Efficient laser isotope separation of 13C using novel linear multi-pass cavity

¹³C isotope has been separated in the form of enriched product C₂F₄ by selective multi-photon dissociation (MPD) of Freon-22 (CHClF₂) using the 9P(26) laser line of a transversely excited atmospheric CO₂ laser. The non-linearity factor, γ, that determines the dependence of the yield of ¹³C isotope on the fluence (J/cm²) has been determined for various laser rotational lines (9P(20)–9P(26)) and the advantage of a lower γ in the case of 9P(26) is highlighted for macroscopic production of ¹³C isotope. It is also shown that a higher value of the optimum fluence at 9P(26) not only results in a higher enrichment efficiency but in a relatively lower value of γ also. The laser pulse energy is efficiently utilized for selective MPD of Freon-22 by focusing the pulse energy repeatedly with the help of a novel linear multi-pass cavity (LMPC). The novelty of this optical arrangement lies in its ability to maintain the laser fluence around an optimum value for a desired enrichment of ¹³C in the product. This also ensures a higher quantity of enriched product because of the higher reaction volume. The advantage of the LMPC over the conventionally used Herriott multi-pass cell has also been presented. The gain in reaction volume in the present optical cavity having 20 passes with a constant fluence in each pass is as high as 12. Isotope-selective MPD of Freon in a LMPC with constant fluence in each pass showed a distinct advantage in energy utilization to separate ¹³C isotope over the gradually reducing fluence case.This revised version was published online in August 2005 with a corrected cover date.     

Transfer of Au Cusp-clusters on Si(111) 7×7 surfaces from a pure Au tip of a scanning tunneling microscope

Received: 16 August 1996/Accepted: 6 March 1997     

Femtosecond thin-disk Yb:KYW regenerative amplifier

We report a compact thin-disk Yb:KYW regenerative amplifier system. Two different concepts are investigated to obtain either subpicosecond pulses with up to 160 μJ or a pulse energy of 20 μJ with a pulse width of about 300 fs. The first concept uses intra-cavity group-velocity dispersion compensation with Gires–Tournois interferometer mirrors to avoid pulse stretching during amplification. The onset of nonlinear effects in this concept inhibits the generation of shorter pulse durations at this energy level. Shorter pulses can be achieved with the second concept, which is based on dispersive pulse stretching during amplification and uses pulse compression after amplification with a grating compressor. Repetition rates up to 45 kHz are demonstrated.This revised version was published online in May 2005. The Article Category was removed.This revised version was published online in August 2005 with a corrected cover date.     

Mode profiles in resonators with a near 90° roof-top mirror

The transverse modes of a laser resonator comprising a near 90° roof-top mirror and a feedback mirror are studied theoretically. We relate this configuration to an almost plane roof-top resonator and apply the Fox and Li technique to compute the lowest-order symmetric and antisymmetric modes. The effects of perturbations to the vertex angle of the roof-top mirror, misalignments of the feedback mirror and curved feedback mirrors are examined.     

Interest-Driven Model for Human Dynamics

Empirical observations indicate that the interevent time distribution of human actions exhibits heavy-tailed features. The queuing model based on task priorities is to some extent successful in explaining the origin of such heavy tails, however, it cannot explain all the temporal statistics of human behavior especially for the daily entertainments. We propose an interest-driven model, which can reproduce the power-law distribution of interevent time. The exponent can be analytically obtained and is in good accordance with the simulations. This model well explains the observed relationship between activities and power-law exponents, as reported recently for web-based behavior and the instant message communications.     

