Scaling the modulation bandwidth and phase efficiency of a silicon optical modulator

We present an optimized design and detailed simulation of an all-silicon optical modulator based on a silicon waveguide phase shifter containing a metal-oxide-semiconductor (MOS) capacitor. Based on a fully vectorial Maxwell mode solver, we analyze the modal characteristics of the silicon waveguide. We show that shrinking the waveguide size and reducing gate oxide thickness significantly enhances the phase modulation efficiency because of the optical field enhancement in the voltage induced charge layers of the MOS capacitor, which, in turn, induce refractive index modulation in silicon due to free carrier dispersion effects. We also analyze the device speed by transient semiconductor device modeling. As both optical absorption and modulation bandwidth increase with increasing doping concentration, we show that, with a nonuniform doping profile in the waveguide, balance between the device operation speed and optical loss can be realized. Our simulation suggests that a TE-polarized optical phase modulator with a bandwidth of 10 GHz and an on-chip optical loss less than 2 dB is achievable in silicon.     

Realization of polymeric optical integrated devices utilizing organic light-emitting diodes and photodetectors fabricated on a polymeric waveguide

Direct fabrication of organic light-emitting diodes (OLEDs) and organic photodetectors (OPDs) on polymeric substrates, i.e., polymeric waveguide substrates to form flexile optical integrated devices is demonstrated. The OELD and OPD were fabricated by organic molecular beam deposition (OMBD) technique on a polymeric or a glass substrate, for comparison. The device fabricated on a polymeric substrate shows similar device characteristics to that on a glass substrate. Optical signals of faster than 100 MHz have been created by applying pulsed voltage directly to the OLED utilizing diamine derivative, or rubrene or porphine doped in 8-hydoxyquinolinum aluminum derivatives, as an emissive layer. Electrical signals are successively converted to optical signals for optical transmission of moving picture signals with OLED fabricated on a polymeric waveguide. OPDs utilizing phthalocyanines derivatives with superlattice structure provide increased pulse response with input optical signals, and the OPD with the cutoff frequency of more than 5 MHz has been realized.     

A monolithically integrated smart pixel using an MSM-PD, MESFET's,and a VCSEL

We have developed a smart pixel that monolithically integrates a metal-semiconductor-metal (MSM) photodetector, metal-semiconductor field effect transistors (MESFET's), and a vertical-cavity surface-emitting laser (VCSEL). This device can perform both NOR- and OR-types of operation with a thresholding function. Optimal device parameters are obtained by using a SPICE simulation. Calculations show that the switching time is mainly limited by the CR time constant of the input stage, which consists of the MSM photodetector, the load-resistor, and the MESFET connected to the MSM photodetector. The fabricated device attained a contrast ratio of more than 30 dB with optical gain. The 3-dB bandwidth was 220 MHz and the switching energy was 700 fJ at an operation frequency of 100 MHz. We also discuss the power consumption and the packing density of the smart pixel including the VCSEL as a function of operation frequency. A MESFET that has high f_T with low bias voltage and a VCSEL that has a low threshold current while maintaining the wall-plug efficiency are necessary to obtain a higher performance device     

Raman-based silicon photonics

This paper reviews recent progress in a new branch of silicon photonics that exploits Raman scattering as a practical and elegant approach for realizing active photonic devices in pure silicon. The large Raman gain in the material, enhanced by the tight optical confinement in Si/SiO2 heterostructures, has enabled the demonstration of the first optical amplifiers and lasers in silicon. Wavelength conversion, between the technologically important wavelength bands of 1300 and 1500 nm, has also been demonstrated through Raman four wave mixing. Since carrier generation through two photon absorption is omnipresent in semiconductors, carrier lifetime is the single most important parameter affecting the performance of silicon Raman devices. A desired reduction in lifetime is attained by reducing the lateral dimensions of the optical waveguide, and by actively removing the carriers with a reverse biased diode. An integrated diode also offers the ability to electrically modulate the optical gain, a unique property not available in fiber Raman devices. Germanium-silicon alloys and superlattices offer the possibility of engineering the otherwise rigid spectrum of Raman in silicon.     

