Photonics

Photorefractive Polymers for Updatable 3D Displays

Photorefractive (PR) polymer composites developed for 3D display applications contain a copolymer as the hole-transporting host matrix. The copolymer approach is followed to reduce the phase separation typical in guest-host polymer systems with low glass transition temperature (Tg), thus allowing increased loading of functional components such as NLO chromophores. The copolymer consists of a polyacrylate backbone with pendant groups tetraphenyldiaminobiphenyltype (TPD) and carbaldehyde aniline (CAAN) attached through an alkoxy linker (PATPD-CAAN). A fluorinated dicyanostyrene (FDCST) NLO chromophore was added to provide sufficient refractive index change and charge generation at the wavelength of interest (532 nm). The plasticizer Nethyl carbazole (ECZ) was also used to reduce the Tg to room temperature. In some composites, a fullerene derivative [6,6]-Phenyl C61 butyric acid methyl ester (PCBM) was used to provide improved sensitization.

Posted in: Briefs, Photonics, Displays, Composite materials, Polymers
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Computational Photonics in Laser Communications Through Clouds

This work explored the concept of creating a partially coherent laser beam consisting of an array of spatially overlapping or separated Gaussian beams with possible individual control of each individual emitter’s wavelength. The idea was to test whether such a transmitter array could propagate more effectively through weak or strong atmospheric turbulence. It was proposed that a versatile, multi-wavelength, multi-emitter configuration could be realized via an array of optically pumped, vertical external-cavity surface emitting semiconductor lasers (VECSELs).

Posted in: Briefs, Photonics, Lasers, Semiconductors, Test procedures, Turbulence
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Threshold Voltage Tuning of Metal-Gate MOSFETs Using an Excimer Laser

This work presents a localized method for tuning the threshold voltages (Vt) of multilayer metal-gate metal-oxide-semiconductor field-effect transistor (MOSFET) devices with a spatial area theoretically limited by the wavelength of the laser beam. This technique allows an independent means to tailor threshold voltage on a device-to-device basis that provides greater design flexibility. This maskless technique allows tailoring of thresholds by tuning the work function of the gate by intermixing titanium and titanium nitride using a laser pulse. The source and drains of the MOSFET are simultaneously annealed by the laser.

Posted in: Briefs, Photonics, Calibration, Lasers, Semiconductor devices, Transistors, Titanium
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Development of GaN-Based Nanostructure Photon Emitters

Gallium nitride (GaN)-based wide bandgap semiconductors are very important material systems for fabrication of photon emitters in a wide range of wavelengths. In particular, the light emitters in ultraviolet (UV), blue, and green wavelengths have been developed and demonstrated in recent years. Besides these UV and visible light emitters, the unique properties of a GaN material system such as large exciton energy and large LO phonon energy, have been proposed as a very suitable material candidate for realization of various photon emitters such as single-photon emitters, LEDs, vertical cavity surface emitting lasers (VCSELs), and quantum cascade lasers (QCL) at room temperature.

Posted in: Briefs, Photonics, Lasers, Fabrication, Materials properties, Semiconductors
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Novel Wavelength Standards in the Near Infrared

The goal of this work was to develop a new class of portable optical frequency references based on sub-Doppler spectroscopy inside gas-filled, hollow-core photonic bandgap (PBG) optical fiber. The change in line width with core size, and narrower transitions inside a new “kagome” structured optical fiber, were demonstrated. A simplified and more compact method for observing saturated absorption spectroscopy in half-sealed photonic bandgap fibers, called the “reflected pump technique,” was realized. Two systems, each consisting of a narrow-line fiber laser locked to the P(13) transition in acetylene, were constructed. By comparing those two systems, it was possible to obtain stability data on the fiber-filled references.

A mode-locked Cr:forsterite laser was developed and stabilized to a GPS-disciplined Rb clock with which to characterize the gas-filled, hollow-fiber optical frequency references. It was found that these lasers offer noisier “f0” beats than Ti:sapphire lasers, but a method was found to dramatically reduce the f0 beat width. A study was initiated into the source of the noise and the exact explanation for the narrowing. In the meantime, absolute frequency measurements of the fiber laser locked to the hollow fiber references are in progress.

A generally useful technique was developed for splicing the photonic bandgap fibers to solid-core fibers using an arc fusion splicer, which makes PBG fibers easier to use in the laboratory. Toward making a completely sealed photonic bandgap fiber cell, PBG was spliced to solid-core fibers inside a vacuum system using a CO2 laser. Efforts to reproduce this in an acetylene vapor proved unsuccessful, most likely due to the thermal properties of acetylene.

