The kinetics of oxidation of kappa-carrageenan (KCAR) as a sulfated polysaccharide by cerium(IV) in aqueous perchlorate solutions at a constant ionic strength of 2.0 moldm−3 have been investigated, spectrophotometically. The results showed a first-order dependence in [CeIV] and fractional-first-order kinetics in carrageenan concentration. A kinetic evidence for the formation of 1:1 complex has been revealed. The hydrogen ion dependence of the reaction rate indicated that the oxidation process is acid catalyzed. The oxidation product was identified by the spectral data and elemental analysis. The activation and thermodynamic parameters have been evaluated and a relevant reaction mechanism is suggested and discussed.

Effect of the linewidth enhancement factor on the operation of InGaAs/InP pumping lasers emitting a wavelength of 980 nm under strong optical feedback is investigated. The investigations are performed based on a new external optical feedback model for semiconductor lasers, which is applicable under any arbitrary strength of optical feedback. The results show that the semiconductor laser operates under strong optical feedback in CW, pulsation and chaos and pulsation operation at lower and higher values of the linewidth enhancement factor respectively. We predict that semiconductor laser subjected to strong optical feedback exhibits much more stable pulsing operation under higher values of the linewidth enhancement factor which, means that the laser is locked at the external cavity frequency.

We present a numerical simulation on the effect of the linewidth enhancement factor on the intensity noise and operation of laser diodes subject to strong optical feedback. The simulation are based on an improved time-delay rate equations model of a single-mode laser that takes into account the multiple round-trips of the lasing field in the fiber cavity and is applicable under any arbitrary strength of optical feedback [1]. The analyses are performed in terms of the temporal trajectory of the laser intensity, bifurcation diagram and relative intensity noise. The model is applied to stimulate output characteristics and intensity noise in InGaAs/InP lasers in a wavelength of 980 nm. The simulation results indicted that the laser under strong optical feedback mainly operates in Cw, pulsation or chaos operation depending on the linewidth enhancement factor value [2]. When the linewidth enhancement factor is low, the regime of strong optical feedback is characterized by either continuous-wave (CW) operation or pulsation. The pulsation frequency is locked at the frequency separation of the external cavity modes. When the linewidth enhancement factor is medium, the laser exhibits pulsing and unstable dynamics like chaotic operation over wide range of strong optical feedback. At higher values of the linewidth enhancement factor the pulsing operation becomes more dominant over wider range of strong optical feedback. The corresponding RIN level is close to or higher than the level of the solitary laser depending on optical feedback strength and the linewidth enhancement factor. The optical feedback noise is found to be as low as the quantum noise level when the laser is operated in pulsing region and at higher values of the linewidth enhancement factor. References [1] S. Abdulrhmann, M. Ahmed, T. Okamoto, W. Ishimori and M. Yamada “An improved analysis of semiconductor laser dynamics under strong feedback”, IEEE J. Select. Topics Quantum., 9(5), 1265-1278, 2003. [2] S. Abdulrhmann and Minoru Yamada “Numerical Simulations of the Effect of the Linewidth Enhancement Factor on the Operation of Pumping Lasers under Optical Feedback”, The Second Arab International Conference in Physics and Materials Science (CPMS), Alexandria, Egypt,October 27 - 29, 2007.

Semiconductor lasers are the most important light sources in optical communication systems. Further improvements of performance and characteristics of the devices contribute to further development of the communication systems. In this regard, two important physical properties affecting performance of semiconductor lasers have been analyzed; namely the quantum noise and optical feedback. The performance of semiconductor lasers, which is used in optical communication systems, is theoretically investigated. Effects of nonlinear gain and nonradiative recombination on characteristics of intensity and phase noises of lasers are investigated. The results showed that both intensity and frequency noises around the relaxation frequency are suppressed when counting the nonlinear gain in the rate equations [1]. An improved theoretical model to analyze dynamics, operation and noise of semiconductor lasers under arbitrary strength of optical feedback ranging from very weak to very strong optical feedback is presented [2]. A new set of modified rate equations of lasers operating under optical feedback are proposed [2]. The simulation results showed that the operations of semiconductor lasers are classified into continuous wave (CW), chaotic, and pulsing operations depending on the operating conditions. The optical feedback noise is found to be as low as the quantum noise level when the laser is injected well above its threshold level [3].

This paper investigates the effect of the linewidth enhancement factor on the operation and intensity noise fluctuations of 980-InGaAs/InP fiber grating lasers operating under strong optical feedback. The analyses are performed in terms of the temporal trajectory of the laser intensity, bifurcation diagram and relative intensity noise. The influence of intrinsic fluctuations in the intensity and optical phase of the lasing field on the laser dynamics were taken into account. The laser diode operates in pulsation under strong optical feedback and at small and relatively large values the linewidth enhancement factor. The optical feedback noise is found to be as low as the quantum noise level when the laser is operated in pulsing region and at relatively large values of the linewidth enhancement factor

This paper investigates influence of the linewidth enhancement factor on the output of 980-InGaAs/InP fiber grating lasers operating under strong optical feedback. The study is based on intensive numerical integration of an improved time-delay rate equations of semiconductor lasers over wide ranges of the linewidth enhancement factor and ratio of the grating reflectivity to that of the anti-reflecting front facet. The results show that the laser operates in continuous wave and pulsation at small values of the linewidth enhancement factor. Under large values of the factor, the laser happens to exhibits chaos and pulsation. However, such pulsation is predicted to be more stable due to locking of the laser oscillation at the external cavity resonance frequency

This paper investigates dynamics of semiconductor lasers under optical feedback during transitions to chaos. Influence of the external-cavity length on the route-to-chaos is elucidated. The study is based on numerical solution of a time-delay model of rate equations. The route to chaos is period doubling when the ratio of the relaxation frequency to the external-cavity resonance frequency is less than unity. The route is sub-harmonic when the frequency ratio is slightly higher than unity, and is quasi-periodic characterized by the compound-cavity frequency and the relaxation frequency as well as their difference when the ratio is far above unity