Susan J. Walker

Two-dimensional array of diffractive microlenses fabricated by thin film deposition.

A 2-D array of 10 x 10 diffractive lenslets was fabricated and tested. Each lenslet has a rectangular aperture and a size of 1.5 mm x 1.5 mm. The focal length of each lenslet is 47 mm. The array was produced by depositing thin films of silicon monoxide on a quartz glass substrate and by using photolithographic techniques. The performance of the lenslets is based on the diffraction of light at a Fresnel zone plate (FZP). The FZP pattern was implemented as a phase structure with eight discrete levels. The diffraction efficiency was measured to be 91%.

Dammann Gratings For Laser Beam Shaping

Dammann gratings are binary phase gratings that are used to produce a one- or two-dimensional array of equal-intensity light spots. Recently, Dammann gratings have become of interest for their ability to provide the optical power supply to an array of optical logic devices from a single laser source. We present here a feasibility study of Dammann gratings, in which we consider several parameters that are important for the computation and fabrication of the gratings. As a result of this investigation we conclude that Dammann gratings are useful for array sizes up to about 40 x 40 spots. Above that size, problems with the computation of the gratings and with the resolution become dominant.

Prototype experience for MIMO BLAST over third-generation wireless system

In this paper, a multiple-input-multiple-output (MIMO) extension for a third-generation (3G) wireless system is described. The integration of MIMO concepts within the existing UMTS standard and the associated space-time RAKE receiver are explained. An analysis is followed by a description of an actual experimental MIMO transmitter and receiver architecture, both realized on digital signal processors (DSPs) and FPGAs within a precommercial OneBTS base station. It uses four transmit and four receive antennas to achieve downlink data rates up to 1 Mb/s per user with a spreading factor of 32 and the UMTS chip rate of 3.84 MHz. Furthermore, different MIMO detectors are evaluated, comparing their performance and complexity. System performance is evaluated through simulations and indoor over-the-air measurements. Capacity and bit-error rate measurement results are presented.

Array generation with multilevel phase gratings

The efficiency of phase gratings used for array illuminators can be improved by increasing the number of phase levels in computer-generated diffraction gratings. This is of interest to provide power to two-dimensional arrays of optical logic devices that are used for optical computing purposes. The theory and fabrication techniques are described, and the experimental performance of a four-level grating that produces a 5 × 5 array is presented.

Optical clock distribution using integrated free-space optics

Abstract The design and experimental results of an optical clock distribution system based on integrated free-space micro-optics are reported. Planar optical components such as lenses, beamsplitters, and mirrors, are monolithically integrated on a single glass substrate to provide a stable and compact system. A single input beam is split and distributed evenly to N output positions using a binary tree of beamsplitters. The experiment shown demonstrates the principle idea for a system with a fanout of eight. Theoretical considerations show that a fanout of 64 or larger is feasible.

Module for optical logic circuits using symmetric self-electrooptic effect devices

An optical module designed to perform cascadable optical logic using arrays of symmetric self-electrooptic effect devices (S-SEEDs) is described. The operation of an array of 7 x 3 devices with optical windows spaced by 20 microm is demonstrated including both array preset and individual device switching. The issues leading to the design of this optical system are detailed. This work illustrates some of the issues which must be considered when designing systems using small reflecting electrooptic devices such as SEEDs and free-space optics in digital systems.

Compressed Transport of Baseband Signals in Radio Access Networks

In current wireless base station solutions it is becoming common to physically separate baseband units and radio subsystems. In many wireless technologies this architecture requires allocation of significant transport network resources. In this paper a low-latency baseband signal compression scheme is presented. The compression scheme significantly lowers the transport data rate while maintaining low levels of signal distortion, thus resulting in a lower-cost transport network. Considering the importance of packet-based networks, a number of additional novel compression schemes are proposed. They are optimized for transport networks that implement a quality-of-service (QoS) mechanism and/or multi-link transmission. The compression schemes are parameterized such that a smooth trade-off between the required signal quality and compression performance can be achieved through operator choice of the suitable parameter values. An attractive feature of these schemes is that they can be applied to different wireless technologies, with appropriate parameter settings, without disrupting the present architecture. The proposed solutions will lead to a cost-effective implementation of collocated and distributed network-centric baseband processing, coordinated multi-point (CoMP) and/or distributed antenna system (DAS) which are critical topics for the entire wireless telecommunications industry and infrastructure.

Design and fabrication of high-efficiency beam splitters and beam deflectors for integrated planar micro-optic systems

High-frequency gratings with rectangular-groove profiles are used to generate high-efficiency beam splitters and beam deflectors. The effects of the grating design parameters, i.e., period, groove depth, duty cycle, number of phase levels, and polarization state (TE and TM) of the incoming signal, are considered. The case of the binary beam splitter grating is analyzed by using rigorous electromagnetic grating analysis. Fabrication techniques are presented in which three different lithographic techniques are considered (optical contact, deep-UV stepper reduction, and electron-beam direct write). Experimental results of 97% efficiency for the beam splitter grating and up to 80% for the beam deflector grating are reported.

Electrodispersive multiple quantum well modulator

The electric-field dependence of optical absorption has been studied extensively for bulk and multiple-quantum-well(MQW) semiconductors. In bulk semiconductors, it is known as the Franz-Keldysh effect. More recently, in MQW semiconductors, it is called the quantum confined Stark effect (QCSE)[1] and turns out to be much larger than in bulk semiconductors. Main consequences of the QCSE are broadening and red-shift of exciton absorption peaks. Direct modulation of optical absorption by the QCSE leads to an electro-optic modulator which has demonstrated fast optical modulation[2]. Major drawbacks of this absorption modulator are poor contrast and/or low absolute transmission. Moreover, since the wavelength of the light is usually close to an exciton peak, there is always strong absorption. This limits the maximum optical intensity to be handled by the modulator to a level below that of exciton saturation.

Array generation with lenslet arrays.

Lenslet arrays can be used as phase gratings, having many diffraction orders with equal intensity. Applications are multiple imaging and illumination of arrays of optical or optoelectronic devices in digital optics. The homogeneity of the intensities within the array can be improved by using field lenslets. The basic theory as well as experiments with diffractive and with graded index lenses are shown.