Large deflection micromechanical scanning mirrors for linear scansand pattern generation

An electrostatically driven silicon micro scanning mirror (MSM) for one-dimensional (1-D) and two-dimensional (2-D) deflection of light is presented. A special configuration of the driving electrodes allows the use of small electrode gaps without restricting the deflection of the plate geometrically. In this paper, the starting of the oscillation and the operation of the scanner is discussed. Experimental results show that scan angles of up to 60° can be achieved at a driving voltage of only 20 V. The 2-D deflection of a laser beam is obtained by a gimbal mounting of the mirror plate. For the fabrication of the devices, SOI-wafers are used as the base material. The mechanical structures are defined by a deep silicon etch. For the electrical isolation of areas on the movable frame, polysilicon-filled trenches are used. The mechanical stability of the scanners is tested. The devices resist shocks of more than 1000 g and show no change of the resonance frequency even after long run tests of 7×10⁹ periods     

A rigorous comparative analysis of directional couplers and multimode Interferometers based on ridge waveguides

We present a rigorous comparison of the unique characteristics of directional couplers and multimode interferometers based on the unique properties of high-index contrast ridge waveguides. The two devices are intimately related as the multimode interference (MMI) is derived from the directional couplers (DCs). We show for the first time the continuous evolution from the two-mode coupling characteristic of DC to the multimode mixing and interference characteristic of MMI, as the DC is structurally transformed into the MMI. We also show that DC can be designed to have the MMI features of compactness and polarization-insensitivity, two traits that reflect their shared lineage. The performance of MMI and DC are compared in terms of coupling length, polarization dependence, crosstalk, excess loss, and fabrication tolerances. We show that the DC, as long as it is designed to have nearly the same coupling length for transverse electric and transverse magnetic, can potentially have better performance than the MMI in terms of crosstalk and polarization sensitivity. Such a DC, however, requires careful control of many design parameters, while the MMI design is more robust and involves fewer design variables. Finally, the effect of higher-order modes and mode filtering are also considered.     

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     

Analysis of Hybrid Dielectric Plasmonic Waveguides

Future ultracompact photonic integrated circuits (PICs) will rely on high-index-contrast dielectric materials, which permit a strong confinement of the optical field in the diffraction limit as well as low propagation losses. This is the case of PICs implemented on a silicon-on-insulator (SOI) platform. To achieve confinement beyond the diffraction limit, plasmonic waveguides (based on metal–dielectric interfaces) have been recently proposed. This new kind of waveguide provides a strong enhancement of the field in the metal–dielectric interface, which is of paramount importance for nonlinear functionalities or sensing. Plasmonic waveguides can also be built on SOI wafers. Thus, it can be reasonably thought that high index contrast as well as plasmonic waveguides can coexist in future ultradense PICs. In this paper, a theoretical and numerical study on the performance of several dielectric and plasmonic waveguides is presented. Thanks to their plasmon-coupled supported modes, ultracompact devices as hybrid ring resonators can be devised and integrated with silicon photonic circuits.      

Microring Resonators Made in Poled and Unpoled Chromophore-Containing Polymers for Optical Communication and Sensors

Poled and unpoled chromophore-containing polymers offer some unique advantages in device functionality and fabrication. UV light and electron beam (e-beam) can bleach out the color of chromophores and reduce the index of refraction of the polymer. The photobleaching and e-beam bleaching methods form optical waveguides in a single step and do not involve solvents or wet chemicals, and can be applied to polymers that are not compatible with other waveguide fabrication techniques. A variety of microring resonator devices for fiber-optic telecommunication and sensors have been realized with chromophore-containing polymers. A novel broadband fiber-optic electric field sensor is presented as an example. The sensor uses a polymer with chromophores preferentially aligned after electric poling, and the microring resonator is directly coupled to the core of optical fiber. The feasibility of vertical integration of a poled electrooptic polymer waveguide device interfaced with silicon microelectronic circuits is also demonstrated.     

