using a tapered optical fiber to probe the guided evanescent field. The fiber probe was maintained at a constant height above the sample surface using feedback provided by performing these near field scanning measurements simultaneously with shear force microscopy topography measurements. Single mode channel waveguides were formed by etching a ridge in planar Si3N4/SiO2 structures and were excited with light of a wavelength of 830 nm. Measurements transverse to a channel waveguide revealed a cosine squared variation of intensity above the ridge and an exponential decay away from the ridge, as expected. Considerations for characterizing AlGaAs waveguides in this manner also are discussed. Multiple scans along the two waveguides of a directional coupler provided a detailed view of optical power transfer from one waveguide to the other and were in agrement with beam propagation method calculations. We anticipate that this type of measurement will provide a more detailed understanding of a central photonic structure, the channel waveguide, and its incorporation into a variety of device configurations.
By fabricating a discrete cell photodiode with multiple edge contacts, and allowing for operation under two different current sensing schemes, a diode which operates optionally as either a continuous or discrete cell position sensitive photodetector has been realized. Inclusion of on-chip CMOS switching circuitry enables a user of such a photodetector to change its operating mode by the use of a single digital input line. Experimental results of the dual mode photodiode with switching circuitry are presented in this paper.
Fast means of etching SiC are required for the fabrication of SiC microstructures. Single- crystal 6H-SiC was etched at rates as high as 640 nm/min in a magnetron plasma reactor using a mixture of CHF3 and O2. Extremely smooth surfaces were obtained, even for etches of several micrometer depth. At a lower rf power, 50 W instead of 250 W, a 12:1 selectivity with respect to aluminum was demonstrated, with a SiC etch rate of 170 nm/min. These preliminary results show that this approach is potentially a very useful tool for SiC microfabrication.
Crystalline oxide thin films have been synthesized at low temperatures from aqueous liquid solutions. A key element of the approach is the use of organic self-assembled monolayers (SAMs) on the substrate to promote the growth of adherent inorganic films. A SAM is a close- packed, highly ordered array of long-chain hydrocarbon molecules, anchored to the substrate by covalent bonds. The terminating functional group on the SAM surface is chosen so as to initiate and help sustain the formation of the oxide film when the substrate is immersed in the oxide precursor solution. Synthesis, microstructural characterization, and properties of TiO2, ZrO2, SiO2, and Y2O3 films are surveyed. Crystalline films were formed either directly from solution, or through subsequent heat treatments at temperatures that in most cases were lower than typical sol-gel or vapor phase deposition processes. All depositions were from aqueous solutions onto single-crystal (100) silicon. The ability to produce patterned films on a micron scale has been demonstrated, taking advantage of the selective deposition characteristics towards different surface functional groups of the SAM. The role of the SAM in oxide film formation is discussed.
An integrated microspectrometer is presented using SiON-slab waveguides on silicon substrates. The microspectrometer is utilized as a versatile detection unit of micro total analysis systems by broad band VIS-spectroscopy. It is designed for efficient diffraction in the wavelength range between 300 - 700 nm, a high spectral resolution and dispersion, respectively. The spectrometer consists of a planar 5 by 5 mm2 transmission grating, a cylindric lens and a commercial silicon diode array positioned in its focus for simultaneous intensity detection of the complete visible spectrum. No moving parts and a compact optical sensor head enable mobile use free of maintenance.
A manufacturing process was developed to reduce the cost of integrated optical components. The process uses a molding operation to form the optical waveguide pattern and dye diffusion process to form the high index region. Waveguide patterns with 5 micrometer lines have been formed in highly transparent thermoplastics.
Gallium sulfide (GaS) deposited by chemical vapor deposition (CVD) is known to passivate GaAs surfaces. In this paper we examine the thin film optical properties of GaS as they relate to the fabrication of optical waveguides. Spectroscopic ellipsometry was used to determine the index of refraction of GaS films deposited on various substrates. Results indicate that GaS has a high index of refraction suitable for waveguide structures. A gallium sulfide waveguide could provide both the optical interconnect and the passivating layer of GaAs integrated circuits. Progress toward fabricating GaS waveguides is also discussed.
