We analyze the performance of a free-space optical (FSO) link affected by atmospheric turbulence and line-of-sight (LOS) blockage. For this purpose, the atmospheric turbulence induced fading is modeled by the M-distribution, which includes the Gamma-Gamma distribution as special case. We exploit the fact that the physical interpretation of the M-distribution allows to split the optical energy through the propagation link into three different components: two coherent components and one incoherent scatter component. Based on this separation, we derive novel analytical expressions for the probability density function (PDF), for the cumulative distribution function (CDF) and for the moment generating function (MGF) of the M-distribution under the temporary blockage of the coherent components, hereinafter referred to as LOS blockage. Further, a new closed-form expression for the outage probability (OP) under LOS blockage is derived in terms of the turbulence model parameters and the LOS blockage probability. By means of an asymptotic analysis, this expression is simplified in the high-SNR regime and the OP in terms of the diversity order and diversity gain is then deduced. Obtained results show that the impact of the LOS blockage on the OP strongly depends on the intensity of the turbulence and on the LOS blockage probability.

To achieve quantum-noise-limited performance, background-limited laser receivers require narrow-band optical filters. We measured and modeled the ultranarrow-band transmission spectrum of a Cs Faraday filter at 852 nm. The transmission spectrum consisted of passbands on either side of the 6 (2)S((1/2))-6 (2)P(3/2) hyperfine doublet lines, making a total of four. The passbands may be simple peaks or highly modulated, depending on the operating parameters. We observed peaked passbands of near-unity transmission with a 0.6-GHz bandwidth and modulated bands with features as sharp as 100 MHz. Excellent agreement with our calculations at 852 nm allows us to predict confidently a 0.7-GHz transmission band for Cs at 455 nm.

No abstract

Ablation of holes with diameters as small as 82 nm and very clean walls was obtained in poly(methyl methacrylate) focusing pulses from a Ne-like Ar 46.9 nm compact capillary-discharge laser with a freestanding Fresnel zone plate diffracting into third order. These results demonstrate the feasibility of using focused soft x-ray laser beams for the direct nanoscale patterning of materials and the development of new nanoprobes.

No abstract