Monolithic integration via a universal damage enhanced quantum-wellintermixing technique

A novel technique for quantum-well intermixing is demonstrated, which has proven a reliable means for obtaining postgrowth shifts in the band edge of a wide range of III-V material systems. The technique relies upon the generation of point defects via plasma induced damage during the deposition of sputtered SiO₂, and provides a simple and reliable process for the fabrication of both wavelength tuned lasers and monolithically integrated devices. Wavelength tuned broad area oxide stripe lasers are demonstrated in InGaAs-InAlGaAs, InGaAs-InGaAsP, and GaInP-AlGaInP quantum well systems, and it is shown that low absorption losses are obtained after intermixing. Oxide stripe lasers with integrated slab waveguides have also enabled the production of a narrow single lobed far field (3°) pattern in both InGaAs-InAlGaAs, and GaInP-AlGaInP devices. Extended cavity ridge waveguide lasers operating at 1.5 μm are demonstrated with low loss (α=4.1 cm^-1) waveguides, and it is shown that this loss is limited only by free carrier absorption in waveguide cladding layers. In addition, the operation of intermixed multimode interference couplers is demonstrated, where four GaAs-AlGaAs laser amplifiers are monolithically integrated to produce high output powers of 180 mW in a single fundamental mode. The results illustrate that the technique can routinely be used to fabricate low-loss optical interconnects and offers a very promising route toward photonic integration     

The integration of III-V optoelectronics with silicon circuitry

The integration of III-V optoelectronics with silicon circuitry provides the potential for fabricating dense parallel optical interconnects with data links capable of Terabit aggregate data rates. This paper reviews many of the current approaches used for the fabrication of integrated optoelectronic devices and then highlights the performance results. Finally, the applied method is reviewed in greater detail with recent results on VCSEL, MESFET, and photodiode integration presented     

Physical Mechanism of p-i-n-Diode-Based Photonic Crystal Silicon Electrooptic Modulators for Gigahertz Operation

In this paper, the physical mechanism governing the optical modulation in a p-i-n-diode-embedded photonic crystal (PC) silicon Mach-Zehnder interferometer modulator is examined. Optical simulations have been performed to study how the slow group velocity of the photonic crystal waveguides enables a significant reduction of device size. The theoretical speed limitation in a PC-based silicon modulator is also explored. The 2-D semiconductor device simulator MEDICI has been employed to analyze the transient behavior of the p-i-n-diode-embedded silicon modulator. Electrical simulations have revealed a significant improvement in modulation speed upon the enhancement of current density in a downscaled PC device. High-speed optical modulation at 1 Gmiddots^-1 has been experimentally demonstrated. The performance degradation in optical modulation at the low-frequency operation region attributed to the thermooptic effect is identified and discussed. Simulations have also revealed that the modulation speed of our device can be improved up to 10 GHz by further reducing the device dimensions with little penalty of the increased optical loss.     

3-D integration of RF circuits using Si micromachining

Micromachined silicon integrated circuits have the potential for providing an overarching circuit integration technology that can significantly reduce the size, weight, and cost of microwave and millimeter-wave components. The capability to integrate diverse substrate technologies opens the door for real multifunction chips, combining analog, digital, RF,and optoelectronic functions. This natural approach to three-dimensional (3-D) vertical integration not only can provide higher density circuits, but, by freeing RF circuit design from the tyranny of the two-dimensional (2-D) layout, can reach levels of performance not possible in a planar geometry. This article focuses on the concept of 3-D circuit integration using silicon (Si) bulk micromachining; however, similar techniques can be applied in any other III-V substrate material. Packaging issues prerequisite for the 3-D integration and component development that led to the capabilities for 3-D integration are discussed. The integration techniques are applied to a 3-D integrated W-band power cube, which provides a vehicle for successfully demonstrating the concept and basic techniques for 3-D integration. A concept study is presented of the use of micromachining to integrate Ka-band 2-D and 3-D corporate power combining architectures     

Silicon-on-Insulator Spectral Filters Fabricated With CMOS Technology

We give an overview of recent progress in passive spectral filters and demultiplexers based on silicon-on-insulator photonic wire waveguides: ring resonators, interferometers, arrayed waveguide gratings, and echelle diffraction gratings, all benefit from the high-index contrast possible with silicon photonics. We show how the current generation of devices has improved crosstalk levels, insertion loss, and uniformity due to an improved fabrication process based on 193 nm lithography.      

