News Article | August 22, 2016
The US White House Council on Environmental Quality (CEQ) issued its final guidance to federal agencies on how to consider climate-change impacts of their decisions during National Environmental Policy Act re-views. The US Chamber of Commerce and Center for LNG (CLNG) separately responded with warnings that CEQ's Aug. 2 action could create more regulatory hurdles for new transportation and other projects.
Novel devices feature improved carrier transport and operate under higher-order modulation schemes to enable increased data transmission rates. Ethernet protocols (i.e., the IEEE 802.3ba 100Gb/s and the forthcoming IEEE P802.3bs 400Gb/s standards) provide the definitions for data transmission systems that are used in long-range (10km) and extended-range (40km) fiber links. These transmission systems include multichannel 25GBd on-off keying (OOK)—the simplest form of intensity modulation in which digital information is represented by low (0) and high (1) amplitude levels—and four-level pulse-amplitude modulation (PAM4). Directly modulated lasers (DMLs) are an attractive option for a light source in such applications because of their low cost, small footprint, and low power consumption. In addition, they allow simple direct detection, while being operated in an intensity-modulation mode. DMLs, however, are strongly limited by their bandwidth. Indeed, much higher data rates are possible with coherent systems in which more advanced modulation schemes and complex transmitters are used. As an alternative to conventional quantum-well-based DMLs, quantum-dot lasers (QDLs) have been studied extensively. QDLs feature several unique properties, e.g., they have temperature-insensitive and ultralow threshold current, and low linewidth enhancement factors.1 Gallium-arsenide-based (GaAs-based) QDLs exhibit at least three confined electronic energy levels, i.e., a ground state (GS) and two very close excited states (ESs). The GS is twofold degenerate, whereas the ES is fourfold degenerate. The larger degeneracy translates to a larger differential gain and smaller nonlinear gain compression.2 Direct modulation of QDLs on the GS, at over 20Gb/s, has been demonstrated for both indium arsenide (InAs)/GaAs lasers at 1.31μm (O-band)3 and for InAs/indium phosphide (InP) devices at 1.55μm (C-band).4 The bandwidth of these structures, however, is limited by several factors, including inhomogeneous broadening (low modal gain),5 the hot-carrier effect, and the slow capture time into the quantum dots (large gain compression).6 To ensure sufficient gain, several quantum dot (QD) layers with wide spacers (limiting carrier transport across the active region) are usually incorporated into QDLs. The holes tend to accumulate on the p-side of the active region because of their short diffusion length. This inhomogeneous distribution of carriers limits the modulation response of the devices.7 In our work,8 we have designed a novel GaAs-based QDL in which we incorporate graded p-doping of spacers to compensate for the hole distribution. We have designed this grading so that there is a small amount of p-doping on the topside of the active region and a large amount of doping on the bottom (i.e., substrate) side. Our lasers feature a larger maximum modulation bandwidth (9.2GHz) compared with standard p-doped samples (7.2GHz). Furthermore, by modifying the reflectivity of one laser facet, our lasers (operating exclusively at the ES) exhibit an increased maximum modulation bandwidth of 11.7GHz. We can also realize InAs/indium gallium arsenide/InP QD structures at 1.55μm, which exhibit a –3dB bandwidth of 12.1GHz, by shortening the distance between the electrical contacts and the active region, and by incorporating seven QD layers. We can thus provide sufficient gain, but do not limit the carrier transport. Our O-band and C-band QDLs exhibit data transmission at a rate of 25Gb/s for direct modulation in the non-return-to-zero OOK scheme. To further increase the digital bandwidth of our devices, we also explored higher-order modulation formats, e.g., PAM4 and eight-level PAM (PAM8). PAM4 results for the O-band and C-band lasers—see Figure 1(a)—show that 17.5GBd (35Gb/s) data transmission was realized with both structures. We thus achieved a 40% increase in the maximum bit rate. Doubling the bit rate, however, was not possible. This is because in PAM schemes (particularly PAM8), the noise level of the lasers becomes the major limiting factor. The 7.5GBd (22.5Gb/s) PAM8 response of a standard p-doped laser structure across 10km of single-mode fiber (SMF) is shown