Imaging with short optical pulses

A new method of three-dimensional optical imaging based on range gating with subpicosecond pulses is being developed. As opposed to other techniques of medical imaging, this new method is characterized by both high contrast and resolution. Preliminary tests demonstrate a longitudinal resolution better than 10 μm. The computer algorithm needed to reconstruct the image from the attenuated backscattering and to extend to three dimensions the one-dimensional depth resolution, is discussed.     

大芯径石英光纤中皮秒脉冲激光的自聚焦

本文报道了对大芯径石英光纤中皮秒脉冲激光自聚焦现象的实验研究和理论分析.文中测量了自聚焦先场的近场光强轮廓,研究了自聚焦光场的时域和频域性质,并对环形自聚焦光场的成因进行了解释.     

Propagation of optical pulses in periodic nonlinear fibers

We study the conditions of formation and propagation of soliton pulses in a periodic two-mode nonlinear fiber with allowance for dispersion of the fiber material and intermode dispersion of interacting modes under the condition of their phase synchronism. State University, Ul’yanovsk, Russia. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika, Vol. 41, No. 8, pp. 1032–1042, August, 1998.     

Dynamics of optical pulses in periodic nonlinear fibers

Dynamics of a two-mode wave packet with strong linear and nonlinear (cross-modulation) coupling is investigated. The effect of the initial conditions of the pulse excitation on its further transformation is considered. The possibility to control the dispersion parameters of the wave packet by varying the initial conditions of its input is pointed out. The effective parameters of the dispersion and nonlinearity that govern the dynamics of an optical pulse in a periodic nonlinear fiber light guide are obtained.     

Reverse propagation of femtosecond pulses in optical fibers

We present a numerical technique for reversing femtosecond pulse propagation in an optical fiber, such that given any output pulse it is possible to obtain the input pulse shape by numerically undoing all dispersion and nonlinear effects. The technique is tested against experimental results, and it is shown that it can be used for fiber output pulse optimization in both the anomalous and normal dispersion regimes.     

Amplification of extremely short pulses in optical media

The dynamics of pulses with durations comparable to the inverse transition frequency that propagate in an optical medium is studied in terms of two integrable systems of Maxwell-Bloch equations. The first model describes the field interaction with a nondegenerate medium with a permanent dipole moment and permanent external pumping. A general formula is derived for the N-soliton solution. Particular solutions are used as examples to investigate the effect of permanent dipole moment and pumping on the soliton dynamics. The second model describes the interaction between two-component electric-field pulses and a two-level degenerate medium with permanent upper-level pumping. For different initial magnetic-sublevel populations, soliton solutions are used as examples to show that pumping causes a change in polarization dynamics. A two-soliton solution is used to analyze the interaction of solitons in a two-level medium with external pumping.     

Self-induced transparency of very short optical pulses

Solutions to a first order wave equation, appropriate to ultrashort optical pulse theory and to the optical undamped Bloch equations are presented. Detuning is shown to play essential role in the physical interpretation of these solutions. Related cases have been discussed by Courtens and by Lee. It is noted that the rotating wave approximation introduces strong restrictions on the range of validity of possible solutions. Pulses with Lorentzian shapes, while implied by the equations, appear to be without physical significance.     

Nonlinear control of the propagation of optical pulses in doped optical fibers

Results of study of nonlinear processes occurring during the propagation of light pulses in optical fibers doped with atoms of rare-earth elements under conditions of atomic coherence and interference are presented. For a three-level Λ scheme of interaction, the linear (χ⁽¹⁾) and nonlinear (χ⁽³⁾) susceptibilities of such a medium are calculated. It is shown that the coefficients of Kerr nonlinearity and nonlinear absorption can reach extremely large values and can be negative. The competition between linear and nonlinear processes in the Λ scheme allows one to obtain compensation regimes when the coefficients of dispersion or absorption of the optical fiber material vanish. The efficient control of the optical properties of such a system over wide limits proves to be possible owing to variations in the parameters of light pulses at the input of the medium. The necessary conditions for realizing regimes with “slow” light, as well as for self-compression of a probing pulse on ultimately small spatial scales in a doped optical fiber, were obtained.     

Analytical description of cross-phase modulation in dispersive optical fibers

The effect of intensity distortion induced by cross-phase modulation in dispersive optical fibers is studied. It is shown that, in most configurations in which distortions that are due to cross-phase modulation can become significant to optical communications, the effect of cross-phase modulation can be described with excellent accuracy by simple analytical expressions. The presented analysis also suggests that intensity distortions owing to cross-phase modulation can be efficiently reduced by linear dispersion compensation.     

Enhanced spectral broadening of short laser pulses in high-numerical-aperture holey fibers

The influence of the structure of holey-fiber cladding on the spectral broadening of femtosecond laser pulses is experimentally studied. These experiments demonstrate that the spectral broadening of 70-fs pulses of 800-nm Ti:sapphire laser radiation transmitted through 2- and 3-μm-pitch holey fibers can be enhanced by a factor of about 1.5 by increasing the air-filling fraction of the fiber cladding from 16 up to 65%. Received: 23 April 2001 / Revised version: 18 June 2001 / Published online: 18 July 2001     

Amplification of chirped pulses in inhomogeneous three-level active optical fibers

The possibility of efficient amplification of self-similar frequency-modulated wave packets in amplifiers inhomogeneous along their length is investigated. Special features of the amplification dynamics in erbium fiber amplifiers that operate according to the three-level scheme are analyzed. It is shown that active optical fibers with a specially designed profile of the normal group-velocity dispersion allow amplifying subpicosecond pulses with high efficiency.     

