Mousumi Basu

Generation of Self-Similar Parabolic Pulses by Designing Normal Dispersion Decreasing Fiber Amplifier as Well as Its Staircase Substitutes

Generation of self-similar parabolic pulse is analytically and numerically demonstrated by designing parabolic index normal dispersion decreasing fiber (NDDF) amplifiers. The pulse transmission is extensively studied for NDDFs in presence of physical gain as well as virtual gain induced by two different dispersion profiles corresponding to two different physical gain coefficients. Here, we introduce the virtual gain arising from the unavoidable spatial nonlinear variation, which helps to obtain the self-similar parabolic pulses at smaller optimum length in comparison to NDDF with constant nonlinearity. The output power profiles resemble with a perfect parabolic shape giving rise to self-similar pulses with very small misfit parameters. Pulse propagation in presence of spatial gain variation is also studied. To avoid fabrication difficulties, we propose equivalent staircase dispersion profiles consisting of a number of constant dispersion fibers (CDFs), which are simple to manufacture and show performances excellently close to that of the proposed NDDF.

Effect of grading on the characteristics of a dispersion compensated fibre

Abstract The effect of grading on the performance of a dispersion compensated optical fibre (DCF) has been studied using spot size technique and been compared with the performance of a conventional step index DCF. The percentage change in the figure of merit, i.e. ( dFOM / FOM (%)), of the graded index DCFs with exponent q values equal to 1, 2, 4 and 8 with respect to the step index ( q =∞) DCF has been calculated. The comparison with step index DCF shows that the maximum positive change in the figure of merit ( FOM ) occurs in the case of a triangular core ( q =1) DCF.

Modified figure of merit for dispersion compensated optical fibres

The conventional figure of merit (CFOM), defined as the ratio of the dispersion to the loss with the units of ps/nm-dB, is the most widely used definition to characterise a dispersion compensated fibre (DCF). Generally, CFOM is estimated by taking into account only the length dependent losses such as bend and Rayleigh scattering losses. But when length independent loss such as splice loss is considered, the CFOM is modified. This gives rise to a new definition of figure of merit and here is termed as modified figure of merit (MFOM). It has been found that in general MFOM is the better choice for characterising a DCF as compared to the CFOM.

Studying the effect of a central dip on the performance of a dispersion compensated fibre

Abstract The spot size technique has been used to study the effect of a central dip on the performance of a dispersion compensated fibre (DCF). A Gaussian dip around the axis of the fibre core has been considered. Such a type of dip in the refractive index profile is obtained during the fibre fabrication by modified chemical vapour deposition (MCVD) technique. The percentage change in figure of merit (dFOM/FOM (%)) of a DCF with a central dip with respect to a DCF having no dip has been estimated for different dip depths and dip widths, respectively. It is seen that for larger values of bend radii with large negative dispersion values the magnitude of dFOM/FOM (%) changes rapidly with change in other fibre losses, dαr, than in case of smaller bend radii with small dispersion values.

GeO2 nanorods: synthesis, structural and photoluminescence properties

One-dimensional (1D) GeO2 nanorods with smooth surface and uniform diameter throughout their length, are synthesized at a relatively lower temperature by hydrothermal technique in the presence of aluminum foil. Further, the nanorods are doped with rare earth element erbium. The products are characterized by XRD, HRTEM, EDS, FTIR, PL techniques. Synthesized nanorods with diameter in the range ~60–100 nm have core–shell type structure. HRTEM and EDS results reveal that the crystalline core is made by hexagonal α-quartz type GeO2 and amorphous shell contains compound of Al, Ge and O2. The role of this amorphous outer layer for unidirectional growth of the nanorods is discussed in detail. PL study reveals that the synthesized nanorods are capable of emitting a strong band in the violet–blue region. Furthermore, the product can emit light in the green and red region. The Er-doped nanorods also show luminescence around 1533 nm under non-resonant excitation confirming the successful inclusion of Er3+ ions in the nanorods. Consequently, the as-synthesized materials can be potentially used in a nano-luminescent device in a broad spectrum and as a material of the core in an optical fiber amplifier.

