Bandpass filters based on pi-shifted long-period fiber gratings for actively mode-locked erbium fiber lasers

We have fabricated bandpass filters based on pi-shifted long-period gratings for application in actively mode-locked erbium fiber lasers. Introducing the pi-phase shift in the middle of the grating opens a bandpass within the core-cladding mode resonance peaks. With a 22-nm bandwidth filter inserted in an actively mode-locked erbium fiber sigma laser, solitonlike pulses are generated, with a power-dependent duration of approximately 3-5 ps , at a 3-GHz repetition rate. These all-fiber filters have the advantages of low insertion loss (<0.5 dB) and a wide bandwidth (10-20 nm), and they do not require that a circulator be inserted into the laser cavity.     

Measurement of pulse width and amplitude jitter noises of gigahertz optical pulse trains by time-domain demodulation

We propose a technique for measuring both pulse width and amplitude jitter noises of high-repetition-rate optical pulse trains and the cross correlation between these noises as well. The technique is based on time-domain amplitude demodulation of three harmonic components of the detected pulse train. We applied this technique to characterize noises of a gigahertz optical pulse train generated by an actively mode-locked Er-doped fiber laser. Correlation between pulse width jitter and pulse amplitude jitter was observed at low frequencies in this laser. Unlike relaxation oscillation noise, low-frequency noise is free from pulse energy jitter. Owing to its ability to measure pulse width jitter in addition to amplitude and phase jitters, this technique is of great interest for characterizing noises of a wide variety of optical pulse train sources.     

Secure communication scheme using chaotic laser diodes subject to incoherent optical feedback and incoherent optical injection

We propose a secure communication scheme based on anticipating synchronization of two chaotic laser diodes, one subject to incoherent optical feedback and the other to incoherent optical injection. This scheme does not require fine tuning of the optical frequencies of both lasers as is the case for other schemes based on chaotic laser diodes subject to coherent optical feedback and injection. Our secure communication scheme is therefore attractive for experimental investigation.     

Comparison of two types of synchronization of external-cavity semiconductor lasers

We study numerically the synchronization of external-cavity semiconductor lasers in a master-slave configuration, based on a Lang-Kobayashi-type model. Depending on the feedback and coupling strengths, the slave laser synchronizes with the injected optical field or with the injected field but lags in time. We show that these two types of synchronization present different robustness with respect to the noise, frequency detuning, and current modulation of the master laser.     

Photodetachment microscopy of the P, Q, and R branches of the OH(-)(v=0) to OH(v=0) detachment threshold

A photodetachment experiment is performed on the v=0-->v=0 OH(-) detachment threshold. The weak O and S branches provide a signal strong enough to make amplitude measurements on all five O, P, Q, R, and S branches possible, which are used to fix the formulas for their relative intensities. Photodetachment microscopy is applied to 15 different thresholds of the P, Q, and R branches. The quantitative analysis of the interference patterns obtained does not show any effect of the dipole moment of OH, but yields a new measurement of the rotational parameters of OH(-)(v=0) and of the electron affinity of the molecule. The new recommended value for the electron affinity of (16)O(1)H is 14 740.982(7) cm(-1) or 1.827 648 7(11) eV.     

Low noise submillimeter SIS receiver with niobium nitride quasiparticle tunnel junctions

We have developed and tested a submillimeter waveguide SIS mixer with NbN-MgO-NbN quasiparticle tunnel junctions. The two junction array is integrated in a full NbN printed circuit. The NbN film critical temperature is 15 K and the junction gap voltage is 5 mV. The size of the junctions is 1.4 × 1.4 µm and Josephson critical current density is about 1.5 KA/cm² resulting in junction R_NC product about 40. The inductive tuning circuit in NbN is integrated with each junction in two junction array. A single non contacting backshort was tuned at each frequency in the mixer block.At 306 GHz the minimum DSB receiver noise temperature is as low as 230 K. The sources of the receiver noise and of the limits of the NbN SIS submillimeter mixer improvement are discussed.     

Effect of a magnetic field in photodetachment microscopy

The effect of an external static magnetic field of arbitrary orientation with respect to the electric field, on the electron interference ring patterns observed by the photodetachment microscope is studied both experimentally and theoretically. The design of the interaction chamber has been modified to superimpose a controlled uniform magnetic field on the whole volume accessible to the interfering electron. Contrary to a previous study in weaker fields, where the overall dimension of the interferogram was not modified, the effect of the magnetic field here encompasses a regime of magnetic refocusing. A quantitative analysis is carried out using a closed-orbit perturbative calculation of the interference phase at the centre of the ring pattern. The essential result of this work is still the invariance of the extreme interference phase whatever the direction and magnitude of the applied magnetic field, up to values 100 times larger than in the previous experimental study. This property can be applied to revise former electron affinity measurements. Partly due to the previously unsuspected robustness of the electron interferograms vs. magnetic fields, partly thanks to the 2006 CODATA revision of the energy conversion factors, one can update the values of the electron affinities of ¹⁶O, ²⁸Si and ³²S to 1.4611134(9), 1.3895210(7) and 2.0771040(6) eV respectively.     

Developments in the application of multibeam sonar backscatter for seafloor habitat mapping

Human impacts on the seafloor environment have reached unprecedented levels. To facilitate ocean management and mitigate these impacts, there is a need to improve our understanding of seabed habitats. Recent developments in acoustic survey techniques, in particular multibeam echosounders (MBES), have revolutionised the way we are able to image, map and understand benthic ecosystems. Using MBES, it is now cost-effective to image large areas of the seafloor, and such surveys provide baseline data from which thematic maps of the seabed environment, including maps of benthic habitat, can be derived and interpreted in conjunction with in situ ground-truthing data. This paper provides an overview of recent developments in the application of MBES for seafloor habitat studies, with a focus on the use of backscatter data for surficial geology and habitat mapping. In March 2006, a MBES backscatter workshop brought together a number of international research teams/groups working on novel methods for interpretive/classification routines for segmentation of the backscatter into acoustic classes with the potential to facilitate the delineation of seabed geological and habitat characteristics. This paper introduces the common data set used as part of the workshop, sets out the research context in which the different studies were conducted, and outlines the main themes of the papers presented in Section 6.     

Experimental and Monte Carlo investigation of visible diffuse-reflectance imaging sensitivity to diffusing particle size changes in an optical model of a bladder wall

We have investigated the influence of scatterer size changes on the laser light diffusion, induced by collimated monochromatic laser irradiation, in tissue-like optical phantoms using diffuse-reflectance imaging. For that purpose, three-layer optical phantoms were prepared, in which nano- and microsphere size varied in order to simulate the scattering properties of healthy and cancerous urinary bladder walls. The informative areas of the surface diffuse-reflected light distributions were about 15×18 pixels for the smallest scattering particles of 0.05 μm, about 21×25 pixels for the medium-size particles of 0.53 μm, and about 25×30 pixels for the largest particles of 5.09 μm. The computation of the laser spot areas provided useful information for the analysis of the light distribution with high measurement accuracy of up to 92%. The minimal stability of 78% accuracy was observed for superficial scattering signals on the phantoms with the largest particles. The experimental results showed a good agreement with the results obtained by the Monte Carlo simulations. The presented method shows a good potential to be useful for a tissue-state diagnosis of the urinary bladder.