Tapered fiber bundles for combining high-power diode lasers

Tapered fiber bundles are often used to combine the output power of several semiconductor lasers into a multimode optical fiber for the purpose of pumping fiber lasers and amplifiers. It is generally recognized that the brightness of such combiners does not exceed the brightness of the individual input fibers. We report that the brightness of the tapered fibers (and fiber bundles) depends on both the taper ratio and the mode-filling properties of the beams launched into the individual fibers. Brightness, therefore, can be increased by selection of sources that fill a small fraction of the input fiber's modal capacity. As proof of concept, we present the results of measurements on tapered fiber-bundle combiners having a low-output étendue. Under low mode-filling conditions per input multimode fiber (i.e., fraction of filled modes < or =0.29), we report brightness enhancements of 8.0 dB for 19 x 1 bundles, 6.7 dB for 7 x 1 bundles, and 4.0 dB for 3 x 1 combiners. Our measured coupling efficiency variations of approximately 1%-2% among the various fibers in a given bundle confirm the uniformity and quality of the fabricated devices.     

Extended Tunability in a Two-Chip VECSEL

We demonstrate a widely tunable vertical-external-cavity surface-emitting laser (VECSEL) with a W-shaped cavity, in which two VECSEL chips serve as fold mirrors and a birefringent filter is inserted at Brewster's angle. These two chips provide much higher modal gain and broader bandwidth of the gain than a single chip does, enhancing the VECSEL tuning range and reducing the variation of tunable output power with the tuned wavelength. This two-chip VECSEL configuration makes it possible to shape the modal gain spectra of the laser or to manipulate the tuning curve of the laser by two different chips with certain gain peak detuning (offset). Multiwatts high-brightness linearly polarized output with a tuning range of 33 nm is demonstrated in such a two-chip VECSEL     

Design of a compact photonic-crystal-based polarizing beam splitter

A compact polarizing beam splitter based on a photonic crystal (PC) directional coupler with a triangular lattice of air holes is designed and simulated. In the employed PC structure, transverse-electric (TE) light is confined with the photonic bandgap effect, while transverse-magnetic (TM) light is guided through an index-like effect. Due to the different guiding mechanisms, TM and TE light have strikingly different beat lengths, which is utilized to separate the two polarizations in a directional coupler no longer than 24.2 /spl mu/m. The two-dimensional finite-difference time-domain method of computation is used to evaluate the device performance. The extinction ratios are found to be around 20 dB for both TE and TM polarized light.     

5-W Yellow Laser by Intracavity Frequency Doubling of High-Power Vertical-External-Cavity Surface-Emitting Laser

We report on the development of a high-power tunable yellow–orange laser. It is based on intracavity frequency doubling of a widely tunable, highly strained InGaAs–GaAs vertical-external-cavity surface-emitting laser operating near 1175 nm. Over 5 W of continuous-wave output power is achieved and is tunable over a 15-nm band centered at 587 nm. This compact low-cost high-power yellow–orange laser provides an innovative alternative for sodium guidestar lasers, medical and communication applications.      

Effect of applied voltage on surface and volume charge density of the jet in electrospinning of polyacrylonitrile solutions

To investigate the effect of applied voltage on jet charge density, one must investigate its effect on the jet electric current and flow rate. However, there are some difficulties in measuring the jet flow rate. Here, the electrospinning jet flow rate is calculated by applying a new method and the effects of applied voltage and feed rate on jet charge density are investigated. The jet flow rate was independent of the feed rate and increased on increasing the applied voltage. By defining an experimental equation for dependence of jet flow rate on jet current, a relation between jet volume charge density and jet current could be defined. By measuring the electrospun fiber diameter, surface charge density of the jet could be estimated. On increasing the voltage, the jet volume charge density decreased whereas the jet surface charge density remained unchanged. POLYM. ENG. SCI., 2010. © 2010 Society of Plastics Engineers     

Dynamic characterization of thin-walled laminated channel beams with warping

The dynamic characteristics of a round cornered C-channel beam made of laminated composite material are studied. The sharp corners are rounded for manufacturing considerations. Thin-walled beam theory is used to formulate the coupled vibration of a rounded C-channel beam with fixed-free end conditions. It is shown that cross-sectional properties used for isotropic case are applicable for laminated composites and the material properties needed for the formulation can be obtained using the law of average. A comparative study is conducted to show the advantage of using composites. The effect of radius of corner and warping on the natural frequency is investigated. The functional of the equations of motion is formed using the variational method. The Ritz method has been used to formulate an eigenvalue problem and its frequency equation. The method proposed is systematic. The computerized procedure can be used as a fast design tool in the design of composite channel beam structures.     

Electrically pumped circular-grating surface-emitting DBR laser onInGaAs strained single-quantum-well structure

The authors demonstrate the fabrication and room temperature operation of an electrically pumped circular-grating surface-emitting distributed-Bragg-reflector laser. An InGaAs/GaAs single quantum well (SQW) graded-index separate confinement heterostructure (GRINSCH) structure was grown by one-step molecular beam epitaxy (MBE). Circular gratings were defined by focused ion beam lithography. The lasing wavelength was 942 nm, and the threshold current was 280 mA. This is the first demonstration of these lasers with no epitaxial regrowth     

A new design of optical add/drop filters and multi-channel filters based on hexagonal PhCRR for WDM systems

Photonic Network Communications - In this research, using photonic crystal dielectric rods with a triangular lattice constant, a photonic crystal ring resonator (PhCRR) has been designed in order...     

Ultra-high-Q optical filter based on photonic crystal ring resonator

In this study, a photonic crystal ring resonator with a triangular lattice is used to design an optical filter. The proposed structure is able to filter the central wavelength of 1548 nm with a transmission coefficient of over 95%. Moreover, this structure has an ultra-high-quality factor (Q) of about 1290. With altering the features of the structure including the refractive index, the lattice constant and the radius of the rods in the resonator core, their effects on the central wavelength of the filter, transmission coefficient, quality factor and bandwidth are investigated. The plane wave expansion and finite-difference time-domain methods are used to extract photonic band gap and investigate the photonic behavior of the proposed structure, respectively.