Generation of a sequence of frequency-modulated pulses in longitudinally inhomogeneous optical waveguides

The conditions for the generation and efficient amplification of frequency-modulated soliton-like wave packets in longitudinally inhomogeneous active optical waveguides have been studied. The possibility of forming a sequence of pico- and subpicosecond pulses from quasi-continuous radiation in active and passive optical waveguides with the group-velocity dispersion (GVD) changing over the waveguide length is considered. The behavior of a wave packet in the well-developed phase of modulation instability with a change in the waveguide inhomogeneity parameters has been investigated based on the numerical analysis.     

Dynamics of frequency-modulated soliton-like pulses in a longitudinally inhomogeneous active optical waveguide

Conditions of the formation and amplification of frequency-modulated soliton-like pulses in longitudinally inhomogeneous active optical waveguides with anomalous dispersion of group velocities are studied. Group velocity dispersion profiles that are necessary for existence and amplification for active and passive optical waveguides are determined, and the dependences on the distance passed along the waveguide, duration, and chirp of the pulse are established.     

Pulse compression of a high-order harmonic

The fifth harmonic pulses of an intense femtosecond Ti:sapphire laser were experimentally shown to be negatively chirped by using an LiF plate as a positive dispersive medium. The chirp of the harmonic pulse originates from the intensity-dependent atomic dipole phase, which is estimated to be proportional to 25 U_p, where U_p is the ponderomotive energy. Consequently, we have succeeded in compressing the chirped pulses to 13 fs by compensating the intrinsic negative chirp. Chirp effects of the fundamental laser on the pulse width of the fifth harmonic were consistent with the negative chirp of the fifth harmonic.     

Pulse compression effect based on stimulated Brillouin scattering light storage in an optical fiber

The pulse compression effect in SBS light storage is numerically investigated. We demonstrate theoretically that the compressed width of retrieved data-pulses is not only related with temporal profile of data-pulse, but also spectrum and temple profile of control-pulse. The data-pulses with steep rising edge can be compressed after they are retrieved. The optimum compression effect takes place when a 2-ns-long exponential data-pulse interacts with a 1.5-ns-long chirped Gaussian control-pulse, where the data-pulse width is compressed to 1.73-ns-long and the readout efficiency is the biggest. A 2-ns-long rectangular data-pulse is also compressed to 1.73-ns-long when it encounters to a 1.5-ns-long rectangular control-pulse, but the readout efficiency isn’t maximum value. The results are significant for increasing storage capacity and variable bits all-optical buffering.     

Pulse compression in dispersion-decreasing-like fibers

Pulse compression characteristics in dispersion-decreasing-like fibers (DDLF) are firstly investigated by employing the second-harmonic generation frequency-resolved optical gating (SHG-FROG) technique, are compared with the numerical results that are obtained using the split-step Fourier method. It is found that the experimental data are consistent with the numerical results. The results show that the pulse compression method is effective. The pulse can be effectively compressed and its spectrum can be broadened in the DDLF. That the pulse compression effect is enhanced with the increase of the input power is in correspondence with the spectrum broadening.     

Wavelength-tunable spectral compression in a dispersion-increasing fiber

We demonstrate, both numerically and experimentally, adiabatic soliton spectral compression in a dispersion-increasing fiber (DIF). We show that a positively chirped pulse provides better spectral compression in a DIF with a large anomalous dispersion ramp. An experimental spectral compression ratio of 15.5 is obtained using 350 fs positively chirped input pulse centered at 1.5 μm. A 30 nm wavelength tuning ability is experimentally achieved.     

Pulse compression of nanojoule pulses in the visible using microstructure optical fiber and dispersion compensation

Experimental results on the pulse compression of 1-nJ, 150-fs pulses from a tunable, 76-MHz Ti:sapphire laser oscillator operating at around 750 nm are reported. The length of the pulses can be compressed to nearly one tenth by applying a high-delta, single-mode microstructured optical fiber exhibiting zero group-delay dispersion at 767 nm, and by a prism-pair/chirped-mirror compressor. The experimental results are verified by theoretical investigations modeling the pulse propagation taking into account non-linear self-phase modulation and fiber dispersion. Received: 10 June 2002 / Revised version: 15 November 2002 / Published online: 12 February 2003 RID="*" ID="*"Corresponding author. Fax: +36-1/375-4553, E-mail: szipoecs@sunserv.kfki.hu     