Collision of extremely short optical pulses in semiconductor carbon nanotubes

The interaction of extremely short optical pulses in semiconductor carbon nanotubes is discussed. An equation is derived for the dynamics of the electromagnetic field in a system of semiconductor carbon nanotubes at low temperatures, whose solutions are analogs to the solitons of the sine-Gordon equation. The behavior of extremely short optical pulses in semiconductor carbon nanotubes on collision is analyzed.     

Nonlinear effects in optical fibers

The basic differential equations necessary to represent nonlinear propagation of short and ultrashort optical pulses in dielectric waveguides are derived. After introducing fundamental and higher-order soliton solutions of the nonlinear Schroedinger equations, the possibilities of realizing soliton-based communication systems are discussed. Higher-order nonlinear effects associated with selj-Raman gain and optical amplification in connection with the use of active fibers are also described.     

Coherence of supercontinua generated by ultrashort pulses compressed in optical fibers

Femtosecond fiber lasers together with nonlinear fibers are compact, reliable, all-fiber supercontinuum sources. Maintaining an all-fiber configuration, however, necessitates pulse compression in an optical fiber, which can lead to nonlinearities for subhundred femtosecond, nanojoule pulses. In this work we show that using large-mode-area fibers for pulse compression mitigates the nonlinearity, resulting in compressed pulses with significantly reduced satellite pulses. Consequently, supercontinua generated with these pulses are shown to have as much as a 10 dB increase in coherence fringe contrast. By using a hybrid highly nonlinear fiber-photonic crystal fiber, the continuum can be extended to visible wavelengths while still maintaining high coherence.     

Characteristics of wavelength conversion of short optical pulses in quantum dot semiconductor optical amplifiers

The characteristics of short optical pulse four-wave mixing (FWM) and amplification in quantum dot semiconductor optical amplifiers (QD-SOAs) are investigated taken into account the effect of the multi-discrete QD energy levels. Different saturation and recovery response for the electron and hole states are observed, which is attributed to different energy spacing between the energy states. We found that the 3 dB saturation energy of QD-SOA depends on the pulse width for short input pulses. Also, the optimum time delay between the probe and pump pulses in QD-SOAs, which provides maximum FWM efficiency in QD-SOAs, is smaller than the optimum delay in quantum well SOA.     

Shaping, propagation and characterization of ultrafast pulses in optical fibers

The characterization of medium- to low-energy shaped pulses at 1.55 7m through frequency-resolved optical gating (FROG) is illustrated. This capability enables the study of ultrafast pulse propagation through optical fibers. The phase dynamics detected furnishes insight on pulse evolution, specifically on soliton formation - a subject of great importance for telecommunication applications. The combination of shaping and propagation of ultrafast pulses in fibers is examined theoretically using an adaptive pulse-shaping model, based on genetic algorithms, that furnishes optimized pulse shapes for fiber propagation.     

On propagation of short pulses in strong dispersion managed optical lines

We show that the propagation of short pulses in optical lines with strong dispersion management is described by an integrable Hamiltonian system. The leading nonlinear effect is the formation of a collective dispersion which is a result of the interaction of all pulses propagating along the line. Pis’ma Zh. éksp. Teor. Fiz. 70, No. 9, 573–576 (10 November 1999) Published in English in the original Russian journal. Edited by Steve Torstveit.     

Application of variational calculus to propagation of coupled pulses in optical fibers

Two nonlinear Schrödinger equations, linked by cross-modulation terms, are used to study the nature of coupled pulse propagation in an optical fiber. The problem is formulated within the framework of variational calculus. Using Gaussian trial functions for the propagating pulses, an expression is constructed for the effective Lagrangian of the system. It is shown that this Lagrangian, via the Ritz optimization procedure, provides a basis to construct approximate solutions of the problem. Some judicious approximations are invoked to investigate how the cross modulation affects the behavior of pulse propagation. Conditions are derived under which both pulses can propagate without distortion.     

Evolution of hyperbolic-secant optical pulses towards wave breaking in quintic nonlinear fibers

Evolutions of the frequency chirps, shapes, and spectra of initial hyperbolic-secant pulses towards wave breaking are numerically investigated in the normal dispersion regimes of optical fibers with quintic nonlinearity and the developing chirps and the characteristic distances of wave breaking are analytically processed approximately. The results show that quite different mathematical expressions from those of initial Gaussian pulses are obtained. Moreover, the wave breaking here will be more intense for more oscillation peaks will appear in the pulse wings and the breaking process will last longer distance before rectangle-shaped pulses form at last. However, the quintic nonlinearity plays a similar role to the case of initial Gaussian pulses in developing chirps and bringing forward or retarding the wave breaking. The wave breaking distance will also decrease with increase of the quintic nonlinearity and the soliton order.     

长脉冲抽运光子晶体光纤四波混频和超连续谱的理论研究

本文数值研究了长脉冲抽运光子晶体光纤四波混频和超连续谱的产生.依据准连续波近似下的相位匹配条件和能量守恒定律,能够理论上确定在光子晶体光纤正常色散区抽运时,四波混频效应产生的信号光和空闲光波长;以及在光纤反常色散区抽运时,调制不稳现象产生的两个对称旁瓣波长.利用自适应分步傅里叶法,定量地模拟了利用波长为1064 nm的亚纳秒激光器抽运两种不同色散特性的光子晶体光纤四波混频效应和超连续谱的产生,模拟结果与实验结果符合非常好. 关键词： 光子晶体光纤 超连续谱 四波混频 自适应分步傅里叶法