Oliver T. Strand

Compact system for high-speed velocimetry using heterodyne techniques

We have built a high-speed velocimeter that has proven to be compact, simple to operate, and fairly inexpensive. This diagnostic is assembled using off-the-shelf components developed for the telecommunications industry. The main components are fiber lasers, high-bandwidth high-sample-rate digitizers, and fiber optic circulators. The laser is a 2W cw fiber laser operating at 1550nm. The digitizers have 8GHz bandwidth and can digitize four channels simultaneously at 20GS∕s. The maximum velocity of this system is ∼5000m∕s and is limited by the bandwidth of the electrical components. For most applications, the recorded beat frequency is analyzed using Fourier transform methods, which determine the time response of the final velocity time history. Using the Fourier transform method of analysis allows multiple velocities to be observed simultaneously. We have obtained high-quality data on many experiments such as explosively driven surfaces and gas gun assemblies.

Velocimetry Using Heterodyne Techniques

At LLNL, we have been using heterodyne techniques for the past year and a half to measure velocities up to several kilometers-per-second on different types of experiments. We assembled this diagnostic, which we call the Heterodyne Velocimeter (HetV), using commercially available products developed for the communications industry. We use a 1550 nm fiber laser and single mode fibers to deliver light to and from the target. The return Doppler-shifted light is mixed with the original laser light to generate a beat frequency proportional to the velocity. At a velocity of 1000 m/s, the beat signal has a frequency of 1.29 GHz. We record the beat signals directly onto fast digitizers. The maximum velocity is limited by the bandwidth of the electronics and the sampling rate of the digitizers. The record length is limited by the amount of memory contained in the digitizers. This paper describes our approach to measuring velocities with this technique and presents recent data obtained with the HetV.

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>>

USING THE HETERODYNE METHOD TO MEASURE VELOCITIES ON SHOCK PHYSICS EXPERIMENTS

We developed a velocimeter system several years ago that uses the heterodyne method [1]. This system is assembled from commercially available components that were developed for the telecommunications industry. There are several advantages of this system over the traditional VISAR method that has made it increasingly popular. This system is compact, portable, and relatively inexpensive. The maximum velocity of this system is determined by the electrical bandwidth of the electronics and the digitizer sample rate. The maximum velocity for the system described here is over 5 km/s.

Low-cost packaging of high-performance optoelectronic components

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.

A data-driven approach for determining time of initial movement in shock experiments using photonic Doppler velocimetry

Photonic Doppler Velocimetry is an interferometric technique for measuring the beat frequency of a moving surface, from which the calculated velocity profile of the surface can be used to describe the physical changes the material undergoes after high-impact shock. Such a technique may also be used to characterize the performance of small detonators and determine the time at which the surface began moving. In this work, we develop a semi-automated technique for extracting the time of initial movement from a normalized lineout of the power spectrogram near the offset frequency of each probe. We characterize the response bias of this method and compare with the time of initial movement obtained by hand calculation of the raw voltage data. Results are shown on data from shock experiments such as gas gun setups and explosives-driven flyer plates.

Multiplexed photonic Doppler velocimetry for large channel count experiments

Photonic Doppler Velocimetry (PDV) is routinely employed as a means of measuring surface velocities for shockwave experimentation. Scientists typically collect ∼4 to 12 channels of PDV data and use extrapolation, assumptions, and models to determine the velocities in regions of the experiment that were not observed directly. We have designed, built and applied a new optical velocimetry diagnostic—the Multiplexed Photonic Doppler Velocimeter (MPDV)—for use on shock physics experiments that require a large number (100s) of spatial points to be measured. MPDV expands upon PDV measurement capabilities via frequency and time multiplexing using commercially available products developed for the telecommunications industry. The MPDV uses the heterodyne method to multiplex four data channels in the frequency domain combined with fiber delays to multiplex an additional four-channel dataset in the time domain, all of which are recorded onto the same digitizer input. This means that each digitizer input records data from eight separate spatial points, so that a single 4-input digitizer may record a total of 32 channels of data. Motivation for development of a multiplexed PDV was driven by requirements for an economical, high-fidelity, high channel–count optical velocimetry system. We present a survey of the methods, components, and trade-offs incorporated into this recent development in optical velocimetry.Photonic Doppler Velocimetry (PDV) is routinely employed as a means of measuring surface velocities for shockwave experimentation. Scientists typically collect ∼4 to 12 channels of PDV data and use extrapolation, assumptions, and models to determine the velocities in regions of the experiment that were not observed directly. We have designed, built and applied a new optical velocimetry diagnostic—the Multiplexed Photonic Doppler Velocimeter (MPDV)—for use on shock physics experiments that require a large number (100s) of spatial points to be measured. MPDV expands upon PDV measurement capabilities via frequency and time multiplexing using commercially available products developed for the telecommunications industry. The MPDV uses the heterodyne method to multiplex four data channels in the frequency domain combined with fiber delays to multiplex an additional four-channel dataset in the time domain, all of which are recorded onto the same digitizer input. This means that each digitizer input records data ...

Low-cost automated fiber pigtailing machine

Automated fiber pigtailing machines (AFPMs) have been designed and built under an ARPA-funded project The AFPM enables many of the critical technologies to perform automated sub-micron fiber pigtailing compatible with a low-cost manufacturing environment. These technologies include low-cost high-precision stages, computer vision to replace the labor-intensive coarse alignment, and many details of parts handling and feeding. Subsequent generations of the AFPM may build upon the design concepts developed here to pigtail fibers to OE devices in more complicated geometries. For example, all applications for this project use epoxy to attach the fibers, so no applications using solder or laser welding have been considered. Also, the stages to manipulate the fibers provide only three axes of translation, so no rotational degrees of freedom are available, including the very important roll axis for polarization-dependent applications. The third AFPM at LLNL will be used to develop some of these capabilities.

Overview of photonics research at Lawrence Livermore National Laboratory

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.