The evolution of two-frequency solitons in an optical fiber with a longitudinally nonuniform nonlinearity

The evolution of two-frequency solitons in an optical fiber, as well as the practically important special case of absence of the second-harmonic wave, in the presence of a longitudinal nonuniformity of the coefficients characterizing the propagation nonlinearity are considered. The solitons found for media with constant values of the nonlinearity coefficients are used as initial distributions for media with a periodic dependence of the nonlinearity coefficients on the longitudinal coordinate. Modulation of the coefficient of cubic or quadratic nonlinearity is shown to result in oscillations of the peak intensity of the solitons (in both their components if two-color solitons are considered). In the case of a weak modulation of the nonlinearity coefficients, oscillations of the peak intensity occur at the frequency coinciding with the frequency of modulation of the nonlinearity coefficients. Under the weak influence of a periodically modulated cubic nonlinearity, parameters of quadratic solitons also oscillate upon the propagation. Regions of stability of solitons in the space of the modulation parameters are established.     

Cross-modulation instability in inhomogeneous normal-dispersion fiber amplifiers

The conditions of the onset of a modulation instability of the signal wave in a doped fiber due to nonlinear cross-modulation interaction with the pump wave are investigated. The analysis takes into consideration the pump depletion and the group-velocity mismatch of the interacting waves. A dispersion relation coupling perturbation parameters with those of pump, analytic expressions governing the integral growth rate of a harmonic perturbation, and other important characteristics determining the dynamics of the signal-wave cross-modulation instability are obtained. Based on computer simulation, the behavior of the above characteristics as a function of the initial signal-wave power, fiber length, and group-velocity mismatch of the pump and signal waves is investigated.     

Pulse dynamics in a passively mode-locked chirped-pulse fiber laser

We report experimental and numerical results on the new pulse dynamics in a passively mode-locked ytterbium-doped fiber laser operating in the chirped-pulse regime. Due to the negligible nonlinearity of added highly-positive GVD segment in the purely-normal-dispersion regime, highly-positive chirped pulses can be formed through weak intra-cavity temporal and spectral breathing. Numerical simulations reveal intra-cavity pulse evolution with local temporal stretching phenomena and pulse shaping properties.     

Pulse compression and fiber delivery of 45 fs Fourier transform limited pulses at 830 nm

A specific scheme is used for fiber delivery of ultrashort pulses using conventional elements. Starting from a standard femtosecond Ti:Al(2)O(3) oscillator (150 fs @ 830 nm), perfectly compressed ultrashort pulses with a duration of 45 fs are produced at the output of a standard two meter long single-mode fiber. The setup allows compensating independently and simultaneously second and third orders of chromatic dispersion as well as management of self-phase modulation in the fiber. It includes an optimized dispersion compensation line made of the assembly of diffraction gratings and prisms. The unsurpassed performances of the device are experimentally and numerically highlighted. Fiber delivery of sub-30 fs multinanojoule pulses is discussed.     

Formation of the amplification regime of quasi-soliton pulses in waveguides with longitudinally inhomogeneous cross sections

The possibility of ideal soliton amplification in a waveguide due to the formation of a specific inhomogeneity of its radius along the length is studied using the example of an active gradient-index (with a parabolic profile of the refractive index) optical fiber. It is shown that, in the case of an appropriate choice of the a(z) function, such soliton dynamics is possible even if the gain increment is constant over the length. The conditions imposed on the frequency dependences of the parameters determining the soliton dynamics are considered.     

Pulse compression techniques in ultrasonic non-destructive testing

The development of a pulse compression system applicable to ultrasonic flaw detection is described. The use of pulse compression technique permits a pulse-echo detection system to operate with long transmitted pulses for increased sensitivity, but without sacrificing resolution. The described system is economically implemented using an aluminium strip dispersive delay line. The operating system achieves a time-bandwidth product of 84 and sidelobe levels of —25 dB. Experimental evidence of the system's capabilities is presented.     

Dark solitons in optical fiber with inhomogeneous effects

We study the nonlinear wave propagation in an inhomogeneous optical fiber core in the normal dispersive regime. In order to include the inhomogeneous physical effects, the nonlinear Schrödinger equation (NLSE), which governs the solitary pulse propagation in optical fiber, is modified by adding terms for phase modulation and power gain or loss. The modified NLSEs are bilinearized and exact dark soliton solutions are obtained. The results are discussed.