Full characterization of Gaussian bipartite entangled states by a single homodyne detector.

We present the full experimental reconstruction of Gaussian entangled states generated by a type-II optical parametric oscillator below threshold. Our scheme provides the entire covariance matrix using a single homodyne detector and allows for the complete characterization of bipartite Gaussian states, including the evaluation of purity, entanglement, and nonclassical photon correlations, without a priori assumptions on the state under investigation. Our results show that single homodyne schemes are convenient and robust setups for the full characterization of optical parametric oscillator signals and represent a tool for quantum technology based on continuous variable entanglement.

Fringe pattern of the field diffracted by axicons.

The far-field intensity pattern of laser beams diffracted by axicons is extensively characterized both theoretically and experimentally. The regular structure of the pattern, consisting of high-contrast fringes, is explained. The experimental results have been interpreted by representing the diffracted field as generated by an extended virtual source shaped as a circle centered on the optical axis of the incident laser beam. The simulations include modifications to the diffraction pattern arising from the laser radiation diffraction limit at the axicon tip, and they reproduce well the measured intensity profile at different distances from the axicon.

Longitudinal study of vasopressin-cell antibodies and of hypothalamic-pituitary region on magnetic resonance imaging in patients with autoimmune and idiopathic complete central diabetes insipidus.

Diagnosis of autoimmune central diabetes insipidus (CDI) is based on the presence of autoantibodies to AVP-secreting cells (AVPcAb) or the coexistence of other autoimmune polyendocrine syndromes; moreover, it can be also suggested by the presence of lymphocytic infundibulo-neurohypophysitis, evidenced by biopsy of pituitary stalk and/or by pituitary stalk thickening on magnetic resonance imaging (MRI). However, so far, in clinical CDI patients with lymphocytic infundibulo-neurohypophysitis, AVPcAb have not been investigated and in those with or without autoimmune polyendocrine syndromes (APS), longitudinal studies on the behavior of AVPcAb alone, or of both AVPcAb and hypothalamic pituitary imaging on MRI are lacking. Aim of this work was to investigate in these patients the occurrence of AVPcAb (by indirect immunofluorescence) and of pituitary stalk thickening (by MRI) and their longitudinal changes during a follow-up period. We studied 22 patients, aged 29-53, with APS and complete CDI, grouped as follows: 10 with recent onset (< or =1.5 yr) of CDI (group 1a) and 12 with CDI of long-term duration (> or = 7 yr) (group 1b); moreover, a group of 13 patients with apparent idiopathic CDI of recent onset (<1.5 yr) were studied. They were divided, on the basis of the detection of AVPcAb as follows: 5 AVPcAb positive patients (aged 19-26) classified as isolated autoimmune CDI (group 2) and 8 AVPcAb negative patients (aged 21-26), classified as true idiopathic CDI (group 3). All patients were evaluated yearly, along 5 yr, for AVPcAb and for hypothalamic-pituitary region imaging. At study entry, 8/10 (80%) of patients in group 1a and 7/12 (58.3%) in group 1b were positive for AVPcAb and persisted positive subsequently, during all the follow-up period, even if at lower titers. All patients in group 2 were positive and all those in group 3 were negative for AVPcAb and persisted positive and negative, respectively, for all the follow-up study. Among the AVPcAb-positive patients, only 5 in group 1a and 2 in group 2 showed also pituitary stalk thickening at the first observations, which however spontaneously disappeared subsequently indicating a possible lymphocytic infundibulo-neurohypophysitis. All patients in the studied groups showed loss of the hyperintense signal of the neurohypophysis on MRI at entry and during all the follow-up period. Results of this longitudinal study suggest: 1) AVPcAb, frequently present at high titers in recent phases of CDI, persist subsequently, even if at lower titers, several years after the onset of disease. 2) The occurrence of a lymphocytic infundibulo-neurohypophysitis suggested by the pituitary stalk thickening on MRI only in patients positive for AVPcAb confirms a further autoimmune variant of CDI also in these cases. 3) The longitudinal behavior of patients in group 3 suggests that the absence of AVPcAb at the onset of clinical idiopathic CDI is able to exclude a subsequent appearance of these antibodies and consequently an autoimmune involvement in CDI of these patients. Instead the finding of AVPcAb in several patients with only CDI, thought at first clinical observation as idiopathic, indicates that the prevalence of autoimmune CDI must be considered much higher than that so far reported.

Mode size and time duration fluctuations in a picosecond Nd:YAG laser.

A new technique is successfully used to analyze in real time the pulse-to-pulse fluctuations of mode size and time duration in a picosecond Nd:YAG laser. In particular we show that the pulse length (30 psec) of our active-passive mode-locked Nd:YAG laser is stable to within 10% when the cavityis perfectly tuned and the saturable absorber is fresh. This technique is experimentally shown to be effective and reliable for real-time analysis of the stability of ultrashort laser pulses undera broad range of experimental conditions.

Alignment and performance of almost concentric resonators for low gain free-electron lasers.

The propagation of mismatched and misaligned laser beams bouncing back and forth between the mirrors of an almost concentric resonator is investigated with reference to the alignment of the cavities used for low gain free-electron lasers. In addition, the excitation of the cavity modes by the spontaneous radiation of a linear undulator is analyzed by considering the reduction of their coupling factors due to the misadjustment of the optical cavity with respect to the axis and the center of the undulator. In particular, analytical expressions are given for the coupling coefficients of two misaligned sets of Gauss-Laguerre or Gauss-Hermite modes and for the coupling factors between the cavity modes and the spontaneous radiation.

Use of indeterminate Padè approximations in the analysis of diffraction at the mirror edges of an optical resonator.

A method is presented for analysis of the diffraction of waveguide modes at the open end of a two-dimensional metallic waveguide, based on a rational approximation of the field diffracted outside the open end. We believe that this method can be used for waveguides with nonplane walls, and it should prove a useful tool in the analysis of optical resonators.