Polymeric optical waveguide films for short-distance opticalinterconnects

We describe three different applications of polymeric waveguide films as short-distance optical interconnects. We fabricated the waveguide films, which were 6.5 cm long and mounted in MT-compatible (MTC) connectors by passive alignment, for MM fiber systems with a 50-μm diameter graded index (GI) core. The average insertion loss of these devices was approximately 0.6 dB at 0.85-μm wavelength. We also fabricated waveguide films with a 350 mirror and an MTC connector for use as 90° out-of-plane optical deflectors, and they exhibited an insertion loss of 1 dB. Two silica planar waveguides for single-mode (SM) fiber systems were also connected by a polymeric waveguide film. Low insertion losses were obtained in both MM and SM films designed to be employed as bending waveguides. This reveals their good potential for use as practical short-distance optical interconnects     

Fabrication of infrared photodetectors

Abstract

This paper presents the fabrication of an integrated optoelectronic circuit consisting of a waveguide and photodetector. Fabrication of the waveguides took place in a RIBE system with 1·5 sccm CH₄/H₂ (60:40) and 0·5 sccm Ar. Wet etching defines the photodetector regions. The detection of surface damage is minimal, using a novel differential optical reflectance technique.     

Tapered rib adiabatic following fiber couplers in etched GaAsmaterials for monolithic spot-size transformation

Design details and demonstration data are presented for an (Al,Ga)As monolithic tapered rib waveguide achieving modal spot-size transformation. The tapered rib adiabatic following fiber coupler structure (TRAFFIC) achieves two-dimensional (2-D) expansion of the output optical mode of single-transverse-mode semiconductor waveguide modulators and lasers using a one-dimensional (1-D) taper between noncritical initial and final taper widths which are compatible with optical lithographic techniques. Measurements are presented of total mode expansion losses between ~1.5-2.0 dB and semiconductor to single-mode-fiber waveguide coupling losses of ~0.5-1.0 dB for doped pin optical-modulator-type waveguides using the TRAFFIC waveguide. A semiconductor laser with a TRAFFIC tapered-rib mode-expansion section and measured coupling loss between the laser output and single-mode fiber of only 0.9 dB is described. Finally, a TRAFFIC Spot-size transformer for undoped waveguide modulators with total mode expansion losses of 1.84 dB and excellent modal behavior at 1.32-μm wavelength is presented. The TRAFFiC structure is particularly well suited for integration with both active and passive etched rib waveguide devices. Fabrication is relatively simple, requiring only patterning and etching of the tapered waveguide and uniform-width outer mesa waveguide without any epitaxial regrowth     

Self-alignment of optical fibers with optical quality end-polishedsilicon rib waveguides using wet chemical micromachining techniques

This paper presents a new cost-effective method for self-aligning optical fibers on silicon platforms and for achieving optical quality end-polished silicon-on-insulator (SOI) rib waveguide devices using wet chemical micromachining techniques. Through accurate alignment to the (011) plane of the (100) device layer of a SOI wafer, rib waveguide devices with self-alignment features are fabricated with the ends of each waveguide wet etched and concurrently polished providing an optical quality facet or fiber-to-waveguide interface. Eliminating the need to saw cut and then mechanically polish the ends of fabricated devices, the overall fabrication process is simplified whilst also providing an integrated optic fiber alignment capability at the ends of the fabricated waveguide devices with an alignment accuracy limited by fiber size tolerance. Experimental measurements were carried out to verify the optical quality of the waveguide facets formed using this new technique which proved excess facet losses of practically unmeasurable quantities     

Grating-assisted codirectional coupler filter using electroopticand passive polymer waveguides

We propose and demonstrate a band-rejection filter based on grating-assisted forward coupling between an electrooptic (EO) polymer waveguide and a passive polymer waveguide. The passive polymer waveguide is used as a low-loss optical path, and the EO polymer waveguide is used for dropping the optical signal at a given wavelength. The grating is etched on top of the EO polymer waveguide, which satisfies the phase-matching condition for the power exchange between the two asynchronous waveguides. Using the grating with a period of 63.4 μm, we achieve significant power exchange between the two waveguides with a 3-dB bandwidth of 4.4 nm at a wavelength of 1533 nm. Next, we demonstrate that photobleaching may be effectively used as a permanent tuning method of the center wavelength in fabricating the proposed device. The center wavelength for the maximum coupling can be adjusted in a range of more than 25 nm by photobleaching without extra loss. Lastly, we investigate changes of the filter characteristics induced by poling and demonstrated wavelength tuning by the poling-induced EO effect     

