Photonics

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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Lasing Consequences of Silicon Nanostructures

While silicon electronics has been a success in modem technologies, silicon photonics is still in development and in need of a laser source. Many approaches have been explored, from anodized silicon luminescence, to generating direct emissions by quantum-confinement, and to indirect down-conversion of a shorter wavelength laser light via silicon's nonlinear dielectric responses. One approach that was developed has led to the demonstration of laser emission in silicon-on-insulator at cryogenic temperatures (<85K).

Posted in: Briefs, Photonics
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Reduced Power Laser Designation Systems

This work contributes to the Micropulse Laser Designation (MPLD) project to develop a six-pound eye-safe micro-pulse laser system to locate, identify, range, mark, and designate stationary and moving targets. MPLD uses laser pulses of much lower energy and higher repetition rates than in existing laser designation systems. Because of this, MPLD presents a range of new circuit design and signal processing problems.

Posted in: Briefs, Photonics
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Continuous-Wave Laser Diodes Based on a Novel InGaAsNSb Material System

Laser sources operating in the spectral region from 2 to 3.5 μm are in demand for ultra-sensitive laser spectroscopy, medical diagnostics, home security, industrial process monitoring, infrared countermeasures, and optical wireless communications. Currently, solid-state lasers and optical parametric oscillators and amplifiers are used as coherent light sources in this spectral region.

Posted in: Briefs, Photonics
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