This work was done by Kristan L. Corwin of Kansas State University for the Air Force Research Laboratory. For more information, download the Technical Support Package (free white paper) at www.defensetechbriefs.com/tsp under the Photonics category. AFRL-0132

Posted in: Briefs, Photonics, Fiber optics, Lasers, Spectroscopy
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Power Enhancement of a Rubidium Vapor Laser with a Master Oscillator Power Amplifier

The concept of alkali lasers was first suggested by Schalow and Townes in the late 1950s. In the 1970s, photo-dissociation of several of the alkali salts produced lasers with wavelengths ranging from the visible to the far infrared. Thirty years later, diode-pumped alkali lasers (DPAL) started rapidly gaining attention as highly efficient lasers as well as brightness converters. These systems partly owe their high efficiencies to the very small energy differences between the pump and lasing levels. Due to recent technological advances in the field of solid-state lasers, direct-diode pumping has provided the efficient, yet compact method for excitation.

Posted in: Briefs, Photonics, Amplifiers, Lasers, Thermal management
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Solid-State, High-Energy Lasers Based on Rare-Earth Doped Gallium Nitride

Laser-based directed-energy weapons (DEW) are important components for future Army missile defense systems. The diode-pumped, rare-earth (RE)-doped, solid-state laser is a very promising path towards achieving a DEW-sufficient level of average power from a reasonably compact device. Even so, the extreme pump power densities, combined with the inevitable non-radiative losses in the pump-lase process, introduce severe thermal loading in the gain medium. Regardless of the sophistication of the heat removal technique and its efficiency, the gain medium itself is the bottleneck for non-distortive heat removal due to the low thermal conductivity of known gain media compared to that of heat-sinking materials. The bestknown laser hosts, e.g., yttrium aluminum garnet (YAG), possess thermal conductivities (10–11 W/(m-K)) that are ~1.5 orders of magnitude lower than those of known heat-sinking materials. In order to eliminate this technical hurdle, an innovative gain medium with a thermal conductivity on the same order as copper (~390 W/(m- K)) had to be engineered.

Posted in: Briefs, Photonics, Lasers, Thermal management, Military vehicles and equipment, Missiles
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Analysis of Analog Photonic Links Employing Multiple-Channel (Arrayed) Receivers

Analog optical links are finding increased application in commercial and military systems ranging from radio-over-fiber applications, antenna remoting, and optical signal processing. As the performance of an analog link improves with received photocurrent, optical amplifiers — predominantly erbium-doped fiber amplifiers (EDFAs) — have been readily incorporated into a variety of systems. It is known that the addition of an optical amplifier (EDFA) raises the electrical noise floor in both digital and analog applications due to the presence of amplified spontaneous emission (optical) noise. To mitigate this additional noise in systems employing EDFAs prior to modulation, dualoutput optical modulators and balanced detection are frequently employed. This technique has been utilized alone to achieve the first multi-gigahertz bandwidth analog optical link with a noise figure

Posted in: Briefs, Photonics, Amplifiers, Architecture, Noise
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Photonic Recirculating Delay Line for Analog-to-Digital Conversion

Aconventional analog fiber-optic link can be augmented with a recirculating optical delay loop so as to realize an optically assisted analogto- digital converter (ADC) that provides improved performance in terms of both speed and resolution using one (slower) electronic ADC (see figure). The overall architecture readily integrates with any electronic ADC system. Moreover, the highspeed ADC performance is fundamentally limited by the performance of the fiber-optic link.

Posted in: Briefs, Photonics, Amplifiers, Fiber optics, Performance upgrades, Test procedures
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Design and Development of a Package for a Diluted Waveguide Electro-Absorption Modulator

Externally coupled electroabsorption modulators (EAMs) are commonly used in order to transmit RF signals on optical fibers. Recently, an alternative device design with diluted waveguide structures was developed. Bench tests show benefits of lower propagation loss, higher power handling (100 mW), and higher normalized slope efficiency. Bench tests were performed in order to characterize the optical coupling of the EAM. The photo current maximum was offset from the optical power output maximum. The transmissions vs. bias voltage curves were measured, and an XY scanner was used to record the mode field of the light exiting from the EAM waveguide in each position. The Beam Propagation Method was used to simulate the mode field and the coupling efficiency. Based on the bench tests and simulation results, a design including mechanical, optical, and RF elements was developed.

Posted in: Briefs, Photonics, Design processes, Fiber optics, Waveguides
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