Silicon oxynitride planar waveguiding structures for application inoptical communication

The refractive index of silicon oxynitride (SiON), a widely used material for integrated optics devices, can be chosen in a wide range between 1.45-2.0. We describe how the consequent large design freedom can be exploited on the one hand for a “standard” polarization independent optical channel waveguide having a favorable tradeoff between efficient fiberchip coupling and small bend radii (compact devices) and on the other hand for special-purpose and hybrid components where the refractive index should be finely adjusted for obtaining the desired functionality. We illustrate the applicability of SiON by describing a few devices for optical filtering in a new architecture for wavelength multiplexing, modulation, polarization splitting and second-harmonic generation     

O/E Integration of Polymer Waveguide Devices by Using Replication Technology

The simple optoelectronic integration of polymer-based optical waveguide devices and the development of the realization processes have been critical issues for cost-effective, high-volume manufacturing of a next-generation optoelectronic integrated circuit (OEIC). We demonstrated the replication technology as a means of implementing the polymer microoptoelectromechanical system (MOEMS)-based packaging structure providing the optical/electrical (O/E) integration of the functional polymer waveguide device. To achieve this, a micromechanical packaging structure consisting of an electric-circuit-embedded polymer optical bench and planar-lightwave-circuit (PLC)-type waveguide chip with alignment microstructure was designed, and the realization process incorporating the UV imprint technique was investigated. To improve optical coupling efficiency, the electric circuit was embedded under the optical bench and the contact pads were opened at the bottom of the alignment pits. In addition, a conductive adhesive-fill space was created at the alignment pits to accommodate the surplus conductive adhesive. Efficient fiber-chip coupling and good electrical contact of upside-down mounted single-mode waveguide chip was accomplished by the simple joining of the electric-circuit patterned micropedestals on the waveguide chip and the alignment pits on the bench. A coupling loss of 0.9 dB per coupling face was measured with a single-mode fiber at a wavelength of 1.5 mum. It was concluded that the replication technology has versatile application capabilities in manufacturing next generation optical interconnect systems     

The application of porous silicon to optical waveguiding technology

The porosification of silicon can be achieved by the partial electrochemical dissolution (anodization) of the surface of a silicon wafer. The degree of porosity is dependent on the anodization parameters and can generally be controlled within the constraints imposed by substrate dopant type and concentration. Control of porosity leads to control of refractive index, and therein lies the concept of using porous silicon as an optical waveguide. We discuss porous silicon wavegides, for the visible to the infrared, produced by a number of approaches: 1) epitaxial growth onto porous silicon (where the porous layer acts as a substrate for a higher refractive index waveguide epilayer); 2) ion implantation (where either selective areas of high electrical resistivity can be produced, which act as a barrier against porosification, or where the surface of a porosified layer is amorphised to form a waveguide; 3) porous silicon multilayers (where the anodization parameters are periodically varied to produce alternate layers of different porosity and thus refractive index); and 4) oxidation of porous silicon (where a porosified layer is oxidized to form a graded-index, dense or porous, oxide waveguide)     

Photonic Bandgap Microcavities With Flat-Top Response

In this paper, the analysis and the design of a photonic bandgap (PBG) microcavity waveguide filter with flat-top spectral characteristics are theoretically presented. The split filter analysis and the bidirectional eigenmode propagation method are applied to the investigation of this waveguide structure to obtain the design parameters of the filter with nearly unity transmittance and a squared passband. It shows that the radiation loss in the PBG mirrors diminishes the transmittance of the filter, and a mirror with anomalous dispersion region in reflection phase could be used to assemble an optical filter with the flat-top passband. A 1-D PBG monorail waveguide microcavity filter centered at 1554 nm with a maximum transmission of -0.55 dB and full-width at half-maximum (FWHM) of 13.6 nm is numerically carried out by using 3-D finite-difference time-domain method     

High-power low-divergence semiconductor lasers for GaAs-based980-nm and InP-based 1550-nm applications

