Thursday, June 13, 2013

Compound eye fabricated by Song et al. Photo from UIUC College of Engineering

Browsing the post deadline papers, whose sessions will run from 8:00-10:00 pm this evening, it seemed the Applications and Technology session exhibited a zoological theme.

Fly in the Ointment:

In postdeadline paper ATh5A.5, to begin at 8:48 pm, Song et al. will present work recently published in Nature on compound eye cameras that mimic the physiology of a fly's eye. Unlike human eyes or the eyes of other vertebrates, most animals use compound eyes that have many optical units (facets), each with their own lenses and set of photoreceptors. Though compound eyes lack the sensitivity and resolution of single-lens eyes which work by forming images on a detection plane, they can have infinite depth-of-field without the need to adjust the focal length of any of the lenses. Because of this, compound eyes are very adept at calculating/perceiving relative motion. A good set of compound eyes allows the fly high-precision navigation while in flight. Digital compound eyes therefore show great promise for  micro air vehicles (MAVs) to be used for reconnaissance, sensing, and diagnostics in tight spaces (say locating people in a collapsed building, or flying inside and around machinery and other cramped environments with extreme conditions: high radioactivity, temperatures, etc).

What makes Song et al.'s work, a multi-institutional collaboration lead by the Beckman Institute of University of Illinois Urbana-Champaign, so compelling is that they make use of recent advances in stretchable electronics and hemispherical detector arrays to create a compact, monolithic, scalable compound eye. Essentially, the collaboration fabricated a planar layer of elastomeric microlenses and a planar layer of flexible photodiodes and blocking diodes that are aligned and stretched into a hemispherical shape. Serpentine-shaped metal interconnections on the electronics aid in flexibility. The Beckman Institute collaboration achieved near infinite depth-of-field and 160 degree field-of-view.

Crocodile Smile:

Postdeadline paper ATh5A.3, to begin at 8:12 pm, from Yang et al. of Case Western Reserve University actually addresses clinical diagnostics of human tooth decay using Raman imaging, though they image an alligator tooth to help demonstrate proof-of-concept (note all alligators are crocodiles so the cute colloquialism above is technically correct, albeit it is reaching a bit!).

Current clinical practices for dental caries (decay) lack early-stage detection. Late-stage cavities often require multiple fillings or more costly measures such as crowns, bridges, or even entire replacement of the tooth over the tooth lifetime because of the insufficiencies of x-ray and visual observation to detect lesions.
If tooth lesions could be detected early, they could be remineralized at an early stage of decay, thereby preventing future costly, invasive procedures. 

Yang et al. use global Raman imaging that implements a 2D-CCD array and images at a single wavenumber over full-field of view rather than point inspection over a spectrum of wavenumbers. Their Raman images show a clear border between the dentin and enamel of an alligator tooth, showing high contrast in mineral signal intensity. They also show similar images for human teeth  indicating their technique shows good promise for early clinical detection of tooth decay.

Wednesday, June 12, 2013

Laser Fusion for Sustainable Energy

View from inside the target chamber at NIF showing the pencil-shaped target positioner. Image from LLNL
Yesterday began two days of laser-driven fusion talks punctuated by a visit to the nearby National Ignition Facilitiy (NIF) at Lawrence Livermore National Lab (LLNL) as part of CLEO Applications and Technology Special Symposium: The Path to Sustainable Energy: Laser Driven Inertial Fusion Energy.

The session began with The Physics of Laser Driven Inertial Confinement Fusion (ICF) and continued with the Technology of ICF Drive Lasers and Laser Facilities, and Optical and Nuclear Diagnostics. After the tour of NIF today, the symposium will pick up again on Thursday culminating in Future Perspective of ICF as Sustainable Energy Source.
A tutorial on ICF on the NIF website gives a cute recipe for creating the temperatures and pressures needed for fusion on earth that are only found elsewhere in our universe in stars,

Recipe for a Star:
- Take a hollow, spherical plastic capsule about two millimeters in diameter (about the size of a small pea)

- Fill it with 150 micrograms (less than one-millionth of a pound) of a mixture of deuterium and tritium

-Take a laser that for about 20 billionths of a second can generate 500 trillion Watts

-Wait ten billionths of a second
-Result: one miniature star
Figure of the hohlraum and a cross-sectional view (right) showing the fuel capsule. Figure from LLNL

Of course the devil is always in the details. Ignition, in which more energy is generated from the reaction than went into creating it, has yet to be achieved.  In 2009, NIF reached its laser energy goal and thought ignition would be achieved by fall of 2012.

