In this paper, we demonstrate the feasibility of using on-chip optoelectronics within VLSI systems to address a wide range of signal distribution issues by examining the following fundamental question: how can we transmit information from one source to many destinations while minimizing propagation delay, skew, jitter, and noise in a way that is compatible with low-cost manufacturing and CMOS circuits? Example systems with such information distribution requirements include banked arrayable memories such as a DRAM or a dense imager with scanned high speed readout, or a clock distribution system. In all instances individual lines are typically connnected to thousands of gates, slowing cell access times and generating skew. We demonstrate how the use of on-chip photonics within VLSI systems can reduce delays introduced by electrical wires in system-on-a-chip interconnects, busses, caches, and control lines at distances shorter than one meter and as short as a few millimeters. We also describe and demonstrate how a simple on-chip optoelectronic system addressing these problems can be realized at low cost, with monolithic photodetectors and on-chip waveguides in a commercial CMOS process, benefiting both ultra-short and one meter link architectures. This unexplored signal distribution architecture promises high optical to electrical efficiency, low noise, and the benefits of monolithic photodetection not previously achieved in existing approaches.
We show high Raman gain in Silicon submicron-size strip waveguide. Using high confinement structures and pico-second pump pulses, we show 13.2-dB peak gain with 14.6-W peak pump power in a 7-mm long waveguide. The effect of free-carrier absorption is observed. We show a pico-second all-optical switch based on the Silicon waveguide, whose transmission is enhanced by the Raman gain.
We have studied tapers that couple light from a conventional ridge
waveguide into a planar photonic crystal (PhC) waveguide. Tapering
is achieved by changing the PhC waveguide width either in steps or
gradually. Lag effects in fabrication provide an additional
tapering due to the fact that the hole depths scale with the
corresponding hole diameter. Our analysis deals with the
out-of-plane loss that arises within such taper sections. The PhC
consists of a triangular lattice of air holes introduced into an
InGaAsP/InP slab structure. For conceptual studies we use the 2D
multiple multipole method (MMP) in conjunction with an extended
phenomenological model. This model covers the out-of-plane
scattering providing a loss parameter and an effective index
correction for the holes under consideration. This realistic 2D
model is retrieved from full-wave 3D FDTD simulations and
Wave propagation in Photonic Crystal Fibers is known for revealing very strong chromatic dispersion characteristics despite the waves being confined in the direction along the fiber axes. 2D and 3D photonic crystal devices offer even more degrees of freedom, as the angular dispersion of the propagating waves can be additionally included into the design. We present the basic principles which can be used for tailoring the overall dispersion characteristics of a single photonic crystal device and of combinations of different structures. Precise calculations and simulations are required in order to obtain reliable information on the dispersion parameters including also higher-order terms. Thus our calculations are based on the propagation of Gaussian beams.
We show that the use of tunable photonic structures opens up unprecedented possibilities in information processing. In particular, we introduce an all-optical adiabatic bandwidth compression and frequency conversion process that overcomes the classical bandwidth-delay constraint in optics. This process requires only very small index modulations (δn/n<10-4) performed at moderate speeds, and yet can alter the spectrum of a photon almost at will. As examples of this process, we show how light pulses can be stopped and stored all-optically in multitude of systems without using any resonant or coherent electronic interactions. We also show how light pulses can be time-reversed using only linear optical elements and modulators. Such systems open up new opportunities in both fundamental sciences and technological applications.
Theoretical and experimental results on ultra-fast all-optical switches based on intersubband transitions for Tb/s operation are presented. Designs for engineering intersubband transitions (ISBT) in GaN/AlN quantum wells near communication wavelengths (~1.55 μm) and for realizing all-optical switches requiring small pulse energies are discussed. Optimized designs show all-optical switching at Tb/s data rates with pulse energies as small as 200 fJ. Experimental realization of narrow line-width ISBT in GaN/AlN superlattices is also demonstrated.