Novel waveguide structures for enhanced fiber grating devices

An emerging class of fiber waveguide structures is being used to increase the functionality of fiber gratings, enabling new devices critical to the performance of next generation light-wave communications systems. These devices rely on advances in the fabrication of optical fiber waveguides, which go beyond the conventional doped silica design and fall into two general categories: 1) local modifications to the waveguide after fabrication and 2) fibers drawn with modified claddings that include nonsilica regions throughout their length. This paper provides a comprehensive review of emerging fiber waveguide structures that enhance the functionality of optical fiber grating devices. Two examples of technologies that fall into the first category are thin metal films deposited onto the cladding surface, which can be used for thermal tuning and infusion of nonsilica materials into the air regions, which change the waveguide structure and can provide enhanced tunability. The second category is typified by air-silica microstructured optical fibers, which contain air-voids that run along the length of the fiber. These fibers have unique cladding mode properties that can be exploited in fiber grating based devices     

Intimate monolithic integration of chip-scale photonic circuits

In this paper, we introduce a robust monolithic integration technique for fabricating photonic integrated circuits comprising optoelectronic devices (e.g., surface-illuminated photodetectors, waveguide quantum-well modulators, etc.) that are made of completely separate epitaxial structures and possibly reside at different locations across the wafer as necessary. Our technique is based on the combination of multiple crystal growth steps, judicious placement of epitaxial etch-stop layers, a carefully designed etch sequence, and self-planarization and passivation steps to compactly integrate optoelectronic devices. This multigrowth integration technique is broadly applicable to most III-V materials and can be exploited to fabricate sophisticated, highly integrated, multifunctional photonic integrated circuits on a single substrate. As a successful demonstration of this technique, we describe integrated photonic switches that consume only a 300 /spl times/300 /spl mu/m footprint and incorporate InGaAs photodetector mesas and InGaAsP/InP quantum-well modulator waveguides separated by 50 /spl mu/m on an InP substrate. These switches perform electrically-reconfigurable optically-controlled wavelength conversion at multi-Gb/s data rates over the entire center telecommunication wavelength band.     

Dielectric investigation of photoinduced chromophore degradation in nonlinear optical side-chain polymer electrets

Organic materials with noncentrosymmetric chromophores are known to be susceptible to a number of photochemical processes, including reversible isomerization reactions as well as irreversible photooxidation or photoreduction reactions. Reversible isomerization is the basis for a variety of applications, such as photoinduced poling, optical data storage and optical grating formation. The irreversible processes that involve the destruction of the chromophores have been found useful for the fabrication of optical waveguides, but they also limit the life times of polymeric photonic devices. In this paper, it is demonstrated that dielectric measurements allow for an in-depth investigation of nonreversible chromophore degradation processes in a typical side-chain polymer. The time- and temperature-dependent dielectric function of the polymer at 1 kHz enables us to follow the chromophore-degradation kinetics and to monitor the bleaching depth as a function of time at room and elevated temperatures.     

Electrostatically actuated mechanooptical waveguide ON-OFF switchshowing high extinction at a low actuation-voltage

This paper reports on an integrated mechanooptical waveguide ON-OFF switch, where an absorbing element is moved into and out of the evanescent field of the guided mode in order to achieve switching. For the electrostatically driven devices, an extinction ratio of 65 dB at an actuation voltage of 2.5 V has been achieved in a 9.5-mm-long device for a wavelength of 632.8 nm. The calculated mechanical fundamental resonance frequency is 1.95 kHz. The device design enables future vacuum sealing that will annihilate the squeeze damping which, in the present device, reduces the response time to 10 s. Full-wafer scale fabrication is enabled by using standard silicon technology, chemical mechanical polishing and aligned wafer bonding. The devices can he used for channel selection purposes in integrated optical sensor arrays     

Densely arrayed eight-wavelength semiconductor lasers fabricated bymicroarray selective epitaxy

This paper describes a novel epitaxial growth technique, called microarray selective epitaxy (MASE), for fabricating extremely small integrated photonic devices. The MASE technique makes it possible to form densely arrayed (pitch <10 μm) multiple-quantum-well (MQW) waveguides without semiconductor etching as well as to control the bandgap energy of each waveguide. The technique is demonstrated for fabricating an eight-channel 10-μm-spacing microarray MQW structure, and the bandgap wavelength of each channel is successfully controlled by changing the SiO₂ mask pattern over a range of 90 nm. The technique is also applied to the fabrication of densely arrayed, eight-wavelength, Fabry-Perot laser diodes. The laser section is only 70 pm wide and 400 μm long. Eight different lasing wavelengths (each over 80 nm), a uniform threshold current of less than 9 mA, and an output power of over 10 mW are obtained     

