We present a new x-ray detection technique based on optical measurement of the effects of x-ray absorption and electron
hole pair creation in a direct band-gap semiconductor. The electron-hole pairs create a frequency dependent shift in optical refractive index and absorption. This is sensed by simultaneously directing an optical carrier beam through the same volume of semiconducting medium that has experienced an xray induced modulation in the electron-hole
population. If the operating wavelength of the optical carrier beam is chosen to be close to the semiconductor band-edge, the optical carrier will be modulated significantly in phase and amplitude.
This approach should be simultaneously capable of very high sensitivity and excellent temporal response, even in the difficult high-energy xray regime. At xray photon energies near 10 keV and higher, we believe that sub-picosecond temporal responses are possible with near single xray photon sensitivity. The approach also allows for the convenient and EMI robust transport of high-bandwidth information via fiber optics. Furthermore, the technology can be scaled to imaging applications. The basic physics of the detector, implementation considerations, and preliminary experimental data are presented and discussed.
At Lawrence Livermore National Laboratory, we have extensive experience with the design and development of miniature photonic systems which require novel packaging schemes. Over the years we have developed silicon micro-optical benches to serve as a stable platform for precision mounting of optical and electronic components. We have developed glass ball lenses that can be fabricated in-situ on the microbench substrate. We have modified commercially available molded plastic fiber ribbon connectors (MT) and added thin film multilayer semiconductor coatings to create potentially low-cost wavelength combiners and wavelength selective filters. We have fabricated both vertical-cavity and in-plane semiconductor lasers and amplifiers, and have packaged these and other components into several miniature photonics systems. For example, we have combined the silicon optical bench with standard electronic packaging techniques and our custom-made wavelength-selective filters to develop a four-wavelength wavelength-division-multiplexing transmitter module mounted in a standard 120-pin ceramic PGA package that couples light from several vertical-cavity-surface-emitting-laser arrays into one multimode fiber-ribbon array. The coupling loss can be as low as 2 dB, and the transmitters can be operated at over 1.25 GHz. While these systems were not designed for biomedical or environmental applications, the concepts and techniques are general and widely applicable.
Optoelectronic component costs are often dominated by the costs of attaching fiber optic pigtails-especially for the case of single transverse mode devices. We present early results of our program in low-cost packaging. We are employing machine vision controlled automated positioning and silicon microbench technology to reduce the costs of optoelectronic components. Our machine vision approach to automated positioning has already attained a positional accuracy of less that 5 microns in less than 5 minutes; accuracies and times are expected to improve significantly as the development progresses. Complementing the machine vision assembly is our manufacturable approach to silicon microbench technology. We will describe our silicon microbench optoelectronic device packages that incorporate built-in heaters for solder bonding reflow.
We will describe research being conducted in the following areas: high-speed, 50 ohm, phased-matched modulators and their applications to digital links; promising new research on flat-panel displays that will be full color, fast response, very thin, and have a very high resolution; all optical switches that are extremely fast, integrable and do not have the latency problems that exist with current optical switches; semiconductor optical amplifiers that are monolithically integrable, more flexible and less expensive than existing fiber amplifiers; novel, semiconductor waveguide devices; and automated packaging techniques that will lower the cost of photonics components.
This paper reports on the operation of lithium niobate electro-optic waveguide modulators at temperatures down to 15 degree(s)K. Commercial and laboratory fiber pigtailed devices have successfully been cooled without any increases in insertion loss from temperature induced stresses in device packaging. Three x-cut devices exhibited a linear increase in Vpi voltage of 8% +/- 1% when cooled from room temperature to approximately 20 degree(s)K. The broadband frequency response improved at lower temperatures. A velocity-matched experimental modulator has shown increased bandwidth when cooled to liquid nitrogen temperature.
High-frequency modulators have been used to record very fast analog voltage pulses in the Nuclear Test Program at LLNL. A system consisting of 810 nm laser diode carrier sources, 7 GHz H-branched, balanced bridge modulators (YBBM), a kilometer of single-mode optical fiber and a streak camera recorder has been successfully fielded. The total system -3 db bandwidth is 4 GHz with a slow roll-off allowing the recoverable data capabilities of the system to approach 10 GHz. This system has provided us with a diagnostic tool that allows us to resolve high-frequency structure in our signals which has been calculated but never measured because of limitations in the resolution of previous measuring systems. A description of the system will be given with particular emphasis given to the YBBM modulator characteristics and performance. Fast pulse data recorded by the system will be shown and an indication of the accuracy with which the pulse can be reproduced will be provided. We expect this system to be a valuable tool that will be used regularly to study the physics of materials at extremely high temperatures and densities. Plans for upgrading the system to have a -3 db bandwidth of 7 - 8 GHz will also be discussed.
An analog fiber-optic data link has been demonstrated for direct charge readout of wire chambers used in high energy particle detectors. The fiber link consists of a Nd:YAG laser carrier, a Mach-Zehnder external modulator, and a low-noise optical receiver. A charge pulse developed on a sensing wire flows directly into the electro-optic modulator with no preamplification. The substitution of passive modulators and fiber cable for active electronics and copper wire near large collider detectors has clear advantages with regard to radiation damage susceptibility, EMI immunity, physical size, and power consumption. Also, the modulator performance was unaffected for an applied one Tesla magnetic field. Reduction of noise contributions from the laser carrier and optical receiver and exploitation of the 'effective optical gain' properties of external modulators resulted in shot-noise-limited link sensitivity. Modulators were packaged with large termination resistorsenabling increased charge sensitivity with an increased system risetime tradeoff. Energy resolution of the fiber link was comparable to preamps conditional on large terminations and subsequent slow readout. For shorter risetimes required in timing diagnostics, small termination resulted in sensitivity 6.8 times inferior to preamps. Several potential improvements in sensitivity are discussed.
Measurements have been made on optical properties of Bicron BCF-91 waveshifting optical fiber. This fiber is proposed as a means of converting UV and blue light emitted from liquid scintillator when exposed to ionizing radiation. The conversion is accomplished by coiling the fiber in a reservoir filled with liquid scintillator and coated internally with reflective paint. UV and blue light is absorbed by the waveshifting dyes in the fiber and reemitted light is channeled into the core of the fiber and output to photo detectors. It has been proposed to outfit the hadron calorimeter sub-system of the GEM detector to be built at the Superconducting Super Collider with 800,000 separate liquid scintillator/waveshifting fiber cells. The measurements described in this work deal with the optical performance of the fiber: spectral emission, response as a function of input wavelengths, response as a function of irradiated length, propagation length and output numerical aperture. The theoretical response of an ideal calorimeter cell is studied based on the results of the measurements presented in this paper.
We discuss the design, fabrication, and evaluation of high speed integrated optical devices for application to
photonics insirumentation systems. Specifically, we have demonstrated integrated optical devices with bandwidths in
excess of 25GHz and implemented these devices in single-shot, streak camera based recording schemes.