Photonic crystals enable a reduction in the size of current photonic devices by virtue of forbidden propagation, except along engineered lines of defects. Furthermore, propagation above the band-gap has unique characteristics such as the superprism effect. Polymer materials which typically suffer from low optical confinement can benefit from photonic crystal structures to increase integration and functionality. Due to its unique advantages, several authors have reported attempts at fabricating photonic crystal structures in polymer materials. However, a clear photonic bandgap (PBG) was not demonstrated. In this paper we describe our recent work in design, simulation and fabrication polymer photonics devices. We will discuss specific slab photonic crystal devices based on 2D hexagonally packed structures achieved in polymethyl-methacrylate films. Supercomputer simulations were used to target optimal geometries that consist of points in a three dimensional space of lattice parameter, hole diameter and slab thickness that enable a design of the photonic bandgap of the structure. Fabrication of the devices was achieved through use of high-resolution electron-beam lithography and etching. A robust air-clad polymer photonic crystal film was enabled by the additional support of a 40 nm-thin low-stress silicon nitride layer.
The "HiPhoCs" program, a cluster of projects supported by the German
Ministry of Education and Research, is focused on the proof of
applicability of planar photonic crystals based on slab waveguides
within telecom transmission lines and optical networks. Results of
the "HiPhoCs" program on modeling and etching aiming at the optimization of device oriented structures in the III-V and SOI material systems are reported as well as their application to key components such as PhC-based WDM-filters, dispersion compensators,
integrated lasers and their coupling to the optical fiber infrastructure.
The high angular dispersion achieved with the photonic crystal superprism effect as well as dispersive non-periodic photonic nanostructures promise compact wavelength division multiplexing (WDM) devices. An important criterion for the usefulness of such WDM devices is the number of channels that a structure can multiplex or demultiplex. Here two different models are developed for calculating the possible number of channels for a given structure. The first model is based on the assumption that different wavelength channels should propagate in mutually exclusive propagation cones within the volume of the dispersive structure. We call these non-overlapping channels "volume modes." The second model assumes that it is sufficient to spatially separate the different wavelength channels on a single output surface, e.g., the plane of the detectors or output waveguides. Since they are only separated along one surface and not in the entire volume, we name these modes "surface modes." As an example it is shown that a dispersive 200-layer non-periodic thin-film stack can be used to multiplex or demultiplex approximately eight WDM channels. The achievable number of channels is not defined by the dispersion alone but by the product of dispersion and wavelength range over which this dispersion is achieved.
We have investigated tunable photonic crystal waveguide lasers and their integration with a photonic crystal based combiner. The devices were fabricated on a layer structure with active and passive sections. The lasers are formed in the active part and consist of two longitudinally coupled photonic crystal waveguide segments. Photonic crystals based waveguides and a combiner, both defined in the passive sections, are used to direct the light of the two sources into a single output waveguide. Tuning over a 30 nm range is demonstrated. The two sources can be operated independently, allowing the simutaneous transmission of two freely selectable wavelengths.
This talk focuses on the high frequency characteristics of red VCSELs. After a short description of important fabrication issues the modulation behaviour of GaInP surface emitting lasers is discussed on the basis of the laser rate equations. The influence of the geometric dimensions of the laser structure and of the operating conditions is investigated. From the S-parameter analysis a modulation coefficient of 3 GHz/(mA)1/2 for VCSELs with a 7 µm aperture and a differential gain of 1.15•10-16 cm2 are deduced. A more detailed analysis reveals, that the modulation behaviour of red VCSELs nearly solely depends on their photon density inside the quantum wells as expected from the rate equations. These results imply that for a certain range of geometries diffusion and diffraction have a second order influence on the high frequency characteristics of red VCSELs. The K-factor analysis indicates very short carrier transfer and relaxation times around 5 ps and a maximum frequency of 25 GHz. Large signal modulation issues such as the properties of the eye diagram are also addressed. From the device characteristics it is concluded that the GaInP-VCSEL is suitable for data communication applications. Low cost fabrication makes the red VCSEL an attractive candidate for both automotive and high-speed data communication.