in Figure 1(b). We successfully demonstrated PAM8 for this particular laser structure because it features a strongly damped (but linear) small signal response, an ability to operate at low currents (threshold of 3mA), and a relatively large modulation-current-efficiency factor.8 Figure 1. Eye diagrams and corresponding bit-error ratio (BER) curves of (a) the O-band (1.31μm) and C-band (1.55μm) lasers under 17.5GBd four-level pulse-amplitude modulation (PAM4) in back-to-back configuration and (b) of a standard p-doped laser under 7.5GBd eight-level PAM (PAM8) across 10km of single-mode fiber (SMF). In (a) the aggregated BERs were determined by direct detection with an error analyzer. In (b) the BERs were retrieved using digital signal processing of single-shot traces that were acquired with a real-time oscilloscope. The improved BERs at lower received powers were achieved by means of equalization. In another part of our work, we have packaged a 39.813GHz monolithic two-section mode-locked laser (MLL) that is based on the 1.31μm graded p-doped laser structure into a module. MLLs (which generate optical and electrical pulse trains) are ideal candidates for various applications, e.g., microwave photonics and radio-over-fiber systems. When combined with modulators, MLLs act as transmitters for optical time-division multiplexing (OTDM) systems in optical communication networks. MLLs operate at frequencies far beyond the intrinsic bandwidth of DMLs. In contrast to DMLs, they benefit strongly from an inhomogeneously broadened QD gain spectrum. When all the longitudinal modes are locked, sub-picosecond pulses are emitted. QD MLLs also exhibit ultrafast recovery, which enables pulse generation up to 100GHz. We achieve jitter reduction and frequency tuning through hybrid mode-locking. In addition, dual-tone injection gives rise to a narrow optical linewidth (essential for coherent systems). We observe a pulse width of 2ps and integrated jitter of 340fs, which makes our MLL suitable for OTDM.9 We generated return-to-zero (RZ) differential quadrature phase-shift keying (DQPSK) data signals by superimposing bit sequences on the MLL pulse train, via successively sequenced dual-drive Mach–Zehnder and phase modulators. We have also conducted data transmission experiments at 40GBd (80Gb/s) across 45km of SMF (see inset of Figure 2). Through the use of DQPSK and OTDM we were thus able to quadruple the bit rate (compared with standard OOK). The corresponding 80GBd (160Gb/s) RZ DQPSK bit-error ratio curves and eye diagrams are also shown in Figure 2.10 Figure 2. Illustration of 80GBd return-to-zero (RZ) differential quadrature phase-shift keying (DQPSK) in back-to-back configuration. Eye diagrams of tributary 1 (Tr 1) and tributary 2 (Tr 2) at the maximum received optical power were measured by a differential receiver, based on a delay interferometer. The signal-to-noise ratios are 6.7 and 6.5, respectively. BERs were detected by means of an electrical demultiplexer (DEMUX). The BER curves of both DEMUX output signals (0P and 1P) and tributaries show error-free performance (i.e., without error floor to below 10−10. Inset: Constellation diagram of 40GBd RZ DQPSK across 45km of SMF, for the maximum received power obtained with an O-band optical modulation analyzer. The error-vector magnitude is 10.1%. In summary, in our novel GaAs-based QDL we incorporate graded p-doping of spacers to compensate for carrier transport limitations across the active region, and thus bandwidth limitations, in DMLs. The ES emission has larger differential gain than the GS emission. These improvements lead to higher cut-off frequencies of 1.31μm-InAs/GaAs devices. In addition, our 1.55μm InAs/InP QD lasers, which have a narrowed active region and barriers, exhibit large 3dB bandwidths. We have demonstrated data rates of 35Gb/s using PAM4 for both wavelength bands, and PAM8 reveals that further optimization of the lasers (in terms of their noise performance) is required. This will therefore be the focus of our future research. We have also shown that integration of MLLs as sources in coherent OTDM systems enables 160Gb/s RZ DQPSK data transmission. Funding for this research was provided by the German Research Foundation in the SFB 787 framework. The authors would like to thank Finisar (Germany) for packaging the quantum-dot mode-locked laser module and C. Meuer at Sicoya for assistance with the digital signal processing. Department of Solid-State Physics and Center of Nanophotonics Technische Universität Berlin Dejan Arsenijević received his diploma in physics from the Technische Universität Berlin