Hierarchy,cities size distribution and Zipf's law

We show that a hierarchical cities structure can be generated by a self-organized process which grows with a bottom-up mechanism, and that the resulting distribution is power law. First we analytically prove that the power law distribution satisfies the balance between the offer of the city and the demand of its basin of attraction, and that the exponent in the Zipf's law corresponds to the multiplier linking the population of the central city to the population of its basin of attraction. Moreover, the corresponding hierarchical structure shows a variable spanning factor, and the population of the cities linked to the same city up in the hierarchy is variable as well. Second a stochastic dynamic spatial model is proposed, whose numerical results confirm the analytical findings. In this model, inhabitants minimize the transportation cost, so that the greater the importance of this cost, the more stable is the system in its microscopic aspect. After a comparison with the existent methods for the generation of a power law distribution, conclusions are drawn on the connection of hierarchical structure, and power law distribution, with the functioning of the system of cities.     

Properties of on-line social systems

We study properties of five different social systems: (i) internet society of friends consisting of over 10⁶ people, (ii) social network consisting of 3 × 10⁴ individuals, who interact in a large virtual world of Massive Multiplayer Online Role Playing Games (MMORPGs), (iii) over 10⁶ users of music community website, (iv) over 5 × 10⁶ users of gamers community server and (v) over 0.25 × 10⁶ users of books admirer website. Individuals included in large social network form an Internet community and organize themselves in groups of different sizes. The destiny of those systems, as well as the method of creating of new connections, are different, however we found that the properties of these networks are very similar. We have found that the network components size distribution follow the power-law scaling form. In all five systems we have found interesting scaling laws concerning human dynamics. Our research has shown how long people are interested in a single task, how much time they devote to it and how fast they are making friends. It is surprising that the time evolution of an individual connectivity is very similar in each system.     

Frequency noise suppression in a diode laser locked to a travelling wave resonator incorporating a Brewster prism

A polarisation locking technique was applied to stabilise an extended cavity diode laser using a travelling wave resonator incorporating a Brewster prism. Despite the fact that the employed unbalanced detection was sensitive to optical power fluctuations, the in-loop photodetector measured 90 dB of noise suppression at 10 Hz in comparison to the free running frequency noise spectrum. Excess intensity noise measured with an out-of-loop detector, indicated the presence of correction-correlated noise in the output of the stabilised diode laser.     

An all-fiber frequency-shifted feedback laser for optical ranging; signal variation with distance

A recent paper [L.P. Yatsenko et al., Opt. Commun. 242 (2004) 581] provided a first-principles prediction for the optical ranging signals obtained when using a frequency-shifted feedback (FSF) laser system, seeded by a phase-modulated laser. Such a system has many useful advantages over other alternative FSF laser techniques. We report here experimental verification of that theory, specifically the variation of the amplitude modulation signal with both distance and modulation index of the seed laser. We describe the operation of an all-fiber FSF laser that uses an Er³⁺-doped active fiber as the gain medium. To improve the signal and minimize the noise we seed the FSF laser with a phase-modulated (PM) laser; the measurement of distance derives from a measurement of amplitude modulation within a narrow frequency interval. We demonstrate that the resulting system is capable of fast and precise measurements. With the bandwidth limitations of our current system we achieved an accuracy better than 0.1 mm. Although measurements based on interferometry offer the potential for much greater accuracy under carefully controlled conditions, the present method does not suffer from the presence of a material-dependent phase shift at the surface of the measured object.     

A crossed optical cavities apparatus for a precision test of the isotropy of light propagation

A novel apparatus for a sensitive test of the independence of the speed of optical waves from the propagation direction has been developed. It employs a monolithic ULE glass structure containing two orthogonal, crossing Fabry-Perot cavities which enable common mode rejection of certain disturbances. Highly accurate locking and cavity frequency read-out are achieved using laser frequency modulation at audio frequencies. Several systematic effects were characterized. Without rotation the root Allan variance (RAV) of the beat frequency reaches a minimum of 0.5 Hz (2 × 10⁻¹⁵) close to the thermal noise floor of the cavities. The performance of the apparatus under rotation is demonstrated by determining with improved accuracy one parameter of the standard model extension test theory,  = (−1.0 ± 2.3) × 10⁻¹⁵, under simplifying assumptions.