A sensitive, high speed optical detector is needed for use in optical communications as well as for laboratory experiments. Recent experiments (1) have shown the GaAs MESFET to be potentially useful as a fast optical detector. We call this device an OPFET. This report summarizes new experimental results which show that the OPFET is both fast and sensitive and which identify the detection mechanism in this structure as photoconductivity. Calculations show that this device should give performance which exceeds that of PIN photo-diodes and rivals avalanche photodiodes. In addition, the structure is easy to integrate for use in optical circuits. EXPERIMENTAL RESULTSTwo sets of experiments were done to investigate the performance of this device and identify the detection mechanism. In the first, the optical pulse response was observed by focussing picosscond pulses of light from a mode-locked dye laser onto a MESFET (Lg = lpm, Wg = 200pm, Nd = lo1 P ) .The device was mounted in a microwave stripline circuit and the response was observed with a Tektronix type S-4 sampling head. Figure 1 shows the observed response of the device under different bias conditions. These measurements are uncorrected for circuit and sampling head response. Fall times on the order of 45 psec. are evident. The response consists of a fast part followed by a slow tail which becomes more pronounced with larger drain currents or smaller gate voltages. The sensitivity (which corresponds to the area under the curves) increases for larger drain currents or smaller gate voltages. The device gain was estimated by calculatingthe total power absorbed in the active region and averaging the pulse response. This gives a gain estimate of 1.3 at a drain current of 2mA increasing to about 2 at a drain current of 4OmA. The peak of the pulse response was plotted vs. input light power (figure 2) and was found to be linear. 'Supported by NSF Grant ENG-76-11729 A01 with sup-Figure 1: Response to a 1 5 psec. optical pulse, plernental support from the Materials Science Center Vu = 5V. (A) ID = 4omA (B) ID = 1 O m A ( C ) at Cornell University and the National Research and Resource Facility for Submicron Structures. ID = 2mA. 120 2In the second set of experiments an OPFET was used with a microwave receiver to observe the longitudinal mode beating of a small He-Ne laser at 641 Mhz. The incident light could be scanned in two dimensions to measure the areal response of the device ( figure 3 ) .An isometric view of the response is shown in figure 4. Figure 5 shows offset sweeps, corresponding to one sweep in the center of the isometric view, in which the drain voltage is kept constant and the gate voltage is varied. Figure 6 shows offset sweeps for a different device (Lg = 2pm,where the gate voltage is kept constant and the drain voltage is varied.Pertinant features of this data are that the total response decreases rapidly for large gate bias, and that for constant gate bias the total response increases and shifts towards the drain with increasing drain bias. The gain of...
A Mission Control Architecture is presented for a Robotic Lunar Sample Return Mission which builds upon the experience of the landed missions of the NASA Mars Exploration Program. This architecture consists of four separate processes working in parallel at Mission Control and achieving buy-in for plans sequentially instead of simultaneously from all members of the team. These four processes were: Science Processing, Science Interpretation, Planning and Mission Evaluation. Science Processing was responsible for creating products from data downlinked from the field and is organized by instrument. Science Interpretation was responsible for determining whether or not science goals are being met and what measurements need to be taken to satisfy these goals. The Planning process, responsible for scheduling and sequencing observations, and the Evaluation process that fostered inter-process communications, reporting and documentation assisted these processes. This organization is advantageous for its flexibility as shown by the ability of the structure to produce plans for the rover every two hours, for the rapidity with which Mission Control team members may be trained and for the relatively small size of each individual team. This architecture was tested in an analogue mission to the Sudbury impact structure from June 6-17, 2011. A rover was used which was capable of developing a network of locations that could be revisited using a teach and repeat method. This allowed the science team to process several different outcrops in parallel, downselecting at each stage to ensure that the samples selected for caching were the most representative of the site. Over the course of 10 days, 18 rock samples were collected from 5 different outcrops, 182 individual field activities -such as roving or acquiring an image mosaic or other data product -were completed within 43 command cycles, and the rover travelled over 2,200 m. Data transfer from communications passes were filled to 74%. Sample triage was simulated to allow down-selection to 1kg of material for return to Earth.
A new photoconductive detector is described that uses the previously reported optical field effect transistor structure. The detector has good sensitivity (photoconductive gain of ∼5) and high speed (∼150 psec). Light was coupled to this detector via a 7059 glass over SiO2 waveguide. The overall coupling efficiency including losses in endfire coupling was about 40%.
It is shown that recent results reported by Sugeta and Mizushima do not contradict the photoconductive model for the OPFET.
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