in 2009. His current research interests include higher-order modulation formats, as well as low-jitter optical and electrical pulse sources for future high-speed data communications. Department of Solid-State Physics and Center of NanophotonicsTechnische Universität Berlin Department of Solid-State Physics and Center of Nanophotonics Technische Universität Berlin Berlin, Germany and King Abdulaziz University Dieter Bimberg received his diploma in physics and PhD from Goethe University, Germany, in 1968 and 1971, respectively. Between 1972 and 1979 he was a principal scientist at the Max Planck Institute for Solid State Research in Germany. He was then appointed as a professor of electrical engineering at the Technical University of Aachen (Germany) and, in 1981, as the chair of the Technische Universität Berlin's Applied Solid State Physics department. From 1990 to 2011 he was the executive director of the Institute of Solid State Physics, and in 2004 he founded the Center of Nanophotonics. In addition, he was the chairman of the board of the German Federal Government's Centers of Excellence in Nanotechnologies between 2006 and 2011. His many honors include the Russian State Prize in Science and Technology (2001), the Max Born Award (2006), the William Streifer Scientific Achievement Award (2010), the United Nations Educational, Scientific, and Cultural Organization's Nanoscience Medal (2012), and the Welker Award (2015). Department of Solid-State Physics and Center of NanophotonicsTechnische Universität BerlinBerlin, GermanyandKing Abdulaziz University 2. D. Arsenijević, A. Schliwa, H. Schmeckebier, M. Stubenrauch, M. Spiegelberg, D. Bimberg, V. Mikhelashvili, G. Eisenstein, Comparison of dynamic properties of ground- and excited-state emission in p-doped InAs/GaAs quantum-dot lasers, Appl. Phys. Lett. 104, p. 181101, 2014. doi:10.1063/1.4875238 5. H. Dery, G. Eisenstein, The impact of energy band diagram and inhomogeneous broadening on the optical differential gain in nanostructure lasers, IEEE J. Quantum Electron. 41, p. 26-35, 2005. doi:10.1109/JQE.2004.837953
News Article | April 28, 2016
Call it personalized medicine for depression -- but the prescription in this case is exercise, which University of Florida Health researchers have found helps people with certain genetic traits. A UF study has found that specific genetic markers that put people at risk for depression also predict who might benefit from exercise, according to a study published recently in The Journal of Frailty & Aging. The researchers found that men who were carriers of two specific genes had the most significant response to exercise. The results suggest physical activity as part of a treatment plan -- exercise as moderate as walking -- could help the carriers of these genes. "I want to better understand who could benefit most from physical activity. I'd like to take the same approach to exercise that we take to medication, which is to have a personalized medicine approach," said Vonetta Dotson, Ph.D., the study's first author and an assistant professor in the College of Public Health and Health Profession's department of clinical and health psychology. "If we show through systematic research that exercise has a good chance of helping a patient because of their particular characteristics, I think that might help with patients' motivation to exercise." The results came from a small pilot study, so more research needs to be done before this work can be translated into clinical practice. But in the future, it's possible that blood or saliva could be tested to determine if a person could benefit from physical activity to lower depressive symptoms. The study used data gathered in the Lifestyle Interventions and Independence for Elders, or LIFE, pilot study. During the LIFE pilot study, 396 sedentary older adults were separated into two groups: those who received health education classes and those who were given moderate physical activity classes for 12 months. A subsequent paper published from the LIFE pilot study found that exercise did not significantly affect depression symptoms across the whole group, but that changed when the research team tunneled down into the data. "When we looked at subgroups, we ended up finding significant response to exercise in men who were carriers of a specific gene." Dotson said. To assess the participants' response to exercise, they took a test called the Center for Epidemiologic Studies Depression Scale, a screening test for depression and depressive disorders, at the beginning of the LIFE study's intervention. They took the test again after the