Nonlinear pulse reshaping in a designed erbium-doped fiber amplifier with a multicladded index profile

A multicladded normally dispersive erbium-doped fiber amplifier (ND-EDFA) is designed for a short length to operate at the wavelength of 1550 nm with a dispersion of −6.5 ps∕kmnm and parabolic pulse gener- ation through the proposed fiber is studied. The proposed ND-EDFA shows a flattened gain spectrum in C-band. The nonlinear Schrodinger equation is solved numerically in presence of fiber gain, nonlinearity, and dispersion to investigate the pulse propagation through the proposed fiber. While continuous wave (CW) sources are considered, parabolic self- similar pulses with structure factor of 0.072 are created at suitable values of optimum fiber length when input pulse properties and fiber parameters are optimized accordingly. Side by side with a low repetition rate laser source, the pulse propagation equation is controlled by the gain dispersion term and dipole relaxation time, such that the evolution of Gaussian pulses may lead to nonparabolic regime. The effects of pulse parameters like power level, pulse width, and dipole relaxation time on the propagation of input Gaussian pulses through the so-designed ND-EDFA are investi- gated. Our results depict that the pulses with same input energy reshape into exactly parabolic shape for CW laser source or nonparabolic profile for

Parabolic and semiparabolic pulse dynamics in optical fibers

Abstract. Nonlinear pulse dynamics in two stages of different active or passive fibers are investigated in this work. Numerical approach of the symmetrized split step Fourier method is used to solve the nonlinear Schrödinger equation in the presence of fiber gain, nonlinearity, and dispersion. An input Gaussian pulse evolves into a linearly chirped perfect parabolic pulse (PP) when it propagates through a standard normal dispersion decreasing fiber amplifier. At the same time, for an erbium-doped dispersion decreasing fiber amplifier with a similar dispersion variation with length, the semiparabolic pulse (SPP) is produced at the output end of the fiber. To our knowledge, this is shown for the first time. In second stage, the so-obtained perfect PP, SPP, and also a chirp-free perfect PP are fed into the input of several normal dispersion fibers and the comparative pulse evolution is studied in detail with the variations of dispersion coefficient, gain, and nonlinearity. While using these pulses as the input of an anomalous dispersion fiber, our result shows that the linearly chirped PP is most efficient for compressing the pulses with a good quality factor without dropping significant pedestal energy.

Dispersion-Compensating Graded Index Multiclad Fiber: Optimization for Dispersion-Managed WDM Transmission Systems

Abstract This work reports the design optimization of a single-mode graded index multiclad dispersion-compensating fiber (DCF) with a central dip, for broadband wavelength division multiplexing (WDM) system in the C- and L-bands of an operating wavelength zone. The index profile parameters of this fiber have been adjusted to simultaneously achieve high figure of merit (FOM) as well as considerably high value of effective core areas of the fiber to minimize the nonlinear effects like self-phase modulation or cross-phase modulation. At 1,550 nm operating wavelength, an effective core area (A eff ) of 46 μm2, which is very large compared to other reported values of DCFs, is obtained here. The average dispersion of the DCFs, in combination with conventional single-mode fiber (CSF) and small dispersion fiber (SDF), are found out to be considerably flat in the entire C- and L-band zone of operating wavelength.

Efficient parabolic similariton geneartion by third order dispersion compensation

Self-similar parabolic pulse generation is studied for a designed fiber having a high value of third order dispersion (TOD) so that the effect of TOD can be realized in the proposed fiber. As TOD destroys the pulse shape leading to optical wave breaking, a compensation method for TOD in practical fibers is extremely needed. In this paper, the deleterious effect of TOD is efficiently compensated by introducing the time reversal system in self-similar parabolic pulse formation and as a result linearly chirped parabolic pulses are obtained even in presence of high value of TOD.

Theoretical Design of Normal Dispersion Decreasing Fiber Amplifier to Obtain Self-Similar Parabolic Pulses and Its Practical Aspects

The evolution of self-similar parabolic pulses is demonstrated by designing a parabolic index dispersion decreasing fiber amplifier with normal dispersion (ND-DDF amplifier), with inclusion of the variation of nonlinear factor (gamma) with distance. The pulse transmission is extensively studied for the proposed ND-DDF in presence of external gain as well as virtual gain induced by its dispersion profile. The output power resembles with a perfect parabolic shape giving rise to self-similar pulses with minimum misfit les 0.003. In view of practical aspects of fabrication we also obtain several equivalent staircase profiles, whose performances are shown to be very close to that of the proposed ND-DDF.