Discontinuities in the Electromagnetic Fields of Vortex Beams in the Complex Source/Sink Model

An analytical discontinuity is reported in what was thought to be the discontinuity-free exact nonparaxial vortex beam phasor obtained within the complex source/sink model. This discontinuity appears for all odd values of the orbital angular momentum mode. Such discontinuities in the phasor lead to nonphysical discontinuities in the real electromagnetic field components. We identify the source of the discontinuities, and provide graphical evidence of the discontinuous real electric fields for the first and third orbital angular momentum modes. A simple means of avoiding these discontinuities is presented.Comment: 10 pages, 4 figure

Coherent power combination of semiconductor lasers using optical phase-lock loops

Heterodyne optical phase-lock loops (OPLLs) enable the precise electronic control over the frequency and phase of a semiconductor laser (SCL) locked to a ldquomasterrdquo reference laser. One of the more interesting applications of OPLLs is the creation of coherent arrays by locking a number of ldquoslaverdquo SCLs to a common master laser. In this paper, we demonstrate the coherent power combination of various high-power semiconductor lasers using OPLLs in both the filled-aperture and tiled-aperture configurations. We further demonstrate the electronic control over the phase of each individual SCL using a voltage-controlled oscillator. It is feasible to combine a large number of SCLs using this approach, leading to compact, efficient, and cost-effective high-power and high-radiance optical sources

Nonlinearity and Noise Effects in Multi-level Signal Millimeter-Wave over Fiber Transmission using Single- and Dual-Wavelength Modulation

We transmit multilevel quadrature amplitude modulation (QAM) data-IEEE 802.16 schemes-at 20 MSps and an orthogonal frequency-division multiplexing (OFDM) 802.11 g signal (54 Mbps) with a 25 GHz millimeter-wave over fiber system, which employs a dual wavelength source, over 20 km of single mode fiber. Downlink data transmission is successfully demonstrated over both optical and wireless (up to 12 m) paths with good error vector magnitude. An analysis of two different schemes, in which data is applied to one (single) and both (dual) of the wavelengths of a dual wavelength source, is carried out. The system performance is analyzed through simulation and a good match with experimental results is obtained. The analysis investigates the impact of Mach-Zehnder modulator (MZM) and RF amplifier nonlinearity and various noise sources, such as laser relative intensity noise, amplified spontaneous emission, thermal, and shot noise. A comparison of single carrier QAM IEEE 802.16 and OFDM in terms of their sensitivity to the distortions from MZM and RF amplifier nonlinearity is also presented

Coherent beam combining with multilevel optical phase-locked loops

Coherent beam combining (CBC) technology holds the promise of enabling laser systems with very high power and near-ideal beam quality. We propose and demonstrate a novel servo system composed of multilevel optical phase lock loops. This servo system is based on entirely electronic components and consequently can be considerably more compact and less expensive compared to servo systems made of optical phase/frequency shifters. We have also characterized the noise of a 1064 nm Yb-doped fiber amplifier to determine its effect on the CBC and studied theoretically the efficiency of combining a large array of beams with the filled-aperture implementation. In a proof-of-concept experiment we have combined two 100 mW 1064 nm semiconductor lasers with an efficiency of 94%

Electrodynamic Response and Stability of Molecular Crystals

We show that electrodynamic dipolar interactions, responsible for long-range fluctuations in matter, play a significant role in the stability of molecular crystals. Density functional theory calculations with van der Waals interactions determined from a semilocal "atom-in-a-molecule" model result in a large overestimation of the dielectric constants and sublimation enthalpies for polyacene crystals from naphthalene to pentacene, whereas an accurate treatment of non-local electrodynamic response leads to an agreement with the measured values for both quantities. Our findings suggest that collective response effects play a substantial role not only for optical excitations, but also for cohesive properties of non-covalently bound molecular crystals

On the Insignificance of Photochemical Hydrocarbon Aerosols in the Atmospheres of Close-in Extrasolar Giant Planets

The close-in extrasolar giant planets (CEGPs) reside in irradiated environments much more intense than that of the giant planets in our solar system. The high UV irradiance strongly influences their photochemistry and the general current view believed that this high UV flux will greatly enhance photochemical production of hydrocarbon aerosols. In this letter, we investigate hydrocarbon aerosol formation in the atmospheres of CEGPs. We find that the abundances of hydrocarbons in the atmospheres of CEGPs are significantly less than that of Jupiter except for models in which the CH$_4$ abundance is unreasonably high (as high as CO) for the hot (effective temperatures $\gtrsim 1000$ K) atmospheres. Moreover, the hydrocarbons will be condensed out to form aerosols only when the temperature-pressure profiles of the species intersect with the saturation profiles--a case almost certainly not realized in the hot CEGPs atmospheres. Hence our models show that photochemical hydrocarbon aerosols are insignificant in the atmospheres of CEGPs. In contrast, Jupiter and Saturn have a much higher abundance of hydrocarbon aerosols in their atmospheres which are responsible for strong absorption shortward of 600 nm. Thus the insignificance of photochemical hydrocarbon aerosols in the atmospheres of CEGPs rules out one class of models with low albedos and featureless spectra shortward of 600 nm.Comment: ApJL accepte

Coherent power combination of two Master-oscillator-power-amplifier (MOPA) semiconductor lasers using optical phase lock loops

Using heterodyne Optical Phase-Locked Loops (OPLLs), two 1W high power 1550 nm master-oscillator-power-amplifier (MOPA) semiconductor lasers operating as current controlled oscillators are phase-locked to a 1 mW reference laser. The signals of the two MOPAs are then coherently combined and their mutual coherence is studied. In each OPLL, the acquisition range is increased to +/-1.1GHz with the help of an aided- acquisition circuit. Control of the phase of a single slave MOPA is demonstrated using a RF phase shifter. The differential phase error between two MOPAs locked to the common reference laser is typically 22 degrees

Attosecond pulse carrier-envelope phase effects on ionized electron momentum and energy distributions

We analyze carrier-envelope phase (CEP) effects on electron wave-packet momentum and energy spectra produced by one or two few-cycle attosecond xuv pulses. The few-cycle attosecond pulses are assumed to have arbitrary phases. We predict CEP effects on ionized electron wave-packet momentum distributions produced by attosecond pulses having durations comparable to those obtained by Sansone et al. [Science 314, 443 (2006)]. The onset of significant CEP effects is predicted to occur for attosecond pulse field strengths close to those possible with current experimental capabilities. Our results are based on single-active-electron solutions of the three-dimensional, time-dependent Schrödinger equation including atomic potentials appropriate for the H and He atoms