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Topic Name: 3-D PHOTONIC CRYSTALS, Highly Efficient Filters Promise Enhanced Data Transmission for Optical Networks
Category: Telecommunication
Research persons: Ames Laboratory Scientists
Location: Ames Laboratory, U.S. Department of Energy, United States
Details
Researchers at the U.S. Department of
Energy’s Ames Laboratory have come up with a potentially perfect way to sort and distribute the massive amounts of data that travel daily over
optical fibers to people throughout the world. The new technology, a three-dimensional photonic crystal add-drop filter, promises greatly enhanced transmission of multiple wavelength channels (wavelengths of light) traveling along the same optical fiber. The innovative filter is a significant achievement in the effort to develop all-optical transport networks that would eliminate electrical components from optical transmission links and guarantee virtually flawless data reception to end users of the Internet and other fiber-based telecommunications systems.
“There are up to 160 wavelength channels traveling through an optical fiber at the same time,” said Rana Biswas, an Ames Laboratory physicist and one of the developers of the new add-drop filter. “That means a lot of dialogue is going on simultaneously.” Biswas, who is also an
Iowa State University adjunct associate professor of physics and astronomy and electrical and computer engineering, explained that as information is transported over these multiple channels, it’s necessary to drop off individual wavelength channels at different points on the fiber. At the same time, it’s essential to be able to add data streams into unfilled wavelength channels.
“When the data being transported in multiple frequency channels over an optical fiber comes to a receiving station, you want to be able to pick off just one of those frequencies and send it to an individual end user,” said Biswas. “That’s where these 3-D photonic crystals come into play.”
Biswas and his colleagues, Kai-Ming Ho, an Ames Laboratory senior physicist and an ISU Distinguished Professor of Liberal Arts and Sciences; Gary Tuttle, an ISU associate professor of electrical and computer engineering and a researcher at the university’s Microelectronics Research Center; and Preeti Kohli, a former Iowa State Ph.D. student now at Micron in Manassas, Va. successfully demonstrated that 3-D photonic crystals could serve as add-drop filters, providing greatly enhanced data transmission.
To prove their concept, the researchers used a three-dimensional, microwave-scale photonic crystal constructed from layered alumina rods and containing a full bandgap – a wavelength range in which electromagnetic waves cannot transmit. Just as electronic bandgaps prevent electrons within a certain energy range from passing through a semiconductor, photonic crystals create photonic bandgaps that confine light of certain wavelengths.
The add-drop filter created by the Ames Laboratory team contains an entrance waveguide and an exit waveguide created by removing rod segments from the layered photonic crystal. A one-rod segment separates the two waveguides. (A waveguide is a system or material that can confine and direct electromagnetic waves.) A defect cavity is located one unit cell above the waveguide layer. The waveguides can communicate through the cavity, allowing a specific wavelength frequency to be selected from the input waveguide and transmitted to the output waveguide, excluding other input frequencies and resulting in near 100 percent efficiency for the drop frequencies.
The idea of using photonic crystals for add-drop filters is not new. Since the mid 1990s, many groups worldwide have been working to develop the technology with two-dimensional photonic crystals.
“It works,” Biswas said, “but there is loss of some intensity to the end user because 2-D photonic crystals don’t confine the light completely. For example, in a phone conversation, the voices would dim out. But with 3-D photonic crystal add-drop filters, the communication would be clear.”
Although Biswas, Kohli, Tuttle and Ho have shown that 3-D photonic crystals would make highly efficient add-drop filters, there are still problems to address. Getting the size of the photonic crystals down to work at the wavelengths used for Internet communications – 1.5 microns – is the big challenge. The Ames Lab group now has some of these photonic crystals working in that range, but to make these controlled structures with one input, another output and a defect … that definitely takes some work. A future direction is to simplify the design of the add-drop filter by reducing the layers in the photonic crystal – perhaps having all the action happen in one layer.
Note for Photonic Crystals
Photonic crystals are periodic optical (nano)structures that are designed to affect the motion of photons in a similar way that periodicity of a semiconductor crystal affects the motion of electrons. Photonic crystals occur in nature and in various forms have been studied by science for the last 100 years.
Photonic crystals are composed of periodic dielectric or metallo-dielectric (nano)structures that affect the propagation of electromagnetic waves
(EM) in the same way as the periodic potential in a semiconductor crystal affects the electron motion by defining allowed and forbidden electronic energy bands. Essentially, photonic crystals contain regularly repeating internal regions of high and low dielectric constant. Photons (behaving as waves) propagate through this structure - or not - depending on their wavelength. Wavelengths of light (stream of photons) that are allowed to travel are known as "modes". Disallowed bands of wavelengths are called photonic band gaps. This gives rise to distinct optical phenomena such as inhibition of spontaneous emission, high-reflecting omni-directional mirrors and
low-loss-waveguiding, amongst others.
Since the basic physical phenomenon is based on diffraction, the periodicity of the photonic crystal structure has to be of the same length-scale as half the wavelength of the EM waves i.e. ~200 (blue) to 350 (red) nm for photonic crystals operating in the visible part of the spectrum - the repeating regions of high and low dielectric constants have to be of this dimension. This makes the fabrication of optical photonic crystals cumbersome and complex.
Photonic crystals are attractive optical materials for controlling and manipulating the flow of light. One dimensional photonic crystals are already in widespread use in the form of thin-film optics with applications ranging from low and high reflection coatings on lenses and mirrors to colour changing paints and inks. Higher dimensional photonic crystals are of great interest for both fundamental and applied research, and the two dimensional ones are beginning to find commercial applications. The first commercial products involving two-dimensionally periodic photonic crystals are already available in the form of
photonic-crystal fibers, which use a nanoscale structure to confine light with radically different characteristics compared to conventional optical fiber for applications in nonlinear devices and guiding exotic wavelengths. The three-dimensional counterparts are still far from commercialization but offer additional features possibly leading to new device concepts (e.g. optical computers), when some technological aspects such as manufacturability and principal difficulties such as disorder are under control.
Note for Fiber-optic Communication
Fiber-optic communication is a method of transmitting information from one place to another by sending light through an optical fiber. The light forms an electromagnetic carrier wave that is modulated to carry information. First developed in the 1970s, fiber-optic communication systems have revolutionized the telecommunications industry and played a major role in the advent of the Information Age. Because of its advantages over electrical transmission, the use of optical fiber has largely replaced copper wire communications in core networks in the developed world.
The process of communicating using fiber-optics involves the following basic steps: Creating the optical signal using a transmitter, relaying the signal along the fiber, ensuring that the signal does not become too distorted or weak, and receiving the optical signal and converting it into an electrical signal.
Modern fiber-optic communication systems generally include an optical transmitter to convert an electrical signal into an optical signal to send into the optical fiber, a cable containing bundles of multiple optical fibers that is routed through underground conduits and buildings, multiple kinds of amplifiers, and an optical receiver to recover the signal as an electrical signal. The information transmitted is typically digital information generated by computers, telephone systems, and cable television companies.
The DOE Office of Science,
Basic Energy Science Office funded the above research on 3-D photonic crystal add-drop filters.
Ames Laboratory is a U.S. Department of Energy Office of Science laboratory operated for the DOE by Iowa State University. The Lab conducts research into various areas of national concern, including the synthesis and study of new materials, energy resources, high-speed computer design, and environmental cleanup and restoration.
In figure 1, Optical fibers
In figure 2, The opal in this bracelet contains a natural periodic microstructure responsible for its iridescent color. It is essentially a natural photonic crystal, although it does not have a complete photonic band gap.
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