Photonics

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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Photon-Counting Chirped Amplitude Modulation Ladar

This work is a follow-on to an effort to develop a method using Geiger-mode avalanche photodiode (GM-APD) photon-counting detectors in the U.S. Army Research Laboratory’s chirped amplitude modulation (AM) ladar receiver to yield sensitivities approaching the shot noise limit. Such sensitivities represent about four orders of magnitude improvement over the sensitivities of the currently used unity-gain, optoelectronic mixing (OEM) metal-semiconductor-metal (MSM) detectors. A variant of the chirped AM ladar has been experimentally assembled and tested, and new single photon-counting detector products were evaluated in terms of their benefits to the chirped AM ladar.

Posted in: Briefs, Photonics, Measurements, Radar, Semiconductor devices, Product development, Military vehicles and equipment
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Evaluation of Performance of WL Fe-Based Spin-LEDs

A study was performed to evaluate the performance of a recently developed type of Fe-based spin-light-emitting diodes (spin-LEDs) that incorporate wetting layers (WLs). [The term "wetting layer" has two slightly different meanings as explained below.] Light beams emitted by the WL Febased spin-LEDs were found to exhibit the same high degree of circular polarization as do those of previously developed Fe-based spin-LEDs, but differ in one very important aspect: they are an order of magnitude brighter than those emitted by their previously developed counterparts. As a consequence, the WL Fe-based spin-LEDs function reliably at room temperature, whereas their previously developed counterparts do not.

Posted in: Briefs, Photonics
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Advances Toward Affordable High-Energy Laser Modules

A multidisciplinary research project entitled “Affordable High-Energy Lasers” has made numerous contributions to the development of several types of advanced laser modules, including not only lasers but also coupling optics and integral laser/ coupling-optic combinations. There are numerous potential applications for such modules, including weaponry, lidar, high-data-rate optical communications, interferometry, spectroscopy, remote sensing, and processing of materials. The devices developed in this project include novel fiber lasers, novel vertical-external-cavity surface emitting lasers (VECSELs), and a radially emitting photonic-bandgap (PBG) polymer fiber laser. Somewhat more specifically, the contributions are summarized as follows:

Posted in: Briefs, Photonics, Design processes, Lasers, Product development, Research and development
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