Focused ion beam nanopatterning for optoelectronic device fabrication

Recent photonic device structures, including distributed Bragg reflectors (DBRs), one-dimensional (1-D) or two-dimensional (2-D) photonic crystals, and surface plasmon devices, often require nanoscale lithography techniques for their device fabrication. Focused ion beam (FIB) etching has been used as a nanolithographic tool for the creation of these nanostructures. We report the use of FIB etching as a lithographic tool that enables sub-100-nm resolution. The FIB patterning of nanoscale holes on an epitaxially grown GaAs layer is characterized. To eliminate redeposition of sputtered materials during FIB patterning, we have developed a process using a dielectric mask and subsequent dry etching. This approach creates patterns with vertical and smooth sidewalls. A thin titanium layer can be deposited on the dielectric layer to avoid surface charging effects during the FIB process. This FIB nanopatterning technique can be applied to fabricate optoelectronic devices, and we show examples of 1-D gratings in optical fibers for sensing applications, photonic crystal vertical cavity lasers, and photonic crystal defect lasers.     

Optical Characterization of High-Order 1-D Silicon Photonic Crystals

In this paper, we present numerical and experimental results on the spectral reflectivity of hybrid, high-order (up to 22nd) 1-D silicon photonic crystals (PCs) in the near-infrared region (wavelength range 1- 1.7 mum). Mechanically robust, vertical 1-D PCs with high aspect ratio and spatial period of 8 mum were fabricated by electrochemical micromachining of silicon, and tested in reflection with an improved optical setup, incorporating standard telecommunication single-mode optical fibers and a lensed fiber pigtail. A detailed theoretical, numerical analysis was performed to assess the effects of both non-idealities of the structures under test and constraints of the optical setup, on the spectral reflectivity. Experimental data were found in very good agreement with theoretical calculations, performed by using the characteristic matrix method, keeping into account an in-plane porosity variation for 1-D PCs, due to surface roughness of silicon walls, and the limited resolution bandwidth of the spectrum analyzer. Best optical performances, measured on the fabricated 1-D PC mirrors, consist of optical losses less than 0.8 dB in a bandgap around 1.5 mum and a -35 dB reflectivity minimum at a bandgap edge.     

Rare-earth-doped GaN: growth, properties, and fabrication of electroluminescent devices

A review is presented of the fabrication, operation, and applications of rare-earth-doped GaN electroluminescent devices (ELDs). GaN:RE ELDs emit light due to impact excitation of the rare earth (RE) ions by hot carriers followed by radiative RE relaxation. By appropriately choosing the RE dopant, narrow linewidth emission can be obtained at selected wavelengths from the ultraviolet to the infrared. The deposition of GaN:RE layers is carried out by solid-source molecular beam epitaxy, and a plasma N/sub 2/ source. Growth mechanisms and optimization of the GaN layers for RE emission are discussed based on RE concentration, growth temperature, and V/III ratio. The fabrication processes and electrical models for both dc- and ac-biased devices are discussed, along with techniques for multicolor integration. Visible emission at red, green, and blue wavelengths from GaN doped with Eu, Er, and Tm has led to the development of flat-panel display (FPD) devices. The brightness characteristics of thick dielectric EL (TDEL) display devices are reviewed as a function of bias, frequency, and time. High contrast TDEL devices using a black dielectric are presented. The fabrication and operation of FPD prototypes are described. Infrared emission at 1.5 /spl mu/m from GaN:Er ELDs has been applied to optical telecommunications devices. The fabrication of GaN channel waveguides by inductively coupled plasma etching is also reviewed, along with waveguide optical characterization.     

Amorphous silicon-based guided-wave passive and active devices forsilicon integrated optoelectronics

Waveguides and interferometric light amplitude modulators for application at the 1.3- and 1.55-μm fiber communication wavelengths have been fabricated with thin-film hydrogenated amorphous silicon and its related alloys. The technique adopted for the thin-film growth is the plasma- enhanced chemical vapor deposition, which has been shown to give the lowest defect concentration in the film. Consequently the proposed waveguiding structures take advantage of the low optical absorption shown by a-Si:H at photon energies below the energy gap. In addition a good radiation confinement can be obtained thanks to the bandgap tailoring opportunity offered by this simple and inexpensive technology. In particular rib waveguides, based on a a-SiC:H/a-Si:H stack, have been realized on crystal silicon, showing low propagation losses. Recently, however, a new interest as low as 0.7 dB/cm. The same structure has been utilized for the fabrication of thermooptic Fabry-Perot modulators with switching times of 10 μs. Modulators based on the alternative waveguiding configuration ZnO/a-Si:H, giving comparable results, are also presented