Mark E. Lowry

10-Gb/s subcarrier multiplexed transmission over 490 km of ordinary single-mode fiber without dispersion compensation

This letter describes the results of a field trial in which four 2.5-Gb/s data channels were transmitted around the San Francisco Bay over ordinary telephone company fiber at 1550 nm with negligible dispersion penalty using subcarrier multiplexing (SCM). Using off-the-shelf microwave components and a lithium niobate external modulator having a modulation bandwidth of around 20 GHz, we were able to multiplex four tightly spaced high-speed data channels at a single wavelength. At the receiving end, we optically demultiplexed the data using optical filters and OC-48 clock recovery receivers.

Multiwavelength parallel optical interconnects for massively parallel processing

We describe a multiwavelength, multifiber (parallel) optical interconnect based on multimode fiber ribbon cables with applications in massively parallel processing systems. By combining the benefits of parallel optics and coarse wavelength division multiplexing high aggregate throughputs are possible in a broadcast and select architecture that provides a single hop to all nodes. We identify the key components needed for such a system and report on our component development efforts for multiwavelength parallel optical interconnects. System components reported herein include a four-wavelength bit-parallel transmitter using a silicon optical bench and hybrid packaging, and two-port and three-port wavelength selective filter modules packaged to be compatible with mechanically transferable ferrule terminated ribbon cables. The transmitters were modulated up to 1.25 Gb/s with a bit-error rate better than 10/sup -12/ and no measurable power penalty due to multiple wavelength bit parallel operation. The filters exhibited insertion losses of between 1 and 2 dB and would support 10 nm spaced channels at -23-dB crosstalk.

X-ray detection by direct modulation of an optical probe beam—Radsensor: Progress on development for imaging applications

We present a progress report on our 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 probe beam through the same volume of semiconducting medium that has experienced an x-ray induced modulation in the electron–hole population. If the wavelength of the optical probe beam is close to the semiconductor band-edge, the optical probe will be modulated significantly in phase and amplitude. We have analyzed the physics of the imaging radsensor, developed modeling tools for device design, and are cautiously optimistic that we will achieve single x-ray photon sensitivity, and picosecond response. These predictions will be tested with Cu Kα xrays at the LLNL USP facility this spring and summer, with a cavity-based radsensor detector suitable for use i...

RadSensor: Xray Detection by Direct Modulation of an Optical Probe Beam

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.

Automated fiber pigtailing technology

The high cost of optoelectronic (OE) devices is due mainly to the labor-intensive packaging process. Manually pigtailing such devices as single-mode laser diodes and modulators is very time consuming with poor quality control. The Photonics Program and the Engineering Research Division at LLNL are addressing several issues associated with automatically packaging OE devices. A fully automated system must include high-precision fiber alignment, fiber attachment techniques, in-situ quality control, and parts handling and feeding. This paper will present on-going work at LLNL in the areas of automated fiber alignment and fiber attachment. For the fiber alignment, we are building an automated fiber pigtailing machine (AFPM) which combines computer vision and object recognition algorithms with active feedback to perform sub-micron alignments of single-mode fibers to modulators and laser diodes. We expect to perform sub-micron alignments in less than five minutes with this technology. For fiber attachment, we are building various geometries of silicon microbenches which include onboard heaters to solder metal-coated fibers and other components in place; these designs are completely compatible with an automated process of OE packaging.<<ETX>>

Static and time-resolved 10–1000 keV x-ray imaging detector options for NIF

High energy (>10 keV) x-ray self-emission imaging and radiography will be essential components of many NIF high energy density physics experiments. In preparation for such experiments, we have evaluated the pros and cons of various static [x-ray film, bare charge-coupled device (CCD), and scintillator + CCD] and time-resolved (streaked and gated) 10–1000 keV detectors.

Multi-mode fiber coarse WDM grating router using broadband add/drop filters for wavelength re-use

We demonstrate a grating-router with 37 nm channel spacing and 6 nm FWHM in the 800-900 nm range for WDM over multimode fiber. Broadband thin-film add/drop filters provide wavelength re-use enabling N/spl times/N fully non-blocking interconnection with N wavelengths.

Characterization of Lithium Niobate Electro-Optic Modulators at Cryogenic Temperatures

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.

Preliminary Radiation Hardness Testing Of LiNbO 3 :Ti Optical Directional Coupler Modulators Operating At 810 Nm

Integrated optics have the potential to replace conventional electronics in many instrumentation applications; in many cases the integrated-optics approach is the only one that will achieve the necessary bandwidth and information-density goals. Thus far, has been done to address the prompt hardness of these devices to intense ionizing-radiation fields. We present preliminary data on the response of optical directional coupler modulators (ODCMs) to radiation from a Febetron. The Febetron produces energetic electrons in the 300-700 keV range in tilde 3-ns FWHM pulses, with a maximum dose rate of tilde 1014 rad/s. The operation of the ODCM is monitored during and promptly after the Febetron pulse impinges the device. Long-term effets are also monitored. These data are analyzed with respect to the operation of such devipes in a harsh inonizing-radiation environment.

Electron Cascades in Sensors for Optical Detection of Ionizing Radiation

A new class of high-speed detectors, called RadOptic detectors, measures ionizing radiation incident on a transparent semiconductor by sensing changes in the refractive index with an optical probe beam. We describe the role of radiation-initiated electron cascades in setting the sensitivity and the spatial and temporal resolution of RadOptic detectors. We model electron cascades with both analytical and Monte Carlo computational methods. We find that the timescale for the development of an electron cascade is less than of order 100 fs and is not expected to affect the time response of a detector. The characteristic size of the electron cloud is typically less than 2 μm, enabling high spatial resolution in imaging systems. The electron-hole pair density created by single x-rays is much smaller than the saturation density and, therefore, single events should not saturate the detector.