interventions ended, at 12 months. The scale assesses four factors, including symptoms of sadness and fearfulness, symptoms such as loss of appetite and concentration difficulties, and a diminished capacity to experience pleasure or perceived difficulties in social relationships. The participants also underwent genetic testing before the intervention, and the researchers tested three genes: the brain-derived neurotrophic, or BDNF, gene, a serotonin transporter gene and a gene called apolipoprotein E. The researchers found the greatest decrease in symptoms such as loss of appetite and concentration difficulties in men who carried the BDNF genetic variation that predisposed them to depression. They also saw an increase in the capacity to experience pleasure in men who exercised regularly who carried specific variations of the serotonin transporter gene. Co-author Taimour Langaee, Ph.D., MSPH, a research associate professor in the UF College of Pharmacy's department of pharmacotherapy and translational research and Center for Pharmacogenomics, is interested in research studies on the effect of antipsychotic drugs on depression. When patients are treated with antidepressants, the level of BDNF expression normalizes, helping them overcome depression, Langaee said. This study was different because it was designed to investigate the effect of physical activity in relation to genetic variations in these genes on changes in depressive symptoms. "We already know that physical activity increases neurotransmitters and endorphins level," he said. "So, we speculated that physical activity increased the expression of BDNF, leading to a decrease in somatic symptoms." Langaee said the study's results were significant, but a larger sample size and more genetic testing is needed to better determine the effect of physical activity on these genes. Dotson said the study provides evidence that physical activity could be explored as an intervention for depression, but warns that this study was not done in people whose symptoms were severe enough to be formally diagnosed with clinical depression. She said it's also important to understand the benefits of exercise because of the impact medications may have on the brains of older adults. "I'm trying to understand how exercise versus antidepressants affect the brain," Dotson said. "The next step for me is to understand from a brain standpoint who is going to benefit and how exercise is going to be beneficial in addition to or as an alternative to medication."
More than 25 percent of the people on the national U.S. waiting list for a heart will die before receiving one. Despite this discouraging figure, heart transplants are still on the rise. There just hasn't been an alternative. Until now. The "cyborg heart patch," a new engineering innovation from Tel Aviv University, may single-handedly change the field of cardiac research. The bionic heart patch combines organic and engineered parts. In fact, its capabilities surpass those of human tissue alone. The patch contracts and expands like human heart tissue but regulates itself like a machine. The invention is the brainchild of Professor Tal Dvir and PhD student Ron Feiner of TAU's Department of Microbiology and Biotechnology, Department of Materials Science and Engineering, and Center for Nanoscience and Nanotechnology. Their study was published this week in the journal Nature Materials. "With this heart patch, we have integrated electronics and living tissue," Dvir says. "It's very science fiction, but it's already here, and we expect it to move cardiac research forward in a big way. "Until now, we could only engineer organic cardiac tissue, with mixed results. Now we have produced viable bionic tissue, which ensures that the heart tissue will function properly." Dvir's Tissue Engineering and Regenerative Medicine Lab at TAU has been at the forefront of cardiac research for the last five years, harnessing sophisticated nanotechnological tools to develop functional substitutes for tissue permanently damaged by heart attacks and cardiac disease. The new cyborg cardiac patch not only replaces organic tissue but also ensures its sound functioning through remote monitoring. "We first ensured that the cells would contract in the patch, which explains the need for organic material," says Dvir. "But, just as importantly, we needed to verify what was happening in the patch and regulate its function. We also wanted to be able to release drugs from the patch directly onto the heart to improve its integration with the host body." For the new bionic patch, Dvir and his team engineered thick