Some Optical Transmission Problems In Optical Fiber Communication And Fiber Grating External Cavity Semiconductor Laser

A numerical approach is often employed for understanding optical transmission process in fibers, for the reason that the equation modeling optical propagation in fibers is a nonlinear partial differential equation (sometimes is called nonlinear Schrodinger equation), which cannot generally be given analytical solutions. Various numerical methods have been presented for this purpose. Among these approaches, the Split-step Fourier method (SSFM) is most widely used for its relative faster speed. To maintain the required accuracy, the implementation of the SSFM requires that the step size be selected carefully, which has been studied by many authors. However, the selective problem of the time window width, which has important effect on the numerical simulation, was rarely reported. It is well known that for an optical pulse, its distribution in time domain is infinite. But only a limited time window width T_w may be selected in numerical calculations because of the limitation of the microcomputer. Therefore, the selection of T_w is also a key aspect that should be considered carefully in numerical simulations. In this work, after giving the selective method of the time window width and the minimum time window width required for corrected calculations of optical pulse propagation in fibers, the shapes of various incident pulses after passing through different fiber lengths have been simulated by selecting different time window widths.The Gaussian model, which assumes that an incident pulse has a Gaussian profile, has been widely used to investigate the optical pulse transmission in fibers. At present, signal pulses produced by directly modulated laser diodes have been used in optical fiber communication systems. The experiment reveals that the optical pulses from directly modulated laser diodes are far from Gaussian and have near-rectangular temporal profiles with much sharper leading and trailing edges, and then a super-Gaussian model, which assumes that an incident optical pulse has a super-Gaussian profile, may be more suitable. In this work, incident super-Gaussian pulse propagation in conventional single-mode fibers has been investigated in detail after taking into account the fiber chromatic dispersion. The results show that for an incident super-Gaussian pulse with steep leading and trailing edges, its shape undergoes a variation from near-rectangular, two-peak, and finally to single-peak. In the meantime, its peak intensity increases at first, after passing a maximum, and finally decreases monotonously.Fabry-Perot filter (FPF) has been intensively investigated because of its wide applications in optical communications, spectroscopy, laser resonators and so on. In most studies, in order to simplify the physical treatment, the laser beam is usually treated as a plane wave and normal incidence is supposed when its transmission through a FPF is considered. However, in fact, a laser beam is Hermit-Gaussian in general and Gaussian in its fundamental mode. Also, normal incidence is difficult to realize in practice. Based on the above considerations, in this work, the transmission of a Gaussian beam nonnormally through a Fabry-Perot etalon has been investigated. After deriving the expression of the intensity of the transmitted beam, several key characteristics of the transmitted beam have been specified. The results show that the peak intensity of the transmitted beam decreases with the increase of the input angle, the position of the peak intensity of the transmitted beam is shifted, and the spot size of the transmitted beam is changed. Under the nonresonant case (i. e., the carrier frequency of a Gaussian beam deviates from the resonant frequency of a Fabry-Perot etalon), variations of the peak intensity, the position of the peak intensity and theDWDM systems, has received considerable attention. In this work, after taking into account the wavelength-depended reflectivity distribution profile of a fiber Bragg grating, the oscillation wavelength #₁ of long external cavity fiber Bragg grat...