John Lindl, of LLNL began today's session speaking about many of these devilish details, particularly on NIF. For example, besides having the necessary peak power, the 20 ns, 500 TW laser needs to have the proper pulse shape, which is a strangely-shaped series of four pulses of tailored delay and power in order to deliver four shocks to the target at the proper intervals. 

The target capsule, which may seem to be a trivial piece of the puzzle, has undergone an intense 20- year effort. Different shells of ablator materials, size, shape, density, concentricity, and surface smoothness are all key factors in a symmetric collapse (the attempt to get the correct "spherical rocket"). Lindl, spent a good portion of his talk showing diagnostics images of the collapse, and efforts to optimize the system to better ensure symmetric spherical collapse and confinement. 
Other factors include whether to use direct drive (hitting the capsule directly with the many laser beams) or indirect drive (hitting a cavity called a hohlraum with the beams to generate a symmetric barrage of x-rays to initiate collapse). NIF uses a hohlraum and 192 beams. Omega in Rochester, NY uses direct drive, which accelerates more fuel to burn, potentially for better energy production (when that day comes). Beam configurations, target placement and position, and much more come into play.
Of course simulation has been a key factor in design, result interpretation, and future direction. The immense effort for ICF at NIF, as well as other the facilities in the U.S. and around the world are extremely impressive and the problems are complicated, beautiful, and rich.
Laser inertial fusion energy (LIFE) is a worthy goal which could deliver a sustainable carbon-free energy source. There is no enrichment, no radioactive waste, and no worries of a meltdown; unlike fission chain reactions, when you turn "off" fusion, it is "off". NIF is an experimental facility made to understand the physics and technology necessary for LIFE and not scalable to a power plant. Scaling ignition towards operable power plants is another direction of physics, engineering, and optics research.
Schematic of how laser ignition fusion may interface with a power plant to deliver a sustainble source of electricity. Image from LLNL
Currently, targets are fixed and the laser delivers a few shots a day so that experiments can be changed out, realigned, and optics and components can cool down. In a power plant facility, the hope is to use a higher-repetition rate laser to deliver 20 shots a second. Targets would be injected at speeds of greater than 100 m/s to continually burn fuel, which would heat up a low-activiation coolant of lithium-bearing liquid metals or molten salts surrounding the target in order to convert water to steam with which to turn a turbine.

Lindl said that NIF is just 2 to 3 times away from achieving ignition, meaning the output energy from the fusion reaction is one-third to one-half of the input photon energy from the laser. Though nature has provided some delays from what was previously thought, ignition is realistically around the corner. Laser ignition fusion power plants may be close as 2030.

Sunday, April 21, 2013

New Trends in Fiber and Fiber Applications

Top: Microplasma ignition in an argon-filled kagome-latticed
hollow-core photonic crystal fiber. Bottom: scanning electron 
micrograph of fiber facet, from B. Dabord et al, CLEO 2013 
talk, CTu3K.6, "Microconfinement of microwave plasma in 
photonic structures." Microplasmas show promise for 
applications requiring small confinement of short-wavelength 
visible or UV light such as photolithography or compact 
UV laser emission sources.
Microwave plasmas, optical vortices, gravitational wave detection, and mode-division multiplexing for high-capacity telecom systems are just some of the topics in CLEO Science and Innovations  11: Fiber, Fiber Amplifiers, Lasers and Devices. I recently had an opportunity to speak with subcommittee chair, Siddharth Ramachandran from Boston University, U.S.A. to discuss this year’s program on fundamental fiber technology and devices. Though at a surface glance we may think fiber and fiber applications to be very conventional or already “all-figured out”, Ramachandran noted the fact that this subcommittee continues to receive so many submissions year-after-year (in fact the second largest in the entire conference for 2013) indicates that this is still an extremely active area of fundamental and applied research.

Ramachandran said that contributed and invited talks for the subcommittee could be divided into to two main categories:  1) Novel Fiber, and 2) Fiber Applications.  The latter represents  breakthroughs in engineering, instrumentation, and devices from fiber technology introduced five to fifteen years ago. It is the product of well-tended ideas, hard work, and ingenuity coming into fruition. The former, on the other hand, will likely be the seeds for cutting-edge instruments and systems five to fifteen CLEOs from now. In terms of novel fiber work, Ramachandran discussed two trends 1) Kagome-lattice structures, and 2) Mode-division multiplexing for high-capacity communications.

