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].