bionic tissue suitable for transplantation. The engineered tissue features electronics that sense tissue function and accordingly provide electrical stimulation. In addition, electroactive polymers are integrated with the electronics. Upon activation, these polymers are able to release medication, such as growth factors or small molecules on demand. "Imagine that a patient is just sitting at home, not feeling well," Dvir says. "His physician will be able to log onto his computer and this patient's file — in real time. He can view data sent remotely from sensors embedded in the engineered tissue and assess exactly how his patient is doing. He can intervene to properly pace the heart and activate drugs to regenerate tissue from afar. "The longer-term goal is for the cardiac patch to be able to regulate its own welfare. In other words, if it senses inflammation, it will release an anti-inflammatory drug. If it senses a lack of oxygen, it will release molecules that recruit blood-vessel-forming cells to the heart." Dvir is currently examining how his proof of concept could apply to the brain and spinal cord to treat neurological conditions. "This is a breakthrough, to be sure," Dvir says. "But I would not suggest binging on cheeseburgers or quitting sports just yet. The practical realization of the technology may take some time. Meanwhile, a healthy lifestyle is still the best way to keep your heart healthy."
News Article | August 31, 2016
To remember events in the order they occur, the brain’s neurons function in a coordinated way that is akin to a symphony, a team of New York University scientists has found. Their findings offer new insights into how we recall information and point to factors that may disrupt certain types of memories. “The findings enhance our understanding of how the brain keeps track of what happened and when it happened relative to other events,” explains Lila Davachi, associate professor in NYU’s Department of Psychology and Center for Neural Science and the study’s senior author. “We’ve known for some time that neurons increase their activity when we encode memories. What our study shows is there’s a rhythm to how they fire in relation to one another—much like different instruments in a symphony orchestra.” The study’s first author was Andrew Heusser, a doctoral candidate in NYU’s Department of Psychology. Its collaborators were David Poeppel, a professor in NYU’s Department of Psychology and Center for Neural Science, and Youssef Ezzyat, also a doctoral candidate in NYU’s Department of Psychology at the time of the research and now a postdoctoral fellow at the University of Pennsylvania. The research, which appears in the journal Nature Neuroscience, sought to determine the validity of a long-standing hypothesis, proposed in 1995 by neuroscientists John Lisman and Marco Idiart, which outlines how the order of memories is encoded. The “theta-gamma phase coding” model states that when our brains create a memory for a specific event, our neurons oscillate in a coordinated fashion, with cells firing at high (gamma) frequencies. To encode the order of multiple events, cells representing each event fire in a sequence that is coordinated by a lower (theta) frequency brain rhythm. To test this, the scientists had the study’s participants view a series of six objects (e.g., a butterfly, headphones, etc.), one at a time, on a computer screen. During the experiment, researchers examined the subjects’ neural activity using magnetoencephalography (MEG), which captures measurements of the tiny magnetic fields generated by the brain. Later, they asked subjects to recall the order of the objects they viewed. In their analysis, the researchers examined the neuronal activity of the subjects when they first viewed the objects, then matched it to the results of the recall test. Their data showed notable differences in the patterns of neural activity when the order of the objects was correctly encoded compared to when it was not. Specifically, when the order of the objects was correctly encoded, the gamma activity associated with each object was temporally ordered along a slower theta oscillation so that the gamma activity for object 1 preceded that for object 2 and so on. By contrast, when subjects incorrectly recalled the order in which the objects were presented, gamma activity was just as high—but there was no discernible pattern. “When particular oscillations are in step with each other, we remember the order,” Davachi observes. “But when they are not, we don’t.” The research was supported by a grant from the National Institute of Mental Health.