Optoelectronic interconnection can provide a practical solution for the ever increasing communication bottleneck problems among a multitude of information processing units. As research activities progress in the field of serial or parallel board-to-board and module-to- module interconnection, some of the research focus has shifted to smaller physical dimensions, like intra-module interconnection, which combines optoelectronic interconnection, multichip module packaging, and microelectromechanical system technologies at the module level. This paper describes novel integrated optical input/output couplers on multichip modules using micro-machined silicon mirrors to be used for optoelectronic multichip modules. The proposed microstructure integrates optical waveguide networks, multilayer electrical transmission line networks, micro-machined silicon mirrors, and flip-chip bonded photonic devices into a single structure. Using both sides of the silicon wafers, multiple metal layers and optical waveguide layers can be successfully fabricated for all metals or optical waveguides. The proposed input/output coupling method utilizes an innovative combination of through-holes across OE- MCM and micro-machined silicon mirrors integrated together into a single package.
A simple optical fiber temperature sensor is under development. The expected temperature range to be measured by the sensor is from minus 50 degrees Celsius to over 1000 degrees Celsius, and the measurement accuracy is 1 degree Celsius. The sensor is robust against harsh environments. It is immune to source fluctuations, surface deterioration of the optical element inside the sensor head, and coupling inconsistency [sensor to electro-optics (EO) board]. The sensor is constructed so that calibration is done automatically upon installation. The sensor uses a high-temperature fiber and a high-temperature metal, and its operation is based on a simple optical interferometer. The source for the sensor is a wideband (white light) source. Data processing software is needed to operate the sensor. The frequency response is 10 Hz or faster. A breadboard sensor made of stainless steel and a silica fiber was built and tested to temperatures up to 500 degrees Celsius. The results show the feasibility of the proposed concept.
We study the possibility of improving the theoretical limit to the resolution of the optical beam deflection method (OBDM) by reflecting the beam from a curved surface. We suggest a new detection scheme by measuring the average intensity over a cross section of the beam after it is reflected from a cylindrical mirror. We show that there is a possibility of decreasing the theoretical minimum detectable angle (MDA) as compared to the usual OBDM when the beam grazes the cylindrical mirror.
A novel concept of a liquid core lightguide in cylindrical geometry is presented. The coating of a hollow core glass tube with Teflon AFR leads together with a liquid filled core to such a guide. Effective coupling was obtained and low losses were observed. Using this liquid core guide as an optical cell in a spectrometer a significant increase in sensitivity, depending on the length of the guide, may be obtained.
A practical self-aligning pinhole (SAP) system, capable of actively aligning a pinhole to an incident optical beam, has been demonstrated. The enabling technology for the SAP is a silicon micromachined pinhole (SiMP). The SiMP is an example of a simple optical element fabricated from silicon in order to take advantage of both the mechanical structure allowed by micromachining technology and the electrical structures allowed by semiconductor technology. To complete the transformation from an enabling technology to a working system, development was necessary in packaging, mechanical mounting and operation, and algorithms.
In this paper a new integrated optical chip containing silica-on-silicon waveguides end-fire coupled to micromachined silicon mesa photodiodes is presented. Contrary to similar devices presented in the literature, buried waveguide structures with a total glass thickness of 15 micrometer have been used, which has considerable potential for new and interesting applications. Unlike the conventional surface-illuminated photodiode, the light is coupled in parallel to the junction in these devices. This configuration results in several advantages, two of which are improved spectral, as well as high frequency response. For the first test series reported here, the aim was high quantum efficiency, thus, lowly doped n-type substrates leading to long diffusion lengths of holes were selected. This leads to low frequency response and a quantum efficiency almost insensitive to reverse bias, due to the very long diffusion lengths of carriers in lowly doped material. Propagation loss for the multimode waveguides was less than 0.5 dB/cm for lambda greater than 700 nm. The dark current of the photodiodes was less than 100 pA and the breakdown voltage above 200 V. In a spectral range from 500 - 1000 nm, the devices showed very flat response with quantum efficiencies up to 82%. In the work now in progress, a p-i-n structure will be used, in order to optimize both bandwidth and quantum efficiency.