“We are still developing all sorts of novel fibers. What a fiber is, in terms of being a high-index region that guides light surrounded by a low-index region, is not a settled issue. There are actually a lot of innovations going on.”

Ramachandran spoke of how a decade back, the excitement in fiber research centered around photonic band gap fiber (PBG) which guides light in air (or a structure of silica/air-cores), but still provides many of the properties of standard single-mode fiber, particularly confinement and guidance over many kilometers of length. “That was very exciting, and then what happened afterwards is people found out these band-gap effects are nice for guiding light but they tend to have very small spectral regions where they can guide light, so it is not as universal as our old fibers.”

Kagome-lattice fibers, named for the trihexagonal pattern of air-holes resembling the weave-pattern of a Japanese Kagome basket, may provide one solution to having the versatility of air-guided fibers, while allowing large-bandwidth propagation.

“What Kagome lattice fibers essentially do is solve this spectrum-limiting problem we had with photonic band-gap fibers. You can get huge bandwidth out of these, albeit with slightly higher (theoretical) losses. And so they have been very interesting for doing nonlinear optics of gasses filled in these fibers, to do all sorts of dispersive applications where you need crazy high-bandwidth, and for instance to create plasmas. And then there are people who are trying to make ignition torches with fibers which one would never have thought of doing maybe even five years ago,” said Ramachandran.

Left: Spiral interference pattern of twelve distinct orbital angular momentum
states (vortex modes)after propagating through 2 m of the air-core fiber shown
on the right. Right: photo of the facet of the core shown on top and index profile
on the bottom. From P. Gregg et al, CLEO 2013 talk CTu2K.2, "Stable Transmission
of 12 OAM States in Air-Core fiber." The potential for simultaneous propagation
of so many modes shows promise for mode-division multiplexing for high capacity
telecom systems.

The other category for submissions on novel fiber development on this subcommittee has centered on mode-division multiplexing for high-capacity telecom systems. Ramachandran discussed,

“The simplest way to scale information capacity might be to not just use a single mode in a fiber, but to start using multiple modes. And that brings with it a lot of complexities of how different modes interact with each other and what impact dispersion has? What does the area of the fiber do, etcetera, etcetera? Which cycles back to being a fiber design and fiber fabrication problem. So there is a lot of innovation going on there. Even figuring out what modes one wants to send. Are they the standard modes that we have seen in textbooks? Or are they these more exotic orbital angular momentum or vortex modes?”

Top: Areal view of the Laser Intereferometer Gravitational-Wave
Observatory (LIGO) at the Hanford Observatory site showing
one of the 4 km arms. Photo from image library. 
Bottom: One of the possible 3rd generation fiber-amplified laser
sources for gravitational wave detection designed by Quest Centre
for Quantum Engineering and Space-Time Research and Laser 
Zentrum Hannover e.V. Photo from Thomas Damm, Quest. Peter 
Wessels from Laser Zentrum Hannover e.V. will be describing 
many of the stringent requirements of laser sources used for 
gravitational wave detection such as high average power 
(~100 W to kW), single-frequency emission, ultra-low amplitude 
and phase noise, and diffraction-limited beam quality in
CLEO 2013, invited talk, CW3M.5, "Single Frequency Laser 
Sources for Gravitational Wave detection." 

In addition to contributed submissions in these areas, four of the invited talks concern novel fibers and their propagation effects. On the other hand, the remaining invited talks, tutorial, and contributed submissions focus on fiber applications. The tutorial, by Michael Marhic of Swansea University, U.K. entitled “Fiber Optical Parametric Amplifiers in Optical communications,” will be given on Thursday June 13, from 2:00-3:00 pm. The invited talks in fiber applications, which are indicative of the contributed submissions,  comprise topics as diverse as fiber parametric devices, microwave plasmas, gravitational wave detection, mid-IR sensing, and ultrafast laser combs.

Ramachandran notes, “And the interesting thing about that space is the fiber itself that people are using is perhaps something that was developed anywhere between five years ago to maybe even fifteen years ago. We are now beginning to see all the promise that we initially thought that fibers could deliver and actually seeing applications across different disciplines of science and technology.”