Theory of concave gratings based on a recursive definition of facet positions

A general theory for concave gratings is presented that is based on a recursion formula for the facet positions and that differs from previous theories that are based on a power-series expansion of the light path function. In the recursion formula approach the facet positions are determined from a numerical solution for the roots of two constraint functions. Facet positions are determined in sequence, starting from the grating pole. One constraint function may be chosen to give a stigmatic point. A variety of grating designs are discussed, including a design that cannot be generated with the power-series approach.     

Observation of the morphology of thin films of solid helium deposited from the fluid onto sapphire

Thin layers of solid helium were grown on sapphire single-crystal substrates at pressures from about 500 bar to 9 kbar. Grain boundaries can be observed in these layer crystals. The morphology of the grains depends on the crystal modification. In the hcp phase (below about 1.13 kbar) a system of parallel bands is observed, probably due to slip and twinning. In the fcc phase (above 1.13 kbar) a polygonal structure similar to a helium froth is found. Melting of this froth in the fcc phase shows grain boundary melting; fluid helium is wetting the fcc grains. Grain boundaries in the hcp phase are, in contrast, not wetted by fluid helium. Near the triple point at 1.13 kbar and 15.0 K one can deposit both crystalline phases side by side. In such structures, the transition fcc hcp⁴He can be observed during isothermal holding. The transition proceeds by the parallel motion of low-energy grain boundaries.     

Planar lightwave circuit eight-channel CWDM multiplexer with <3.9-dB insertion loss

This paper describes the demonstration of an improved eight-channel coarse wavelength-division multiplexer. The fully packaged channel losses range from 2.3 to 3.9 dB, the 1-dB bandwidth is 16.3 nm, and the crosstalk is <-28dB. This silica-waveguide device could have very low cost.     

High efficiency high-speed photodetection in a monolithically integratable InGaAs/InP MQW laser structure at 1.5 mu m

Time-resolved photocurrent measurements in a reverse biased InGaAs/InP ridge waveguide multiquantum well pin laser structure at 1.54 mu m are reported. The pulse response of this monolithically integratable detector is approximately 150 ps FWHM with an internal quantum efficiency of approximately 100% at reverse bias voltages of approximately 5 V.<>     

Low divergence electrically pumped circular-grating surface-emitting DBR laser on an InGaAs/GaAs structure

Reports the fabrication and low divergence operation of an electrically pumped circular grating surface emitting DBR laser on an InGaAs/GaAs SQW structure. A threshold current below 85 mA, output power of more than 20 mW and a divergence of less than 1 degrees FWHM were obtained.<>     

Arrayed waveguide gratings for wavelength routing

Wavelength routing can be performed in the optical domain for both long-haul and passive optical networks. Arrayed waveguide gratings (AWGs) can perform wavelength routing for a large number of optical channels and provide a high level of functionality on an integrated chip. The AWG guides light on a planar lightwave circuit into an array of waveguides that provide dispersion to separate the different wavelengths of light. Routing functions can be performed on individual wavelengths. With this technology, optical cross-connects, optical add/drop multiplexers, and passive optical routers have been demonstrated. Performance issues-such as insertion loss, polarization dependence, passband shape, passband position, crosstalk, and temperature dependence-are being addressed so that AWGs will be practical for deployment into systems     

Gating demultiplexer integrated with MSM detector array inInGaAs/AlGaAs/GaAs for WDM

The authors demonstrate the integration of a grating demultiplexer with curved output waveguides and a MSM photodetector array in InGaAs/AlGaAs/GaAs operating in the 1-μm wavelength region. The structure provides 38 channels with 1-nm channel spacing. The total loss, including scattering from the unmetallized grating, is about 17 dB. A channel crosstalk of -11 dB is obtained. The FWHM of the channel pass band is 0.5 nm