The photonic crystal (PC) coupler and planar photonic band gap (PBG) coupler were theoretically investigated. Also the conventional step-index coupler was investigated for comparison with these couplers. The PC-coupler is a structure formed by the gradual tapering of the photonic-crystal holey fiber and the conventional step-index coupler -- by the gradual tapering of the conventional fiber. The planar PBG-coupler is a planar PC waveguide, in which defect region has taper-like geometry. The photonic crystal couplers are characterized by alternation of the material refractive index along the wave propagation direction, i. e. along the optical axis of the taper. In our investigation an effective refractive index model for two-dimensional photonic crystal was used. This model allows to analyze the waveguide structures, which work on the effective index waveguiding in a defect of the photonic crystal. The optical field in photonic crystal couplers, PBG couplers and conventional couplers was investigated. Also losses and mode shape distortion of all types of the couplers were calculated.
We use evolutionary algorithms to search the space of two-dimensional photonic crystals with maximum bandgaps for a given index contrast. The unit-cells of the photonic crystal are represented as bitmaps which are either directly encoded or generated through bottom-up and top-down construction trees. The fitness criterion rewards for partial band gaps and is evaluated by solving for the photonic crystals bands using an eigensolver in a planewave basis. Starting from random patterns and with no prior knowledge, the process discovered a number of novel photonic crystals, some with bandgaps that are 12.5% larger than any previously reported human-designed crystals.
Photonic band gap (PBG) structures or photonic crystals have attracted a lot of interest since one of their promising applications is to build compact photonic integrated circuits (PIC). One of key components in PICs is a 1 x 2 optical power splitter or a 2 x 1 combiner. Design of 1 x 2 optical power splitters based on photonic crystal has been investigated by several research groups, but no attention has been paid to the design of 2 x 1 optical combiners. In conventional dielectric waveguide based circuits, optical combiners are obtained just by operating the splitters in the opposite direction and the isolation between two input ports in the combiners is naturally achieved. In photonic crystal based circuits, however, we have found that reciprocal operation of the splitters as combiners will not provide proper isolation between the input ports of the combiners. In this work, microwave-circuit concept has been adopted to obtain isolation between two input ports of the combiner and compact optical power splitters/combiners of good performance have been designed using 2-D photonic crystal. Numerical analysis of the designed splitters/combiners has been performed with the finite-difference time-domain method. The designed splitters/combiners show good isolation between input ports in combiner operation with small return losses.
Although the success of demonstrating tellurite glass as a waveguide material in many applications, including tellurite fiberization, Nd3+-doped tellurite fiber laser, and 1.5 μm ultra-broad band Er3+-doped optical amplifier, the advance of tellurite is still necessary in the areas of improving the quality of waveguide and understanding the correlations among processing, structure, and desired property, such as nonlinearity, rare-earth spectroscopy, nanocrystalline doping, and microstructured holey fiber. In the paper, we report some initial experimental results on fiberization processing of KNbO3-Na2O-ZnO-TeO2 and Er2O3-WO3-TeO2 systems. The report, in particular, focuses on the thermal characteristics of these glasses.
There is strong need for low cost, optically active materials whose high electro-optic (EO) and second harmonic generation (SHG) properties can be engineered flexibly, in bulk and fiber forms. Therefore, we have fabricated transparent ferroelectric composites consisting of strontium barium niobate crystallites in a refractive index compatible tellurium oxide (TeO2) glass matrix. Several glass compositions, in the series x SrO-(10-x) BaO-y (Nb2O5)-(90-y) TeO2 (where x=2.5, 5 and 7.5 and y = 10, 15, 20 and 25), have been prepared by a conventional melt quenching technique. The compositions have been selected on the basis of thermal stability data obtained from differential thermal analysis (DTA). X-ray diffraction studies indicate ferroelectric phase formation in the controlled crystallized glasses. The non-centrosymmetric nature of the crystallized regions has been monitored via observing the second harmonic signal.
Planar integrated photonic devices are typically designed for telecommunications wavelengths in the 1.55 micron range. For strong mode-confinement at these wavelengths, very high index contrasts are required and semiconductor materials are often used for the waveguide core. Recently, planar devices designed for the visible range were demonstrated with relatively large dimensions on the order of 0.5 - 5 mm. Here in contrast we demonstrate micron-size photonic devices with single-mode operation in the visible range. Devices made for light propagation in the visible range are designed for tapping specific wavelengths of light vertically out of the plane of integration. The structures are based on high confinement waveguides with an effective mode size on the order of 0.5 μm2.