High-power diode lasers with low-vertical divergence and high-fiber coupling efficiency were developed for GaAs-based 980-nm pump lasers and InP-based 1550-nm Fabry-Perot and distributed-feedback (DFB) lasers. Narrow divergence at 980 nm was made possible by a large optical-mode waveguide design, with full-width at half-maximum (FWHM) far-field angles of 11.7°×17.8° and coupling efficiency of 80% into a cleaved single-mode fiber (SMF). A vertical taper processing technique was developed for InP-based laser structures. Fabry-Perot lasers produced over 90-mW output power, 17°×16° FWHM beam divergence angles, and 63% coupling efficiency into a lensed SMF. The vertical taper was successfully integrated in 1550-nm DFB lasers, and over 80 mW single-mode output power with beam divergence angles of 12°×14° was obtained     

Self-Aligned Single-Mode Polymer Waveguide Interconnections for Efficient Chip-to-Chip Optical Coupling

Single-mode (SM) ultrashort optical interconnections between the fibers and waveguides using self-forming polymeric waveguides with low optical losses at 1300 and 1550 nm were demonstrated. The localized refractive index in the SM regime is estimated by measuring the surface topography induced by monomer diffusion during the waveguide formation. A loss less than$-1$dB can be obtained from self-aligning SM-to-multimode (MM) fibers and SM-to-SM fibers interconnections, respectively. A self-formed waveguide-to-fiber interconnection is fabricated and measured with loss less than 0.2 dB at 1550 nm. The polymer waveguide relaxes the positioning requirements for single-mode chip-to-chip optical interconnections, showing great potential to improve the short-term yield and long-term reliability.     

APPLICATION OF CONFORMAL MAPPING IN MODELLING THE RESPONSE OF AN MSM PHOTODETECTOR

In this paper we present an efficient method for numerical solution of the transport equation of photogenerated carriers in MSM photodetectors. The method is based on conformal mapping of both the MSM photodetector structure and the system of transport equations into a new space. The mapping enables analytical calculation of the electric field, allowing one to take one of the co-ordinates as a parameter and thus reducing the problem to quasi-one-dimensional. In addition, the method of conformal mapping enables calculation of the normalized current response of a photodetector in the new space. © 1997 by John Wiley & Sons, Ltd.     

High-Speed Miniaturized Photodiode and Parallel-Fed Traveling-Wave Photodetectors Based on InP

A periodic parallel-fed traveling-wave photodetector (TWPD) with monolithically integrated power splitter is presented. The device exhibits an impedance match to the 50-Omega environment as well as a velocity match between the optical and electrical signals and shows a 3-dB bandwidth up to 85 GHz. The electrical output power reaches +10 dBm at 10 GHz. The device is based on evanescently coupled low-capacitance waveguide-integrated p-i-n photodiodes (PDs) with optimized optical matching layer which reveal a bandwidth of 145 GHz when employed as a stand-alone photodetector     

Electrooptical Modulating Device Based on a CMOS-Compatible -Si:H/ -SiCN Multistack Waveguide

In this paper, we report results on a field-effect-induced light modulation at $lambda$ $=$ 1.55 $mu$m in a high-index-contrast waveguide based on a multisilicon-on-insulator platform. The device is realized with the hydrogenated amorphous silicon ($alpha$ -Si:H) technology, and it is suitable for monolithic integration in a CMOS IC. The device exploits the free-carrier optical absorption electrically induced in the semiconductor core waveguide. The amorphous silicon waveguiding layer contains several thin dielectric films of amorphous silicon carbonitride ($alpha$ -SiCN) embedded along its thickness, thus highly enhancing the absorbing action of the modulator held in the on state.      

Novel on-chip fully monolithic integration of GaAs devices with completely fabricated Si CMOS circuits

We monolithically integrated polycrystalline GaAs metal-semiconductor-metal (MSM) photoconductive switches with a completely fabricated Si-CMOS amplifier and obtained a properly functional optical receiver, without altering the Si circuit performance. To our knowledge, this is the first time a fully monolithic on-chip integration has been achieved.     

Silicon micromachined hollow optical waveguides for sensingapplications

Novel micromachined optical waveguides useful for sensing applications are proposed. The waveguide is designed as hollow-core antiresonant reflecting optical waveguide (ARROW) and can be easily fabricated using standard silicon micromachining techniques. The hollow structure permits to use the core to confine simultaneously the light and the substance to be probed, leading to an increase of the interaction efficiency. Numerical simulations, performed using finite element method technique, show that with a suitable design these waveguides can be used in sensing applications, where the substances under test can be gases or liquids